Replacing signal caps during "blackface" mod

[otter]

Short version:

Would you replace the blue drops with orange drops in a silverface Twin, while you have the thing apart and you're doing extensive surgery anyways?

Long version:

The impetus for these modifications is that I'm facing the prospect of moving out of a large house into a small apartment, and I really won't need a 100w Twin and a 50w Silvertone. I'm planning on selling the Twin because I can probably get more money for it than I can for my homebrew Silvertone, and I can probably rebuild the Silvertone into something a bit smaller anyways (with 6V6s).

Anyways, I've had the Twin (1976 Silverface) for about 5 years now, during which time I've modified the daylights out of it (it wasn't entirely original, so I didn't feel so bad about it). What started with a simple recap (electrolytics) led to new speakers, sag resistors, 4*6V6S, cathode biased power tubes, bias modulating tremolo (on the phase inverter no less, sounded really freaking cool), bigger bright caps, and some 220k plate resistors on the normal channel. With the prospect of selling it, I realized that no one was going to want my frankenstein-of-an-amp-that-probably-gets-slightly-too-hot-to-be-completely-safe-even-though-it's-the-best-sounding-Twin-in-Texas.

So I decided to go about de-modding it and cleaning up some of the sloppy work that previous owners had done to it (I swear I removed about a gallon of solder). While I had it apart I ended up blackfacing (AB763 spec) the bias supply, phase inverter, and reverb circuits since I could probably use that as a selling point and I had the parts on hand (in my secret stash of carbon comps). What I don't have is caps. Despite three generations of hacks working on this amp, most of the original blue drop signal caps still remain (except for the four I replaced in the tremolo and phase inverter with orange drops).

So the question is, after all my rambling, if you've already rebuilt 80% of the circuit would you go ahead and "upgrade" all the signal caps? Are the blue drops as maligned as the earlier brown caps? I don't particularly subscribe to any cap mojo, and I'll admit this is mostly driven by my obsessive compulsive need to have all the caps match the four I've already replaced. I quite enjoy working on the thing (since I don't have enough money to buy something new to work on), and it'll only cost about $20 to get the 15 or so caps I need.

Other questions that have nothing to do with the cap question:

How hot does it get inside these things while they're running? Would it burn a piece of paper? I'd like to put a note inside the amp that says what I've done so that anyone who works on it in the future will know to reference the AB763 schematic/layout rather than the silverface one. If I just put it in the cabinet I don't think anyone will read it, so I'd like to put it in the chassis, but I'm not sure if this is safe.

Would you remove the master volume, just to say it's 100% blackfaced, or is a master volume necessary for a Twin in the 21st century? I've always quite liked the master volume on this amp, but I know some people won't even look at a Fender with a master volume (I already removed the pull boost "feature"). Is there a more useful control I could put in the master volume position?

While I'm on the topic of master volumes, I'd like to ask about a discrepancy I've noticed among silverface schematics. On the schematic without the pull boost the bottom lug of the master volume goes to ground, but on the schematic with the pull boost the bottom of the master volume is connected through the 100R resistor (at the node between the 820R feedback resistor and the 22k phase inverter tail resistor) to ground. Would this make an audible difference? Also the non-pull boost schematic doesn't have a bright cap on the master volume, while the pull boost schematic has 120p between the top lug and that weird tap on the other side of the pot. If I leave the master volume in, should I leave this cap in there?

Sorry for the long winded post; I'm obviously thinking way too much about an amp that I intend to sell. I'll be sad to see it go. This was my first (only) "vintage" amp, and the amp on which I learned how amps work.

[xtian]

There's no vintage value in your Twin any more, so put in whatever caps you like! Make it pretty and clean, and show photos of your work, and you'll probably get a better price.

The power tubes will get over 200 degrees F, maybe hot enough to boil water, but not hot enough to ignite paper (451 F? or is that just Ray Bradbury?)

