Fender Bandmaster/Showman mod reversal

[ziggy007]

G'day All,
I have a mates '63 bandmaster that he wanted re-biasing. I discovered that it had been modded to a 'showman' standard with 4x 6l6s and bigger PT & OTs.
He would like it returned to original but besides the big drilled out hole for the new PT i have a question re the OT. It has no ident but is a 1700 Ohm 4/8/16.( prob a Marshall?) Would there be any major dramas running the original 6l6 pair into this load. Nominal should be about 6K from what I've read, but I've also read that shouldn't be a drama. At least I would think there should be more plate current ?
Thanx as always,
Gavin

[MWJB]

Fine to run the transformers that are in there at the moment, but you would have to double the speaker load, so the OT secondaries become 8/16/32ohms. So the owner might want to make decision on whether a new OT might be in order if he only has 2-4ohm cabs.

"At least I would think there should be more plate current ?" the OT will take more current, but the tubes should be biased as normal (30-35mA per tube depending on plate voltage).

"Nominal should be about 6K from what I've read," typical Fender primary Z for 6L6 is 4K or so per tube, A Twin OT is normally around 1900ohms per tube, so the OT you have is close enough.

[MWJB]

"A Twin OT is normally around 1900ohms per tube" Apologies, I meant 1900 per primary (3800ohms per tube).

[ziggy007]

Thanks Heaps for yr reply MWJB. really appreciate it.
Not quite sure what you mean with running the secondary taps as 8 /16/ 32. Do you mean now running a 8ohm speaker load on the secondary 4 ohm ,16 on the 8 tap etc etc?
(this is cool as the owner was running a 2 x12 8ohm cab)
Also so I can get my head around the theory, would the plate voltage not need to be lowered as to reduce current in the plate cct, or is this the reason for the doubling of the secondary load?
thanx again.
Gavin

[MWJB]

By using only 2 power tubes, you effectively double the primary impedance of the OT, 1700ohms becomes 3400ohms. The knock on effect is that the impedances at the secondaries also double, so yes, you would plug the 8ohm cab into what was the 4ohm tap.

How would you lower the B+? You would need to install a zener diode. It isn't necessary, but you may need to rebias the remaining tubes. Measure the plate voltage when the tubes are drawing 30mA each & report back. Correcting the impedance mismatch has a bigger effect on the AC voltages & tube life, than dc idle (bias) specifically.

[ziggy007]

Thanks again mate,
I will do so hopefully this weekend when I next get a chance to play.
I also found a great link on the Harp Amps site which helped.
( Output Transformers and Impedance)
The (no load) voltage is about 470v (due to replacement PT so will be high for the 2 x 6l6.
Have since found 'bulging' caps, a re wired standby switch (gnds centre tap on PT) and several other questionable mods that are going to require a bit of re work. Why cant people leave classic amps alone
cheers & ta
Gavin

[MWJB]

No load voltage of 470vdc is fine for 6L6s (it will drop some with tubes installed), they'll take 500v+ with a reasonable current draw.

[ziggy007]

Its all in the too hard basket, he wants it left alone now, so I'll replace the filter caps and rebias and close her up. Thanks for yr help.

Could I pick everyones brain on why someone would have modded the standby switch to gnd out the PT centre tap from its original method. I know its an older way of doing this but is there any method/advantage rewiring & doing it this way that I'm not seeing ????
ta in advance.
Gavin

[Enzo]

Seems to me standby switches work fine wherever you stick them, so claiming one way is "better" than another strikes me as pointless.

The only thing I can think of would be that opening the center tap of the transformer winding keeps high voltage off the switch. Current will never be an isssue on a B+ switch. I may be overthinking this, but with no ground connection, the high voltage winding has no particular potential above ground. You might read something on a meter, but a meter has some impedance, however high, so it completes a circuit to that extent which can appear as a reading. But unlike switching 450-500v to an unchagred cap - which would be switching 500v, it doesn;t see that. Opening the switch might lead to some inductive arcing, I don;t know.

[ziggy007]

Thanks Enzo,
Having no voltage thru the switch was my only reasoning as well. I just thought the chance of the switch line becoming open was a way of unknowingly having full herbs in the amp with the switch 'off'. Just a safety,"what if" thing I guess. (have been working on aeroplanes for too long)
Cheers
Gavin