807 (maybe) amplifier build. Now EL34
 

In parallel with Miguel it was my intention to build an amplifier using 807's if possible for no other reason that they're cheap and look good. However as you may know 807's are limited by they're maximum g2 voltage of 300v which limits to an extent the output power somewhat. One further disadvantage I've read, is that they're prone to throw out 3rd order harmonics, though TBH I've only read that in one place - DIY audio. There's one last reason that I may not be able to use them, and that is they may not work out with the OPT I have.

I've started out using a EF37A pentode using the layout/component values of the Leak TL12. With 360v on the anode and 500mv P-P on g1 I'm getting a good clean sinewave (1khz) of 6v P-P. The TL12 uses an ECC33 as the phase splitter, which I hav'nt got and at £250 each, I'm highly unlikely to have. I have 6SN7GT and 6SL7's in octal triodes, so will probably use one of those, which gets me to the tricky bit.

Rather than build an existing amp, which I've already done, I'd like to try and build from scratch as much as possible. The problem is I'm short on knowledge, I hope you can guide me a little.

My first problem is despite all the reading I've done I can't understand how to work out the output stage. For no other reason that I got it cheap, my OPT is a Hammond 1650TA with an A-A Z of 1900 CT, which I understood to mean in reference to the CT, but I'm not sure. The OPT has taps at 40% for UL connection and is capable of 120w. It has the usual 4/8/16 secs. I don't need an amp that kicks out that much power, so 60w is a reasonable figure to head for.

My main sticking point is trying to match the OP valves anode R to the tfmr. If I was to use the 807, the datasheet says that the 807 has an Ra of 3k, however it then goes on to say that it is anything from 3k to 12k depending on various values etc. So how do you calculate the Ra of a valve? Nothing I've read makes sense. I can work out the impedance present at the primary as Hammond kindly gave me the turns ratio,it's the valve end I'm stuck on. For the reason's I gave at the start I'll probably use 6L6GC's or EL34's and will need 4 or 6 in parallel to get 60w +. The OP stage will have to be class B.

My next query regards the front end. In the EF37A and other datasheets I've looked at, it states certain resistor values for anode and cathode self bias etc but also resistance/z for g1 of the following stage. I'm unable to find any reference or value of g1's R in datasheets. Does it refer to the grid resistor? If I use a long tailed pair for the phase invertor for instance, this is typically 1meg or so.

I realise I'm up against trying to build an amp with such a high OP with several rather than two OP valves. I also realise I have gaping holes in my knowledge which I've tried to fill by reading as much as can. I've read Morgan Jones's book - Building Valve Amps, which I thought was bl**dy useless as far as amp design was concerned, as well as a tome by JLH and several others. Though they all have snippets of useful info, none have the answers to my questions, therefore the best way I can think to learn is have a go and hopefully I won't kill myself or blow anything up too badly. : )

TFL, Andy.

The pics are of the prototype and hence all wiring etc is temporary and a bit hows your father. I know enough about layout hopefully to do a far neater job on the finished amp, which is a long way off.

Re: 807 (maybe) amplifier build.
 

Hi Andy
I think you mean Class AB1, this circuit shows a standard amp, but you can connect
the screen grids to the UL transformer instead of as shown ;
http://audioroot.net/807-pp/

Re: 807 (maybe) amplifier build.
 

If you parallel the 807s, that will produce quite a low output impedance. Can your transformer handle that?
This page has some practical advice on how to calculate output impedances of push-pull stages.
good luck!
Jeremy

Re: 807 (maybe) amplifier build.
 

Don't confuse 'anode impedance' with 'required load impedance'. They are two quite different concepts. A pentode or tetrode output will typically have a load impedance much smaller than its anode impedance, while a triode output may have a load impedance somewhat higher than its anode impedance.

Datasheet circuits for voltage amplifiers may assume a grid resistor in the following stage which is for AC purposes in parallel with the anode resistor of the stage you are designing. The value of this grid resistor is for you to decide, but some valves have a maximum safe value. It is not a resistance of g1 of the following valve, but a circuit component. At audio frequencies you can regard a valve input resistance as being almost infinite, provided that no appreciable grid current is flowing.

Re: 807 (maybe) amplifier build.
 

That transformer isn't ideal for 807s- 1950ohms a-a is quite low- suited to multiple parallel pairs of ELL34 KT88 etc. A single pair of 807s needs more like 9-10k a-a.

