JTM45/5F6-A pre amp into a '68 Bassman

[C Moore]

Is there any electrical/electronics reason you could not replace the "bass channel" of a SFB head with pre amp components similar to a 5F6-A/JTM-45 normal channel?
Thank You

[stubbyfex]

Hello!
It would be a pretty involved mod. The main problem I see when I compare the two circuits is the way the tone controls are implemented. Mojo Musical Supply carries the eyelet board for the 5F6. If I were doing it, I would get the new eyelet board, and remove the existing one and perhaps keep it in case I didn't like the new amp. Then you'll most likely have to remove a few pots from the second channel that won't be used.
Here is a link to the fiberboard.

http://www.mojomusicalsupply.com/item.a ... id=7126270

[Sonny ReVerb]

There are a couple designs here:

http://www.thegearpage.net/board/showth ... p?t=459071

[C Moore]

I took the old eyelet board out of the Bassman head, and bought a turret board set up for the AA864 BF circuit. My thinking was to make the "bass channel" similar to the 5F6-A normal channel, and keep the "normal channel" of the Bassman the stock AA864 circuit. I can remove one of the input jacks from the "bass channel" and use that hole for the Midrange pot of the 5F6-A normal channel.
So what I am proposing is this. 1968 Bassman head that is AB165. Make the first channel similar to the normal channel of 5F6-A. Make the second channel similar to AA864 second channel. The preamp would consist of 1x12ay7, 2x12ax7, 1x12at7. Would use 2x6L6 power tubes. My thinking was that it is two separate preamps that use a similar B+ supply. They use separate tone controls and are fed to a common phase inverter and then to the common 6L6 power section.
I am not concerned about having an exact clone of the first channel 5F6-A, just something similar. Something that would be much better for guitar than the stock, first channel of a Bassman head.
Hope I am able to get my point across, tried to be as precise as my knowledge permits.

[swarty]

Not tough at all if you use both channels. You have just enough pots and inputs and one extra tube.

[C Moore]

Not tough at all if you use both channels. You have just enough pots and inputs and one extra tube.

Hey Thanks,
The only trouble I have encountered so far is the AA864 turret board. I ran out/will run out of room for components. Looks like I can add one, maybe two terminal strips and I will be OK. Guess the problem is that the preamp side of the turret board has a lot of empty/wasted realestate. Think I can squeeze a 6 lug strip on that end if I turn it sideways.
Thanks Again

[aclempoppi]

Hi there Very interesting, I've been tinkering with a '68 Bassman myself, for guitar use. The amp came my way as a grossly modded head.Since then, I've put it in a custom 2X10 cab, setup the basic bias circuit(no balance), addressed the bogus(hi gain) mods,recapped the power filters, recapped the tone and coupling caps on the board with oil in foil to my tastes, replaced a few resistors, nos pre amp tubes :(12AY7,5751,12AX7,12AT7), Mercury PT,OT,choke, and few other minor tweaks. It's gettin' there. Any way, I'm curious as to how your project progresses and turns out. Good Luck! Art

[C Moore]

I am getting confused at the Phase Inverter. Both channels of the 5F6-A seem to enter the grid of the PI from the cathode of the previous preamp tube via the Treble Pot.
Both channels of the Bassman AA864 enter the grid of the PI from the plate of the previous preamp tube.
Sense I am trying to use one channel from each amp does this present a problem? Can I tie both channels to the same PI grid? One channel from the treble pot/cathode, and one channel from the plate of the preamp.
Thanks

[solderstain]

Sense I am trying to use one channel from each amp does this present a problem? Can I tie both channels to the same PI grid? One channel from the treble pot/cathode, and one channel from the plate of the preamp.
Thanks

I've never tried your specific build, but I've done all kinds of weird Fender-inspired mix-n-match builds over the years.

You can definitely mix two different preamp styles at the PI. Even stock Fender reverb/trem amps mix different preamp 'styles' at the PI grid.

Here's my $.02 worth:

Like any other place where two channels are passively mixed, you need mix resistors on each channel prior to connecting them through the PI input coupling cap. The mix resistors isolate each channel from the other. With your build, you may need to experiment with the value of the mix resistor for each channel. Look at an AB763 schemo of your choice - you'll see what I'm talking about. Since this is a Fender-style tweak, start with the 220k value for mix resistors and adjust slightly up or down. And I do mean slightly...

Good luck!

[C Moore]

solderstain -
Thanks for the help. Just talked to a friend/amp tech and he gave me very much the same advice. I will use the 220k resistors and a 500pf coupling cap. This is the biggest project I have taken on so far, and all of you guys have been a great help. I will have to post some gut shots and take my punches so to speak. It is only fair that the Master get his chance to critique the Apprentice. Will get some pictures up in the near future.
Thanks Again

[C Moore]

So I finally got this guy together. There is a real bad, high pitch, oscillation. This occurs with no instrument plugged into the amp. I have checked the wiring layout twice, and think I have everything in the right place. But I will check it again. I am thinking I have a ground and/or lead dress problem. I really do not know. If you see anything obvious in my pictures....
Thank You

[IMG:800:600]http://i111.photobucket.com/albums/n143 ... M45001.jpg[/img]

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[ampdoc1]

I haven't looked at the schematics of the preamps you put in, but do they both have the same number of gain stages? If not, you may be getting positive feedback. If the gain stages differ, lift one channel at the mix point, and the feedback should disappear.

ampdoc1

[solderstain]

Based on the sypmptoms you described, I'd look at the whether the blue and brown wires for the primary of the OT are backwards. I did that by mistake once and got EXACTLY what you're hearing.

One thing to keep in mind also is that the SF Bassman (AB165) has a 'backwards' wired OT secondary too. If you're trying to rewire the weirdo AB165 power section to the much-better AA864 design, the NFB, the primary AND the secondary of the OT all need to be altered so everything's in phase.

Don't worry about the differing number of gain stages guess - any number of Fender amps have different numbers of gain stages between the channels.

[C Moore]

Based on the sypmptoms you described, I'd look at the whether the blue and brown wires for the primary of the OT are backwards. I did that by mistake once and got EXACTLY what you're hearing.

Do you honestly believe, for one second, that switching two little wires on the OT could possibly........
.......Do you take out of state checks?

Unbelievable, always try the easy stuff first. I am sure to be back with problems more difficult. At least now I can plug in and test this Frankenstein.
Thank You, Again

[ampdoc1]

Solderstain is correct, but most Fender amps don't share an input as this appears to do. If the stages are both being fed by a common input and one is in phase with the input, it's possible it could be feeding back thru the other stage.

ampdoc1