6AV6 heather-cathode voltage

[cdemike]

I have a Marshall-style amp with some very high preamp voltages that I've adjusted to put the cathode follower very close to the heater-cathode potential right under the datasheet maximum of 200V for a 12AX7. Trouble is that I have a spare triode in the amp and don't particularly want to to use it for a tremolo, effects loop buffer, etc., so I'm thinking I'll just swap the noval socket for a 7-pin socket and load a 6AV6 in place of the 12AX7 which could be used for other amps.

Trouble is that I'm finding contradicting values for maximum heater-cathode potential. The RCA datasheet (https://tubedata.edebris.com/sheets/049/6/6AV6.pdf) lists 200V just like a 12AX7, whereas the Tungsol (https://frank.pocnet.net/sheets/127/6/6AV6.pdf) and GE (https://frank.pocnet.net/sheets/093/6/6AV6.pdf) datasheets list 90V. It seems odd that different manufacturers would rate them differently, so I'm wondering if that's possibly just a misprint by RCA. Or did RCA just make a more robust 6AV6?

[pdf64]

There's the design centre to design maximum apples to oranges thing muddying the water.
A 200V design maximum limit would be perhaps 180V design centre.
Tied up with that is older and more recent info, RCA is 1959, GE 1955, TungSol is a 1959 print of 1948 info.

[maxkracht]

I don't have answers about the 6av6, but I would probably just float the heater and call it a day. Use a diode or neon for arc suppression on the cathode follower for good measure.

[B Ingram]

... The RCA datasheet (https://tubedata.edebris.com/sheets/049/6/6AV6.pdf) lists 200V just like a 12AX7, whereas the Tungsol (https://frank.pocnet.net/sheets/127/6/6AV6.pdf) and GE (https://frank.pocnet.net/sheets/093/6/6AV6.pdf) datasheets list 90V. It seems odd that different manufacturers would rate them differently, so I'm wondering if that's possibly just a misprint by RCA. ...

Assume "misprint by RCA."

A note for the RCA sheet you supplied says "DC component must not exceed 100 volts" for one of the values, implying the thing printed is an "AC value" or a "Peak Voltage value."

Looking at the 6AV6 entry in RCA's tube manual reveals an "Average Value" rating of "100 max Volts." Then +/-200v is listed as a Peak Voltage value.

Assume the 6AV6 is no more-capable than a 12AX7, because it's just a single 12AX7 section with a couple diodes.

[cdemike]

Thanks everyone! It sounds like it probably makes most sense to elevate my heaters. I had some concern about insulation ratings on my PT's secondaries since the heaters would need to be elevated a little over 100V, but I emailed Heyboer and they said they rate their secondaries for 1500V. Certainly seems like a wide enough margin to elevate those heaters 110V or so, but then I think I run into an issue then during the period of time between when the amp powers on and when the tubes start conducting after the cathodes warm up; with the heaters elevated ~110V, they'd be 110V more positive than the cathode prior to flipping the standby switch at which point they'd be safe again with the cathode ~80V over the heaters.

Maybe I'm over-complicating this and it'd make more sense to just switch the order of the 12AX7 and 6AV6 in the signal chain with the 6AV6 appearing to operate equivalently to a 12AX7 but with a less robust h-k potential rating. In other words, maybe rather than using the 6AV6 as a dedicated cathode follower, I'd just use that as the input gain stage with the 12AX7 doing its usual double-duty as gain stage and CF. I originally meant for the setup with the 6AV6 to be so that I could highlight a particularly nice NOS Brimar 12AX7 for the actual amplification to get the most out of it, but I'm not really sure it's possible to keep the operating points anywhere similar with the 6AV6 as a CF.

[B Ingram]

... I'm not really sure it's possible to keep the operating points anywhere similar with the 6AV6 as a CF.

A cathode follower is an excellent place for you to use a JFET instead of a tube.

The cathode-follower has 100% negative feedback, so the tube wasn't "contributing tube-y-ness" anyway.
And the JFET has to heater, so no heater-to cathode voltage rating issue.

[cdemike]

... I'm not really sure it's possible to keep the operating points anywhere similar with the 6AV6 as a CF.

A cathode follower is an excellent place for you to use a JFET instead of a tube.

The cathode-follower has 100% negative feedback, so the tube wasn't "contributing tube-y-ness" anyway.
And the JFET has to heater, so no heater-to cathode voltage rating issue.

I've been wondering about this after reading R.G.'s suggestion to use a MOSFET in lieu of the cathode follower. I recognize there's an important difference between JFETs and MOSFETs in terms of depletion vs enhancement mode transistors, but I'd be lying if I said I really understood how to predict how a JFET would behave in those circumstances (I'm very, very green when it comes to solid state design). I understand a critical part of "the sound" for a Marshall is the CF rounding off the positive waveform peaks due to grid current in a high plate resistance triode like 12AX7. Would a JFET draw gate current in a way that would mimic that effect? As I understand it, a MOSFET shouldn't draw any gate current?

[LOUDthud]

There are some 300V JFETs, but it's probably cheaper to use a tube. A MOSFET will draw Gate current just after you killed it. You could use a low Voltage zener diode (4V to 5V) on the gate, but it wouldn't be anything like the soft clipping rounding effect of a tube cathode follower.

[Bergheim]

... I'm not really sure it's possible to keep the operating points anywhere similar with the 6AV6 as a CF.

A cathode follower is an excellent place for you to use a JFET instead of a tube.

The cathode-follower has 100% negative feedback, so the tube wasn't "contributing tube-y-ness" anyway.
And the JFET has to heater, so no heater-to cathode voltage rating issue.

I'm not gonna say a JFET doesn't have that characteristic as well, because I've not tinkered with those, but a DC coupled CF has a very soft and assymetrical distortion when overdriven. Merlin Blencowe has this topic covered on his site (http://www.valvewizard.co.uk/dccf.html). A preamp with a DC CF as its last stage has a very different character (more compression and 2nd order harmonics) than one without it. I'm usually not a big fan of the DCCF sound myself, so I even tried to bypass the CF in a Marshall style preamp once, and although the gain didn't drop that much, the character changed a lot.

[LOUDthud]

Technically speaking a 12AX7 triode doesn't make a very good cathode follower because when the Plate to Cathode Voltage goes below 100V at 2mA you start drawing grid current. The lower the Plate to Cathode Voltage goes, the higher the Grid current. So it rounds off the top of the waveform. If you connected the Plate of the Cathode follower to a higher DC Voltage than the Voltage used for the gain stage, it wouldn't distort the waveform as much. Other tubes like a 12AT7 or 12AU7 make a lower distortion cathode follower if you want lower distortion.

In the RCA tube manual if you want to look at the Plate curves of a 12AX7, it says look at the 6AV6 data sheet.