Fender Super Farting?

[doctorvoltz]

This is my first post on the forum. I've got a '72 Fender Super Reverb that was brought to me in desperation and has been to 2 other techs with no success. The complaint is that the amp sometimes develops an awful sounding clipping distortion when low bass notes are struck. To my ear the amp doesn't seem to have a great deal of headroom either.

In an attempt to cure this problem the other techs completely recapped it (all electros and coupling/bypass caps changed except for ceramic discs). Speakers were replaced plus the amp has been tested with different cabinets with the same results. All Tubes are good and properly biased. A different tube set was subbed out to double check. All resistors are in spec. All voltages are correct as per schematic There are no loose or intermittent connections. I'm frankly stumped.

I've been reading on the net that this is a fairly common problem with Fenders, though I've never experienced it. Does anyone know of a definitive fix? As best as I can tell the amp will need some kind of mod. One mod I've seen mentioned is reducing the value of the 25mf/25v preamp cathode caps. Has anyone done this with success? If so, can you provide specifics? Thanks.

[tedmich]

this link might help:
Circuits - Grid Blocking

maybe this one too
http://www.harpamps.com/micKfender/S...Much-Bass.html

[Chuck H]

The OT could be upgraded. The stock OT is good but you could get better bottom end from a top shelf replacement. Other than that...

The thing to remember about the BF type arrangement is that the tonestack is very early in the signal chain. If you turn up the mids for a thicker OD tone and don't SUBSTANTIALLY reduce the bass setting the amp will fart out. The thing about the BF arrangement is that under OD conditions you need to fiddle with the mid and bass knobs until they just start to fart out, and then back them down a hair. At that point the amp is producing all the lower mids and bass it can. Trying to add more with higher knob settings will just result in flab and farting but not more bass.

IME changing the bypass caps on BF's is a good thing for most players. The guys that use single coil tele's will like the stock amp. But LP's and even strat's do better with lower bypass cap values. Dumble used 5uf for both preamp bypass caps. I use a 3.3uf on V1a and adjust the bypass on V1b for each build. 1uf, 2.2uf,3.3uf, 4.7uf are values I use for V1b depending on other things in the amp. It's easy enough to experiment with.

Chuck

[doctorvoltz]

Thanks guys. Question for Chuck. I'm working on a SF Super Reverb that follows the AA270 schematic layout. The owner of the amp usually plays out of the vibrato channel. I see on the schematic that V1b and V2b share the same bypass cap. So I guess I will try 4.7mf caps on V1a and V2a and a 4.7mf cap on V1b/V2b a la Dumble for starters. Does this sound correct?
Thanks again.

[guitician]

Does the amp do this played at low volume, or cranked, or both?

[doctorvoltz]

It starts doing it at moderate to high volumes but not consistently.

[guitician]

OK, look at the plate voltage on a scope and see if your getting anything unusual like occillation or spikes, etc.

[Chuck H]

Thanks guys. Question for Chuck. I'm working on a SF Super Reverb that follows the AA270 schematic layout. The owner of the amp usually plays out of the vibrato channel. I see on the schematic that V1b and V2b share the same bypass cap. So I guess I will try 4.7mf caps on V1a and V2a and a 4.7mf cap on V1b/V2b a la Dumble for starters. Does this sound correct?
Thanks again.

Thats not exactly correct. In the Dumble and most other BFish type amps other than actual Fenders the cathodes are separate, no shared cathodes. You only wan't to change the bypass cap value on the first two preamp stages. All currently shared stages would have their own cathode circuit. Use 1.5k cathode resistors throughout for all the newly separated stages. For the cathodes who's bypass cap value you are NOT reducing just use the standard 22uf value.

Chuck

[doctorvoltz]

Hi Chuck:

Let me rephrase what I'm saying. On my particular amp the 1st 7025 preamp tubes of both the normal and vibrato channels have separate 25 mf caps on their input cathodes (pins 3) and share the same 25 mf cap and 820 ohm resistor on the return cathodes (pins 8). Are you saying that in order to use the lower value caps I need to modify this circuit with separate 1.5k resistors in place of the 820 ohm resistor and a separate 5 mf cap for each? Is this the way this mod is typically done on Fender BF and early SF circuits?

[Chuck H]

Ok...My last post was a very general description. I just looked at the AA270 schem and what you propose should be fine.

And yes, this mod usually includes changing to separate cathode resistors. But it really isn't needed at all unless you intended to voice each channel differently.

Typically with shared cathodes you would half the resistance and double the capacitance (roughly). So if you wanted to tell your customer that your installing "Dumble" part values you would need to change V1a bypass cap to 4.7uf and the V1b/V2b bypass cap to 10uf, still using the 820R shared resistor. Although IMHE 4.7uf would be fine, maybe better for V1b/V2b.

Sorry for the confusion.

Chuck