Supro build, more breakup.

[DRH1958]

It provides a ground reference for the input jack, but why they chose a low value, especially with only one gain stage is beyond me. This is the first time I have come across this particular setup. I've worked on grid leak biased amps before, but not like this one. This resistor and the 47k grid stoppers form a voltage divider, so if the resistor going to ground is larger, less signal is grounded out. This translates to a stronger signal to the input. At least that is what I see. Maybe someone else knows better. I don't really like grid leak biasing. Not a very stable way to do it. I suppose that's why they quit using it.

[g1]

Yep, not a big fan of grid bias either. A lot of grid biased stages get updated to the relatively "modern" cathode biased, I guess more so when they have the weird "batteries" or what have you.
And there's nothing to say that wasn't done to the amp in the video either, is it supposedly stock?
My earlier comment wasn't to say I was opposed to tweaking, just to first make sure you're not tweaking a defective circuit.

[Enzo]

I see three inputs, the bottom two have the voltage divider for a 6db pad, the top one does not have the resistor, just a cap, so it does not see the voltage division and would be a bright input.

[Pryde]

I see three inputs, the bottom two have the voltage divider for a 6db pad, the top one does not have the resistor, just a cap, so it does not see the voltage division and would be a bright input.

Sorry I did not make it clear I only did a single input (47k) with my build and omitted that top treble input altogether.

Attached is a copy of the turret layout I designed if anyone cares to have a look.

thanks

[Chuck H]

I'm going to call out the elephant in the room here...

The circuit in post #1 is relatively low gain. Even for a single triode preamp. How do we know this hand drawn reference is accurate to the actual circuit? I mean, if the originals sound different and there's no official schematic then the amp in question may not even be a Supro Supreme 1600. A couple of circuit errors or misinterpretations can destroy any effort to clone.

And +1 to Enzo's notes on the input. Using the bright input, that doesn't include a series resistor, would eliminate the voltage division. 47k (or even 100k) is still very low for an input load and will have a large affect on the tone and trim output voltage of most guitar circuits.

[Pryde]

I'm going to call out the elephant in the room here...

The circuit in post #1 is relatively low gain. Even for a single triode preamp. How do we know this hand drawn reference is accurate to the actual circuit? I mean, if the originals sound different and there's no official schematic then the amp in question may not even be a Supro Supreme 1600. A couple of circuit errors or misinterpretations can destroy any effort to clone.

And +1 to Enzo's notes on the input. Using the bright input, that doesn't include a series resistor, would eliminate the voltage division. 47k (or even 100k) is still very low for an input load and will have a large affect on the tone and trim output voltage of most guitar circuits.

Most certainly an elephant and that is why I posted to get opinions on tweaking a bit more gain as I (or anyone else really) know if this is "exact". Being a valco/supro amp it would certainly stand to reason there are discrepancies/variations all over.

On a side note, Terry Dobbs (Mr. Valco) drew this schematic from some original Supro 1600 Supremes he either owned or serviced. I verified this with him before building from this schematic. He also pointed out it is the same circuit as the Valco 6400 which there is a "factory" schematic for. Not having an actual vintage unit to dig into I am merely attempting to get as close as I can here.

The late 50's early 60's Supreme all had grid bias V1, paraphase PI, 6973 power section so I am at least in the ball park for some crunchy Supro tone...I think.

[DRH1958]

Good to hear that you verified the schemo as best you could. The paraphase PI does have a huge effect on the tone, so don't change that for sure.
Enzo is definitely one to listen to. It might be good to add an input with just the cap on it and you could jumper the inputs or just use that one. Maybe this is the jack the guy in the video plugged in to. My bet is the 1Meg resistor will help as the amp sits right now. If you aren't getting the sound in the video you either have a mistake, which you are reasonably sure you don't, or you need to mod it for more crunch. It seems these are the only two options. But adding that third jack seems a good place to start.

