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Old 11th December 2011, 02:31 PM   #1181
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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
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Old 11th December 2011, 03:43 PM   #1182
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Quote:
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... )
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Old 11th December 2011, 04:53 PM   #1183
vgeorge is offline vgeorge  Greece
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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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Old 11th December 2011, 05:37 PM   #1184
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Fine business !

DO NOT try to balance the voltage at cathode but insure that plate currents are balanced.
The goal being to DO NOT have any DC component in the OPT.
Of course, tubes have tolerances, this means that cathode voltages will be slightly different.

Yves.
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Old 11th December 2011, 05:56 PM   #1185
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Thanks Yves,
that means I should have to adjust cathode currents in order to have the same plate voltage between the two EL84?
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Old 11th December 2011, 06:01 PM   #1186
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No, except if the two half windings have exactly the same resistance (easy to check), this is not always true for all OPTs.
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Old 11th December 2011, 06:10 PM   #1187
vgeorge is offline vgeorge  Greece
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Ok, will check then.
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Old 11th December 2011, 06:39 PM   #1188
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Quote:
Originally Posted by Soonerorlater View Post
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


Thanks for your help.
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Old 11th December 2011, 07:15 PM   #1189
Yvesm is offline Yvesm  France
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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.
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Old 11th December 2011, 07:22 PM   #1190
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Quote:
I just find it interesting that something has changed in a very short time.
Commercially built equipment after having passed function and quality tests are usually powered up and run ("soaked" ) for at least 24 hours before being delivered to the customer. Change in temperature is an important factor in the development of faults during this period.
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