7591 to 7868's

Bradbrews

Bradbrews

Super Member
Has anyone had any experience using adapters to convert amps that run 7591's into 7868's?
LTube adaptor.PNG IMG_0834.jpg
I've got a couple sets of 7868's that test good, and am thinking about using them in my Scott LK-72.
Looks like a set of 4 adapters would run about $40.
 
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No experience but it should work fine as long as the adaptor is well made and the cosmetics don't bother you.
 
You also have to consider the clearance inside any case you might put the amp/receiver into. Will it fit with the adapter in place? Those adapters are like a pedestal that raises up the tube to make it taller.
 
Bradbrews said:
Has anyone had any experience using adapters to convert amps that run 7591's into 7868's?

I've got a couple sets of 7868's that test good, and am thinking about using them in my Scott LK-72.
Looks like a set of 4 adapters would run about $40.
Click to expand...
I would wonder about the quality of the adapters and the height problem. I have converted amps from 7868 to 7591 with no problems. It is a straightforward conversion and tubes like new sockets. Most people go the other way but, you need to do what you can. One thing for sure the results will sound better than adapters but changing out the sockets may hurt the Scott's resale value.
 
Electrically the tubes interchange without issues, physical height and construction quality on the adapter would be the only real questions here. I converted an amp from 7591 to 7868 before stock-size new production 7591 existed. That one wouldn't take the fat body 7591 then available, and there is no space for adapters.
 
shelly_d said:
According to the pin outs on Duncanamps website, the 7591 is 8 pin and the 7868 is 9 pin. Adaptors must of necessity be one way only.
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That's what I suspected...the adaptor he show's won't fit his existing socket. He'd need a 7868 to 7591.

Height probably wouldn't be an issue, as most likely he doesn't have a case.

Diameter would matter, because the transformers are pretty close.
 
B6D00298-24A3-49AA-B827-BB8E7EC03CD3.jpeg Here are the two tube bases. The pins of the tubes are as mentioned 7 vs 9 but also notice the diameter difference of the pins. The amp takes the large diameter pins and the adapter has male large pins on the 7591 pattern. I suspect it will work. I don’t have a case for the LK-72 so the height won’t be an issue.
 
I'd be leery, as Novar sockets haven't been available for quite a while. Some history: Western Electric introduced the Magnoval tube base with .050" (1.25 mm) pins. RCA was never happy with OTHER companies' ideas, and introduced the Novar base with pins spaced exactly the same as WE, but using .040" pins - the 7868 was one of these. European and later Russian tubes used the Magnoval base with larger pins, and sockets for these are still made. A vintage 7868 will be a very loose fit in a Magnoval socket. Modern ones will be fine in a new socket (though they're very hard to fit into a vintage Novar socket). I'd bet the adapter is made for modern ones.
 
honestly the fatter pins work better. I've got some old production 7868 and the pins bend easily. The Russian ones, not so much.
 
D0CBFD64-B208-44FF-8597-743BDD7CC4DA.jpeg
Here’s confirmation the pins are 0.040”. Curiously the only comment on the adapter was that the sockets were too tight and needed to be opened up to get the tube in. Of course no indication of what size pins the tube used had. Thanks for the info, I would not have thought about that.
 
Bradbrews said:
View attachment 1476324
Here’s confirmation the pins are 0.040”. Curiously the only comment on the adapter was that the sockets were too tight and needed to be opened up to get the tube in. Of course no indication of what size pins the tube used had. Thanks for the info, I would not have thought about that.
Click to expand...
Again: keep using 7591 tubes in your amp. There is several new in addition to NOS to choose from.
Sell you 7868 tubes if they irritate you. Don't risk ruin your fine amp!
 
if the new production 7591 tubes had existed, I wouldn't have changed sockets in my Sherwood. At this point I'm not motivated to change it back either.
 
gadget73 said:
if the new production 7591 tubes had existed, I wouldn't have changed sockets in my Sherwood. At this point I'm not motivated to change it back either.
Click to expand...

I got my 7591s from Jim McShane http://www.mcshanedesign.net/tubes.htm

I had to use the Tung-Sol tubes as:

1) I have not heard good reports about JJ 7591s
2) the Electro-Harmonix 7591s would have required me to modify the case on my Scott 340B and I didn't want to do that.

It's been a few years though; are there any other options on the market today?

Straight from Jim's page:

7591A Electro-Harmonix tubes in stock! $21.50 each single, $44.50/matched pair, and $89.50/quad. Here's a typical customer comment...
"I put the EH 7591's in my Pilot SA-264 power amp. Set the bias and let 'em cook a while. Then I listened. I was really surprised.
They sound like the real thing. What a sweet sounding amp that is." They sound like it because they are the real thing, just in a larger bottle!

7591A Tung-Sol - some stock on hand and more on order! Please email me for details and/or to purchase these tubes!

Important note - updated! There have been some cases where the new production 7591A (and 7868) tubes have had trouble in circuits using
a very high value grid resistor - as vintage Scotts, Fishers, and others sometimes do. The best solution is changing all the grid resistors to 200K-220K
from the original value, since often the factory spec resistors exceeded the maximum recommendations. For instance, Fisher routinely used 330K carbon
resistors that would often drift up to 360K over time! As well, to prevent loss of deep bass energy a change of the output coupling cap value to .1 uf
(preferred) or .22 uf. is needed. I stock all the parts you need, the parts cost is only $15.00 to $30.00 per amp, depending on the coupling cap you need
to use. Please note I do not recommend the use of JJ tubes under any circumstances, not even with the changes above!

Click here for a look at a typical 7591 output circuit. Resistors R129 & R130 and caps C87 & C88 in the outlined
boxes are what may need replacement.
Click to expand...
 
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