FX Loop Advice for Hot Cat with VVR Build

[bcmatt]

I've been really loving the Hot Cat that I built out of an old Traynor YGL- Mark 3. The main differences from the stock Hot Cat are that it has VVR and Garter biasing. Also, The filtering is a little different because it also doesn't have a rectifier tube or a choke.

Garter Hot Cat.png

I let an acquaintance use it and tried to buy it off me. I said it wasn't for sale but I could build another one for him if he really wanted. However, his main complaint was that he wanted it to have an FX loop. I actually agree that it might be time I gave it one. I actually even reserved a tube socket and real-estate for some sort of tube buffered loop when I built it... but now I'm wondering what sort of design I should use. Should I build in a dumbleator? How would it be affected by the VVR as it currently scales the whole amp?
I had already been considering switching the scaling to be only on the power tubes, but I'm reluctant to add a PPIMV since I'm not eager to over-complicate the controls and the regular master volume actually works quite nicely in conjunction with the VVR to dial in the desired level of Power-section squishiness/tightness. I just can't lower the VVR below about 40% when using the gain channel because it just begins to disappear. I'm a little unclear if it is due to the grid-leak bias for v1 or just the number of gain stages in use...

Anyways, an thoughts on what might be an appropriate way to tackle an FX loop on this amp?

[bcmatt]

This is what I think I'll try. Thanks to Martin Manning for his new improved Dumbleator schematic:

Garter Hot Cat FX LOOP Schem.png

I have space saved for it including a tube socket already with heater wires. I'll need to drill more holes for the pots and bright switch:

20250702_072619.jpg

[pdf64]

4 successive level controls (master volume, send drive, recover input, recover output) each / all of which act as a master volume, seems a recipe for confusion to me.
I suggest to insert the loop before the master volume, add a instrument / line loop level switch if necessary.

If the VVR is causing strange behaviour with the preamp, then consider taking the preamp supply from before the VVR.

[bcmatt]

4 successive level controls (master volume, send drive, recover input, recover output) each / all of which act as a master volume, seems a recipe for confusion to me.
I suggest to insert the loop before the master volume, add a instrument / line loop level switch if necessary.

If the VVR is causing strange behaviour with the preamp, then consider taking the preamp supply from before the VVR.

Thanks. Ya, I had been playing with the idea of scaling the power tubes only, but then that is yet another control needing to be added to the front panel for a PPIMV. I still might do it, but I'm wanting to not make this thing overly complicated to actually use... and I'm tipping over that edge now as I plan this loop as well.

But, now you got me thinking maybe I could scale the power tubes only, and use the current Master Volume knob position for a PPIMV instead. Then I would delete the regular master, feeding the Input of the loop circuit right off of the treble control with some sort of voltage divider before hitting that first cap and grid.
I could still put all the send and return controls on the back panel for the loop. thoughts? I could see there still being the issue of the back panel controls needing to be accessed all the time in order to hit the PI with the right amount of signal.

[pdf64]

I suggest to use the VVR on the power amp, ie the output valves and LTP phase splitter. Maybe the clean input stage too.
Keep the supply to the overdrive preamp and loop fixed, then the loop level will be more predictable, and all those level controls unnecessary, just left with a master volume after the loop.

[bcmatt]

I suggest to use the VVR on the power amp, ie the output valves and LTP phase splitter. Maybe the clean input stage too.
Keep the supply to the overdrive preamp and loop fixed, then the loop level will be more predictable, and all those level controls unnecessary, just left with a master volume after the loop.

That's a great point. I Actually thought the clean channel would be the issue, but now realize the clean channel volume is actually after the single triode but right before the PI, so that would negate the need for a PPIMV if scaling the PI and Power tubes only. Great plan! I will implement that first before adding the Loop.

But then, you are right, I need to simplify the number of pots on this loop. The recovery output could take the place of the Master Volume on the front panel. Do you think I ought to keep the send control and recovery input controls as they are and just try to figure out the right values with resisters to hit the first loop triode grid appropriately off of the treble control?

[bcmatt]

OK, how does this look to you? Any other suggestions? Replacing the 100k Master volume pot with a 56k resistor to ground after the 220K out of the treble pot is rather arbitrary, but I'm just taking a bit of a stab at something to reduce the amount of pots needed.

Garter Hot Cat FX LOOP Schem.png

[bcmatt]

I came home from work early and switched the VVR to scale just the PI and power tubes. It definately improved things as far as the gain channel being more usable at lower voltages. I'll need to check all the voltages again to see where everything is running now. I may wait till after the loop is installed because that will affect things as well.

[pdf64]

Yes that looks good, especially if the B+3 feed is iaken from the VVR also.
I'm not keen on the loop drive and recover controls, they'll get inadvertently knocked and cause problems on a gig. Just tweak the potential divider after the tone stack (maybe use an internal trimmer preset) to achieve a suitable loop level.

[bcmatt]

Yes that looks good, especially if the B+3 feed is iaken from the VVR also.
I'm not keen on the loop drive and recover controls, they'll get inadvertently knocked and cause problems on a gig. Just tweak the potential divider after the tone stack (maybe use an internal trimmer preset) to achieve a suitable loop level.

Yes, the B+3 is also fed from the VVR so now it's just the PI and Power tubes scaled. I gotta say, it is a huge improvement for this amp to do that. Now everything works properly at any voltage. Now it will be way easier to balance the two channels' at any volume.

Regarding the send and return controls, I'll see if I can build it with just presets... that would save me some drilling. I doubt I have appropriate trimpots, but I may need to order some parts anyways. Some of my stock of capacitor values are starting to dwindle. And I doubt I have 10M resistors either.

[bcmatt]

It will be tuesday when the rest of my missing components arrive for the loop. But in the meantime, I measured my preamp plate voltages since they will no longer be variable. They range between 20V-65V over the original Hot Cat Schematic plate voltages:

Garter Hot Cat Preamp Voltages.png

So, I guess by splitting the rail before the Sag resistors and the 1K "Choke" resistor, I'm pushing even higher voltages to the preamp. I'll try a couple 1k 5w resisters in series for the unregulated side of the B+ rail.

[pdf64]

The only trimmer I would use would be a 100k in place of the original master volume.
Set it to avoid clipping pedals in the loop.

[Colossal]

Hello Matt,

This is slightly off-topic but since you changed the power supply, your schematic shows 360R for your garter bias cathode resistors. Is that what you still have in there or did you warm them up to 270R? Where did your cathode voltages end up?

[bcmatt]

The only trimmer I would use would be a 100k in place of the original master volume.
Set it to avoid clipping pedals in the loop.

Ok Thanks. Ya, I was thinking I would actually make all three trimpots to start with the recovery output on the front panel in place of the Master. Then, once I feel like I've found a resonable setting, I would maybe swap them out for resistors.

[bcmatt]

Hello Matt,

This is slightly off-topic but since you changed the power supply, your schematic shows 360R for your garter bias cathode resistors. Is that what you still have in there or did you warm them up to 270R? Where did your cathode voltages end up?

I don't think I'll ever end up with 270R. That would likely make it too hot.
I'll have to measure again now that I'm messing with the power supply voltages. I forget what the final result was at full voltage. I think it ended up at about 100% dissipation with the 360r Resistors for the Garter Circuit. Those values were intentionally purchased after a bunch of testing. Hopefully they will remain appropriate when all is done.
Since, the preamp will now always run at full voltage, I'll want to get them closer to spec, which may push up the power section voltage due to some adjusted resistor values in the dropping string. I won't go out and buy new cathode resistors until after the loop is finished and I think everything is settled in.