This table is informative:

https://www.hammfg.com/electronics/t...ic/1608a-1650a

You could try a parallel p-p pair of triode strapped 807s at 400v Va- that calls for 1500ohms a-a,not too far off, and should give a nice clean 30W of audio. The data is in the Philips datasheet for QE 06/50 in Frank's Electron Tube pages.

Something similar with three pairs running UL with around 300V Va/Vg2 might be interesting to try.

Re: 807 (maybe) amplifier build.
 

Who's confused? Parallelled valves do halve effective output impedance of the valves. That in turn reduces the turns ratio required in the output tranformer. The calculation of required load impedance and the exact turns ratio is another matter.

Some hifi amps use parallelled valves partly to make the turns ratio easier to achieve, partly to get a better wideband performance.

Re: 807 (maybe) amplifier build.
 

1900 ohms for push pull parallel 807's wired as triodes will, as pointed out fetch 30 very clean watts.
push pull parallel EL34/6CA7 into 1900 ohms in pentode mode will EASILY fetch 100 watts.
Its the basis for the larger Marshall guitar amps, and 1900 ohms is what Marshall amps use.
"Optimum" load is usually given in valve data sheets. You can use other than optimum loads but output power, distortion and valve life usually suffer.
As Herals1360 pointed out, transformer manufacturers usually give examples of valves that will be loaded close to optimally in their specifications.

Joe

edit:
What I forgot to mention, is the power supply required to go parallel in the output stage!!! Suffice to say for either 6CA7 or 807 its LARGE!!!
about 250-300 mA and its nice to have some change in your pocket so a design for 350 mA is nice.

Re: 807 (maybe) amplifier build.
 

I had a look at my book of leak amp circuits.
If you use a 6SL7 then why not go for the later circuit, the pentode is about the same, ef36/37/40/86 can be treated the same to most intents. Also the American 6SJ7 is a cheap (less than a tenner each for NOS and less than a fiver for Russian ones) equivalent to EF37 with the advantage of no topcap and grid wires waving about on top. Just my preference.
Anyway, looking at the circuit and this is from memory these days lol, its 100k and 91k for the anode resistors and 1k + 22k off the cathode with 3M3 from each grid to the junction of 1k and 22k on the LTP. its fairly universal through the range and works well.
If you're running EL34 type HT volts ( I dont know what 807's need) then you are looking at around 150k on the anode of the pentode first valve and roughly 2.2k on the cathode.
That will give plenty of drive for a pair of EL34 and I measured over 35 watts at Clip on my TL25+ clones.
Theres also the Mullard circuit with direct coupled first stage into the phase splitter.
The Leak circuit seems less fussy and I can confirm it works very well.
I'm just going off circuits I have in my library but I hope it gets you a start.
Andy.

Re: 807 (maybe) amplifier build.
 

Yes sorry, I meant AB1, and thanks for clearing up my confusion re a following stages grid Z.

I sat down yesterday and drew a few load lines for an EL34 and tried to calculate bias point etc for the output stage. The problem I ran into is that with a 1900 ohm pri Z and 400v HT, the EL34 load line when running in the class B part of AB1 ends up over the max power dissapation curve. Rob (Glowing Anode) had a look and gave me some advice and cleared up a few misconceptions I had. I suggested that I could connect my 8 ohm speaker Z to the 4 0hm tap say in order to raise the pri Z of the OPT. Rob pointed out that I would loose bass response.

At the end of the afternoon we got a little further but with no fixed design. Whatever I do, I'll have to parallel 2 - 3 pairs of valves be they 807/6L6GC or EL34's. I have 807's and EL34's so will try again today to fix a design of sorts and try it out.

Re the mains tfmr, for 60w possibly more output it will obviously have to be beefy - 6 EL34's need 9A for the htr's and 400mA at least for HT. The tfmr isn't a problem as I can wind my own with whatever secs I need.

I'm doing a lot of reading. The valve Wizard site is invaluble even though it is primerally for guitar amps. Thanks for your input, will have more to report later.

Andy.

BTW, this site deals with a similar amplifier and my OPT.
www.turneraudio.com.au/100w-monobloc3-2014.html

Re: 807 (maybe) amplifier build.
 

That 'Maximum Power Theorem' in some college textbooks has a lot of confusion to answer for. It specifies that for maximum power output, load impedance should equal source impedance , but it's dangerously misleading in active electronics, where the key factors are maximum available voltage and maximum safe current.

