12AU7 substituted for a 6922

[john65b]

I just received a Soundstage Vacuum 2 Pre off ebay and it uses a pair of 6922 tubes. The previous owner had sent a set of pair old RCA 12AU7 tubes that he said sounded much better.

I looked at the 6922 and the 12AU7 and the pinout are near identical, except 6922 pin 9 is No Connection. while the 12AU7 pin 9 is Heater Centertap.

Both tubes have same heater pins for the heaters, as well as grid, cathode, plates....So at first glance it looks like the only difference is the 12AU7 only had 6.3V on the heaters...

Anyway, should I expect any damage to the unit if the previous owner used 12AU7 in this unit? I would expect the 12AU7 to have most of its life sucked out of it if he were using it long period of time without proper heater voltage.

Don't have a schematic, and if anyone has one, I would be interested in a copy. Looks like a cheap PCB, and a pair of Solen caps.
Sound decent (with 6922) and very little hum.

[Geek]

Hi,

I find it difficult to comprehend any 12AU7 sounding better than a 6922 unless some component was right haywire...

Got a voltmeter? If the B+ on the anodes (pins 1 and 6) are 130V or less, you can pop in a 6DJ8 for a whirl.

Cheers!

[anatech]

Hi john65b,
Make sure the previous owner didn't mess with the heater wiring. You never know what mods have been done by people these days.

-Chris

[john65b]

It does not look like the previous owner messed with the heater wiring. There's a cheap PCB with no mods. I plugged in the pre with the 12AU7 (I thought it was the 6922) and the heaters didn't even light up.

I checked voltage on the plates of the 6922 - a whopping 20V. Pretty cheesy. Simple pot (no Alps Blue Velvet as I was hoping) and basic caps.

I can always gut the thing and replace the internals with a 5687 or 12B4. I am hoping at least the transformer is over 250V, but doubt it. It is a nice looking unit and goes well with my other stuff..

I really should do some critical listening before I bash the thing. I was just hoping for better "guts", like my Foreplay III

[anatech]

Hi john65b,

It does not look like the previous owner messed with the heater wiring. There's a cheap PCB with no mods. I plugged in the pre with the 12AU7 (I thought it was the 6922) and the heaters didn't even light up.

That's good to hear.

I checked voltage on the plates of the 6922 - a whopping 20V. Pretty cheesy. Simple pot (no Alps Blue Velvet as I was hoping) and basic caps.

What is the supply voltage on the other side of your plate resistors? Normal pots aren't as bad as they are reputed to be.

I can always gut the thing and replace the internals with a 5687 or 12B4. I am hoping at least the transformer is over 250V, but doubt it. It is a nice looking unit and goes well with my other stuff..

That was were I was headed with my earlier question. Try to measure the raw B+ after the rectifier. There is a possibility there is a simple problem that dropped your B+ low. It may sound better than the guy who sold it to you knows.

-Chris

[john65b]

Well this is getting interesting...

I had a feeling the B+ was going to be a bit low, as after initially looking at the guts there didn't seem to be a cap in there that is rated over 35V...just didn't hit me until now...


Anyway, I checked the Transformer secondary and it is 13VAC. After the caps it is +38VDC and -38VDC. A Dual Rail B+ setup? Hmmm...

I measured the voltage between the plate and the cathode and it is 58V.


After a little more research, the Musical Fidelity X10D Tube Buffer also has a 6922 with +30V and -30V on the plate and cathode...

[MRupp]

You may be able to use the ECC86, it is specifically designed to run at low voltages up to 30V.

[Geek]

Yup, 6922 (and 6DJ8/ECC88 at that low voltage) will hold the linear portion of their curves to very low voltages.

The ECC86 looks like a good idea to try too. It crosses to 6GM8, which being in the US, you might find easier to locate.

Cheers!