Randall Thrasher 50 observation & bias issue UPDATE

napalmdeath

napalmdeath

Well-known member
As I previously stated when I got the amp, the bias was at 8.3mA, and when I tried KT77's, I had no reading until I cranked the bias, and maxed out at 12mA. I put the 6L6's back in, and biased it up to suggested, (in the manual) to 32mA, all is well..

I picked up my Bias One at my brother's today, and a new pair of E34L's I had over there to try, and check the plate voltage.

The plate voltage on this sucker is 511! And, I could only get the E34L's to 18mA, maxed out. WTF? They're rated 35, which I wouldn't consider cold, would you? Do I need to get my buddy Eddie at Eurotubes to send me a majorly hot, maybe upper 40's, or am I missing something here?

On another note, my math puts recommended 32mA seriously cold! And they recommend EL34's at 38mA which would be seriously hot! A 30 watt 6L6 with plate voltage at 511 should be 41, at 70%, (which I never really go that high), so I settled at 38mA. A 25 watt EL34 or KT77 should be closer to the 32mA.

So, WTF is going on? I'm an EL34 fanboy, and couldn't get E34L's or KT77's remotely close to where they should be. Just over half, maxed out. What gives?

I also have another stupid question. There's only one bias adjustment, yet two test points? What's the reasoning behind that?

Everything aside, I still love it. The 6L6's bias up just fine, but I want to try EL34's, KT77's, or 6CA7's!
 
Have you checked the manual? It has some bias info in it..

(I was reading the PDF. It's online)
 
So, here's what my buddy at Eurotubes says:

"it sounds like you would do better with some hotter EL34 types.  It appears the Thrasher 50 is geared more for 6L6’s, which take a lot more negative bias voltage to come into the safe operating range than EL34’s, so if the bias voltage range doesn’t get to less than about 38 negative volts it will choke the cooler EL34’s down a bit too much.  We can definitely find you E34L’s and KT77’s that are significantly hotter.  However standard EL34’s and 6CA7’s hotter than about 28 are few and far between".

They hand-picked an insane hot pair rated 66, so, I'll give them a whirl.

To me, it seems that Randall has bad info in the manual, I believe their suggested settings for 6L6 and EL34's are ass-backwards, for starters. Why it won't accept EL34's without it being a ridiculously hot pair is some kind of flaw.
 
Interesting. My Thrasher 120 biases up fine with any tube variant and has with 6550's E34L and 6L6. Wonder what is up with your amp. My only issue is with the
JJ E34L I haven't gotten more than 6 months with those tubes but ask most tube retailers and they will tell you they are known for their reliability issues sound killer though am thinking about trying another duet or quad in my Thrasher again lol. Have had JJ 6L6 GC's in the amp for about a year and no issues...just love the sound of E34L's...
 
shred-o-holic":jqd8gv2r said:
Interesting. My Thrasher 120 biases up fine with any tube variant and has with 6550's E34L and 6L6. Wonder what is up with your amp
Click to expand...

Don't know. I know seller had some issues when he received it, (no details, other than his post when he bought it), retubed it, and when I got it it had a cracked headshell, (he handled it like a champ), a super loose chassis bolt or 2, and the bias on the new 6L6's was 8.3mA, (crossover distortion os why it didn't sound right). It's now at 38mA, (41mA would be 70%), sounds fine. However, it was my intentions to run KT77's, and it just wont take 'em!

Randall suggested returning it, (though I told him I bought used), and/or taking it to Diversified Audio, (but that owner Bruce, is such an asshole, and they take a month to replace a fuse!)

I am suspecting it has something to do with the 50 watt vs. the 120 not being the "same", with regards to tubes it'll handle. I love the amp, however!
 
Yeah I have never used Diversified as I have heard the same complaints. There isn't another local amp repair shop that can take a look? Although if the 6L6 are doing ok it's not a huge deal. I really shouldn't and probably won't for a while stray again from the 6L6 in mine either. They have been rock solid and really they sound fine.........it's up to you how bad you want to try other tube types. If it sounds good screw it and leave it be maybe..
 
Another thing you could do is attempt to contact the designer Mike Fortin and ask him. Not sure how available or responsive he might be. When I had issues with the RM M100 I had I called it's designer Bruce Egnater and much to my surprise the guy was super nice and I even shipped my amp to him which he repaired for relatively cheap. But I don't know Mr. Fortin or his availability I think he used to be on these forums. If you do talk to him tell him thanks for designing killer amps lol.
 
shred-o-holic":x23j1nd3 said:
Yeah I have never used Diversified as I have heard the same complaints. There isn't another local amp repair shop that can take a look? Although if the 6L6 are doing ok it's not a huge deal. I really shouldn't and probably won't for a while stray again from the 6L6 in mine either. They have been rock solid and really they sound fine.........it's up to you how bad you want to try other tube types. If it sounds good screw it and leave it be maybe..
Click to expand...

I ordered a super hot pair of KT77's, we'll see if they get to range. If not, fuck it. The Tungsol 6L6-STRs bias up, no problem, and it sounds damn good. If EL34 variants won't work, so be it. I'll rock the shit out of it with 6L6's, and won't lose sleep!

The 35 rated KT77's got me to 18mA. Eddie at Eurotubes is sending me a pair rated 66! That's HOT! Should work like a charm!
 
Well, I got a hot pair of JJ KT77's, and sure enough, they biased up just fine! Amp sounds meaner than ever - what a KILLER amp!
 
The bias is controlled by the amount of negative DC voltage applied to the grids. Regardless of what the tube is rated for, the amount amount of negative voltage needed to bias them in this range can be extreme between tube types. This is why you will see a switch on boogies and why a resistor change is needed when changing from 6550s to el34s in older Marshalls. In the case of the extremely cold bias, a larger bias resistor would need to be swapped as to lessen the amount of -DCV. Without this it limits what a user can use for off the shelf tubes from those models and rely on a bias pot alone. As in your case, you had to order tube that drew fell within that range
 
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