[Pryde]

Good to hear that you verified the schemo as best you could. The paraphase PI does have a huge effect on the tone, so don't change that for sure.
Enzo is definitely one to listen to. It might be good to add an input with just the cap on it and you could jumper the inputs or just use that one. Maybe this is the jack the guy in the video plugged in to. My bet is the 1Meg resistor will help as the amp sits right now. If you aren't getting the sound in the video you either have a mistake, which you are reasonably sure you don't, or you need to mod it for more crunch. It seems these are the only two options. But adding that third jack seems a good place to start.

Thank you. In the video comments section below he indicates plugging into the normal jack for the clips. I will try the 1M input resistor this evening and go from there. I appreciate everyone's help thus far.

[Enzo]

What I described was the action of the stock circuit. If you only built the voltage divided input, then that is what you have, a permanent 6db pad. Eliminate the series resistor and get your 6db back. I think you need a lot more than those 6db, but why waste them?

[Pryde]

What I described was the action of the stock circuit. If you only built the voltage divided input, then that is what you have, a permanent 6db pad. Eliminate the series resistor and get your 6db back. I think you need a lot more than those 6db, but why waste them?

I see. So for experimenting you suggest to remove the series 47k from the input. Would this also include increasing that input ground resistor to 1M as others suggest?

[Enzo]

All or nothing? I'd short across or remove the series resistor and see what it does, maybe little, maybe more than you think. THEN consider changing the other resistor. if you just wholesale change a bunch of parts, then you never know which ones really made the difference. If you change the lower one to 1M, then the whole voltage divider becomes moot. Two ways to do a similar job.

[DRH1958]

I obviously can't speak for Enzo, but go ahead and jumper the 47k grid stopper and see what happens. There is no danger of ruining anything. The signal is too low here. And yes, go ahead and replace the 100k with 1Meg. In fact before you do, try jumpering the 100k and play that way. See if the resistor is really needed or not. If it doesn't sound right, then put in the 1Meg to see what it will do.
I would think the elephant in the room is valid if you want to say you built a Supro clone. If you want your amp to sound closer to what you built it for in the first place, I would go ahead and try some of these suggestions. It really comes down to that decision.

There will not be any one change that will get you to what you want if it is as clean as you say. It will probably take a few mods that give you some gain for each. In modding amps, I have found this to be true of most. Lots of small gains add up. Actually, they multiply. If you have two stages that have a gain of 50 each, 50 X 50= 2500. If you increase the gain of each by only 5, you get 55 X 55=3025. You increased the total gain by 525 for only an extra 5 in each. This is how to get your sound.

The other easy one to do if the input mod isn't enough is to replace your volume control with a 1Meg audio taper pot instead of 500k.

[Tom Phillips]

...If you aren't getting the sound in the video you either have a mistake, which you are reasonably sure you don't, or you need to mod it for more crunch...

Also note that we have no proof that the amp used in the video is in stock condition.

[Pryde]

Well I think I will call this one resolved

I tried bypassing input resistor, it helped marginally with a bit more volume.
swapped in 1M input resistor to ground. Got some more volume and breakup but it was pretty harsh sounding so went back to 100k.

Got into my pile of NOS 12ax7s. Threw in another Amperex bugleboy and hey instantly some more breakup. On the right track I swapped in different ones with varying results until I put in an old Westinghouse (England made) and WOW there it is just like that. Actually it is getting probably a bit more breakup even than the video. Stays clean til 4/10 then starts breaking up great with fantastic compression and growl at 8/10. Sounds so nice I will leave it here.

The westinghouse is measuring -0.76v on the grid compared to -0.49v with the amperex. Apparently this is the sweet spot. Crazy I tried new production and a few NOS tubes with not great results then this old beat up one is a gold nugget

I appreciate all the great suggestions, the important thing is that I learned a good deal more about these funky grid bias preamps so for that I thank you.

[eschertron]

I appreciate all the great suggestions, the important thing is that I learned a good deal more about these funky grid bias preamps so for that I thank you.

Grid-leak bias is kind like Bigfoot from what I've read. Everybody has an opinion, but nobody's ever seen it. I applaud your experiment and the clear results you returned. So we have your confirmation that this kind of bias is hugely dependent on the tube. More so than I would have ever guessed. Thanks