Tube effects loop

[audiosalvage]

So I've successfully built and implemented this loop circuit. My question is which tube to boost the return, ecc832 or ecc823. I just need a double check but I would think the latter.

[maxkracht]

https://www.jj-electronic.com/en/ecc832-12dw7 pins 123 are lower gain on a, 832/dw7 opposite on the 823

[Dennis Perusse]

Hello,


Quick question on this. I have Merlin's book so I went back to it to reread that page that was listed. Now he uses a 12ax7 in his version. Now, you want to use a 12dw7 which I know has one side as a 12au7 and the other side as a 12aX7. If I may ask as to why do you want to use a 12dw7 in place of what Merlin uses in his version? Would their be other sonic characteristics in this apart from the signal going in with less gain then getting recovered with a higher gain?

The PTP layout looks really good here, but I am a bit confused on the tube choice.

Dennis

[TUBEDUDE]

It's not about the gain per se. The 12DW7 optimizes the driver and recovery functions.
The AX side provides high gain for the recovery signal. The AU side provides a low impedance output capable of driving the cable without high frequency loss due to cable capacitance.

[Ten Over]

So I've successfully built and implemented this loop circuit. My question is which tube to boost the return, ecc832 or ecc823. I just need a double check but I would think the latter.

ECC823, but that is a little bit of a rare bird that could go extinct at any time. Why not lay it out for a standard 12DW7?

[Ten Over]

It's not about the gain per se. The 12DW7 optimizes the driver and recovery functions.
The AX side provides high gain for the recovery signal. The AU side provides a low impedance output capable of driving the cable without high frequency loss due to cable capacitance.

You might want to check my math because it appears to me as though a 12AU7 will result in an output impedance at the cathode of 1.384k while the 12AX7 results in 0.85k, which would make a 12AX7 the better choice for driving capacitive loads.

[TUBEDUDE]

I guess I'd have to see your math. This loop uses a cathode follower topology doesn’t it?

[Ten Over]

This loop uses a cathode follower topology doesn’t it?

I believe it looks something like this:

Tube Loop Merlin audiosalvage.png

[Ten Over]

I guess I'd have to see your math.

I used a mu of 100 and an internal anode impedance of 62.5k for the 12AX7. I used a mu of 17 and an internal anode impedance of 7.7k for the 12AU7. The external cathode resistance is 2.2k in parallel with the 10k pot for a total of 1.8k. The 1M grid leak resistor is big enough to ignore.

CF 51 PNG.png

[TUBEDUDE]

Thanks for the thorough explanation. The grid leak is 15k rather than 1M.