DSL bias?

JTyson

JTyson

Well-known member
Just put my biasmaster on my DSL 100 watt, I'm getting 80-83.4 ma per tube on a matched set of KT77's I had in my tube stash, Cant get it below 78ma.. But it will go much higher. I'm not dividing this by 2 am I??
Its been awhile, and another DSL I have has bias drift issue's, so I cant use it as a reference point.
Thanks in advance :scared:
 
I think this is for 2 tubes, so you divide by 2. I think they should usually be in the low 40 range or so. Maybe someone else will chime in as well.
 
OK, I have the bias set all the way down, I'm at 70.8/72 ma (first 2) with 151mv on the pins for the left side, 74/71.8 ma (second 2) with 154 on the pins for the right side..
The vid showed 1 probe on each pair of KT77's at roughly 36ma. He did not say anything about dividing by 2. Looks like I am WAY off??
I stuck in a quad of 6CA7's that are old, I'm at 73/57ma with 198mv at the pins for the left pair, and 127/123 ma with 194.3 mv at the pins for the right pair. I know these 6CA7's are old and not matched, but 127/123ma's?? I guess the 6CA7 readings are not relevent since they are probably badly mismatched, so I'll stick to the KT77 readings.
What the hell is going on? :doh:
 
MississippiMetal":3b3u89qj said:
The readings are for two tubes. One pin measures cathode current of the inside pair, the other pin measures the outside pair.
Click to expand...
But I'm still way over what the vid shows. He was saying 99mv was a bit too hot, and I'm at 151/154mv. Is there something I'm missing?
Am I really at 74/77mv, and should bump it to around 160? (80/80)
Thanks guys, I dont recall ever running into this on my older heads :doh:
 
tsel34quad.jpg



80 / 80 [80 per side for 100 watt].

Thats the problem with old and mismatched tewbs the bias is all over the place.

Call Laura @ Valvequeen.com or PM her here for a matched quad of Tungsol EL-34B's, about $100 bucks.

I bought 2 matched quads and they biased perfectly.
 
that should be per pair. have you looked at any tech sheets for kt-77's? it seems odd to me that they would bias at that high of a range on 7-8. :confused: i wonder if there is a possibility that this head also has bias drift issues.. or if there might be a component in the circuit that is causing the problem?
 
Its a matched quad of Gold Lion KT77's that only have a few hours on them. I have another DSL that does in fact have bias drift issue's, but it acts differently than this. The bias on this one stays where I put it, it just seems to be too high (ma's per tube) unless I'm dividing it by 2.
I watched that vid on eurotubes, and the guy is reading way less ma's per tube than I am. I have a probe on each tube to read each one by itself (I thought) and the individual readings are way over what he got, as well as the mv readings at the pins...
So the overall consensus seems to be its half what I am actually reading on the tubes, and the mv's at the pins are per pair, I just dont understand how I am miles from the readings in the vid. I'm checking mv's exactly the same way he did, and I'm WAY higher.
Maybe I'll stick a matched quad of SED EL34's that I have in and see where I end up with those?
Thanks for the responses :rock:
 
Your amp needs a tweak from a tech if you're running KT-77's

DSLs are EL-34 amps which is a Pentode.

KT's are a Tetrode.
 
Also don't bias these amps at the arbitrary numbers marshall gives out. They're way too hot. Check your plate V and bias it at about 65%. The sound much better that way.
 
well.. the tetrode/pentode difference would explain a whole bunch. :) i'd try the el 34's and see if that makes a difference when divided by 2. the marshall tech bulletin says 45 mv per tube.. or 90 mv for the pair.. but i set mine at 38 mv.. or 76 mv per pair.
 
So I put in a matched quad of SED 34's, I'm at 88 on one side and 90 on the other with the bias pulled all the way back on both sides.
Still slightly hotter than I would like to see, but its better. I think somethings up with the amp at this point. I have no idea what, and Mike F is still too busy for me to bother him with it at the moment. I'll wait for a bit and see if he gets any free time later.
I appreciate the replies, I'm gonna have to come back to it later.
Thanks guys
 
it stands to reason that if i can get mine down to 76 mv per pair you should be able to as well. good luck. :thumbsup:
 
Also agree with Bloodrock and 1big1 that they seem to sound best around 37/38 mv. I just had mine up around 40 for a while, then just dropped them back down a little bit. Sounds better now I think.

Good luck with it. I just picked up my DSL and it's a pretty darn good amp for not much $$.
 
at one time i had tried "old schooling" mine at 31mv based on 450 plate volts like the plexi's.. because that's what i metered.. but that was too cold. a few helpful folks on this forum had suggested 38mv.. so i ran with that. :thumbsup:
 
Agreed, I'm running most of my Marshalls at 36-38, I still get good tube life and they sound much better :yes:
 
You must log in or register to reply here.
Back
Top