JCM 2000 Bias Mod and other mod advice

[12AX7]

I have a JCM 2000 DSL 50 watt and a JCM 2000 TSL 100 watt head and both have the thermal bias issue. I'm waiting on delivery of the bias kits from DrTube now. Looks like they may be lost in the mail so they are shipping me out another order asp and they should be here in a couple weeks (great customer service). From what I've read a lot of these boards were populated with the wrong resistor values in some spots from the factory. I have already ordered and received 20 of each value 10k,33k,220k and 40 5k6 1 watt metal film resistors to replace any of the resistors on the board that may have been installed wrong or damaged from the thermal bias issue. I've also got a new matched quad of Groove Tubes EL34-M and a new Matched set of Ruby EL34BSTR to install after the mod is complete. I also went ahead a bought 2 Classic Tone vintage plexi style 3H chokes to install while I'm in it. Is there any other resistor values I may have missed or should have on hand or have I covered all my bases? Is there any other must have mod I need to think about while I'm in there? Something I was thinking about doing is adding a toggle switch to switch between the choke and the factory resistor. Does that sound at all usable or make any sense? Also I think I read somewhere that the resistor I need to pull out and replace it with the choke is located in a different # spot on the DSL and TSL boards. If there is anyone that has done these mods or any other mods to these amps that has any advice or instructions It would be much appreciated.

 

[neikeel]

The grid stoppers were the ones that were incorrect, they should be 5k6.
The bias board can be bought unpopulated or fully populated (the boards are pretty small and the mf resistors are probably less than 1w so might be a bit of a squeeze)
The kit should come with a drill to totally isolate pin 5 of the octals you then wire the board in and it sits on standoffs above the board. You can do it with a dremel if you are careful.
I’ve not done the choke mod, but a few, incl AdrianR have and quite like it. I always think the DSL100 transformers are tiny, even compared with my JTM45s.

 

[12AX7]

I have 3 unpopulated kits coming from DrTube with the hollow bit and all the components. Since I am going to be populating them myself I was going to go up to the 1 watt metal film resistors for a little more protection if they will not fit on the board I'll just use the 1/4 watt that comes with the kits. I am thinking about swapping the output transformer for a ClassicTone # 40-18074 has anybody installed this OT? Is there a noticeable tone improvement to be worth the $88? I'm going to do a visual inspection of all the caps for any bulge or physical leaking since the DSL 50 is a 2002 model and the TSL is a 2004 model. Does anyone have any info or photos of the speaker jack ground mod? I figure this needs to be done while I'm in it. I've also thought about doing some version of the Joey mod with a 3 position toggle switching between 3 different cap values. Has anyone ever did this or does it even sound like a good idea? If I did this mod is there a spot on the amp I should avoid installing the toggle and running the wires to avoid noise ect..? What value caps would work best in this?

 

[ampmadscientist]

I have a JCM 2000 DSL 50 watt and a JCM 2000 TSL 100 watt head and both have the thermal bias issue. I'm waiting on delivery of the bias kits from DrTube now. Looks like they may be lost in the mail so they are shipping me out another order asp and they should be here in a couple weeks (great customer service). From what I've read a lot of these boards were populated with the wrong resistor values in some spots from the factory. I have already ordered and received 20 of each value 10k,33k,220k and 40 5k6 1 watt metal film resistors to replace any of the resistors on the board that may have been installed wrong or damaged from the thermal bias issue. I've also got a new matched quad of Groove Tubes EL34-M and a new Matched set of Ruby EL34BSTR to install after the mod is complete. I also went ahead a bought 2 Classic Tone vintage plexi style 3H chokes to install while I'm in it. Is there any other resistor values I may have missed or should have on hand or have I covered all my bases? Is there any other must have mod I need to think about while I'm in there? Something I was thinking about doing is adding a toggle switch to switch between the choke and the factory resistor. Does that sound at all usable or make any sense? Also I think I read somewhere that the resistor I need to pull out and replace it with the choke is located in a different # spot on the DSL and TSL boards. If there is anyone that has done these mods or any other mods to these amps that has any advice or instructions It would be much appreciated.

Besides doing the Dr tube update for stable bias, you should also replace the 2X bias adjust pots.
The bias adjust is available as a replacement board with the pots already installed...
The bias adjust pots are frequently bad in these amps.

 

[12AX7]

Where would I order these replacement boards with the pots from if mine are bad? I was able to adjust the bias to the correct settings in both heads but after it gets hot the TSL runs away the DSL seemed to be more stable . Whats the range on these pots? Maybe I sould pull the chassis a check them while I'm waiting on the bias mods to arrive. Will Deoxit not straighten them out?

Besides doing the Dr tube update for stable bias, you should also replace the 2X bias adjust pots.
The bias adjust is available as a replacement board with the pots already installed...
The bias adjust pots are frequently bad in these amps.

 

[Gunner64]

Amprepairparts.com has the 22k bias pots, they are a snap to install on the little bias board. No need to change the whole board if you can't find one. Both pots for my 2k dsl100 cost me like 8 bucks, and took probably less than 5 minutes to install while I had the amp apart for the Dr. Tube fix.

 

[ampmadscientist]

Where would I order these replacement boards with the pots from if mine are bad? I was able to adjust the bias to the correct settings in both heads but after it gets hot the TSL runs away the DSL seemed to be more stable . Whats the range on these pots? Maybe I sould pull the chassis a check them while I'm waiting on the bias mods to arrive. Will Deoxit not straighten them out?

Replace the pots.
When the board is bad it causes high voltage to bleed into the bias circuit. This burns out the wiper of the adjust pot.
After the wiper is damaged the bias is no longer reliable.
It's worth the price of the pots to save your output tubes.

 

[12AX7]

Amprepairparts.com has the 22k bias pots, they are a snap to install on the little bias board. No need to change the whole board if you can't find one. Both pots for my 2k dsl100 cost me like 8 bucks, and took probably less than 5 minutes to install while I had the amp apart for the Dr. Tube fix.

Yea I found the pots last night on this site and I will replace them while I'm in there.

 

[12AX7]

Replace the pots.
When the board is bad it causes high voltage to bleed into the bias circuit. This burns out the wiper of the adjust pot.
After the wiper is damaged the bias is no longer reliable.
It's worth the price of the pots to save your output tubes.

Thanks I'll order 4 (enough to replace them in both heads) tonight.

 

[12AX7]

Well amprepairparts.com checkout page is broke it will not let me pay for my order. I created an account and everything it just want complete my order. Ill try calling them tomorrow.

 

[12AX7]

fixed it popup blocker was blocking paypal.