Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jennings on April 26, 2010, 09:24:27 am
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Hi there...my first ever build (18 watt clone) seems to fire up fine, but the one of the EL84s keeps red-plating. Doesn't sound too bad with a guitar plugged in and strummed. I've drawn a voltage table below, and it looks rough ball park. What do you think I've missed?!
V1 (12AX7)
Pin 1 2 3 4 5 6 7 8 9
150 0 1.18 H H 151 0 1.19 H
V2 (12AX7)
Pin 1 2 3 4 5 6 7 8 9
239 63 90 H H 244 63 90 H
V4 (EL84)
Pin 1 2 3 4 5 6 7 8 9
n/a 0 12.82 H H n/a 308 n/a 287
V5 (EL84)
Pin 1 2 3 4 5 6 7 8 9
n/a 0 12.82 H H n/a 335 n/a 288
V6 (EZ81)
Pin 1 2 3 4 5 6 7 8 9
308 n/a 333 H H n/a 308 n/a n/a
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You many not have missed anything. That EL84 might be bad. Try a matched set of known good EL84's and see what happens.
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Hi there...my first ever build (18 watt clone) seems to fire up fine, but the one of the EL84s keeps red-plating. Doesn't sound too bad with a guitar plugged in and strummed. I've drawn a voltage table below, and it looks rough ball park. What do you think I've missed?!
V4 (EL84)
Pin 1 2 3 4 5 6 7 8 9
n/a 0 12.82 H H n/a 308 n/a 287
V5 (EL84)
Pin 1 2 3 4 5 6 7 8 9
n/a 0 12.82 H H n/a 335 n/a 288
I'm inclined to say the same as Tommy. That first one up there is pulling to much current and the bias voltage you have is the same on both tubes. Check the sockets and maybe reheat the solder joints with a little fresh solder just to rule out a cold solder joint. Providing the grid blockers and screen resistors are fine, I would say the tube.
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You're both right!!! Tried another set of tubes at first that I was 90% sure were OK. Same result. So I pulled a good set I'd recently used in another amp, re-flowed all the solder joints around the output stage, and bob is indeed your mother's brother!!! It now works fine. What's more, I think this actually sounds slightly better than the 18 watt clone I'd previously bought ready made a couple of years back!!! Ace!!!!!! Cheers for your help!
Now all I gotta do is wait for the cab I'm having made to arrive, and in the meantime decide what speaker to go for!!!
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In the future when you have a tube misbehaving, try to swap sockets with the tubes.
If the problem follows the tube then you know it's the tube, if the problem stays at the socket then you have other troubles, solder joint, bad resistor, etc.
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I think this actually sounds slightly better than the 18 watt clone I'd previously bought ready made a couple of years back!!!
That's just the euphoria of building it yourself coloring your hearing. :laugh:
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FWIW I've come across bad runs of EL84s before, particularly the Sovteks in those newer red boxes. Same thing with red-plating.
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I think this actually sounds slightly better than the 18 watt clone I'd previously bought ready made a couple of years back!!!
That's just the euphoria of building it yourself coloring your hearing. :laugh:
And when you start selling them they become MUCH better than the clone you bought a couple years back!!! :grin:
Nice work, post a pic when it is all done - we love amp porn!
Jim