Fender 1961 Concert Brown face 6g12a re-cap time, need input

[Smitty02]

Hi all So I got a pretty cool amp in for a restore, its pretty beat and blowing fuses, leaking caps bad tube sockets ..........yeah it a project but I love these kind, its beat to crap but soon will be running smooth and looking....well better than it does now LOL I see the caps under the dog house are leaking and I noticed they are rated 20uf 525 My first question is will F&T 22uf 500 suit this job as that is what I plan on using and have done much back ground on it and the general opinions are to use the 500v F&T's which was my first thought BUT This is the oldest Fender Ive done and would like some input, Bias supply has drifted up to -68v from the -55v on the schemo..and its running about 512 plate volts..448v on the schemo also.. I plan on doing a full recap of all the electrolitics.. Also what do you think would be the best or correct replacement rectifier diodes ?? 1n2002 or 4's...? Oh yes too, what would be the best way to make this adjustable bias? It has dual 8uf 150v caps in the bias supply, or do you think I should just leave it original ? Thanks guys any input would be helpful... http://ampwares.com/schematics/concert_6g12a.pdf link to schematic

[MWJB]

"Bias supply has drifted up to -68v from the -55v on the schemo..and its running about 512 plate volts..448v on the schemo also.."

Bias has drifted, or been rebiased? Do you have a 56K to ground in the bias circuit (may have burnt out, gone open, neg dc should be ~12% of B+) ? Could be 10K or 15K dropper feeding that. Are you measuring this -68vdc at the junction of the 2x220K 6L6/5881 grid load resistors?

Your B+ voltage is ball park, no issue here.

6x1N4007 are typical replacements.

Keep the 2x8uf cap set up (or 10uf won't hurt), 150vdc.

Replace the 56k bias load resistor with 39K to ground in series with a 50KL cermer trim pot wired as a vairable resistor (unless you are a harp player, then sub out the 15K/10K load with a 10K cermet pot wired as a variable resistor, in series with a 1K fixed resistor, between the 2 cap leads).

Forget 500/525/550vdc rated caps for the main filters, do this...

[Tom Phillips]

... I plan on doing a full recap of all the electrolitics.. Also what do you think would be the best or correct replacement rectifier diodes ?? 1n2002 or 4's...?

Comment - I don't find many bad rectifier diodes in these old amps. Therefore, unless they test bad, I don't replace them at all.

[Smitty02]

Hi guys Thanks for the brain food. I am into this one full time now, I just finished up a red knob twin Cabinet was in pieces baffle board falling out Good thing he brought it in it was on the verge of a melt down among all the other issues and he told me it was sounding fine LOL, man...It wasn't going to last much longer I think. I spent some quality time with it and got it all good to go, I taken to rebuilding the pots as you cant get them anymore 3 white groove tubes one blue it had smoked the balance resistors at some point and been fixed but that was all they fixed.. Glue screws nails clamps, full recap some were leaking and drifted values the 470 ohm resistors on the tubes were reading 700's and 600's new set of JJ's only to find the bias and balance woudn't even come close the one ohm bias resistors were fried fixed them up ad it smooth and right on the money, then a Crate GT 50 stealth with barely audible output, op amps and a open resistor new reverb tank good to go, not to shabby for a crate nice clean channel, 6v6's dirt channel isnt to bad if you dont go nuts, it can get ugly fast LOL. Re worked a botched pedal attempt a mod tremolo kit... Pretty nice pedal really, now.. Anyways... thanks for the diagram MWJB was leaning towards just putting F&T 500v in it but have read up some, seems to be 2 trains for thought on the recapfor sure which do you think is better I think the 500v could handle the standby voltage as they have a 20% + tolerence and have read about putting a resistor across the standby to calm the in rush...I guess how would the modded series caps effect the tone of the amp ?? is what Im thinking now I know the new stuff will change it anyways but I did find some interesting things in the Bias supply some one put a 68k in place of the 56k and a 10k for the 15k, one diode is a newer style looks like a 4002 for what I could male out the rest are black bullet style and the bias diode is a old blue one I couldnt read that one either...It had been modded in the past but put back it looks like but it does have one wire running the length of the board, why I dont know it does not show it on the print that I can see. Yes it is beat the tolex has been pulled off and varnished but bet to crap and changed over to 2 12" speakers but I see a diamond in the rough here and am looking forward to doing it some good, old recap with cheapo caps and one 47uf50ov replacement in the dog house. I will ty and get some pics up of it but thanks for the info and looking forward to some more replies I hope Thank you all.. starting my parts list now, Ive got a 150 dollar allowance to make her right as I can thanks again Peace,,,, time to get some sleep

[MWJB]

"thanks for the diagram MWJB was leaning towards just putting F&T 500v in it but have read up some, seems to be 2 trains for thought on the recapfor sure which do you think is better I think the 500v could handle the standby voltage as they have a 20% + tolerence and have read about putting a resistor across the standby to calm the in rush...I guess how would the modded series caps effect the tone of the amp ??"

