New valve amp build - 30w 6146 AB2 PP.

[Kalee20]

Can you put up the circuit diagram as it is at present?

3 stages of decoupling, each adding 60 degrees of phase shift, could conceivably form a phase-shift oscillator.

And I have had a push-pull amplifier doing low-frequency common-mode oscillations. You hardly see any oscillation on the output itself, but if the oscillations are wild enough to drive the output valves into cutoff, the audio periodically stops and starts.

[Diabolical Artificer]

Will do Paul. At present it's stripped it back, no tone controls or vol pot, OP stage, CF/driver, PS & gain stage, latter three decoupled = quiet as a lamb. BTW de-coupled = de-coupled from HT. Will re-fit tone controls and see if the fun starts and will then re-draw schematic.

Andy.

[Kalee20]

Will do Paul.

It's Peter actually... One extra character to type, but they are all on the same row of the Qwerty keyboard whereas Paul requires 3 changes of row, so hopefully I can sell it on basis of overall a slight efficiency improvement nonetheless 

[Amie]

Will do Paul.

It's Peter actually... One extra character to type, but they are all on the same row of the Qwerty keyboard whereas Paul requires 3 changes of row, so hopefully I can sell it on basis of overall a slight efficiency improvement nonetheless 

Actually Andy, Peter doesn't mind what you call him as long as its not "late for Dinner"

[Diabolical Artificer]

Sorry, where have I go Paul from? Well I got the first letter right,sorry Peter. I often feel like Lord Emsworth " Stop squawking Connie, like a.... (puzzled look) multi coloured thing, sits on your shoulder..... mind your fingers"

Ker ching. Thanks April for the comedy.

A.

[Amie]

No problem Anthony:

Yes folks, we're here all week...

Back to the amplifier now...

Regards,
Avril.

[Diabolical Artificer]

Thanks Amie, it'll happen to you one day for real, remember, it's downhill all the way : ) This is what I tell our Molly who's a bit younger than you in an effort to get her to not waste her youth.

Back to yon amp, put it all back together yesterday, not a whistle, chirp or other euphemism for oscillation. I've either done something right or the Gods feel benevolent at present.

I improved the HT decoupling - those three blue caps - and improved the ground bus. HT wise now all individual stages are de-coupled apart from the tone control hi gain stage and OP CF, which have one stage. Trying to fit all this in is a nightmare, there's plenty of room, but it's in the wrong place. One more ting, I re-jigged the tone control de-coupling too.

Pics attached to give some colour and interlude to waffle. There's still some tidying to do as you can see. I've used 100r in the last two de-coupling RC circuits as I don't want to drop V,  just de-couple.

Thanks Peter for your help, Andy.

[Diabolical Artificer]

Getting there but also encountered yet more problems. As you see from the pics it's all painted and pots/knobs fitted. I no longer have oscillation but do have leakage of signal between left & right channels. I suspect this is a ground issue or a bogup of wiring on my part. Grounding of stereo amps is tricky, in my case I've gone for each IP goes to each channels gain stage ground. This is slightly complicated by the cathodes followers on each IP, these are two triodes in one ECC82. Each CF is grounded separately instead of both triodes sharing a common ground as would usually be the case; this may the seat of the issue. Anyhoo, no biggy.

What is more of a problem is self induced PSU issues. The amp uses two tfmrs, one supplies HT & heater supply for both OP stages/CF-drivers & phase splitters. The other tfmr supplies a negative supply/bias, screen grid supply and IP/tone stage heater supply. I wound both tfmrs myself but have bogged up on the latter - the negative rail is sagging, should be -100v say -80v loaded,it's dropped to -62v. The regulated screen grid supply is also going into SC protection mode, probably a mistake on the PSU board.

I tested all these supplies but have obviously made a mistake or mistakes somewhere, ce la vie, I'll sort it but am suffering from a lack of enthusiasm at present so I've parked the amp out of the way for now and will drop back onto when I have a few other jobs/projects sorted. Anyroad,here's a few pics.

[BusyBee]

It's looking pretty Andy and, being so neat, it should be far easier to find the problems.

I know what you mean by lack of enthusiasm. I had that earlier in the year so was pleased when I got my new laptop just after Christmas so I had something to keep me going. I am beginning to ramp up on radio again but am trying to modulise so avoiding big 'all or nothing' projects.

Tracy

[Diabolical Artificer]

Thanks. We all get days like those Tracy, I find I have to feel well in myself to do a good job. Often I wonder into my workshop, potter about a bit but my heart ain't in it 100%. In which case I SW off the iron and go and read a book or something else I'll screw up. Also I have several "big" jobs like fixing a HP Spectrum Analyser which needs sustained enthusiasm for long periods of time, this is hard to sustain especially as I haven't a scooby doo what I'm doing most of the time, in which case it's a matter of doing a bit, then putting it away till next time. Takes yonks and I often forget what I've done before, but slowly slowly catchee monkey : )

Andy.