Bassman rebuild/hotrod

[clarkleach]

NFB is the least of my worries at this point.

[Randall]

What part of Maine?

[clarkleach]

What part of Maine?

Born and raised in the Farmington area. Lived all over from Sanford to Orono...

[Enzo]

I spent a couple days in Rumford in 1969. I watched the first moon landing on TV there.

[clarkleach]

Here's the schematic

I can finally post attachments! So here goes nothing.
I'm going to try to attach the schematic.

bassman-lefty_special-2.pdf

[Enzo]

C3 in power supply is wired backwards.

Bias, since you don't have individual tweaks or a balance, may I suggest, instead of two 1 ohm resistors above the cathode resistor, perhaps a single 1 ohm between the cathode resistor and ground. I know you lose the ability to probe individual tubes - other than pulling one at a time. So just my opinion. I don't care for the "floating" test points.






Got a couple friends from Capac I've known for 55 years.

[clarkleach]

C3 in power supply is wired backwards.

Derp... that's because I copy-pasted from the bias supply.

Bias, since you don't have individual tweaks or a balance, may I suggest, instead of two 1 ohm resistors above the cathode resistor, perhaps a single 1 ohm between the cathode resistor and ground. I know you lose the ability to probe individual tubes - other than pulling one at a time. So just my opinion.

I was actually contemplating adding a balance pot.
Moving that resistor there would accomplish what? Other give me a headache rearranging the switching circuit..

I don't care for the "floating" test points.

Not sure what you're referring to.

Got a couple friends from Capac I've known for 55 years.

I know roughly where that is. Only been here a year and most of that in the basement laboratory/studio.

Any thoughts on the Channel A pre-amp circuit?

[Enzo]

Capac, a tiny town half way between you and Sarnia.

My 1 ohm won't complicate anything. In fact you could leave the two up at the tube too if you wanted. My 1 ohm would go... Well follow left from R44 and C28 until you hit the vertical line to ground. From that junction, insert the 1 ohm to ground. In fixed bias it is your sensing resistor. In cathode bias, the extra one ohm won't upset the operation at all, and still serves the same sensing function.

By floating test point I mean they are not referenced to ground. Well, they are in fixed bias mode, but not in cathode bias mode.

Channel A? V2.2 has no grid return resistor. Needs one. Just my impression, but that 470 dropping resistor at the input is lopping a third of your signal off before it gets amplified. C11,12 ought to make it super bright. C12 seems kinda large for a brightness cap, but may I assume you have dummied up this circuit for a listen?

There is no wrong if it sounds good, but generally we want the first stage to take the weak guitar signal and boost it up above the noise floor before we get serious about gain. That is why you rarely see a volume control before the first triode.

[clarkleach]

My 1 ohm won't complicate anything. In fact you could leave the two up at the tube too if you wanted. My 1 ohm would go... Well follow left from R44 and C28 until you hit the vertical line to ground. From that junction, insert the 1 ohm to ground. In fixed bias it is your sensing resistor. In cathode bias, the extra one ohm won't upset the operation at all, and still serves the same sensing function.

R44 and C28 are not in the circuit in fixed bias mode. Thus, putting a resistor in series is pointless, no?

By floating test point I mean they are not referenced to ground. Well, they are in fixed bias mode, but not in cathode bias mode.

Still don't know what you mean by "test point".

Channel A? V2.2 has no grid return resistor. Needs one. Just my impression, but that 470 dropping resistor at the input is lopping a third of your signal off before it gets amplified. C11,12 ought to make it super bright. C12 seems kinda large for a brightness cap, but may I assume you have dummied up this circuit for a listen?
There is no wrong if it sounds good, but generally we want the first stage to take the weak guitar signal and boost it up above the noise floor before we get serious about gain. That is why you rarely see a volume control before the first triode.

I don't know what a grid return resistor is... This channel is pretty much a Marshall 2204/JCM800 pre-amp "low channel" No more triodes available.
I'm not a big fan of attenuation before the first stage (just another guitar volume knob) but that's how they done it...
I have not breadboarded this, my high-voltage breadboarding facilities are very limited.

[Enzo]

R44 and C28 are not in the circuit in fixed bias mode. Thus, putting a resistor in series is pointless, no?

Look at your circuit. They are always in the circuit, you just either use them or short across them. In any case, that last portion of vertical line to ground is the ground path for either setting. I realize the actual wiring might not look like the schematic, but the idea is to find the final connection to ground and insert a 1 ohm there. In fixed mode, this 1 ohm is in the circuit. In cathode mode, the 1 ohm is in series with the R44. It is just a suggestion, I am not married to the idea.

OK, sorry I am unclear. You have two 1 ohm resistors, and I proposed a single one. But either way, you intend to use it to measure tube current. SO the ends of the resistors are test points. In some amps they wire little jacks on the rear panel for your meter. That would be actual physical test points. But in many schematics, like Fender, test points are just nodes in the circuit where test measurements are made. So my "test points" are I guess imaginary, or implied. So: my test points are the ends of the 1 ohm resistors, wherever you have them.

In my single resistor idea, imagine a circuit thus: cathode, R44/C28, 1 ohm, ground. And shorting switch across R44. That is essentially what you have now, except your 1 ohm is on top instead of bottom, and you used two.

A grid return resistor is the resistor from grid to ground. Right now V2.2 grid has no path to ground. V2.1 has one in the form of the PRE control. V1.1 is a special case as a cathode follower, but R12 does the job. V1.2 has R25. Two sides of V3 have either resistors or a volume control. You will find every triode except the cathode follower has a resistor from grid to ground. Either a resistor or a control. COntrols are just variable resistors. The 2204 has the pair of 470k resistors to cut signal level in half going into that triode. If you don't want that reduction, just put the 470k to ground from that grid.

By breadboard I was just thinking about sending a signal through the couple resistors and caps to see how they affect tone. No voltage needed.