Power Tube replacement question

[metalmm785]

Hi all,

I have a Jcm2000 DSL100 that I’m trying to retube but I need some help to get replacement power tubes with the correct current and TC numbers to get it to bias correctly.

I got a set of JJ E34Ls with a PC rating of 40 and TC of 5.6 and I can only get the amp to bias at 70ma per side with the bias trim pots maxed out.

I just got another set of JJ tubes with a PC rating of 26 and TC of 6.6 and they only go up to 75ma per side this time around. I’m trying bias my amp at 78-80 per side with a little spare room. It seems my amp need tubes that bias warm but could someone tell me what PC and TC numbers I should be trying to get? I don’t want to change the bias resistor on my amp.

 

[mickeydg5]

Hi all,

I have a Jcm2000 DSL100 that I’m trying to retube but I need some help to get replacement power tubes with the correct current and TC numbers to get it to bias correctly.

I got a set of JJ E34Ls with a PC rating of 40 and TC of 5.6 and I can only get the amp to bias at 70ma per side with the bias trim pots maxed out.

I just got another set of JJ tubes with a PC rating of 26 and TC of 6.6 and they only go up to 75ma per side this time around. I’m trying bias my amp at 78-80 per side with a little spare room. It seems my amp need tubes that bias warm but could someone tell me what PC and TC numbers I should be trying to get? I don’t want to change the bias resistor on my amp.

Who are you buying from and what is their site?
The ratings are relative to their testing equipment. I believe most everyone dealing with tubes and testing like this do not understand.

 

[Jon Snell]

What happens when you adjust the bias pots, PR1 & PR2 and what are the grid1 voltages with regard to the dissipation?

 

[metalmm785]

Who are you buying from and what is their site?
The ratings are relative to their testing equipment. I believe most everyone dealing with tubes and testing like this do not understand

Who are you buying from and what is their site?
The ratings are relative to their testing equipment. I believe most everyone dealing with tubes and testing like this do not understand.

I got them from Tubes and More (AES). They are willing to exchange the set but they asked me what numbers I need.

 

[metalmm785]

What happens when you adjust the bias pots, PR1 & PR2 and what are the grid1 voltages with regard to the dissipation?

I don’t have a bias probe so I don’t have those numbers. I’m just using a multimeter and the external trim pots to bias the amp. The head is a 2009 MLB model.

 

[Jon Snell]

You don't require a probe, that is what the test points are for.
The voltage across the test point is the power dissipation of each side.
Take it to a suitably qualified repair shop for fault finding.

 

[PelliX]

You mentioned that you're putting in new valves, what was the deal with the old ones? Did they bias "normally"? I'll presume you don't have the amp on a variac, dropping the AC input voltage, correct?

Was the amp previously modded for a different type of output valve?

 

[metalmm785]

You mentioned that you're putting in new valves, what was the deal with the old ones? Did they bias "normally"? I'll presume you don't have the amp on a variac, dropping the AC input voltage, correct?

Was the amp previously modded for a different type of output valve?

I bought the amp used and it had the original Marshall-labeled EL34 power tubes from 2009. It looks stock and very clean. The bias reading from the test points comes out at 67 per side with those tubes. I figured it’s low since those tubes are old, which is why I wanted to install new ones.

 

[PelliX]

I bought the amp used and it had the original Marshall-labeled EL34 power tubes from 2009. It looks stock and very clean. The bias reading from the test points comes out at 67 per side with those tubes. I figured it’s low since those tubes are old, which is why I wanted to install new ones.

Those would indeed likely be stock. While those valves may indeed be past their optimum, it could be indicative of an underlying issue. What's your wall AC voltage? How comfortable are you opening it up and measuring inside the amp? Do you have an ESR meter or capacitor tester?

 

[mickeydg5]

Hi all,

I have a Jcm2000 DSL100 that I’m trying to retube but I need some help to get replacement power tubes with the correct current and TC numbers to get it to bias correctly.

I got a set of JJ E34Ls with a PC rating of 40 and TC of 5.6 and I can only get the amp to bias at 70ma per side with the bias trim pots maxed out.

I just got another set of JJ tubes with a PC rating of 26 and TC of 6.6 and they only go up to 75ma per side this time around. I’m trying bias my amp at 78-80 per side with a little spare room. It seems my amp need tubes that bias warm but could someone tell me what PC and TC numbers I should be trying to get? I don’t want to change the bias resistor on my amp.

I got them from Tubes and More (AES). They are willing to exchange the set but they asked me what numbers I need.

So AES uses APEX for matching. As I figured the values given are as tested plate current (PC) and transconductance (TC).
A lower PC with a higher TC is a stronger tube.

It seems you are using a multimeter and applying it to the CON2 bias pins of the amplifier.
Are you reading DC voltage of the appropriate range or on auto-ranging from Common/Ground pin #2 to pins #1 and #3?

 

[metalmm785]

Those would indeed likely be stock. While those valves may indeed be past their optimum, it could be indicative of an underlying issue. What's your wall AC voltage? How comfortable are you opening it up and measuring inside the amp? Do you have an ESR meter or capacitor tester?

Wall ac voltage is 121.7v 60hz). I have a regular multimeter that can measure capacitance but no specific capacitor tester. I’d be cautious but I can try to get a reading if I know what component needs to be checked.

 

[metalmm785]

So AES uses APEX for matching. As I figured the values given are as tested plate current (PC) and transconductance (TC).
A lower PC with a higher TC is a stronger tube.

It seems you are using a multimeter and applying it to the CON2 bias pins of the amplifier.
Are you reading DC voltage of the appropriate range or on auto-ranging from Common/Ground pin #2 to pins #1 and #3?

You are correct. The tubes are apex labeled. I’m using the external con2 pins on the back of the amp. I’m trying to bias it similar to how Eurotubes does it in this tutorial for DSL amps

I’m on the milliamp setting of my multimeter and testing pins 1 and 3 to check each side.

 

[mickeydg5]

You are correct. The tubes are apex labeled. I’m using the external con2 pins on the back of the amp. I’m trying to bias it similar to how Eurotubes does it in this tutorial for DSL amps

I’m on the milliamp setting of my multimeter and testing pins 1 and 3 to check each side.

Whoa, that is the problem. You kept saying mA so that is why I was asking. It has to be set for mV.

Just make sure you have the multimeter set for DCmV (0-200mV range or auto-ranging) and start biasing with the bias adjusters from center position working it up on both sides.

 

[metalmm785]

Whoa, that is the problem. You kept saying mA so that is why I was asking. It has to be set for mV.

Just make sure you have the multimeter set for DCmV (0-200mV range or auto-ranging) and start biasing with the bias adjusters from center position working it up on both sides.

Got it. Totally mixed those two up somehow. I will try to get readings again tomorrow with that in mind. Thank you!