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6550 screen voltage
6550 screen voltage
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Old 12th February 2018, 02:30 AM   #11
famousmockingbird is offline famousmockingbird  United States
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It's not cold in my house.......................now LOL.


I didn't engineer the dang things so I can't tell anyone why they chose to do it this way.

There is not much room under the hood for any boards and the chassis is shallow, I don't think it's even 2" deep.

I do agree, I shook my head a few times at the amount of energy put forth into heat trying to drop the screens.


Here is the original schematic attached inside the amp. I circled where the 10 watt 2k serial resistor and 20 watt 8k resistor are. The replacement 20 watt resistor inside the chassis was VERY hot so I replaced them with 50 watt ones inside the transformer cover. This is what i used and it's much better than the wirewound 20 watters.

EDIT: I also replaced the 2k resistor with a 20 watt ceramic so it doesn't get as hot either. I went with 3k3 instead of 2k and I changed the 8k to 8k2, I have 360v on the screen and 520 on the plate.

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Last edited by famousmockingbird; 12th February 2018 at 02:37 AM.
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Old 12th February 2018, 07:37 AM   #12
toprepairman is offline toprepairman  United Kingdom
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The absolute max screen volts for 6550's is 550 Volts, pentode mode only. this means UL is definitely out for 550V supplies. Some manufacturers choose to ignore this, CJ for one. They also seem to think we have 230V mains supply, when it can get up to 253Volts. this means the unfortunate valves are seriously overdriven and flashover and consequent damage follows.
You could try KT120's which seem more tolerant in this respect.
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Old 12th February 2018, 03:45 PM   #13
mike567 is offline mike567  United States
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Fender had a FS400 amp that ran 6550 ge's at 350 volts on the screens and 700! on the plates
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Old 12th February 2018, 04:33 PM   #14
famousmockingbird is offline famousmockingbird  United States
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Looking at output power you don't really gain much bringing the screens past 350v, and I don't really trust the screens on new production power valves.
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Old 12th February 2018, 04:52 PM   #15
kevinkr is offline kevinkr  United States
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6550 screen voltage
I'd run them at 350V - performance will be just fine.

I suspect the resistor picture is just a stock photo and not what you put in your amps. (It's 8.2 ohms)

That voltage divider is consuming something like 50mA, if you have space you could install a pair of gas tubes running at 20 - 30 mA or so, greatly improve screen regulation ( = less distortion) and reduce power consumption slightly.
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Old 12th February 2018, 05:54 PM   #16
famousmockingbird is offline famousmockingbird  United States
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Yes there is the divider + the screen current across the 3k3 and then just the divider current through the 8k2. So about 55mA through the 3k3, and 45mA through the 8k2. I don't like it but that's how it was designed.

I like the idea of gas tubes, I just actually put some 0D3's in my 6HJ5 amp (mainly for looks) but I want to keep the looks of these original, and the circuit as original as possible for resale purposes. People that buy this stuff want it as stock as possible.

So it looks like I will keep it as is with the screens down around 350, the tubes should be happier and the performance should not suffer.
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Old 13th February 2018, 12:27 AM   #17
danFrank is offline danFrank  United States
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Quote:
Originally Posted by mike567 View Post
Fender had a FS400 amp that ran 6550 ge's at 350 volts on the screens and 700! on the plates
I run 6550s and KT88s this way all the time with no problems. You will need good quality sockets though as 700+ volts on the octals is pushing it, but the tubes hold up fine if the screens are limited to 300-350V. Where else can you get around 100 watts clean with a pair of common tubes?
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Old 14th February 2018, 12:23 PM   #18
AquaTarkus is offline AquaTarkus  Canada
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The 200W Marshall Major ran its KT88s in UL at 620V..They became notorious for their power stages blowing up when overdriven.
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