Fender Bassman circuit questions

[Eric_Allard]

I was recently restoring a Twin Reverb and a few people here helped me troubleshoot a few issues (Thanks!). The amp is now restored and sounds absolutely beautiful. Here are a few photos if anyone is interested: https://plus.google.com/photos/11359400 ... 8097595905

I'm now restoring a Fender Bassman and hope the experts could chime in with answers to a few of my questions about this amp.

My Bassman is a 1969 and has the AC568 circuit. I have already removed the 150 ohm cathode resistors and converted the bias supply to match the AA864 circuit. Also, I have removed the suppression caps on pin 5 of the power tubes. These two changes have improved the tone tremendously already, but I feel that more can be done. Of course, I have replaced the original 45-year-old electrolytic capacitors.

I find that compared to the Twin (or most other Fender amps I have heard), I find the Bassman's tone to be rather "raw", "stiff" and a little more "high gain" than I prefer. Is this normal for a Bassman? I tried changing the tube in V3 to a 12AT7 and that seemed to help a bit, but it's still a little more "gainy" than I prefer. I also find the the volume seems to jump up significantly quite low on the volume knob in the "normal" channel. Could this indicate something amiss? I notice that the AC568 seems to have higher voltages than most other Bassman circuits I have seen. Might that have some effect?

Comparing the many Bassman circuits, I notice many differences and hope you guys can help me choose the modifications that will suit my purposes best. I am primarily a bass player, but also own a few guitars and would like to use this amp for both (I am aware that it's not powerful enough to be a very good gigging amp for bass). As a result, I cannot completely compromise one function or the other.

First: the phase inverter and negative feedback loop. What would be the effect of changing the phase inverter and negative feedback loop circuit to the AA864 design? Higher or lower gain? Better distortion characteristics? From what I have read, I think that using the AA864 phase inverter circuit would improve the guitar sound, but would it compromise its utility as a bass amp?

Second: the local feedback resistor around V2 There is a 470kohm local feedback resistor around V2. What is its purpose? What would be the effect of removing it?

Third: The coupling caps. This amp has the so-called "dog turd" capacitors. Is it worth replacing them? I replaced them in the Twin I was restoring because several were leaking DC. In the Bassman, I only found one leaking DC and it's in the phase inverter which will likely get some modifications. I have the necessary parts, but won't bother changing them if it's not necessary. For the cost of a few capacitors, however, I will put them in if it improves the sound.

Thanks for the help and your patience.

Regards,
Eric

[Eric_Allard]

I just read an article and think I have an answer to one of my questions.

In changing the bias supply circuitry, I changed the phase inverter coupling caps from the stock 0.047mfd to the blackface 0.1mfd value. I think that this change should be only be be done in tandem with rewiring the remainder of the phase inverter and negative feedback circuitry. This may be causing the excessive gain. Is this correct?

[fevzay]

Hi Eric,

I am new here, so I'm going to limit my answer to something that I know for sure. I bet other more experienced people can help with all of the fine details.

Regarding the extra gain, the best thing to do would be to convert the preamp back to AA864 as well. This bypasses V2 in the Normal channel. So there's an "extra" gain stage after the preamp in the AC568 that the AA864 doesn't have. This is a simple fix, so you could reverse it if for some reason you weren't sold on it; however, after reading your posts I think you should give it a shot.

Good luck! I've recently acquired a handful of BF and SF Bassman heads that are on deck for restoration/modification. Enjoy!

jordan

[Firestorm]

V2 is a mixer stage and the local feedback sets the gain of the stage (mostly the ratio of the series resistor to the feedback resistor). If you remove the feedback, the gain will become unuseably high.

[5dollazz]

Yeah removing that 470k local negative feedback resistor will not remotely reduce gain. I personally like what it and v2 in general do for a guitar. That stage definitely adds some gain and compression, but I think that's part of the charm of a bassman, particularly for grindy rhythm guitar. With medium hot humbuckers my amp starts to break up around 3 on the dial. By 5 it kicks ass. I find plenty of clean sounds to get out of it, but my volume pot is touchy. Most of the time it's on the verge of break up. I know I'm not special tho - I think everybody probably likes amps on the verge of break up.

