Mk III cap mod (cathode)

[HelpingFriendly]

I was wondering if anyone has experimented with different values on the cathode caps on the mark 3? I've read that it is a pretty popular mod on mark ii's to lower the uf some to reduce unnecessary bass in the first stage. I feel I could benefit from a small reduction in bass on my purple stripe. Which cap makes the most sense for this? (cathode). I have done some other mods with great success and was hoping to make this one my last. I tried reducing with the bass knob but I feel that it looses it's punch. Same with the GEQ. Any input on this mod is greatly appreciated. Thanks

 

[Boogiebabies]

I was wondering if anyone has experimented with different values on the cathode caps on the mark 3? I've read that it is a pretty popular mod on mark ii's to lower the uf some to reduce unnecessary bass in the first stage. I feel I could benefit from a small reduction in bass on my purple stripe. Which cap makes the most sense for this? (cathode). I have done some other mods with great success and was hoping to make this one my last. I tried reducing with the bass knob but I feel that it looses it's punch. Same with the GEQ. Any input on this mod is greatly appreciated. Thanks

This is like a "What is your favorite ice cream" question.

Regardless of what value you choose, your ears will have to cope with the effect down the signal chain.

My thoughts are to first install some terminal or test points so you can swap out 50 caps if you want and not lift the
pads on the circuit board, I hate working on MK III's because of this.

So, the positive. You can listen to everything from .22, .47, .68, 1uf, 2.2uf, 3.3uf and 5uf to whatever and be sure that the results
are what you want to hear. I would start at 10uf and see how low you can go without making the lead channel a buzz saw. 15uf passes
a fair amount of lows but it really is what makes that circuit sing. The original caps are 6V tantalum, but you can use just about any type of cap construction but tantalum were chosen for there smooth quality. When I prototype circuits I use a decade, or multi-pole switch with multiple caps on and run through the different values in different situations, volumes, rooms and settings until I am positive that
value is what I really want. If I really like two, I make them switchable and keep them.

 

[HelpingFriendly]

I was wondering if anyone has experimented with different values on the cathode caps on the mark 3? I've read that it is a pretty popular mod on mark ii's to lower the uf some to reduce unnecessary bass in the first stage. I feel I could benefit from a small reduction in bass on my purple stripe. Which cap makes the most sense for this? (cathode). I have done some other mods with great success and was hoping to make this one my last. I tried reducing with the bass knob but I feel that it looses it's punch. Same with the GEQ. Any input on this mod is greatly appreciated. Thanks

This is like a "What is your favorite ice cream" question.

Regardless of what value you choose, your ears will have to cope with the effect down the signal chain.

My thoughts are to first install some terminal or test points so you can swap out 50 caps if you want and not lift the
pads on the circuit board, I hate working on MK III's because of this.

So, the positive. You can listen to everything from .22, .47, .68, 1uf, 2.2uf, 3.3uf and 5uf to whatever and be sure that the results
are what you want to hear. I would start at 10uf and see how low you can go without making the lead channel a buzz saw. 15uf passes
a fair amount of lows but it really is what makes that circuit sing. The original caps are 6V tantalum, but you can use just about any type of cap construction but tantalum were chosen for there smooth quality. When I prototype circuits I use a decade, or multi-pole switch with multiple caps on and run through the different values in different situations, volumes, rooms and settings until I am positive that
value is what I really want. If I really like two, I make them switchable and keep them.

Thanks boogiebabies! Great input indeed. I would probably just want a small decrease in bass. What do you suggest I start with? And most importantly, which # cap is it on the board? Can you point it out for me? I will probably do as you say and line up a number of values to find my magic value

 

[Boogiebabies]

What is the value of the cap on V1B pin 8 ? Both caps for the V1 are right behind the bass pot. The one on V1 A has a .47uf cap and a 15f cap connected to the pull shift function.

 

[HelpingFriendly]

Thanks man! I will check when I get home. I know the blue cap is .47uf. All the blue caps in the preamp section on the board are not factory installed. They were put on by the person's tech I bought the amp from. I'm trying to find a good schematic for the purple stripe so I can make sure those secondary blue caps are the right value. I assume they are. I wish there was a good schematic with all the caps numbered like on the actual circuit board. Thanks! Will keep you posted

 

[HelpingFriendly]

Ok so the values seem to be good. Just so I get this right I'm posting another pic of the circuit, I circled the cap that I think is the one I want to reduce in white. Is this the correct one? It's an orange drop. I'm confused cause you mentioned tantalum. Thanks

 

[HelpingFriendly]

I'm going crazy over here! Can someone tell me which cap it is? Now I think it's the little tan one that I circled in pink. Please help me.

 

[Boogiebabies]

The caps circled in white are for the tone circuit.

250pf Treble
750pf Treble- Pull Shift on 1000pf
.1 uf Bass
.047uf Midrange

The ones circled in pink are for the pull shift circuit. The B side cathode cap is right below it (Yellow Kemet 15uf).
Pin 8 is where it is connected to. The higher value cap on the cathode allows more lows to pass, but can also effect midrange
to some extent.

The 12AX7 has 9 pins. Count from right to left on the V1 solder pads.

1. Plate
2. Grid
3. Cathode
4. Heater
5. Heater
6. Plate
7. Grid
8. Cathode
9. Heater

 

[HelpingFriendly]

Thanks boogiebabies! You are the man! I actually reduced the "other" yellow 15uf cap right above the one connected to pin 8 and I got an improvement. I brought that one down to 1uf and I swear it got rid of the congestion a bit. Unless its in my head but..idk . I will experiment tomorrow with the cathode cap on pin 8 and see if that get me in the right direction even more. My goal is to keep the bass knob around 3.3 and have it be present but not over powering. Thanks again man. I'll keep you posted.

 

[Henz]

Thanks boogiebabies! You are the man! I actually reduced the "other" yellow 15uf cap right above the one connected to pin 8 and I got an improvement. I brought that one down to 1uf and I swear it got rid of the congestion a bit. Unless its in my head but..idk . I will experiment tomorrow with the cathode cap on pin 8 and see if that get me in the right direction even more. My goal is to keep the bass knob around 3.3 and have it be present but not over powering. Thanks again man. I'll keep you posted.

The 15uf that you reduced only affects the Bass pull shift frequency. Correct?