807 Valve P-P Amplifier

[Backtoreality]

Hi everyone,
I have some 807 valves, holders and top cap connectors which I would like to use to construct a beefy stereo amplifier. Has anyone got a reasonable circuit diagram I could use? I've googled a number of times but not been particularly happy with anything I've found.
Regards, David

[G6Tanuki]

I used to build push-pull-807 amps quite a bit in the 1970s/early-1980s.

My favourite design was to triode-connect the 807s and run them in zero-bias: you'll need a 750V supply at about 130mA peak. For each channel. This way a pair can give you 120 Watts of audio.

Bearing in mind that the pair will draw grid-current you need a driver stage capable of delivering some power, from a low impedance. My way was to use a "100-Ohm Line" matching transformer connected in reverse [grids connected across the outer ends of the 100-Volt winding, with the centre-tap to earth] then use a 6L6 or another 807 as a Class-A 'cathode follower' conected to what would normally be this little transformer's secondary winding.

[Anthony Thomas]

Practical wireless publish a circuit for a guitar amplifier back in the early 1960s that used two 807s in P-P

Wouldn't a circuit that uses KT88 be something that could be adapted to suit the 807 valve.

A suitable output transformer will be a challenge I would think, Savage made one years back.

Tony

[maninashed]

Hi David, perhaps this is something to consider
http://www.chambonino.com/work/linear/lin2.html
This is a Linear Conchord, made in Leeds and popular with kit builders in the 60's. The one at the bottom of the page is actually one of mine. Someone on the forum may have the circuit diagram to hand. Could be a start, and actually quite a good sounding amp, let down by a cheap o/p transformer.
Bill

[Herald1360]

The AB class 47W suggestion in here might be a good starting point:

http://frank.pocnet.net/sheets/155/8/807.pdf

[G6Tanuki]

If you're after things like power-supply or output transformers have a look at http://www.hammondmfg.com/1608.htm

They have several tha are specifically listed for 807-type amps.

Pricing here: http://www.hammondmfg.com/pdf/uk_trans_trade_oct12.pdf

[Leon Crampin]

The Brimar 25P1 amplifier used push-pull 5B/225M valves (which are re-based and SQ 807s) and the design claimed 25W output at 0.1% THD using a Partridge P5352 transformer.

It's very like the Mullard 5-20 but uses in my view, better valves. I'm not sure how Brimar got away with running the screens in ultra-linear configuration at about 420V when the data for the 807 gives a limit of 300V. I would imagine its performance would be very comparable with that of the Mullard 5-20.

The circuit is in the back of the Brimar valve data book No. 8 - and is probably published elsewhere online.

Leon.

[ScopeMonkey]

I'm astonished the thread's got this far and nobody's mentioned the Williamson yet

Ed.

[Mark James]

I was building one of Chas Millers designs last year and he suggested using some 807's in my next one. A bit of research pulled up this Williamson variant from a few months after the original article was published in Wireless World. http://dalmura.com.au/projects/A515.pdf I might be tempted to look at it again but the limiting factor appears to be the cost of the transformers and choke.

Mark

[Ed_Dinning]

Hi David, a good choice of amp valve. Geloso made several PA amps with these valves and they sound surprisingly good. They normally ran theirs with about 35v of negative bias. It is easy to arrange the extra winding on a transformer to supply this voltage at minimal current.

Transformers for both mains and output are easily designed; if you are looking for good fidelity that should be possible with the UL connection then you should look at a sectionalised and interleaved op trans, but probably no need to go to the 14 or so sections that were used in the Williamson.

PM me if you need more details.

Ed

[Stevie342000]

Ah yes I see the Williamson has already been mentioned, but there is this one as well which can be found here nekhbet.com/STC807.pdf or try this 807 - Nekhbet

I hope that works I did a search on 807 75 watt class AB2 valve amplifier its an old STC data set for the 807. It can be found an an Australian website which I can not locate at the moment but the site includes the AWA515.

I can not attach the original file as it exceeds the limit.

[Backtoreality]

Wow, ten replies hours before I've had the chance to check the forum. Thanks everyone, lots to look into now!
David

[Stevie342000]

Hello David

Sent you a PM with the data for the 807 but just in case (belt and braces) I will post the link here. As I have found the website which is called Retrovox here is the link: http://www.retrovox.com.au/

Lots of interesting circuits on Australian broadcast equipment, for the life of me I could not remember the site but focused on my visual memory of the site some moments ago and it came back to me when I was replying to your PM.

