5F8-A Adjustable Bias MOD 6550

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DaveWell
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5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

Hey !

I plan mod the 5F8-A tu use 6550

The fix bias resistor 56k going to be replace by a trim pot 25K

But, to use the 6550 tube, i plan to use a 47K in line with the pot...

Question, ; to get the 70% ( or more ), is the 47 are ok ??? Higher or lower ?

Thanks
pdf64
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Re: 5F8-A Adjustable Bias MOD 6550

Post by pdf64 »

DaveWell wrote: Wed Nov 29, 2017 1:39 am Hey !

I plan mod the 5F8-A tu use 6550

The fix bias resistor 56k going to be replace by a trim pot 25K

But, to use the 6550 tube, i plan to use a 47K in line with the pot...

Question, ; to get the 70% ( or more ), is the 47 are ok ??? Higher or lower ?

Thanks
This seems an example of how, whilst it's a useful guideline when applied as intended, the 'bias to idle at 70%' thing is akin to looking down the wrong end of a telescope.
The 5F6A is an AB1 amp designed around the 5881, a 23 watt plate. The rationale regarding the 'idle at 70% of plate limit' is applicable to that tube, ie the amp's operating point and intended conduction angle was likely designed around that tube's limits.
It's probably fine to swap a suitable 35 or 42 watt plate tube in there, but nonsensical to consider it valid idling them at 70% of their plate limit.
Plate dissipation is purely a limit, not a characteristic; an AB1 6L6 amp will work just the same idling at eg 25mA, whether it's got 6L6G, GB, WGB, 5881, GC etc in there, the tube characteristics don't change just because one variant can idle hotter than another.

If you want to fit beefier power tubes in order to turn it into a class A, or near class A amp, ie much larger conduction angle, then the iron will need beefing up to accommodate that.
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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

Hi PDF, thanks for the infos,

I already mod , on paper, the rest of the amp,

Reduce 4 input to 1
V1 50uf/50v with a 1K cathode
6550 Power tube,
Powercap, 20uf/20uf to 47uf/47uf , second one 20uf to 47uf and pre-amp one 20uf to 22 uf.
coupling cap between v1 and volume 0.022 to 0.05
PI entry 0.022 to a 0.05
Solid State rectifier

My quastion was about the Adjustable BIAS resistor value for 6550
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sluckey
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Re: 5F8-A Adjustable Bias MOD 6550

Post by sluckey »

DaveWell wrote: Wed Nov 29, 2017 2:37 pm My quastion was about the Adjustable BIAS resistor value for 6550
Sometimes it just makes more sense to determine the exact value for a resistor experimentally. 47K should be a good/safe ballpark. I would rather start with a 50K pot and 47K resistor and hope to be able to swing the bias voltage between approx -40v and -70v for 6550s.

This all depends on the PT you will use. Will it have a bias tap that will provide the proper AC voltage to be able to swing between -40 and -70vdc? Or will you need to get your AC voltage from one of the HT leads? Plenty of time to determine the actual value of that resistor later on.
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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

Sluckey,
The PT it a Mercury Magnetic
I got the 50V tap on it.

Thanks for the infos !

Really appreciate !
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Colossal
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Re: 5F8-A Adjustable Bias MOD 6550

Post by Colossal »

Dave,

Just so you know, that PT you showed is not the one used in the High Power Tweed Twin. It is for a rare White Twin. The B+ on that secondary is higher than the High Power Tweed Twin. That said, you might want the higher plate voltage it will deliver with your bass amp.
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DaveWell
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Re: 5F8-A Adjustable Bias MOD 6550

Post by DaveWell »

Hi Colossal,

I'm going to use 6550 Power Tube, I think this transformer is going to be fine...or is over powered ?
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Colossal
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Re: 5F8-A Adjustable Bias MOD 6550

Post by Colossal »

DaveWell wrote: Wed Nov 29, 2017 7:33 pm Hi Colossal,

I'm going to use 6550 Power Tube, I think this transformer is going to be fine...or is over powered ?
It will be fine for your application and definitely not overpowered.
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Re: 5F8-A Adjustable Bias MOD 6550

Post by sluckey »

DaveWell wrote: Wed Nov 29, 2017 7:33 pm I'm going to use 6550 Power Tube, I think this transformer is going to be fine...or is over powered ?
The filaments will want 7.3 amps but that PT is rated for 7 amps. If I had a 7 amp PT on hand I'd use it, but if I had to buy one, I'd want one with a higher filament current rating.

The 50vac bias tap will provide 70vdc unloaded with no dropping resistors. I'd want a 60vac bias tap if I were buying a new PT. I have a Sunn amp with two 6550s. My bias voltage is set to -62 and -64 on the tubes.

None of this is a show stopper. Just stuff to think about. Have you looked for a PT that is designed for 4 x 6550?
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didit
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Re: 5F8-A Adjustable Bias MOD 6550

Post by didit »

What is the output transformer planned to mate into this?

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

Post by DaveWell »

The output is A FTHPT-OMA From Mercury Magnetic :)
2K primary 4/8/16 ohm Tap
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Colossal
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Re: 5F8-A Adjustable Bias MOD 6550

Post by Colossal »

DaveWell wrote: Wed Nov 29, 2017 9:27 pm The output is A FTHPT-OMA From Mercury Magnetic :)
2K primary 4/8/16 ohm Tap
That's the one to go with. It is a big block of steel 8)
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martin manning
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Re: 5F8-A Adjustable Bias MOD 6550

Post by martin manning »

PT spec shows 360VAC, which will yield about 500V B+ with a SS rectifier. The OT has a 2k primary, which looks good for 4x 6550 at 500V B+, except that the screen voltage should be about 325V, and the 5F8-A power supply doesn't provide for that. Also, the output power will be close to 200W, and I don't think the Twin PT or OT will be up to that.

One other comment... it's bad form to start multiple threads on the same project. It's very confusing for anyone trying to follow along or offer help.
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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 1:31 am PT spec shows 360VAC, which will yield about 500V B+ with a SS rectifier. The OT has a 2k primary, which looks good for 4x 6550 at 500V B+, except that the screen voltage should be about 325V, and the 5F8-A power supply doesn't provide for that. Also, the output power will be close to 200W, and I don't think the Twin PT or OT will be up to that.

One other comment... it's bad form to start multiple threads on the same project. It's very confusing for anyone trying to follow along or offer help.
That's why I mentioned about the higher voltage secondaries with the chosen PT not being that of the traditional Tweed Twin HP transformer. Also didn't know if he was going tube rectified as with the Tweed Twin or SS, but presumed tube rectified since it was a 5F8A platform. But SS would make more sense for bass. 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.
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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 1:31 am PT spec shows 360VAC, which will yield about 500V B+ with a SS rectifier. The OT has a 2k primary, which looks good for 4x 6550 at 500V B+, except that the screen voltage should be about 325V, and the 5F8-A power supply doesn't provide for that. Also, the output power will be close to 200W, and I don't think the Twin PT or OT will be up to that.

One other comment... it's bad form to start multiple threads on the same project. It's very confusing for anyone trying to follow along or offer help.
That's why I mentioned about the higher voltage secondaries with the chosen PT not being that of the traditional Tweed Twin HP transformer. Also didn't know if he was going tube rectified as with the Tweed Twin or SS, but presumed tube rectified since it was a 5F8A platform. But SS would make more sense for bass. 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. Mercury specs their voltages as unloaded, FWIW.
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