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Does anyone have specs for the "new" Tung-sol 7591s
Does anyone have specs for the "new" Tung-sol 7591s
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Old 18th November 2019, 10:27 PM   #21
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by charnich74 View Post
I don't get why multiple resellers of the tube, including Jim Mcshane are telling me I'll b need to rebias the amp and the dissipations are different on the new tube when all written specs are the same. WTH is going on? On paper it looks like it is a drop in replacement.
Short answer:it is NOT a true 7591, even less "Tungsol" but some Soviet designed tube which is "close enough" to be roughly used as an equivalent.

Tube **resellers** can do nothing about that, they canīt publish real datasheet nor original name (if anything because itīs all written in Cyrillic) and specially because it kills sales.

Mind you, itīs probably better than the original one, military specs, etc. but that doesnīt cut ice with buyers, who want nostalgia, Mojo and a "famous name" , even when anybody thinking 30 seconds about it must realize thereīs no connection whatsoever between old and new Tungsol.

And prefer inaccurate but magical datasheets than real ones.

That said, honest resellers at least hint at the difference and suggest proper biasing.
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Old 18th November 2019, 10:48 PM   #22
Jim McShane is offline Jim McShane  United States
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"If you want to measure the total dissipation of the valve it is neccessary to measure the Anode AND Screen grid current."

That's what the cathode current is - the sum if the screen and plate current (and grid current if there is any). And that's what I wrote about.

With all the explanation you gave - how would you measure dissipation in a triode?? They don't have screens.

I'm sure that in general what you are saying is worthwhile but it has little or nothing to do with establishing the safe operating area for the tube in question.

That's all I intend to say on this topic.
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Old 18th November 2019, 11:17 PM   #23
Jim McShane is offline Jim McShane  United States
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Quote:
Originally Posted by JMFahey View Post
Short answer:it is NOT a true 7591, even less "Tungsol" but some Soviet designed tube which is "close enough" to be roughly used as an equivalent.
Do you have evidence or a reliable source to support that?

The reason I ask is that there was a tube a number of years ago called the 7591XYZ that was produced as a sub for the real 7591A when the old stock supply dried up and there were not yet any new 7591s in production. The "XYZ" was a repinned 6P3S-E Russian tube and it came with a cathode resistor that had to be added if you wanted to use the 7591XYZ in a real 7591 socket since electrically the 6P3S-E is much closer to a 6L6 than a 7591.

Ever since them there has been a steady drumbeat of people who insist that ALL Russian 7591 tubes are just repinned 6L6s. I do not believe - based on my own measurements and on the people I've spoken to from New Sensor - that is the case.

So if you're certain that the Tung-Sol branded tube is a rebranded/relabeled tube could you share the source of that information ? And could you share what the number is of the Russian tube that's being sold as a 7591? I'd appreciate it very much.
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Old 18th November 2019, 11:17 PM   #24
6vheater is offline 6vheater
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It might be all you want to say, but the easy fix is not messing with the control grid voltage, (bias) as I have pointed out over and over again.

The easiest is to put a simple zener in series with the screen grid supply to drop the supply voltage.
No doubt selecting a few random values from 6V to say 18V, will get the thing under control.
How come we have to go round and round to get to this?

As for the following comment:-

Quote:
Originally Posted by JMFahey View Post
Short answer:it is NOT a true 7591, but some Soviet designed tube which is "close enough" to be roughly used as an equivalent.

Tube **resellers** canīt publish real datasheet nor original name (if anything because itīs all written in Cyrillic) and .....inaccurate but magical datasheets than real ones.
This statement is inaccurate, I will correct because I am often there.
Most all these valves from ex-USSR were based in the old 6L6 copy the 6P3S.
This was basically not a Soviet design, but as I made clear, a US design made in the USSR.

That was modernised to be the 6P3S-E with "coin" base and is really suprisingly small.
Hey suprise suprise, the last 7591 had coin bases too...and is small.

