Dumble mods for '65 Bandmaster

[sonicmojo]

I recently picked up an AB763 '65 Bandmaster. It had a hole in the chassis and no head cab (termites) so I patched it up as well as possible and built a cabinet for it. I'm now thinking of modding it.

I'd love to try an Ultraphonix-flavored mod on it. I won't ask for that information directly since I've read all the posts but here are my thoughts based on what I understand may get me towards it:

I'll do most of the tonestack mods on just the Normal channel and make it a Bluesmaster tonestack. I'm definitely looking to get the amp to open way up from standard Blackface on that side.

Tie the Normal channel into the tremolo effect.

Use Bluesmaster values for the PI also and tune it for a 12AX7 instead of 12AT7.

It will be a fixed mid EQ and I'll set it up for hard coded "Rock" and "Mid-Boost", leave in the Bright switch and not install any extra switches. Does anyone have any suggestions on what values I should use on the mid EQ? I don't want to go too crazy with things but if I did some sort of a Preamp boost, what method do you think would work best?

I plan to leave the Vibrato channel stock although I'm sure the PI change will make it sound different as well.

Should I consider using EL34s instead of 6L6s?

Thanks for any tips and suggestions I would be happy to document my progress and any clips or pictures as I go.

[Guy77]

As far as tube types goes....I tried EL34, 6L6 and 5881's in my Overdrive #124 build and i preferred the rich smooth sound I got from the 5881 tubes. Overdrive channel was nice with all 3 tube types but the EL34's had more sustain happening there than the other 2 tube types.

My 2 cents worth.
Cheers

Guy
Keep the iron hot!

[sonicmojo]

Thanks for the opinion on tubes. I do like the sound of EL34s in my Bluesmaster. I plan to wire this mod to accept EL34s. I've got some parts ordered but I'm now debating using a beat up looking '66 Bassman instead. I think it has stock values but there are some Orange Drops in it and it sounds bigger than other similar Bassmans that I have access to from the local shop that I work with. The owner is really cool when I want to play with something. Kid in a candy store....

[Smokebreak]

I'm working on a clean channel of an amp that's basically set up like a non-verb old Fender, like a Bandmaster. The other channels are high gain stuff, so the PI is Marshall, EL34 outputs. I'm sure you see where I'm going with this

I'm using a decidedly more Fender stack, but that's what I need for this application. Nonetheless, lookout for all the bass, if you are looking to get some great cranked tones, especially if you are using the .022 PI entrance like a BM(Marshall). I used a .01 coming off the 2nd plate, ignored what would be the vibrato pot loading, and split with 470Ks before the PI. The cleans are beautiful, and when cranked it gets gnarly, but doesn't fall apart. 470K split is slightly less bright than 220K. I'm sure you can tell by this point that I'm referencing a Fender clean preamp, not Dumble though a BM clean as drawn sounds great too.

Whatever pot you were going to use for mids, cut it in half for the fixed value. Boost can be anything from 10K - 22M from mid to ground, depending on what you like. I think 68K is probably common? My boost here is dual function, adding to the treble cap, and adding 10K to my fixed 6.8K mid resistor. So mine is just kinda like turning a mid control up a bit, as opposed to a massive lift.

[sonicmojo]

Whatever pot you were going to use for mids, cut it in half for the fixed value. Boost can be anything from 10K - 22M from mid to ground, depending on what you like. I think 68K is probably common? My boost here is dual function, adding to the treble cap, and adding 10K to my fixed 6.8K mid resistor. So mine is just kinda like turning a mid control up a bit, as opposed to a massive lift.

Thanks for the tips. My parts arrived. What I'd like to do is document the changes as I go and the effect that it has, so I will try to keep track so someone else may get something out of this. Really need to get my computer based-oscilloscope going......

Here is the amp to be modified:

AB763 stock circuit to start - just some replaced electrolytics and my one purple wire to make tremolo available to both channels. I knew the OT had been replaced and upon further inspection it appears to be a Bassman OT which, as I understand, was one of the things that HAD changed if starting from a Bandmaster, so that is actually a bonus!

I need to spend a little time designing layout as I realize that V1 and V2 on the standard Fender BF amp share Cathode resistor/caps on pin 8. I want to keep the Vibrato channel as stock as possible, but I don't want to punch any new holes in the board to set it up differently so I'll have to think about that a little bit......maybe use some terminal strips.

[img652]http://www.sonicmojo.com/d-amps/beforefront.jpg[/img]
[img612]http://www.sonicmojo.com/d-amps/beforeback.jpg[/img]
[img532]http://www.sonicmojo.com/d-amps/bmguts.jpg[/img]
[img960]http://www.sonicmojo.com/d-amps/bmot.JPG[/img]

[sonicmojo]

I did a few things tonight, trying to not drill any new holes or otherwise doing anything irreversible....

Changed the PI output cap values to .022 ala Bluesmaster.
Divorced the shared Caps/Resistors on V1 pin 8 from V1 to V2, added a terminal strip to handle the extra wiring on V1 and changed up the cap/resistor values for experimentation. Going with tone cap values from 183 and resistor values from a Bluesmaster in the V1 area for the most part.
Replaced the 6K8 resistor to ground on the Bass pot with a 1K5 and moved the "mid" wire from that lug to a 100K trimmer to set the mid value. I set it at about 82K for now.
Changed the value of the resistors on the output tube sockets to 1K 5W/3k3 1W and rewired with a terminal strip to better accommodate EL34s.

