audio-talk

[andrew Ivimey]

well, it exists and it works. I'll post a piccy later but I am not good at such simple technology,preferring to strip clean and rebuild a bicycle bottom bracket set....

This little monoblocked amp was on eBay. The chap had done the difficult, drilling cutting filing, spraying, buying lovely expensive components including getting power transformers wound by Majestic and Sowter chokes and outputs. He'd begun to mount to odd component but lost interest about five years ago. So I snapped it up, thought a bit, guessed what voltages would be where and put in roughly appropriate components.

switched on .... and yep, that sounds nice. I removed the 45 cathode cap, removed 100k grid leaks on 45s, removed cathode cap on 7F8W, (immediate loss of power) removed and put back decoupler cap on the top of the Lundahl interstage, added 220ohm grid stoppers, just in case, and have had overall pretty obvious improvements.

It is a very small pair of monoblocks, so serious changes to the structure is difficult. I may have to keep it just as it is!

But it aquitted itself well up the hill at Andy's driving a pair of monster Tannoys. Compared to Andy's huge monoblocks of course there were huge differences and his amplifier is more like a Bentley to this Mini, but blimey this Mini can't half cut it! In a smaller room; (I don't have big rooms at all), with that long searched after, more sensitive speakers, this amp could be a stunner - more work! more thought!

choke loaded PSU for purity, CCS on the 7F8W and voltage regulator tubes for looks (no room!) adapt the output stage to take 2A3s, well why not.

Start again and build a giant version of this one!

here's the schematic. The PSU is boring; 6Z4 into 47mfd to 10Henry choke to 47mfd (could black Gate them, I suppose. A.C. heaters throughout. NO hum whatsoever so no need of humbuckers - thanks Mr Majestic.

[andrew Ivimey]

here's a piccy that doesn't so much say 1000 words, but does look about as attractive as an amplifier I've built is ever going to look. - ah but the purity of sound!

[andrew Ivimey]

I was going to take another photograph and then thought, hold on, it looks just the same!

Indeed it does. I determined it was time to swap out the 45s and put in the 2A3s but TJs, JJs and Shuguang Cs are too big. I thought there was drilling to be done and was reminded that I could make up a socket for trial purposes out of a 4 pin base and a socket - I've done it before for 83s so why not? Well I'm lazy and overworked and tired... so I remembered, what about the old RCA twibn plates and those ultra cheap Chinese copies? They fit ... and so do the rather grey Sovteks and the much better EHGGs.

And then I agonise about the correct cathode resistor. 750 ohms is great for 45s but ... I try and find I'm too high at around 500ohms; too darn'hot. So I plump for a cool running 390ohms - fine.

So now I have a cool 2A3PP which sounds 'pretty good'; louder and more detail. The only cap left is the decoupler, mentioned above. A cap on the driver valve cathode would add welly but let's leave that for a while.

Since I built it, the amp has been on LS35as (15ohm too). Apart from the trip up the hill, where it must return to drive to the monster Tannoys, I think it is time to go downstairs and se what happens there but just right now I think I'll let 'Mysterious Traveller' end.

[andrew Ivimey]

I swapped to the Elacs (beloved of HiFiWorld they have ribbon tweeters and are big bookshelfs) - same music playing - WOW!

improvements -nice - no point in hyperbole - this is too good to stop. Downstairs can happen tomorrow.

[Andrew]

Smashing, Andrew, have you been fully converted to push pull then? Not that I don't mind a bit of push pull now and then, when done properly, it so rarely is tho'....

t'other....

[Dave the bass]

Red amp...blue carpet.... we need a 'talk' Mr I

Glad to hear you're achieving maximum pleasure.

DTB

[SteveTheShadow]

Nice to see another pp amp being built Andrew.

How does it compare to the Amity, or is that an impertinent question?

Steve

[andrew Ivimey]

Questions, questions and hey... red and blue is cool!

Amity is, or was..... great but come on, 5 volts to drive it - and low impedance too, No! And I tried, as did the boys who know what they are doing, but Amity has to become Karna; three stages better.

