So after I biased by amp with new JJs it appeared that the amp wasn't as loud as it was before. It still sounds good but seems like I had to turn the volume knob up a notch than before the new tubes and bias. Also, the Master volumes have no effect anymore. When I get home I'll swap back in the original power to tubes and see what happens. I'm pretty sure I had the Master volume engaged when I did the bias. Does that matter?
JVM Bias question
- Thread starter GerryL98
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Matt_Krush
Well-Known Member
What did you bias it at?
The master volumes should still work...
"What'd you do?"
The master volumes should still work...
"What'd you do?"
did you swap preamp tubes too?
those mastervolumes should be turned completely down when setting bias
i don't understand why they would be non functional anymore btw
those mastervolumes should be turned completely down when setting bias
i don't understand why they would be non functional anymore btw
I didn't swap out the preamp tubes. Eurotubes is telling me it shouldn't matter if the volume was turned up, although he didn't specifically say anything about the Master volume. I will rebias with the master volume off and see what happens. If no change I'll see what happens if I put the old power tubes back in.
BTW, does it matter what channel you have engaged during the bias process? I was on OD1 Orange with the Master and Reverb engaged. Volume was only around 2
Matt_Krush
Well-Known Member
Still curious as to what you biased the tubes up/down to.
Its not impossible to have a bad tube just because they are new...anything can happen in transit.
Its not impossible to have a bad tube just because they are new...anything can happen in transit.
I biased at 86 millivolts. The Eurotubes site says to bias between 80-90 millivolts.
Even though I didn't replace the preamp tubes I did reseat them. Michael from Eurotubes said to use a known good preamp tube and check each one starting from V1. I'll try that before troubleshooting the new powertubes.
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Gerry I think you should bring the bias down to:
35mA per tube = 70mV reading. Some people go as high as 40mA -a 80mV .
35mA per tube = 70mV reading. Some people go as high as 40mA -a 80mV .
mickeydg5
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"What or why do you think the old tubes need replacing?" is my first question.
I mean I would probably not exchange a 6900 umho set with a 6100 set.
I mean I would probably not exchange a 6900 umho set with a 6100 set.
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yep! it seems that a lot of guys on here, like EL34s at around 39mA/ea (78/pair).
Still, something doesn't sound right...
JCMDOUG
Well-Known Member
I just did a power tube swap on My JVM with the Master both up on OD 1, I set the Bias to 34ma and it works like a champ, I bet you got a bad JJ power tube. I always say what was the last thing that was changed or touched when trouble shooting a problem.
So the issue was that when I slid the chassis back into the cabinet, a cluster of blue wire snagged on a protruding amp handle screw and unplugged the Molex connector to the Master volume. Issue resolved last night. Sounds fine at bedroom levels. I'll semi-crank it after work.
glad you got it sorted out & was something easy!I'd still try biasing it down around 78, instead of in the 80s+ My amp came alive & breathes a bit. Seems less compressed & more dynamic...
It's really amazing what a few mA/mV can do to change the response of the amp.
I'd still try biasing it down around 78, instead of in the 80s+ My amp came alive & breathes a bit. Seems less compressed & more dynamic...
It's really amazing what a few mA/mV can do to change the response of the amp.
I'm pretty sure I will turn bias it down. Weird thing is that before I started the bias process the reading I got was 89.
what's the AC voltage from the wall socket???
Matt_Krush
Well-Known Member
89mV stock?
I think we have a new high record...jk
Seems most Marshalls have been sent out on the cool side.
My JVM was at 66mV/33ma...(many people state this about the average that they've seen as well).
During the mods, I installed KT77's and moved the bias up to 77mV/38.5mA.
It had Winged C's so...I'll be hanging on to those.
My Jubilee reissue was set at a cold 58mV/29mA..
I bumped it up to 80mV/40mA and it sounds much better (It came with Tung sol EL34's).
I think we have a new high record...jk
Seems most Marshalls have been sent out on the cool side.
My JVM was at 66mV/33ma...(many people state this about the average that they've seen as well).
During the mods, I installed KT77's and moved the bias up to 77mV/38.5mA.
It had Winged C's so...I'll be hanging on to those.
My Jubilee reissue was set at a cold 58mV/29mA..
I bumped it up to 80mV/40mA and it sounds much better (It came with Tung sol EL34's).
spacerocker
Well-Known Member
Are you sure your meter is accurate? I agree with others here that stock is usually 65-70 mV.
What is your plate voltage? I bias at 78mV which gives me 65% dissipation. Biasing too high can shorten the life of your valves, and cause red-plating!
To be more accurate, is was 89mV when I checked it and it had the stock tubes. It's a Craigslist find. Amp is dated 2007. I think I'm either the 3rd or 4th owner. Don't know what, if anything, the previous owners did to it.
