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Let's talk 12BH7
Let's talk 12BH7
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Old 8th February 2017, 12:12 PM   #1
mashaffer is offline mashaffer  United States
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Default Let's talk 12BH7

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
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Old 8th February 2017, 12:32 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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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.
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Old 8th February 2017, 12:33 PM   #3
Kay Pirinha is offline Kay Pirinha  Germany
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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 double triode is favourable over a low or medium 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!
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Old 8th February 2017, 11:32 PM   #4
mashaffer is offline mashaffer  United States
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Thanks for the input. The reason I was thinking of the BH7 for LTP is that it doesn't seem to mind operating with a relatively low plate to cathode voltage leaving more for the tail if using a passive tail.
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Old 9th February 2017, 02:01 AM   #5
Sodacose is offline Sodacose  United States
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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.
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Old 9th February 2017, 08:49 AM   #6
legendre is offline legendre  United States
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Quote:
Originally Posted by Kay Pirinha View Post
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 double triode is favourable over a low or medium one.
The 12AT7 / ECC81 is also excellent for cascode service.. particularly in a differential circuit with a CCS.
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Old 9th February 2017, 04:36 PM   #7
Kay Pirinha is offline Kay Pirinha  Germany
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I've tried a PI circuitry, based on the Stax SRX, You see, it is Van Scoyoc-like design with two 12AT7 in Cascode. Never managed to get it workung sufficiently, due to a great lack of response in the higher audio range.

Best regards!
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Old 9th February 2017, 05:12 PM   #8
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Quote:
Originally Posted by Kay Pirinha View Post
I've tried a PI circuitry, based on the Stax SRX, You see, it is Van Scoyoc-like design with two 12AT7 in Cascode. Never managed to get it workung sufficiently, due to a great lack of response in the higher audio range.

Best regards!
Cascode circuits have a very high output impedance, like +2 times the rp of the lower triode. I found an easy solution for this trouble: Neutralizing An Amplifier
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Old 9th February 2017, 05:31 PM   #9
skidave is offline skidave  United States
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The 12BH7 pulls a bit more heater current than the 12A__ series. Just be aware of that if you try tube rolling.

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Old 9th February 2017, 06:06 PM   #10
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Cascode circuits have a very high output impedance, like +2 times the rp of the lower triode. I found an easy solution for this trouble: Neutralizing An Amplifier
Thanks, Osvaldo!

Sadly I've forgotten to link to the circuit. I didn't find any schematics in your thread, so where in the 'Stax' circuitry from Buonavolta's pages should I place those trimmers?

Best regards!
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