Preamp B+ Voltage High/Low Switch?

[FourT6and2]

Hot-rodded 1972 Marshall Super Lead with factory polarity switch with "death" cap. I'd like to disconnect the death cap (amp has 3-prong already) and repurpose the switch for a preamp voltage high/low switch.

The preamp has two main B+ dropping resistors leading into the PI B+ node. Each one is 10K (20K total). Bypassing one of them raises preamp voltages by about 50v, which sounds very nice. I was wondering how safe it would be to repurpose the polarity switch as a preamp voltage high/low switch to simply bypass one of the 10K droppers? It would mean about 165v – 180v would be across the switch. I suppose the standby switch has more than this, so it should be ok?

[Roe]

place a 10k across the switch, so that there is is around 80-90v across it. use the same type of switch used for standby and power switches. it should work just fine

[FourT6and2]

place a 10k across the switch, so that there is is around 80-90v across it. use the same type of switch used for standby and power switches. it should work just fine

Yeah the polarity switch already is the same as the standby/power so that should work.

I was thinking of leaving both 10Ks on the turret board then placing leads on either side of one resistor, going back to the switch. Is that grossly different than placing the resistor itself across the switch?

high low switch.jpg

[Roe]

place a 10k across the switch, so that there is is around 80-90v across it. use the same type of switch used for standby and power switches. it should work just fine

Yeah the polarity switch already is the same as the standby/power so that should work.

I was thinking of leaving both 10Ks on the turret board then placing leads on either side of one resistor, going back to the switch. Is that grossly different than placing the resistor itself across the switch?

high low switch.jpg

should work. higher voltages gives a faster response and drives the power amp harder. also it adds treble (and possibly bass) and clean headroom, giving less mids and compression

[martin manning]

...higher voltages gives a faster response and drives the power amp harder. also it adds treble (and possibly bass) and clean headroom, giving less mids and compression

I'll give you the increase in headroom, but I don't see raising preamp voltage as having any significant effect on frequency response or gain.

[Roe]

...higher voltages gives a faster response and drives the power amp harder. also it adds treble (and possibly bass) and clean headroom, giving less mids and compression

I'll give you the increase in headroom, but I don't see raising preamp voltage as having any significant effect on frequency response or gain.

usually the effect is subtle. but it is noticable if you raise the voltages significantly for several triodes. it adds treble, clarity and makes the amp a bit more unforgiving, with less emphasis on preamp compression and distortion and more emphasis on power amp drive

[martin manning]

I think those are likely to be perceptions that stem from the increase in headroom.

[Roe]

I think those are likely to be perceptions that stem from the increase in headroom.

not just a perception but rather results of an objective increase in headroom?

[R.G.]

Hmmm. Sine wave input, spectrum analyzer or distortion analyzer.

[Roe]

Hmmm. Sine wave input, spectrum analyzer or distortion analyzer.

ok for sound, but not so much for dynamic response and feel?

[martin manning]

not just a perception but rather results of an objective increase in headroom?

Yes, and a lot of things could be going on. The actual increase in gain is negligible, but the increased headroom might be used to play louder, resulting in the perception that there is more treble and bass when there actually isn't any in the frequency response.

[FourT6and2]

I've already tested this by simply bypassing one of the resistors with alligator clips. The difference is mostly in feel. But there is a subtle tonal difference as well. Yes, amp gets slightly brighter, sharper attack, faster low-end response. The difference is big enough that having it on a switch is desirable. BUT, ultimately, I would want to tune the preamp for even higher B+ at one of the nodes. But I can't get there without redoing how B+ is tapped for that specific node, which I'm not going to do for this particular amp.

My only concern is that the switch will have 400v+ DC across it if I do this.
Vp / Screen voltage = 513v
With both 10K resistors (20K total), that drops to 344v. So bypassing one with the switch will send a good amount across it.

[Roe]

I've already tested this by simply bypassing one of the resistors with alligator clips. The difference is mostly in feel. But there is a subtle tonal difference as well. Yes, amp gets slightly brighter, sharper attack, faster low-end response. The difference is big enough that having it on a switch is desirable. BUT, ultimately, I would want to tune the preamp for even higher B+ at one of the nodes. But I can't get there without redoing how B+ is tapped for that specific node, which I'm not going to do for this particular amp.

My only concern is that the switch will have 400v+ DC across it if I do this.
Vp / Screen voltage = 513v
With both 10K resistors (20K total), that drops to 344v. So bypassing one with the switch will send a good amount across it.

compared to many standby switches that is not too bad. A 45/100 had 600v across the standby swich, but here it is only around 85v.

[FourT6and2]

I've already tested this by simply bypassing one of the resistors with alligator clips. The difference is mostly in feel. But there is a subtle tonal difference as well. Yes, amp gets slightly brighter, sharper attack, faster low-end response. The difference is big enough that having it on a switch is desirable. BUT, ultimately, I would want to tune the preamp for even higher B+ at one of the nodes. But I can't get there without redoing how B+ is tapped for that specific node, which I'm not going to do for this particular amp.

My only concern is that the switch will have 400v+ DC across it if I do this.
Vp / Screen voltage = 513v
With both 10K resistors (20K total), that drops to 344v. So bypassing one with the switch will send a good amount across it.

compared to many standby switches that is not too bad. A 45/100 had 600v across the standby swich, but here it is only around 85v.

It's more than 85v, no? 85v or so would be the voltage drop across one of the 10K resistors. Not the switch. With both 10K resistors in-circuit (20K total), the Vin = 513v and Vout = 344v. That's a voltage drop of 169v. Using a switch to bypass one of the resistors (the second one in series, for example), reduces the voltage drop to about 85v (assuming current remains constant), which means total Vout will be about 428v DC. That 428v would be flowing through the switch. The voltage drops happens across the resistor, not the switch. The switch is in series after that voltage drop.

[martin manning]

My only concern is that the switch will have 400v+ DC across it if I do this.
Vp / Screen voltage = 513v
With both 10K resistors (20K total), that drops to 344v. So bypassing one with the switch will send a good amount across it.

513-344=169
169/2=84.5

That's the voltage across the switch when it's open. Yes it will have 513-84.5=428.5V potential on it, but its dielectric strength should easily handle that.