[PaisleyTube]

blue drops are imo nicer/more mellow than most modern ODs and way nicer than the earlier brown/red drops in late 60s Fenders

[Gordie]

I'd leave those shiny blue caps alone. Why not staple an envelope into the inside of the cab with your important circuit info? The amp might even be more desirable with the 6V6s!

[vibratoking]

I have a 70 Twin. I changed all the signal caps to ODs. Glad I saved them all! After about 3 months I changed them all back. Ahhhh, my amp returned from its alien abduction. That amp was very hard sounding with the ODs...some may like that...not me.

[rp]

+ 1 on leaving them. I've played some stock silverfaces that sounded terrific. I'd get it all running up to spec and see what it sounded like before replacing coupling caps, even the brown turds. And yeh those blues drops seem sweeter (less bright) than modern orange drops.

Why not do a clean faithful BF conversion with a PPIMV/larmar, get it sounding really good and put it on consignment in a store that understands what was done and can use the changes to help sell it rather than it just being sold as a repeatedly violated '76 twin.

BTW I've never succeeded upgrading the disc caps in the rev and trem, always sounded awful, best left along or use discs again - best is put the orig discs back. Maybe others have had better luck here.

[Phil_S]

See if you can find brown turds to put in place of the blue ones.

Why would you replace tone caps that aren't leaky? Besides, your amp is no longer what it once was. Do the minimum required to get it in shape to sell.

[rp]

Why would you replace tone caps that aren't leaky? Besides, your amp is no longer what it once was. Do the minimum required to get it in shape to sell.

'cause it was very much vogue to do it in the 80s-90s when word was the brown or blue turds sucked, and maybe they weren't as special as the yellow or blue molded ajax or astrons but they had a murky warmth and top end softness that made a good silverface sound right. Shotgun it w/ orange drops and mica and the amps most often became even worse, harder colder than what CBS had done. And, the SFs used to be cheap so sophomore tinkerers like me and the OP bought them specifically to mess with - You changed them 'cause you could.

If the OP has the skills to do nice work I still think doing a nice BF restoration with a Larmar and consigning it w/ a real musician's store is the way to get the most out of it. Tell the store you got it that way and don't recall who did the conversion but it was some renowned tech in NY, LA, Nashville... It's got to sound real good though, otherwise it's $350 on craigslist.

[tsutt]

time for more of my ignorance. how do test for Leakey caps what does mean or?

[tictac]

Leaky coupling caps would have DC voltage on the side not connected to the plate load resistors. It would be something you'd notice audibly as well, there's no need to test your caps for leakage...

Stay with the Blue caps your amp will sound so much better with them in...

TT

[jelle]

I disagree about testing the caps in circuit. Many times the board is more conductive than the caps so this will offset the reading. I recommend to take the caps out to verify if they leak.

I have seen very very few polyester caps that leak.

[tictac]

I have seen very very few polyester caps that leak.

That's the point that I was making, those Blue poly's are almost never bad.

If the poster doesn't know what a leaky cap is surely he doesn't have the test equipment to check capacitors for leakage, in or out of the circuit...

Correct me if I'm wrong but my advice would be to do the Blackface Mod and use the original caps; if the amp fires up and sounds good... mission accomplished. If not let the troubleshooting begin... leakage testing or whatever else is needed...

TT

[tsutt]

i wasnt the op. just ? as i learn. read a lot about leaky caps but not how to test.

[jelle]

I have seen very very few polyester caps that leak.

That's the point that I was making, those Blue poly's are almost never bad.

If the poster doesn't know what a leaky cap is surely he doesn't have the test equipment to check capacitors for leakage, in or out of the circuit...

Correct me if I'm wrong but my advice would be to do the Blackface Mod and use the original caps; if the amp fires up and sounds good... mission accomplished. If not let the troubleshooting begin... leakage testing or whatever else is needed...

TT

I agree 100%.

[rp]

time for more of my ignorance. how do test for Leakey caps what does mean or?

scratchy pots sometimes indicate leaky caps. on some circuits the presence could be scratchy but normal.