I got an RCA 12BH7A from ebay, it looks like it's made by Tung Sol, it has the "12BH7A" printed inside an octagon near the top of the glass. The tube lettering and box are in perfect shape. Price was just under $20, much better than more popular audio tubes.
Sound is a step up from the JJ ecc99. It's a bit more forward and crisp sounding, which I like.
I tried feeding the amp direct from the DAC and using foobar's volume control, and there isn't enough gain for some material, but I think this driver is worth considering if you're using an active preamp and building the Simple SE as a power amp.
Sound is a step up from the JJ ecc99. It's a bit more forward and crisp sounding, which I like.
I tried feeding the amp direct from the DAC and using foobar's volume control, and there isn't enough gain for some material, but I think this driver is worth considering if you're using an active preamp and building the Simple SE as a power amp.
Actually, you could try it as-is. I think the 12at7 uses ~10 mA plate current, while the ecc99 and 12bh7 work best around 20 mA. I changed the resistor between the anode and CCS (I forget which # it is) to adjust the plate current, the value of resistor you need is on the IXYS datasheet. You could rig a resistor in parallel with the resistor used for the 12at7 so you can try the 12bh7 at 20 mA instaed of 10.
Looking at the ecc99 and 12bh7 datasheets, the tube looks very linear and I wonder if the ccs is helping or not. Might be worth it to try running the 12bh7 resistor loaded instead of ccs loaded.
Interesting with the 12bh7. I would think you would have to adjust cathode resistance to properly set the plate voltage. Did you measure that with stock?
I was looking at implementing a similar cathode bias set-up as in the tubelab SE. This way all you do is set the IXYS reference resistor for current and adjust your voltage to put it in the linear range.
I was looking at implementing a similar cathode bias set-up as in the tubelab SE. This way all you do is set the IXYS reference resistor for current and adjust your voltage to put it in the linear range.
I remember checking on the plate voltage and I decided to use the same voltage the 12at7 uses, so I did not change the cathode resistor value.
May I ask, other than gain is there any difference? in my system more gain is a problem as I am using the amps in a full active setup and my tweeter is close to 100 db sensitive while my mids are about 93 db. The only reason I would change is for sonic benifits. I am using a mullard 12at7 and I also have a mullard (Labeled Phillips) 7062 which is also very nice.
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I drew out the load lines when I built the amp, and it looked like the bh7 curves were more linear than the at7, but the ccs makes the load line horizontal, which linearizes the tube to some degree. Also, the bh7 can run at double the current the at7 can, making it a more powerful driver tube. But again, I think the ccs will make the resulting difference smaller.
None of that matters though, I think you have to try it and see. You could just add a resistor in parallel with R13 and R23 using clips to see if you like it. I think I used a 150R resistor, but check the IXYS datasheet, it's been a while... You'll only be out 2 resistors and a $20 tube (if you go top of the line) if you don't like it. IMO, reducing excessive gain is a good thing, although my preamp has 28 dB gain, which is a bit excessive. I'll build a preamp eventually...
I have tried with different driver tubes (no other things changed) and got the following values:
12AT7:
Ugk: 2.1V
Ia: 10mA
Ua:250V
Vout:20Vrms
12AU7:
Ugk: 2.2V
Ia: 10mA
Ua:140V
Vout:6Vrms
12BH7A:
Ugk: 2.2V
Ia: 10mA
Ua:110V
Vout:6Vrms
Would it be a good idea to change kathode resistor to even up the Ua? Or would it be better to change current source to ie. 20mA? It might get too hot?
Alternative is to change CCS to a 22K power resistor...
12AT7:
Ugk: 2.1V
Ia: 10mA
Ua:250V
Vout:20Vrms
12AU7:
Ugk: 2.2V
Ia: 10mA
Ua:140V
Vout:6Vrms
12BH7A:
Ugk: 2.2V
Ia: 10mA
Ua:110V
Vout:6Vrms
Would it be a good idea to change kathode resistor to even up the Ua? Or would it be better to change current source to ie. 20mA? It might get too hot?
Alternative is to change CCS to a 22K power resistor...
Hi,
I'm using a 12BH7 and started with a jj ecc99. Both run 20 mA Ia, and I changed the resistor on the current source to do so. The resistor values and corresponding amperages are on the ccs datasheet, I think it was linear so I use a 150r resistor, but it's been a couple years or so now so best to double check. They have done fine so far putting out that much current, no heat issues so far.
I like the way the bh7 sounds, but I think you need to give it more current to really evaluate it correctly.
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