Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: panhead on April 16, 2012, 11:55:53 am
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Hi,
I'm refreshing a 1972 Bandmaster Reverb head for a friend of mine. So far I've measured all the voltages against the schematic, rebalanced the bias for the two 6L6GC's, and replaced a couple of microphonic preamp tubes. All in all, the amp checks out pretty well. I ordered a matched pair of 6L6GC's anyway, because the ones in there are from two different manufacturers and of questionable age (even though I was able to set bias for 47V for each - schematic says 44V).
This amp has a surprisingly gritty tone for what I would have expected for a Silverface. Is that normal for this amp?
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This amp has a surprisingly gritty tone for what I would have expected for a Silverface. Is that normal for this amp?
Yes it is... I did one 2 months ago where it was too dirty for the customer. I replaced the 5U4 with a solid state plug-in type rectifier and blackfaced the Phase Inverter and bias circuit and rebiased the tubes. That brought it to his liking. Mainly I blame that 5U4 for it's saggy gritty tone. IMO it's not a good saggy grit like a Tweed, rather it's a nasty one.
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It is a harsher grit than anything Tweed I've ever heard, but the customer likes it.
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It is a harsher grit than anything Tweed I've ever heard, but the customer likes it.
If you Chang the small pF ceramic disk tone caps and the 500pF reverb cap to silver mica (to same value) it will help get rid of some of the grit and the high end will have more bell like tone. But if he likes it..... :don't know:
What are the PI plate R's, 47K, instead of 82K & 100K? What are the PI grid R's values? You could try setting the PI R's back to AB763 values.
One more thing would be to try different type of coupling cap from the PI to the output tubes. OD pp 715's, Malory 151 polyester, or the small sized MPP's their a very clean sounding cap yet warm. Doug's got all of them.
Does this amp have the dreaded grid bleed caps on the 6L6's?
Brad :icon_biggrin:
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Haven't looked for the bleed caps. The layout doesn't exactly match the schematic I have. In fact, the bias circuit looks like a cross between a Vibrolux and a Twin, with a Bandmaster bias board.
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I had a SF Bandmaster Reverb in the '70s. With a 335 I could get very close to the CCR sound. Plenty gritty. I never thought of it as harsh though. I think the grit was largely due to the small OT.
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I forgot to add that I cut those bleeder caps off the power tube sockets. This player tours the nation as a Johnny Cash tribute artist and needed more headroom and fatter tone. He's got no complaints now.
j.
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Isnt the Super reverb and Bandmaster reverb the same amp? The power transformer secondary voltage only differs by 10-15 VDC
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Isnt the Super reverb and Bandmaster reverb the same amp?
Same family, same basic schemo, different iron.
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As I mentioned before, all the voltages checked out and it sounded fine. I fired it up again and got a thin distorted sound, with one of the 6L6GC's redplating. I shut it down and swapped the two power tubes. This time the same tube redplated in the other socket. I shut it down again. Then I fired it up to measure the bias. No redplating. I adjusted the bias balance to 46v for both tubes, and it has been fine. I am replacing the two power tubes. Not sure why the redplating happened, but I'll be getting back into it when I install and bias the new power tubes.
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As I mentioned before, all the voltages checked out and it sounded fine. I fired it up again and got a thin distorted sound, with one of the 6L6GC's redplating. I shut it down and swapped the two power tubes. This time the same tube redplated in the other socket. I shut it down again. Then I fired it up to measure the bias. No redplating. I adjusted the bias balance to 46v for both tubes, and it has been fine. I am replacing the two power tubes. Not sure why the redplating happened, but I'll be getting back into it when I install and bias the new power tubes.
I have had the redplating happen twice in the past 2 years. One was a Bassman and the other a Twin reverb last week. In both cases's i saw a tiny flash of a spark at the bias pot. The bias pots were just plain wore out and needed replacing. The Bassman's pot read good on my meter with no dead spots. But that spark had to mean something was going on inside the pot just the same. The pot wires were soldered good when this happened
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Thanks, that's interesting about the bias pot. Installed two new JJ 6l6GC's this morning. As soon as I hit the standby I got a squeal and redplating. Shut it down, turned the volumes down, measured the bias voltage, turned the standby back on and brought up the volume. No problems. I may end up replacing the bias pot if I can't find anthing else. I chopsticked the hell out of it and got no glitches or instabilities.