70s ODS with footswitchable FET

[Matt J]

Hello everyone

I've been slowly putting together a 50watt 70s circuit ODS with one of Erwin's chassis since late last year. One thing for this amp I am including is a footswitchable FET circuit.
Compared to wiring an OD or PAB relay, this is my first time wiring up a FET circuit like this and I would appreciate some feedback.
I have wired everything according to Martin's diagram (which I am enterally thankful for, btw!). It may be hard to discern everything from the attached photos though because of the crowded space.
The only thing missing is using a push-pull pot for the pedal/manual switch since I only want to control it via footswitch and didn't have that value 10k push-pull pot on hand anyway.
I would like to see if there is anyway to test that the relays signals are working properly before I progress any more. Finding errors later on would involve resoldering in an already crowded area of the amp.
I've done a power test to see that the relays were getting the correct power from the relay rectifier/filter board. Everything worked as it should, and the footswitch LEDs worked as designed.
Any further tests I should do before progressing on? The actual FET board does not have any power do it but if there is a way to see if the relay is properly switching between sending the normal signal and the FET signal to the V1 grid I would love to try it.

Thanks!
Matt J.

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[martin manning]

Pretty easy to adapt the drawing to footswitch only... just forget about the black and purple leads going to the pull switch, and run the orange wire (relay coil lead) to the footswitch FET pin, and I assume that is what you have done.

To check the function, I would put a 1/4" plug into the input jack and verify that
When the relay is energized
- You have continuity from the input jack tip to the FET board input and 3M to ground from there
- You have continuity from the FET pot wiper to the lead going to the V1a grid and resistance to ground from there depending on FET gain pot setting
When the relay is not energized
- You have continuity from the input jack tip to the lead going to the V1a grid and 1M to ground from there
- The FET input is grounded

[Matt J]

Pretty easy to adapt the drawing to foottswitch only... just forget about the black and purple leads going to the pull switch, and run the orange wire (relay coil lead) to the footswitch FET pin, and I assume that is what you have done.

To check the function, I would put a 1/4" plug into the input jack and verify that you have continuity from the tip to the FET board input and 3M to ground from there and 1M to ground from the lead going to V1a grid when the relay is energized. When it's not energized Check that you have continuity from the input jack tip to the lead going to the V1a grid, and 1M to ground from there.

Thank you very much, Martin!
I followed you guide. Everything came out ok as you described, but I did notice that when the FET is on I get continuity to ground on the V1 grid lead. I double checked that there wasn't a short, and it is not there when the FET is off. Should this be?
Matt J.

[martin manning]

No, doesn't sound right. Note I edited above...

edit: oops, when the FET is engaged the V1a grid resistance to ground should be set by the FET gain pot. Maybe it was turned all the way down when you checked?

[Matt J]

Found the error!
I had miswired NO1 and NC2 on the relay board so that was causing the ground issues.
Rewired the board and checked everything again. All is as you described it should be!
Thank you very much, Martin! I will post more pictures as I work on it further.
Trying some Pacific transformers with this one, interested to hear how they come out.
Matt J.

[Matt J]

Worked on wiring up the pots and control switches today. I'm always a bit antsy with wiring the deep switch since I'm used to wiring up the later mid switches instead. Followed everything per every diagram I've seen of circuits from the time. I've decided to go with the 3rd Gen circuit with a 250KA treble pot and omitted the .002uf cap on the bass control. From reading on her other people's accounts of wiring this circuit that seems to make the EQ a little more interactive.

Martin, I'm using your updated bias design for this well.

Matt J

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[martin manning]

Martin, I'm using your updated bias design for this well.

This is a good idea in light of the bias pot failures Raoul had. Let's see if the voltage range comes out right ;^)

[captaincoconut]

Curious why there are IC sockets installed on the relay boards?

[jabguit]

Curious why there are IC sockets installed on the relay boards?

easier to change a bad relay.

[captaincoconut]

Curious why there are IC sockets installed on the relay boards?

easier to change a bad relay.

Well yes, obviously. That's the point of sockets. Thought there might be some deeper thought behind it.

[erwin_ve]

Curious why there are IC sockets installed on the relay boards?

easier to change a bad relay.

I never had a bad relay, I have had bad sockets after multiple in/out while testing.

[Matt J]

Curious why there are IC sockets installed on the relay boards?

easier to change a bad relay.

That is exactly why. In a dozen years, I've encountered two faulty relays out of the box, and having to completely remove one that was soldered directly to the board was enough for me. I build a #183 about 9 years ago and the PAB relay had issues. It would work intermittently, then stop or stay in the PAB 'on' position. I thought the relay board might have had an issue. I had a spare so I desoldered the original relay and added it to the new board, same issue. Tried a new relay on the original board to see if it was the old one after all. That fixed every thing, but only after a lot of aggravating desoldering. Decided them to IC sockets even though they have their own risks.
Sure enough, last month I was working on a high plate classic like #94 and the OD relay was having issues. Either a weak OD signal would pass or it would just cut out after a few seconds. Popped a new relay in the socket and it fixed it. Just to double check it wasn't the relay board I tried the one I expected to be faulty in the PAB relay, did the same thing.
I threw the faulty relay away but wish I had kept it now to try to test it somehow. I don't know if there's been discussion of a 'relay test before installing procedure' or building some sort of jig to do that but it would be an interesting project. (I mean, who among us here doesn't like building thing?)
Matt J.

[Matt J]

Did some test fitting of the circuit boards last night the make sure everything is fitting ok. I wanted to double check everything before I wired up the shielded cables and installed the buss wire. A tight fit in a few places but it gives me warning to what to keep an eye one once their leads are installed and they ready to be permanently wired in.
Notice that I did add three time pots to the board: 1) A trimpot on the entrance to the V2 grid as shown on one of Martin's board designs to fine tune to 'funky OD entrance' to my taste, 2) A PI balance pot because I've really come to like having them, and 3) A 1K resistor instead of the fixed 100r resistor in the PI tail. This way I can fine tune the amount of 'accent' that gets engaged with the accent switch.
I briefly thought about drilling panel holes for these but decided against it since I'm building this for myself and will mainly use it for overdriven tone, plus I really didn't want to dig in any more into this really nice chassis I got from Erwin!
BTW, Ewin, I still have your boards for this, BTW. I'm saving them for a project my bother wants me to work on.
Matt J.

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[martin manning]

3) A 1K resistor instead of the fixed 100r resistor in the PI tail. This way I can fine tune the amount of 'accent' that gets engaged with the accent switch.

You mean a 1k trimmer, I believe. I would use a 100R trimmer and wire it up like the old scratchy presence pot circuit. That way it will preserve the NFB ratio.

[Matt J]

3) A 1K resistor instead of the fixed 100r resistor in the PI tail. This way I can fine tune the amount of 'accent' that gets engaged with the accent switch.

You mean a 1k trimmer, I believe. I would use a 100R trimmer and wire it up like the old scratchy presence pot circuit. That way it will preserve the NFB ratio.

Correct, 1K trimmer. Would that be too much or should I just focus between the 0 to 100r range and put a different one in?
Thanks!
Matt J.