70s ODS with footswitchable FET

[martin manning]

The layout shows 820R feedback resistor and 100R to ground. That divider sets the feedback ratio, so you're reducing the NFB by a factor of ten. Accent puts 1u across the 100R, and that combination is what dominates the shape of the high frequency boost. Your 1k trimmer will lower the frequency where it comes on substantially, a factor of 5. I'd use a 100R trimmer and connect the accent cap to the wiper.

[Matt J]

The layout shows 820R feedback resistor and 100R to ground. That divider sets the feedback ratio, so you're reducing the NFB by a factor of ten. Accent puts 1u across the 100R, and that combination is what dominates the shape of the high frequency boost. Your 1k trimmer will lower the frequency where it comes on substantially, a factor of 5. I'd use a 100R trimmer and connect the accent cap to the wiper.

Gotcha, I'll check my parts pile again for a 100r trimmer, if I don't have a 100r trimmer I do have a 100r resistor I can run parallel across the 1k trimmer for ~90.9r so I could adjust that for taste with the accent cap engaged and off and solder a permanent value in from there.
As always, Martin, thanks for your input!
You, sir, are a scholar and a gentleman!
Matt J.

[Matt J]

Decided to stick with the stock 100r from the design and removed the 1k trimmer. I'll see how things are from there once the amp is done and see if I need to tweak it.

Finally found time to tidy up the deep switch wiring, install the ground bus wire, and connect all the shielded cables. Going to try to wire up the preamp board after Easter.

Matt J.

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[GAStan]

Looks good so far with the exception of the mounting bolts for the Output Transformer. They look too long, like they will be too close to the boards when installed. I'd either cut down the bolts you have or find shorter ones to use.

[Matt J]

Looks good so far with the exception of the mounting bolts for the Output Transformer. They look too long, like they will be too close to the boards when installed. I'd either cut down the bolts you have or find shorter ones to use.

Good catch!
Im waiting on some shorter #8 bolts to arrive in the last order of parts to swap those out.

Oh, and I decided to omit the FX loop and plugged the holes. I don't really use effects that often and prefer to run them in the front of the amp.
Matt J.

[martin manning]

Oh, and I decided to omit the FX loop and plugged the holes. I don't really use effects that often and prefer to run them in the front of the amp.

Those can be handy for troubleshooting this or another amp, but it's true that a bad switching contact on the power amp in jack can cause problems.

[Matt J]

I had an illness, then death, in my family so I have been away from building for a while.

Had a chance to start back on this the other day. I redid the wiring around the output tubes, removed the bias probe jacks (I don't generally use them to bias anyway) and wired up the PT primaries and secondaries for a power test.

No smoke or issues on start-up, first through my current limiter then bypassed. At 120 VAC from the wall voltages were all what I expected with no load connected the B+ line.

Martin, I used this as an opportunity to check out the bias sweep since I'm using your updated bias design. Bias pot sweep went from -39.11 VDC to -47.44 VDC.

I'm going to work on wiring up the preamp board next. I'm still debating whether to leave the 50K od trim on the board or move it like the FET trim to one of the holes meant for the FX loop.

Matt J.

[martin manning]

Martin, I used this as an opportunity to check out the bias sweep since I'm using your updated bias design. Bias pot sweep went from -39.11 VDC to -47.44 VDC.

Thanks for the feedback. Depending on where your B+ lands you might need to shift the range down (more negative). The ~8V sweep is good, but a smaller input resistor might be needed to center the bias range.

I'm still debating whether to leave the 50K od trim on the board or move it like the FET trim to one of the holes meant for the FX loop.

In my experience, once you find out where you like, it you'll tend to leave it there.

[Guy77]

I had an illness, then death, in my family so I have been away from building for a while.

Had a chance to start back on this the other day. I redid the wiring around the output tubes, removed the bias probe jacks (I don't generally use them to bias anyway) and wired up the PT primaries and secondaries for a power test.

No smoke or issues on start-up, first through my current limiter then bypassed. At 120 VAC from the wall voltages were all what I expected with no load connected the B+ line.

Martin, I used this as an opportunity to check out the bias sweep since I'm using your updated bias design. Bias pot sweep went from -39.11 VDC to -47.44 VDC.

I'm going to work on wiring up the preamp board next. I'm still debating whether to leave the 50K od trim on the board or move it like the FET trim to one of the holes meant for the FX loop.

Matt J.

I do a cool little trick with my 50k Overdrive Trim pot. I switch them out to normal pots and place them right below the edge of the preamp board (close to where the regular small trim pot would be on the board) and then you can reach under the back of the amp if you ever want to adjust it!

This way you dont need to label anything extra on your rear faceplate either.

Cheers
Guy