what plate load for SE 6G6-G ???

[HTH]

I messed around with using a 12AU7 in parallel SE, but the tone doesn't have the grunt of a pentode (although it otherwise did sound very nice).

That brings me up to date and I'm wanting to add an octal socket in that amp to run a 6G6-G (2.75w pentode). I figure that since its a pentode, I should get that ballsier tone, but the volume will be much lower than using a single EL34.

The stumbling block is what primary Z I'll need on the OT - anyone?

Also, if anyone can give me a quick lesson in how to extract this info from the data sheet, I'd be most grateful.

[Steve Conner]

It's on the first page...

http://www.r-type.org/pdfs/6g6g.pdf

"Load Resistance... 10k"

[HTH]

Thats a different data sheet than I found, but thanks for that anyway Steve.

One more thing, the data sheet you linked to states the following primary Z at the two plate voltages below...

* 12k @ 135v
* 10k @ 180v

I'll be likely running it closer to the max plate and screen voltage of 300v (probably at 275v) so I get the voltage I want in the preamp. So, I take it that the primary Z will decrease with a higher plate voltage ???

Would I be ok with a primary around 5k at 275v on the plates ????

[ThermionicScott]

People have a bad habit of looking at datasheets, finding an example operating point and concluding that "X" is the load that the tube wants to see in all conditions.

In truth, it depends not just on the B+, but where you're running the screens in relation to the plates. Drop the screen voltage relative to the plates, and the optimal Z goes up. Raise the screen voltage, and the optimal Z goes down. That's why a Deluxe Reverb has a 6.6K OT when the 6V6 datasheet examples have 8K and 10K.

It's hard to say what the 6G6 will want to see at its max voltage. Might be instructive to purchase one of those "universal primary" OTs from Hammond, hook up a scope, and see which load makes the most power. My guess would be something lower than 8K, but I dunno how much.

(BTW, Steve, this wasn't aimed just at you. I see a lot of blanket statements like "6V6s want to see an 8k load" that people arrive at based on one example in the datasheets, and the tweed Deluxe OT.)

- Scott

[HTH]

thanks for the replies so far - much appreciated.

I'd be running the plates and screens pretty close like the majority of amps out there. I'll be shooting for 275v plates, 270v screens.

With that in mind, how do I draw the load lines to find the ballpark primary Z ???

Also, the universal Hammond SE OTs have optional 2k5, 5k and 10k primaries - would be spot on if there was a 7k5 one right in the middle. I guess that 5k would be a close enough primary Z for a SE 6G6-G (bearing in mind this is being built with low volume/big tone in mind). A SE 6G6-G is only going to do around 2w at best, right?

[Steve Conner]

How do you know the 6G6-G will even take 275V? While researching this, I found a few references where people claimed to have burnt them out with less than that.

The example operating point in the datasheet is mainly for the benefit of people who want to use a tube tester on the tube: it shows how to set up the tester and what readings to expect.

The primary Z might be decreasing with increasing screen voltage, but I doubt that trend will continue all the way to 275V. You might run into the limit of screen dissipation or cathode emission, and that would require you to stop increasing the screen voltage to get acceptable tube life. Now with constant screen voltage, but plate voltage increasing, the required primary Z starts to go up again.

Don't have time to give a tutorial on load lines, try Merlin's Valve Wizard site.

[HTH]

How do you know the 6G6-G will even take 275V? While researching this, I found a few references where people claimed to have burnt them out with less than that.

The example operating point in the datasheet is mainly for the benefit of people who want to use a tube tester on the tube: it shows how to set up the tester and what readings to expect.

The primary Z might be decreasing with increasing screen voltage, but I doubt that trend will continue all the way to 275V. You might run into the limit of screen dissipation or cathode emission, and that would require you to stop increasing the screen voltage to get acceptable tube life. Now with constant screen voltage, but plate voltage increasing, the required primary Z starts to go up again.

Don't have time to give a tutorial on load lines, try Merlin's Valve Wizard site.

The data sheet gives max plate and screen values of 300v for the plate and screen, so I went slightly under with a voltage that I know will work for the preamp too; using a fairly typical 'marshall' HT line.

I've found a website where a guy was running a 6G6-G at 180v with a 10k OT, but thats it. Plus, he was using it as a headphone amp, so wouldn't have been driving it hard.

I think for the sake of a £20.00 OT and £10.00 on the tube, I can throw this at the wall and see if it sticks.

Had a look on Merlin's site for load lines - still clear as mud.

Cheers, Ian