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Old 20th September 2005, 11:05 PM   #1
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Default very odd thing happening in my amp... help

ok... this is strange.

when i first power my amp up one channel works fine for a bit... but after say 30 secs of music it pops and goes quieter and hums more.

looking at voltages its all messed up with cathode resistors of output stage saying 40v and like 100v... supposed to be 60 and 60.

amp is a 6sn7 cap coupled to 6bx7 transformer phase splitter to PP 6B4Gs with seperate cathode resistors and filament transformers.

the other channel is very loud static when i turn it on....

its very bizzare whats going on... i have tried just about everything.

anyone in Minnesota want to help me???????????????

anyways at about wits end.

Thanks!!

lemme know if you need more info.
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Old 20th September 2005, 11:17 PM   #2
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Tubes not good? (Does the bias shift follow the tube when swapped?)

Grid bias bad? (Unlikely if transformer coupled...)

Etc.

I'm guessing the one tube drops to 40V bias because the other is running hotter, pulling B+ down.

Tim
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Old 20th September 2005, 11:25 PM   #3
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Default hmmm...

just tried this:

pulled out both input tubes (shared between channels).

no hum for 30secs or so then one channel here pop and loud hum then other channel does same thing 10 secs later.

very odd as i have never had any problem with these output tubes.

its cathode bias also.

1K resistors and 100v 220uf cathode bypass caps.

running them at 340v plate to cathode and 60v bias at 60mA each.

these are the sovteks.

don't get it at all.
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Old 20th September 2005, 11:28 PM   #4
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Default cont...

i also have grid resistors to load the previous IT stage. I think they are 10 or 20K resistors with centerpoint connected to CT of IT and returned to ground.
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Old 20th September 2005, 11:42 PM   #5
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A diagram (including power supply) is required for any chance of diagnosing the problem at a distance.
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Old 20th September 2005, 11:44 PM   #6
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Default hmm.....

would overvoltage on the filaments cause this?

measure 7.5vac on filaments which is a bit too high?

should i just use 1 filament transformer for each channels output tubes?

problem is that then I cannot individually adjust each bias... unless i go partial fixed / cathode bias
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Old 21st September 2005, 12:43 AM   #7
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Sounds like something is breaking down and loading the supply. The voltage will be pulled lower (loss of gain) and the caps won't be able to filter the ripple (hum).

Leaky coupling caps can do this too. Replace them, even if they look good.
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Old 21st September 2005, 02:16 AM   #8
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Default tried just output stage....

ok tried just output stage without any input tubes.... will be ok for about 20 secs or so then pop and hum from one channel then pop and hum from other.

pulling off the cathode bypass caps did NOT make any difference.

i guess i don't know what could be breaking down... there is not much to break down.

i will try and do a schematic but here it is in text:

PS is 374BX to 5u4G to 15uF oil cap to 10H 100 ohm choke to 47uF cap to 47uF cap to 47uF cap

this is to output stage

off the the 47uF cap is a 5.1K resistor to another 47uF cap to input stage

Input stage is:

6SN7 with 22K plate resistor 680 ohm cathode bypass 79K input resistor on grid.

this is cap coupled 1uf cap to 6BX7 with 750 ohm cathode bypass and 200uF cathode cap

loaded with phase splitting tranny to 6B4G's

6B4G's are run at 340v plate to cathode 60v bias with 1K cathode resistor and 220uF cathode caps.

all 6B4G's run from seperate filament trannys. The amp is dual mono other than a common power cord and a common filament supply for the input tubes

the input tubes are shared for the channels.

that is about it really.

star grounded.
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Old 21st September 2005, 02:48 AM   #9
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Default HF instability?

grid resistor on 6BX7?
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Old 21st September 2005, 07:20 PM   #10
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I think you are overdriving the 6B4 tubes with 7.5v on the filament and 340v anode to cathode.

Try lowering your HT and filament voltages to more respectable levels, your tubes are working too hard.

-Eric
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