Bandmaster mods...

[Panther35]

I've got a dripedge 1968 Bandmaster and it has the wrong output transformer in it. The transformer in it works fine, but it gives it a bit too much clean headroom for my taste. Here are the following things I'd like to do to the amp, let me know if it's feasible... or crazy:

1) Install a Deluxe Reverb OT and 6V6 power tubes in place of the 6L6's

2) Install a 5Y3GT rectifier tube by adding a 5v transformer somewhere on the chassis

I'm going for more tube sag (I love a compressed sound) and a bit more crunch at a lower volume. I know I'll need to rebias for the 6V6's. I realize I could just get an old Deluxe Reverb, but they have just gotten too pricey for me. I've got a single 12" Jensen (Special Design, 8ohm) already mounted in an openback cabinet, so I was going to run the modified Bandmaster thru that.

Any input, tips, or dire warnings would be greatly appreciated. I don't want to burn up my amp, but I'd like to experiment with the tone a bit. I've done plenty of work on tube amps before and I'm aware of all the precautions, but I've never done any mods to this extent.

Am I barking up the wrong tree? Or am I on the right track?

[Gibsonman63]

I wouldn't be putting any new holes in that amp chassis. The Silverface amps are going up in value as the supply of blackface amps dwindles and the dripedge amps were only made for a short period. Back in the day, a lot of guys changed the OT for the Bassman OT to eek out a bit more power or at least a little more clean headroom.

In my experience, the old Fenders sound better when you get them cranked up to 6 or 7 on your volume. Going from 6L6s to 6V6 will not get you half the volume, it will be more like 2-3 dB less, which will not be a huge difference for the work involved.

Some suggestions:
Try a closed back cabinet. The bandmasters came with 2x12 closed back cabinets. Being closed, it will dampen the speakers a bit, plus only project forward. Less efficient speakers.

The tone controls are subtractive. Try starting with all the tone controls on ten and dial back as needed.

Both channels are in phase, so you can jumper them together. That gives you lots of things to play with and you get both preamps feeding the phase inverter.

Try some power tubes that break up a little earlier.

I like the Boss pedals for Fenders. Try an OD-1 dialed back. The SD-1 works well for leads.

[Panther35]

Another option?

Try some power tubes that break up a little earlier.

Thanks for the response Gibsonman.

That brings up another option I had considered. I understand that I can run 2 6V6's in place of the 6L6's if I get JJ's which apparently can handle the voltage. I may just try that option first (and rebias of course). If I'm not mistaken, that'll take my current output of 8ohms to the speakers down to 4ohms? Or is it the other way around? The oddball OT in there right now only provides 8ohms, which worked out fine for the single 12" speaker, but it's just a bit too powerful (50ish watts). I'm going for a lower volume amp I can use at home or in the studio... won't be used for live shows really. That's why I'm looking for the breakup a bit earlier on the volume control... I don't want to blast my neighbors out.

[Gibsonman63]

I am not really sure what that will do to your output impedance. I have built an amp that will run either EL34's or 6V6s and I am not aware of any output impedance issues.

Another couple of thoughts, if you need a more dramatic decrease in volume, you could use an attenuator between the amp and the speaker.

If you are ambitious, you can pull out the ground switch, order a VVR from Dana Hall and mount it there. If you haven't already, you really should have a three-prong cord installed for power to give you a solid chassis ground for safety. Once you do that, the ground switch can be removed and you have a nice mounting hole in about the right place. If you are not familiar, the VVR (variable voltage regulator) works on the secondary side of the power transformer. The effect is similar to using a variac, but the advantage is the heater voltages are not reduced as you turn down B+. This should allow you to lower the B+ enough to use any 6V6 you like. Google Hall Electronics.

I installed it in a cathode biased amp and really like it. I will be installing one in one my adjustable fixed bias amps in the next couple of weeks.

[Panther35]

My next question is this: will anything be at risk if I put an OT for a Deluxe Reverb in my Bandmaster (assuming I'm running JJ's 6V6 tubes)? I'm looking at the fact that the CT for the normal Bandmaster OT is about 4000 ohms and the Deluxe Reverb OT is about 6600 ohms (even 7000 ohms if I were to get the beefier DR output transformer that Allen sells, Heyboar TO26... which I'm considering).

[hasserl]

Look at the dimensions of the mounts for the different trans, the DR type trans is smaller than the Bandmaster, meaning you'll have to drill the chassis to mount it. Not really a desirable thing to do! And it really won't get you what you're looking for. You could go with the JJ's 6V6's that will help you get more breakup at a lower volume, but it's still going to be loud. (about your question on the impedance difference, to keep the impedance ratio correct with a change to 6V6's you would want to use a 16ohm speaker in place of the 8 ohm; but it won't hurt anything to run the 8 ohm speaker, so you don't have to change it) There are a lot of ways to get more break up at lower volumes, you could go with the VVR mentioned above or some other sort of power scaling; or you could add a master volume (I really like post PI master volumes in Fender amps, they work great IMO) or you could simply use an attenuator. There are lot's of attenuators out there, lot's of choices to fit every budget. IME the simple resistive attenuators tend to be very transparent and provide a nice stable load for the amp. Weber has a 50 watt Load Dump attenuator that works just fine for this. You could spend a whole lot more on much fancier attenuators with more options if you wished, but the simple Load Dump works fine. That way you aren't hacking up a nice old Fender, and you can use the attenuator on other amps that you may pick up in the future.

[olddawg]

Just buy an Epiphone VJ head or something like it for around $100. It will break up nicely atlow volume, take pedals well, you can easily modify the crap out of it if you want, it will be less expensive than what you plan to do, and you won't have to butcher a fine old amp. I run a VJ through my 63 Tremolux cabs all of the time for the same reasons you mentioned in your posts.

[Bob M.]

Before switching out transformers or adding aux heater transformers, try some easier stuff first like triode mode, cathode bias and/or sag resistors (ala Kevin O'Connor). There are lots of potential solutions without resorting to the expense of new transformers. I'm sure you can get this amp to sound how you'd like with less radical, and less expensive, measures.

Bob M.