Pros and Cons of this technique?

[G]

And for anyone interested in building this here is the PSU. One note though. I'm running this at 55mA bias on the output tubes because of the current limitation on the output transformers I'm using. If you can I would run the bias at 70mA on the output tubes. You will have to use a power transformer with a little higher secondary voltage in that case. The transformer represented in the schematic is a Hammond 372JX.

G

[Brett]

Put the bypass cap around both resistors, otherwise the variable one will also add to the output Z.

[G]

Oops. Good eye Brett. Will do.

G

[G]

Here is the correct one.

[fdegrove]

Hi,

Gavin,

I would put the 1000µF on the 5687 cathode and vice versa.

Positron,

Just some thoughts of my testing and experience.

Very good remark, it happens to concur nicely with test run here locally in Western Europe.

When I first mentioned it here I met with some rather fierce opposition though...

Cheers,

[45guy]

Hi Frank,

Wouldn't the 470uF on the 5687 give a -3dB point of around 1Hz? Did I do the math wrong...or the wrong math? Or is there something else coming into play that I'm not aware of here?

Tom

[Positron]

Thanks Fd.

I know what you mean.

Not too many have the time and expertise to experiment as much as we do.

Have a great week.

[fdegrove]

Hi,

Wouldn't the 470uF on the 5687 give a -3dB point of around 1Hz? Did I do the math wrong...or the wrong math? Or is there something else coming into play that I'm not aware of here?

Nothing really wrong here but why go so deep in the bass department when you know that the OPT won't pass it anyway?

Maybe I could just as have well said, keep the 470µF on the input but use no more than 220µF on the powertube...

IME it is not going to do transient response any good, making for a rather slow amp.

Cheers,

[G]

Guys my recipe for designing a tube amp is simple and in your eyes probably leaves a lot to be desired. I have no idea what calculation you are talking about as far as the bypass cap on the 5687 goes. Maybe you would be kind enough to explain. I put the cap there strictly to lower the "ra". Or is it "Ra"? I put the rather large bypass cap on the output stage to maximize the bass extension as triodes are weak in this area. At least triodes in SE amps are. Haven't gotten around to PP yet. I assume the calculation is similar to the calculation for a coupling cap. Sometimes ignorance is a blessing and sometimes a curse.

G

[45guy]

Hi Frank,

I just wondered if there was reason to lower the -3dB point on the driver tube and raise it on the output tube. Now I see that you were just working with what G had already ordered.

G,

You probably have the correct equation. I use 1/ (2*pi*R*C) I think that's the correct one in this case. That's about as complicated as my math skills will allow at this point. I'm still having a tough time juggling all the variables in circuit design too, so you are hardly alone. I think it's a life-long thing..learning that is.

Thanks,
Tom

The more I learn, the more questions I have.