Have fun, it's a beast of a circuit but is very flexible, uses a lot of iron but has the details on the specifics of the iron. So you could either wind the transformers yourself or find someone here who could do that or go to someone like Sowter (but they will not be cheap). It's not a cheap design, no valve amplifier was once you get above Mullard 5-10 20 watter level.

Another alternative site would be Bonavolta Audio, lots of vintage circuits and some modern ones here. The one you would be interested in would be the 6146 or for the more adventurous there is a 400 Watt KT88 design.

Which can be found here:http://www.bonavolta.ch/hobby/en/audio/audioel.htm#Amps

[lesmw0sec]

Happy memories of a rather large bass amp I made in the 60's with 4 x 807's in par. push-pull, working at the max ratings in class AB1 or thereabouts. I recall that it worked so well that the cone of the 15" LS joined me on stage at a particularly noisy do. Rather embarrassing!.

[Diabolical Artificer]

You don't say what kind of amplifier you want to build, a guitar amp is completely different to a hifi amp.

This sort of amp has been a bee in my bonnet, and a bit of an obsession for a while. I have quite a few schematics stashed away in my backup files, which will take some digging out.

I have the Geloso circuit, that Ed mentions as well as others, but AFAIK having looked into this, any circuit that uses a 6L6, KT66/88 can be adapted to use 807's. I've also seen EL34 PA amps like the Carlsbro PA 200 adapted to use the 807.

Also don't forget that there are several incarnations of the 807 out there, like the 12v htr version 1625 and numerous others.

I'll dig those schematics out for you, when I have time. Andy.

[n_r_muir]

Hi,

If you have an existing amplifier that uses 6L6 or KT66/88 etc and you just want to try out a pair of 807's to hear how they sound then 807 base to 6L6 base adapters are available from usual sources.

[joebog1]

This thread seems to have died a bit.
One of the BEST valves ever created for audio was the humble 807!!!
It was never taken seriously by the yanks, unfortunately.
I absolutely love them!!
You DONT need 75 watts!!! trust me. an pair of 807's rumbling along at 350 volts and efficient speakers will lift the rooves of most houses!!
Here is a link to an amp I massaged together a few years ago.
Its over the top if you just want Hi-Fi, but as usual I am crazy
All the transformers are hand wound by me ( except the toroids) and are 5 times oversized but I think will work. I wired the 807's as triodes and still got about 28 watts with less distortion than I could accurately measure. By that I mean WAY less than 1%
and as its ALL ( mostly) second harmonic its inaudible to even the most refined ears.

Heres a link, and yes I am the "amp_mangler"

http://www.vt4c.com/phpbb3/viewtopic.php?f=18&t=2203

best regards
Joe

[joebog1]

I didnt think during my last post, BUT AWA ( Amalgamated Wireless Valve Australasia) made quite a few broadcast band transmitters that used 807's as pre modulators.
I can remember them with the plates glowing dull red, and they lasted years !!!
The red colour would change with the modulation, so that the glow was an early "musicolour".
Origionally designed as a tough military transmitter output, or driver valve for mobile work.
Many here will remember the backpack radios from WWII with a single 807 as a class C amp. and could be coupled to:
A piece of wire thrown over a tree branch!!.
A whip that screwed direct to the output insulator.
A top of the fence, piece of fence wire !!!!
You could not have made a better choice.
There are many variants that are not commonly known,
and as such dont command E-blech prices by the Hi-Fi fraternity, one of my favorites is the CV428, which is a loctal version ( and a MUCH better base than octal or five pin)
there is also a loctal version which has the plate brought out to a base pin, instead of a topcap ( safer if ya have kids) (( but for the life of me cant remember the number))

Anyway you have picked a superb project. The ONE piece of advice I would offer is DONT scrimp on output transformers!!! The valve is SUPERB in well designed audio, and loves a high inductance to work into. The actual impedance, if you do some research, can be almost anything!!
Just adjust the volts to suit.

regards
Joe

[MALC SCOTT]

Hi Joe, I tried the link but message states "unable to deliver file", Malc.

[Colin Ames]

I got the same message.

Colin