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MC225: EL84 with adapter instead of 7591?

oldmustang

oldmustang

New Member
I was looking at what is available in the way of 7591s for my MC225 at Tube Depot today and they had this listing:

www.tubedepot.com

YJ7591 EL84 to 7591 Adapter

YJ7591: An adaptor to allow EL84s to replace 7591 tubes. Converts from Class AB to Class A operation. Output is slightly less than the 7591 Price is for a Matched Pair with JJ EL84. Height with EL84 is 5 inches.
www.tubedepot.com www.tubedepot.com


Screenshot 2025-07-11 at 19.34.35.png
I'm not dissatisfied with the new manufacture Tung Sol 7591s I'm currently running, and I have a set of NOS with plenty of life left in them set aside, but I wasn't aware substituting EL84s for 7591s was possible. I'm further confused by the statement, "converts from Class AB to Class A operation".

Comments? Caveats?

Thanks in advance.
 
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damacman said:
Given that the 7591 is among the most revered tubed from an SQ standpoint, why?
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To be clear, I'm not thinking about doing this and asking to be talked out it. My question, like yours is, Why?

I didn't know this could be done, and given the operating characteristics of the MC225, wondering if this is even advisable. So I'm wondering if someone more knowledgeable than I can explain this to me.

That said, the EL84 can be a really good sounding tube in its own right.
 
You can't. An EL84 will run out of oomph real quickly because of its plate dissipation of only 12 W. The 7591 is 19 W. EL34 comes a bit closer (different pinout). The AB->A could mean the adapter lowers the grid voltage from -16 to -7 V. See the datasheets for both tubes.
 
oldmustang said:
To be clear, I'm not thinking about doing this and asking to be talked out it. My question, like yours is, Why?

I didn't know this could be done, and given the operating characteristics of the MC225, wondering if this is even advisable. So I'm wondering if someone more knowledgeable than I can explain this to me.

That said, the EL84 can be a really good sounding tube in its own right.
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Even if it was a decent idea to sub an EL84 for a 7591, which it isn't because of wildly different operating parameters, it is worse to do so in a Mc225 because of the unique operating conditions of the McIntosh Unity Coupled output transformer. Read Mike Gibson's re-post of Dave Gillespie's explanation here: https://audiokarma.org/forums/index.php?threads/mcintosh-technology.336389/

Get some 6GM5 to 7591 adapters If you want to experience what NOS 7591 tubes can do. Same tube inside, different bottle and pinout and not new manufacture.
 
I was thinking the main purpose of these adaptors would be in an old Ampeg amp that many people thought were too clean sounding for rock and roll. The owner of Ampeg was a jazz guy and switched over from 6V6s to 7591s in the early '60's. Rock guys, of course, preferred the old 6V6 version.
As Billyz said, these would be the ticket to add a little crunch to an old clean sounding Reverberocket.
-I have an MC 225 and wouldn't change a thing on it ;-)
 
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PE9ZZ said:
You can't. An EL84 will run out of oomph real quickly because of its plate dissipation of only 12 W. The 7591 is 19 W. EL34 comes a bit closer (different pinout). The AB->A could mean the adapter lowers the grid voltage from -16 to -7 V. See the datasheets for both tubes.
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Thanks. I hadn't gotten to the step of checking the data sheets yet but your explanation makes sense. I appreciate your time in replying.
 
ramjet said:
Even if it was a decent idea to sub an EL84 for a 7591, which it isn't because of wildly different operating parameters, it is worse to do so in a Mc225 because of the unique operating conditions of the McIntosh Unity Coupled output transformer. Read Mike Gibson's re-post of Dave Gillespie's explanation here: https://audiokarma.org/forums/index.php?threads/mcintosh-technology.336389/

Get some 6GM5 to 7591 adapters If you want to experience what NOS 7591 tubes can do. Same tube inside, different bottle and pinout and not new manufacture.
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Thank you @ramjet. A wealth of information in that thread and as I've been exploring the AK McIntosh forum Dave Gillespie's posts are always an education.

