Supro Inspired VVR Question

[Pryde]

Hey folks I built a Supro 1600/Valco 6400 build recently (schematic attached) and it sounds great...when cranked. I am considering voltage scaling to get the crunch at lower volume and would appreciate some advice on what might be the best way:

a) your crazy don't do that
b) scale PI/PA only
c) scale entire amp

As you can see it uses the old/archaic grid bias input and paraphase inverter setup so not really sure what my results might be with the different options. I don't really want to add any sort of master volume either. The amp is "done" except I can still change the power switch with a switched pot for the VVR control and not alter the front/faceplate.

I appreciate the thoughts

[J M Fahey]

a) your crazy don't do that

Not much chance of getting this answer here
And in the rare case you do, it must be for very good reason (such as son't mess with hot chassis amps) .

b) scale PI/PA only
c) scale entire amp

Not the entire amp, it will be way too mushy.

I'd start with scaling just the power plates and screens, cathode biased power amps scale beautifully and sort of self adjust, the fact that the amp becomes easier to overdrive is not exactly a defect in my book, if you know what I mean.

Then try scaling also V2, the PI.

You tell us which way you like better, a couple YT/MP3 even better

You can even add a small switch so PI either gets dimmed or not.

Since you will feed the first tube straight from the first main cap (because you won't dim it), I'd add an extra 1k/10uF filter stage for it, even better increase the 1k to 2k2 , so first tube gets same voltage and filtering as before, we don't want increased hum.

I *guess* the amp will be killer

[Pryde]

Not much chance of getting this answer here
And in the rare case you do, it must be for very good reason (such as son't mess with hot chassis amps) .

Not the entire amp, it will be way too mushy.

I'd start with scaling just the power plates and screens, cathode biased power amps scale beautifully and sort of self adjust, the fact that the amp becomes easier to overdrive is not exactly a defect in my book, if you know what I mean.

Then try scaling also V2, the PI.

You tell us which way you like better, a couple YT/MP3 even better

You can even add a small switch so PI either gets dimmed or not.

Since you will feed the first tube straight from the first main cap (because you won't dim it), I'd add an extra 1k/10uF filter stage for it, even better increase the 1k to 2k2 , so first tube gets same voltage and filtering as before, we don't want increased hum.

I *guess* the amp will be killer

Thanks for the input JM. So if scaling just the PA (or PA/PI) in this design you wouldn't think a master volume would be needed? I don't want to add one because, really there is not room and I don't want to alter the chassis any more.

Good to know that doing the entire amp would be mush. Don't want that. BTW: I tried an attenuator and did not like the results of that either, it mushed it up good.

[riz]

Hey folks I built a Supro 1600/Valco 6400 build recently (schematic attached) and it sounds great...when cranked. I am considering voltage scaling to get the crunch at lower volume and would appreciate some advice on what might be the best way...

I have a somewhat similar Valco I'm fond of. Bought all the bits to power scale it, but it sounds so good with a Weber Mass attenuator, I've never bothered. Seems very reliable run hard too--the voltages are on the low side in these amps. Not to dissuade you from some late night soldering sessions, just throwing out another solution!

[Pryde]

I have a somewhat similar Valco I'm fond of. Bought all the bits to power scale it, but it sounds so good with a Weber Mass attenuator, I've never bothered. Seems very reliable run hard too--the voltages are on the low side in these amps. Not to dissuade you from some late night soldering sessions, just throwing out another solution!

Interesting. I tried a Weber mass with mine and it really sucked the life/sparkle out of her. I just couldn't get along with it so hoping some voltage scaling might work for me.

[Gregg]

The easiest thing to do is to scale the power tubes' screens only.

[Pryde]

The easiest thing to do is to scale the power tubes' screens only.

OK I will give it a go but do you think a master volume would then be necessary with this design?

[riz]

Interesting. I tried a Weber mass with mine and it really sucked the life/sparkle out of her. I just couldn't get along with it so hoping some voltage scaling might work for me.

Fair enough, I happen to really like the way it sounds... Curious to see how VVR turns out for you!

[lowell]

+1 Gregg. It's easy, and really has an audible effect. You may have to scale the bias along with it depending on Rscreen value. I've gone to 5k and really liked it, but I like SQUISH.

[Pryde]

+1 Gregg. It's easy, and really has an audible effect. You may have to scale the bias along with it depending on Rscreen value. I've gone to 5k and really liked it, but I like SQUISH.

What do you mean by 5k?
How would you scale the bias along with the power tube screens?

[lowell]

5kohm as a screen resistor value. In your schematic there are no screen resistors. However, the 1k (1000) resistor in the power supply from the B+ to the screen node is something. So if you were to insert screen resistors, you would accomplish earlier breakup and more squish. The higher the value the larger the effect. They would go between the filter capacitor node and the tube screen. I'd try between 470-5k values. The higher the Rscreen value, the more the bias will cool. This may be less so in a cathode-bias amp but I'm pretty sure it still changes the bias.

Google any fender amp from the 70s and you'll see where there are 470ohm screen resistors. Finally, you'll want to use 1watt metal oxide resistors for this.

I'd love to hear this amp...will you post some sound samples?

[Gregg]

OK I will give it a go but do you think a master volume would then be necessary with this design?

With this design maybe not.

You may have to scale the bias along with it depending on Rscreen value.

If an amp has negative bias then yes but this one doesn't look like one.