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Diabolical Artificer 4th Dec 2015 3:07 pm

807 (maybe) amplifier build. Now EL34
2 Attachment(s)
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.

Restoration73 4th Dec 2015 4:50 pm

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 ;

Pamphonica 4th Dec 2015 4:52 pm

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!

G8HQP Dave 4th Dec 2015 5:46 pm

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.

Herald1360 4th Dec 2015 6:08 pm

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:

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.

Pamphonica 4th Dec 2015 6:16 pm

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.

joebog1 4th Dec 2015 10:46 pm

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.


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.

bikerhifinut 4th Dec 2015 11:49 pm

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.

Diabolical Artificer 7th Dec 2015 1:00 pm

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.


BTW, this site deals with a similar amplifier and my OPT.

Hartley118 7th Dec 2015 1:25 pm

Re: 807 (maybe) amplifier build.

Originally Posted by G8HQP Dave (Post 804180)
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...........

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


Diabolical Artificer 7th Dec 2015 2:17 pm

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.


Diabolical Artificer 7th Dec 2015 2:58 pm

Re: 807 (maybe) amplifier build.
2 Attachment(s)
Heres the pdf's.


G6Tanuki 7th Dec 2015 5:18 pm

Re: 807 (maybe) amplifier build.
What about a zero-bias P-P setup?

As recommended by RCA:

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 !!

Hartley118 7th Dec 2015 7:05 pm

Re: 807 (maybe) amplifier build.
Maybe you'd get that 1960s transistor sound with nostalgic crossover distortion!


Herald1360 7th Dec 2015 8:10 pm

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.

tri-comp 8th Dec 2015 4:55 pm

Re: 807 (maybe) amplifier build.
1 Attachment(s)

Originally Posted by Herald1360 (Post 804893)
I'd stay away from AB2, it's problematic to drive properly.

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):

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.

G6Tanuki 8th Dec 2015 6:35 pm

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.

tri-comp 8th Dec 2015 8:25 pm

Re: 807 (maybe) amplifier build.

Originally Posted by G6Tanuki (Post 805102)
The 5687 is a new one to me

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 !

G6Tanuki 8th Dec 2015 9:18 pm

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 and the earlier CV415 - were more my idea of fun for low power stuff. The QQV03/10 was better for VHF stuff.

tri-comp 8th Dec 2015 10:01 pm

Re: 807 (maybe) amplifier build.
1 Attachment(s)
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.

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