Supro build, more breakup.

[Pryde]

Hello folks

I recently built a Supro Supreme 1600 schematic attached.

I used some NOS 6973 and 12Ax7 in V1 and am testing it through a 10" low-efficiency Weber speaker which will eventually be a 1x10 combo. It is actually is a GREAT sounding build but it is not getting the breakup/overdrive I expected (listening to audio clips and video of original one). This one stays pretty loud and clean up to 3 o'clock then it will break up some last quarter turn but still kinda tame.

Notice V1 is grid biased with cathode tied straight to ground. I did change the 47k off the input ground to 100k and that helped a bit but still not enough dirt/attitude. So what changes would you folks make on this circuit to achieve more and earlier distortion/breakup? Maybe up the 3.9k and 15k dropping resistors in value? PI changes? Other ideas? Thanks for looking.

[DRH1958]

You only have one gain stage before going into the PI. It looks as though you have an unused half of the 12AX7 used at the input. I wonder if you have enough gain from just the one stage to get any grind. You might want to use the other half of the 12AX7 between your vol and tone controls.
The other way to add gain the way it is would be to change the resistor you raised to 100k to 500k-1Meg at the input.
What is the grid voltage on pin 2 at the input? If it is somewhat close to center biased like 1.2-1.5 volts, you could lower the 6.8 Meg bias resistor to 3.3 Meg to lower the bias voltage and get more grind that way. Try for .8-1 volt.
You could also change your volume control to 1Meg audio taper.
You could change the grid leaks on the 6973s to 470k although I don't know if that will work because you have a paraphase PI and it might screw up the voltage divider to the lower half of the 12AX7 PI tube. You might have to change the 6.8 and 4.7 k resistors to 20k to keep about the same ratio.
To get more out of your 6973s, you could put a 50uF bypass cap on the cathode resistor.
You don't have to do all of these. They are just individual ways to get more out of the amp. Try one at a time and see what happens. I would try the 500k-1Meg. resistor to ground at the input first. Then change your volume to 1Meg next. Then the bypass cap on the power tubes.

[Pryde]

You only have one gain stage before going into the PI. It looks as though you have an unused half of the 12AX7 used at the input. I wonder if you have enough gain from just the one stage to get any grind. You might want to use the other half of the 12AX7 between your vol and tone controls.
The other way to add gain the way it is would be to change the resistor you raised to 100k to 500k-1Meg at the input.
What is the grid voltage on pin 2 at the input? If it is somewhat close to center biased like 1.2-1.5 volts, you could lower the 6.8 Meg bias resistor to 3.3 Meg to lower the bias voltage and get more grind that way. Try for .8-1 volt.
You could also change your volume control to 1Meg audio taper.
You could change the grid leaks on the 6973s to 470k although I don't know if that will work because you have a paraphase PI and it might screw up the voltage divider to the lower half of the 12AX7 PI tube. You might have to change the 6.8 and 4.7 k resistors to 20k to keep about the same ratio.
To get more out of your 6973s, you could put a 50uF bypass cap on the cathode resistor.
You don't have to do all of these. They are just individual ways to get more out of the amp. Try one at a time and see what happens. I would try the 500k-1Meg. resistor to ground at the input first. Then change your volume to 1Meg next. Then the bypass cap on the power tubes.

Thank you for the great suggestions. Yes only one triode is used at V1. I considered utilizing the other half but listening to originals they are getting plenty of breakup with this configuration so I figured some other tweaks might get it there.

I am not familiar with the grid-biased preamps so wasn't sure what to do to get more push out of them. I will try some of your suggestions step-wise and see what I get.

[g1]

If you built a copy, and you have heard the originals sound with more gain (without pedals), then it might be better to treat it as a fault first, rather than trying to modify a potentially defective circuit.
If it is not a copy, then detail what parts are different from the original.

[DRH1958]

As always good advice from g1. And with the grid biased preamp, the suggestion I gave won't give it any more push or gain, it will just cause more signal clipping because off-center bias. It might actually give less gain, just more distortion. That system is really outdated. They stopped using it back in the 1950's as the normal method partly because it caused distortion.

