'74 twin reverb blackface mod

[Trojanhead]

Hello, I purchased a 1974 twin reverb and am in the process of converting to AB 763 circuit. Any advise and or help would be appriciated. I have disconnected the push/pull master volume and changed values on several resistors, replaced all electrolytic caps and cleaned all pots. My problem now is that I have a very weak volume out of the amp even with all controls maxed. Where should I start looking at to run down this issue?

[Jazz P Bass]

Did the amplifier function before you "modded" it?
If so, then you must go back & check what you may have done wrong.

[pdf64]

One method would be to undo all the changes that you've just made, assuming that it worked ok before; but that's probably not acceptable.
Another would be using http://support.fender.com/schematics..._schematic.pdf as the reference, note all the voltages in the rectangular boxes for the same test points in your amp. The voltage levels in a 74 TR are likely to be higher, maybe 500V at TP27, so use whatever it is as a multiplier on all the other voltages, eg if it is 500v, then multiply the other TP Vdc by 1.13 (=500/442).
If there's much discrepency in the comparitive Vdc of the various test points, then that's where the problem may be. Pete.

[Ronsonic]

Where should I start looking at to run down this issue?

Sorry, but the best answer, pretty obviously, is check your work. I'd be especially curious about the MV replacement.

[überfuzz]

Where should I start looking at to run down this issue?

Number one bet: Your local tech.

Why don't you start by posting a list on all the tinkering you've done so far. That way someone on this forum might get an idea of what you've actually done to the amp.

[Tom Phillips]

Troy,
Without knowing your level of expertise I suggest the following.
Print a copy of the Twin Reverb AB-763 Wiring Layout. If you don't already have a copy it is available at http://www.webphix.com/schematic%20h...b763_schem.pdf (see page 2 of 2).
Then trace out the wiring currently in your amp and mark it off on the layout drawing with a highlighter looking for differences. You will need to use the schematic for wiring that feeds through to the other side of the chassis and there will be some physical differences.

Additional comments:
1) There are so many variations in the layouts and component values that it is difficult to compile a list of exact steps to convert a "Silverface" circuit to a "Blackface" circuit.
2) There are features of the Silverface that are improvements over the Blackface such as improved grounding, bias balance (Although a bias level control is still needed) and hum balance, shielded wiring etc. Some of these are theoretical improvements that were ruined by bad workmanship and sloppy wiring dress. (My rant)
3) We don't know what was done to the amp since it left the Fender factory and you started you blackfacing project. Hence the question above Re: Did it ever work properly since you have owned the amp?

Regards,
Tom

[Tom Phillips]

...I have disconnected the push/pull master volume .... My problem now is that I have a very weak volume out of the amp even with all controls maxed. Where should I start looking at to run down this issue?

Sorry I didn't address this in my first reply. When you disconnect the MV you then need to re-connect the resulting open circuit. My advise is to follow the information in my first reply with emphasis on the circuitry between the pre-amp and the phase inverter.

Regards,
Tom

[Trojanhead]

Thanks Tom, The amp did work but the MV push/pull feature did litterally nothing but the volume control did adjust the volume. Also, the intensity control had to be turned up or there was no sound? The amp has a bias adjust pot and there is no bias balance or hum balance on the rear panel. I have checked voltages and they are present and the bias reads -51 V. I have used a jumper wire to connect between points where the MV was but the sound is weak. I have a layout, I think my best move now is to start at the first preamp tube and work toward the phase inverter tube and rework as neccesary.

[überfuzz]

If you go about this in this way you might hit the issue quite a long way down the road. So be patient, and you'll learn heaps of stuff as you go along. The best tip you'll ever get is, triple check everything.

Oh, one thing. You probably have some issues in the tremolo circuit. If it's not due to bad wiring, it's often easier to just replace most of it in one blow... Rather than pin pointing a single bad component.

[Bruce / Mission Amps]

Hmmmm.... First, lift the two wires closest to the pots off the tremolo bug on the eyelet board and see if it's cell is shorted... also, make sure you plug the speaker into the shorting jack, the one with the three lugs.

[mac1amps]

"BF"ing a Twin as late as '74 can run into problems with it's 135 Watt power supply (also with a different supply configuration), as opposed to the 80 Watt power supply found on BF Twins. I've "BF"ed many-a-Fender over the years (I've been a pro-tech since '73), but after attempting to perform it on a later, 135 Watt Twin, I ran into various issues that required "adjustments" from the BF era design. In the end, it was questionable as to whether it was worth it. Fortunately [for me], my client was happy, but did it sound ANYthing like a BF Twin?.......Not to me!

[Jazz P Bass]

Right on!
Personally, if it is not looked at as a BF amp, I like the 135 watters.
If you do not expect too much out of the amp, the MV can help to get "different" tones.
Along with the channel volume (which is now a gain control).
It can be a pleasing, slightly dirty, clean amp.
And Loud!

[Zipslack]

Why mod?

Why exactly are you modding a perfectly fine amp? I understand the "mystique", but I find the silverface amps to be just fine for what they are. My father-in-law has a '73 Twin Reverb with the push-pull MV and it is great for clean to hint-of-dirt stuff. If I want to get more crunch, use a Valvecaster/TubeScreamer/Boost-du-jour/whatever. I wouldn't dream of modding it, other than the 3-prong cord and removal of the death-cap (done years ago). The only problem I have with it is it's BIG and LOUD.

If you want a "blackface" sound, maybe you should try an effects-loops EQ - or modify the existing tone-stack closer to BF values...would be a lot cheaper, quicker, and easier.

[melvin]

Why mod? Rubbish. Seek. Mod. "No volume" is a minor setback in the search for your sound.

[Ronsonic]

The early 70s SF Twins sounded so good and were central to so much great music I really don't get the BF fad. These were great sounding amps. Yes, there was the cathode biased abomination that Fender built for about three weeks in the late 60s. Those should be fixed. But the SF Twin of the 70s is a classic in its own right. They sound brilliant.