5F8-A Adjustable Bias MOD 6550

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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

I'm sorry about post multi thread... but the tread was for different question... sorry for the confusing,

I didn't find a better place to find very knowledgeable people and tips/help...

Best regards

Dave
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martin manning
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Re: 5F8-A Adjustable Bias MOD 6550

Post by martin manning »

Colossal wrote: Thu Nov 30, 2017 2:43 am...Mercury specs their OTs to be able to deliver 50% more than the vintage equivalent, but 200W is more than 50%. The chosen Mercury OT is 2k2. The original Triad 7993 PT was rated for 200mA and the Mercury equivalent is 300mA on secondaries, 4A on the rectifier winding, and 9A on the filaments.
DaveWell posted the spec sheet for the FTWHPT-P PT above. It shows 5V@4A, 6.3V@7A, 360-0-360 with no current specified, and a bias tap with no voltage specified. WTF.

This plan is not well thought-out. At 500V, 2x 6550 with a 4k load (producing ~100W) would be reasonable for this transformer, but a screen supply at 325-350V would still be needed.
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didit
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Re: 5F8-A Adjustable Bias MOD 6550

Post by didit »

DaveWell wrote: Thu Nov 30, 2017 3:02 am I'm sorry about post multi thread... but the tread was for different question... sorry for the confusing,
Better approach is a single thread that captures that larger question: "help please, how best to design this modified amp?"; with all these various issues as sub-topics.
martin manning wrote: Thu Nov 30, 2017 11:12 am
Colossal wrote: Thu Nov 30, 2017 2:43 am...Mercury specs their OTs to be able to deliver 50% more than the vintage equivalent, but 200W is more than 50%. The chosen Mercury OT is 2k2. The original Triad 7993 PT was rated for 200mA and the Mercury equivalent is 300mA on secondaries, 4A on the rectifier winding, and 9A on the filaments.
DaveWell posted the spec sheet for the FTWHPT-P PT above. It shows 5V@4A, 6.3V@7A, 360-0-360 with no current specified, and a bias tap with no voltage specified. WTF.

This plan is not well thought-out. At 500V, 2x 6550 with a 4k load (producing ~100W) would be reasonable for this transformer, but a screen supply at 325-350V would still be needed.
Agreed. Not the best way to resolve re-engineering. A single project thread, with clean redrawn schematic with updated parts, and their specification would be best. A layout is a poor substitute for a correct schematic.

My estimate of first B+ is closer to 475-480V, though depends on too many unknowns to be accurate. Bigger individual screen resistors (2K ohms or greater, at a guess), perhaps with a common dropping series resistor to lower shared current and reduce voltage can solve that. Key goal is screen dissipation <= 5W.

Best .. Ian
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Colossal
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Re: 5F8-A Adjustable Bias MOD 6550

Post by Colossal »

martin manning wrote: Thu Nov 30, 2017 11:12 amWTF.
Indeed. Their "spec sheets" don't tell the full story. You have to call or email them and ask specifically. It's pretty maddening how they handle their business.
martin manning wrote: Thu Nov 30, 2017 11:12 amThis plan is not well thought-out. At 500V, 2x 6550 with a 4k load (producing ~100W) would be reasonable for this transformer, but a screen supply at 325-350V would still be needed.
I figured since Dave was getting the modified Tweed Twin HP OT with the 4/8/16R taps (the original has 4R only), he would change the reflected impedance to 4k4 (using say a 8R speaker on the 4R tap) and dial the power back to something manageable. But the bottom line is that this should really just be built as a bass amp instead as a collection of compromises to fit the 5F8A circuit.
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martin manning
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Re: 5F8-A Adjustable Bias MOD 6550

Post by martin manning »

Somewhere it was stated that the the plan is to use 4x 6550.
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didit
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Re: 5F8-A Adjustable Bias MOD 6550

Post by didit »

martin manning wrote: Thu Nov 30, 2017 3:18 pm Somewhere it was stated that the the plan is to use 4x 6550.
Yes, read that in a few spots. And depending on the transformer choices, details of screen supply, and fixed bias set-point should be doable. Mercury reprehensibly has minimal to zero detail on their actual specs. Hard to advise much more.

Best .. Ian
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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

I email Mercury magnetic for more detail on the PT.

Yes, i should take a basse schematic of a bass amp, the thing is I didn't find a Layout.

Ian, the screen resistors seem's ok at 1K 5W... you mention 2K ?, never see that before... Why ?

And Yes, 4 x 6550 gonna be use
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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

Ok guy's I Got , finally , the specs From Mercury Magnetic

B+ 250 ma

Bias is 60V at 100 ma

6.3 8-10 amps ( I know this is not ecaxt...)

5V 4 amps
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Colossal
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Re: 5F8-A Adjustable Bias MOD 6550

Post by Colossal »

DaveWell wrote: Thu Nov 30, 2017 7:50 pm Ok guy's I Got , finally , the specs From Mercury Magnetic

B+ 250 ma

Bias is 60V at 100 ma

6.3 8-10 amps ( I know this is not ecaxt...)

5V 4 amps
Well, that's different than what I was told with respect to current handling for that PT. I keep very careful notes. I would err on the side of caution (250mA).
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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

The 60v on the BIAS tap is on my mind..

circuit was for a 50V wire, I must have to adjust the 15K arfter the diode a litthe hier ?

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Re: 5F8-A Adjustable Bias MOD 6550

Post by sluckey »

I doubt it. Remember, you're moving up from little 5881s to 6550s. Those big tubes will need a little more bias voltage.
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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

Yep ok,

So for BIAS adjustement a 47K and a 50K pot will be fine
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martin manning
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Re: 5F8-A Adjustable Bias MOD 6550

Post by martin manning »

If the PT is for the 6G8-A blonde twin, it ran 4x 5881 at 430V B+ with a SS rectifier. The bottom line is this transformer set is good for about 100W. If the OT primary is 2k2, and the screen voltage is close to the plate voltage, it will be fine for 4x 5881 or 4x 6L6GC, or 2x 6550.
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Re: 5F8-A Adjustable Bias MOD 6550

Post by Stevem »

That PTs bias tap has 100 ma?!
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Re: 5F8-A Adjustable Bias MOD 6550

Post by didit »

Is it possible to get a measure of static resistance on the PT primary and high voltage secondary?

Best .. Ian
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