Blues Jr Conversion Really Loud

[acheld]

Could the issue I'm experiencing have anything to do with this mod? Maybe I'm getting a much stronger signal than what is needed?

No.  Frankly it doesn't change much, though in my own experience there might be less noise.

[shooter]

Maybe I'm getting a much stronger signal than what is needed

only the scope will tell
subjective guessing usually results in subjective outcomes


you're driving tubes that need about 1/5th the signal a 6v6-ish tube would need for the "same outcome"
your PA "limiting" factors in a nutshell is your bias VDC whether fixed or self, that is the "clean swing range" before the tubes hit clipping or cutoff


once you exceed that the un-expected happens, sometimes it "Oh ya baby" other times it's "WTF was that???"


$35 gets you the tool, 35 hours reading gets you the knowledge, together great things can happen

[mresistor]

I would say "unparallel the tube" and then see if it helps.

[AlNewman]

You said you could get clean sounds with the master volume up...
So, with the master @ 12:00, does it still break up at 2 on the volume dial?
Or do you have to turn the volume higher to get the same amount of distortion?

[Tbone55]

Could someone explain to me the reason for R6, 120k resistor, that is connected to the volume pot wiper terminal and ground? Is it acting as a grid leak resistor? Curious minds would like to know.

Thanks.

[Tbone55]

No.  Frankly it doesn't change much, though in my own experience there might be less noise.
[/quote]

Thanks kindly. I really didn't want try and unsolder the connections. 😁

[Tbone55]

only the scope will tell
subjective guessing usually results in subjective outcomes

you're driving tubes that need about 1/5th the signal a 6v6-ish tube would need for the "same outcome"
your PA "limiting" factors in a nutshell is your bias VDC whether fixed or self, that is the "clean swing range" before the tubes hit clipping or cutoff

once you exceed that the un-expected happens, sometimes it "Oh ya baby" other times it's "WTF was that???"

$35 gets you the tool, 35 hours reading gets you the knowledge, together great things can happen
[/quote]

Thanks for the reply. Nothing replaces knowledge and experience. I had an opportunity once to get a scope and let it get away. Sure wish I had it now. Pictures are worth a thousand words. I sure have been reading a lot lately 😂.

[Tbone55]

I would say "unparallel the tube" and then see if it helps.

Thanks. I might try it but not yet.

[Tbone55]

You said you could get clean sounds with the master volume up...
So, with the master @ 12:00, does it still break up at 2 on the volume dial?
Or do you have to turn the volume higher to get the same amount of distortion?

With the master around 12 the volume stays clean when it's at 2. It stays pretty clean up to around 6 where it starts to break up. The amp seems to jump in volume when I move the volume from 2 to 3 and stays around that level until it starts to breakup around 6-7.

I'm borrowing a 12AT7  from a friend to try in the V1 position. Once I've had a chance to install it and see what kind of a difference it makes I'll report back my findings.  If that doesnt work I still have a few other recommendations I can try.

Thanks for your help.

[AlNewman]

With the master around 12 the volume stays clean when it's at 2. It stays pretty clean up to around 6 where it starts to break up. The amp seems to jump in volume when I move the volume from 2 to 3 and stays around that level until it starts to breakup around 6-7.

I'm borrowing a 12AT7  from a friend to try in the V1 position. Once I've had a chance to install it and see what kind of a difference it makes I'll report back my findings.  If that doesnt work I still have a few other recommendations I can try.

Thanks for your help.

Personally, I would think that has something to do with the 10k/120k voltage divider off the volume control, as far as the weird pot reactance goes.
I don't have an answer for you, but things i would be trying is measuring resistances from your wiper to ground at the point when these changes take place.  And then maybe do some math or experimentation.

It sounds like you can get all the in betweens at the right place on the knob, but it just isn't where you'd like it to be.

[Tbone55]

[quote author=Tbone55 link=topic=31828.msg351587#msg351587

Personally, I would think that has something to do with the 10k/120k voltage divider off the volume control, as far as the weird pot reactance goes.
I don't have an answer for you, but things i would be trying is measuring resistances from your wiper to ground at the point when these changes take place.  And then maybe do some math or experimentation.