I can tell there is some extra dynamics and feedback going on with the modified Normal channel compared to the Vibrato channel but I plan to crank it up during the day, record some comparison clips of the two channels, and tweak this some more....

[img960]http://www.sonicmojo.com/d-amps/pi.jpg[/img]
[img960]http://www.sonicmojo.com/d-amps/v1.jpg[/img]
[img521]http://www.sonicmojo.com/d-amps/midmod.jpg[/img]
[img960]http://www.sonicmojo.com/d-amps/el34mod.jpg[/img]

[sonicmojo]

I do like where I am heading with this. I had a little time to crank it up today. It is noticeably different on the modified Normal channel at similar volume/tone settings. The mids are boosted but not over the top. I'm a little unsure of the effect my method of implementation is having on the Bass control so I may rework that.

Tonight I experimented with bumping up the negative feedback resistor to give it a little more hair and I settled on a 1.5K resistor. I may go back to 820R if I feel it is over the top once I crank it up again. I'll play around with some different tubes and bias settings.

[M Fowler]

Bill W. noted in a post he likes these changes:

Use 470K mix resistors in stead of 220K, adds just a bit more HF rolloff which I feel helps. One of my favorite configurations these days is a 25W 2x6V6 with a BM PI and a Classic tone stack (.05 mid cap, 100K slope) and a 300pF or 250pF treble cap.

Unquote: Did you try a different slope resistor value? I changed the slope resistor to 150K on a Bassman 50 4x10 amp and that was what sealed the deal for one happy customer. Also, is that picture old or did you change the PI tail resistor as well?

Mark

P.S. love the looks of your bare wood cabinet.

[sonicmojo]

Did you try a different slope resistor value? I changed the slope resistor to 150K on a Bassman 50 4x10 amp and that was what sealed the deal for one happy customer. Also, is that picture old or did you change the PI tail resistor as well?

I used an entrance just like #183 because when I had a clone at one point, I remember it being more aggressive and I was looking for that a little on this so my T/M/B is .002/.01/.1 with 150K for the slope.

The picture is old now but I did not change the 100R NFB tail resistor and after just now reading up on that a little, I understand it is more about the ratio on the NFB. I suppose once you hit a certain ratio, it is as if you have disconnected NFB altogether. That explains why there was only a very small range of difference over about 3K resistance so I settled on 1.5K to kind of be in the middle. This was at low volumes so I may revisit once I crank it up again.

I will have to try the 470K/470K PI bias split if I feel it is getting a little bright.

I built the cabinet for tolex but I liked it so much without that I stained and left it!

[sonicmojo]

I realized I had my mid and bass caps reversed. Fender and Dumble layouts are usually the opposite for these caps. Something to watch out for....

Also, since I had separated the shared Cathodes circuit between v1 and V2, I changed the Cathode resistor going to V2- Pin8 from 820R up to 1.5K to keep my Vibrato channel as stock sounding as possible. I was reading that the reason it is 820R in the first place in the stock AB763 circuit is because two separate Cathodes sharing that resistor need to be that way to bias similar to if it was not shared. I got that little tidbit from a book I picked up called "How to Hot Rod your Fender amp" which I recommend to anyone who wants to modify Fender amps.

I put in a new set of JJ 6L6GC tubes and everything is sounding pretty good. I may add a PI trimmer.

[img960]http://www.sonicmojo.com/d-amps/v1-2.JPG[/img]

[sonicmojo]

I decided I had to have an effect loop so I drilled two new holes in the back (yikes).

I only had a few minutes in between a sick baby in and out of the house today but I turned it up (to 4) and recorded something very quickly.

This quick and dirty clip is with new JJ6L6GC tubes, about 65% dissipation tube bias, a Dumbleator in the effect loop, and a light OD/Boost pedal in front. Amp volume on 4 into an 8 Ohm 1x12 open EV12L cab. Fender Stratosonic guitar with Lollar P90s. The solo part is on the bridge except for the end where I switch off the OD and move to neck pickup. It got a little loud at that point since my boost was a little high and it is prior to the OD which was a little lower.

http://soundcloud.com/bryan-suthard/bm-test2/s-knTiu

Also, one thing I've noticed going through this is that the stock coupling caps for AB763 are .047 on the Normal Channel and .1 on the Vibrato. I am sharing the stock Vibrato cap for both channels now as I tied into Tremolo for both sides so the .1 will allow a little more bass response to go to power section for me. I may revisit if I feel it is too bass heavy but for now with my tone stack, it doesn't seem to be.

In most cases, Dumble used lower values coming out of the clean channel, usually around .05 and in the case of the BM, he used even smaller - .02, presumably to tame the bass a little coming out of the preamp.

If anyone has seen what coupling cap values were used in the Ultraphonix, I'm curious. I'd have to guess either .05 or .02. Seems like that would fit with ODS and/or BM topology.

Just my observations....