12AU7 plate voltage

[igiu83]

Hi guys!!

In my research for a JCM/JMP low watt type amp build, i discovered this JCM 800 MICRO of RobRobinette https://robrobinette.com/RR2104_Master_ ... M800_Micro

Really great great site... But, based on that schematic, i see that on those 12AU7 plates there are circa 370 V DC, am i right?

If this is correct, considering what datasheets say about a max plate voltage of 300-330 V DC, is it safe to apply that high voltage on plates???

Or am i missing something??

Thanks in advance for any reply...


Andrea

[ChopSauce]

Did you attempt to ask Rob directly, for example on the TDPRI forum?
(sorry, I don't know for sure)

Outside the US, you might as well consider Tube Town (Germany) low Watts amp (verstärker) kits, whose schematics - and sometimes demos - are available for free.

[Snicksound]

Hi guys!!

In my research for a JCM/JMP low watt type amp build, i discovered this JCM 800 MICRO of RobRobinette https://robrobinette.com/RR2104_Master_ ... M800_Micro

Really great great site... But, based on that schematic, i see that on those 12AU7 plates there are circa 370 V DC, am i right?

If this is correct, considering what datasheets say about a max plate voltage of 300-330 V DC, is it safe to apply that high voltage on plates???

Or am i missing something??

Thanks in advance for any reply...


Andrea

Reading the JCM800 Micro thread on the TDPRI forum, it seems that one reason voltages are so high is so the preamp can sound as close to a real JCM800 as possible. This makes sense to me, but it's also playing a bit of lottery with the power tube. They're rated for 300VDC as you say, so some may tolerate it, others might self destruct early. You can always upgrade to an ECC99, which is like a 12BH7 but in a taller envelope and good for 400VDC with 3.5W max TDP per triode. Might require a different bias resistor. I'm building a high gainer with an ECC99 right now, can't wait to hear it.

Or you can use a 269GX PT instead of the JX which will give you just over 300VDC (factor in OT efficiencies and voltage drop across the cathode resistor and we're good). That will "brown" out the preamp though due to lower voltage, which you may or may not like. (Sorry, I thought it was a Hammond 269JX based on specs, I don't know if ClassicTone makes an equivalent to the lower voltage GX).

Another option is running the preamp and power amp supplies in parallel and adding a sag resistor to the power node to reduce that voltage further. But given the low currents here, we're talking over 3K ohms so that might be too much sag under load.

[Stevem]

Don't forget that like any Cathode biased tube the voltage that's developed on the Cathode gets subtracted from the plate voltage when your determining the actual plate voltage on the tube!

[Snicksound]

Don't forget that like any Cathode biased tube the voltage that's developed on the Cathode gets subtracted from the plate voltage when your determining the actual plate voltage on the tube!

It's not nearly enough here though, we're talking roughly a 10VDC drop which would only bring it down to 360VDC, aka 60VDC or 20% over max.

[Phil_S]

If you are truly concerned about plate voltage, use a 6SN7 or a 12AN7 (12V heater) instead of the 12AU7. It is the same tube in an octal bottle, with max Va = 450v. Problem solved! You can buy old stock of this tube relatively cheap. New production EH, Tung Sol, JJ, Sovtek, maybe others run around $20. 6N8P Chinese is another alternative.

I realize many people do not like to buy tubes on eBay. I suggest, however, there are deals so cheap that it is worth rolling the dice. Examples are plentiful:
https://www.ebay.com/itm/Pair-Sylvania- ... Sw1zReokiU
https://www.ebay.com/itm/6SN7GT-6SN7-Tu ... SwZrteo68I
https://www.ebay.com/itm/Tested-6SN7-GT ... Sw185eopF4
https://www.ebay.com/itm/Vintage-Tests- ... SwhBFd9zRB

[Snicksound]

If you are truly concerned about plate voltage, use a 6SN7 or a 12AN7 (12V heater) instead of the 12AU7. It is the same tube in an octal bottle, with max Va = 450v. Problem solved! You can buy old stock of this tube relatively cheap. New production EH, Tung Sol, JJ, Sovtek, maybe others run around $20. 6N8P Chinese is another alternative.

I realize many people do not like to buy tubes on eBay. I suggest, however, there are deals so cheap that it is worth rolling the dice. Examples are plentiful:
https://www.ebay.com/itm/Pair-Sylvania- ... Sw1zReokiU
https://www.ebay.com/itm/6SN7GT-6SN7-Tu ... SwZrteo68I
https://www.ebay.com/itm/Tested-6SN7-GT ... Sw185eopF4
https://www.ebay.com/itm/Vintage-Tests- ... SwhBFd9zRB

I'm always wondering why the 6SN7 isn't used more by tube amp manufacturers for low power options. It's usually the 12BH7.

I tried to ask this question once and never got a straight answer.

[Phil_S]

To be accurate, the 6SN7 isn't an exact match to the 12AU7. It matches for Ra = 7700. Other parameters are very close; close enough.

The 12BH7 is noval and maybe that is why it get suggested. I really have no idea.

Several years ago, I built an amp with a pair of 6SN7 (bought on eBay cheap.) I wired a pair in parallel so that pulling one 6SN7 will also work. I haven't used it in a dog's age, so memory is a bit faded. As I recall, I tried 3 different PI's (in this order LTP, paraphase a la Ampeg, and concertina) and ended up with the concertina. The other configurations drove the power stage too hard to get anything I considered useful. In the end, it was a very clean amp. The build was what xtian calls "beeks and feet." I used up a low mA high voltage PT and ended up with something like 445v on the plates. The advantage of using a pair (4 triodes) was that I was able to use a ~8K OT. It was not in-your-face loud, but it was surprisingly loud when cranked and clean all the way up. I have no recollection of the source of the circuit, but here it is from my archive.

[ChopSauce]

Interesting ...

By reading Merlin's "pre-amp book", I thought the biasing was more relevant than the tension of the plate, soundwise - as a comment to the use of very high voltages to reproduce the sound of big amps. Maybe someone amongst you guys, could clarify?

[JD0x0]

The AT7 in most Fender reverb circuits runs around 400VDC, IIRC.

[Snicksound]

Interesting ...

By reading Merlin's "pre-amp book", I thought the biasing was more relevant than the tension of the plate, soundwise - as a comment to the use of very high voltages to reproduce the sound of big amps. Maybe someone amongst you guys, could clarify?

Maybe so, but it affects the gain staging: where and how distortion happens.

My experience with JCM800 type amps is that lower voltages in the preamp results in a softer warmer sound (which can be a nice thing, but I like my JCM800 bold and aggressive)

[Phil_S]

I am thinking the trick here is to create the sound in the preamp and then run it into the 12AU7 only for amplification. In other words, figure it out so you aren't relying on the power amp to color the tone, or not much color. If this is the case, reducing power amp voltage shouldn't be an issue and might actually be a good thing. Anyone else?