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Bias Marshall TSL60 combo


 
7/26/2005 2:38 AM
Pete Bias Marshall TSL60 combo
G'day guys,  
I think I need a new set of output tubes in my marshall TSL60 combo. It's got two EL34's in it. From what I can see there's a connector (CON2) at the back which has I think a 22K pot I'd assume adjusts the bias. From the schematic I have, it seems pins 1&3 of CON2 are connected, and pin 2 goes to ground.  
Am I suppose to be measuring a voltage across pins 1 & 2? And if so what should I adjust it to for EL34s?  
If you could steer me in the right direction it would be much appreciated!  
 
As an aside question, do you guys find that the highs tend to go when the output tubes are worn? They've been in there from new (about 5 years) and I've changed all the preamps with sovteks. The clean channels a bit better, but the lead channels still seem to be laking highs.  
 
Is there any common faults people find with this particular amp??  
 
Cheers  
 
Pete
 
7/26/2005 6:01 PM
Jack Collins
email

Yep. You'll lose the highs when your power tubes wear out.  
 
5 years is a long time for EL34's. I'd say you'll  
definitely notice a tone improvement by installing new ones.  
 
As far as the biasing procedure goes I grabbed this from Dr. Tubes Website:  
 
"The EL34 tubes are biassed at 40mA each. To adjust the bias, turn the bias pot (PR1 22k) so that the voltage between pins 1 and 2 of CON2 (BIAS) measure 80mV (= 80mA over 1)"  
 
Of course you're going to need a digital multimeter to do this.  
 
I'd recommend you try some of the new JJecc803s in your preamp. I have em in my Rivera and love em.  
 
IMHO, Sovtek 12ax7 have just about the deadest tone of any tube of that type. They are rugged though.
 
7/28/2005 6:51 PM
Pete Thanks!
Thanks Jack,  
I thought it would be something along those lines. I just checked the bias of the old tubes and it was reading about 140mV ! No wonder they wore out. Or, as they wear do they use more current? Might be something I'll have to watch out for.  
 
Five years I suppose is pretty good, but I generally only get the time now to play once a week if I'm lucky. But when we're jamming, it's a good 4-5 hours.  
 
BTW The JJecc803s you mention, who are they distributed by. I'm in Australia, and it's usually a bit hard to get hold of anything other than Sovtek and Svetlana tubes. But I'll enquire around. Any of the Aussies on the forum know of anything around Canberra that might keep 'em??  
 
Thanks again  
 
Pete
 
7/29/2005 11:12 AM
Jack Collins
email

I get my JJ's from:  
 
www.eurotubes.com  
 
He's in Oregon USA. Not sure if he ships down  
under.
 
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