Troubleshoot my JVM

[LPMarshall hack]

I googled some old MF threads that say they were discontinued.

 

[avspecialist]

Based on your 457vdc voltage 76mA per pair is max (unless the bias points are straight cathode current (ie plates plus screens) then you can allow 4-5mA for the screens.

I know I might be a little hot, I actually called The Tube Store told them my numbers, they thought they would be fine for their tubes. So far no red-plating, over the last month or so, just the best sound ever out of this amp.

 

[Jethro Rocker]

You are correct and I tried to find them yesterday and I couldn’t, I was like WTF. Its a great tool, I know many that have them and love them also. I dont know why Weber would stop making them. I saw a few used ones for sale.

I got a Eurotubes bias meter. Same basic thing. Does 1 tube at a time. It was like 100 bucks. Shows PV and bias current. Still have to open up amp to adjust but hey, it's miles easier.

 

[LPMarshall hack]

I got a Eurotubes bias meter. Same basic thing. Does 1 tube at a time. It was like 100 bucks. Shows PV and bias current. Still have to open up amp to adjust but hey, it's miles easier.

Serious question tho...

so if you have to open up the chassis and adjust the little bias thingy anyway...what is the benefit of it?

Is it doing something else for you that a multimeter won’t? Just trying to figure out how these things work as opposed to the “traditional” way...

edit: I watched some videos and it looks like the thing does the plate voltage and milliamps equation for you. All you have to do is adjust the bias pot to the desired tube reading. Is that correct?

 

[Shae201]

Also, adjusting bias on one side affects the other. It's a balancing act to get both sides close to same DC millivolts.

 

[Geo]

Also, adjusting bias on one side affects the other. It's a balancing act to get both sides close to same DC millivolts.

You have to find the balance yes,but the voltage on the plate what has to do with the tubes adjustment,even its going up or down,i don't get it..(sorry but im new to this,just bought my first tube amp and im on the same way)

 

[KraftyBob]

I've not done this myself but figured it may be worth looking into for some.

 

[Shae201]

You have to find the balance yes,but the voltage on the plate what has to do with the tubes adjustment,even its going up or down,i don't get it..(sorry but im new to this,just bought my first tube amp and im on the same way)

If at all possible, take it to a technician and see if they will let you watch how they bias. There are important steps to prepare for biasing that you need to know. It can be dangerous. Learning to read a schematic and knowing how the power section works by having someone show you is a great start.

 

[Jethro Rocker]

Serious question tho...

so if you have to open up the chassis and adjust the little bias thingy anyway...what is the benefit of it?

Is it doing something else for you that a multimeter won’t? Just trying to figure out how these things work as opposed to the “traditional” way...

edit: I watched some videos and it looks like the thing does the plate voltage and milliamps equation for you. All you have to do is adjust the bias pot to the desired tube reading. Is that correct?

It beats taking MM probes and touching live contacts on tube sockets firstly. Simply plug in, heat up, I still do tye math quickly but it measures the PV and current. Easy peasy,no MM, no poking atound inside with probes.

 

[anitoli]

I love the smell of arcing mulit meter probes in the morning...................

 

[anitoli]

I've not done this myself but figured it may be worth looking into for some.

That exposed voltage thingy could be very dangerous. I would rather use a meter inside the chassis.

 

[LPMarshall hack]

It beats taking MM probes and touching live contacts on tube sockets firstly. Simply plug in, heat up, I still do tye math quickly but it measures the PV and current. Easy peasy,no MM, no poking atound inside with probes.

But you’re still poking around the bias pot correct?

Hopefully they’ll stay selling those again soon.

 

[guitronics]

If at all possible, take it to a technician and see if they will let you watch how they bias. There are important steps to prepare for biasing that you need to know. It can be dangerous. Learning to read a schematic and knowing how the power section works by having someone show you is a great start.

That is the best advice I have read...There were 3 guys quoting 3 different Bias voltages!

 

[Jethro Rocker]

But you’re still poking around the bias pot correct?

Hopefully they’ll stay selling those again soon.

Yeah. Relatively way safer to turn a small pot w a screwdriver though, to me, than connecting meters to live sockets. It elimates those steps. Totally worth it to me. Eurotubes likely still does.
I like bias points and adjustments on the outside better still.

 

[LPMarshall hack]

Yeah. Relatively way safer to turn a small pot w a screwdriver though, to me, than connecting meters to live sockets. It elimates those steps. Totally worth it to me. Eurotubes likely still does.
I like bias points and adjustments on the outside better still.

I need to get one. I called them and they said they should be ready for sale again in about 2-3 weeks.

 

[Jethro Rocker]

I need to get one. I called them and they said they should be ready for sale again in about 2-3 weeks.

I find it really easy. Esp when you have a few amps to bias here n there

 

[LPMarshall hack]

I find it really easy. Esp when you have a few amps to bias here n there

I also like that you can play the guitar while the thing is hooked up so you can tweak on the fly without disconnecting it.

 

[Jethro Rocker]

I also like that you can play the guitar while the thing is hooked up so you can tweak on the fly without disconnecting it.

Good point. I have done that to set by ear but know I am in a safe area of bias.

 

[LPMarshall hack]

Good point. I have done that to set by ear but know I am in a safe area of bias.

I know you know good tone, Jet. So if you like this bias thing, it’s good enough for me.

 

[Jethro Rocker]

I know you know good tone, Jet. So if you like this bias thing, it’s good enough for me.

Thanks! Well it is convenient.