CLEAN module question

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ilearn

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I just bought an used CLEAN module with black stripe.
I tried it and it's very dark sounding, too dark even for Jazz.
I remove the tube bracket to see if there's any mod were done.

I found out that C7 cap were removed. (C7 is in between tubes)
I can still see what's left of the legs of the cap.

https://goo.gl/photos/V88HzCAMYpwvBCyYA

Can anyone with Clean module verify if there's any cap on C7?

Thanks in advance.
 
If you're interested in some DIY mods for the clean module to give it a little gain boost and sparkle, then I'd recommend the following:

Easiest - add another 22uF electrolytic cap into C7. Make sure you get the polarity right (neg lead goes to the left when looking at the module from the front). C7 is the cathode bypass cap for V1A. Adding it will provide a high mid/treble boost for that first module gain stage.

Slightly harder - transfer the 22uF you've got currently in C13 over to C7 and add a 1uF into the newly vacated C13. Once again, make sure you get the polarity right. C13 is the cathode bypass cap for V2B (second gain stage on the module). This will provide a further treble boost. This really makes the modules using this PCB come alive.

Final tweak - replace R10 with an 820Ohm resistor. R10 is a coupling resistor between gain stages. Dropping it from 1k to 820Ohm will give you a little extra gain.

If you do all three of these mods you will end up with a module that is no longer dark or muddy. With the gain cranked it'll feel really dynamic, touch sensitive and open.

I've experimented with an absolute truckload of changes to my Blackface module, some good and many bad. The three changes suggested above are what I'd recommend as a good starting point.
 
I've owned 2 Clean modules, and both had no capacitors in C7.

Don't confuse CLEAN mods with BLACKFACE mods.

If you download my spreadsheet file (see DIY topic - component spreadsheet), this has all of the standard resistor and capacitor values for both the Clean and Blackface modules as well as various others.
 
Graham Pearson said:
Don't confuse CLEAN mods with BLACKFACE mods.
Click to expand...

My suggested mods aren't really "Blackface" mods. They are mods that should work with the common PCB that the clean and blackface modules use.

Adding appropriately sized cathode bypass caps is a common way to lighten a dark tube pre-amp circuit by preventing the high frequencies from bleeding to ground through the cathode bias resistor.
 
Graham Pearson said:
I've owned 2 Clean modules, and both had no capacitors in C7.

Don't confuse CLEAN mods with BLACKFACE mods.

If you download my spreadsheet file (see DIY topic - component spreadsheet), this has all of the standard resistor and capacitor values for both the Clean and Blackface modules as well as various others.
Click to expand...

yes, I did refer your spreadsheet. According to the sheet there aren't supposed to be any caps in C7 and C13.
But my CLEAN module has 22uF in C13.
This made me confused..
 
CrunchBunch said:
If you're interested in some DIY mods for the clean module to give it a little gain boost and sparkle, then I'd recommend the following:

Easiest - add another 22uF electrolytic cap into C7. Make sure you get the polarity right (neg lead goes to the left when looking at the module from the front). C7 is the cathode bypass cap for V1A. Adding it will provide a high mid/treble boost for that first module gain stage.

Slightly harder - transfer the 22uF you've got currently in C13 over to C7 and add a 1uF into the newly vacated C13. Once again, make sure you get the polarity right. C13 is the cathode bypass cap for V2B (second gain stage on the module). This will provide a further treble boost. This really makes the modules using this PCB come alive.

Final tweak - replace R10 with an 820Ohm resistor. R10 is a coupling resistor between gain stages. Dropping it from 1k to 820Ohm will give you a little extra gain.

If you do all three of these mods you will end up with a module that is no longer dark or muddy. With the gain cranked it'll feel really dynamic, touch sensitive and open.

I've experimented with an absolute truckload of changes to my Blackface module, some good and many bad. The three changes suggested above are what I'd recommend as a good starting point.
Click to expand...

Thanks for the info. I will try tonight and let you know.
Regarding R10, if I replace it with 1.2K or 1.5K, will it improve headroom?
I want my CLEAN and Blackface stay as clean as possible with no drives or breakups even with Gain knob over 2 o'clock or higher.
 
To keep things as clean as possible you will want to keep R10 at 1k or above. You can try 1.2k or 1.5k and it may give you more headroom but not if any of the previous gain stages are causing breakup. R10 is between V1A and V2B, so if you're getting breakup via V1A, attenuating between it and V2B will not do you any good.

