Any suggestions for things to do with this amp?

[Ptron]

This is the schematic for an amp I built a few years back. It works fine and actually sounds pretty good at low gain settings, but as you can see, it has some goofy things going on. It also has a pretty limited range of useful settings.

The original idea was a 5F6-A Bassman with an SE output. I added a switch that paralled the first triodes, and later changed it to the cascade setup it is now. The last triode I think was my kooky idea of replacing some of the gain/mojo/whatev lost by not having a PI. It’s pretty useless; blocking distortion city.

I left R13 at 820 ohms out of clueless-ness when I split V1s cathodes. I should probably bump that up to 1.5k-ish.

It would be nice to have some sort of decent high gain available out of all those tubes. Right now, switching in V1B things get fairly ratty even with the 470/470 divider.

The tonestack seems not terribly effective to me although I’ve heard that cathode follower tone stacks can be that way. I don’t see anything wrong with how I have it hooked up.

The filtering/decoupling in the pre is left over from the original 5F6-A design (which I knew no better than to copy exactly). Should probably bump that up a bit, especially if I try to do anything gainy. It is stable as of now though.

Current voltages:
Va – 305
Vs – 260
Vd – 255
C - 255
B - 240

6V6 plate 255
6V6 k – 17.4

[txstrat]

You might have a look at the preamp section of this: http://www.schematicheaven.com/marsh..._100w_2203.pdf
It is sort of a further developed bassman circuit with a preamp and a master volume.
Compare to the values of your schematic and you can see, there are only minor changes to make, to get that preamp running like a Marshall MV/ Bassman. You could use only one input and add a switch to skip V1A.
Hope this helps.

[loudthud]

V3A will clip the positive side of the signal going to V4 when SW3 is in the up position. V3A needs a grid stopper or divider network. When SW4 is open, there is no grid leak path to ground for V4. V2A should have a grid leak resistor to ground to suppress the pop when SW1 is switched.

[Ptron]

You might have a look at the preamp section of this: http://www.schematicheaven.com/marsh..._100w_2203.pdf
It is sort of a further developed bassman circuit with a preamp and a master volume.
Compare to the values of your schematic and you can see, there are only minor changes to make, to get that preamp running like a Marshall MV/ Bassman. You could use only one input and add a switch to skip V1A.
Hope this helps.

Everything helps! I like that suggestion because as you point out, it's not that far off. I don't know if I like the way the the first vol knob precedes everything when V1A is bypassed though. Makes it sort of a redundant guitar vol doesn't it?

[txstrat]

That's right. But worth a try.

[Ptron]

Thanks Loudthud,

V3A will clip the positive side of the signal going to V4 when SW3 is in the up position. V3A needs a grid stopper or divider network.

V3A is pretty much just a poorly executed, bad idea from the get go (no coupling cap?) but since it's there, I may try to do something with it anyway. Maybe a cathode follower drive for the 6V6?

When SW4 is open, there is no grid leak path to ground for V4.

I saw that but I decided to throw out the schem as-is-warts-and-all first. I figured the tone stack must be acting as the grid leak path for now as long as V3A is switched out.

V2A should have a grid leak resistor to ground to suppress the pop when SW1 is switched.

I will add such a thing in assuming the topology remains similar.

[Ptron]

I finally did a couple preliminary things with the amp but nothing real exciting yet.

First of all, the original schematic had some mistakes, so just for the record, I included the correct original schematic. Turns out I did have a coupling cap on V3A. Also, the heater virtual center tap resistors are 100ohms, not 3.3k and the center is tied to the cathode of V4, not ground. This time I put all the voltages in the schematic. (funny side note: some of the voltages in the “orginal” schematic are wrong because after the mods, I was just about pulling may hair out trying to figure out how I could suddenly be getting 258V on Vs, 262V on node C, and 268V on node B. The voltage was going up! Turns out one of the LCD bars is out on my “good” meter causing 9s to look like 5s and 8s to look like 6s. $5 meter to the rescue.)

First I changed R13 from 820 to 1.8k so as to be more correct for a single triode. Seemed to make a small improvement. Then I beefed up the filtering. The first four stages all off one 8uf cap?! I see the 5F6A Bassman really was done like this. If this made a tonal change, it was subtle. The amp might be a little quieter now.

SW4 is built into tremolo level control (VR75) so for the meantime I just turn it on and leave all the way down. It definitely loads down the signal a bit that's not necessarily a bad thing.

Speaking of the tremolo, *ahem*, you'll notice I put a bias modulation trem in a single ended amp. It still trems but I assume it just moves the q point up and down. If the level is high enough it will start to push it against cutoff and saturation alternately. At least that's what it sounds like it's doing. It's an interesting sound but I may convert it to something more conventional by hooking it to one of the pre stages ala this or use a roach or something.

So as it stands, with V1B and V3A switched out, the amp starts to breakup at like 2. I almost always like a little dirt but that seems excessive. I suppose I’ll start by bumping up R13 even further, and maybe then moving R20/21 to the other side if the lead switch so the gain reduction happens weather or not the extra stage is switched in.

I was also going to complain that the amp sounds a little honky but I was running it through an unmounted Jensen P12R. I finally tried it through my Bandmaster cab and that took care of that (In fact it may have been a little boomy). Kind of embarrassing because I’m a believer in the speaker/cab is everything and wasn't paying attention to my own advice.

EDIT: OH yeah, this is the amp I reference in this thread where the tone controls hardly do anything. Any ideas on this?