Reducing plate voltage on 5G5 Brownface Fender Pro Amp?

Marc Stone · From: Louisiana, USA

Just scored a lovely center vol 1960 5G5 Pro for a song. Cool amp but some harshness and doinky upper mids that hurt my brain. My tech said that as far as he can tell, the 45217 output transformer (late tweed era and early brown era triad black open frame tranny used mostly on the smaller mid sized amps ala Bandmaster) is a bit too small to work properly with it's larger, bassman sized power tranny. Currently has Tung Sol 5881s but looks happier on the scope w 6L6 GCs. Tech says the GCs will prob sound better but stand a good chance of eating that output tranny.
He suggested 2 fixes - 1. larger OT, which will push the wattage up higher than I probably want to go with this amp, and endanger the speaker.
2. use a zener diode to reduce the plate voltage (he said around it's around 450 and the diode would bring it down to high 300s) reducing the wattage slightly as well as reducing strain on the OT, and probably warming the amp up a bit.
I'm partial, in theory, to option 2. There are already some circuit mods that have been done and it's a recovered amp w a cpl non-orig parts incl speaker (68 C15N) and probably the OT (despite being correct for the 5G5), so I have no qualms about further mods if it makes the amp more usable. I find the potential for a little sag and sweetness from lower output preferable to a harder, cleaner sound from this amp. But I don't know if there are any potentially negative consequences from that approach.
One concern I have is that I read that the harmonic trem likes high plate voltages. Is that true? The trem is weak as is and my tech said he will most likely have to replace most of the resistors in the trem circuit. I definitely wouldn't want to sacrifice the trem if the lower plate voltages would impair it's operation.
Thoughts, ideas, info all appreciated! Thanks!

Donny Hinson · From: Glen Burnie, Md. U.S.A.

If you're looking for a 100v drop, I'd just add a big choke or change out the power tranny. Other solutions, like a zener or dropping resistor, are going to be creating a lot of heat, especially if you're going to be running it in distortion all the time.

Ken Fox · From: Nashville GA USA

45217 output transformer is actually a 4 ohm output transformer. I would opt for a correct 125A7A or have Mercury Magnetics build one.

The forward drop of a single diode is only 0.7 volts. Perhaps he is talking about a large Zener Diode?? If it were mine I would not mess with the plate voltage until you have a correct output transformer.

Both the 6G5 and 6G5A amps used the same 125A7A output transformer. The 6G5 used 5881 tubes and the 6G5A used 6L6GC tubes.
Again, if it were mine I would put it back to the stock schematic and then evaluate the amp

It appears there are no schematics for the 5G5 amp, but some research indicates they did come with the 45217 transformer mentioned! A rare amp for sure.

From the net:
This is the short-lived, 5 preamp-tube circuit that immediately followed the late, narrow-panel tweed version and preceded the darker, chocolate brown tolex one. While these other designs were in production for 2-3 years each, this early brown circuit was only in production for about 10 months making it among the most short-lived and uncommon Fender production amps"

Good link on the amp here:

http://drewsgeezeramps.com/Pro-5g5-1960.html

"

Bruce Derr · From: Lee, New Hampshire, USA

Hmm, I'm curious how he would add a diode to reduce plate voltage by 60 volts. Unless he means replacing the silicon diode rectifiers with a tube diode (rectifier tube)? If so you might consider just having him add a "sag resistor" between the silicon diodes and the first filter cap. Also if you are trying to get away from a cleaner, firmer sound you might consider sticking with 5881s rather than 6L6GCs. Maybe a different speaker would help tame those upper mids, an easy thing to experiment with. Good luck, sounds like a sweet amp.

Paul Arntson · From: Washington, USA

I had a Bandmaster from about that time frame and I had the same issues with tone. Rather than devalue the amp with mods, I sold it for a profit and moved on to another amp.
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Excel D10 8&4, Supro 8, Regal resonator, Peavey Powerslide, homemade lap 12(a work in progress)

Marc Stone · From: Louisiana, USA

Thx for the info so far everyone. I would really like to keep this amp. If I can dial out the aggressively doinky upper mids, keep the wattage at 40 or lower, and not destroy what appears to be a rather pricey OT, that would be my preference.
If someone offers me enough money to say "hell with it, next!" I might consider that but I loved my brown Concert, just too much amp to lug around and too much power for a lot of situations. That's why the idea of getting a tweedier situation w lower wattage and plate voltage is appealing to me. And I'm not sure that my tech wants to take it down 60v. Just spoke w him he said it might make sense to get it down to about 420. He's a who has spent a lifetime working the "bring it back to orig spec" concept, but is open to doing mods. He builds his own amps for his owning gigging/recording use, modeled mostly after a tweed super, and they are stupendous. He's worked on all my vintage amps (the ones I gig and record with all the time) and anything leaves his hands to mine always sounds great and works well.
I wonder if this amp (from a highly experimental time in Fender's development), which was redesigned after only a few months w the larger OT, wouldn't benefit from a little circuit massage. the basic tone is lovely, just want to shape it a little to tame that upper mid spike. My 62 Concert did not have that spike. I believe it had the 125A7A OT and a similar PT to the Pro, but going that route leaves me with a surplus of power.
BTW A/Bed w a different speaker and the upper mid doink was still very much there.

Howard Steinberg · From: St. Petersburg, Florida , USA

I have a vintage 1962 bandmaster with the same issue. It currently has nos Sylvania 5881's. It's also been set up with 6L6GS's and KT88's. I've had this amp since it was new. It was never a great sounding amp. Once I was able to discern this issue, I moved onto other amps but kept it for whatever reason. I will occasionally take it on a gig where I'm playing a 6 string. I find that the use of an overdrive pedal (fulltone fulldrive in my case) fattens the sound and makes the amp very usable. Another interesting thing is that I lose the edgy mids and highs when the vibrato is on.
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Justice Pro Lite (4-5), Justice D-10 (8-5)x2 , Quilter Steelaire, Hilton Pedal, BJ's bar.

Matthew Dawson · From: Portland Oregon, USA

Hmmmm, I noticed the same "honking" upper-mids with the Brown-Era Vibrolux I got to try out recently with steel. The sound was a little shrill which I attributed to how the amp matched with the Altec speaker that was in it. The amp used 6L6GCs per the tube diagram and had the 125A7A.

Howard Steinberg · From: St. Petersburg, Florida , USA

I had Altecs in my Bandmaster for several years. They actually made this issue more apparent. I played steel through it, at a rehearsal, not too long ago. Not a good combination. The amp currently has 2 15 watt Eminence alnicos which didn't help with steel. I wish that I had purchased a bassman 54 years ago.
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Justice Pro Lite (4-5), Justice D-10 (8-5)x2 , Quilter Steelaire, Hilton Pedal, BJ's bar.

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