• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 Amp - Baby Huey

Hmm ok so it was up and running fine, it did have a bit of an intermitent fault, that seems to of come back perminantly.

One channel is very very quite, so on further inspection the anodes of the drivers are way out. And no change of the bias balance pot changes the voltages at all. It isnt a ccs problem, and I also went back to a resistor bias and no change there either.

Like I said though there is audio passing but very quitely.

Im checking through everything again, could someone suggest some reasons for this to help me along?

Many thanks

Charlie
 
so on further inspection the anodes of the drivers are way out

Charlie,

Would need some numbers here. Do you mean the voltages have gone high again.

Swap valves from one channel to another to eliminate a damaged valve.

I take it you swapped rca input leads over as first test. ( Sorry if that sounds a bit like asking if someone has any petrol in their tank when their car engine won't start...;) )
 
Just finished building my Baby Huey.
Very happy with the sound right now, and the build was not so difficult with all the information inside this thread.
I use a Hammond 370HX mains transformer and Hashimoto output ones.
All plastic caps for the HV and tube rectification. HV now is at 290V, I will increase the first capacitor later to go a little bit higher. Also will add a balance input.
Interstage caps are FT3 100n, as the amp will be used from 80Hz up.
The big choke is a Lundhal 1673.
El84 bias is with dn2540 current sources a la Bas, and here I have one question how to adjust them in the circuit. Should I balance the voltages on the cathodes of the El84 exactly or should I leave them as I tested outside of the amp for 38mA? Right now they are around 9V.
Thanks everybody here.
 

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Would need some numbers here. Do you mean the voltages have gone high again

Yeah with a resistor bias one plate changed with the value of resistor, and the other plate (upper) stayed at about 170V. Yesterday with the ccs bias I had to put in a resistor to one side of the balance pot to bring both plates inline, and all was fine, but then the channel started cutting out. I checked the plate voltages again and they wouldnt match (no change when turning the bias pot)

I pulled the pot out, working fine. The valve works fine in the other channel, as does the ccs. I'm going to hunt around for any dry joints etc. I just find it interesting that something has changed in a very short time. I'll keep hunting :rolleyes:


Thanks for your help.
 
With the ccs in the cathodes, the quiescent plate voltages of the LTP WON'T be equal because of tube dispersions, this is NORMAL, do not try to balance them.

But signal at plates SHOULD be balanced if you mesure WITHOUT global NFB.
The NFB could introduce some unbalance in an attempt to compensate for output tubes unbalance ! That's life.

Anyway, a perfect balance will only reduce the amount of produced even harmonics and this may not be a good idea sonically speaking ;)

Yves.
 
But signal at plates SHOULD be balanced if you mesure WITHOUT global NFB.
The NFB could introduce some unbalance in an attempt to compensate for output tubes unbalance ! That's life.

Yvesm,

My amp has no GNF. Is the balanced signal the result of correctly setting the pot under quiescent conditions? Could you give us instruction on how to balance the " signal" at the plates?

Brgds Bill
 
With the ccs in the cathodes, the quiescent plate voltages of the LTP WON'T be equal because of tube dispersions, this is NORMAL, do not try to balance them.

But signal at plates SHOULD be balanced if you mesure WITHOUT global NFB.
The NFB could introduce some unbalance in an attempt to compensate for output tubes unbalance ! That's life.

Anyway, a perfect balance will only reduce the amount of produced even harmonics and this may not be a good idea sonically speaking ;)

Yves.


Thanks again Yves. I do not use NFB for the time being. Only a 18k shunt feedback. I will do some more measurements tomorrow.
 
Took it into work today to get it on a scope. Found the fault pretty quickly when I started thinking sensibly. Turns out it was a faulty input switch. My boss said that sometimes the contact on silver switches goes bad if less than 50mA is passing through it. News to me.

Until the next drama :rolleyes:
 
Some pics.

A friend is building a nice wooden case for it, so I am looking forward to that, but not looking forward to the extra weight!

I call it Soho because of the tall buildings and the sleezy RLD :)

Thanks to all for your help

Charlie
 

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Thanks Bill. It's only my 3rd amp, still learning a lot but Im glad the layout worked... it did get a bit tight!

As for the James OPT, they worked out to be about £95 each including delivery and the huge import tax. So only a few quid more than some Hammonds. Had the PT and choke wound from majestic, they are huge!

I am now completely broke, and therefore ready for the next project :rolleyes:
 
... For a new preamp Allen Wright sent me his latest schematics for the SPV2 about 2 months before he died. They were provided on a "Comercial in Confidence" for my personal use ONLY basis, so please, no-one ask to have them posted because I won't.

Cheers,
Ian

Ian,

Although a bit off-topic, I will pose my question here, since it has to do with the subject appeared in this thread:

Does the SVP2 have great differences in relation to the published SVP?

I have already built the FVP preamp (phono + line) a long time ago (1999), before late Allen Wright decided that he would no longer use a MOSFET anywhere in the audio signal. I have been thinking of modifying it to a SVP.

Thanks.