The amp will sound better (less loose/mushy) with series main filters, you could use a 550v/600v cap for the screen supply node. I just wouldn't run 512v on a 500v cap. Rewiring the main & screen filters doesn't take that long as part of a recap. The number of shot screen supply caps (usually the first cap in big BF/SF Fenders to spill their innards) I have seen suggests the caps don't know about this 20% over tolerance. ;-)

[Smitty02]

as this one probably did. I did call Mojo and ask them for the TAD 550v ones of fabled posts,, but they said they provide no such caps, also looked at their over sized doghouse covers Hmm....Yes the #1 had been switched out for a 47uf the remaining 6 ...4 are leaking. what do you make of the choice of resistors 68k instead of 56k and 10k instead of 15k..could this be the reason the bias voltage up to -68v ? I did just buy some F&T 22uf 500v for that Red Knob and they are pretty small, but man I would really like to keep it as stock as possible and preserve the mojo amap

[Jazz P Bass]

I never heard of a 'plus' only capacitor tolerance.
Now plus OR minus, yeah.
I will have to agree that placing 512 volts on a 500 volt cap is asking for trouble.

[Smitty02]

They are the special ones only available to me Jazz....yes I am only speaking of the + due to the amp running a bit over. I did see on the print where they mention the 110v, it didnt dawn on me that this is why its high...Is it possible to do the diode on the center tap trick, Ive seen that mentioned I think by you in a old post of mine or wait it may have been Loud thud that told me that sorry.. yes i do realize that the #1 47uf replacement is not the screen filters. I am looking over the posting you gave me.....Can this be done and still all fit in the original cover guys ? Ive never done this type of thing before Im a bit intimidated by it, I am trying to see how I could do this and keep it all inline, plus what of the remaining caps would 22uf 500v due ?

[Tom Phillips]

If it were me I'd use the series cap mod that MWJB posted for the first stage filter cap. The first cap voltage really rises high when the standby switch is opened in that amp. I'd then use the 500V caps for the rest of the filters.

[MWJB]

as this one probably did. I did call Mojo and ask them for the TAD 550v ones of fabled posts,, but they said they provide no such caps, also looked at their over sized doghouse covers Hmm....Yes the #1 had been switched out for a 47uf the remaining 6 ...4 are leaking. what do you make of the choice of resistors 68k instead of 56k and 10k instead of 15k..could this be the reason the bias voltage up to -68v ? I did just buy some F&T 22uf 500v for that Red Knob and they are pretty small, but man I would really like to keep it as stock as possible and preserve the mojo amap

15K/56K gives us a .79 reduction divider.
10K/56K gives us a .85 reduction divider.
10K/68K gives us a .87 reduction divider....about -6vdc swing, about a volt and a half between scenario 2 & 3. Barely enough to constitute a "mod". You shouldn't be seeing -68v at the junction of the 2x220K 5881/6L6 grid load resistors with any of these combinations...I can only guess at 2 possibilities - 1) You are measuring right after the diode, before the voltage divider? 2) The amp's PT has been replaced.

This amp, supposedly, ran 64vdc less B+ when it was built. What it did over the last 50yrs is less pertinent than what it will be expected to do over the next 20 ;-). Those F&T 22uf 500 will be fine for everywhere except main & screen nodes.

[Smitty02]

I would hate to change the character of the amp to much for the man wish they made 550v caps as I have seen marshalls with the same 550v cap rating is this not America where we have the best of everything..health care good paying jobs pretty girls awesome Fender amps Telecaster guitars....plenty of food for the country, cheap gas BUT NO 550v caps, My god. Maybe Ill hop on my private jet and shoot over the pond and get me those caps they seem to have but we cant get th over sized cap pan is starting to look good to me Ha ha. I am going to go do some voltage readings and would like to post some pics of this totally cool amp.. mojo factor priceless

[Jazz P Bass]

TVA1966-E3 - VISHAY SPRAGUE - CAPACITOR ALUM ELEC, 20UF, 600V | Newark

[Daver]

Antique Electronics has the 20uf 600V Spragues for $19. FWIW.

[bob p]

If you're thinking about doing the totem pole cap arrangement, you could always take a look at the Super Twin Reverb schematic for reference. The STR has a B+ of about 530 VDC on modern 125VAC supply voltages and It uses the same totem pole cap configuration that MWJB is describing.

Many people like to strap bleeder resistors across capacitors, but I'm not one of them. I don't even like to put them inside of the doghouse. Whenever possible I prefer to locate bleeder resistors, cathode resistors, Pi filter resistors, etc., as far as practicable from their associated capacitors. Resistors generate heat, and heat is one of a capacitor's worst enemies. If you take a look at the attached gut shot of a stock Super Twin Reverb chassis, you'll see that Fender mounted the bleeder resistors for the totem pole caps as far away from the caps as possible. Instead of putting the bleeders in the doghouse, which would retain all of the heat in an enclosed space and threaten capacitor life, Fender put the bleeders on the terminal strips that I've circled on the attached photo. Then they just run wires from the caps in the doghouse to the terminal strips.

[sgelectric]

Weber VST offers 600v rated filter caps.
Not USA made, but perhaps an alternative to series connected lower voltage rated caps.
Prices are not off-the-wall.
I've used them for 500+v applications without a problem.YMMV.
Just a suggestion......Check it out.
SG