Attempts to apply the 'Maximum Power Theorem' in electronics can easily result in smoke! :o

Martin

Re: 807 (maybe) amplifier build.
 

I managed to find a more complete 807 datasheet. Looking at this it says with 360v HT, g2 @ 225v output z is 6000 ( 6000 /3= 2000, so near our 1900 ohms of the OPT) = 31w OP @
8% THD, this is in Class AB2 though. Not brilliant but a start in the right direction.If I put a 100k R in series that will get my g2 down to 225v @3.5mA which is what is given for g2 current no signal.

That said, Pout max is given as 25w, so why the discrepancy. If I use fixed bias (-18v) could I use a low ohm R on the cathodes , say 1- 2 ohms to even things up a bit?

Does this sound reasonable or am I talking out of my proverbial hat? I've affixed a PDF of the datasheet, in it there is a schematic for a class AB2 75w amplifier. The front end and phase splitter look like a good basis to start with.

Andy.

Re: 807 (maybe) amplifier build.
 

Heres the pdf's.

Andy.

Re: 807 (maybe) amplifier build.
 

What about a zero-bias P-P setup?

As recommended by RCA: http://n4trb.com/AmateurRadio/RCA_Ha...amtips0702.pdf

Strap the screen-and control-grids on each 807 together with a resistor or two, drive them from a centre-tapped transformer. Centre-tap to ground - no complicated bias supplies needed. Feed with 750v HT, needs to be able to deliver 120mA or so - you'll get a nice solid 120 Watts output.

(Rather than the push-pull driver shown, I'd use something like a single-ended 6V6)

Wish we could still get genuine RCA 807s for $2.30 !!

Re: 807 (maybe) amplifier build.
 

Maybe you'd get that 1960s transistor sound with nostalgic crossover distortion!

Martin

Re: 807 (maybe) amplifier build.
 

I'd stay away from AB2, it's problematic to drive properly.

The AB1 6600 load suggestion looks better. 2200 isn't that far from 1950 and you'll get more like 80 watts at about 2% distortion.

Re: 807 (maybe) amplifier build.
 

I don't subscribe to that theory.
AB2 can sound VERY sweet and driving it isn't difficult to set up.
Especially if you don't mind mixing a little sand in.
The sand goes in as a CCS (Constant Current Sink) as a replacement to the cathode-resistor in the cathode-follower drive.
Here's an example on how I did it (Posting #68 har the final schematics that are still in force after 4+ years as is the amplifier in daily use):
https://www.vintage-radio.net/forum/...t=67511&page=4

I'm attaching an interesting doc regarding ways of driving the 807 ao.
If you're planning to do an 807 amp., you should read this.

Re: 807 (maybe) amplifier build.
 

Interesting to see the 6CL6 used in some of the circuits: I can see the sense though - it's originally designed as a video output valve (delivering significant power into a load over a wide bandwidth) so would be a good choice as a driver in relatively undemanding audio applications. I'm more used to seeing it as a HF transmitter driver! Same reasons though.

The 5687 is a new one to me: I reckon it'd be good as a Class-B modulator for a traditional 10-Watt entry-level HF AM transmitter, too.

Re: 807 (maybe) amplifier build.
 

It's been around for quite a long time by now.
I believe it was first introduced in early/mid 50'ies.
Perhaps you know it better as E182CC or as the Russian variant 6Н6П ?
I belive there's even an ECC99 that's close.
It has it's drawbacks: Price and Power-Hungry Filaments.
Other than that it's one tough little cookie !

Re: 807 (maybe) amplifier build.
 

They're not bottles I'm that familiar with; in that space the 12BH7 was the "one bottle twin-triode" I thought of for low-power audio-output (preferably class-B when you were on a restricted HT-supply budget).

As a RF enthusiast the http://www.r-type.org/pdfs/qqv02-6.pdf and the earlier CV415 - http://www.r-type.org/exhib/aab0035.htm were more my idea of fun for low power stuff. The QQV03/10 was better for VHF stuff.

Re: 807 (maybe) amplifier build.
 

I do know a little about QQE's.
In the beginning of the 70'ies I made it to VHF (144MHz) with a rebuilt all-valve Storno Taxi Transceiver that featured a QQE03/12 as final.
I now stock 30 NOS Tesla & Miniwatt QQE03/12's as they are specified for audio Class AB2 and I want to go there some day.
Quoting Philips they're good at LF for 17,5W out running on Va=300V, Vg2=200V and R(a-a)=6,5K
With a cathode-follower drive that's doable.

