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

[Smitty02]

52 years on these 6 of the 7 caps is pretty good .......Not a bad feat for Fenders original design. Granted the parts were made in AMERICA back then. I think it will be fine, the way it is. I know Im not as savy as most of you guys on this forum but I am shooting to make this true to the original schematic.. That's the whole point... I apprecieate everyones input and like to learn all the time, Love this forum, but it is just a amp..Im not trying to start a jihad over whos got the best ideas...... Cause Leo does or did..... I think Ill take a vintage Twin Reverb and a Telecaster over any rig out there.. ha ha Peace my brothers peace and i dont charge a bench fee either I do a job for one quoted price no matter how long it takes, I dont make a living doing this, I do it cause I love too...and i dont charge ridiculus money that is why I stay busy..A good deal, done right for a fair price.. word of mouth does the rest..

[Bruce / Mission Amps]

...
Maybe you didn't notice that the amp I was referring to didn't have 220k resistors, so those calculations won't apply.
The photo I posted shows that the color bands on the bleeders are orange-white-orange, or 39k. Schematic says the same thing.
530v/78,000 ohms = 6.8mA; 6.8mA^2 * 39,000 ohms = 1.8 watts each resistor.
So far we've got 3.6 watts being dissipated just by the 2 bleeders.
Looking at the first stage of the Pi filter, over 100V is being dropped across a 2k7 10W resistor; that's 100^2/2700 = 3.7 watts.
So just the bleeders and the first Pi resistor are dissipating over 7 watts.

...

No I must not have noticed because I saw no other pictures other then the art with the blue caps and 220K resistors.
However, swamping the series E-caps with 39K seems kinda over kill though.
I was always under the impression that the idea of the resistor voltage divider is only to swamp the variable reactance of the caps, which can not be equal (+-10% or so), with a known fixed resistance, .... not to intentionally draw excessive current. It kind of forces the caps to charge at a similar rate of time... etc.
Book learned and smarter folks then me probably know the real story there! ha ha

[bob p]

I did notice that the site wasn't serving locally stored images last night. And it seems to be doing it again right now. Maybe that's why you couldn't see the photos I posted.

I have to agree with you, I don't understand exactly why the designer chose 39k bleeders. But then I don't understand why they did a lot of the things that they did with the STR. When I ran through numbers I thought that 220k would work just fine as balancing resistors. the only reason I can think of for them to use a lower resistance value would be to speed-up the time constant for bleed down at power off.

This amp is like a furnace. With a sextet of 6L6 it bakes it's parts. I think I'm going to end up adding a fan.

[Smitty02]

sounds like you know more than you claim lol I have stopped on the fender for the moment to wait on the caps also i got a dsl 401 come my way with reverb or lack there of and it needs a couple pots and a good tune up, it the 1998 model and I cant find the schematic for it dl40 60 00....not the 02 that is readily out there..Flying blind its givein me a run for my money, tracing the signal it all looks good Im stumped for the moment this one dont have zd5 or t15 sooo ?? Ive got 15v through the ic's and t6 that is where i stopped They put the wrong BR in it a nte 600v calls for a 1000v I hate those 401 & 201's Marshalls LOL and Crates..

[bob p]

I think this explains the problem with the disappearing images:

http://music-electronics-forum.com/t32799

[Smitty02]

Wow I guess it does. I hope all the schematics didnt get lost....computers are going to be the end of us all. Thanks for showing me this bob

[Smitty02]

