Selecting 7581A in the correct bias range for a Mesa/Boogie DC-5

[Solinger]

I found some 7581A tubes to buy and I would like to use them in my Mesa/Boogie DC-5 combo.
Somebody could tell me what is the correct bias range for a Mesa/Boogie? What is the plate voltage of this amp?
Thank you

[John Phillips]

The plate voltage is about 465V. Don't worry about the bias; just put the tubes in and it will be fine. You can't easily adjust it anyway since Mesa amps do not have bias trimmers.

I run 7581As in my DC-5 too; some time ago I tested about 40 Philips 7581As and Philips/Sylvania 6L6GCs (essentially the same tube) in my amp - every single one biased in the correct range, and the total variation across all the tubes was only about 10mA. These are a very high quality tube, built to close tolerances.

You will get a very noticeable improvement in depth, clarity and output power compared to the stock Mesa-labeled 6L6s. I would also fit old-stock preamp tubes too - I use GE 12AX7s in V1, V3 and V4 and a 5751 in V2 (lead channel main gain stage; improves detail while reducing gain and the tendency to harshness) and JAN/Philips 12AT7s in V5 (reverb; improves depth and reduces noise) and V6 (phase inverter; improves clarity and dynamics). The type changes are more important than the brands.

It's a good amp anyway, but better tubes make it a truly great one (IMO).

[Solinger]

Great John!
Are you sure that Philips 7581 and Sylvania 6L6GC are the same tube?
Now I can buy GE 7581 or Sylvania 6L6GC. Do you have experience with GE 7581 in your DC-5?
I have EH 12ax7 for preamp but I'll change for Tesla 12ax7 and maybe some 12at7 as you told me.
Please help me with your experience.
Thank you

[John Phillips]

If we're talking about Philips/Sylvania large-bottle 6L6GCs and 7581As, they are the same tube, yes. I'm not sure about other makers. The 7581A is essentially a military-spec 6L6GC, with ruggedised construction, higher maximum ratings, and tighter tolerances. I'm not sure if Philips made them as 6L6GCs and found they were good enough to be 7581As, or designed them to the better spec and then simply sold many as 6L6GCs in order to get economies of scale.

I had a pair of 7581As in my DC-5, but one went rattly (so much for the ruggedised construction! :) - actually not bad enough to matter at a gig, but annoying for playing at home) so rather than buy another pair of 7581As, I substituted a spare 6L6 which biased the same; no detectable difference in performance.

I don't have any experience with the GE 7581As, but I would expect them to work just as well - the spec for the 7581A is more demanding than for a 6L6GC, so even if they are not the same tube they should be fine in the amp.

[mad dog]

Just got NOS 7581s in my Sewell Wampus Cat, plus a fixed/cathode bias switch and test points. As I understand it, the bias points check both tubes at once. My tech set it to 67 if I'm remembering correctly. Said anywhere between 50 and 75 would work for 6L6s, in this amp. He did say the 7581s like it on the warmer side, so I'm going to experiment a bit with this, maybe bump it up some. Sure sounds sweet right now.

[jcs]

i have a substantial collection of sylvania 6l6gc and 7581,imo,be advised that these tubes can vary as much as 20-25 ma apart..

always check current draw,if nothing else check for red plating in a darkened room (in which case shut the amp off immediately)..

remember the high plate volts put strain on any tube especially if run over 70% dissipation...good luck!

[CitizenCain]

Does anyone know, would these tubes work in a Mark 1 reissue also?

[John Phillips]

Yes.

They are actually the closest available tubes to the ones Mesa designed their amps around originally - the Sylvania STR415 6L6GC.

[CitizenCain]

Woohoo!