New project

[Tom Phillips]

...It looks like you have pin 8's grounded through a 47K...

WOW! Talk about hidden in plane sight! I totally missed that.
The effect would be to pretty much keep the output tubes turned off so I don't know how it could be related to the awful sound your build is generating. Let us know what happens when you ground the #8 pins on the output tubes.

By the way, there is no need to ground pin #1. They were grounded in very old designs because the metal tubes of the day had the metal case connected to pin 1 so that the case served as a shield. In more modern Fender amps the #1 pin was used as a tie point since it was not connected to anything inside the tube. In fact, there wasn't even a #1 pin included in the glass 6L6 tube base.

Cheers,
Tom

[gui_tarzan]

Well I took that off (what the hell was I thinking?) and grounded pins 8 and I'm still getting a loud noise but it's less of a farting sound than it is just a loud oscillation that starts in the 3-400hz range and goes down, then when I kick off the power you can hear it going back up as it fades out. Bad filter cap maybe? I also tried a different set of tubes throughout.

[Tom Phillips]

...I'm still getting a loud noise but it's less of a farting sound than it is just a loud oscillation...

I suggest that you disconnect the power amp feedback. Do this by disconnecting the wire that leads from the speaker jack to the 27k resistor on the parts board. Then fire up the amp and report if the sound changes.

Does this mean you just turned the amp on with everything connected as the first try?
Do you have gut shots to post?
How are the power rail voltages and ripple %?

[gui_tarzan]

Here's the gut shot. I removed the wire at the speaker jack and the oscillation went away. I could only hear some faint 60hz hum so I plugged the guitar in and I have no sound which means I have a wire in the wrong spot most likely. I get some light buzzing in CH2 but not in CH1 and no sound plugged into either. I'll go back over my wiring tomorrow. Again. lol

So why would that feedback create such a horrific noise?

What do you mean "as the first try"?

Not sure about ripple but the voltages were 303v, 273v and 232v at points A, B & C respectively.

[Tom Phillips]

...So why would that feedback create such a horrific noise?...

This indicates that you had positive phase feedback.The solution is to swap the primary lead plate connections at pins 3 of the 6L6s OR swap the secondary connections to the speaker jacks. (You can do whichever is easier. This situation is not uncommon when using a salvaged OT that doesn't use the same color coding as the Fender part.

[g1]

OR swap the secondary connections to the speaker jacks.

Is this a solution that only works in some amps and not others? Or is it just because you can't flip the speaker jack connections when there are multiple impedance taps?

[Tom Phillips]

Is this a solution that only works in some amps and not others? Or is it just because you can't flip the speaker jack connections when there are multiple impedance taps?

I believe that the solution will work for any amp. With some extra hassle one could still flip the secondary leads on a multiple tap OT. However, that makes the process unnecessarily complicated. So...we are back to doing whatever is easier. Personally, I swap the primary leads since the wire color codes used there are less universal than those used for the secondary leads. At least with respect to black = common.

The posted photo helps but understanding this build was complicated by the fact that the discussion started with a marked up 5F6-A layout diagram instead of a schematic of the actual build. Since there are still problems the task at had is to find the wiring mistake or the faulty component. The feedback line can been reconnected after the OT leads are swapped.

[gui_tarzan]

Well I let this sit for a couple of weeks and took a look tonight to draw a schematic exactly as I have it laid out. Turns out I made a rookie mistake and connected what I thought was a bias tap wire (turns out this Hammond M-100 transformer doesn't have one) to the fixed bias circuit and it was actually the 6.3v center tap lead. The heater leads on this are brown and very heavy solid wire with a plastic cover just like the 5v leads, the wire in question is much smaller, stranded and has a blue-ish green cloth cover so I naturally ASSumed it was for the bias tap.

(banging head on desk)

I also flipped the OT primary leads as was suggested earlier in this thread (blue & brown) on the power tube sockets. I may end up switching them back as this may have been one of the problems the entire time. Now I need to come up with a new bias circuit since there's no tap, but it's late and I'm not going to work on it anymore tonight, I'll just end up making another STUPID mistake.

(grumble)

I know, I know, make the schematic before building.

[Tom Phillips]

Jim,
Do you have other test equipment in addition to your DVM?
Tom

[gui_tarzan]

I have a tube tester and a signal generator but no scope yet.

[g1]

No big deal with the bias, you can just take it off the high voltage winding. Have a look at the 6G3 Deluxe. You might have to tweak the value of the 100K & the 22K to get the bias level you want.

[Tom Phillips]

...I also flipped the OT primary leads as was suggested earlier in this thread (blue & brown) on the power tube sockets. I may end up switching them back as this may have been one of the problems the entire time...

There is an easy test you can do using your signal generator and your ears to determine if the feedback phase is correct. Lift the end of the 27k resistor on the parts board that is connected to the speaker jack. This disconnects the feedback signal as we discussed in post #18. Leave the end of the resistor hanging above the eyelet such that you can just push on it with a chopstick or other non conducting tool to make it touch the eyelet. The rest of this is done with the amp powered on so only proceed if you know you can do it safely. Connect the amp to a speaker, feed a tone to the input with your signal generator and adjust so you can easily hear it. The volume can be at any comfortable listening level. Now push on that loose end of the 27k Ohm resistor so that it touches the eyelet where it was originally connected. The volume of the tone will either go up or down. If it goes down the feedback is negative and all is well. You have verified that the OT connection is phased correctly. If the volume goes up when you reconnect the resistor then the feedback is positive and you need to reverse the OT leads to change to negative feedback. Note that, if the feedback is positive the amp may squeal or howl when you connect the resistor which is another indication of positive feedback.
Tom

[gui_tarzan]

Ok, I spent several hours yesterday and today drawing a schematic of this amp, exactly as it's laid out. While doing so I discovered a missing cap and three wires that were in the wrong place. Those are taken care of now and this is exactly what I have. Before I fire it up to check voltages I'd like you guys to check it out and see if I missed anything, or if I still have something wrong. If you don't mind.

[Tom Phillips]

Before you turn the amp on you need to correct the bias supply. Currently you have components shown but no input voltage and the filter cap across the diode.
Reference g-one's comments in post 26 for an idea of how to create a bias supply when your transformer does not have a dedicated bias tap.
Re-check your feedback connection and presence control. Reference the original Bassman schematic you started with. The feedback connects to the phase inverter circuit, not to the tone stack.

EDIT: Just noticed that the Ch 1 input is missing its 1 Meg grid leak resistor. Also with single jack inputs the grid stopper could be lowered to 34k. (Reference post #7 above)

When you power up the amp for the first time do it without any tubes installed and the feedback resistor lifter as I described in post #27.
Follow these steps:
1) Set the bias voltage for the most negative reading at the pin(s) 5 of the power tube sockets. You will have something like -55V with respect to chassis ground.
2) Install the rectifier tube and verify your B+ voltages. They will be higher than normal because there is no load on the power supply yet.
3) If all is well then power down, install the other tubes and power back up. Verify the B+ voltages and the cathode voltages and set the bias.
4) Do the feedback phasing test I described in post #27. Power down, reconnect the 27k feedback resistor and correct the OT wiring if necessary depending on the outcome of the test.

Do you know the output impedance of your OT? I doubt it is 2 Ohms like the original Bassman OT. Because of that you will probably want to increase the value of the 27k feedback resistor. More on that later after you correct the fatal problems mentioned above.

Cheers,
Tom

[loudthud]

Your schematic shows a center tap on the 5V winding connected to ground. This will kill the recitfier tube. Disconnect the 5V CT, insulate it with heatshrink tubing and stow it or connect it to an otherwise unused eyelet.