About cathode-follower drive I remember a producer using it extensively.
I'm talking about Altec-Lansing and their Cinema-amplifiers.
Have a look at the attached.

Replacing R14 & R15 with 2 x CCS will improve on frequency response tremendously !
This circuit shows 6L6G as output valves. 807 is just an alternative version of the 6L6 with mostly the same characteristics. Very little change to the schematic needed if the 6L6 is substituted by the 807. Possibly only the bias-setting needs to be touched.

Re: 807 (maybe) amplifier build.
 

Thanks all lots to think about. T, I'm not averse to using sand and will do so in the PSU. I've been reading up on my current sinks/sources and agree it makes sense to use one.

I have a few QQV03-10's and thought about using them as a phase splitter similar to Baileys' "New phase splitter" in WW. I remember the discussion on the forum mentioning it would work better with a double pentode, the QQV03-10 is a double beam tetrode which is near enough for experimentation.

Later, Andy.

Re: 807 (maybe) amplifier build.
 

Thanks for posting that link to the original Seybold article, which I have not seen before. Although the circuit has been featured in various Handbooks over the years, they only give the 750V version. It's very useful to have the graphs, and the additional data for lower voltage supplies.

I suspect the distortion figures with Class-B will be a bit high for the serious hi-fi buffs but I have used that circuit with excellent results in a few radio transmitter modulators.

Re: 807 (maybe) amplifier build.
 

Actually he used a PCF82, so only one pentode to reduce the Miller-effect resulting in better high frequency response.
It caused quite a stir on Wireless World.
I previously uploaded all articles here:
https://www.vintage-radio.net/forum/...=108186&page=3
Posting # 55

Re: 807 (maybe) amplifier build.
 

I've been struggling with equations and formulae for a few days and in the end I knocked up a wooden chassis and used component values that I thought right or as in the case of screen resistors, had. As regards running in AB1 or 2- as I'm using UL any data is meaningless as I can't get the screen down much below anode/HT. Here's a great link on UL - www.oestex.com/tubes/ul.html.BTW, could someone tell me the purpose of the 1k R and 1n cap in the OP/anode section of this KT88 amp please. At first I though it might offer a solution to reduce screen/g2 voltage or something like it, but to reduce g2 voltage from 360v to 220v ish as on datasheet would require a 300k approx R.

I'm going to use my valve tester for the HT and bias for the OP stage ( 0-300v @ 250mA x 2 - 0 - -30v ) and see what happens and adjust values. I'm going to use fixed bias with pots on each valve. There is also a 10r R on each valve for metering. Screen R is 330 ohm, g1 R's are 10k in series with grid, 100k grid stopper going to a 20k preset wiper with adjustable bias in the left and the right to 0v/busbar. See pic.

For now, I'm using the front end of the Brimar 25SP1, but with a 6SL7GT instead of the ECC83 lashed to 6 807's in parallel pairs.

Andy.

Re: 807 (maybe) amplifier build.
 

Ah I remember that circuit well. I built an amplifier using 10 off Kt88's back in 79 or so
the sound was awesome I think that was about the last time you could get decent branded bottles

Trev

Re: 807 (maybe) amplifier build.
 

I think the 1K resistor and the 1n capacitor, may be to improve stability. Modern transformers may not need it, due to their better winding methods and materials.

Nice experiment!

Cheers

Aub

Re: 807 (maybe) amplifier build.
 

HHMM I think if I remember rightly, after the williamson amp was published, AWV in australia set up a whole bunch of 807's and ran them with voltages found in the williamson amp. Afterwards they increased the screen ratings to 400 volts. Ill try find the publication. I do know its in the AWV Radiotronics magazines!!.
As I have about 500 of them it might take a while.

Joe

Re: 807 (maybe) amplifier build.
 

Andy, see attached which I hope may be of interest.

Courtesy RSGB Radio Communication Handbook, fifth edn.

Cheers, Rob.

Re: 807 (maybe) amplifier build.
 

Since my last post I've done a bit more work on the amplifier. I changed the circuit I'm using as a template to one given in the STC 807 datasheet which is for a 75w class AB2 amplifier. I'm only using the the first two stages and phase splitter/cathode follower. I got it running today after a fashion, but it's not inverting one side as far as I can see- see pic. Other than that, with a few changes I'm getting a good clean sinewave out at about 40vRMS. (On the scope the two OP's are quite different in amplitude but read off my DMM arnt that out.

