My take on a 2nd Gen ODS

[martin manning]

The more I have thought over, the more I am leaning towards changing it to have the separate masters for OD and Clean.
The design I have seen the most for this is in the attached PDF.
I have two questions on this;
1. Why is the clean master not connected to the ground bus? The outer (brown in this drawing) wire runs to COM 2 which, from Martin's relay layouts, should just have the OD entrance running to it.

It's grounded along with the OD entrance in Clean mode, and lifted to remove that load when Clean output goes direct to OD input.

2. Is there any benefit to switching from the 2nd Gen Deep Switch to the to the version from the 4th/5th eras as this design does? From trying it when this amp would work, I was not that impressed with the 2nd Gen's function.

I didn't think the 4th/5th had Deep... the switch was repurposed as Mid Boost.

I was about to say that to keep the 2nd gen's loading on the OD output, delete the 180k to ground and the 1Meg series resistor and go straight into a 250k pot for the added Ratio control.

[Matt J]

The more I have thought over, the more I am leaning towards changing it to have the separate masters for OD and Clean.
The design I have seen the most for this is in the attached PDF.
I have two questions on this;
1. Why is the clean master not connected to the ground bus? The outer (brown in this drawing) wire runs to COM 2 which, from Martin's relay layouts, should just have the OD entrance running to it.

It's grounded along with the OD entrance in Clean mode, and lifted to remove that load when Clean output goes direct to OD input.

2. Is there any benefit to switching from the 2nd Gen Deep Switch to the to the version from the 4th/5th eras as this design does? From trying it when this amp would work, I was not that impressed with the 2nd Gen's function.

I didn't think the 4th/5th had Deep... the switch was repurposed as Mid Boost.

I was about to say that to keep the 2nd gen's loading on the OD output, delete the 180k to ground and the 1Meg series resistor and go straight into a 250k pot for the added Ratio control.

Thank you, Martin! Should I pull the 180K and 1Meg resistor as the amp is now or is that for when it has been changed to the hybrid design? I'm wondering if that has been overloading the OD and causing it to cut out.
Matt J.

[martin manning]

Should I pull the 180K and 1Meg resistor as the amp is now or is that for when it has been changed to the hybrid design? I'm wondering if that has been overloading the OD and causing it to cut out.

The 250k pot would be for the 2nd gen but with added Ratio/Level control like #124. I doubt that your OD cutting out is a loading issue. It's more likely to be an intermittent grounding of the signal, such as a stray bit of shield braid touching the center conductor in one of the coax runs. Does the Clean channel work as expected?

[dbharris]

...
2. Is there any benefit to switching from the 2nd Gen Deep Switch to the to the version from the 4th/5th eras as this design does? From trying it when this amp would work, I was not that impressed with the 2nd Gen's function.
...

You will find most 4th Gen amps that still have a classic tone stack are also equipped with a deep switch. Though some have a mid switch function on a chassis with a deep switch label. I am not aware of skyline tone stack amps with a deep switch. But they very well may exist.

I find deep useful with a strat and mid useful with a les paul. But ymmv, either way if you don't like it you can always change the function later.

-Dan

[Matt J]

So I have played around a little more and I think I have narrowed it down to an issue in the preamp, possibly in the wiring of the FX send/return jacks.
I plugged into the return jack and got a nice clean signal, only affected by the Presence control. Plugging in to the input jack got nothing.
I desoldered the connections to the FX jacks and by passed it with a jumper clip, going from the wiper of the Master volume pot directly into the PI input. This gave me both and clean and OD signals, but the clean channel was very anemic and both were very noisy.
I haven't found any shorts yet or other issues. Does anyone see anything amiss with the FX loop wiring? The blue shielded cable feeds to the Master volume pot, the yellow to the PI input.

Matt J.

[dbharris]

Hi Matt,

Some of those solder joints on the jacks look a bit dry, but may just be the photograph. Have you tried inserting a patch cable in the fx loop jacks?

-Dan

[Matt J]

Hi Matt,

Some of those solder joints on the jacks look a bit dry, but may just be the photograph. Have you tried inserting a patch cable in the fx loop jacks?

-Dan

I reflowed the solder joints, and tried a patch cable, no luck. The power amp still gets a signal if I plug directly into the FX send or return, but nothing from the preamp.

Very odd. I've never had this happen before with a D-style amp.
Matt J.

[martin manning]

Have you made a voltage survey of the preamp?

[martin manning]

I was about to say that to keep the 2nd gen's loading on the OD output, delete the 180k to ground and the 1Meg series resistor and go straight into a 250k pot for the added Ratio control.

...Should I pull the 180K and 1Meg resistor as the amp is now or is that for when it has been changed to the hybrid design?

The 250k pot would be for the 2nd gen but with added Ratio/Level control like #124.

The more I think about it the more I like the idea. It gives you almost the same OD output load as the 2nd gen, but adds a ratio control. All you have to do is delete the 180k to ground and the 1Meg at the OD2 output, and connect the OD2 coupling cap direct to a 250k linear Ratio pot. The pot wiper goes to NO1 on the OD relay (NC1 goes to the Clean2 output coupling cap, and Com1 goes to Master CW).

