touch sensitivity

[daz]

I finished my tweaking the other day on my el34 build but since then i realized it's not as touch sensitive as it should be. Less than most of the marshalls i've had. The tone is great but for that. Can you guys please look at my schematic and tell me what you think may be contributing to this? (It's not the choke, i tried a res.)
http://img528.imageshack.us/my.php?i...l345ar4am7.jpg

[Bruce / Mission Amps]

B+ ?

How about listing the tube's plate and cathode voltages too.

[daz]

EL34 plate=413v, biased at 40v across a 1 ohm cathode R. So i assume 40 minus 5 for the screen R=35V bias. I have gone as high as 45 tho and it's the same.
V1A, V1B, V2A, V2B PI in order=176, 205,177, 326, 217, 214

[mooreamps]

On the power tubes, on pin 4 ; replace the screen grid resistor with 5.6K ; 3 watt flame proof ; each. . . . . .


-g

[Steve Conner]

Huh? I don't know what that'll do for touch sensitivity, but I do know it'll cut the power output to about half what it was, or even less.

[daz]

Well, i tried it anyways but no cigar.

[tubeswell]

Hi Daz - do you mean "a tube current measuring 40mA when you read 40mV"? (when you say biased at 40V across a 1R cathode resistor)?

[daz]

Yes. I use a 1 ohm R at each cathode and measure across it to get the bias current. However, AFAIK you are suppose to measure across the screen R and take the voltage drop you get (5v in my case) and subtract that from your reading of the bias current to get the true current reading. IE: screen is 5 v drop. That=5ma. bias current reads 40 ma, so subtract the screen drop of 5 from that and you get 35ma.

[Shea]

You could try a 5U4GB rectifier to make it a little saggier. The B+ should go down, so you'll have to rebias.

If the phase inverter circuit isn't exactly the same as Marshall's, try making it so. Some people believe it's an essential part of the Marshall sound.

Shea

[daz]

It has plenty of sag already, and i had a sag r in there before but removed it because it was too much with the tube, which gives enough sag by itself. The PI is copied from a marshall.

[tubeswell]

Pardon my ignorance but out of curiousity, why is your PII filter between the plate and the screen? (For some reason I thought those things were supposed to go between the Rectifier and the Plate supply). There doesn't seem to be any filtering between the rectifier and the Plate.

[Shea]

Pardon my ignorance but out of curiousity, why is your PII filter between the plate and the screen? (For some reason I thought those things were supposed to go between the Rectifier and the Plate supply). There doesn't seem to be any filtering between the rectifier and the Plate.

The capacitor to the right of the choke is the filter for the plate supply. That's how it's normally done in guitar amps.

I'd use 50 uf if I were him, but I don't think using 32 is the cause of his lack of touch sensitivity.

Shea

[Shea]

It has plenty of sag already, and i had a sag r in there before but removed it because it was too much with the tube, which gives enough sag by itself. The PI is copied from a marshall.

Maybe your perception of touch sensitivity is different than mine, because for me, more sag usually means more touch sensitivity.

You could always try putting a bypass capacitor on the cathode resistor for V2A for more gain.

Shea

[Russ]

Shoudn't pin 1 (the suppresor grid) on the el34s be connect to pin 8, not to the bias supply?

russ

[daz]

LOL ! I went thru this ad nausium already. the last time someone was spanking me for having pins 1 and 8 togther instead of putting the suppressor grid to the bias ! It never ends!

Maybe your perception of touch sensitivity is different than mine, because for me, more sag usually means more touch sensitivity.

to me is when the difference between picking hard and soft goes from normal to brighter, less distortion to more, and less volume to more. On this amp that all happens but to a lesser degree. especially the brightness which doesn't change near as much as with most touch sensitive amps.

You could always try putting a bypass capacitor on the cathode resistor for V2A for more gain

Nooooo! i have more than enough already.