It sounds like you can get all the in betweens at the right place on the knob, but it just isn't where you'd like it to be.

Thanks Al. I've been thinking the same thing. I just don't understand what the purpose of the 120k resistor on the pot is for. It looks almost as if it would act as a grid leak resistor somehow but a much lower value than what you normally see. I'm still wrapping my head around all the circuits work and have done quite a bit of reading in the last while.

I will do as you've suggested and measure the resistance at the point where the volume changes. I've also gotten hold of a 12AT7 tube to try. Will definitely report back with my findings.

Cheers!

[AlNewman]

Yes, it's a grid leak.  There's just  lot of gain in the pre amp, so looks like they were dumping some signal.  It wouldn't hurt anything to lift one leg of the 120k resistor and see how that changes the response on your volume dial.  But you'll get to a point and everything will start oscillating.
It might be a way to see if you can increase that grid leak some.  Just remember being it's parallel it works hand in hand with the setting on your pot.  Once you find the magic ratio, (which could be 12:1), then you can increase both the grid stopper and grid leak at the same ratio to change the response on your pot.

[Tbone55]

Thanks AI for confirming what I thought. Since the volume pot is in parallel to the 120k resistor they act the same way as two resistors in parallel would and the overall resistance is always less than 120k?

[Tbone55]

Ok. I have some results to report.

1. 12AT7 in V1 position
- a bit less gain but not as much as I would have expected. The volume still gets loud quick at 3 without much change until around 6-7.

2. Lifted one end of C1, 47uf by-pass capacitor
- gain dropped significantly and the volume control had a much more gradual increase in volume with break up around the 7-8 mark. This is much more in line with what I want.  It allows me to make more use out of my distortion pedals. It seems to have a bit more bass in it as well.

 I need to decide if I'm going to use a much smaller value capacitor in place of the 47uf in order to get a bit more highs. Not sure what value I should be considering. If anyone has a suggestion let me know.

Cheers!

[dwinstonwood]

Use this calculator to determine the gain with different cap values:
https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

Here is the Valve Wizard's graph showing the frequency cutoff for different cap values:
Starting in the middle of page 25 here: https://www.valvewizard.co.uk/Common_Gain_Stage.pdf

[Tbone55]

Thanks so much d. Those two references should really help me out a lot.

Cheers!

[Tbone55]

Those references were amazing d! I have to admit that I don't fully comprehend everything I read but based on that calculator and the valve wizards chart it would seem I need to use a capacitor value that's orders  of magnitude smaller than 47uf. It looks to me that I might want to use  a 1uf or maybe even a 470nf capacitor. Does this make sense?

[dwinstonwood]

Tbone55, I think this ultimately comes down to making a choice by listening. 
FWIW, some Marshall's used 0.68uF bypass caps. I used 1uF in the last amp I built.

[Tbone55]

Tbone55, I think this ultimately comes down to making a choice by listening. 
FWIW, some Marshall's used 0.68uF bypass caps. I used 1uF in the last amp I built.

You're absolutely right. My ear will be the best judge. I was leaning towards a 1uf cap myself so I'll try that first. Thanks for your help.

Cheers!

[Tbone55]

Another update. I bought a 1uf and 0.47uf cap to try in place of the 47uf bypass cap C1. They both worked quite well and made the volume work in a more gradual fashion. The amp starts to break up around 7-8 and the bass is a lot tighter. I settled on the 0.47uf cap as it was a Mallory.

The original 47uf cap is directional but the Mallory isn't.  Also the Mallory is rated at 630vDC and the 47uf at 25v. That's all I was able to find. Is this okay?

[Tbone55]

Did some reading and found out you can use a non polarized capacitor in place of the polarized ones. Electrolytics are polarized and come in compact sizes for very high values. The 0.47uf mallory I used is huge compared to tge original 47uf cap.

Just ordering some parts from Doug as spares including a 0.68uf cap to try out. I have learnt quite a bit from this post and would like to thank everyone for their contributions and patience. This has been an invaluable learning experience from a great group of people.

My sincerest and utmost thanks to all. Cheers!