1974 Twin Reverb needs work - should I just BF it?

[nickpaul]

I know there's tons of advice on blackfacing a silverface amp, and most seem to point to liking the sound of the BFs better.

I bought a 74 twin a while ago that needs some work so I have yet to even hear it. I was thinking about just rebuilding it to Blackface specs. Anyone have any recommendations on whether or not do to this?

The other option would be just to get it up and running and see how I like the sound of it's stock circuit...

Part of the reason that I was thinking about just doing a rebuild is that it has already been modded (probably with some blackface mods). I really want to understand what is in the amp so I figured it probably be best to rebuild it to the exact circuit of either the BF or SF.

I don't have a ton of experience with amp building (built a 5 watter and have built a bunch of pedals)... another reason why I figured I'd need to stick to a stock circuit to even get this working for me to be able to troubleshoot...

[nickpaul]

I also have a AB763 bandmaster that was modded like crazy and I was going to restore that one to the original circuit. I'm thinking I should probably start with this one as it seems to be the easier one to work on. Also, figured I'd mention this as in case this would be a good reason to keep the twin as a silverface...

[Tom Phillips]

If the amp were mine I'd blackface it and I'll add the following comments / advice
1) I have never seen an accurate and complete description of the steps necessary to convert a SF circuit to BF. There are just too many variations the circuits shipped. Plus, in your case, you have a 36 year old amp that you already say has been modded. I'd just get a copy of the Twin Reverb AB763 service information and follow it. That will address SF variations, past repairs & mods and original factory mistakes.
2) Not everything about the SF was totally bad.
a)You can retain some features such as the bias balance control and add an additional bias level control. Then you have the best of both.
b) Some SF amps actually have a little better grounding scheme. This is not reflected on the service information though. I find it interesting because it shows that someone at CBS Fender recognized the deficiencies in the original semi random grounding. Note: we do have to remember that the performance was good enough and met the market needs. It's not a HiFi amp and when you plug your single coil pickup guitar into the amp the internal background noise and hum is swamped out anyway.
3) A significant quality problem with the SFTR is the bad wiring dress. In my opinion, it's worth the effort to clean it up. Some of the work is simple and some could take a lot of time. I've done several and it's amazing how much extra wire can be left over after a re-dress. This step could be limited or become a labor of love that eats up lots of time. The law of diminishing return applies.

Hope this helps. Post pictures of the amp inside and out if you have them.

Regards,
Tom

[Tom Phillips]

Good idea to start with the Bandmaster then do the twin.
I'd Blackface both. Then add mods to taste.
Regards,
Tom

[Gibsonman63]

Blackface vs. Silverface

Blackface vs Silverface is mostly about playing style. Fender was trying to make the cleanest sounding amps possible during the CBS years, so if that fits your style better stay with Silverface. Otherwise, go with the Blackface mod.
FWIW, I blackfaced a 72 Super Reverb and the change was definitely worth it for my playing style (classic rock/blues).

I would get the amp working before I started to mod so there is less to troubleshoot.

As far as the lead dress, if you remove the supressor capacitors from the power tubes and replace with wire and your amp plays without parisitic oscillations, that is a good indication that the lead dress isn't completly horrible. You should be able to get some good ideas for lead dress from the Bandmaster.

Personally, I wouldn't worry about keeping the bias balance pot since most of us order matched power tubes anyway. I would just convert it to bias adjust.

[hasserl]

IMO the SF phase inverter is better than the BF circuit. With the SF values the amp has better headroom with a more open sound when played clean; and when pushed hard it will drive the power tubes farther into non-linear range for more power tube distortion and with less grid blocking distortion and farting out. The change to the PI in SF amps was something Fender got right.

[nickpaul]

Thanks for the info!

Tom, I think I'll start with your advice #1 just so that I am sticking to a certain schematic before doing any mods to blend the two amps together. Here's a couple pics I just took of the Twin:
'74 Twin Reverb


Not the prettiest of amps but that's not much of a concern to me. Anyone have any idea of that speaker that's in there? I'm going to need another anyway and was going to just put in two new Jensen Neo's (One tornado, and one neo)...

I'm going to take your other advice and start w/ the Bandmaster first...

I don't have any pics of the Bandmaster but I can take a couple if you are interested.

Gibsonman63, I wouldn't say clean is necessarily my thing (although I was kind of expecting this to be more of my clean amp) so I think I'll stick w/ the BF route... I've heard others say the same about those bias pots... Think I may go this route at least to start with...

hasserl, Sounds good, I'll try out both amps values for the PI and see what I like best when all is said and done!

.................
When doing a complete overhaul, do you guys find any reason to keep any of the components? I was thinking all new pots, jacks, switches, tube sockets, resistors, and caps. I've heard that the coupling caps may be worth keeping? Not sure there's any reason to not reuse the carbon comps that are in the amp and try and just replace the ones that I need that are missing to match the schematic?

[Tom Phillips]

...When doing a complete overhaul, do you guys find any reason to keep any of the components? I was thinking all new pots, jacks, switches, tube sockets, resistors, and caps.

That doesn't leave much except the chassis and the transformers.
I would start with a good cleaning and change the parts that are out of tolerance plus the filter caps which appear to be original and appear to be date coded 1977. Then listen to it and go from there.

[Gibsonman63]

Not that rough

Aside from some cosmetic work on the cabinet, your amp looks really clean. No rust on the transformers or anywhere I can see on the chassis. I wouldn't go crazy replacing pots, jacks or tube sockets until I got it going and saw what I had.

[Sweetfinger]

if you remove the supressor capacitors from the power tubes and replace with wire . . . . .

You will make smoke and fire! It rhymes! ...maybe not fire, but you'll definitely be unhappy with the results.

IIRC, you just snip them out. I think you meant to say, remove the cap and replace the 1.5K series resistor with a straight wire.

[Gibsonman63]

Hmmm...

I must be missing something. Last year I blackfaced my 1972 Super Reverb. When I cut the snubbing capacitors from the circuit, the power tubes were no longer grounded and were effectively turned off. It took a while to figure out what was going on, but the gentlemen of the forum were kind enough and patient enough to walk me through it and I got it sorted out.

Now that I look at the schematic I see that the capacitor on the schematic does not go to ground, but on my amp it did... so I must admit that I am confused.

I will say that this experience has taught me to double and triple check my work against the schematic before powering up an amplifier.

[pdf64]

'Now that I look at the schematic I see that the capacitor on the schematic does not go to ground, but on my amp it did... so I must admit that I am confused.'
What you had there were cathode bias resistors, not capacitors. Some mid period SF amps used combined fixed and cathode bias, like an ac50.
But the (ceramic) capacitors on the power tube grids should have connected to ground - it's just that you couldn't replace them with a wire link (else the tubes would have no bias).