Fender Super Twin- New Project

[pbman1953]

Gotcha!


Re-bias?

[sluckey]

You've probably read this before...

     If it squeals or howls you'll likely need to swap OT primary plate leads.

[pbman1953]

After the install there wasn't any howl and it's quite, but there's a little hum.


I sent a chart to show the voltages and they are very close.


The odd finding is that cathode is off with the first and last tube and that's why I'm writing. I'd suspect the hum would disappear if tubes 1 & 6 had a cathode reading like the rest.




Tubes 1-3 is one side and 4-6 is the other side.


I'm curious what you all think. Unless the too hotter tubes are on the way out. Before the OT update they were  all very close.
 

[thetragichero]

try swapping tubes 3 and 5? that might balance out the sides

[tubeswell]

try swapping tubes ... balance out the sides

^That's^ how I bias up an Ampeg SVT-CL

[pbman1953]

I'm back with a question-


This amp has been great but I wonder if I softened the attack. Before most of the work the amp had a scary attack to it. I performed a cap job throughout with a kit designed for this amp. The kit was part for part at the most of the same values. If they were not , they were slightly higher for the filter caps. 


In the front end for V1A pin8, the 750 cap I subbed with a 10 uf. No other stock values were changed besides the resistor off pin1 of V4B- from 220k to 180k


Thanks

[sluckey]

I'm back with a question-

Well, what's the question?

[pbman1953]

Did any of these changes effect the attack sensitivity?

[nandrewjackson]

The 1st stage cathode bypass cap, to my ears, is a big tone shaping component.


If 750 means 0.75uF changed to a 10uF, that'd be a big difference.

[sluckey]

750 means 750µF. Using such a huge size on the input triode is supposed to reduce or eliminate heater induced hum. The Fender 5F6A used a 250µF. All the old Sunn amps used 250µF as well.

[pbman1953]

Using the 10uf was suggested from an amp tech to calm down the gain.  I could install the 250 and try a 12at7 to see if that controls the gain more.


Another thing I noticed between the Fender, Sunn and Traynor YBA series:


The resistor for the v1 pin 8-


Fender & Sunn use a 1.5 k


Traynor uses a 820 ohm


What are the differences?


For V4B, should I stay with the 180k on pin 1? If I remember ,that's a Fender update.


Thanks

[pbman1953]

Any suggestions on my last post?

[PRR]

> Any suggestions on my last post?

Two resistors and a cap? Hot-up the iron and try it.

[pbman1953]

It's true I could experiment, but I'm simply asking for an answer to my question. I know it's a cop out but I can live with that


Thanks

[sluckey]

It's true I could experiment, but I'm simply asking for an answer to my question. I know it's a cop out but I can live with that

Sometimes the answer is "just because" or "just the way it is".

[shooter]

 
those were my canned answers to "why did it break?"
"Boss, you pay me to keep things runnin, to fix broke things, I want $1,000/hr to figure out "why" it broke 

[soundfish]

I've just acquired one of these Super Twins (just chassis and iron) and plan on rebuilding it to spec, or possibly with a standard Twin output section.  Either way, I'd like to use the multi-band EQ and have a silly question;  The pots in the EQ section show a fourth wire going to ground (in addition to the usual upper, lower and wiper connections). I've not seen this configuration before.  Is this just a wire soldered to the chassis of the pot, or some other type of pot?  I've studied all of the photos but can't see the details on the pot wiring and there doesn't seem to be a layout diagram available anywhere.   

Any help would be appreciated, thanks

[sluckey]

The fourth wire is a fixed tap that's connected to the resistive element in the pot. It's not connected to the metal housing of the pot. Treat with care. May be impossible to find a replacement.

[soundfish]

that's unfortunate, because I didn't get any of the pots with the chassis.  Oh well,  I guess I'll have to go in another direction.  I'm open to suggestions.  Thanks for the info and the quick reply.

[tdvt]

Fender must have bought boat-loads of those tapped pots during that time.


I just removed 2 out of a 79 Pro Reverb I am rebuilding; one for the MV & one in a power tube balance circuit. Different values than you would need anyway, but looks like they used them all over the place.