120W PP EL34 amplifier re-born.

[Refugee]

Surely if you drive the non grounded grid of the LTP with enough audio at some point it will over drive into distortion far enough for the cathode to go above ground so that the other cathode being linked to it will follow.
The one with the grounded grid will be cut off so that only the input one takes all the current.
The CCS will need a voltage rating to cover for this otherwise you will for ever be replacing components.

[Diabolical Artificer]

Still plugging away at this amp but have managed to solve some issues. I've completely changed the drive and IP stage circuits. I did away with the cathode followers and am now using a very beefy LTP as PS and drivers using Russian 6H6N/ECC99's, a bit like a 12BH7, these are paralled, EG anodes, grids and cathodes are tied together. This will drive the amp easily up to and over 120w with only about 3v RMS IP - open loop gain.

For the IP stage I'm using another LTP but as a differential amp to deal with hum. IP isn't of the balanced type just a shielded twisted pair onto one grid with NFB going to the other grid. Ouput is SE off the non inverting side, this makes stability very good, since the change I've had far less issues with spurious oscillation. I'll do a new schematic when I get a mo.

Andy.

[Crackle]

But can you turn it up loud enough to get feedback through the record pickup.
Its looking very professional Andy
Mike

[Terry]

But can you turn it up loud enough to get feedback through the record pickup.

That doesn't prove much, Mike - you can do that with a 2W output Dansette record player if you clamp the autochanger down using the transit screws!

[Diabolical Artificer]

"But can you turn it up loud enough to get feedback through the record pickup" Not sure that's a good idea Mike, Molly's guinea pigs share my workshop, they'd probably oscillate too : )

Struggling getting my head round what level of NFB to apply, THD is 0.4% at 1v RMS OP and actually goes down at 10v RMS OP which is weird. I'm going to do some proper tests tomorrow, get some figures to get a handle on things.

Andy.

[Mark Hennessy]

Bear in mind that your THD measurement is almost certainly THD+N. So it's not unusual for the apparent distortion value to get worse as you go down in level.

The way to be sure about this is to observe the post-filter signal on a 'scope (or listen to it if you can). If I'm right, then that post-filter signal will be mostly noise, rather than 2kHz and 3kHz.

With practice, it's easy to hear what sort of distortion dominates from listening to the "residual" (the post-filter signal). Also, mains hum and other problems can often be heard. We have to really encourage our students to listen to the residual - they simply don't realise how much useful information is contained in that signal.

Of course, I could be wrong about this specific case, but in general, watch out for that - it's a trap for the unwary. Always look at and listen to the residual.

I could also be wrong about your measurement - what bit of kit are you using? It might genuinely do THD (rather than THD+N) if it's based on spectrum analysis in some form. THD is still subject to noise problems, however.

Sorry I haven't commented earlier - I've been really busy lately - but it's really looking the business now! Really nice work

[Diabolical Artificer]

Thanks Mark, I know you've suggested listening to the post filter signal before, I tried this but the distortion meter can't drive a speaker so it needs an amplifier, I've been meaning to knock one for ages. Am using a Marconi TF2331, which I think as you say measures THD+N.

Andy.

[Mark Hennessy]

Yes, that one is THD+N as it is based on a notch-filter.

Few audio test sets can drive a speaker directly* - you will need an amplified speaker. Anything will do - even crappy PC speakers, for example. The quality is unimportant.

* Some audio test sets include a speaker, and sometimes a headphone socket which could be used to drive an external speaker. They are relatively rare.

I ought to apologise for repeating myself, but I won't. It's so important to helping your understanding that you really must do it as soon as possible. And do it always. Trust me

You might have seen the one I built: https://www.markhennessy.co.uk/activespeaker/ - something like that ought to be a walk in the park for someone with your skills and experience. This unit is usually plugged into the Y output of my 'scope (something else that audio people hardly do, and that's completely mystifying), but when I'm doing distortion measurements I swing the cable over to whatever instrument I'm using.

Keep up the good work!

[Diabolical Artificer]

Will do a powered speaker as you suggest Mark, it's on The List.

Amps finished... kallooo kalay oh frabjous day... : ) Everythings tweaked, mucked about with thrice, altered, redesigned, put back as it was then redesigned and tweaked again. They sound right gradly, THD 0.28% @ 112.5w, F response 3dB at 33khz, N = 20mV, no idea what that is in dB, here's some pics....

Re the meter after mucking about trying to draw a scale I thought sod it, and decided to paint it black. I quite fancied a meter as per the Disater Area spaceship in hitchhikers, EG black on a black background with black lights, but decided to cover the pointers in gold leaf by way of contrast.

Andy.

[BusyBee]

Looking very nice Andy!

Tracy