6v6's in my plexi ?

[jamme61]

was thinking of running jj 6v6's in my 1987x plexi ri (plate volts 445) I think the jj's will be fine after correct bias. My question is, do I half the ohms. So running a 8 ohm cab, do I swith the amp to 4 ohms? thanks for any help finding conflicting info on this.

[brewdude]

I have a hard time keeping the impedance mismatch straight, but I thought you would double the speaker impedance to double the reflected primary impedance.

That is plug the 8 ohm speaker into a 16 ohm output.

Then again as I already stated I am easily confused by this stuff.

[jamme61]

yea i'm confused too. I'm going to keep it at 8 untill I find out for sure. This way only a mismatch of one up or down.

[Firestorm]

If you want to double the reflected impedance, it's the way you stated it originally: put the 8 ohm load on the 4 ohm tap. Whether that's right or not depends on the original ratio.

[jamme61]

ok so I have a plexi that runs EL34's. I'm putting in jj' 6v6's and biasing to around 20 ma (the amp is putting out 455 plate volts). MY 2x12 is 8ohms so, I switch the amp to 4 ohms, because of the difference between el34's and 6V6's right? sorry just trying to get it right

[martin manning]

You've got it. Doubling the speaker load relative to the selected output impedance doubles the primary load. The normal primary impedance for a pair of EL34's is 3600, so you'll get 7200. A pair of 6V6's normally uses 8000, so you're a little low and your voltage is a bit high... look out for redplating.

20 mA and 455V is 9.1W, which if you're measuring cathode current, should be about right. If you get redplating you might bring that down a bit and see if it helps.

[ToneMerc]

A 3.4K OT @ 4-8-16, becomes 6.8K @ 8[4] - 16[8] - 32[16]. Thus, connect your 8 ohm speaker load to the 4 ohm tap.

TM

[Colossal]

I ran this exact circuit and voltages, but into a 7k6 primary so could run the secondaries normally. Bias was right around 19-20mA across 1R, IIRC. JJs held up and sounded very good.

[xtian]

Theory question: Won't the plate voltage vary depending on the current draw of the tubes?

BTW, my 20-watt build with 450v on the plates of 6v6's works well.

[martin manning]

Theory question: Won't the plate voltage vary depending on the current draw of the tubes?

Yes, The plate voltage will rise a bit when the idle current drops ~40 mA or so, and the power supply will be relatively stiffer.

[jamme61]

this is working out very well. I have a skipz circuits VVR installed, which gets me more into the sweet spot at a better volume then the el34's. 6v6's are still very loud at full voltage (loud enough for my gigs) I like the 6v6's a lot better then the yellow jackets (el84). The 6v6's sound closer to the el34's then the el84's do. I know that shouldn't be but, just my opinion.

[ToneMerc]

. The 6v6's sound closer to the el34's then the el84's do. I know that shouldn't be but, just my opinion.

I don't find it strange at all, I've said more than once than the JJ 6V6 is more like a small bottle EL34.

TM

[katopan]

The curves are different, but the tone is very similar.

[Darkbluemurder]

6v6's are still very loud at full voltage (loud enough for my gigs) I like the 6v6's a lot better then the yellow jackets (el84). The 6v6's sound closer to the el34's then the el84's do. I know that shouldn't be but, just my opinion.

I agree with this. EL84s do sound very different to my ears than either EL34s or 6V6s.

[cxx]

I did this for a while. The JJ 6V6's worked fine. While the amp was slightly quieter, I much preferred the sound of the el34's especially for cleaner tones. If you are trying to reduce the volume the combo of inefficient speakers, lamar PPIMV and attenuator work best, although I haven't tried the VVR which might be even better.