Bias drifting question on a TSL 60 Marshall

[Ascension]

I just got a used Marshall TSL 60 head a week or so ago and snagged a fresh set of JJ EL34L's locally. I biased the amp with my DVOM off the test points in the back setting it at 85 MV. I ran the amp for an hour or so at idle played it a little and rechecked it had increased to over 95 MV so I reset it. The amp was played at my Church for two services and started sounding really stiff and harsh by the end of the set so I brought it back home and the bias checked at over 100MV this time:eek: . Why is it drifting so badly this quickly?? I have to wonder if I just have a bad set of tubes that are changing as they burn in. I wanted to A/B a set of SED EL 34's with the JJ EL34L's so I have a set of SED's coming from Doug over at Dougs Tubes. Should I look elsewhere in the amp or just bias the SED's up and keep an eye on it to see what happens.

[fusionbear]

What cabs are you using and what ohm rating are you using. I have some ideas, but will wait for your set-up explanation.

[GuitslingerTim]

You said you set the bias to 85 mV--the DSL50 has one test pin for each tube, and a third pin for ground. Each tube should be set to between 35-45 mV--the TSL60 should be set to similar values. If you're setting the bias to 85mV per pin/tube, then it's twice as high as it should be--85mV is a bias setting for the DSL/TSL 100 series with a pair of tubes wired to each test pin.

The drift is probably caused by a set of tubes that were not burned in or matched properly. If the bias setting is incorrect, fix it and then re-evaluate for drift.

[Ascension]

You said you set the bias to 85 mV--the DSL50 has one test pin for each tube, and a third pin for ground. Each tube should be set to between 35-45 mV--the TSL60 should be set to similar values. If you're setting the bias to 85mV per pin/tube, then it's twice as high as it should be--85mV is a bias setting for the DSL/TSL 100 series with a pair of tubes wired to each test pin.

The drift is probably caused by a set of tubes that were not burned in or matched properly. If the bias setting is incorrect, fix it and then re-evaluate for drift.
35-45 MV is correct for the DSL 50 but according to Marshall on the TSL 60 80MV is correct and Bob at Eurotube tells me 80-90 MV. The outside pins on the TSL are both connected to the pair of EL34's not the singles like the DSL.

[Ascension]

What cabs are you using and what ohm rating are you using. I have some ideas, but will wait for your set-up explanation.
At home I'm running the head on a 2/12 Fender cab with 16 ohm Celestian 70's wired for 8 Ohms. At Church a Yamaha 4/12 with 70's wired for 16 Ohms.

[BassHog]

I have had one of these family of amps in my shop before with the same drifting bias problem you seem to be having (DSL 100 though).
It took me a while to figure it out as it only happened after the amp got really warm.
The problem was a loose connector on the output transformer. On this series of amps Marshall used push on connectors for the transformers and this amp had one that had broken and was sorta loose. It went very loose once the transformer heated up, it was actually split down the side. I cut off the push on connector and soldered the wire on and it solved the problem. This was the center tap on the primary.
It had been to several shops before me trying to solve the problem, I don't blame them for giving up, it was a hard one to track down.

[Ascension]

I have had one of these family of amps in my shop before with the same drifting bias problem you seem to be having (DSL 100 though).
It took me a while to figure it out as it only happened after the amp got really warm.
The problem was a loose connector on the output transformer. On this series of amps Marshall used push on connectors for the transformers and this amp had one that had broken and was sorta loose. It went very loose once the transformer heated up, it was actually split down the side. I cut off the push on connector and soldered the wire on and it solved the problem. This was the center tap on the primary.
It had been to several shops before me trying to solve the problem, I don't blame them for giving up, it was a hard one to track down.
Thanks for that tip I will take a look at the conectors on the Transformer. Right now I have the amp running without a chord plugged ito the imput just attached to the cab. I will let it burn in for a few hours at idle and see if that helps it stabilize if not I will then check the Transformer connections. These JJ's don't have but about 4 hours on them total so maybe they just need run for a while to burn in and stabilize.

[jezzzz2003]

Just for the record, JJ's are not available to buy from a huge retailer who buys up almost 40 percent of the worlds tubes at the moment because theyre currently too unreliable.

[fusionbear]

Thanks for that tip I will take a look at the conectors on the Transformer. Right now I have the amp running without a chord plugged ito the imput just attached to the cab. I will let it burn in for a few hours at idle and see if that helps it stabilize if not I will then check the Transformer connections. These JJ's don't have but about 4 hours on them total so maybe they just need run for a while to burn in and stabilize.

I recently replaced a DSL trannie with a Mercury Magnetics and I ALWAYS solder the terminals. This alone makes good sense and gives a good stable connection. Hope all goes well, let us know......:D