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previous: danhei The pair that glows red was the lef... -- 7/9/2000 8:10 AM View Thread

Re: Ignore those other posts; THIS is the right link

7/9/2000 1:00 PM
Ray Ivers
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Re: Ignore those other posts; THIS is the right link
Danhei,  
 
In deference to John Martin's post, I checked and found that he is indeed correct; the Sovtek 7027 is definitely NOT a 5881, and may well be a relabeled Sovtek 6L6 (I didn't have any of these to compare the 7027's with, but the 7027 plate structure is very different from the 5881). I was just parroting info I read in another post somewhere - sorry about that! I read something somewhere about Sovtek 5881's not holding up well in Ampegs, and also saw a post about the Sovtek 7027's being rebased 5881's (or maybe it was Sovtek 7591's that were rebased 5881's?)... you get the idea - a lack of mental clarity.  
 
The Ampeg book ('Ampeg - The Story behind the Sound') states that 6L6GC, 7581A, Sovtek 5881, 6L6WGB/5881, 6550, KT88, or KT90 tubes may be substituted in this amp. I would say a matched quad of Svetlana or JJ 6L6GC tubes, or NOS Sylvania 6L6GC's or 7581A's would come closest to the original sound, and Svetlana 6550 or KT88 would make for a slightly louder, cleaner sound (the KT88's also distort pretty nice - wear earplugs!). All these tubes, even including a set of NOS 7027's, would require rebiasing, of course. I feel it would be worthwhile to install a variable bias pot at the same time, to make all future biasings less of a pain. If this sounds like something you want done to your amp, let me know and I'll figure out the component changes needed.  
 
If you want, you can pull the glowing tubes one at a time (leaving the other three in, amp off while pulling tubes) and see if the tube that also glowed before stops glowing; this would indicate that the glowing tube is pulling the grid bias voltage of the other one down. This would leave you with three spare tubes instead of two - no big deal.  
 
I don't think I can recommend "leaving it be", though. Once a tube starts drawing grid current at idle, it seems to draw more and more, potentially leading to disaster. I'd rather see you pull the two glowing tubes and run the amp with the two good ones than leave the glowing ones in.  
 
Ray

 
Replies:
Tom L Re: Bias pot -- 7/9/2000 5:18 PM