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Old 28th June 2012, 04:38 PM   #1
noland0 is offline noland0  Australia
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Default New amp a success, then dead.

Well, I've been building a tube amp and it worked perfectly for about 10 minutes. After such time i cranked the amp (master, gain, tones all to 10), and it faded out (similar to what an tube amp would sound like if you were to turn it off and strum) and the only thing that remains is a small hum, which is constant (doesnt change if i alter volume etc).

Absolutely shattered.

I cracked it open and WITHOUT power tubes, everything's fine.

With power tubes, the heaters go to a ridiculous 27 volts. !!! I then tested it with a dim bulb tester, and sure enough, a short is detected.

I've done basic checks on the trannies (shorts, open circuits) and all seems to be fine in that department. Oh, and i've tried 4 different output tubes and 2 sets of 2 preamp tubes.

Any suggestions?

Thanks heaps for your time.

Last edited by noland0; 28th June 2012 at 05:34 PM.
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Old 28th June 2012, 05:52 PM   #2
kevinkr is offline kevinkr  United States
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This looks like a guitar amplifier thread, please look at the headers on the Tubes / Valves page and the Instruments & Amplifiers page for the explanation as to why I moved it.
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Old 28th June 2012, 05:53 PM   #3
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Quote:
Originally Posted by noland0 View Post
Well, I've been building a tube amp and it worked perfectly for about 10 minutes. After such time i cranked the amp (master, gain, tones all to 10), and it faded out (similar to what an tube amp would sound like if you were to turn it off and strum) and the only thing that remains is a small hum, which is constant (doesnt change if i alter volume etc).

Absolutely shattered.

I cracked it open and WITHOUT power tubes, everything's fine.

With power tubes, the heaters go to a ridiculous 27 volts. !!! I then tested it with a dim bulb tester, and sure enough, a short is detected.

I've done basic checks on the trannies (shorts, open circuits) and all seems to be fine in that department. Oh, and i've tried 4 different output tubes and 2 sets of 2 preamp tubes.

Any suggestions?

Thanks heaps for your time.
First, don't give up, it takes patience.
Second post lots of clear pictures and a schematic.
There is no way anybody can troubleshoot the problem without SEEING.
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Old 28th June 2012, 05:56 PM   #4
kevinkr is offline kevinkr  United States
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Need to give us a lot more detail, a hand drawn and scanned schematic of what you have built would be a good start. (Do not post other people's copyrighted drawings here without their permission)

I assume you had a speaker connected to the output?

All voltages look normal with the output tubes removed?

When you say you measure 27V on the filaments is this across them or to ground?

If to ground it indicates that a power tube has lost its filament to cathode insulation and was probably defective from the get go..

Is this an SE or PP amp, what type of output tubes are you using?
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Old 28th June 2012, 06:38 PM   #5
noland0 is offline noland0  Australia
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Thanks guys,
Sorry about the blunder with the forum

.. and for my vagueness

Anyway for clarification it's a 10 watt Phaez Daisycutter kit single ended

-Tried it with KT66, EL34, 6L6 and 6V6 and 2 different sets of 12ax7's after it died.
-Heater's are 27v AC to ground (~6.8-7.0v AC across the secondary windings)
-All voltages look normal with the output tubes removed
-When i'm debugging i'm connected a dummy 8Ohm load.

I don't think i can post the schematic, but there was a youtube vid of pictorial instructions: Phaez Daisycutter 10 Assembly Pictorial - YouTube

My work is definitely not what you'd call perfect, but I tried my darnedest.

Thanks again
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Old 28th June 2012, 07:00 PM   #6
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Absolutely shattered.

I cracked it open and WITHOUT power tubes, everything's fine.

With power tubes, the heaters go to a ridiculous 27 volts. !!! I then tested it with a dim bulb tester, and sure enough, a short is detected.


Any suggestions?

Thanks heaps for your time.[/QUOTE]

Don't be shattered. It takes time.
Could just be a bad tube, a short in a tube to the filament is not impossible. Pictures Please?
Now put just the preamp tubes in, whats the filament voltage?
Now put just the output tubes in, whats the filament voltage?

Last edited by soundguruman; 28th June 2012 at 07:03 PM.
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Old 29th June 2012, 02:42 AM   #7
noland0 is offline noland0  Australia
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Filament voltage for Preamp tubes only
6.8v AC w.r.t the windings
3.3v AC wrt ground

Filament voltage for power tubes only
6.8v AC wrt to the windings
27v (DC?) wrt ground
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Old 29th June 2012, 03:55 AM   #8
Enzo is offline Enzo  United States
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If you had 27v across your filaments, they would get VERY bright, then burn out. You have 6v across them, which is fine. I suspect your 27vDC to ground is just an elevation to reduce hum. It doesn't matter what voltage the heaters are to ground (within limits) it matter what is across the heater itself.

Without a schematic, I;d bet the 27v you find is borrowed from the power tube cathodes, which would only be present when those tubes are. Is the amp cathode biased?

I would initially suspect a solder connection not made or not well made. I might also suspect a missing grid resistor somewhere in the preamp, allowing a grid to float up to some voltage it doesn't like.
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Old 29th June 2012, 05:21 AM   #9
shanx is offline shanx  Canada
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OK I don't have a verified schematic,but a quick check showed the heater voltages are elevated on the output tubes. If they are tied to the cathode of the output tubes you are OK there. A 27 DC offset is probably normal..EnZO is bang on, normal hum reduction. Something else is crapping out, not filaments.
BTW It is a nice clean build from the pics, good thing. You should be able to trace out the voltages easily. I just hope the OT is not toasted..check that.
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Old 29th June 2012, 01:42 PM   #10
noland0 is offline noland0  Australia
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@Enzo
Yes the amp is cathode biased. Thanks for verifying the heaters to gnd.
@Shanx
Thanks. I've done basic checks on the OT, basically just checking windings. Something that may be of some significance is the 16Ohm connection i forgot to solder, but my cab is 8 Ohm so i didnt use it. Apart from that i'll retouch the amp in troublesome looking areas with solder and see how we go.
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