[Pryde]

If you built a copy, and you have heard the originals sound with more gain (without pedals), then it might be better to treat it as a fault first, rather than trying to modify a potentially defective circuit.
If it is not a copy, then detail what parts are different from the original.

Obviously the transformers are not vintage/original but I did use Classictone which are good quality and spec to this (and many other similar valco/supro) similar designs. All other part values are per schematic. My Layout was verified by another builder as well so I am pretty confident between us it is ok as well.

Being that the build is probably as close as I can get to an actual vintage one I figured some circuit tweaks would be necessary from this point to get it even closer.

BTW here is a video of what I am striving for sound wise:

[Pryde]

As always good advice from g1. And with the grid biased preamp, the suggestion I gave won't give it any more push or gain, it will just cause more signal clipping because off-center bias. It might actually give less gain, just more distortion. That system is really outdated. They stopped using it back in the 1950's as the normal method partly because it caused distortion.

Thanks. I checked the V1 grid voltage per your suggestion and am showing - 0.49v so it is quite low as it stands.

[DRH1958]

This might be a stupid question, but are you using a Les Paul or a guitar with comparable pickups? Single coils can have roughly half the output of these 'bucker pups. It really would make a difference with only one gain stage going into the PI using single coils. I bet the 1Meg resistor at the input will make a lot of difference compared to the 100k.
Definitely don't go to the 3.3meg for the bias resistor. I'm surprised you don't have more distortion. With that low of a voltage on the grid, it shouldn't be anywhere near clean.

The other variable is the speaker. You said low efficiency Weber. You can lose a lot of your output with a low sensitivity speaker. If you have one with 3dB less than the amp's speaker in the demo, it's like cutting your power in half.

[uneumann]

Thanks. I checked the V1 grid voltage per your suggestion and am showing - 0.49v so it is quite low as it stands.

A grid leak bias stage is sensitive to the tube itself. Different tubes produce different bias voltages. It's hard to say which way it will change the distortion, but bias voltage can change by 1/2v or more. Try a few before you start soldering.

[Pryde]

This might be a stupid question, but are you using a Les Paul or a guitar with comparable pickups? Single coils can have roughly half the output of these 'bucker pups. It really would make a difference with only one gain stage going into the PI using single coils. I bet the 1Meg resistor at the input will make a lot of difference compared to the 100k.
Definitely don't go to the 3.3meg for the bias resistor. I'm surprised you don't have more distortion. With that low of a voltage on the grid, it shouldn't be anywhere near clean.

The other variable is the speaker. You said low efficiency Weber. You can lose a lot of your output with a low sensitivity speaker. If you have one with 3dB less than the amp's speaker in the demo, it's like cutting your power in half.

Yes I am testing with a Les Paul, PAF output pickups (~8.5k). The Weber (20 watt) is a fairly comparable speaker to the Jensen C10R (stock to originals). The Weber should be giving me breakup earlier and with lower volume which is my goal. I will try that 1M resistor at input ground as well as swap a few 12AX7 tubes and see what happens. Thank you.

[Jazz P Bass]

Put a boost pedal in front of it & call it a day.

[Pryde]

Put a boost pedal in front of it & call it a day.

Yes I know but that would be to easy

[Pryde]

Also wondering on that input resistor to ground (47k in schematic) what is the point of it? I see other examples of grid bias V1 (i.e. Fender 5C3 or Supro 1624t) that have no resistor at all in this position. I assume no resistance equals less gain from input?

[g1]

Well, it's all about the knees isn't it?

He does mention the right side is going through his "wet rig" and left side is dry. Have you listened to just the left? Probably not the difference maker, but you might as well hear it "as is".

[Pryde]

Well, it's all about the knees isn't it?

He does mention the right side is going through his "wet rig" and left side is dry. Have you listened to just the left? Probably not the difference maker, but you might as well hear it "as is".

Yes I did adjust my monitor balance to get an accurate listen. I don't know really, just hoped for more grit outta this one I built but I will keep trying some options. It really is quite clean and very loud until damn near full crank then it will break up some. I will still be happy with it as at least the clean sounds very good and it is dead quiet/little hum even up full.