The schematic gives -78v bias, which seems excessive, lowering it appears to make no difference. I'm still trying to catch up on theory, when I've ironed out the wrinkles I'll have a go at connecting up the OP valves and OPT and at posting some graphical analysis and a few figures.

Thanks for your interest and help so far, Andy.

Re: 807 (maybe) amplifier build.
 

The schematic's right and inversion does take place.
You should connect scope ch. 1 & 2 inputs to the cathodes of the Cathode-follower drivers and observe equal amplitude, 180 degrees out-of-phase signals.
If not, your Phase-Inverter is not hooked up correctly.
The P/I being the input (left) triode of the upper 6SN7GT
To me this looks like a not-so-optimal, possibly poorly balanced P/I.
I would go for a Split-Load or a Long-Tailed-Pair P/I circuit.
Actually the circuit will work full output with any kind of P/I that's able to swing around 50-60V RMS between the 807 grids.

The 807 Bias is NOT -78V.
That's just the negative reference (Bias-Line) for the cathode-follower.
807 Bias voltage is measured directly at the cathodes of the cathode-follower drivers.
You can easily make the Bias adjustable by replacing the 2 x 22KOhm resistors with a 47KOhm linear pot., center connected to the junction of the 2 470KOhm grid resistors.
Carefull, that's not an optimal solution for a final circuit when the 807's are connected.
The voltage-variation will be far too high and the range should be limited by using suitable (experiment) resistors on both the hot and ground sides of the pot.

Re: 807 (maybe) amplifier build.
 

Thanks T that helps. I knocked up a LTP which is a lot simpler circuit but with less gain obviously. I have a pair of opposite phase signals about 35v P-P so I can start sorting out the OP section.

I have a few questions regarding this, I realised last night I may have made a few false assumptions while drawing a load line for the 807's. From the datasheet I got the Ra 6k or 6k6 then divided by 3 to get three pairs, is this wrong? With R's in parallel of the same value you halve it, so two 1k R's in parallel = 500k, but what about 3 valves with the same Ra? I've tried to calculate with the calculator using 1/ 1/6000 + 1/6000 1/6000 ( / = divide ) and get a meaningless ( to me ) result. Something like 3,0303,03030 -04. My calculator is a scientific one but the operator isn't used to calculating formulae *.

I'm making assumptions here as I said and trying to opperate the OP valves near to the values given on the data sheet, for an Ra of 6k anode voltage is 360v, g2 is 270v so my thinking is run both anode and g2 at 300 - 350v ( because UL connection). After drawing a load line, I get an OP power of 25-34w approx using a bias of -7.5 - 15v. Which brings me to my second question, the bumf I read about drawing a load line for a PP OP stage said to use the 0v bias line to find peak current and minimum anode voltage, why? Doesn't the AC signal swing higher IE more + than 0v? The datasheet sheet gives a peak G1 - G1 voltage of 40- 60v . AFAIK at low power the amp opperates in class A so on all the time then a higher power in class B part of the time in cutoff, but at some point the signal must swing above the 0v line.

My figures are all approximate at this stage because 1) there is no datasheet or figures given for an 807 at 300v HT UL, and 2) I'm still struggling with the maths * ( the various texts/books I've read, use notation in formulae I'm not 100% familiar with so it's hard to translate them. EG some use / to denote divide, * to represent X while others nothing, so trying to work out what 2(AB\Z)^ dash with wavy line on top etc for example is tricky so say the least) Therefore I'm trying to get as close as I can with calculations to get a rough schematic, then hopefully when built and powered up I can change resistors etc. I've tried to find a guide or key for notation as used in equations but the best I found was a O level Physics paper circa 1960, useless for American texts.

Understanding schematics and the maths has been very challenging and I havn't got to NFB, phase compensation, LF/HF filters etc yet. Still I understand more than when I started so am making some progress.

Below is an oscillogram of the OP's of the LTP ( top in phase bottom inverse- anode R needs adjusting) and a rough load line for the OP stage. Pink line is class A, red class B at 300v HT, blue is at 350v HT ,assuming Ra is 1000 ohm for Class A ( 1/2 OPT 2k primary), 500 for Class B (1/4 OPT pri ). The green is the max line, it isn't accurate only there as a guide.

Andy.

Re: 807 (maybe) amplifier build.
 

Once you're up to so many 807s, it may be time for the next size up, the 813. The drawback is the higher HT and the higher output transformer ratio. It's do-able though. I was involved with a bit of a beast sporting 4x813 per channel. It needed blown air just for the filament power (8x50W).