Now being as I happen to have the magic "cyrillic" data for the 6P3S-E (yes it's almost impossible to find, the REAL bias and current) , it's clear it differed quite a bit from the 6L6, AND they suffered from wide variations in quality and gassiness, a guy called "Wavebourn", an expert in USSR valves will confirm...

You only need to ask him.
The 6L6 really has a different more compact anode shape than the Russian one, I don't know about the Czech and Serbian ones.
It appears this 6P3S-E was altered or reverse engineered in various ways, the grid spacings alterred and hey presto it's a 7591.

Quote:
Originally Posted by Jim McShane View Post
Ever since them there has been a steady drumbeat of people who insist that ALL Russian 7591 tubes are just repinned 6L6s.
So yes that is "fake news".

What is not fake is the fact it is being produced by Reflektor Saratov, which has basically rebranded everything with fake names on the instructions of a certain person...ahum..and even invaded Svetlana's St Petersburg's space so badly they got so almighty p..ssd off with it, they stopped production of all consumer audio devices...

In my opinion SVET, would be the only company that could make the 7591 properly, being as I deal with another high quality company there.
Melz no longer make valves - specialising in military optics, Novossibirsk is dead, Foton Tashkent Uzbek is dead, I can't think of many more.

Last edited by 6vheater; 18th November 2019 at 11:28 PM.
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Old 19th November 2019, 12:13 AM   #25
smoking-amp is offline smoking-amp  United States
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The grids are how one gets the current set up, sure.
Icathode = k (Vg1 + Vg2/Mu2 + Vp/Mup) ^1.5

Once the current is established however, power dissipation is Pdiss = Vp x Ip
and G2diss = Vg2 x Ig2 with Ip and Ig2 summing to Icathode

Since Vg2 is roughly similar to Vp (or B+ actually) for the usual audio tubes, to a good approx. Tube diss. = Vp x Icath.

Last edited by smoking-amp; 19th November 2019 at 12:18 AM.
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Old 19th November 2019, 12:20 AM   #26
6vheater is offline 6vheater
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Quote:
Originally Posted by smoking-amp View Post
Since Vg2 is roughly similar to Vp for the usual audio tubes,...
What makes you say that?
Most of the normal examples Vg2 is half that of Vp...
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Old 19th November 2019, 12:23 AM   #27
smoking-amp is offline smoking-amp  United States
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The grid 2 current is typically only 5% to 10% of plate current at idle. It's just an approximation to simplify the formula to use Vg2 at B+ too (a small inaccuracy), it -will- be B+ for ultralinear mode.

If one is specifically calculating the grid 2 dissipation to check against tube grid 2 max spec, then of course use the real Vg2.

Last edited by smoking-amp; 19th November 2019 at 12:39 AM.
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Old 19th November 2019, 12:42 AM   #28
kodabmx is offline kodabmx  Canada
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Does anyone have specs for the "new" Tung-sol 7591s
In most examples, I see screen tied to B+. At least all the low power tubes like EL84 and 6V6. Sure, an 829B wants 200V screens and 600V plates, but it's a transmitting tube, and it still works at 400V as a triode... Watch out for runaway though LOL
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Old 19th November 2019, 12:50 AM   #29
Chris Hornbeck is offline Chris Hornbeck  United States
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I'm wondering if the OP's best solution might be conversion to cathode bias. It soaks up some of the excess anode and G2 voltages from modern power lines, proves a small cushion for G1 current runaway, requires no chassis holes, and generally is friendlier to use with modern valves. Four resistors of - what? - maybe 750 or 820 Ohms, four big 50 Volt 105C electrolytics. Another possibility.


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Chris
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Old 19th November 2019, 01:06 AM   #30
kodabmx is offline kodabmx  Canada
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Does anyone have specs for the "new" Tung-sol 7591s
Only downside is less power, usually more distortion, and maybe 10W-20W of waste heat? That and can you find the space in the chassis for 10W resistors and the caps?
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