Thanks too for the tip about 6GM5. If I get the itch to tube-roll the output tubes and I can find the adapters I'll definitely give that a try.
 
Dave_in_Va. said:
I was thinking the main purpose of these adaptors would be in an old Ampeg amp that many people thought were too clean sounding for rock and roll. The owner of Ampeg was a jazz guy and switched over from 6V6s to 7591s in the early '60's. Rock guys, of course, preferred the old 6V6 version.
As Billyz said, these would be the ticket to add a little crunch to an old clean sounding Reverberocket.
-I have an MC 225 and would change a thing on it ;-)
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Thank you Dave. That is a good theory. As I commented above, there appears to be more musicians leaving feedback on their tube purchases than hifi users. Tone, breakup and distortion are all very important to amplified musicians.
 
I always wondered what exactly was inside of those adapters. Cant be just a pin conversion, the voltages would be all wrong for most of the tubes being subbed.
 
I loved the MC 225 while they were being made. But they weren't quite enough for my Bozak Symphony speakers. I used two MC 240's in mono at the time. My two favorite amps at the time were the smallish MC 225 and it biggest cousin the MC 3500 pair. 34 watts versus close to 470 watts before clipping. I had MC 60's, 240's and 275s's before going SS. I should have waited until the 7000 series SS amps came along. Heard the new 3500's and prefer the older ones in case you are wondering. So stick with your MC 225 as built and enjoy the sound. I would.
 
Trying to improve upon OEM designs or substituting parts that a given circuit had been engineered around is a fool's errand. In addition to potentially compromising performance, it may even be harmful to the amplifier.

It's the last thing I would consider doing, least of all to a valuable McIntosh unit. My 2 cents.
 
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twiiii said:
I loved the MC 225 while they were being made. But they weren't quite enough for my Bozak Symphony speakers. I used two MC 240's in mono at the time. My two favorite amps at the time were the smallish MC 225 and it biggest cousin the MC 3500 pair. 34 watts versus close to 470 watts before clipping. I had MC 60's, 240's and 275s's before going SS. I should have waited until the 7000 series SS amps came along. Heard the new 3500's and prefer the older ones in case you are wondering. So stick with your MC 225 as built and enjoy the sound. I would.
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Thanks very much. I love the sound of my MC225 and I'm kicking myself for taking so long to get to the party. I've always admired the various McIntosh tube amps but for some reason have never owned one until now. Now, I can't envision any of the other amplifiers I have here kicking it out of my system.
 
62caddy said:
Trying to improve upon OEM designs or substituting parts a given circuit had been engineered around is a fool's errand. In addition to potentially compromising performance, it may even be harmful to the amplifier.

It's the last thing I would consider doing, least of all to a valuable McIntosh unit. My 2 cents.
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Point well taken.

My original question was more to express puzzlement/amazement that this is even possible with two such different tubes and given the Unity Coupled topology of the MC225 to question the advisability. Given my much lower knowledge level than some of the folks here, I solicited comment and explanations.

If I were to experiment with something other than 7591s for the MC225 it would be with something like the 6GM5 and a quality socket adapter.
 
I have a pair of Yellow Jackets intended to stick EL84s in place of 6L6s. Used them a little to try to tame the volume of my Fender Vibrolux. It was a very long time ago, I don't remember much about them. I don't think I disliked them, but I also know that I stopped using them. As I recall, it converted the amp from fixed bias to cathode biased. That's probably why they make the claim about class AB -> class A.
 
JonL said:
I have a pair of Yellow Jackets intended to stick EL84s in place of 6L6s. Used them a little to try to tame the volume of my Fender Vibrolux. It was a very long time ago, I don't remember much about them. I don't think I disliked them, but I also know that I stopped using them. As I recall, it converted the amp from fixed bias to cathode biased. That's probably why they make the claim about class AB -> class A.
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When I first posted this I didn't even think about guitar amplifiers but after doing some research and thanks to the replies here I've found out that Yellow Jackets are a device to do just what you say, allow the musician to tailor their sound to just the way they like it by means of tube substitution.

Thanks for taking the time!
 
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