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

[Matt J]

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.

I'll play some with the bias input resistor and see what results I come up with. Currently I have a 330r in place. I have several 100r metal films to use but with try to find a ~250r in my stash or parts as well.

Martin, I will measure the results and report back. Thank you again for sharing this design!
- Matt J.

[Matt J]

I also just noticed the preamp socket were installed in the wrong direction: The side with the heater junction between pins 4 and 5 should be facing downward!

[Matt J]

Reinstalled the preamp sockets and switched out the 330r bias dropping resistor to a 100r.

Bias now swings form -49.67 VDC to -41.54 VDC so still roughly a ~8 v swing.

Martin, should I stick with that or try adjusting the 15k resistor to ground off the bias pot?

Thanks!
Matt J.

[martin manning]

I would go with that and see if it’s workable. You could reduce the input resistor further, to 47R say. Increasing the resistor going to ground is not ideal. It will reduce the voltage range since the 10k of adjustment is fixed.

[Matt J]

Spent time wiring up the preamp board and sockets this weekend.

Before going any further, I've been double checking several points with my continuity meter to make sure there's not an accidental connection to ground somewhere.

Everything has been ok so far, except for the output jacks. I keep getting a connection to ground at the positive lug of the main and external jacks where the 4 ohm tap from the OT connects and the NFB line feeds back to the preamp board.

I get the same result when I install a dummy jack, so I wanted to double check on this before going any further. I originally thought the extension jack being a Switchcraft 12A might have something to do with it, so I switched it out with a Switchraft 11.

I know the solution is probably right in front of me, but the more eyes to double check my work, the better!
Matt J.

[sluckey]

Everything has been ok so far, except for the output jacks. I keep getting a connection to ground at the positive lug of the main and external jacks where the 4 ohm tap from the OT connects and the NFB line feeds back to the preamp board.

No problem here. That's perfectly normal. Your meter is reading the very low resistance of the OT secondary winding which is just a fairly short length of fairly heavy wire. Put your Switchcraft 12A jack back in and wire the switch lug to the sleeve(ground) lug.