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Old Yesterday, 12:12 am   #61
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,437
Default Re: 120W PP EL34 amplifier re-born.

I'm going to play a bit of devils advocate here Andy so please take this in the spirit it's meant.
3 pairs of EL34 in parallel, for starters thats a hell of a brave thing to do, 2 pairs could be challenging enough!
If you want to persist with using 3 pairs of El34 then why not run them as individually cathode biased valves, you'll still get 25 to 30W per pair (you can squeeze more but these are realistic achievable targets) which adds up to a stonking 75 to 90W and I don't think unless you want to light up an auditorium that you'll notice the difference betwen that and 120W. Your ears will have been bleeding long before that in any normal to large sitting room.
There are a lot of advantages to this approach.
Not least of which, you'll be running your expensive power valves nicely below their limits. Right now you're squeezing their poor little spheroids rather hard. Not at all necesarry in domestic audio, we aren't amplifying a rock band here.
Now another curved ball.
for an amp and a half less heater current you could run a quad of 6550/Kt88 or equivalents there may even be some cheap telly valves that would respond to this,I don't know but others will. I stick to mainstream Audio valves as they are mostly still manufactured somewhere and dont want to upset the guy who might want a PL something or other to keep his 405 line telly working.
Ok so why do that?
You'll get 100W with cathode bias and an HT of 500V without breaking yours or the amps sweat. Or an easy 70W at around 410V Ht.
Of course I do not know what your output transformers are rated at in terms of a to a resistance. That will rule the valve lineup.
However those big well built transformers now in your possession will handle 100W and are well suited to a quad of KT88 etc.
Like I said, these are only my feelings and I can see you are riding a steep and exciting learning curve.
i've spent too many years noodling around with valve power amplifiers and have come to the conclusion that in the average UK living room it simply isnt worth trying to improve on the classic circuit configurations, the gains arent audible. Better invest in the absolutely VERY BEST output transformers you can find, this is an area where mostly the ancients had it nailed. (Theres a couple at least on here who understand the Black art of transformers).
Then build the neatest and well laid out circuit you can, using the reliable high tolerance and high stability passive components that were'nt available even as recently as the 60s or even 70s for some bits.
Don't even think about valve rectifiers, but you know this.

And if after all this, you continue on your journey of self flagellation, I'll be there with what encouragement and very limited help I can give.
Because at the end of the day I know why you are doing it. But don't get disheartened when things don't work out.

The other Andy.

Last edited by bikerhifinut; Yesterday at 12:18 am.
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Old Yesterday, 2:07 pm   #62
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 3,913
Default Re: 120W PP EL34 amplifier re-born.

Joe's humbucker circuit - see attached - is really interesting and has got me thinking about phase, in relation to the AC heaters of the OP stage. PJL also makes a good point about the naff EMF properties of PSU plate.

Regarding the latter, I'll see if I can get some thin steel plate, it will be relatively straight forward to pop this on top of the existing plate.

Re phase, at present there are two separate 18.9v AC heater windings, each is referenced to ground on the amp chassis, if they are referenced to ground on the same phase, could this be causing ground/amp chassis to pulse at 50hz? If I were to ground them out of phase, would this cancel?

This brings me to a topic I've often thought about. It's easy to think of ground, be that a ground bus or chassis, as some sort of rock or magic thing that sits at zero volts with not a lot happening, but obviously there are currents of various sorts flowing through it, in my case HT current, 6.3v DC current, AC OP stage HTR current, etc, but how would one check or monitor it? You can't swop a scope probe around and stick the probe black wire on HT say, probe tip to ground or any of the other separate voltage/current sources above. I'll admit to being a bit foggy on the use of differential scoping and what have you, that might help in cases like this, something I need to read up on.

It's an idea Andy to convert the OP HTR's to DC, there might be enough room. I'm not sure the winding's will be beefy enough though, although I did overwind them. Your right about series HTR's, but like a lot of design descisions this was one I took way back at the start of the amp build and would do differently now.

This brings us to some of your other suggestion's, which are always welcome BTW. As regards running the OP stage cathode biased I'm reticent to make too many changes as the ****** has already taken me two years to build and every change means undoing hours of work, not to mention causing damage in some cases to the existing structure of the amp. I do have an extra 20-30v for the HT if needed, when I wound the mains tfmr I wound a few more turn's on the HT secondary, just in case.

Regards the power OP, your right, it is daft, 70w is more than sufficient and there is no big difference in SPL between 70w and 120w. The reasons why I'm a bit set on squeezing out every last watt is 1) because already the amp is inefficient with over 56w to just run the OP stage HTR's, seems a good idea to ensure every watt counts 2) I have this silly preoccupation with big power OP amplifiers, despite there being several good engineering reasons why it doesn't matter 3) I'm stubborn and illogical : )

So at present I just want to get the amp finished and although there are several things that arn't perfect, if I can end up with an amp that has low THD, good BW and looks good, I'll settle at that. Else I'll be here till doomsday or have to start from scratch again.

Next amp though using Joe's OPT's will be reet, I'll be able to apply all I've learned on this one and hopefully do the job correctly from day one. Hope some of that makes sense; every suggestion and idea you put forward Andy, gets absorbed and logged away ready for the next amp build.

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