Basic Q on tube amp design

[mrstone]

Hi all,

I'm new in this forum and want to say Hi to everybody!

I got some simple and basic questions about the driver stage of the power amp.

Let's say I'm using 71a under the following condition:

71a:
Z = 5K
B+ = 210V
Vp = 205V
Ip = 20mA
Vg = -40V

My questions are:
- What is the min driving voltage?
- If I'm going to build an integrated amp, what is the min gain required to drive the power tube?
- Other than these, any other things I have to pay attention to?

Thanks so much!!

Cheers
Mr. Stone

 

[arnoldc]

Maybe this JE Labs WE71A will interest you...

An externally hosted image should be here but it was not working when we last tested it.

 

[Brett]

Let's say I'm using 71a under the following condition:

71a:
Z = 5K
B+ = 210V
Vp = 205V
Ip = 20mA
Vg = -40V

I presume your talking about it as an output tube and not a driver for a bigger tube.

My questions are:
- What is the min driving voltage?

40V peak, 28Vrms

- If I'm going to build an integrated amp, what is the min gain required to drive the power tube?

Somewhere between 60 and 120 depending on how sensitive you want the amp to be. I'd go for about 90.

- Other than these, any other things I have to pay attention to?

PSU, layout, earthing, chassis construction (microphonics), passive components, topology.....

 

[PRR]

Brett is correct. (Of course.)

For most normal tube amps, the peak swing on the Grid is about equal to the grid bias. You get more output power the more positive the grid goes. However at about zero grid volts, the grid current changes from about-zero to very-high. So most tube amps swing the grid from bias-point (which you say is -40V for this tube and operating condition) to zero, or about 40 volts peak (80 volts peak-to-peak).

Gain? What is your input signal? In a straight power amp, without feedback, the input signal is often 1.4V RMS or 2V peak. So you need a gain of 40V/2V= 20:1 to build a basic power amp from a 71 and a gain/driver stage.

For integrated amps expected to accept the sometimes low output of some phono preamps, which may be as weak as 100mV-200mV on some recordings, you want an extra gain of about 10 between the phone preamp and the power amp. However with hot phono preamps or about any self-amplified unit like a CD or tape player, you can use a little less. Brett seems to like 5X20= about 90 or 100, and I'd go along with that.

If you run feedback around things, you will want much more open-loop gain, but about the same closed-loop gain. (And no, I don't advocate using NFB on Type 71s... with NFB you might as well use something less rare because it all sounds the same.)

 

[fdegrove]

Hi,

with NFB you might as well use something less rare because it all sounds the same.

Wise words indeed.

Cheers,