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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 19th November 2019, 01:24 AM   #31
Chris Hornbeck is offline Chris Hornbeck  United States
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There are well understood downsides, but total heat for a given bias is constant. I wouldn't ordinarily recommend this significant a modification, but this isn't an ordinary case. Horses for courses.


Restoring vintage electronics in 2019 requires some adjustments from ideological purity. There won't ever be true type 7591's again, or 115 VAC lines (although easy to make oneself). But life goes on.


All good fortune,
Chris
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Old 19th November 2019, 01:27 AM   #32
kodabmx is online now kodabmx  Canada
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Does anyone have specs for the "new" Tung-sol 7591s
If anything, the ~120V line gives old equipment a boost like overclocking a computer
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Old 19th November 2019, 02:04 AM   #33
charnich74 is offline charnich74  United States
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Quote:
Originally Posted by Chris Hornbeck View Post
There are well understood downsides, but total heat for a given bias is constant. I wouldn't ordinarily recommend this significant a modification, but this isn't an ordinary case. Horses for courses.


Restoring vintage electronics in 2019 requires some adjustments from ideological purity. There won't ever be true type 7591's again, or 115 VAC lines (although easy to make oneself). But life goes on.


All good fortune,
Chris
Hi Chris, I'm planning on running it on a Variac permanently so at least 115 will be assured.. I'm not too sure that there is any room for additional caps etc in the box. It's absolutely crammed full. Currently I'm waiting on parts to restuff the cans. Once I get them I'll update with cathode currents and go from there. I'm considering going ahead and drilling the case for bias pots. We'll see how I feel about it or I get there.
To everyone who has contributed to this thread, a big big thank you. There has been some amazing knowledge shared here, a lot of it is currently over my head. Nonetheless, it is valuable that it can be shared with green guys like myself. Cheers
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Old 19th November 2019, 04:36 AM   #34
Chris Hornbeck is offline Chris Hornbeck  United States
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Note that it's possible to adjust "fixed" bias by changing resistor values in the bias voltage dropping stream. That's how it was done on McIntosh's even into the early transistor era. Extra work, but extra reliability.


All the best fortune with your restoration,
Chris
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Old 19th November 2019, 06:06 AM   #35
6vheater is offline 6vheater
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As I said, the easiest method to lower the idle current of the valve is to drop the screen voltage anything from 20-40V less will drop it back to "normal values".

The anode voltage is not important. It can be easily 50V over the max without any harm at all.
Think of the screen like a 2nd control grid, and you are right in there.

That's easily proved by smoking amp's screen drive, where he uses mosfets to drive the screen instead of the g1.
All screen grids are sensitive to noise and voltage variation in high gain valves.
This isn't hearsay it's fact.
It's actually why ultralinear connections work the way they do, adding negative feedback to the screen grid.
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Old 20th November 2019, 02:36 AM   #36
charnich74 is offline charnich74  United States
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Quote:
Originally Posted by 6vheater View Post

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?
I PM'd you regarding this. Thanks
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Old 20th November 2019, 10:04 AM   #37
6vheater is offline 6vheater
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I did mention briefly in passing, - these high gain valves need low impedance drives or they self bias and run away. (Quicksilver did exactly that and constantly melted valves).

Sherwood appears to have done what quite a lot of 60s amps did, use a 12AX7 as an output driver/ phase splitter.
For a low gain valve it works fine (eg. the beautiful Bogen DB110), but 10W was no longer enough in 1963.

They seem to have carried over the circuit from their EL84 which also was a tube killer, chasing high output on the edge of what is sensible.

For a high gain higher performance valve it can rapidly become suicidal.

With a little bit of drift a 150k resistor can become 180k or higher.
I know RCA quote their maximum g1 circuit as 0.3Mohm, but that's at an anode/screen voltage of 300V aa -10V bias.
here we have over 400 and -18>-20 (double), and a repro valve.

so,-
Otherwise, off we go into self bias which rapidly becomes a vicious circle as it drags the entire secondaries down with the extra current.. (I note they derive their bias from an extra secondary winding), but the silicon diode will allow as much current as you like to dump into a dying valve inc a DEAD SHORT direct through the output tranformer primary.

What you would get is the equivalent of running a big end bearing in an engine, max damage and a rod sticking out of the side of the block!

I did point out in PM both Scott & Bogen managed to make amps, using 7591s (or the 7868) which not only has higher anode & screen volts but doesn't go wrong, and makes a lot more power.
How do they do it?
Look at the LK72.

Low impedance drive.
Scott uses a 6U8 pentode DC coupled into a strong triode... 15k source,
not
a 68k, split through a high impedance anode .AX7 thing.
They can safely then let the output valve bias still sit on 200-220k DC source resistance.

Fisher did the low Z thing with their 8417 amp but differently, using instead a dual power pentode strapped as a triode. They don't go wrong and they make good power with low distortion.

In my opinion I cannot understand why Sherwood didn't use a 7247 as driver/phase invertor to drive their output stage.
It's pin for pin plug in.
That would give it the low impedance needed, cure all the ills and make it ultra stable.

I can't even say their power output and distortion is very nice.
As is, originally it's at the bottom end of 'fi.
IMD is very high at higher powers, the amp maxes out at 24Wper ch, & it appears to be limit reliable.
I would suspect the 1v biased 12AX7 input stage is also to blame.

Scott managed easily 35-40W with basically the same line-up, reliably and Scotts output transformers are brilliant, much under-rated kit.

The other silly idea was Sherwood using a series of dropping resistors to feed anode and screen supplies.
OK the quick fix is to remove one and substitute a 30V zener.

I don't reckon fooling with the bias line is neccessary.
In any case with a higher mains voltage, the bias voltage will increase in exact proportion to the line voltage.

Last edited by 6vheater; 20th November 2019 at 10:16 AM.
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