I restored my AB165 bassman a couple years ago and read a lot about that circuit (which has some significant similarities to AC568) in comparison to the AA864 circuit and tried them out, though I am soooo far from an expert. In the end I kept mine stock AB165 (excepting the mod to adjustable bias) because I just liked the way it sounded. Frankly AA864 wasn't that different than my bandmaster. Beefier than the bandmaster to be sure, and looser than AB165, but I like the uniqueness of the bassman tone in my fender stable and I figured I'd embrace that oddball whole-hog. It's its own thing which I think is cool when I consider that my few other fenders are so closely related. Sorry, I'm not exactly trying to dissuade you from going AA864, I just remember being inundated with info saying that AA864 was "so much better" than any other blackface and beyond bassman circuit. I totally got swept up in that thinking (too much time reading the internet probably) and had to check myself before I wrecked my amp. Self. Good luck with the restore, that Twin sure is shiny. Nice.

[Eric_Allard]

Thanks for the replies. I've since found one of the coupling caps in the bass channel is leaking DC pretty badly. I also read that a leaking coupling cap can cause excessive gain in the following gain stage. I'll change it and report back. Another symptom is that the deep switch makes an unnerving "pop" when engaged and the volume control is scratchy (despite having been cleaned and lubricated).

One more question: Coming off pin 1 in the guitar channel preamp tube, there is a 500 pf capacitor in parallel with the plate resistor. If I understand basic RC circuits correctly, that would be bleeding off all frequencies above about 3.2 kHz to ground. Is this correct? Is it safe to assume it would be a good idea to remove it?

Following that same logic, a similar RC network is in the bass channel except the capacitor has a value of 0.01mfd. This would mean that it's bleeding frequencies above about 160 Hz to ground. Is this correct? Has anyone tried replacing this capacitor with a smaller value, say 0.001MFD or 500PFD to increase the rolloff frequency? I like a fat bass tone, but it's pretty "woofy" at the moment.

[5dollazz]

That cap across the plate resistor is essentially a shunt to ground for AC. I forgot about that. Mine apparently never came with it on the guitar channel (it's a very early 1965), but does have one on the bass channel. I found that I liked the amp more without that cap for guitar. Sparklier. I can't speak to bass tones, but my buddy the bassist loves the tone of the bass channel (which has .01) for recording.

[Eric_Allard]

I've been experimenting with this thing all day and it still doesn't sound right to my ears.

I have the bias supply converted from the mixed cathode / fixed / balance circuit to that found in the AA864. I also converted the phase inverter to the AA864 specs. The bias mod is a no-brainer. From what I have read, the phase inverter mod should allow breakup to come on more gradually and be more pleasing than with the silverface phase inverter circuit.

I found this discussion that appears to indicate that I should go back to the 47k resistors and the .047MFD caps in the phase inverter coupling stage, but much of it is over my head:

http://www.blueguitar.org/new/schem/fen ... bmtips.pdf

Based on the above, would it make sense to go back to the 47k resistors and the .047MFD caps in the phase inverter coupling stage?

Thanks for the patience. I previously restored the Twin Reverb, but this is my first attempt at modifying an amp.


Edit:

I have since swapped the parts I took out in these locations back in and it appears to have made a huge improvement in terms of volume and clarity.

On another note: I have no equipment to test capacitors. I usually just check if they're leaking DC to the other side. For the Phase inverter / power amp coupling caps, there's DC on both sides so my usual criteria won't work. The caps are black molded ones made by GE. Do they have a tendency to go bad? Should I replace them or just leave them?

[5dollazz]

Hey, I can't answer all your questions, but I know a lot of people think the 47k plate resistors on the PI are something the Fender actually got right in the silver face years. I've got a SF vibrolux that I tried both the BF and SF values in and preferred the SF. Glad to hear you're making progress!