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I have an Ampeg Gemini on the bench with a pair of 7591s on the output. If I were to replace these with something more generic, what would it be? (6V6,6L6, 5881,6550, EL34 etc)

Don

Here's a link to some good info on converting to 6L6's
on Triode's web site ....

http://www.triodeel.com/7591.htm

Don, before you go ripping and rendering, get some 7591EHs. I've used them in a couple of Fisher 500C restorations, and they worked just fine.

There's also the Sovtek 7591XYZ, which, according to New Sensors' Alan Otto, is nothing more than a 6L6 rewired to emulate a 7591. You have to change the cathode resistor, which, I believe, they supply with the tube. If not, they tell you what the new value needs to be.

HTH

Thanks Guys! I will see if the local music store has them in stock. This is my gauge for tube desirability, if the musician can buy one one the road, it's good. eg an output tube on the Hammertone Viper broke and a replacement was found in a nearby Fender, the kind of thing I like to hear.

Don

Just went to the link Tom posted above, and noticed a couple of errors.

If the voltage is too high, make the resistor higher in value, or vice versa. . Actually, you want to increase the cathode resistance to decrease the cathode current in a cathode biased stage. The higher the cathode voltage is with respect to the grid (which is at 0 volts), the less the tube will conduct.

Since the gain of the amp is cut more or less by half, try cutting the value by half. Another name for negative feedback is inverse feedback. If you want to increase the gain of a circuit, you decrease the feedback voltage by increasing the feedback resistance.

I would be very tempted to change to fixed bias, although cathode bias is part of the sound of the amp and I don't know how much I want to fiddle with that.

If I stayed with cathode bias I would put a 1k pot in the cathode find the right value and solder some fixed resistors in. It all depends on what I find B+ to be.

In any case I will probably be changing away from 7591s

Don

I just started looking for 7199s, maybe I should be looking for some way to get rid of this one too.

Don

Don, I think they make a Yellow Jacket tube converter for 7591s. It might be the simplest but not the cheapest way to go.
http://www.thdelectronics.com/product_p ... acket.html

Blurb from the THD website (including the typo):
In the cases of the YJ7591 converters, the overall ouput power remains the same, but the sound becomes fuller and the extinct 7591 are replaced with common and inexpensive EL84 tubes. In old Ampeg Reverb Rockets and the like, you experience a warmth and sweetness that you could never get with the now unavailable 7591 tubes.

Cheers

I know your philosophy is to design amps with common tubes, but personally I like the sound of the 7591's... plus JJ/Tesla and Sovtek both have decent offerings in that department. I always carry spare tubes with me when I gig, so having an amp with odd tubes (especially the triple-triode 6U10 in my Reverbojet) ins't a concern.

Chris

"While the 7591 output tube is becoming more scarce and expensive, it is actually one of a family of three tubes with essentially identical characteristics. The three tubes are the 7591, 7868, and 6GM5. Ratings are identical, and the only electrical differences are very, very minor differences in inter electrode capacitances. The 7591 has an octal base, the 7868 has a 9 pin Novar base (9 pin compactron), and the 6GM5 has a 9 pin miniature base. Basing diagrams and outline drawings are provided below. ... Substitution (of a 6GM5 for a 7591, for example) is a simple matter of either changing sockets or making adapters by following the basing diagrams. It is interesting to note that RCA lists the basing diagram for the 7868 as "9RW" and Sylvania lists it as "9NZ". Both are identical."...source http://www.vacuumtubesinc.com

Hope this helps.

erichayes wrote:Just went to the link Tom posted above, and noticed a couple of errors.

If the voltage is too high, make the resistor higher in value, or vice versa. . Actually, you want to increase the cathode resistance to decrease the cathode current in a cathode biased stage. The higher the cathode voltage is with respect to the grid

I think guitar amp. employs fixed bias, you can simply adjust the bias pot would do the job. Not alter the cathode resistor.

7591 is a hi-fi audio tube, usually found inside Fisher 500, 500C. Replace with 6l6GC will give you more shocking power.

Gain will not be affected by altering power tubes, it depends on the preamp. stage.

dhuebert wrote:I just started looking for 7199s, maybe I should be looking for some way to get rid of this one too.

Don

Don't think about 7199, the price is terrible. Try to convert to 6an8 or 6gh8, or 6aw8. These tubes are the best replacement for 7199. Only need to alter a few pins to change to the converted tubes.

Well, I got the Ampeg working and it sounds very good. As for replacing the 7591 and 7199; the amp has a printed circuit board and so would require butcher work to modify, which I will not do. I have left it to the owner to source and supply these tubes, so if one of them goes, he has to do something about it if he wants to keep using the amp. Over all I was very happy with the way it turned out considering the condition it was in when I got it. One thing about old guitar amps tho, the cigarette smoke is so thick on the chassis you can scrape it off with a spoon.

Don