David

Re: 807 (maybe) amplifier build.
 

Ra etc- there's often confusion here between Ra (the slope resistance of the valve) and RL or R a-a (the optimum load resistance for the application). Pentodes and beam tetrodes have high Ra but the optimum load R is related more to the operating supply volts and anode current. Three valves in parallel will have a combined optimum load R of 1/3 that for one since you will need the same voltage swing at the anodes at three times the current swing.

The bumf you refer to is just talking about operation in class A1 AB1 where no grid current is allowed. For A2 AB2 just use the line appropriate to whatever degree of +ve grid voltage is involved.

Re: 807 (maybe) amplifier build.
 

I wont be able to use 813's for this amp David as I don't have any, they're not as readily available as 807's and using them would complicate the PSU and make it more expensive. I'm already struggling finding enough HV rated caps.

When I say Ra, it is to Ra-a I'm referring. In the STC datasheet the term "plate load impedance" is used, but so is "output load impedance" .

I've sorted out, I think, biasing and class of operation. Class A is biased at the midpoint, Class AB is more negative, near to cutoff where the second valve takes over and "fills in" the bottom half of the wave form. Interestingly, nothing I've read on valve amplification mentions x-over distortion which is can be a problem on tranny amps.

Andy.

Re: 807 (maybe) amplifier build.
 

Distortion at low levels in Class B amplifiers has, I believe, always been recognised since the early valve days. It's certainly described in Langford-Smith (1953) I guess though it was only with the advent of transistor amps that the zero crossing 'kink' could be really sharp and obvious, with the generation of lots of horrible high order harmonics at low levels that we associate with the term crossover distortion.

Martin

Re: 807 (maybe) amplifier build.
 

Good point Martin, I was doing some more reading last night and as you say distortion in Class B is mentioned but not called crossover distortion which is a term I guess came into use with trannys.

Andy.

Re: 807 (maybe) amplifier build.
 

Here's an example demonstrating crossover distortion in an incorrectly biased Marshall guitar amp. https://www.youtube.com/watch?v=54aKKedAMX4 .

Martin

Re: 807 (maybe) amplifier build.
 

Here's what I've built so far - see schematic. There are two mistakes in the drawing, anode of EF37A connected to g3/cathode and g2 connected to 250k R. Oh and a third, the resistor on the cathode of the EF37A shown dotted is going to be a 2k pot for NFB, it should be to the right hand side connected directly to the cathode and the 0.1u g2 bias cap will be connected there also when NFB is applied.

The amp works, kind of, but there's a problem with the second stage. Up to .5v AC sig is a clean sine, but over that, it's clipping, not sure why. Here's some figures. There is 236v HT to the first three stages - EF37A ... a 107v, K1v, g3 40v ( bit low ? ) input 1khz @ .535v AC P-P, output up to 50v AC P-P. 6SN7 ( 2nd stage ) a 108v, k 3.5v so bias a bit too positive, load line suggests - 6-8v bias better, still as it's only taking 500mV P-P before clipping, this suggests something wrong. Have tried more HT - no improvement. This second stage can put out up to 109v P-P into the PS.

The phase splitter is a LTP which started with two 83k anode R's, I increased the out of phase to 120k to balance the two signals. Measurements = a1 173v , g1 44v , both k are 68v, a2 is 164v, g2 49.5v so sections off by about 5v so not too bad.

The six 807's should have 300v HT, but my valve tester PSU ( which I'm using to power them )which should be capable of 250mA will only go up to 269v, I think the cap on the PSU is too small at 220u. 807's biased at -15v, 1v on cathode. As you see in the schematic, the pots are to adjust each valve to get them running as near as possible to each other. However when I adjusted the first four valves, adjusting the fifth - bottom left say threw out the rest. I have to look again at this set up. The valves are all of different makes but all have a similar gm. When tested I ended up with 3 pairs of matched valves - to within 10%, 5% on one pair.

I'm going to see if I have a hefty tfmr for the HT. Rough guesstimate says I'll need 500VA or above. With a 12v g - g on the 807's I got a 15w clean sine out, but stage 2 is messing things up. As soon as I turn the volume up, the signal clips and the valves or something start ringing. I tried cutting the second stage out but the sine out of the OPT was all wrong; picture a line of "n's " joined up.

