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Old 16th Jan 2019, 12:12 am   #61
bikerhifinut
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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!
So.
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.
Ok.
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; 16th Jan 2019 at 12:18 am.
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Old 16th Jan 2019, 2:07 pm   #62
Diabolical Artificer
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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.

Andy.
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Old 17th Jan 2019, 12:36 am   #63
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Default Re: 120W PP EL34 amplifier re-born.

Hi to both Andy's.
Andy biker has said some very sensible things. 6 off EL34/6CA7 are hard enough to get running in known circuits, and without raising the ire of too many people, the eastern bloc versions are not a patch on those from Mullard, and I have had MULTIPLE failures with them, unless you just trickle them along. You can get 100 watts/pair, but they wont last long. To biker Andy's point1: cathode bias is always pretty safe!! Its built in protection to some degree
Point 2: running modern EL34/6CA7 @ 500 volts is the absolute top end limit!! 400 volts or so will still fetch better than 60 watts in pentode mode, and a very pleasant sounding 35 watts in UL.
Dont get too dissappointed with your results so far !!! You have done a masterful job even completing the project!! AND to such a degree of visual perfection.

To my transformers: They are SPECIFICALLY wound to suit 4 off EL34 in Ultra Linear mode with around 400 - 450 volts maximum. They will produce 100 watts @50 Hz with little distortion. They were designed to run around 60-70 watts @ 30 Hz. I did use hard biassing in my design!! around 30-35 volts, depending on your exact HT.
I did incorporate 1K5 G2 resistors between the UL taps and the commoned G2's for each side of the HT.

This brings me to G3 or the suppressor grid. Mullard did a wonderful thing by bringing this grid out so it could be used. Instead of tying it to the cathode as 99% of designs do, tie it to the raw negative bias voltage, that way its impossible to exceed maximum ratings of the valve. Traynor guitar amplifiers ( Canadian firm) used this technique, and are famous for their reliability, even when using Russian valves. They did produce some pretty big amps BTW.

Try a "standard" design, forgetting the CCS thingies! About the only modification I would suggest is to use a largish valve for driving 4 output bottles. 12BH7 comes to mind, also 6CG7.
12 volt heaters in series for the output valves is very easy to implement using single twisted pair heater runs, and couples straight to your 12**7 voltage amplifier, phase splitter/driver or separate phase splitter and then driver. Although for 4 off EL34 cathode followers are not really necessary.
I can send you more theory on all the above mentioned suggestions, especially the work done by Traynor.
I think its time you downed tools for a time and forget the amp. I always find that when I have forgotten the ONE specific aspect of an amp I am trying to improve, my head clears and I can come back to it with an open mind, rather than more running around in circles.

It is a new year and that means a new project.
I have started my pre amp/phono stage. Watch this space.

Joe
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Old 17th Jan 2019, 10:54 pm   #64
bikerhifinut
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Default Re: 120W PP EL34 amplifier re-born.

Just to clarify joe, I was thinking 500V if using 6550/KT88 types. I agree 450V as a maximum HT for EL34 is more sensible and gets you a very decent bang for the buck. The new rework has Ht around the 400V area and I really can't hear the 5 watt difference from the older job with 450V ht. As I'd expect of course. It's still loud enough to make my ears bleed in a normal UK living room.
I don't think any current production output valves are really up to their original versions durability, but that said it isnt so much of an issue on domestic audio amplifiers as we arent looking to squeeze the last milliwatt out.
My feelings are known to you both by now. My take on a lot of the fancy circuitry that gets applied around a valve is merely an attempt to get it to sound like a solid state amp, so in that case I'd buy or build one and do without the supplementary room heating. As no matter how well you getthe circuit working, even the best output transformer is going to compromise the final output?
And you should try and get the best quality OPTs you can afford, again we'd all agree on that.

Well I'm closing the workshop for a fair while now as the building work is about to kick off and there won't be room or time to spread my mess about................
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Old 18th Jan 2019, 7:28 am   #65
Diabolical Artificer
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Default Re: 120W PP EL34 amplifier re-born.

Don't know what my EL34's are, Langrex Premium whatever that means, they look well built inside. Another reason I'm reticent about using cathode bias is that I've bought 12 Welwyn 10r W21 resistors to use as sense R's, they cost quite a few bob. Although the amp can do 120w, it will hardly ever do so. as mentioned on recent threads about OP power. At normal listening level it's bunging out about a watt. So I'm a bit like yon numpty's who buy Ferrari's then sit in traffic all day. Still, I take on board your points, 40w per pair is pushing it somewhat.

Not using your OPT's on this amp Joe, using Hammond 1650TA's, 120w max UL, yours will be under run on next amp.


"This brings me to G3 or the suppressor grid....tie it to the raw negative bias voltage" That's an idea.

I know you don't like CCS joe but it gives you better balance, better BW, more OP swing and is adjustable. PS/driver stage at present is ECC99/6H6N paralleled LTP using said CCS, works well. Have ditched the cathode followers.

Havn't done any work on it for a few days, time out as you say is a good idea.

I'd understood about the 500v HT KT88 Andy and other points. the only fancy circuitry in the audio circuit is a CCS, which isn't in the scheme of things that fancy. Good luck with the building work, A.
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Old 18th Jan 2019, 1:10 pm   #66
bikerhifinut
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Default Re: 120W PP EL34 amplifier re-born.

They look very like the Chinese shuguang bottles Andy.
The Russian ones I've seen and have had seem to be more like the original Mullard style with a less wide glass envelope.
If they are new current manufacture they are either Russian saratov (i think, could be wrong), JJ or chinese, I don't know of any other new production on any scale. Doesnt mean there are'nt of course.

OK enjoy theesen, hope you get your 'onda fettled as the days are getting longer again.

Andy.
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