Supertwin help needed

[howardmoon]

Hi,
I'm working on a fender Supertwin to use as a bass amp. It wasn't working when it was given to me and with a bit of an impoverished country mindset make-it-work-somehow, I was somehow able to get it running again for a bit. Thankfully it broke again, forcing me to reevaluate my Forrest Mims level skills and pay more attention to safety issues.


Here's where I am with the amp:
built a current limiter and learned to use with variac
checked resistance of transformers, they appear to be good
rebuilt rectifier and replaced filter caps
replaced old mallory bypass caps
changed all but 2 of the old blue film caps.
replaced power tube sockets-there was one blown 470 resistor which I replaced, but I will actually be replacing all the power socket resistors with metal film.
purchased a new set of tubes-tung sol and sovtek
scrubbed all boards with alcohol, serviced pots, ground bus, replaced power cord with 3 prong.
learned that installing electrolytic with value facing up is a thing.


Voltages:
6L6 sockets
pins 3 and 5 515V
pin 4 -67


All preamp tubes
Pin 6 of V1A is supposed to be 195V but it's -7.7, and all the preamp tubes are like that.
So I checked the voltage of the filter caps and everything seems right until I get to the first 20@500 and I get -4.9V where it should be 400V.


I've attached the schematic that I'm working from (two pager) which seems closest to mine. Thankfully all the circuitry around the power supply seems the same across schematics.


Thanks in advance to everyone in the control tower working to talk me down!

[pdf64]

Needs the other schematic page.
If this is correct https://el34world.com/charts/Schematics/files/Fender/Fender_super_twin_180w_schem.pdf
the issue may be that the 5k7W HT dropper, or a connection to it, is bad.

[sluckey]

Show us the entire schematic.

[howardmoon]

Weird, looked like the second page loaded but lemme try that again...

[howardmoon]

[PRR]

.

[howardmoon]

That's better, thanks! My amp has another clipping diode in parallel with a 2.2k resistor on the other side of the inductor in the distortion circuit and a 5uf cap at the junction of the 3.9k V3A cathode resistor, whose function I don't understand.

[howardmoon]

Thanks everybody, I got it working again! The 5k 7W resistor was the culprit. I tried not to look at those resistors because that terminal strip of with all those wire wound resistors is such an unholy mess.


Anyway, not sure if I should make this a new post, but I have a question about voltage rating for screen and grid stopper resistors. I bought some metal oxide 3W 470R that have a minimum dielectric withstanding voltage of 700V, and a maximum working voltage of 350V. The 1/2W 1k5 resistors I have are rated at 250V for both max working and dielectric voltage, and since the voltage on pin 5 is around %00V or a little higher, I'd say I got the wrong resistor. But then I've seen people arguing that it's the power rating that matters. 


 I have tried to research this a little bit but could definitely use a bit of guidance.

[sluckey]

...the voltage on pin 5 is around %00V or a little higher...

Huh? Say that again.   

[howardmoon]

Haha! It's that kind of attention to detail that's needed when working around high voltages. That would be 500V. And I should have also mentioned that the 1k5 resistors would be the grid stoppers on pin 5.

[sluckey]

Haha! It's that kind of attention to detail that's needed when working around high voltages. That would be 500V. And I should have also mentioned that the 1k5 resistors would be the grid stoppers on pin 5.

No way is there 500v on pin 5. More likely -50v.

[howardmoon]

Gah, counting in the wrong direction. So yes pin 4 has about 436V on it right now and pin 5 is about -57V.

[PRR]

> voltage rating for screen and grid stopper resistors

The voltage *across* these resistors is small, 1V-50V at most.

If you smash the resistor against the chassis or other parts, you could have breakdowns. Don't do that.

[howardmoon]

Aw man, light dawns on Marblehead! Thanks for the help, and in only two sentences!

[Big Iron]

Lord have Mercy, I bought one of those New and took it on the Road for a couple of years.
Great sounding amp, Loud as Hell and weighed as much as an aircraft Carrier.

It certianly has the Fortitude to be a great Bass Amp.

Good Luck.

[howardmoon]

Hehe! Who knows, maybe this used to be your amp! Yeah it's crazy heavy and I've noticed gravity seems to be getting stronger by the year for some reason. It's probably heavier now that it's disguised as a bass amp but at least it's in two sections. Someone was getting rid of an old enclosure for a Fender Studio Bass amp for $30 in some part of Brooklyn that's really far from the subway so I found myself wheeling its rattly carcass forever in the middle of winter haha! Then I chopped it and recovered it which maybe is sacrilege to some but it fits in a car better and I have other cabs I can use it with.

[dwinstonwood]

Is that a double bass head scroll in the bottom right corner of your second picture? Is that what you're playing through the Super Twin? Cool!

[howardmoon]

That is my upright photobombing my super twin pic. I confess haven't tried it through this amp yet but my electric upright and fender electric both sounded awesome. Looking forward to getting it all boxed up again and maybe playing a gig with it one of these days!

[RadioComm]

Lord have Mercy, I bought one of those New and took it on the Road for a couple of years.
Great sounding amp, Loud as Hell and weighed as much as an aircraft Carrier.

It certianly has the Fortitude to be a great Bass Amp.

Good Luck.

In the mid to late 70's, I lived on the 7th floor with no elevator. Tight and steep stairway, so only one person could lug that twin reverb at a time, once at the beginning of the evening, and then another in the wee hours of the morning when the gig was over. Also had to set up, and then play the gig with a 13 pound gibson les paul custom. All this with no aches and pain what so ever. Now a days, I'd be out on Aleve for a full week... lol.

[howardmoon]

Haha! Well at least it pays! Hey wait a minute..doh!!