Fisher X-101-C - what modern 7591A would be best ?

[david winter]

Hi,

I'm currently working on a Fisher X-101-C. The 7591A were all done.
It's recaped and I haven't checked posts about improvements .
I wanted to know what modern type of 7591A would work best considering the voltages.

I also read somewhere that fixed bias models should have the 330K grid leak resistors replaced by 180K-220K and the coupling caps increased around 0.068µF but I don't know if this applies to this model as the bias can be adjusted using a common potentiometer.

What tubes would you recommend ?


Thanks,

David

 

[kward]

I know others have used new production Tung Sol 7591's in their Fishers. I recall their comments being they work and sound great, but I also recall a few people saying they run a bit hotter than the old stock stuff, without any other mods.

I think your comment about the grid leak resistor size is accurate from my recollection also. But all this is second hand, as I have no direct experience with the TS reissue tube. I do need to get some new 7591's in my Fisher 500C. Just haven't pulled that trigger yet.

 

[Tim D]

Might want to collect a list of the new production manufacturers to ask opinions. The list isn't that long.

 

[triode17]

I still advocate the JJ 7591. I use them in my Scott 299C and they're great. No drifting in bias, solid power.

 

[RUmad]

I redid my x-101-c about two years ago. I set it up at first with the original Westinghouse tubes, but decided to preserve originality for the future and retube it. I used EH 7591’s first. They biased well and sounded good but the cabinet wouldn’t fit. I left it naked for a while, until one of the tubes blew up. I don’t hold it against EH, I think I just got a random bad tube. I use them in other equipment and never had an issue. When that happened I ordered the reissue Tungsols. They had a good reputation AND fit inside the cabinet. They also biased well and they sound very good. Hard to tell if they run a little hotter as the x-101-c runs pretty damn hot as it is but it sounds great. As far as the grid leak resistors and coupling caps, I was told directly by a knowledgeable AK’er that because of the way this model is biased it’s not necessary. You CAN do it, but you don’t NEED to. It was either Larry (D) or Dave (G). It’s in a thread here somewhere. Good luck with it.
If you can’t find it, I’ll try a search for it later.

 

[Tom Bavis]

For the X-101C, I would lower the grid resistors and increase the coupling caps in proportion. The Scotts generally have a low enough grid resistor already and Fishers generally used the max value of 300K and a little more... (Westinghouse said 300K is MAX for grid return). And Heathkits used 470K - specs? who needs 'em! Higher value makes it more sensitive to grid leakage, and modern tubes may not be quite what the NOS ones were.

 

[david winter]

I have applied the following mods and still wonder about the grid bias vs cathode voltage which supplies the preamp tubes heaters.
Mods done:
- Added 4x 100 ohm (metal, 0.6W) screen resistors
- Replaced 4x 1K (metal, 0.5W) grid resistors
- Replaced 4x 330K grid leak resistors by 200K (metal, 0.5W).
- To be done once received: replace 47nF coupling caps by ERO 68nF / 630V

So far, the schematics says to set 28.5V between grid and ground, and shows 40V between cathode and ground (to heat 4 12AX78 in series).
The thing is, I can set to 28.5V but get 42.5V at the cathode (no big deal I think). If I want 40V to the cathode, I need to lower the bias to 25.5V.
Can this be due to changing the 330K grid leaks to 200K ? If so, I'll keep the bias to 28.5V.

 

[Sound Dragon]

I run JJ's. Affordable and reliable. Just be sure to get them from a reputable source. I have heard good things about the Tung Sols and will be trying them at some point also. I haven't really heard enough of a difference to justify the insane prices that quads of vintage 7591's command. And I don't believe that there is a pallet of NOS 7591's hidden somewhere. They are almost all used tubes at this point.

 

[primosounds]

Hi,

I'm currently working on a Fisher X-101-C. The 7591A were all done.
It's recaped and I haven't checked posts about improvements .
I wanted to know what modern type of 7591A would work best considering the voltages.

I also read somewhere that fixed bias models should have the 330K grid leak resistors replaced by 180K-220K and the coupling caps increased around 0.068µF but I don't know if this applies to this model as the bias can be adjusted using a common potentiometer.

What tubes would you recommend ?


Thanks,

David

This might be too late for you but it is an alternative that really worked well for me. I recently rebuilt for my son, an EICO ST70 , which is fixed bias PP 7591 output tubes. But not wanting to use my Westinghouse 7591 tubes in it i rewired the sockets to accomodate the Russian 6p3se or 6l6gc/5881 output tubes. Along with the comprehensive rebuild i also modded the power supply with a choke. I must say the the result was excellent. The amp sounds very clear and strong. It powers his B&W speakers better than any other amp that he tried before. The Russian tubes are also short enough to use the original tube cage. Anyway the matching quad of 6p3se were about 80.00 shipped to me. which is not bad at all.