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Old 10th Dec 2015, 8:13 am   #21
Diabolical Artificer
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Default 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.
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Old 10th Dec 2015, 5:20 pm   #22
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by G6Tanuki View Post
What about a zero-bias P-P setup?
As recommended by RCA: http://n4trb.com/AmateurRadio/RCA_Ha...amtips0702.pdf
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.
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Old 11th Dec 2015, 8:53 am   #23
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by Dr Wobble View Post
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.
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
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Old 12th Dec 2015, 8:23 am   #24
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Default 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.
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Old 12th Dec 2015, 10:42 am   #25
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Default 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

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Old 12th Dec 2015, 2:00 pm   #26
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Default 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

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Old 13th Dec 2015, 12:09 am   #27
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Default 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.

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Old 13th Dec 2015, 1:59 am   #28
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Default Re: 807 (maybe) amplifier build.

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

Courtesy RSGB Radio Communication Handbook, fifth edn.

Cheers, Rob.
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Old 20th Dec 2015, 3:26 pm   #29
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Default 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.
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Old 20th Dec 2015, 11:47 pm   #30
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Default 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.
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Old 22nd Dec 2015, 8:01 am   #31
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Default 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.
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Old 22nd Dec 2015, 10:09 am   #32
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Default 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
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Old 22nd Dec 2015, 1:49 pm   #33
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by Dr Wobble View Post
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?

........

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.
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.
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Old 27th Dec 2015, 8:45 am   #34
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Default 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.
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Old 27th Dec 2015, 3:50 pm   #35
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by Dr Wobble View Post
?......
................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.
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.

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Old 28th Dec 2015, 7:57 am   #36
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Default 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.
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Old 28th Dec 2015, 9:11 am   #37
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Default 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 .

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Old 30th Dec 2015, 1:24 pm   #38
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Default 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.
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Old 30th Dec 2015, 3:49 pm   #39
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Default 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.
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Old 30th Dec 2015, 5:09 pm   #40
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Default 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.
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