SF 135Watt: Dropping the B+ voltage, and bias mod

[FFM]

add adjustable bias in addition to the balance circuit.

[Pedro Vecino]

You can maintain the balance control and replace the 33K resistor with a 22K pot and one 15K resistor in series.
To avoid problems, I use good pots as Morganite:



If you are going to make the adjustment of balance control fixed in the exact center (to use a matched quartet), itīs suitable to equalize the resistor values that are connected to every side of the balance pot (47K/68K). I use always 68k/68K.
Schematic is here: http://www.ampwares.com/schematics/t...erb_sf_135.pdf

Regards

[pdf64]

KT66 draw 1.3A heater current, 6L6GC draws 0.9A. With 4 of them there will be an extra 10 or 11 watts load on the power transformer. Probably not a problem, but if you do go that way, monitor the heater voltages and how hot the power transformer gets, compared to when it's got 6L6GC.
KT66 want to see about twice the load impedance of a 6L6GC. Therefore it might be better (in terms of power output etc) to connect your 4ohm load to the 2 ohm output (shown as green/yellow wire).

[FFM]

is the NFB needed for this amp?

[FFM]

So to get the 2 ohm I just connect it the second speaker out
or do I need to resolder?

I removed the nfb and the amp breaks up
earlier which is nice. I will install a switch
to run it either on or off so I can get the loud
clean and when I want some break up I can switch the nfb off.

[Gtr_tech]

This is the UL output section right? I'd leave everything else alone and replace the output xfmr with the std type and put in a proper screen supply. Amps that run screen taps sound that way due to the design....dropping plate supply won't do much for that. A std output section will shift the tonal balance back to what you expect to hear from a Fender amp.

Or...sell the thing to a bass player or a steel player....they love'em just the way they are.

[FFM]

Actually it sounds fine and very much like
a fender. I happen to like the ul - just wanting to
tidy up a few things. The voltage problem is solved
by using the 240v ac tap.

[Gtr_tech]

And the voltage on your heaters is what?

[Prairie Dawg]

HI,

What is the safe and simple mod for dropping the B+ of a 135 watt UL SF twin?

I am getting over 500 Volts on both the plate and screen grids.

Also, to add a bias pot of the output matching pot: can one just replace the resistorcoming off the pot with a 100K pot?

Thanks,

Sekt88

Mouser will sell you any number of nice 50w zeners in all sorts of ratings. There's no trick to using them except that you've definitely got to use a good heat sink compound. I'd install it into the chassis rather than that little bracket Weber sells. If you want the good stuff order an NTE zener diode.

I did this on a Rose Morris AC30 that was pushing 395 volts at the plates, dropped it down to 345. I installed the zener directly into the steel chassis in a remote area.

Weber has a calculator to help you select the proper zener diode rating.

https://taweber.powweb.com/store/vdump.htm

[FFM]

the voltage problem is soved by using the 240ac tap. thanks

[majwild1]

Ok, I'm still learning about tube amps, but I've got a problem. I have a 77 twin reverb 135 I lent it out for a couple of months and I got it back yesterday I plugged it in and played it and noticed that the volume had dropped at home I can normally play it on 2 now same volume is 3-4, so I looked at the power tubes I noticed that one was red plating so I shut it down, and ordered a new set of tubes, but the curious guy I am I fired it up again now nothing even if I pull 2 tubes inner or outter no sound unless you crank it. anyone have any ideas other than it being tubes I don't want to put new tubes in and have them take a dump off the bat. also I want to add the adjustable bias to the amp what type of pot do I need? also the hum balance pot is getting alittle funky what type of pot is it? I would appreciate any info.

Thanks