Marshall TSL 122 biasing

[ampbldr2]

I just bought a tsl122 combo and want to be sure I bias it properly. Any good reference docs I could review?

Im sure alot of techs on this forum that have had these models come across there bench. Any advice appreciated.

[ampbldr2]

other owners have found bias drift issues with these amps. Here is one article I found. http://www.lynx.bc.ca/~jc/TSL122.html -very good

Have others replaced the motherboard or tweeked it to single bias pot and found negative temp coefficient resistors in the bias? I would be interested to find others with this amp and what there experience has been.

[ampbldr2]

I found my answer
http://www.marshallampforum.com/foru...topic.php?t=17

Here's the Marshall Service Bulletin on biasing the JCM2000 series amps:

Procedure:
1. Make sure the amp is connected to a load with the proper impedance selected.
2. Power up amp on Standby, and let the circuit stabilize for a couple of minutes.
3. Locate the male three pin molex connector (CON2) with the two mini pots (PR1 and PR2) (located on each side of the molex connector) on both ends found at the bottom of tube bay.
4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2.
5. Take amp off Standby with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to mV voltage that is listed below.
6. Repeat steps 4 & 5 for pin3 mV and until pin1 & pin3 mV readings are the same.

Bias Settings:
DSL50 45 mV TSL 601&602 80 mV
DSL100 90 mV DSL 401 1.375V
TSL100/122 90mV DSL201 .675V

Here's a quick bias lesson:
What you want to do is measure the plate voltage (B+) from pin3 of one of the power tubes. It should measure between 450-500V. Now the max plate dissapation (Pa Max) for EL34s is about 25W. To calculate the correct bias, you want to:
(Pa Max/B+)(.7) = 38-45mA depending on the exact plate voltage of your amp. Check and adjust via the molex connector and pots from above.

[ampbldr2]

Id still ike to know what others think of this amp or any of the DSL/TSL builds that are now discontinued.

[Dave Curtis, dB AudioTech]

Id still ike to know what others think of this amp or any of the DSL/TSL builds that are now discontinued.

Solely from the repair tech's point of view? They suck.

Discontinued? For good reason, IMHO.

[Gtr_tech]

Solely from the repair tech's point of view? They suck.

Discontinued? For good reason, IMHO.

Ditto that....twice And from the tone department....see above. Unless bumblebees in a tinfoil bag is your idea of fun....

[MWAMP]

other owners have found bias drift issues with these amps. Here is one article I found. http://www.lynx.bc.ca/~jc/TSL122.html -very good

Have others replaced the motherboard or tweeked it to single bias pot and found negative temp coefficient resistors in the bias? I would be interested to find others with this amp and what there experience has been.

I wrote a reasonably detailed diagnosis which appears near the bottom of the TSL122 section of JC's website. Replacing the PC board with one made from proper materials is the only course of action guaranteed to fix this amplifier.

Malcolm