6Av5GA questions

2nd harmonics for the masses

6Av5GA questions

Postby joncar » Sat Jun 09, 2012 8:35 am

I have just finished my first GetSetGo and am very pleased with the results. I am using 6AV5GA tubes and have read the forum entries relative to that version. My knowledge is pretty limited and I have some questions I am hoping someone can help me with.

In one of EWBrown's entries he says he used a 470 ohm resistor between plate and screen grid instead of the 100 ohm I have seen mentioned in other entries. I am wondering how the value is chosen and what effect it has.

He also mentions using 1K grid stopper on the output tubes. My understanding of the use of these is hazy--to prevent oscillation by the tube but what other effect is there. Am I right in assuming that it limits the high frequencies produced by the circuit? I get the impression that there is a benefit to using them all the time but is there a downside to doing so?

My apologies if these seem pretty fundamental but it would help me in furthering my somewhat limited understanding.

I would just like to add how pleased I am with what Shannon has done with his designs and these projects as well as the support he offers.

Thanks for any help with my questions.
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Re: 6Av5GA questions

Postby Shannon Parks » Tue Jun 12, 2012 5:30 am

joncar wrote:In one of EWBrown's entries he says he used a 470 ohm resistor between plate and screen grid instead of the 100 ohm I have seen mentioned in other entries. I am wondering how the value is chosen and what effect it has.


I didn't re-read Ed's original post, but the value isn't critical. I see 100 ohms used as the default triode strapper resistor in many old datasheets. 470 ohms might have been what he had on hand. Also, if the screen's voltage rating is low, the larger value resistor does help quell any meltdowns if prone to runaway. I don't believe that is a concern here, though 6AV5 usage is relatively new audio-wise, so I can understand being careful.

joncar wrote:He also mentions using 1K grid stopper on the output tubes. My understanding of the use of these is hazy--to prevent oscillation by the tube but what other effect is there. Am I right in assuming that it limits the high frequencies produced by the circuit? I get the impression that there is a benefit to using them all the time but is there a downside to doing so?


The 1k grid stopper isn't rolling off any audio frequencies for you, so don't worry. In this application, it isn't even needed for oscillation protection, as you are operating with no feedback. But this resistor *does* calm things down if the driver tube drives the power tube grid into the positive region. Indeed, this ends up being one of the keys to guitar amp sound. We hope to never drive our hifi amps into the positive grid voltage, but if we do, it will prevent too much grid current. We'll know it happens due to the obvious distortion.

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Postby EWBrown » Wed Jun 13, 2012 2:49 pm

The resting current going thru the 470 ohm SG resistor on the 6AV5GA is on the order of a couple milliamps, the voltage drop through it is just under one volt. Haven't measured across it during operation.

I've used screen grid stopper resistors from 100 ohms to 1K, it doesn't really seem to make a difference in the sound or stable operation. I'll use at least a 1 W resistor, just in case something goes "wrong", but still it's small enough to "fuse" if there is a serious problem.

The 1K grid stoppers are there to prevent any RF oscillation or instability issues, the resistance, combined with the tube's internal grid to cathode (and plate) capacitance effectively behaves as a low-pass filter, but does not affect the audio response, the roll-off is in the magahertz domain, at least a couple orders of magnitude above the normal 20 KHz upper audible range. Once again the value isn't critical, I've seen values from 220 ohms to 10K, carbon comp resistors are the best choice, as they aren't inductive as some metal films might be. 5% or even 10% carbon comps are more than good enough here. Since, in normal Class A operation, the control grid does not draw any current, the voltage loss across the resistor is essentially zero, at all audio frequencies.

From running many SE-AMP CAD simulations, and comparing them with actual working circuits, I found that the triode-strapped 6AV5GA (and 12AV5GA) , at B+ voltages between 300 and 385VDC, behaves a lot more like a small 300B (within it's real 20W PD limitations) , than a 6B4G or 2A3, just don't expect the 300B's maximum voltage and 40W plate dissipation capabilities, with the smaller 6AV5GA, in triode operation.

Using 6B4G as the SE-CAD substitute, I had an error range of about 10%, with using 300B, it was within 2% of my actual measured voltages and currents.

/ed B
Last edited by EWBrown on Sat Dec 28, 2013 9:52 pm, edited 1 time in total.
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Postby joncar » Wed Jun 13, 2012 7:46 pm

Thanks to Shannon and EWBrown for clarifying these issues for me.

It is very helpful and appreciated.
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Re: 6Av5GA questions

Postby HighCtenor » Fri Dec 27, 2013 10:41 pm

Can anyone tell me if a 6fw5 is a 6av5ga replacement or upgrade. It has different v/max values but the same pinout wiring: 550V/5500 to 770v/6500. Is it superior in sound to standard 6av5gas? I imagine it does not pick up detail in the same way a 300B does, but I would ask if it is on par with a 2a3/6b4g. I am educating myself before I either order or build based on a gsg as a possibility. Thanks
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Re: 6Av5GA questions

Postby EWBrown » Sat Dec 28, 2013 9:56 pm

6FW5 has higher 19 Watt plate dissipation rating, a swith most TV horizontal deflection power tubes, it can be pushed harder,
25 to 30 watts, very similar in basic operating characteristics to 6GE5 and 6GV5. 6FW5 can be a "drop in" replacement for 6AV5GA,
may have to tweek the cathode resistor value slightly.

Different transconductance and internal plate resistance, triode strapped, it behaves like a 6B4G, with higher B+ and plate current capabilities.

They are happy with a 3K primary OPT, but anything between 2.5K and 5K is good.

It is best to run any of these deflection tubes in the triode-strapped, cathode biased mode, it is more "forgiving" and reduces the possibility of
unwanted / unexpected meltdowns.

/ed B
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