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Post subject: Bandmaster questions
Posted: Thu Mar 29, 2012 11:35 am
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Hello again,
I have a couple of questions regarding my bandmaster. It is a 73' Silverface.
1. As I increase the intensity control for the vibrato the volume starts to decrease. By the time I've reached max intensity volume has dropped by maybe a couple db. Any suggestions as to where to start to remedy this, or is this normal?
2. My Bandmaster already has a three pronged power cord, however it is still wired as if it only had the two pronged power cord with the "death cap". Is it o.k. to leave it this way or should I rewire it as shown in the "updated layout" below:

This is how it is currently wired but with the green wire from the cord going to chasis ground.
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Thanks in advance.

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Post subject: Re: Bandmaster questions
Posted: Thu Mar 29, 2012 1:22 pm
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The tremolo circuit always extracted a toll in pre-amp gain due to inherent insertion loss -- not so with onboard reverb whose recovery tube compensates for this anomaly by restoring the signal level subsequent to the transformer and tank. I have always observed a marked difference between the channels when both volume and tone controls were set to the same parameters. One of Cesar Diaz's mods to SRV's amps was to add a switch which completely disabled the trem and took it out of the signal chain. You can easily do this without drilling a single hole anywhere -- merely use the rear panel's "ground" switch location and install a DP/DT toggle to wire around the input and output of the trem circuit.

Arjay

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Post subject: Re: Bandmaster questions
Posted: Thu Mar 29, 2012 1:28 pm
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Retroverbial wrote:
The tremolo circuit always extracted a toll in pre-amp gain due to inherent insertion loss -- not so with onboard reverb whose recovery tube compensates for this anomaly by restoring the signal level subsequent to the transformer and tank. I have always observed a marked difference between the channels when both volume and tone controls were set to the same parameters. One of Cesar Diaz's mods to SRV's amps was to add a switch which completely disabled the trem and took it out of the signal chain. You can easily do this without drilling a single hole anywhere -- merely use the rear panel's "ground" switch location and install a DP/DT toggle to wire around the input and output of the trem circuit.

Arjay


Not a bad idea, thanks for the heads up Arjay. I will look into this mod via the internet. Any thoughts on the 2nd question?

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Post subject: Re: Bandmaster questions
Posted: Thu Mar 29, 2012 2:53 pm
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Hey socal....as far as changing the three prong cord wiring,change it to the "updated"way shown in the second drawing,the main reason being your hot black wire will be the one that's fused,I don't know why but almost everyone who changes the cords just hooks white to white and black to black,but still only one leg is fused....I change all of mine to the configuration of the second drawing...as far as the "death cap" is concerned,just clip the lead going to the chassis instead of trying to unsolder it...it's easier that way.


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Post subject: Re: Bandmaster questions
Posted: Thu Mar 29, 2012 4:03 pm
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Rebelsoul wrote:
Hey socal....as far as changing the three prong cord wiring,change it to the "updated"way shown in the second drawing,the main reason being your hot black wire will be the one that's fused,I don't know why but almost everyone who changes the cords just hooks white to white and black to black,but still only one leg is fused....I change all of mine to the configuration of the second drawing...as far as the "death cap" is concerned,just clip the lead going to the chassis instead of trying to unsolder it...it's easier that way.


Thanks rebelsoul, that's the route I'll take then.

On another note, If I decided to blackface the amp, and it's still up in the air, one of the suggested and last modifications is to remove the 5u4 and replace with a 5ar4/gz34 and then rebias. I read that the 5ar4 draws considerably less current but also increases the voltage. The pro's to doing this is that it produces less sag, about half of the 5u4 and will give extra power to the tubes. The con is will the transformer be able to handle this extra voltage? Right now im getting 448v and 446v at the power tubes measuing from pin 3(plate) to ground. Am I putting the amp at risk with this component swap out? Does anyone have any experience with this or has anyone tried it? Any feedback would be greatly appreciated.
Thanks,
Abe.

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Post subject: Re: Bandmaster questions
Posted: Fri Mar 30, 2012 1:35 am
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IIRC, the power tranny never changed through the various permutations of the BMR -- from the initial AA768 revision up through the AA270 variant. It was the same Fender P/N regardless of the year. Thus, a rectifier swap to a GZ34 isn't likely to harm it. The 5U4GB required some 30% more heater current and this was accomplished by tying each leg of the heater tap through a 100K resistor to ground. These should be removed for more efficient operation when using a GZ34 -- a relatively simple mod should you wish to try it.

Arjay

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Post subject: Re: Bandmaster questions
Posted: Fri Mar 30, 2012 9:20 am
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Retroverbial wrote:
IIRC, the power tranny never changed through the various permutations of the BMR -- from the initial AA768 revision up through the AA270 variant. It was the same Fender P/N regardless of the year. Thus, a rectifier swap to a GZ34 isn't likely to harm it. The 5U4GB required some 30% more heater current and this was accomplished by tying each leg of the heater tap through a 100K resistor to ground. These should be removed for more efficient operation when using a GZ34 -- a relatively simple mod should you wish to try it.

Arjay


Excellent observation Arjay, thanks for pointing that out to me. Compared to all the other mods, that one seems like the simplest. I will report back my findings should I elect to blackface the amp. Im still on the fence about that one.

Thanks again to all for your contributions.
Abe

p.s.
Three prong conversion to updated schematic, check.
And no smoke.... :D

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Post subject: Re: Bandmaster questions
Posted: Fri Mar 30, 2012 11:43 pm
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Would be interesting to compare your Bandmaster Reverb blackfaced to a BF Bandmaster, since yours has the tube rectifier whereas mine has the SS one -- guessing it would sound more like a Tremolux?

I had my death cap dealt with the way Arjay did as well on both the Bandmaster and Showman.

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Post subject: Re: Bandmaster questions
Posted: Sat Mar 31, 2012 2:11 am
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:lol: :lol: Socal..."no smoke" is always a good thing when dealing with these amps!


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