Tsl100 bias pot maxed

[Shardik]

So I picked up an 02 TSL100, when I got it the preamp tubes were original and the power tubes had seen better days.

When I checked the bias when I first got it. Side A read 93mv and side B read 29mv. So I got a complete retube with kt77s. Put all the new tubes in, I watched for run away bias... sure enough it was.

So I pulled the board drilled out pin 5 on all sockets, replaced the 220k with 5.6k.
Replaced R67,R68,R69 with new metal film resistors. Got it all back together, checked the bias again and side A was at 75mv and side B was at 29mv. So I turned down side A to about 61mv and then side B I got up to 61mv but it maxed out the pot.

I want to try out biasing higher where do I start?

 

[Gunner64]

First off change the pots. The pots seem to be cheap, and problematic. They control the bias, and if they are not up to par all your other work was for nothing.

 

[Shardik]

The bias was set at 61mv on both sides before I pulled the board and the pots were both in the middle of their sweep.

Should I just replace them with the same marshall pots? Or is there something better?

 

[Gunner64]

I thought you just said you had runaway bias before you pulled the board, and bias was set at 93mv and 29mv. Which is it? 61mv both sides before you pulled the board or 93/29mv?.

I looked for better pots than the factory ones when I repaired my dsl100, and I could not find any that had the same footprint that would fit on the pcb. I ordered the same original pots from amprepairparts.com, and haven't had an issue with them.

You could try swapping one tube from each side with the other. Maybe your tubes are just that mismatched.

 

[Shardik]

The 93/29 was the old worn out power tubes that came in the amp. I swapped them for brand new JJ kt77s.

I set the bias on the new tubes at around 61 and watched the bias and it didnt take long to start climing up. But I shut it off when it got to around 71mv. That's when I decided to just pull the board and do the fix job.

I put the tubes back in in the same order they were in before and now side Bs trim pot is maxed at about 64mv.

 

[Gunner64]

I would double check my work, especially if you didn't have an issue initially. And make sure the 1ohm cathode resistors associated with the bias points are ok. And I still would change the pots regardless, just for piece of mind.

I didn't have any issues with my 2000 yr. Dsl100 after I installed the Dr. Tube kit. (Factory el34s) For $80 I feel it was a bargain. Bias is rock solid at 72 a pair. I recommend it highly.

 

[Shardik]

I guess technically it was an issue when I first got it. The side that was 93mv I could only turn down to that 61mv mark and that pot was all the way down. But I chalked that up to old bad tubes and it was fine when I put in new tubes.

 

[Shardik]

SOLVED: turns out I missed hooking up W3, the white wire from the power transformer. Thanks for your help gunner

 

[Gunner64]

SOLVED: turns out I missed hooking up W3, the white wire from the power transformer. Thanks for your help gunner

Sweet. Glad you got it figured out, and it was something simple! Rock on.