Fender Tweed Twin Reissue JB Edition

[a j shaner]

I have a curious question. What rectifier tube is being used.
If it is a GZ-34, how can it supply enough current to adequately power a quartet of 6l6GC's.

[martin manning]

If you zoom in on the chassis pic in the first post you can see it's a 5AR4/GZ34. Must be cutting it close.

[a j shaner]

Thank you, Martin.
I am considering building a 5f8-a circuit with a 5V3/5AU4 rectifier though I believe I will need a custom made power transformer to supply the necessary current requirements.

[tictac]

Interesting filament circuit..... It looks like there's the commonly seen virtual CT of two (100 ohm?) resistors on the filament winding, even though the winding has a center-tap.

Then on the center-tap it appears like two back-to-back zeners to ground. Could this prevent a voltage spike during power up, kinda like a poor-man's soft-start for the filaments?

Never seen anything like it...

TT

[pdf64]

Re the GZ34 perhaps being over stressed, I agree that would likely happen if regular BF 4x6L6 parts were used.
But it should be borne in mind that in the absence of this amp's p-p load impedance, loaded HT voltage etc, we can't know the loaded current draw. So it may be that the dynamic operating point has been designed to keep everything in a happy place.

Interesting filament circuit...the winding has a center-tap.

Then on the center-tap it appears like two back-to-back zeners to ground. Could this prevent a voltage spike during power up, kinda like a poor-man's soft-start for the filaments?

Never seen anything like it...

TT

I can't make out the CT wire or those parts on the pic http://ampgarage.com/forum/download/fil ... &mode=view
It's a big ask, but would you be so kind as to mark them out somehow in Paint etc?

Whatever, I think that such an arrangement would act to prevent collateral damage in the event of a power tube 'plate-heater' short, ie the rather common failure mode that blows up the 100 ohm balancing resistors.
Should that happen, until a fuse blows, the entire heater string may have its dc reference pulled up towards HT voltage, thereby potentially damaging the heater-cathode insulation of all the amp's tubes.

[Deric]

It does indeed have the typical pair of 100r resistors to ground as well as a back to back pair of 12v zeners from center tap to ground.

[tictac]

I can't make out the CT wire or those parts on the pic

That's because you can't see them in the pic of the amp. It's the pic of Joe holding the schematic. Kinda teasing us with the schematic but not close enough to get a clean image of parts values.

You can definitely see that the filament circuit is a bit unusual but in a good was. Fusing is a nice touch too...

TT

[Colossal]

Glad to see this thread getting some interest. I have been debating a Tweed Twin build for a long time so it was interesting to see 7ender reissue it as a signature build. I have no place to play such a beast but you guys know how it is

Tweed Twin and Burst.jpg

[a j shaner]

It appears to be possible to use the 5AR4/GZ34 rectifier if you are using 5881 tubes.
The quiescent operating point(@ 70% plate dissipation calculates @ 41.28mA per tube at a plate voltage of 390 volts
For four tubes it is 165.13mA
6L6GC's calculate @ 215.38mA for four tubes.
This is right at the edge of the maximum current rating fo a 5AR4/GZ34(225mA)

Does anyone know how to calculate max signal current draw?
I would appreciate any input on this.

[pompeiisneaks]

It appears to be possible to use the 5AR4/GZ34 rectifier if you are using 5881 tubes.
The quiescent operating point(@ 70% plate dissipation calculates @ 41.28mA per tube at a plate voltage of 390 volts
For four tubes it is 165.13mA
6L6GC's calculate @ 215.38mA for four tubes.
This is right at the edge of the maximum current rating fo a 5AR4/GZ34(225mA)

Does anyone know how to calculate max signal current draw?
I would appreciate any input on this.

If you're following the 70% for idle, I believe that's trying to keep max signal current around or below 100% so that you'd say max dissipation should be that 100% dissipation rating, so you don't roast the tubes too quickly.

Max really is going to depend on how the bias is actually set, not the theory though You could calculate max by putting in a square wave and turning everything wide open, but that's also going to really beat things up

~Phil

[pdf64]

It appears to be possible to use the 5AR4/GZ34 rectifier if you are using 5881 tubes.
The quiescent operating point(@ 70% plate dissipation calculates @ 41.28mA per tube at a plate voltage of 390 volts
For four tubes it is 165.13mA
6L6GC's calculate @ 215.38mA for four tubes...

As their characteristics / curves are virtually identical, what's the rationale for running 6L6GC hotter than 5881?
http://www.mif.pg.gda.pl/homepages/fran ... 5/5881.pdf
http://www.mif.pg.gda.pl/homepages/fran ... /6L6GC.pdf

What if tube development had continued, and 6L6GZ with 100 watt plates was now available; would it make sense to stick those into an amp designed around around 5881 etc and try to idle them at 70%?

[M Fowler]

It appears to be possible to use the 5AR4/GZ34 rectifier if you are using 5881 tubes.
The quiescent operating point(@ 70% plate dissipation calculates @ 41.28mA per tube at a plate voltage of 390 volts
For four tubes it is 165.13mA
6L6GC's calculate @ 215.38mA for four tubes.
This is right at the edge of the maximum current rating fo a 5AR4/GZ34(225mA)

Does anyone know how to calculate max signal current draw?
I would appreciate any input on this.

I've asked how Little Walter Amps Phil Bradbury says his 89 series amp gets 100W GZ34 tube rectifier, with four 6L6GC PTP build either Fender or Marshall JTM45/100? He never shows gut shots for good reason.

Mark

[a j shaner]

Perhaps I should clarify the subject a bit more.
My concern was mostly the power transformer and rectifiers ability to supply sufficient current to the power tubes.
Generally speaking I bias my amps at 70% plate dissipation.
In my 5f6-a Bassman the power transformer has a 200mA current capacity and is more than sufficient to run two 5881 or two 6l6GC tubes.
A four 5881 or 6l6GC amp(such as a 6g8-a or AB763 Twin has a 450mA capacity and uses six 4007 diodes in a full wave rectifier configuration.

My concern was that the 5AR4/GZ34 and the power transformer will be unable to supply the necessary current if the amp is pushed hard.

I have not been able to find the specs on the original Triad 7993 power transformer and Mercury Magnetics does not publish theirs on their website.

All thoughts and comments welcome.

AJ

[Colossal]

I have not been able to find the specs on the original Triad 7993 power transformer and Mercury Magnetics does not publish theirs on their website.

MERCURY MAGNETICS
 FTHPT-P
Triad 7993
348 - 0 - 348 @ 250mA UNLOADED
50VAC @ 100mA BIAS UNLOADED
5VAC @ 4A RECTIFIER
6.3VAC @ 9A FILAMENT

GZ34 rectified and under load, this will be 400VDC on the plates. Right where it should be for a 5F8A.

[a j shaner]

Does anyone think 250mA is sufficient to run a quartet of 6L6GC's?