Question about Pentode reverb drive circuit

[SoulFetish]

The 68K Rg2 with the .056uF bypass capacitor works out to 41.8Hz. This is where the bass cut is almost complete, and it is a considerable cut. Just as an educated guess I would say that the bass frequencies start getting noticeably cut around 200Hz. I like it because those lower frequencies just muddy-up the reverb. For a 47K Rg2, a 0.082uF Cg2 might be a good choice.

I don't like high frequencies like 3KHz and above, either. If you eliminate the 27K grid stopper and put a 250pF capacitor from g1 to ground, the cut-off will be about 3.3KHz.

Other people may have different preferences.

Okay, that sounds reasonable. I’ll make the adjustments.
As far as the 220pF cap, I suppose I could make the grid leak a 1M pot, tie the 250pF to the wiper, and dial it in to taste.

[nickb]

The direct drivers like the channelroadamps one have a problem around the resonant frequency. The signal gets real ugly and the reverb doesn't sound good. In actual practice, there is usually so much stuff going on with the guitar signal that this phenomenon goes unnoticed. If you put a 470r resistor in series with the capacitor and coil, the total impedance of the combination at a given frequency doesn't change much, but it never drops below 470r at any frequency. The current delivered to the coil is essentially the same with no ugliness around the resonant frequency.

Just a mo... Isn't the whole point of the design is that is approximates a constant current? The source impedance in this case nominally 15K so adding a further series 470 ohms makes negligible difference. It might be an appropriate tweak for a much lower source impedance driver, but not here, it's just a waste of a resistor.

[66 Kicks]

Just a mo... Isn't the whole point of the design is that is approximates a constant current? The source impedance in this case nominally 15K so adding a further series 470 ohms makes negligible difference. It might be an appropriate tweak for a much lower source impedance driver, but not here, it's just a waste of a resistor.

From a current point of view, Ra (15K) is in parallel with the load (tank + capacitor). The observed phenomenon is that the tube chokes when the load becomes very small. The solution is to add a small resistor in series with the load so that the total impedance never drops to such levels. The RC impedance is nearly the same as the RCL impedance with the values used in this case.

[66 Kicks]

As far as the 220pF cap, I suppose I could make the grid leak a 1M pot, tie the 250pF to the wiper, and dial it in to taste.

I don't think that you would want to change the 1.5M / 220K voltage divider because that adjusts the amplified guitar signal to the appropriate level for the reverb driver. I would just change the 27K grid stopper to 220K for anti-oscillation purposes and see how it sounds.

[SoulFetish]

I don't think that you would want to change the 1.5M / 220K voltage divider because that adjusts the amplified guitar signal to the appropriate level for the reverb driver. I would just change the 27K grid stopper to 220K for anti-oscillation purposes and see how it sounds.

Ah Ha! That's is the correct answer.
I, um, was just making sure we were all still paying attention.... carry on.

[SoulFetish]

Here is the full drive and recovery stage.

[nickb]

From a current point of view, Ra (15K) is in parallel with the load (tank + capacitor). The observed phenomenon is that the tube chokes when the load becomes very small. The solution is to add a small resistor in series with the load so that the total impedance never drops to such levels. The RC impedance is nearly the same as the RCL impedance with the values used in this case.

Agreed, but it the current through the tank that matters, not the plate voltage swing. All the 470 does is reduce the current in the tank slightly.

[66 Kicks]

Agreed, but it the current through the tank that matters, not the plate voltage swing. All the 470 does is reduce the current in the tank slightly.

I didn't say anything about any plate voltage swing. I said the tube chokes when the load impedance becomes too small, and it does. That's why we have output transformers. You have posted simulations that clearly illuminate the problem and the simple remedy. What they don't show is the garbled signal at and around the resonant frequency. But if a 470r resistor violates a religious belief that you hold, then by all means leave it out.

[nickb]

Agreed, but it the current through the tank that matters, not the plate voltage swing. All the 470 does is reduce the current in the tank slightly.

I didn't say anything about any plate voltage swing. I said the tube chokes when the load impedance becomes too small, and it does. That's why we have output transformers. You have posted simulations that clearly illuminate the problem and the simple remedy. What they don't show is the garbled signal at and around the resonant frequency. But if a 470r resistor violates a religious belief that you hold, then by all means leave it out.

If you cannot see it from the results above then I'm sorry, I don't think I help you. There is nothing bad happening in the tank current around resonance of 500Hz and that is all that matters. It a matter of current in the tank, not religious belief. If there is some other parameter that I've over looked that is relevant, I'd be genuinely interested, to learn about it.

[Helmholtz]

While I agree that 470R in series with 15k cannot change much, I suggest to show the simulated current wave shapes at the resonance frequency and compare the max. current levels before clipping with and without the resistor.

[nickb]

While I agree that 470R in series with 15k cannot change much, I suggest to show the simulated current wave shapes at the resonance frequency and compare the max. current levels before clipping with and without the resistor.

Good idea

Here you go. I threw in an FFT of both to allow further comparison. Beware the change in color between the two plots!!!

[66 Kicks]

If you cannot see it from the results above then I'm sorry, I don't think I help you. There is nothing bad happening in the tank current around resonance of 500Hz and that is all that matters. It a matter of current in the tank, not religious belief. If there is some other parameter that I've over looked that is relevant, I'd be genuinely interested, to learn about it.

This is why I yank on your chain.

If it's all about the current, then why don't you use the Norton equivalent?

[66 Kicks]

Good idea

Here you go. I threw in an FFT of both to allow further comparison. Beware the change in color between the two plots!!!

Here is what it really looks like.

[nickb]

Here is what it really looks like.

These are not relevant results as
1) You are showing the plate voltage not tank current
2) You are driving it into clipping

Back down the drive to keep in in the linear region, add a small resistor on the ground side of the tank and monitor the voltage across that.


I see the same if I crank it into clipping, but that is not how we are operating, or at least shouldn't be. it Even then there is no significant difference with and without the 470 ohm.




A Norton equivalent gives the same result. Mr Thevinen was great relieved to learn of that Since we are using idealized sources here the frequency response is different from when driving with the tube, but the result is the same.

[Helmholtz]

What is the DCR of the driver coil? This acts just like the 470R as an extra series resistor in this highly damped series resonant circuit. (The resonant frequency could be varied by using a different coupling cap.)