JVM biasing?

[JTyson]

So I opened up my JVM to check the bias, I hear they come pretty cold from the factory, I dont see the regular sized bias pot, but I see two smaller ones at V2-V3 (?) Are these two 1 for the outside and 1 for the inside pair, or are they not the bias adj at all?
If not, where is it?? I dont see any other adj pots in there
If its one of the two, which one? I did not want to change anything till I knew
I'm assuming its like my others, 25-40ma? ( I usually end up around 36-38ma)
Thanks


Sure is a lot of stuff crammed in there, looks like the insides of my Mac

 

[RedB4Black]

I know there are 2 bias pots - so you can use matched pairs of tubes rather than quads. Likely one pot biases the outter pair and the other the inner pair.

 

[155]

yeah thats them the 2 small pots , actually the only pots i think ,it may be 2 for left and 2 right ? but either way dial them in the same .got mine at like 38-39 sounds great , good luck

 

[bognerluv]

So I opened up my JVM to check the bias, I hear they come pretty cold from the factory, I dont see the regular sized bias pot, but I see two smaller ones at V2-V3 (?) Are these two 1 for the outside and 1 for the inside pair, or are they not the bias adj at all?
If not, where is it?? I dont see any other adj pots in there
If its one of the two, which one? I did not want to change anything till I knew
I'm assuming its like my others, 25-40ma? ( I usually end up around 36-38ma)
Thanks


Sure is a lot of stuff crammed in there, looks like the insides of my Mac

Those are them. There should also be test points very close. There are 3 "pins" with the middle being common and the left for the left trim pot and right for the right trim pot. It is a very easy amp to bias. There is a good tutorial here http://jvmforum.com/phpBB3/viewtopic.php?f=7&t=237

 

[JTyson]

Thanks guys, I appreciate the info

 

[killertone]

I think the amp sounds best at around 32mA. Hotter is not always better. I have had mine since the first run (early '07) and have tried a bunch of different settings and 32 is where it's at.

 

[blackba]

I am going to be helping my buddy bias his JVM, this thread helps alot, thanks guys.

 

[JTyson]

Looks like my tubes are not matched very well, I'm getting readings of 31.4 - 35.3 - 39.7 - 32.6 with stock Marshall tubes
I'll have to get some SED's and recheck, I'd heard of readings of 25ma, I guess this one was not as bad as that.
Killertone, I'll try that setting once I get them matched up, thanks

 

[JTyson]

I am going to be helping my buddy bias his JVM, this thread helps alot, thanks guys.

If you dont have a bias meter, the link bognerluv posted tells how to measure it off the bias pot pins, it would be the reading divided by 2. It looks like mine wasent that far out, hard to tell, the readings are not consistant with those tubes
I'm getting those readings with a Biasmaster off each tube

 

[blackba]

I am going to be helping my buddy bias his JVM, this thread helps alot, thanks guys.

If you dont have a bias meter, the link bognerluv posted tells how to measure it off the bias pot pins, it would be the reading divided by 2. It looks like mine wasent that far out, hard to tell, the readings are not consistant with those tubes
I'm getting those readings with a Biasmaster off each tube

Thanks. Saw that in the link. I do have a compu bias meter. Its really nice actually. Did you happen to measure the plate voltage too? Just curious...

http://www.compu-bias.com/

 

[JTyson]

I am going to be helping my buddy bias his JVM, this thread helps alot, thanks guys.

If you dont have a bias meter, the link bognerluv posted tells how to measure it off the bias pot pins, it would be the reading divided by 2. It looks like mine wasent that far out, hard to tell, the readings are not consistant with those tubes
I'm getting those readings with a Biasmaster off each tube

Thanks. Saw that in the link. I do have a compu bias meter. Its really nice actually. Did you happen to measure the plate voltage too? Just curious...

http://www.compu-bias.com/

I did not. I felt like proceeding with mis matched tubes was a waste of time, so I just stopped. My BiasMaster does not show plate voltage, I would have to calculate it, and I'm not sure I remember how to The stock tubes have blue dots on them like they were supposed to be matched I'm assuming the readings I was getting that were all over the map was most likely the tubes, right?
That Compu meter is very nice

 

[noob_pwn]

I used to bias my JVM at about 32ma, lows held together better at high volume, i also use a biasmaster

 

[blackba]

I did not. I felt like proceeding with mis matched tubes was a waste of time, so I just stopped. My BiasMaster does not show plate voltage, I would have to calculate it, and I'm not sure I remember how to The stock tubes have blue dots on them like they were supposed to be matched I'm assuming the readings I was getting that were all over the map was most likely the tubes, right?
That Compu meter is very nice

Plate voltage is Pin3 to Pin 8. And yes it sounds like you had some somewhat unmatched tubes. A range of 3mA's is about average in my experience for matched tubes. Might try switching them around and put the 2 ones at around 32 together and the 2 higher ones together.