12bh7 for direct coupling

[Chipotle]

Hello,

I'm having some problems trying to direct couple my output tubes via a 12bh7 cathode follower. So, I have a couple 12bh7 cathode followers with 47k cathode resistors connected to -90 volts instead of ground. I'm putting -bias voltage on the grid of the cathode followers, lets say -60 volts. I'm putting around 350 volts on the plates of the cathode followers. The cathodes are direct coupled to the grid of the power tubes via 1.5k resistors.

Here's the problem, when I apply the plate voltage to the cathode followers, the bias voltage goes positive! I've tried a bunch of different tubes, and the problem is not present when the tubes are removed, so I don't think my wiring is wrong. Are my voltages out of line?

[Ray Ivers]

IMO 350V B+ is way too high for the CF plates; you'll have to bias them very cold to drop the required 410 volts plate-to-cathode for -60V at the output grids.

I would try measuring your -90V raw-bias supply before and after application of the 350V B+; my guess is that it will be pulled less negative by current flow thru the CF's.

Ray

[Chipotle]

Ray, thanks for your post. I'm kind of trying to hit SVT voltages, but not quite getting it, of course. Can you suggest some good starting voltages for the plate and cathode of the 12bh7 cathode follower?

I'll drop the voltages and see if it helps. I managed to get the 47k cathode resistor -voltage down to -130 but my bias voltage is not even close! I'll drop the plate voltage.

Yes, the -voltage supply goes positive with application of the plate voltage. I've been increasing the -voltage to try and reach equilibrium.

[Ray Ivers]

The earliest (6146) SVT wasn't too bad; +220V and -150V:

http://www.freeinfosociety.com/elect...view.php?id=40

IMO "ideal" voltages for a 12BH7 CF driver might be +100V and -200V, although a symmetrical +/-150V supply would work just as well and be easier to build, using a single 240VCT grounded-CT secondary winding and bridge rectifier.

I've been increasing the -voltage to try and reach equilibrium.

I would highly recommend not doing this, as you'll end up with with an extremely wimpy CF-driver stage with performance probably no better than an RC-coupled LTP or similar. Both positive and negative driver supplies should be able to sink/source at least 20mA of current without appreciable sag. IME you'll be much happier with a stiff or regulated -90V supply that remains steady after application of CF B+, than a 'bouncy' -130V one that doesn't - with a stiff -150V supply being about the best for the SVT/12BH7 application.

If you're trying to utilize an existing B+ supply, you could try adding a plate resistance to the CF, bypassed by a large-value cap, to drop CF plate voltage and allow a hotter bias point (it would be better to drop the B+ voltage, though). Also, I wouldn't consider a 47K Rk value 'set in stone' by any means; the early SVT circuit uses 123K total Rk, for example.

Ray

[Chipotle]

Ok, now I realize that I've been doing 2 things when I've been increasing my -voltage: increasing the negative voltage and correspondingly increasing the resistance from cathode to ground of the 12bh7 cathode follower! I was many times over 123k. The 47k resistance was from the -voltage to cathode. Also, my reworking of my -voltage supply corresponds to the onset of all my latest problems. So, I'm going to reduce that 47k resistor to somewhere around 10k and adjust my -voltage supply to get around 120k from cathode to ground. I was also running the 12bh7 driver and cathode follower from the same power supply node, so I had increased the B+ to get decent voltage on the plate of the driver. I split them out and have reduced the voltage on the plate of the cathode follower to the 200-250 volt range. It's hard to tell exactly where it's going to end up, since I can't really run the power tubes in the amp without the bias voltage. I think it's likely I will be able to get -90 on the cathode and appr. 225 or so. on the plate. Probably not optimum. Currently, I'm getting everything from one 300 volt, non center tapped secondary, bridge rectifier, with the negative voltage derived through a capacitor off one leg per ampeg v4.

I've got another set of 270 volt non center tapped taps on the same winding and I was thinking perhaps if I bridge rectified them to get -380 volts and used 2 totem filter caps, I could get -190 from the junction of the filter caps and likely get +100 by doing the v4 style supply off the 270 tap. So, I think that would be pretty close to the optimum voltages you suggested. It might be a valid plan B, however, it would require a lot of deconstruction/reworking.

Thanks again, Ray, I appreciate your help tremendously!

[Ray Ivers]

The 47k resistance was from the -voltage to cathode.

Sorry - I mis-spoke about the 6146 SVT, it actually has just the single 47K Rk to the (AC-grounded) raw bias supply. The remaining resistances - both fixed and variable - just adjust the negative voltage to the CF grids.

Also, my reworking of my -voltage supply corresponds to the onset of all my latest problems. So, I'm going to reduce that 47k resistor to somewhere around 10k and adjust my -voltage supply to get around 120k from cathode to ground.

My suggestions would be to a) get your raw bias supply capable of supplying 20mA without sag, regardless of the specific resistances needed (BTW, the Ampeg cap-coupled circuit is unsuitable for this duty IMO), and b) choose your CF Rk based on both your desired bias point for the CF's and available CF B+/raw bias under CF current loading; your chosen 10K Rk should be a good place to start, but you may end up using an even lower value.

I had increased the B+ to get decent voltage on the plate of the driver.

You shouldn't need a whole lot of B+ for your pre-driver gain stage; whatever you use for the CF driver plates should be fine. I'd just set up the GS appropriately for the lower B+; an 18K 2W Rp and 680R Rk should work well for a 12BH7 GS @ 220V B+, giving a >90V positive plate swing.

Ray

[Chipotle]

Thanks, Ray. The Ampeg v4 style bias has not been treating me well. I increased the cap to 3.3mfd, which stiffened the supply, but it still drops 50 volts when I apply the B+. I'm going to bite the bullet and bridge rectify the second set of taps.

[Ray Ivers]

I would keep in mind that you won't be able to get -190V from the center stacked-cap connection - this is available only with a CT connection, which you don't have. Why not just use a 110V, 5W Zener with a 10K, 10W dropping resistor from the rectified 350V? The Zener is Digi-Key P/N 1N5379BMSCT-ND or Mouser 526-NTE5157A, and should give you a rock-solid -110V @ 20mA (there are plenty of other Zener values too, and Digi-Key has a good selection of the higher-voltage values).

Ray