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6550 screen voltage
6550 screen voltage
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Old 11th February 2018, 08:11 PM   #1
famousmockingbird is offline famousmockingbird  United States
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Default 6550 screen voltage

I have two Stromberg-Carlson Model AP-437 mono power amplifiers and the schematics under the hood for the two are different but they are wired the same. One schematic, the one that is how they are wired shows a 2k serial resistor to the screens and an 8k shunt resistor for a screen voltage of 375. The plates are ~520v. The other schematic shows a 470R serial to the screens and zero shunt resistor, the screens are only 5v less than the plates, which to me is too high.

Has anyone seen these amps before and why are there two variations? My thinking is maybe the 6550s were failing with such high screen voltage and so they decided to lower them via the resistive divider.

The 8k shunt resistor is above the chassis, the type that stands straight up and they both are failed. Someone else added large wirewounds that I do not know the rating but look like 20 watts and they are screaming hot and blackening stuff underneath. I currently have mounted 50w chassis mount resistors under the transformer cover using the chassis as a heatsink and it seems to be a little better. But I am curious about why there are two different versions. And maybe going with just the 470 serial screen resistor can be fine but I have little experience with the 6550 and that seems too high to me.

What do the 6550 experts think out there? What should I do? Leave the large chassis mount shunt resistors and lower screen voltage or go ahead and run the screens at 515v?
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Old 11th February 2018, 08:22 PM   #2
kevinkr is offline kevinkr  United States
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6550 screen voltage
I suspect it might be the other way around as a cost saving measure, although there will be parametric changes due to the differences in screen voltage.

I designed and built a 120W UL amp for a client/friend 2 decades ago with a quartet of 6550 running at 550V and 25mA quiescent current each, those tubes were finally replaced at my insistence after 18 years of continuous service. (Svetlana 6550C - apparently I got some good ones) Note that the screens would have been a couple of volts higher than the plates except for the stopper resistors on each screen. (I no longer remember the value, I think it was 100 ohms, but I also used 330 ohms in some amplifiers)

I am not sure there are any good 6550 these days..
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Old 11th February 2018, 08:30 PM   #3
famousmockingbird is offline famousmockingbird  United States
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Originally Posted by kevinkr View Post
I suspect it might be the other way around as a cost saving measure, although there will be parametric changes due to the differences in screen voltage.
Oh okay so you are thinking that maybe the extra 20 watt resistor top side was failing and so they said lets just run the screens at anode voltage. That makes sense to me since both the resistors have failed. The bias voltage must be higher with the higher screen voltage which if I change it I would modify for this.

I also don't have any plate characteristic graphs with the screen at 515v, and I do not know what the load impedance the output iron is but I could easily calculate it, the former is the more difficult thing to obtain. So I don't know which operating condition the output transformer is optimal for. If your assumption is correct in that the first iteration is the one with the 8k shunt resistor then I would think the output transformer load is optimal for the lower screen voltage.
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Originally Posted by kevinkr View Post
I am not sure there are any good 6550 these days..
This is what worries me. Although the amp does have tired looking Tung-Sol 6550's with metal bases in them currently they won't last forever.

What's a guy to do?
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Old 11th February 2018, 10:27 PM   #4
kevinkr is offline kevinkr  United States
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6550 screen voltage
I think you'd find with a commensurate shift in bias and screen voltage that the operating conditions will be virtually the same. (This is the problem and benefit of pentodes.. LOL)

Makes sense if you are using them as a pair to make them both the same.

Reliability and cost both could have been considerations for the change.

Your best bet is probably a set of Russian made Genalex Gold Lion KT88, these are what I have been using to retube old client amps. So far so good over a couple of years use. (Get them matched and burned in, this usually weeds out the questionable ones)
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Old 11th February 2018, 10:33 PM   #5
famousmockingbird is offline famousmockingbird  United States
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Thanks for the tip about the Genalex Gold Lion KT88, I have heard good things about them. II was using Penta Labs KT88's a few years ago with a 6550 tube amp I had.

I already outfitted the both with the chassis mount 50 watt resistor for the lowered screens and I will see how that goes for a while. The chassis where I mounted the resistor doesn't seem to get too hot so I am happy. If there are any issues or anything I will just run the screens at 515v.
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Old 11th February 2018, 11:08 PM   #6
MelB is offline MelB  Canada
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Here's a plan. Get these and regulate the screen voltage:
21st Century Maida Regulator

You can then adjust the screen voltage.

You are using 27 watts with the 2K 8K resistors!!!
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Old 11th February 2018, 11:43 PM   #7
Eli Duttman is offline Eli Duttman  United States
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Shortened service life is an ongoing expense that never ends. IMO, 1 way or another, the cost "bullet" of holding g2 B+ down has to be bitten. AFAIK, the only 6550/KT88 variant with truly tough screen grids is the scarce/costly GE 6550A.

While the regulators already mentioned are not low cost, they are, in my mind's eye, clearly superior to any sort of dropping resistor arrangement.
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Old 12th February 2018, 12:54 AM   #8
kevinkr is offline kevinkr  United States
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6550 screen voltage
Another option could be gas tube or zener stabilized screen supplies at 350V. If you use gas tubes you will need to make sure you can start them both. (You will need two gas tubes)
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Old 12th February 2018, 02:04 AM   #9
PRR is online now PRR  United States
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You've got over 25 WATTs in that divider. Is it cold in your house?

That amp can't need over 300V on screens. Use a MOSFET to make a low-loss dropped supply. Total screen circuit dissipation will be 1W-2W.

I don't think 500V/300V in hi-fi, with reasonable idle current, is any kind of 6550 abuse. I've run them much harder no-problem. I did not have any 6550 from this decade-- but a ~2005 Sovtek appeared as robust as a Kentucky G.E. or a vintage Dyna 6550.
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Old 12th February 2018, 02:13 AM   #10
rayma is online now rayma  United States
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Originally Posted by PRR View Post
I don't think 500V/300V in hi-fi, with reasonable idle current, is any kind of 6550 abuse.
ARC ran the 6550 at around 575V plate and 350V screen with 50mA in their D51/D75/D76/D150 amps.
The screen was regulated by another 6550 and a Zener string.

Last edited by rayma; 12th February 2018 at 02:23 AM.
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