Issue With Bassman 5F6A Build

[Guitarnut]

Since I'll be using this amp for playing guitar...one at a time is all I'm capable of...can I just simplify things by using standard jacks and hooking up the mix resistors like so?

[img:800:600]http://www.crenshawweb.com/bassman/input_jacks.jpg[/img]

[Phil_S]

You can hang them on the jacks, on a turret board, or on the tube sockets. The treatment you show is typical. I think I'd call them grid stoppers. An amp can be built without them. AFAIK, they are there to reduce noise.

[Guitarnut]

...The treatment you show is typical.

Interesting. The Weber and Mojotone schematics use switching jacks.

Peace,
Mark

[Firestorm]

They're partly grid stops, but mostly to isolate the guitars from each other if you plug more than one in. You should maybe add 1M grid loads (don't have to, tho). But why are you fiddling with the established circuit? You're having trouble getting a by-the-book build to behave.

[statorvane]

The way you have your schematic, the unused jack inputs aren't grounded when not in use. You'll get a lot of buzz that way. Use shorting jacks and wire them up like this:

http://www.18watt.com/files/high_and_lo ... tion_2.gif

[Guitarnut]

The way you have your schematic, the unused jack inputs aren't grounded when not in use. You'll get a lot of buzz that way. Use shorting jacks and wire them up like this:

http://www.18watt.com/files/high_and_lo ... tion_2.gif

Excellent explanation. Thanks!

The way Mojotone has it drawn is confusing.

Peace,
Mark

[martin manning]

V2 still looks ok, let's get V1 going first and then go after the PI. I'm wondering if there is a problem with the ground reference for V1a and b. I think I can see the 1M resistors on the input jacks, but verify that the jacks are wired like the picture below. I would put the 1M on the other side of the jacks, and I would run a wire tying all of the ground lugs together and connect that to the ground side of the V1 cathode cap and resistor on the board.

[Guitarnut]

V2 still looks ok, let's get V1 going first and then go after the PI. I'm wondering if there is a problem with the ground reference for V1a and b. I think I can see the 1M resistors on the input jacks, but verify that the jacks are wired like the picture below. I would put the 1M on the other side of the jacks, and I would run a wire tying all of the ground lugs together and connect that to the ground side of the V1 cathode cap and resistor on the board.

I couldn't work with the Mojotone drawing so this ^ is the one I followed. I tied the grounds together but didn't take them to the cap/resistor. I'll add that jumper and see where we are.

Thanks as always,
Mark

[Guitarnut]

I verified the jacks are wired to the drawing your posted. Each pair of jacks had the grounds jumped together. I added a jumper from one bright jack to one normal and from there a jumper to the ground side cathode cap/resistor. Before powering up, I checked for continuity from the ground bus to ground tabs on all 4 jacks. And neg side of cap/resistor to ground. All good.

Powered up and no change in voltages. Pin 6 194v and pin 1 119v. Cathodes are at 3.3v. I checked the ground side of the cathode cap and it's at zero.

Here's a thought. The cathode cap is what Mojo called a bipolar. It's 2 caps set opposite of each other in one package. neg-pos-pos-neg. So it doesn't matter which way it installs. Could this be an issue?

Peace,
Mark

[Guitarnut]

A curiosity as I look over the numbers. I noticed that V2 pin 7 shows a high voltage compared to the other two 9 pin tubes. Is this because it's a cathode follower and it's voltage is taken from the plate of the previous stage vs from a coupling cap?

Peace,
Mark

[martin manning]

Bipolar is ok. Check the resistance across that cap resistor pair. You should see pretty close to 820 ohm. Do you know that the 12AY7 is good? You could put a 12AX7 in there temporarily to see if that changes anything.

[martin manning]

A curiosity as I look over the numbers. I noticed that V2 pin 7 shows a high voltage compared to the other two 9 pin tubes. Is this because it's a cathode follower and it's voltage is taken from the plate of the previous stage vs from a coupling cap?

Peace,
Mark

Yes. There is no plate load so the plate is at B+, and the cathode will come up to a bit above the grid, which as you note is sitting at the plate voltage of the previous stage.

[Guitarnut]

Bipolar is ok. Check the resistance across that cap resistor pair. You should see pretty close to 820 ohm. Do you know that the 12AY7 is good? You could put a 12AX7 in there temporarily to see if that changes anything.

Thanks again Martin,

Thru this process I've swapped tubes in both V1 and V3. I've had 2 different 12ay7's and a 12ax7 in V1. At least 3 different 12ax7's in V3.

The R you wanted me to check is 790.

If you have other ideas please post them but I have to go to work and deal with some difficult clients to get my mind off of this.

More this evening.

Peace,
Mark

[Guitarnut]

Yes. There is no plate load so the plate is at B+, and the cathode will come up to a bit above the grid, which as you note is sitting at the plate voltage of the previous stage.

The cathode follower is a current amp instead of a voltage amp? It doesn't appear to be amplifying voltage.

Peace,
Mark

[Reeltarded]

Here, Mark..

http://www.ampbooks.com/home/classic-ci ... ltage-amp/

Two things I want to do in this life.

1. Discover something astronomical that freaks people out.

2. Figure out what the problem with this build is before the experts do.

My brain has been boiling for long enough to call it soup by now.


-Miles