Hi I just got my first tube receiver yesterday,a Fisher 500C restored from a dealer. But somehow I either broke one of the output tubes durring unpacking or it could have been defective. Anyway I need to get a matched pair of 7591 output tubes for my Fisher 500C,the good pair of matched tubes are the ones made in Russia. Should I get a new quad set or a pair like the Russia ones? And where should I get them from.
Advice greatly appreciated.
Thanks,
Robert

Although I've not heard them, the Russian 7591A are supposed to be very good. You shouldn't have to pay more than $60-80 for a matched quad. There was a guy on ebay unloading them for I think $65 a quad. I called CE Distribution and they were willing to come close to that price for I think $77 for a quad if I remember right.

I think Paul at Bizzy Bee, www.2baudio.com, sells quads of EH's for $65 plus shipping.

Clay

You can't go wrong with the EH-7591A's. I purchased two sets of quads from Paul at Bizzy Bee over a year ago and they work and sound just as good or better than the originals in my Fisher 500B and Sherwood 5500III. The Sherwood amp originally had 7868 outputs but I liked the EH-7591's so well I changed the tube sockets so I could use the 7591's.

Bill

The EH-7591s are in larger bottles (small 6L6) than the vintage but I see no problem with them fitting into the 500C. If your 500C has Russian 7591s in it you had better check with the guy who restored it and see if he changed anything bias wise as the earlier Russian "7591"s were indeed only 6L6s with the pinout of a 7591. Later versions have the same specs and don't require any changes but you are better off asking.
I have heard the EH 7591s and agree they are very good tubes, recommend that you buy a matched quad of them and forget the others.


Bill,
What did you do with the 7868s?

Thanks Bill,
I currenty have (2) Electro Harmonix 7591A made in Russia tubes in there now.
I just ordered two more a matched pair from Bizzy Bee.

Thanks for your help.

Robert

Thatch, The EH-7591A's, other than their larger size bias and work identical to oem 7591's. If the guy who restored
the 500C set the bias to a minus 20-22 volts the EH's will work perfectly. The ones you are thinking about are the Sovtek 7591XYZ. The Sovtek's are basically a re-pinned 6L6 and require you to put a resistor in the cathode circuit to get them to bias correctly. Also you lose a little power output with the Sovtek's. The 7868's I took out of the Sherwood amp I am keeping as backups for my Sherwood 8000IV receiver. The 7868's are getting a little pricey and scarce so I thought I better keep them.

Bill

Bill,
I have a few 7868s stashed as well and since I want my Fisher 400 reciever to run forever,.......well. I think the pointy tops are Syls (what I am running) and have a pair of round top RCAs.
Yes it was the old Sovteks that required the rebiasing done. The new EH are drop in replacements as far as I know. I will be needing a quad soon myself. I like the wafer base better than the old ones, the heat will dissapate better.
The 500C has no resisters in the neg bias, only chassis ground, our at least mine does.



Robert,
There have been a few varients of 7591s out of Russia the last 10 years. Only the latest that is being sold under the EH label is a direct drop in replacement for the old US made tubes. If the early Sovteks had been in your 500C alterations would, or at least should, have been made to use them. These alterations would have to be removed to use the EH.
Just stating that the tubes were Russian meant that there was the possability of 2 different tube types being used with only one being a direct replacement for the stock US made. I wasn't trying to confuse you.

I wanted to roll my original tubes back in before commenting on EH's

I got a matched quad from Parts Express. The first quad had one dead tube and two bad ones. The second set were matched pretty close but tested just below the "?" on my tube tester.

My original tubes were Sylvania's. I think they sounded pretty damn good, but I ordered a new quad thinking that they should sound better than these tubes that are testing questionable also.

I've changed out the bias power supply caps, the SR's and rebias'ed the outputs to about 20 volts. To my ear there is no comparison, the EH's sound like crap.

I learned electronics on tubes, but really haven't played with them in years. I picked up this 500C just to play with. I'm not sure if there is anything I can do to make the EH's sound better, right now I just want to leave the old Sylvania's in and get rid of the EH's. Could it be possible that Parts Express sent second bad quad to me?

Right now I'm listening to John Lee Hooker on the 500C and its sounding very sweet.

I'd like to change out the coupling caps next, any suggestions on something affordable. Sources would be much appreciated.

X

Put in new diodes while your at it. I did my 400 with Sprague Orange drops and it sounds fine. The 500C looks like it has mostly W.German oils in the amp section and most of the US made are quite a bit larger in size.
I might have enough Oranges on hand if you give me a list. I'll let you pay me when I replace them.
I have,
.2 600V
.33 400V
.82 600V
.047 600V
.33 200V

There is enough to do a 500 and if you tell me what the little electros are I might have those too. Not all these caps have long leads. If you want oils I can look next time I go to Nortex, but you might have a hard time getting them to fit.

X, the EH's I got from Paul at Bizzy Bee sound just as good as the Sylvania's that I have. I won't say they sound better. The Fisher 500B I have them in has been gone over by me. I redone the bias with new caps and changed the voltage divider resistors to get the output bias to a minus 22 volts. I also replaced the coupling caps with film foils. By the way the two sets I bought from Paul do check different than the old Sylvania's. Just about every one checks on the line between the good and ? mark on my emissions style checker. While the Sylvania's checkck half way over in the green. Maybe you are hearing something I don't. At my age the old ears ain't as good as they used to be. Hey if you like the Sylvania's better, use them. That's what this hobby is all about, trying differnt things and see how they sound and work. All I'm saying is that the EH's work for me. Sorry you are having problems with them and Good luck with the 500C.

Bill

Thanks Thatch. I'm trying to find my scat right now so I can align the tuner, I'll see what the 500C uses.

I did change out the diodes and the bridge rectifier. The bias power supply caps and changed the voltage divider network, If I remember correctly I got it at right around -20 volts. I know it runs a lot cooler now. Maybe I'll play around and try to get it down to -22.

I wish I could describe what my ears hear better. But the EH's didn't sound as detailed or delicate as the Sylvania's. The bass seemed a little over powering and muddy with the EH's. Right now I'm using a pair KLH 17's with the Fisher, and they really don't need much help on the bottom end.

X

I have a 101-B with 7591s driving a pair of Shahinian Obelisks and it sounds fine. I haven't touched it except to put a balance control from my non-funtioning 500C into it. Those X series integrated had nasty dimension/balance controls, but other than the 5AR4 the pre and amp sections are on parr with the recievers. 202s have larger steel.

If you add a 10 Ohm resistor in each cathode to ground you get a convenient point to measure idle current and a "fuse" to protect the transformers if a tube shorts. If they're not matched within a few mA, you may get some bass distortion.

quarter watt?