12BH7 sub=hum

[Lavane]

So, I was doing a bit of tube rolling in my ST-70 this morning and popped a 12BH7 in the driver socket and got a hum. None of the different 12AU7's I have caused this. Could current draw be the problem? The bigger power trany in these bob latino amps are supposed to be able to run 12BH7's and even E80CC's. has anyone had this problem?

 

[Brice]

I had exactly the same problem on a W5M. Slight hum with the 12BH7.
I kept first signal driver tube as 12AU7 while the phase splitter was a 12BH7 also and no hum.
So I stick with a 12AU7 as first tube.

 

[Lavane]

I had exactly the same problem on a W5M. Slight hum with the 12BH7.
I kept first signal driver tube as 12AU7 while the phase splitter was a 12BH7 also and no hum.
So I stick with a 12AU7 as first tube.

Thanks for the responce. I would like to try an E80CC but im afraid of the same result. Ill try the 12bh7 in one of the splitter sockets.

 

[Retrovert]

I had looked into different ST-70 front-end tubes. Here's what I have in my notes about the 12AU7 vs. the 12BH7.

The 12AU7/12BH7 has µ = 17, which is fine for the splitter but less fine for the gain stage. The ST-70 doesn't need huge amounts of gain, but a 12AT7 has µ = 60, for comparison.

The 12BH7 was designed for TV vertical deflection and oscillator circuits, and the -A version has some special heater warmup tweak. It reportedly can be noisy in audio use, probably from heater to cathode leakage. The 12BH7 draws twice the current draw of a 12AU7, 0.6 mA instead of the usual 0.3 mA, but that shouldn't be a problem for your power supply unless it is at its limits.

You might have a bad tube, a version prone to noise, or idiosyncratic wiring issue with an AC imbalance that causes the tube to hum.

 

[Lavane]

I had looked into different ST-70 front-end here's what I have in my notes about the 12AU7 vs. the 12BH7.

The 12AU7/12BH7 has µ = 17, which is fine for the splitter but less fine for the gain stage. The ST-70 doesn't need huge amounts of gain, but a 12AT7 has µ = 60, for comparison.

The 12BH7 was designed for TV vertical deflection and oscillator circuits, and the -A version has some special heater warmup tweak. It reportedly can be noisy in audio use, probably from heater to cathode leakage. The 12BH7 draws twice the current draw of a 12AU7, 0.6 mA instead of the usual 0.3 mA, but that shouldn't be a problem for your power supply unless it is at its limits.

You might have a bad tube, a version prone to noise, or idiosyncratic wiring issue with an AC imbalance that causes the tube to hum.

Thanks. I know they are used mostly as splitters. I popped one in as a driver just to see what it would sound like. Mark told me it could be gassy. It doesn't look gassy but I guess it could be a bad tube. The Sylvania 12au7's I have in there now sound pretty good. Even if the 12bh7 didn't hum I wouldn't have left it in. The highs sounded great but the mids sounded a bit flat.

 

[Brice]

Even as a phase splitters, I wasn't thrilled with the sound quality. I had more gain, but I found I lost clarity over the 12AU7. I also did not use them.