Any reason you want things super clean AND the gain cranked? I've modded my blackface to give a nice amount of grit with the gain cranked, but its still super clean with the gain low. If I need to match volume with the gain low, I just up the module level to compensate.
 
CrunchBunch said:
To keep things as clean as possible you will want to keep R10 at 1k or above. You can try 1.2k or 1.5k and it may give you more headroom but not if any of the previous gain stages are causing breakup. R10 is between V1A and V2B, so if you're getting breakup via V1A, attenuating between it and V2B will not do you any good.

Any reason you want things super clean AND the gain cranked? I've modded my blackface to give a nice amount of grit with the gain cranked, but its still super clean with the gain low. If I need to match volume with the gain low, I just up the module level to compensate.
Click to expand...

Dear CrunchBunch,

I just finished mods as you recommended and my CLEAN module is much better sounding now. The darkness is still there but not in a bad way.

I also applied the mods to my Blackface module and it's more lively sounding.

I tried 820 ohm for R10, but I had to get it back to 1k.
The reason I want the big clean headroom is for playing Jazz stuff.
Even with the same volume level, gain knob around 10 o'clock is better sounding than 8 or 9 o'clock to my ears. well, maybe it's just me.

Anyhow again many thanks for your great tip.

Cheers!
 
No worries ilearn. Glad to help. You may also want to take a look at the value of R28. It's a grid leak resistor and basically acts to dump signal between V1A and V2B (first and second module gain stages) to ground. The lower its value, the more signal is dumped to ground. The standard value is 220k, but you could try going a little lower to reduce gain and maintain maximum headroom through the second stage. 200k or 180k resistors would be good starting points for experimentation. Maybe try the 180k first.

Also, double check that your values for R17 and R29. These two resistors will influence the bias point of the tubes. For maximum clean you want the tubes biased closed to their center. 1.8k should get close.
 
CrunchBunch said:
No worries ilearn. Glad to help. You may also want to take a look at the value of R28. It's a grid leak resistor and basically acts to dump signal between V1A and V2B (first and second module gain stages) to ground. The lower its value, the more signal is dumped to ground. The standard value is 220k, but you could try going a little lower to reduce gain and maintain maximum headroom through the second stage. 200k or 180k resistors would be good starting points for experimentation. Maybe try the 180k first.

Also, double check that your values for R17 and R29. These two resistors will influence the bias point of the tubes. For maximum clean you want the tubes biased closed to their center. 1.8k should get close.
Click to expand...

Wow!

Great info.
I'll try them soon.
This is the info that I've been looking.

R28 of my Blackface and CLEAN are 220K, so I'll try 200k and 180k.

https://goo.gl/photos/ZjAiGLbg7La3HZGw7

For R17 and R29, Blackface has both 1.8kohm, but my CLEAN module has 4.7k for R17 and 1.8k for R29. Should I change R17 to 1.8K?


Thank you so much.[/url]
 
ilearn said:
Should I change R17 to 1.8K?
Click to expand...

Interesting that the CLEAN module has 4.7k for the cathode resistor on V1A. What is the value of the anode resistor (should be R25)?

A center biased arrangement for a 12ax7 is normally 100k for the anode resistor and around 1.5k for the cathode resistor which should you give you maximum clean headroom on that stage.

However, increasing the ratio of anode to cathode resistor (by lowering the cathode resistor value from 4.7k to say 1.5k) will increase the stage gain and could result in more distortion at V2B.
 
Sweet. If that's the case, then dropping R17 to 1.8k or 1.5k should get you more "clean" from your clean module.
 
CrunchBunch said:
Sweet. If that's the case, then dropping R17 to 1.8k or 1.5k should get you more "clean" from your clean module.
Click to expand...

ok, I tried changing R17 and R28.
But somehow it added more gain to my CLEAN module.
I changed them back.

My Blackface module had 1.8K for R17, but the 'MTSmodulevalue spreadsheet' has 4.7K for R17. So I tried 4.7k and suddenly my Blackface got more headroom.
I have no idea whether this make sense or not. I don't have a good understanding on how tube amplifiers work.

Anyhow, I have both modules working way better than it was for my own purpose.

Many Thanks for your help, Crunchbunch.

Cheers!
 
Got a CLEAN module that isn’t working so I opened it up and discovered that the pot values vary greatly from the spreadsheet of module components (thanks Graham). The cap values are off too, so not sure what’s happened here. It doesn’t look as though it’s been modded.

VR1 is cabbaged so that’ll be the obvious place to start with the solder iron, then I’ll make those cap changes as advised.

Many thanks.
 
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