One other problem I had was ripple, I'm going to sort out the power supply for the first three valves with some filters and bigger caps. Another problem I've yet to sort out concerns applying NFB. From what I've read this is found by applying a 10khz square wave to the input and adjusting the pot on the cathode on the EF37A. This should improve stability, improve the speaker damping etc. I've been unable to find a good text on phase compensation and how to calculate the value of the capacitor that is usually in parallel with the NFB resistor. It may be among the books and texts I've recently downloaded, but have yet to read.

First pic schematic, 2nd, oscillogram of output/input 1khz sine ( top trace, probe x10 = 11v P-P - bottom trace probe x1 535mV 1k. ) 3rd pic temporary chassis and PSU's, 4th 6 x 807's.

TFL, any suggestions most welcome, Andy.

Re: 807 (maybe) amplifier build.
 

I think you seriously need to consider some math here.
6 x 807 each dropping 1V across a 10R cathode resistor equals total quiescent current of 600mA!
That's about 160Watts dissipated into heat, not counting the filaments.
No wonder your powersupply sags if it was built for a total output of 1/4 amp.
What I do wonder is how your output transformer is able to cope with that !
I'm convinced it wont but since your powersupply fortunately is giving way everything survives so far.
I suppose I'm asking why you would ever consider running a 6 x 807 push-pull amp. into so heavy class-A as a beginners project ?
Triple the bias and get into class AB1, adjusting the cathode-resistor drop voltage to around 0,6V each.
Your powersupply will still be overtaxed, so pull a couple of 807's until you get things going. 4ea. or even 2 will do nicely while experimenting.

Re: 807 (maybe) amplifier build.
 

Your right, not thinking. Trying to juggle too many variables at once, I've been a bit obsessed with the anode to anode impedance and trying to match it to the OPT. I need to step back a bit and focus on the whole.

The cathodes ended up at 1v because the 20k trimmers don't allow me enough adjustment; I ended up at 1v ish not by design, but because that was the value I could get all the valves near to. I have some 100k trimmers ready to put in, and as I said I need to change the trimming set up so each one is independant. My tfmr is overwound and is quite hefty, so as you say, I'm lucky.

I have a big isolation tfmr/variac I will use tomorrow suitably rectified and filtered for the power supply but not before I've made some changes.

Ta for your advice, Andy.

Re: 807 (maybe) amplifier build.
 

Replacing the 20K pots with 100K ditto will not cure the problem.
You need more negative Bias voltage.
I suggest to feed the Bias-circuit around -25 ~ -30VDC.
Then your 20K pots will fit fine taking the amplifier to class AB1 which I find more sensible.
About -20V at Grid1 should set the cathode resistor drop around 800mV, each, depending on Va & Vg2.

Re: 807 (maybe) amplifier build.
 

I made the bias more negative on the last test, but the 20k pots still didn't give me adequate adjustment. For instance on one valve a pot reduced cathode voltage, whilst on another I couldn't adjust downwards as the pot was on it's end stop. Not too worried about this at present as I'm going to use auto bias until I get the front end sorted.

Re the front end, I have problems here. I've taken out the second stage as it complicated things and wasn't really doing anything except volume control. That and the PS has stopped working. I also had a problem with attenuation of the signal from the PS by the OP stage. The OP stage has a g1- g1 low impedance, 100k ish when measured.

So I tried building a cathode follower first using a 6V6 triode strapped. I encountered problems here trying to bias the grid. I used a resistor above the cathode load resistor with the bias tapped off at the node where they join.No matter what value R I used, I couldn't get the grid to bias properly. I tried changing the value of the grid leak R also but feeding the bias from a pot across HT and 0v got the best results. However, I ended up with distortion and a square wave riding on top of the signal. The other problem I had was that I really had to reduce my input signal to get a clean output. I tried a triode as well, with similar results. In the end I thought I may as well use bigger grid leak/stopper R's on the OP stage to get bigger Z for the PS to see.

So after two days at the bench, I'm no further forward, with as I said the LTP phase splitter not inverting. A bit frustrating, but hey ho that's how it is sometimes. I have a few ideas to try today, if I still hit a brick wall I will put the amp aside for a few days, I havn't a deadline to meet fortunately. I'ts good to step back as you can come back refreshed.

Andy.

Re: 807 (maybe) amplifier build.
 