This nifty little 45PP number came along as a sideline to inevitably and wretchedly realsing that I have to do a 'Karna' and it has quite taken my heart. 2A3 is just to see if a tad more welly hits the spot. Hm.... nice.

Don't get me wrong, I have not only not given up on SE, oh no. A good looking 2A3 parafeed (inspired by Bottlehead) is on the table and as for 6DN7s, cripes! you can use them everywhere but this parafeed concept needs a jolly good looking at!

46s and 47s need a good compare and contrast too and yes..... another PP-er in the pipelines.

I am no nearer to deciding one or the other, and it isn't just horses for courses. I have heard a PP amp that is far and away, 'not bad at all' and I suppose shortly if not sooner, Ongaku will have to be directly compared in all the appalling glory of Shostakovich #11.

I have never heard Amy Winehouse's Back to Black so, well everything and utterly astonishing before or since, either.

But my house is small and listening room is small too so don't need the luxourious enormity of wonderful Tannoys etc (maybe! )

onwards ....!

[Max N]

Hi Andrew
Did you ever publish a circuit for this lovely amp?
Which Lundahls are you using?
Cheers
Max

[andrew Ivimey]

Hello Max, Yes I did but I don't know where it is. It is about as minimal as your 211 machine. Paralleled 7F8W into Lundahl 1660s into PP output pair. 12k anode resistor for 7F8W and a cathode resistor(can't remember) and (BG100Mfd)cap. Output pair have a cathode resistor only. PSU is bog standard and cap input! Sowter ouputs and Majestic power transformers.

[Clive]

The circuit is


Andrew, the power supply, what size choke, any idea of the PTX volts and rating (I know, it's not difficult for me to work out....).

Edit....sorry, just noticed: 6Z4 into 47mfd to 10Henry choke to 47mfd, hmm choke loaded should be a good option.

Oh yes, one more question, approximately what sensitivity would you say the amp is?

[Mike H]

I'm looking at this wondering if any more resistors could be removed.

[andrew Ivimey]

Yes Clive that's the circuit. I'd guess the transformer is 300-0-300 at more than 100ma!

Yeah I know choke input but you haven't heard this little beast. Could it be improved? I wonder.

The PSU caps are bog standard BHC and the 20mfd is not BG but Aerovox. Perhaps you could get rid of the gridstopper but this valve is very high Mu and paralleled, so what's the harm in keeping it. The cathode cap just obviously gives a touch more welly for obvious reasons and doesn't hurt the sound so it stays. And that's about your lot.

I changed the 45s to 2A3s with appropriate cathode resistors and yes it was louder but not better sound, and the mains transformers got hot (could have been the heater windings, I know).

So 45PP is good enough for me (not bad at all) and does seem to raise appreciative eye-brows when heard by others.

[Clive]

I must admit I'm tempted, it would I'm sure be great with my Bastanis.

I assume it's the PTX that is the large TX as it's close to the rectifier. Any idea what Sowter is used for the OPT? Is it 5K?

[Max N]

Thanks chaps. OK, the 1660S transformer is doing the phase-splitting. So it really is a kind of smaller cousin of the old WAD PP 300B. There is definitely some magic in this configuration! Possibly in both cases the single-ended driver gives just enough second harmonic to give the sound some magic. Or perhaps it is because the interstage transformer has standing flux in the core, so it is is working in the linear part of the BH curve. This contrasts with the PP:PP interstages in say Amity and its cousins (and in my PP 211 come to think of it) where the transformer is traversing the non-linear zero-flux 'hysteresis' part of the curve all the time.
Andrew, like Clive I would also be interested to know what spec the Sowter OPTs are?
On the choke input/cap input question - one of the major benefits of choke input is of course better PSU regulation. This is obviously much more beneficial with a single-ended amp than with a push-pull amp, because the push-pull amp is drawing more or less constant current anyway.
But it still might be worth considering choke input in this case because it will reduce the current fluctuations in the mains transformer, and in theory there will be some magnetic coupling from the mains Tx to that interstage.
On the other hand, having heard the amp on several occasions driving all sorts of speakers and always sounding so nice, I would say don't change anything!