Tips for increasing touch sensitivity??

Prattacaster

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These amps have the potential to sound pretty good, there is some goofy stuff going on in there.

The amp lacks touch sensitivity because it is lacking mids; the cluster of components between the first stage and the volume control is a type of notch filter. This has to go.

Snip C4 and C3, jumper over R9 and play. If the bottom is flabby when you crank it then do the "Ampeg mod" to the cathodyne phase inverter.

These two simple mods will do a lot to improve this amp.
 

12strings

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Hi, thanks for your response! I am sorry, should have mentioned I already did those mods. Helped out a ton. Do u think lowering the 20uf filter caps in the preamp section to 10uf will help?
 

Prattacaster

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If you feel the amp is a little to tight then lowering to 8uF or 10uF will help. I'd use an 8uF/450v TVA.

Lower the first plate resistor R5 to 100k and the a cathode resistor to 1.5k. Or you could go to 220k with a 2.2k cathode, maybe try that one first. When you do this you also need to increase the coupling cap to have the same low frequency roll-off. I'd go with a .01 or .02 for the first stage coupling.

You could do a similar mod to the second stage R17, C18, R16. I would also think about increasing C5 to 1uF or 2uF to beef things up a little.

If it is still lacking I'd be playing thru a different speaker, or swapping in different tubes.
 
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Take a look at your plate voltages on the preamp tubes. The 6EU7 has an amplication factor of 100 so you're good there. Bringing up your plate voltages around 200 vdc on V2 would go a long way to bring some life to that amp.
 

Prattacaster

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Hi HT, thanks for your response. What's the best way to increase the plate voltage on V2?
Start out by changing R18 to 1K ohm then check plate voltage on V2 and see how the amp sounds to you.
This is true but if you lower the plate resistor value then it will drastically change the plate voltage. I'd lower the plate resistor first before tweak the Power Supply.
 
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This is true but if you lower the plate resistor value then it will drastically change the plate voltage. I'd lower the plate resistor first before tweak the Power Supply.
Lowering the plate load resistor value may seem like it could "drastically" increase the plate voltage but that is far from the truth as the plate load resistor does not function as a dropping resistor. In fact if you dropped the value of the plate resistor without lowering the value of the cathode resistor the gain would decrease. The only way to significantly increase the plate voltage is to increase the dc supply voltage.
 

Prattacaster

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Lowering the plate load resistor value may seem like it could "drastically" increase the plate voltage but that is far from the truth as the plate load resistor does not function as a dropping resistor.
Have you ever plotted a load line? Of course the plate resistor drops voltage, I*R=E.
Lowering plate resistors in these amp is a proven way to gain touch sensitivity.

Lower the first plate resistor R5 to 100k and the a cathode resistor to 1.5k.
This would increase that plate voltage by about 60-65vdc. Not that I think the plate voltage in the greatest concern, merely a side effect from choosing a more suitable plate load. Yes there will be a slight loss in gain(2db) but the feel and sound will be improved.

It might help to define "touch sensitivity". For some players this means getting the amp a spot where pic attack controls the overdrive of the amp, back off and its clean, dig in and it is distorted. Some amps do this much better than others. Others see touch sensitivity as purely the feel of the amp, tight vs. loose.

What matters most is the OP's definition of touch sensitivity so people can offer good suggestions.
 
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Have you ever plotted a load line? Of course the plate resistor drops voltage, I*R=E.
Lowering plate resistors in these amp is a proven way to gain touch sensitivity.

You are right the plate load resistor does drop voltage but it is in the circuit to provide a load for the plate, not for lowering the plate voltage. I would agree that what you are proposing is a valid way to improve preamp performance but is more a function of biasing the preamp tube and wouldn't be done to solely bring up the plate voltage which is what I am proposing.
 

12strings

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Gents, thanks so much for your responses. You both know way more than I do. The type of touch sensitivity I'm looking for is where pic attack controls drive of the amp.
 

Prattacaster

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solely bring up the plate voltage which is what I am proposing.
You are proposing an increase in B+ for the preamp node. I guess I'm interested in why you propose this. Is it because the filter becomes less effective thereby loosening up the front end?



The type of touch sensitivity I'm looking for is where pic attack controls drive of the amp.
This is done by having a careful balance between output and preamp overdrive. The "ampeg" or paul C mod should have help this a lot, lowering the plate resistors will warm the preamp stages up and help touch sensitivity especially in the 2nd gain stage.
 

Prattacaster

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I recommend raising the plate voltage to increase gain and headroom.
The gain would remain unaffected, the headroom would be increased a small amount, but the filter for that node become less effective as you lower it's dropping resistor, which by historic standards is already a pretty low value for a preamp node.

Sometimes things don't have to make sense to sound good tho.
 
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The gain would remain unaffected, the headroom would be increased a small amount, but the filter for that node become less effective as you lower it's dropping resistor, which by historic standards is already a pretty low value for a preamp node.

Sometimes things don't have to make sense to sound good tho.
I have not seen it mentioned anywhere or know of anyone that makes the claim that increasing plate voltage does not increase gain, until now I guess.

Maybe you are right. I would just like to know where you get your tube theory from. I have personally increased plate voltages on preamp tubes by reducing dropping resistor values and the results seem to support what I'm suggesting so I'm not just living in the theoretical.
 




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