I've rebuilt the whole front end using 1/2 of a 6SN7 as the input stage and another as a LTP phase splitter. I tried direct coupling but it was tricky to keep it stable. I connected it all up this afternoon and got 45v P-P before onset of clipping into an 8 ohm dummy load using a 1khz sine. So that's 32w "RMS" output power. Also used a speaker in series with 1/2 the dummy load, there was hum but's that's expected as my "layout" is untidy.

I'm quite chuffed, this is my first homebrew amp that I've designed and built myself, with your help here and on VRAT. There's lot's to do to make it into a finished amplifier like applying NFB etc. I'd advise anyone wishing to really learn about valves and amplifiers to have a go and build one.

Andy.

Re: 807 (maybe) amplifier build.
 

Andy,

So far, so good.
It's very rewarding listening to music streaming from an amp you created yourself :)
I take it you're only going with one pair of 807's at the moment ?
32 Watts isn't terribly much for a pair so at least you may expect the valves to live long.
Is there a chance to have a glimpse at the schematics ?
It would also be interesting to know what the 807 is now dropping across it's 10R cathode resistor i.e. are you still in Class A or more reasonable into Class AB1 ?
For AB2 you'll of course still need the cathode-follower drive and it isn't all that difficult getting it up and running.
You'll notice the difference in bass-output quality if you do. VERY firm !

Re: 807 (maybe) amplifier build.
 

That's three pairs of 807's with .6v across the sense R so 60mA quiesent. I'm running them with a common to all 50ohm R ( with the 10r sense R's on each cathode ), - 20v bias at the moment to get it going, so it will kick out a bit more. I'll use fixed bias in the finished amp.

The power output is 200w if calculated from it's P-P output - 40v which brings us back to the old chestnut, " What is the power rating of my amplifier?" It's loud on my WS test speaker (100w 4 ohm car speaker in series with 4 ohm dummy).

Thanks T for your interest and comments,I'll post a schematic later, Andy.

Re: 807 (maybe) amplifier build.
 

Here's the schematic of progress so far. I need to reduce the 62.5k R on the PS. Yesterday the two signals were spot on. Today I've tidied up the layout a bit as I have quite bad 100hz hum/ripple as a result the in phase OP is too big.


TFL,A

Re: 807 (maybe) amplifier build.
 

Some serious overkill for driving a car speaker ;D

I like your schematic showing the LTP with a CCS in the tail.
You need to get as high an impedance as possible towards the cathodes and using just 1 transistor isn't optimal.
The amplification factor of a single transistor is too low.
The CCS should eliminate any need to balance the P/I output with different anode loads.
Do you know how to calculate/dimension the CCS circuit ?
If not I'll be happy to post a page on the subject from a Morgan Jones publication.
Very easy to follow, theory as well as practical dimensioning.

Re: 807 (maybe) amplifier build.
 

I'm going to try the tranny CCS when I've sorted other issues. Thanks for the offer T, but found MJ's article online, that and I have a few books about CCS's and current mirrors. A lot of "designs" use LED's or diodes as the reference but I think a zener may be better. From what I've read a low voltage CCS is better as were not dumping heat as would be the case using HT. I'm going to use fixed bias so I'll knock up a little PSU for that and tap off a bit for a CCS and any other LT applications I might use.

Talking of fixed bias, what can I do to protect the OP stage in the invent of the bias supply failing?

I've eliminated the hum completely, now it's quiet as a the proverbial mouse. I upped the capacitance and tied all grounds to one point, still hum, so I gave it a poke with a plastic pen, the hum stopped after a poke on anode 2 of the PS, looks like a dodgy pin on the base.

The power OP is feeble for what it should do, so I need to alter the input stage and maybe the PS for more gain. I may need to add a driver stage too.

I have a problem though that I could do with a bit of help on. There is a 4v P-P saw tooth on the OP of the phase splitter - see pic. So the sine or music is riding this, not good. It looks like ripple, IE ramping up/down as the caps are charging and discharging. Could it be not enough capacitance on the PSU? (I have two 220u's in parallel as reservoir caps to drive the OP stage with a 47u for the input and a 47u for the PS) Therefore the HT is sagging? I'm sure the transformer and variac I'm using to power it all is big enough.

Andy.

Re: 807 (maybe) amplifier build.
 

What is the frequency of the sawtooth and is it related to mains frequency?
Also is the nfb connected yet?
Rob.

Re: 807 (maybe) amplifier build.
 

To protect the OP in case of bias failure simply drive a relay from the bias supply. If the bias fails the relay will relax and cut HT to the output stage.
An even better method would be to disconnect the cathodes using the relay when bias fails.

Joe