Stereo 100w Valve Amp

[Mr Clock]

For some time now I,ve been collecting parts for a stereo 100w valve amp. Why? I have a few 807 valves, a couple of hefty rectifier valves as well as a very big Partridge mains transformer- but its not HT only 20v @ 10A.(Possible voltage doubler/tripler PSU? And I figure with a bit of help it will be fun and a learning experience. This was inspired by the Williamson amp and an on line conversation I had with Tri comp over on another channel-in that the power of the Williamson amp is a bit puny. So something like the above with a high anode voltage and maybe using four 807,s in P/P per channel but not driven hard,as the OP valves,with either 6N7 or ECC triodes in the pre stage. I,m wary of using a EF86 pentode as I,ve had problems in the past with them.

I,ve looked at hundreds of 807/6L6/KT66 etc schematics,as well as lots of silly American amps with custard yellow/with gold fleck paint jobs. The nearest I,ve come for a basis is the Geloso G727A which uses a pair of 807,s in push/pull for output,but I find the amplifier overly complex in its pre amp stages and it has far too many I/O's.

I want to build something relatively simple, that can amplify a MM cartridge or line level source.With as little distortion as possible,so it,ll need to use FB. Just trying to bounce a few ideas around,Andy.

[pwdrive]

Hi, you need a decent HT transformer, they do come up for sale, I built an 807 amp many years ago I think it ran on 400-500v HT.

Lawrence.

[Steve_P]

There's a choice here...

http://www.ebay.co.uk/sch/i.html?_trksid...m570.l1311&_nkw=ht+transformer&_sacat=0&_from=R40

Cheers,

Steve P.

[tiv48]

i built an amp years ago useing 807's class a pp ,ht was about 450 volt output was about 20 watts rms i used an ecc82 as phase splitter.
the circuit was based on a mullard 5-10 .i must admit it was not perfect very unstable .but that was maybe my lack of knollage at the time.
ps the heater volts on an 807 are 6.3 volts so your 10 volt tx would need a regulator dropper.
to get your 100 watts i would look at class ab2 guitar amps for circuit ideas.
pps why do you need 100 watts ? most people who use high quality valve amps ar happy with 10/40 watts.
as for building a valve mag pickup eq circuit best of luck.
i dont want to put you off but look at the size of the output tx on say a leak stereo 20 and think this only outputs 12 watts.
rob t

[pwdrive]

I must admit that I was thinking that too, you would need a lot of m/a for 100w, I remember the transformer was quite big in the one I built, I seem to remember I based it on something that was in PW, 20 to 30 watts max (mono) I think.
I built it for our 1st mobile disco gig! A great foot warmer, it served us well until we got a couple of transistor jobies from Henrys Radio, we went stereo then, real progress!

Lawrence.

[pwdrive]

Managed to find the design mine was built on in the link below.
I only built the output stage, I built a seperate preamp in the deck consol, the preamp used BC107's I think, decks were 2x Garrard SP25's.

Lawrence.

[Skywave]

Many, many years ago when I was very enthusiastic about Amateur Radio 144 MHz transmissions (amplitude modulation), I found a cct. diagram in a 'Short Wave Magazine' (March 1956, I think) for a 120-watt output modulator. It used a pair of 807 with fixed bias, the rest on the valves were 6SN7. Of course, the O/P transformer had a relatively high-impedance secondary (mine was a Woden UM3), to modulate the HT for the RFPA: a QQV06-40A, but the cct. would tolerate a different transformer with the appropriate secondary winding for a 3 ~ 15 ohm speaker load. The PSU for it was a separate affair - and it was massive! Although that modulator vanished many years ago, I do still have the mains transformer (somewhere). I seem to recall it has a H.V. winding of something like 500 - 0 - 500 v. at about 500 mA: it is large - and heavy - and at those voltages and currents, positively lethal !

Al.

[Mr Clock]

Thanks Lawrence for the link,interesting. Firstly I have a good pre-amp for a MM cartridge,also if I was to make one it would be IC based. A preamps only there to boost the low MM cartridge signal with as little distortion as poss. I was talking about the pre stage,rather than a pre-amp. All PP valve amps I,ve seen use a pre stage,splitter and OP valves.

Why 100 watts? I must admit I dont really know. I started this thread to bounce a few ideas about. I have a big pair of Mission 730 speakers,which say they need a minimum of 30w going up to 150w. As usaul I take specs with a pinch of salt. Is that RMS/ my info doesnt say. I had an idea of using them in my recording studio (bedroom full of recording stuff) Re technical specs-I,m still learning about dBv/m/u etc but most importantly headroom. Wether I use the Mission;s or some proper monitors,I need power to spare.

I ,m also toying with a PA amp. I have two 15 inch Celestion woofer's,some Kef woofers (as Mids?) and some Fane horns (for highs) . Two cabs with that lot in will need some power. I want to do some gigs at some point. I want to record a backing track (drums/bass/backing guitar/keys/FX) with myself up front on lead vocals.

Lastly,I have some hefty speakers down stairs-currently driven by a transistor Yamaha amp. Its loud....but sometimes when the neighbors are out I want to FEEL the bass in my gut. I hope that addresses the 100w question. I,m a bit deaf too. : )

As for iron,(Al,if you still have the TX,will you let me know) I will most likely ask Ed D to make a proper mains/OPT TX's. I,ve been trying to upload the Geloso G272A schematic only,but my computor skills are rubbish. i dont want to upload the whole file which is in Italian. The Geloso G272A uses a mains TX with three HT windings
267/242/206volts @ 0.48A/0.5A/45mA bridge rectified with what looks like a secondary transformer. sorry,I,m still learning. Andy.

[Joe]

You can e-mail the schematic to me and tell me which bits you'd like posting. ThePillenwerfer (at) GolborneVintageRadio.Org

This goes for anybody else who has problems. The staff are here to HELP members.

- Joe

[Nick]

Not wishing to sell other sites here, but if I were you I'd tiptoe over to the UK AudioTalk forums http://www.audio-talk.co.uk/phpBB2/index.php - very similar "hands-off" type forum full of people in the UK who design and build all their own kit.

Folk there will know about 807 amps, PP UL designs etc.

Personally, I'd say that 100W was ambitious and that you'll get a hernia lugging it around - normally, the "iron" in big amps is the main expense - more power also means HEAVY, so big amps tend to be made as 2 x monoblocks with a separate PSU block (or two). I believe 807s are really only good in class AB up to maybe 20 or 30W... most speakers for SS use are not nearly efficient enough for use with valve amps...

The OPTs will be EXPENSIVE - Expect to pay well over GBP 100 EACH for s/h, and probably over GBP 200 each for new (Sowter etc.) though Amplimo are also worth a look (if you don't mind toroidal) as are Hammond.