Well I have finished the Brownface Concert. Put in a bias trim pot to boot. This amp some really shoddy work done to it years ago, the center taps for the heaters and bias and B+ came off in my fingers, barely grounded the wires were not even tinned, the grounded cord they installed was wrong and the ground wire was finger tight on the tranny bolt the very last cap wasnt even connected, it was broken at the center node of the cap which I didnt notice till I replaced the caps after I did the recap and bias board the bias supply came down to 58v and the plate voltage come down to a nice 485v, put in some TAD 6L6wgc-str black plate modeled power tubes. It wen extremly well and i biased it a couple ma shy of 43ma at 41 and holding steady. The Weber 20uf 600v caps worked perfect so I didnt have to totem pole it I have to snap some pics before I button it up and will post them for all to see and hopefully be able to use for future reference i had a $140 budget spent 150 but got a few cosmetic like chassis straps bolts and nuts new handle and feet/glides and a new knob.. full re-cap, bias mod, fixed all the grounds, 2 new bakelite tube sockets, converted one of the hole in the chassis to a 1/4 foot switch input to go along with the rca..considering using the other hole for Bias test point or isolated bias adjustment pot and removing the small trim pot. Got to be something I can do with that other hole, also wondering what i can do with the empty half of unused 12ax7 ?? customer would like to add a reverb mod, but dont see that in the cards LOL. It was a good project that went very well. Now I have to go on to a DSL401 with no reverb send and other issues common that I already repaired. Hope to be able to get pics up and wish I could have recovered the cabinet, it sure would have been sharp, By the way after the cap job the plate voltage come down to a nice manageable 485v and the bias supply to 58v much better than the starting voltages, Im sure he will be very happy with his golden nugget of a amp

[MWJB]

42mA is a bit hot for this amp. It's not the cap job that brought the voltage down, it is the high plate current.

These amps (67233 PT) usually run 495vdc to 520vdc on the plates (-60vdc bias, or more) depending on year & bias, anything under 530vdc is "manageable". These are the coldest biased 6L6 Fenders made, they can often stand to go a little hotter, but 30-35mA (15W to 17.5W-ish per tube) is as much as you should need for good performance.

[Smitty02]

It sounds a bit hot after playing it for awhile it get pretty hot also, you think I should dial it down to about 30? would crossover distortion be a issue? These were sapose to run that way ? Even though the plate voltage was 485v, I usually use the Weber Bias calculator. Could I have caused any damage running around 40ma ?? I am not real familiar with these and value any input I can get on them. I want it to last and the man to be happy without any trouble but he is quite a blues man, very good player.. Thanks for the insight I had a 47k bias resistor in it with a 22k trim pot and it topped out at 36ma so I went a switched the bias resistor to 33k and the got me right where i needed to be fairly centered at about 36 to 56ma. So do you feel I should swap back to the 47k and bring it down to 30-35ma and cross over wont be a issue ?

[Justin Thomas]

Smitty,

I've got one of these with a homemade cap pan. I couldn't find one @ Mojo to match the screwholes. You mentioned an "oversize" one. What are the dimensions on your pan? Mine needs about an inch in extra length more than the Twin Reverb pan that Mojo sells.

And yours still has the blue caps. Someone went through mine and replaced EVERY cap with generic replacements. I can't believe ALL of them were bad. And the amp still sounds great, but I like those old blues. These are great amps - I'd have a hard time parting with it... Congrats on getting it done!

Justin

[MWJB]

Running at 42mA for a short period is unlikely to have caused any issues. Crossover will not hinder clean W RMS rating at 30mA...probably not even if in the mid to late 20's either, but no need to run that low for reliability. The amp will perform as intended (well better than intended, probably didn't run much more than 20mA when new) at 30mA, or a little over, at 495vdc.

[Smitty02]

Thanks Justin it was a nice job and a amp I wish was mine, I want to build one now I would like to hear it with the 4x10 that it was meant to have, yes blue molded caps are drifted and one or 2 could stand replacing but it dont seem to be a deal breaker vibrato works great, these usually hold up for some time I think it is part of the mojo of the old fenders Ive done a bunch of black and silver faced work and have never replaced any of the blue caps. Ive got a collection of death caps LOL Tight punchy, earthy grind when you dig in. I want it . what are the exact dimentions of the pan you need ? I may be able to get one made for you from aluminum, just tell me the rectangular inside size and how much of a lip you would need to screw it down. For a small charge of course, plus shipping. PM me if you are interested for details. My pan is stock, I used Weber 20uf 600v caps that fit just fine actually they are small like F&T caps. Mwjb I will open her up and re adjust thanks for the info, Ill give it a go see how it sounds..

[Smitty02]

I took your advice and opened it up and set the bias at 33ma Thanks MBJW It still sounds marvelous, I love it.. I would love to hear 4x10's through it

[Smitty02]

UPDATE RE-Tolex along with rebuild

http://music-electronics-forum.com/a...4&d=1367981684

[Smitty02]

61 concert refinish complete

I been away for awhile but I did refinish this amp I know it is late in the post but here are some pics of the re-tolex job. I came out pretty nice