5687 Srpp

[Scrappydoo]

Hi, I've just finished making a lampizator output on a tda 1541 dac using the 5687 as opposed to the 6h6p used in lampizators circuit.

It uses 180v B+, 13ma plate current and 220ohm resistors for the cathodes.

I don't understand valves well enough to determine if this is the most suitable setup for the 5687. Looking the characteristics chart it looks like it's running at the low end of the lines where they start to curve, but this may not be an issue running in srpp. I don't know, can anyone explain this or suggest resistors that may be better?
Thanks.

[kevinkr]

Originally posted by Scrappydoo
Hi, I've just finished making a lampizator output on a tda 1541 dac using the 5687 as opposed to the 6h6p used in lampizators circuit.

It uses 180v B+, 13ma plate current and 220ohm resistors for the cathodes.

I don't understand valves well enough to determine if this is the most suitable setup for the 5687. Looking the characteristics chart it looks like it's running at the low end of the lines where they start to curve, but this may not be an issue running in srpp. I don't know, can anyone explain this or suggest resistors that may be better?
Thanks.

It's always an issue, at low amplitudes you may get away with operating near the less linear portion of the characteristic curves, and in this case this may be so - that said a few more volts and a few more mA would move things in the right direction. (Just watch out that you do not exceed the combined plate dissipation rating.)

[Scrappydoo]

Ok thanks,
I'll try cranking the voltage up.
What would be the limit on the plate current?

[Poindexter]

If you look a at triode plate curve plot, the most linear region is in the upper left hand corner - low volts and high current.  Keep the voltage down, and increase current (reduce the value of the cathode resistor) until the voltage across it is a little higher than the peak single direction swing you need to have on the grid (half the p-p swing, or 1.4 × the ac (rms) voltage).  Easy to try; all you need is a resistor.

Aloha,

Poinz
AudioTropic

[Scrappydoo]

Originally posted by Poindexter
If you look a at triode plate curve plot, the most linear region is in the upper left hand corner - low volts and high current.  Keep the voltage down, and increase current (reduce the value of the cathode resistor) until the voltage across it is a little higher than the peak single direction swing you need to have on the grid (half the p-p swing, or 1.4 × the ac (rms) voltage).  Easy to try; all you need is a resistor.

Aloha,

Poinz
AudioTropic

As this circuit is srpp, does that mean I need to change both cathode resistors?
At the moment they are 220ohm. And with the circuit being srpp do I have to change both to the same value?

Also the rms output from the tda1541 was 65mv to the grid ,with a 50ohm resistor to ground and a 100hz signal.
Thanks.

[Poindexter]

Yes, you should change both cathode resistors.  They should stay about the same value. I've operated the top tube with a little less resistance on the cathode so that its gm is a little higher, thus balancing the circuit just a bit better, but this will decrease the gain slightly, and you're going to need all the gain you can get.

If the voltage on the output of the DAC is 65mV, I don't think this circuit is going to give you enough gain.  You may end up plugging a 6SL7 into it and adjusting the circuit values.  On the plus side, you can certainly operate your devices low and hot if they only have .065Vac on the grid.

Try changing those 220Ω Rk to 150 or 120Ω, and if necessary bring the PS voltage down a little by increasing the value of the dropping resistor (it'll also give you a little smoother rail).  This sort of tweaking is normal in the development of a new circuit; listen in between, have fun!

Aloha,

Poinz
AudioTropic

[Scrappydoo]

Yes, gain is quite low, I'm only getting 600mv on the output.
Is there anything I can do to get more gain apart from going to a different tube?
Thanks.

[Poindexter]

No.  The problem you are going to have in the larger sense is that the gain you need for your DAC is so much different than for a standard line level source, so the piece will not be useful for both.

Plug a 6SL7 into the circuit, adjust the cathode resistors to get a reasonable OP for the device, and hear what you can hear.  If you are getting about 100v on the tube (200v B+ voltage), try about a 470Ω resistor on the cathode and see what the current is.

Aloha,

Poinz
AudioTropic