cathode resistor and cap, SFTR vs AB763

[m-fine]

I acquired a silver face twin reverb that was making some truly horrific noises. Valve 3, the reverb drive 12at7 got very hot quickly so I figured it was a good place to start debugging. Sure enough, replacing it fixed the noise, but the new tube got hot as well. The B+ measured 439-440 (spec on this amp and the ab763 is 440) but the cathode voltage only measured 4.85 (spec is 6.0 on this amp and 8.6 on the blackface). The cathode resistor only measured 280 ohms, and to my surprise, there is no bypass cap.

The spec on a MV twin is 470 ohm for the cathode resistor with no bypass cap, but on the AB763, it shows as 2200 and there is also the standard 25-25 bypass cap.

First step, I want to replace the resistor pronto before it burns up another tube. Can I go with a 2200 without the bypass cap, or should I go with a new 470 until I can get a cap? I figure I can get a resistor at ratshack to hold it over until I can place a parts order for a more substantial overhaul. Not having the cap just reduces the gain on that stage right? If I need more gain (seems like it has more PLENTY of reverb such that I could spare some) I can always run a 12ax7 in that stage correct?

Longer term, my plan was to swap out a few of the components to "blackface" the amp. After finding one resistor that is so far off in value, and realizing that I have no way to measure the caps other than to check for shorts, I am a little weary of everything in there. I am thinking of getting an entirely new main board and all new resistors and caps. I figure the parts should be between $50-$75 but the kits I see are more than double that. Am I missing something? The B+ and filament supplies are spot on, the pots all work and are quiet (except the MV which I bypassed while I had the chassis open anyway) so I don't think I need to touch the power supply or replace the tranys or anything else. Again, am I missing something I should be worried about?

I am not looking to invest too much since it is much bigger LOUDER (it has a pair of Celestion Vintage 30's!) and cleaner than I generally prefer and I may just fix it up and sell and then build a Mission 5e3 or similar with the proceeds.

Thanks.

[acorkos]

if you may possibly be selling it, i would just replace the resistors that are out of spec and leave the rest of the amp alone...it will bring the most $$ being as original as possible

[m-fine]

if you may possibly be selling it, i would just replace the resistors that are out of spec and leave the rest of the amp alone...it will bring the most $$ being as original as possible

I don't think so. First, this is not a showpiece amp, the grill is dingy, the tolex has tears in the corners and the face plate is pretty beat up. Also the speakers were replaced already, and I do not have the factory originals.

Second, I don't think anyone is buying a master volume silver face for collectors value. People like them because they are a fraction of the cost of a "real" black face but you can get decent tone from them, or with some minor mods get black face tones. I did some research and it looks like people will pay a bit more for a silver face that has new speakers (usually Jensens, not v30's though) and for amps that have been converted to black face. This is especially true if it is well done (brand name caps) and the amp is rewired neatly like the pre-CBS amps and not the mess that it left the factory with. Anyone buying one for tone will certainly prefer a BF modded amp to stock if they have heard both.

That said, I bought it as a project, not to make money. If I do a full board replacement it will probably cost me about $75 in parts plus many hours of labor to add maybe $50-$100 in resale value so this is not a wise business move. OTOH, I think it will be a better sounding amp as long as I keep it, and if I do sell it, I will feel better about passing on a reliable amp with quality components that has been wired with care. I can save the old circuit board and leads if a buyer really want to go back to it, but I can not imagine anyone would.

FWIW, my search of completed sales shows a 100 watt SF with the push pull MV to fetch about $650-$700 in better condition than mine (all original). I have seen BF modded ones with new Jensens or similar speakers going for $800-$900. I see AB763 kits selling for $1100 and up, so there is definitely not a lot of nostalgic value built into the silver face prices.

[acorkos]

If I do a full board replacement it will probably cost me about $75 in parts plus many hours of labor to add maybe $50-$100 in resale value...

wishful thinking....more likely spend $75 only to lose $100 in value for not having the original eyelet board and most original components. even for a silverface. my opinion, with which you may choose to disagree.

[m-fine]

wishful thinking....more likely spend $75 only to lose $100 in value for not having the original eyelet board and most original components. even for a silverface. my opinion, with which you may choose to disagree.

What is your opinion based on? I spent some time searching classifieds, and especially completed auctions, and I didn't see any evidence of a discount for a BF modded silver face.

As I said, I am not doing any of this for the money, I am fixing the amp first, and then seeking out tone and sound quality improvements. Take the fact that I will definitely be modding it as a given. I am not a collector and this is not a collector piece, it is a tool that is meant to make sounds. I have never heard anyone say a silver face Fender sounds better than a blackface and that is even more true the further into the 70's you get. Even if you ignore the component value changes, the quality of the work in these things is terrible. For example the filament leads are long and sloppy and barely twisted. I am confident I can all but eliminate the hum in this amp, and to anyone who actually wants to use it to make sound, that is a big plus.

So beside the don't mod it, advice, does anyone have any input on my original questions?