need help subbing 7581a output tube

ke4mcl

ke4mcl

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i just got a bogen MO-60 monoblock. tube lineup is (3) 5ar4, (2) 7581a, (1) 12ax7, (1) 6c4.

this thing cost as much to feed as it does to just buy a different tube amp!

my question to you guys is...
i see that the 7581a is basically a 6L6GC with slightly higher ratings. is there a drop in rectifier i can put into this amp that will give me lower voltage so i can run 6L6GB's or GC's?
 
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5R4s will reduce the voltage, but 6L6GC should be fine as long as it's biased within its ratings.
 
Funny how only certain companies used that hard to find tube. HH Scott used them on one of their top of the line amplifiers, the 296 too...
 
My newly completed MC30 mono blocks are running 7581a outputs. Well, after someone recommended I use them in the amps, I discovered I had 2 in another amp, and a buddy had one NOS. I am on the hunt for a 4th but it will cost me. You can use the others if you check voltages. John
 
Modern Winged-C 6L6GCs will handle the voltage. I've seen them used in some guitar amps with as much as 500v on the plates.

Sylvania/Philips ECG 6L6GC STR tubes will also handle the voltage and current. In fact, they're what the Winged-C 6L6GCs are based on...

Regards,
Gordon.
 
When supplies of US NOS 6L6GCs first starting drying up in the '90s you could still get 7581As pretty cheaply. I have a pair of Sylvania's (I think) in my Super Reverb. Mesa Boogie relabeled a whole bunch of them also back in the late '80s I guess. They tend to be a cleaner tube with more headroom in guitar amps.
 
My HK Citation V calls for 7581s. Jim McShane and others recommended Winged SED 6L6GCs. Their attitude is, you can buy new and find old stock later. No worries about stretching socket pins or having to modify feeding voltages.
Common sense ruled and the Winged SEDs are now here. Oh, and a new matched quad of 4 was under $100.
Don
 
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No, the 5AR4 and 5R4 have the same pinout, and the same heater current draw, and similar plate current capacity (as long as you use the 5R4GYA or 5R4GB- the early coke-bottle 5R4 may not cut it).

The 5R4 will drop more B+ voltage... that's the main difference.

Regards,
Gordon.
 
You can sub 7581s or 6L6GC for 5881s, but you can't sub 5881s for the others because they're rated to tolerate more than the 5881. I mean, it'll probably work, but you will shorten the 5881 tube life.
 
audiodon657 said:
My HK Citation V calls for 7581s. Jim McShane and others recommended Winged SED 6L6GCs. Their attitude is, you can buy new and find old stock later. No worries about stretching socket pins or having to modify feeding voltages.
Common sense ruled and the Winged SEDs are now here. Oh, and a new matched quad of 4 was under $100.
Don
Click to expand...

I should do the same. I have heard good about the winged SED. John
 
GordonW said:
No, the 5AR4 and 5R4 have the same pinout, and the same heater current draw, and similar plate current capacity (as long as you use the 5R4GYA or 5R4GB- the early coke-bottle 5R4 may not cut it).

The 5R4 will drop more B+ voltage... that's the main difference.

Regards,
Gordon.
Click to expand...

One other thing - the 5R4 is directly heated, unlike the indirectly heated 5AR4. In some gear the caps are marginal without an indirectly heated recto or some sort of time delay. Remember that until the tubes draw current the voltages can be VERY high in some spots in an amp.

To the original poster - stick with the 5AR4 and use SED or Tung-Sol reissue 6L6GC-STRs (or 6L6G "coke bottle" T-Sol reissues).
 
audiodon657 said:
You can sub 7581s or 6L6GC for 5881s, but you can't sub 5881s for the others because they're rated to tolerate more than the 5881. I mean, it'll probably work, but you will shorten the 5881 tube life.
Click to expand...
Thank you, understood............ now. I was aware that these tubes are all of a "family" but i wasn't sure how much the ratings differed or in which direction.
 
any experience here with the new issue 5ar4's?

i want to hold onto my NOS 5ar4's for a future dynaco project.
 
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