Understanding the cathode follower (D-ulator)

[Bob-I]

If you using 250KA pots unity is approx. noon and noon on the knobs. Like Gary said the cathode should be at 30 volts with the new values

That's been my experience so far, but not even close with this one. With the 12AT7 I have 330 on the CF plate, 160 on the CC plate, 64V on the CF cathode and 2.4 on the CC cathode. I'm still at about 40% on both send and return for unity.

[glasman]

Bob, what is the order of your string?

The real deal used the the following order. I have seen a lot of confusion on this over the years.

Gil and I have discussed this many times as well. I am still not a big fan of the 100K series resistor. The time constant on the second stage filter is measured in days .

[glasman]

Actually it takes just under 17 seconds for the second stage to come to max voltage.

[dogears]

Because it is a 12AT7.

If you using 250KA pots unity is approx. noon and noon on the knobs. Like Gary said the cathode should be at 30 volts with the new values

That's been my experience so far, but not even close with this one. With the 12AT7 I have 330 on the CF plate, 160 on the CC plate, 64V on the CF cathode and 2.4 on the CC cathode. I'm still at about 40% on both send and return for unity.

[glasman]

Scott is correct, to use an AT7 you would have to juggle all of the values.

Bias points are different as are the plate loads for the recovery amp.

There "could" be a benefit as the AT7 has a broader frequency response than the 12AX7.

[Bob-I]

Bob, what is the order of your string?

String? What string?

I'm using a single node off the screen supply. I did this on another amp this way and it's a very effective loop, in fact the loop alone adds harmonic content to the tone even without anything in the loop, but the loop sounds great with a cheepy Alesis Midiverb.

[Bob-I]

Scott is correct, to use an AT7 you would have to juggle all of the values.

Bias points are different as are the plate loads for the recovery amp.

I put a 12AT7 in my other amp accidently, it was in the wrong box. I liked the way it sounded.

There "could" be a benefit as the AT7 has a broader frequency response than the 12AX7.

Especally on the high end with a delay.

[butwhatif]

Cool, I need to get back on my bench and try this -have you guys also increased the cath bias resistor, or does the increased tail suffice?

[heisthl]

I've had good luck using a 12ax7 and this dropping string:

[butwhatif]

I'll investigate this, thanks. Does increasing the 10k tail generally suffice to
rebias the cf stage, or have you found that the cathode resistor also needs to be increased?

[glasman]

I'll investigate this, thanks. Does increasing the 10k tail generally suffice to
rebias the cf stage, or have you found that the cathode resistor also needs to be increased?

Increasing the tail would probably do the trick, but I have not tried it. I would say increasing the cathode is still a good idea as it raises the potential difference between the cathode and grid a little bit.

[butwhatif]

I took a look at my internal dlator, and was able to fix most of the one sided clipping , here's a scope trace of the before signal out of the CF and by raising the cat bias up to 32 volts I was able to get most of the clipping out. Actually, it had to go to 60-70vdc to pass a very hi clean sine wave, but stopped at 32v as that is where the output section clips. Also, here is a trace of an OD waveform with a 1khz sig in. One thing I realized is the importance of the .05 blocking cap on the input grid of the CF stage. If that's not there--and one might not put it in, the CF stage will get rebiased by an amp that is plugged into it--like an OD master from a clone amp.

[glasman]

Very good info and you took the exact steps I took when I was searching for the problem.

Gary

[Bob-I]

I took a look at my internal dlator, and was able to fix most of the one sided clipping , here's a scope trace of the before signal out of the CF and by raising the cat bias up to 32 volts I was able to get most of the clipping out. Actually, it had to go to 60-70vdc to pass a very hi clean sine wave, but stopped at 32v as that is where the output section clips.

That's exactly what mine looked like. I put in 1.8k/27k as suggested and my cathode is at 62V. It's completley symetrical now up to the point where the clean stages clip. I hadn't thought about the output clipping but that's a good point.

What values did you end up with to get 32V?

[butwhatif]

I used 2.2k/30K, but my amp is a mod, (Mesa Mk III) not a build- My power is off the main string also, but modified, I remember around 310v on the plate of the CF.