New project

[gui_tarzan]

I figured it would be easier to build what I originally wanted from an existing plan than it would be to totally modify something so here is a back-to-basics rig. I think I got everything covered so here is a fixed bias Fender Bassman 5F6-A take-off with a couple of modifications based on recommendations from several of you (to me and others).

The green stars are my marks, double-checking my circuit. Can you guys see if I missed anything before I fire it up and check voltages? I've been over the schematic & boards twice, once forward and once backward and it looks like I got everything but like they say, the more eyes the better.

[Tom Phillips]

First impressions.
The bias supply filter hookup is incorrect. There is no ground reference connection.
Since this is a new build I would also include a bias adjustment pot.
Can't tell the connections of your grounding scheme from the layout diagram. Are you following the advanced grounding guidelines discussed in other threads or trying to duplicate the scheme that Fender originally used?
Cheers,
Tom

[gui_tarzan]

Corrected pic in first post. I did leave out the grounding, sorry.

[gui_tarzan]

Like this? I cleaned up the power supply connections as well and notated the drain resistors that I forgot on the first pic.

[Tom Phillips]

The adjustable bias supply you added will work. However, you need to check the voltage on the bias filter cap. It could easily be higher than 50 V so I'd recommend a 100V rated cap.
Cheers,
Tom

[gui_tarzan]

Yeah, I have the 150v cap for that. How does the rest of it look? I updated the pic again to reflect that cap.

[Tom Phillips]

Looks OK as a circuit cloning the Fender layout and grounding.
My comments are:
1) I'd use more than 8uF bias filter. Like around 50 to 100uF. Fender used 8uF but they also used a 2 stage filter.
2) Re-check your input circuit. It's hard to trace a layout and imagine the schematic which is the real circuit. The attached schematic shows what's really happening with the switching jacks.
3) First turn on is best done without tubes installed. Make voltage checks set bias voltage to max negative then add rectifier, check again, add power tubes etc.

[gui_tarzan]

Comparing the 8uf to 50 or 100uf - some say 100uf will kill the response and make things sound muddy. Doesn't this cap just ensure stable negative voltage and if so, would 8uf not be enough?

[J M Fahey]

1) cap value will not affect frequency response in any way, so it can't make it muddier.
2) 100uF much better than 8uF, simply because that PSU (that's what the bias supply is) will have less ripple.
That you don't hear bias ripple in a balanced amp does not mean it's not there messing things up.
And that ripple will modulate useful audio signal.
Do you think you need a 60 Hz bias modulation Tremolo "on" all the time?
No? ..... thought so

[Leo_Gnardo]

Comparing the 8uf to 50 or 100uf - some say 100uf will kill the response and make things sound muddy. Doesn't this cap just ensure stable negative voltage and if so, would 8uf not be enough?

Yes I've heard that canard & even seen it on this website. "Some" are wrong. There's absolutely no harm in having a well-filtered bias supply. OTOH if you like to hear background hum plus lots of gargle along with your notes, go ahead and skimp on your bias filter, and while you're at it cut the values of your hi voltage filters to the minimum.

[gui_tarzan]

Sarcasm. I like it.

When I fired it up it made an awful sound that reminded me a lot of a cross between a thhhhhpppppppppp and a fart. I'm getting really discouraged...

[tedmich]

I'd build the vanilla Bassman (like countless millions before you) get it running like they all do and then add in the mods one at a time. Stop shy of the fart mod.

[gui_tarzan]

The simplicity of this circuit is why I chose it, but maybe I should look at the Champ circuit instead. Other than moving the PS caps to a separate board and adding the cathode piece on the power tubes & bias control I didn't change the main circuit at all. Besides upping the cap values of course. The rest is bone-stock according to the schematic.

The original had the pins 1 & 8 on the power tubes grounded, could adding the resistor and cap possibly be my source of the raspberry? It sure doesn't seem like it. I will check voltages with the tubes out tonight but this transformer pair came out of a working Hammond organ amp so I doubt that's the issue.

[g1]

The original had the pins 1 & 8 on the power tubes grounded, could adding the resistor and cap possibly be my source of the raspberry?

It certainly could. It looks like you have pin 8's grounded through a 47K ?
47K wouldn't even work for cathode bias, but you are fixed bias, so ground pins 1 & 8 and see how it sounds.

[gui_tarzan]

I saw that on the schem but it doesn't sound right. I'll check when I get home tonight.