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.
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.
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
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
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.
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.
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.
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.
A diagram (including power supply) is required for any chance of diagnosing the problem at a distance.
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
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
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.
Leaky coupling caps can do this too. Replace them, even if they look good.
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.
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.
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
Try lowering your HT and filament voltages to more respectable levels, your tubes are working too hard.
-Eric
I saw a similar effect in Sovtek 300B's. Even though the spec sheet says that they would handle 450 volts, they would not. After 1 or 2 minutes they would go into a runaway condition, leading to a loud hum and a red glow from the plates. Reducing the B+ voltage to 400 cured the problem. Their "6B4's" are derived from their 300B technology. Be careful with the filaments on Sovtek tubes, they will die easilly.
The RCA tube manual shows a 6B4 wants 6.3 volts at 1 amp. With a Sovtek, who knows.
Since both your filament and B+ are too high, see if you can borrow a Variac, and try to turn down the line voltage until your filament voltage is close to 6.3 volts, this should also lower your B+ by 15%. See if this solves your problem. Then you can figure out how to fix it.
Since both your filament and B+ are too high, see if you can borrow a Variac, and try to turn down the line voltage until your filament voltage is close to 6.3 volts, this should also lower your B+ by 15%. See if this solves your problem. Then you can figure out how to fix it.
A resistor would be required to drop 1.2v @ 1 Amp, so you would require a 1.2e resistor at about 3W rating for a single 6B4. Or otherwise you could try two diodes in series, say 1N5403. Either way, the diodes or resistors would get hot whilst in operation.
The HT you can safely bring down with a resistor and extra decoupling cap. You only need to come down some 30-40V to be safe.
-Eric
The HT you can safely bring down with a resistor and extra decoupling cap. You only need to come down some 30-40V to be safe.
-Eric
fixed it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
hey i put in dropping resistors on the filaments and ferrite beads on all grid inputs.
stable and sounds good.
seems i may have fried a coupling cap earlier as the grid of the 6BX7 arcs a little when i turn it on.
gotta get a new 6BX7.
still seems to work though.
anyways bought all the new stuff at a local surplus store.
anyone got a good 6SN7 and 6BX7?
all i got is some old GE ones.
thanks for all the help.
hey i put in dropping resistors on the filaments and ferrite beads on all grid inputs.
stable and sounds good.
seems i may have fried a coupling cap earlier as the grid of the 6BX7 arcs a little when i turn it on.
gotta get a new 6BX7.
still seems to work though.
anyways bought all the new stuff at a local surplus store.
anyone got a good 6SN7 and 6BX7?
all i got is some old GE ones.
thanks for all the help.
i have yet to pick some up.
http://search.store.yahoo.com/cgi-b...y=6sn7&.autodone=http://www.thetubestore.com/
http://search.store.yahoo.com/cgi-b...y=6sn7&.autodone=http://www.thetubestore.com/
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