audio-talk

That choke is 57 henry without dc current (measured). Its spec is 35 henry at 330 mA, though nothing printed on it, just my memory of the spec the seller passed on to me. Going on its size weight and the testing without dc current it seems the right data. The power transformer is 725-0-725 on 230v supply rated at 500mA, originally bought after reading the Herb Flesh and Blood article. I brought a px25 amp to the first Eggborough with it in.

I dont yet know what b+ it’ll provide, as there are many factors to take into account, but the signal section design has vr150/30’s regulating each triode d3a and the 801a’s are set up autobias with slightly larger value cathode resistors to allow for b+ greater than intended.

The “many things” are:

a. transformer primary wound for 230v most of uk 240v + a bit most areas.
b. transformer current rating to provide the spec’d voltage 1/2 amp. The signal section even with d3a shunt regs 220 mA.
c. diodes are over spec’d for the design, but 5u4 or 5r4 are on the edge for the role and would require doubling anyway so no brainer, and future proffed for higher voltages.
d. eagle eyed will have noticed the repurposed microwave capacitor, I can estimate its effect from past hard earned experience.

All these factors push or pull on the b+ as found on test, so in the final analysis all filaments up to temp then b+ brought up on variac. If any surprises; find them on the way up and stop!

Still lots to do and from an older man trying to recover knowleds which was taken for granted at first eggborough. On that occasion I also hooked up a 212 amp. But the px25 was all that was powered from this transformer without any oower supply capacitors. Ive provided for that thought in this whole design, but due to my mind as it is right now Im holding back on capacitorless power supplies, though ive had great results in the past. In time all things are possible mentioned here. For now just seemed better to stop at the crutch of Black Gates. They arent a bad thing, and a lot easier to juggle. They are in series so only 100uF, should be enough with that choke, time will tell.

If I’m to get this done, need to start with a clean up.

Wish the opts had same inductance as that choke. But at 27h for one, 30h the other (you’d be quite shocked how common that variance is, hardly any Leak Stereo 20 opts match but nobody ever hears the variance) likewise these transformers were used on the best sounding amp I’ve ever hooked up. The other main factors stood out and any lack from the opts was swamped out by the enormity of the two other unique features of that design.

But there you have it, transformers vary more than most things, not to forget loudspeakers.

It’s finished it’s safe the operating point is bang on with normal mains voltage. Obviously I brought it up on the variance. Found it beneath the operating voltage maybe 10 v at variacs max. So plugged direct to mains took it to bang on. 72mA two valves plate voltage 561 620 ohm resistor combined parallel cathodes. 60uF pio bypass.

On the speakers Ant gave me bass way too boomy. Might be the speakers or top end has a problem. See what it’s like on better speakers tomorrow.

But it’s done for now, as am I!

Good effort !

Turned out to recess the voicing significantly and rolled off chronically.

Early hours today googled for phasing issues as Andrew Le and Nick both in help mode independantly suggested phase issues.

Up early doors today and found at Bartoli Valves and found “ Andy Evans suggested that secondary green should be negative so phase is 180. Aparently there is a thread meantioning the distortion introduced by the way I was using the OPT.” Exaclty opposite phase to what Im using.

Also learned I had omited to check dc current capability of Hammond 126c is only 15mA: I had lost myself absorbed in the combined s/ri/mu graph of te triode d3a, to bring the ri down mu up s up by operating it at 20mA, also mindful of the parallel 801a topology. But I had neglected to check the spec of the 126c, a transformer Ive never hooked up in an amp. It’s only rated at 15mA. I didnt want to point this out but I heard transformer saturation in the bloom of the mid to lower bass, in the form of break up at those frequencies. In its worst manifestation this saturation spreads distortion at the so important midrange. I think the amp was right on the cusp of gross distortion, and had the amp sounded any good (it didnt due to the phasing issue) it would have crashed and burned into nasty distortion at higher levels.

The good thing to bring out of this experience is how to use bifilar wound interstage transformers best. Phasing it appears is best reversed.

If I had a suitable value cathode resistor to alter the operating point of the d3a to 15mA 150v - 1.5v and changed secondary phase to opposite of primary I could probably make it sound good. I’ll have a quick look around at home. But most likely for today back at Owston, which according to the staff there is pronounced OH! Ston! Ston pronounsed like the Ston in Stonk., the much simply adea is leave bias alone take the AE interstage transformers and clip leed them in place of the Hammond’s to a/ eliminate saturation risk, and smple to experiment which phase is right. Then I’ll probably have a great amp. It already showed promise inspight of the phase issue and saturation issue, there were lovely imaging experiences at home the night before at 2 am.

So for me, Owston has had a good glimpse of the past glories now, when we experimented together with long conversations of these types of issues, as great heads got together. I particularly enjoyed expolring what might have gone wrong with Nick and Andrew together in the closing moments of yesterday afternoon.

Today I recon we’ll have a the fix.

Nothing in my life has been learned better than failure, in my work Im one of the best boiler repair engineers everywhere I go to work, and every single one of those greater skills than the masses I have is due to my own pushing the boundaries of my ability and failing. Old Amos a Yorkshire sage says “experience is summat ye get the minute after ye need it!” i.e. failure is the precursor to success!

I’ve found suitable resistor to bias D3a at 15mA. Discovered AE interstate is 50mA tolerant , but inductance halted compared to Hammond.

So I’ll put in the bias alteration and correct phase of of Hammond first.

I’ll take the AE’s in case that doesn’t work.

Sounds like you’re well on the way to getting it sorted Paul.
Bet it was great to be able to get into a serious conversation aimed at solving an engineering problem. These meets are often the only way to get face to face conversations, which are always better than email, Zoom or whatever other electronic crap.
Your experience of these meetings goes back way further than mine and it’s good that the old spirit as you put it, is back, at least in a small corner of a South Yorkshire pub/restaurant function room. A bit less grand than the ETF, but it’ll do for now.

It was good to catch up yesterday Paul, albeit briefly.

Given your concerted efforts to get the amp ready for 'Oh! ston', I guess it was pretty much inevitable that some tweaking would be required - I'm sure you'll nail it down (there is a hammer on your work bench picture ) and we can look forward to a treat next time.

It was the IT phasing.

I also changed the D3a cathode resistor @15mA.

Those remaining this morning heard it was fixed.

At home now I take it back Ant, the speakers you gave me are very good match for this amp. I’m using cheap Chinese titanium compression bullet tweeters about £35 delivered from China, they fix the dull high end of the 10khz drivers.

The 6 watts power output lacks nothing on these speakers and drove Phil’s loud enough for Owston.

Very greatful to Richard Nick Phil and the other man whose name I don’t know for waiting to allow me to fix the amp and be an audience. Special thanks to Andrew Le and Nick for confronting the issue with encouragement support and knowledge. We got it right together, that is the magic of Ohston(k)!

At home I’m in 7th heaven, probably one of the best amps I’ve built. I’ll do full circuit diagram in the fullness of time.

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Bartoli Valves, quoting Andy Evans, who I dont know, understated the phsing issue of Hammond IT’s. It copletely ruins the sound, they should come with a HMgov warning like bacca!

They are grossly wrong out of phase!

Which makes you look a pratt at Owston!

This amp is so good I seriously might go into production and sell them through Dave. After Ive found current production valves of a quality not lacking.

Congrats on the fix!

I've got a pair of 126C to use when I build a driver stage for the 833a, but I might do something with them on a MoFo gain stage first.

When I checked the Hammond documentation (a few years ago) the wiring up looked simple.

Great to see it glowing away in situ. Pleased for you Paul.