[Matt J]

I was about to say that to keep the 2nd gen's loading on the OD output, delete the 180k to ground and the 1Meg series resistor and go straight into a 250k pot for the added Ratio control.

...Should I pull the 180K and 1Meg resistor as the amp is now or is that for when it has been changed to the hybrid design?

The 250k pot would be for the 2nd gen but with added Ratio/Level control like #124.

The more I think about it the more I like the idea. It gives you almost exactly the same OD output load as the 2nd gen, but adds a ratio control. All you have to do is delete the 180k to ground and the 1Meg at the OD2 output, and connect the OD2 coupling cap direct to a 250k linear Ratio pot. The pot wiper goes to NO1 on the OD relay (NC1 goes to the Clean2 output coupling cap, and Com1 goes to Master CW).

I worked on it some more last night but forgot to write the voltages down. From testing things earlier everything seemed ok, just a tad lower than what is on the layout and schematics since I had my variac running at ~115VAC to get the heaters right at 3.15VAC on both sides.

I'm going remove the 180k to ground and 1Meg at OD2 and see how that goes. From trying things out last night, I've isolated the issue to something that is definitely in the preamp. Just strumming a guitar connected to the input jack will get you no signal, but millivolts on the grid, plate, and tone caps coming off of V1A and the coupling cap from V1B. Somewhere the signal is getting muddled in the preamp. I'm probably going to hard wire the Master's output direct to the PI and remove the FX loop (I don't really use effects with D-style amps anyway) and see how that comes out. I will just keep trying to narrow things down from there.

All the shielded cable tested ok with the continuity function on my DMM. Nothing was getting shorted to ground. This was my first time working with RG188 from Pasternack though. I had run out of the previous supply I bought eons ago from a different vendor. I do have some replacement shielded wire coming from Apex Jr.

I keep thinking I've missed something in the tone stack wiring, or in the front panel switches. I'm going to go over it some more this afternoon. I might disconnect the switches and tack solder a jumper into each tone cap to see if something is amiss there.

Matt J.

[martin manning]

I worked on it some more last night but forgot to write the voltages down. From testing things earlier everything seemed ok, just a tad lower than what is on the layout and schematics since I had my variac running at ~115VAC to get the heaters right at 3.15VAC on both sides.

I was thinking verify that all of the preamp tubes are biased at reasonable operating points, i.e. nothing in cut-off stopping the signal.

[Matt J]

I worked on it some more last night but forgot to write the voltages down. From testing things earlier everything seemed ok, just a tad lower than what is on the layout and schematics since I had my variac running at ~115VAC to get the heaters right at 3.15VAC on both sides.

I was thinking verify that all of the preamp tubes are biased at reasonable operating points, i.e. nothing in cut-off stopping the signal.

Updated voltage chart:

[Matt J]

Made some updates and still chasing gremlins in the preamp.

• Removed the 180k and 1Meg resistors from the OD output and ran it straight to the Ratio control

• Removed the FX loop completely. The wiper from the Master now goes directly to the PI input cap and the PI is now ground at the main preamp ground by the input jack.

Some issues:

• I now get full signal into the amp from the input jack. But, the OD sounds very weak with little overdrive at all.

• Switching on the OD, PAB, or any of the preamp switches, especially the R/J switch, sometimes causes the signal to cut out completely.

Could I possibly be using the wrong set of relays and overloading them? I'm using Omron G5V-H1 listed at 12 volts with 12 volts coming from the relay power supply.

Just gotta keep digging!
Matt J.

[martin manning]

The relay spec should be fine. More likely some wiring error, but you say “sometimes” cuts out, which sounds like a faulty switch or relay.

[Matt J]

Did a lot of rewiring this evening and I'm still chasing something very odd in the preamp.
Here are some updates:

• I deleted the 2nd Gen Deep Switch and converted it to a 4th/5th Gen Mid switch. This solved the issue of the preamp cutting out. OD and PAB switching works fine. Volume level is still very low though. Even with the Volume and Master on 10 it is maybe a quarter of the volume it should be.

• I've rolled several 12AX7s in V1 and V2 and it made no difference.

• I'm wondering if something is getting cut out with the shielded cable I'm using, or if it has high capacitance that is bringing the volume down. I temporarily replaced the shielded cable from the input jack to V1A and from V1B to the volume pot with standard wire with standard wire to test this. The volume did go up slightly but so did the noise floor. This is the cable from Pasternack I have used:
  https://www.pasternack.com/50-ohm-flexi ... ulk-p.aspx

• In case they were faulty, I replaced the 1Meg volume and 500kL treble pot to see if something was faulty with them. Also, I changed the treble pot of a 250kA and redid the bass pot cap to a .005uf from the outer lug to the wiper like in a 3rd Gen. This really had no effect.

Still dealing with preamp volume issues, the OD is very harsh when engaged too. I tried a different set of 6L6GCs to see if that could be an issue, same result.

Matt J.