I bet this has been asked before but here goes. Where would a fellow find a matched quad of 7355s? I know if available they will cost some money but likely a lot less than the mod's needed to use something else in a H-K TA5000, which has a very finicky fixed screen/cathode biasing scheme.
If unmatched devices are less expensive, matching a pair of pairs yourself could do the trick. The equipment needed is not single use. 🙂
Resocket for 7591 which is the octal equivalent? The 7355 is pretty rare and expensive these days.
I also came across that but was hesitant to suggest it given some tubes are really favourites! 😀
Rewiring does seem like the universally better choice though.
Rewiring does seem like the universally better choice though.
So it looks like the Sovtek 7591XYZ is long gone, so we're back to square one. I see vacuumtubes.net have 7355 listed, so I would start there first.
jeff
jeff
There are several manufacturers making real 7591 these days. Expo-Pul (New Sensor) is responsible for two of them - the EH7591A and the Tungsol 7591A which is more compact than the EH and will fit places where the EH won't. There is also the JJ7591A which has been good IMLE.
All should be purchased in tested matched pairs, and I would talk to Jim McShane to make sure that you choose the most suitable type.
None of these are very likely to give you the long trouble free service life of the originals, but if you replace them periodically you should be OK.
NOS and good used 7591A are available sporadically on eBay, but unless you can test and match and otherwise assess their condition I would pass on this option.
As far as I know the original 7591 was only made by Westinghouse and Sylvania, and this would also likely be true of the 7355 since it was a repackaged 7591.
All should be purchased in tested matched pairs, and I would talk to Jim McShane to make sure that you choose the most suitable type.
None of these are very likely to give you the long trouble free service life of the originals, but if you replace them periodically you should be OK.
NOS and good used 7591A are available sporadically on eBay, but unless you can test and match and otherwise assess their condition I would pass on this option.
As far as I know the original 7591 was only made by Westinghouse and Sylvania, and this would also likely be true of the 7355 since it was a repackaged 7591.
7355 was made by GE only, regardless of the name on the box. Basing is only one pin different from 7591, biasing quite different as the 7591 has a third more transconductance. But it's not hard to reduce the negative bias.
I stand corrected, I confirmed in one of my GE tube manuals, the 7591 is not an exact sub for the 7355 but it has been used as such. Requires a socket change and re-biasing to accomplish.
I should have double checked, this bit of lore has been circulating for a long time, and it's not correct. It's possible that I confused with something else, but I don't know if that is the case or not.
There are 2 matched pairs of 7355 on eBay right now if you are quick.
I should have double checked, this bit of lore has been circulating for a long time, and it's not correct. It's possible that I confused with something else, but I don't know if that is the case or not.
There are 2 matched pairs of 7355 on eBay right now if you are quick.
Kevinkr - Thanks I got em. I ventured down the rabbit hole of trying to retube and never made it off the schematic. Firstly, very few bottles will fit in the cabinet. Also, the 7355s have different bias shift and plate load ratings than most other 6L6 type tubes and the H-K design has a combo grid/cathode biasing circuit that also uses the filaments of the pre-amp front end tubes. They did this to get really quiet filament voltage and to bias the tubes; this is a very elaborate and well designed circuit but specific for these exact tubes. To change to different tubes, I would have to do a lot of mods including decoupling the filaments from the cathodes and screens then providing a new source for the pre- filaments and negative dc to the screen (there is no DC- source currently). Too much for me and this old receiver to go through. I currently have 10 not matched UOS (used-old-stock) 7355s that all work but test all over the place. I'll let you know how the pair of matched pairs of NOS 7355s sound after I get them in and balanced.
So I purchased two matched pairs of 7355s. Both pairs are RCA and all four tubes test 140 on a serviced & calibrated Eico 667. One pair is date coded 1971 and the other 1974. I feel like I could call this a matched quad. Anyone disagree?
Yes. The two parameters that need to be matched are mutual conductance and current.
The problem here is that your tester is not really capable of matching tubes. I have an Eico 666, which is essentially the same unit. Despite being labelled as "Dynamic Mutual Conductance" testers, these don't really test for mutual conductance, they are just emissions testers. Testers that only have a % scale on their meter are only capable of emissions testing.
After owning the Eico for many years I picked up a Hickok which tests for mutual conductance but not current.
Current is best checked under normal operating conditions. There may be some testers that test current like that, I'm not sure. Some sellers may also have specially built test setups which allow them to match current.
The Eico is good for some things, I still use mine, but it will not match tubes.
Thanks Charlie. So if you had two matched pairs substituting for a quad, would you put the matched pairs together or put one of each pair in each channel so the channels have the same combination of tubes?
Keep the pairs together, their DC balance at idle reduces the net dc flux present across the output transformers which is what matters most.
Hopefully they are reasonably well matched despite the simplistic tube tester used.
Hopefully they are reasonably well matched despite the simplistic tube tester used.
Billingtons in the UK list 4 x 7355 NOS for £80, and they would probably balance them. You have to buy £240 worth of gear for that price, but maybe there is a market for a group buy type of thing, where we add small orders and get a wholesale price?
If only the EL81 was the Holy Grail of tube audio; they are just 90p in bulk.
If only the EL81 was the Holy Grail of tube audio; they are just 90p in bulk.
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