Tsl 100 bias wacky

[bmd]

My Tsl 100 has a bias problem.I've had the amp 4 years and re-tubed (GT 12ax7M & EL34LS)& had service shop bias about a year after purchase.I've gigged with it moderately,as I usually run a '74 JMP 50 W( which now has developed problems,any v2 glass cracks after 5 min.)Anyway I checked The Tsl bias recently and the right side was about 130 mv. and there was crackling sounds from the cab.I adjusted both sides to 90 mv and turned the amp off.I re-checked it about 15 minutes later and the right side was about 40 mv. and I re- adjusted and turned off.I've done this 3 times and it is definitely still doing this.I believe I am doing this correctly,hook up to 16 ohm cab,16 ohm jack on amp,turn power on,wait about 5 min.,turn amp on hook up connector and multi -meter set on mv.Any help would be appreciated. Brian

[Chuck H]

I think the most likely cause is a bad power tube. But it could also be the bias circuit. If it were my amp I would get a new set of tubes (probably needs them anyhow after 4 years) and I would shotgun repair the bias circuit (replace all componants in the bias circuit). This would be alot easier than trying to troubleshoot the exact cause and will most likely fix the problem.

"I usually run a '74 JMP 50 W( which now has developed problems,any v2 glass cracks after 5 min.)"

This is bizzare. Nothing about the circuit or it's operation should cause the glass to crack, even if there is an electronic problem. This is purely a physical vibration problem. You need to replace either the tube socket with the retainer, or at least the tube retainer.

HTH

Chuck

[drewl]

I had one which one of the bias pots had an intermittant solder connection on the bias board.

[bmd]

Tsl bias wacky

Thanks Chuck, I will try that with the Tsl.The jmp had a telefunken that went cloudy, then a tungsram that cracked. Both tubes tested good on a tv-7 a local electronics store has.The jmp has never been re-capped ,just pots cleaned,tubed & biased.The tsl probably has less than 100 hours.Thank you very much.Brian

[Enzo]

A tube can fail at ANY TIME. New out of the box, or 20 years old. Never dismiss tubes as a cause of trouble. Your on again off again bias woes sound to me like a failing power tube as well. SOme little bit of wire inside sometimes shorts out to another little bit and that affects current. You adjust for it one way, then when it touches - or untouches - it reverts to the other way. Happens all the time. Swap a different set of tubes in there and see if it behaves. They don't have to be new, just tubes that work.


Try this: You know which side is the one that won't act right, so powr off, take the two left tubes and put them in the two right sockets, and take the two right tubes and put them in the left sockets. Now with the tube reversed, see if the up and down bias thing has moved to the other side. If so, then at least one tube is bad. DOn't care how many hours they have on them.

I think what you are seeing is one tube blinking off and on. When it is off, you set the bias for that side, but there is only one tube running, so it is really running at twice the current it should to add up to 90. Then when it pops on, all of a sudden you are way over, so you back off to get back to 90. But then the tube pops off again and the colder result is just one tube's current.

So one intermittent tube can do this.

If it is not the tubes, then the amp has an isse. But I still think it will be one tube cutting in and out. In this case, one of the sockets will have an issue. First place I'd look then would be the screen resistor. A 5w cement resistor up on stilts. See if one is a bit loose, like its solder is letting go.

Obviously other things could cause this, like loose socket pins, failing caps, etc. but the things I described are the most likely.

[bmd]

TSL bias wacky

I changed the power tubes back to the Svetlana's which still measured good on my cheap Jackson tube tester model 49. Crackling was gone. Right side was down to 50mv brought both to 88mv and played the amp some. I left my tester hooked up and switched back and forth between sides and channels. On crunch and lead channel with master and pre-amp on about 4 my tester indicated that my bias voltage (both sides) was spiking up to 120, even 138mv on crunch and lead channels while playing. Is this normal??? If I quit playing or played softly on clean channel bias stayed pretty much at 88mv.

[Enzo]

Bias measurements are always done with no signal present. Your bias meter has no way to tell the difference between the idle current - what you set the bias with - adn the extra current when teh tube is amplifying something.

When you play, you are not measuring bias current, you are measuring total tube current. That 100 watts has to draw current through somewhere, and that somewhere is your tubes. Always set bias at idle.

[bmd]

TSL Bias

Thank You Enzo,and all others.As you can see I am not an amp tech.Tsl seems to maintain bias now.GTEL34 LS must have been bad.NOS RCA in v2 of JMP seems to fix that.Odd that Telefunken & Tungsram went bad so quickly,as they both tested very good in tv7.Again thank you all. Brian