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Let's talk 12BH7

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12BH7 is an interesting tube. It looks like it might be outstanding in LTP or Cascode service. I have one used one to play with I think so it begs a few questions (in no particular order).

1. Are they prone to microphonics?
2. Are the new production any good?
3. In a cascode using a 12BH7 and a 12AU7 would I get most of the linearity and gm advantage by using the BH in the bottom position?
4. Would you agree that the BH is well suited to these two applications?

Mike
 
The 12BH7 double triode is pin compatible with the ECC81, ECC82 and ECC83 series from Mullard but has slightly different characteristics. Brimar give the intended use as TV frame oscillator and frame output. The construction is compatible with audio through to low VHF use.

The thin glass tube envelope is 19 mm in diameter and, excluding the B9A base pins, is 68 mm tall.

Type 12BH7 was first introduced in 1950.
 
Hi Mike,

in Cascode services I'd prefer to use tubes that were dedicated for that, e.g. 6922, 6DJ8 or the European ECC84 (I don't know if there exists an US equivalent). In LTP's a high µ double triode is favourable over a low or medium µ one.

As 12BH7 is about in the region of 6SN7/6CG7/6FQ7, you certainly could draw the most out of it if you'd use it like them.

Best regards!
 
I recently finished a SET with 12BH7 as the input (choke loaded). I think it sounds very nice. The first 12BH7 I tried was microphonic but that could have been just bad luck.

You're definitely right about perveance. That and a 5687 seem to stay fairly happy at lower anode voltages.
 
I've done some simulation with 12BH7. I use it as the input-voltage gain stage for a headphone amplifier. Coupled parallel it gives a gain like x6. My output is a JFET-MOSFET inverting buffer constructed as a discrete opamp. The overall distortion is like THD 0.010%

I just love the 12BH7A behavior. Especially in parallel configuration. The only tube that comes close is the 6SN7.

All this I have found out using Multisim spice simulation. I have not tried it in real life.
 
Sometime in the past I had an AN Soro SE amp, which was equipped with ECC82 SRPP line stage. I was very nervous about this stage because nearly all of the available ECC82s were whistling temporarily. The final solution was a pair of GE 12BH7As, as direct substitution, without any modifications in the circuit nor in the PSU. Sound was also better with 12BH7s.
 
I am currently designing a power amp with MOSFET output. The input is the 12BH7 tube, which also provide all the voltage amplification. The transistors in the output end just acts like an inverting buffer. As the tube inverts the outcome is in phase with the input.

I tested 6SN7 in the front. But it is not as good as 12BH7 when it comes to keep 3rd harmonics low. But it is no big difference between 6SN7 and 12BH7. 12BH7 has a margin of 45-50 dB from 2nd downto 3rd harmonie in my circuit at 1 Watt RMS out.
 
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2 types

There are 2 types of 12bh7A , those manufactured by RCA license have an expending curves centered at -12Vg , and those manufactured by GE license have linear expansion always centered -12Vg. Only Toshiba from the Japanese is RCA type . RCA ,Westinghouse and Sylvania 12bh7a written Japan are manufactured by Toshiba .

The expending character of RCA type is the most sought for , as in MC 225 generates odd order harmonics but of opposite phase and expands the dynamic .
 

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