Power Scale Footswitching

[Steve Ward]

Hi,

I have an SV2-M36 in a cathode-biased 4×EL84 Rocket-style amp. 
In this amp the Powerscaling delivers an exceptional Overdrive or distortion sound. 

I would like to implement a footswitchable OD function which, when engaged, reduces the Power Scale setting by a proportional amount while simultaneously increasing a Lar-Mar PPIMV setting. It would also need to simultaneously reduce the reverb level.

I’ve read the switching section in TUT4, where preset PS levels are selected using separate Power Scale controls/units. On the SV2-M36, is it practical to switch an additional resistor/network around the existing 1M PS control to lower the output voltage, or would you recommend a different method because of the DC voltage present on the PS control and any concerns around instant changes of power?
I also have an ERK available. Can ERK/JFET switching safely be used anywhere in the SV2 control network, or should the PS function be switched with a relay/MOSFET arrangement instead?
Current SV2 setup: 1M PS pot, R5 calibrated to about 262k effective, PS3 fixed at 164.2k, full-power regulator drop about 6V.


best

Steve

[K O'Connor]

Hi Steve

Because the Power Scale is not ground-referenced in the kit version you have, you can only use a relay to switch it.

When selecting between power levels in a Power Scaled amp you have to switch BOTH the Power Scale control and the Drive Compensation control. You did not mention the latter.

You can switch the power level as you described. Have the added resistance inline all the time and short/unshort using a relay pole.

The master volume you refer to is a post-PI MV directly wired to the power tube grid-leaks, with the pot-0 of both sections tied to the bias-feed, wiper to grid-stop, pot-X to the coupling cap from the splitter. Since it is a dual pot, each half of the output stage can have its own bias pot to assure hum-balancing. Switching for this requires two relay poles.

Changing the reverb level at the same time requires a fourth relay pole.

Two DPDT relays are required to achieve the switching you desire. Since the Power Scale relay pole is working within a resistive path there should be no noise issue.

Have fun

[Steve Ward]

Hi Kevin,

Many thanks — that's very helpful and clears up the switching issue with the kit-version SV2.
I went back through the SV2-M36 notes after reading your reply. I think I may simply have been using different terminology for the Drive Compensation function.
My existing master volume is a Lar-Mar/type-2 PPIMV: dual 250kA, PI coupling caps to pot-X, wipers to the EL84 grid stoppers, pot-0 to ground as the amp is cathode biased, with 1.8M safety resistors from each wiper to ground.
Looking at the Drive Compensation circuit in the SV2 notes, it appears to be doing essentially the same job — a dual post-PI control varying the signal into the output valves. So am I correct that my existing PPIMV can serve as the Drive Compensation control, rather than adding a separate DriveComp pot?
If so, I understand the switching as four relay poles: one to short/unshort the added Power Scale resistance, two for the two halves of the PPIMV/DriveComp, and one to reduce the reverb level — hence two DPDT relays.
Thanks again for the quick help.

Steve

[K O'Connor]

Hi Steve

If the switching merely has to bypass the DriveComp, then there is no need for an extra control.

If you want a Volume pot at the same point for the non-Power Scaled mode, then a second control is necessary.

have fun