Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jasonvilla on April 12, 2016, 01:26:04 am
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Hi all. I have(had) a working DIY 6v6 jcm800 type amp that I built. Pretty straight forward 2204 circuit with a few changes for the 6v6s. Well it was working fine... but the other day I decided to hook up the 115 primary tap up to see what it sounded like with more voltage and bone head me hooked up the 115v tap and the 125v tap instead of the 115v and ground. Well the fuse immediately blew. I wired it back up correctly and replaced the fuse. Now the amp isn't too happy, but the voltages seem normal. It makes hissy crackling noises, but still plays. I swapped out the power tubes, thinking i killed them,(running jj6v6s) and as soon as they warmed up the amp made weird radio/oscillating noises. Put the old tubes back in. Checked the bias, seems to be fine. Tried checking the tube plate voltage and it immediately starts making that fm/oscillation noise and freaks out my RMS multimeter with flashing display, on one tube. The other tube plate voltage checks out fine - around 425v. If I check the weird tube plate voltage with my cheapy multimeter it makes the noises but gives me a reading of around 425v. Soo Im not sure where to go from here. Any help would be greatly appreciated. Thank you!
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It seems as though you might have done some damage to the PT's primary windings? :dontknow:
A PT is an expensive mistake, as I can attest too.
+425V is already rather high voltage for 6V6s.
I'm not sure what you expected from feeding them +450 or more. :w2:
A variac that could give you 5-10% overvoltage would have been a better way to try to experiment, IMO.
You probably don't have another PT to swap in to see if this is the source of your problem.
I didn't when I blew mine. :sad2:
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+425V is already rather high voltage for 6V6s.
Look at this amp with 6V6's @ 485vdc;
http://el34world.com/charts/Schematics/files/_Miscellaneous/jimkelly_facs.pdf (http://el34world.com/charts/Schematics/files/_Miscellaneous/jimkelly_facs.pdf)
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The reply you got over at ampage pretty much tells you what you need to know. You fried the amp's power supply including the rectifier and filter caps, and all the coupling caps too probably. The tubes also may not have liked too-high a voltage on the heaters and the plates. The transformers are probably okay.
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+425V is already rather high voltage for 6V6s.
Look at this amp with 6V6's @ 485vdc;
http://el34world.com/charts/Schematics/files/_Miscellaneous/jimkelly_facs.pdf (http://el34world.com/charts/Schematics/files/_Miscellaneous/jimkelly_facs.pdf)
I know there are plenty of amps that push 6V6s very hard, and somehow they seem to survive.
The Data Sheet states a Maximum Plate Voltage of 315.
Obviously, the engineers were quite conservative with their maximum voltage rating.
IMO, exceeding the max voltage by almost 40% is rather high voltage for their rating,
50% or more just seems to be asking for trouble to me. :dontknow:
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the JJ brand will take higher voltages than most other brands. You could try replacing the tubes one at a time with known good tubes and see if it stops the noise.
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The Data Sheet states
my understanding, "max design center values" are 'ol skol USA happy tube values, the Russians or Chinese might have different ideas about happy, or center
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Thanks everyone. I have new filter caps on order and ill let you know what happens. :)