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PostPosted: 10 Mar 2018, 21:36 
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Joined: 09 Dec 2011, 07:04
Posts: 12
Location: Australia
I have been having a very challenging time trying to get some assistance in incorporating a circuit/modification to a linestage valve preamp (JP200) which uses 6 x 12AX7's --- to use with a low Z-value input SS amp (ME550) of around <10ohms. I have spent 6 years trying to find a way of doing this via a large diy forum, but the idea was frowned on and I was continuously told the 12AX7's had 'no business in a linestage' and to either use the SS preamp made for the ME550 or find another alternative valve preamp design, however there was never a schematic suggested and the thread was closed with no resolution nor any form of communication.

However in my travels online for a solution, I noticed that on this site the 12AX7's and other similar valves were used in linestage designs posted...and that the 12AU7 (ECC82) Cathode Follower Tube Preamplifier Schematic - as listed in this site - appears to show promise in being able to be used/modified into the cathode follower stage of the JP200 preamp design. Does anyone have any suggestions on incorporating the 12AU7 (ECC82) Cathode Follower Tube Preamplifier Schematic to the JP200 to lower the Z-value as much as possible?? (see schematic attached)

...a person on Utube who did this used 12AT7's in the cathode follower section, but I have a feeling that the 12AU7 design on this site may be preferable as a possible modification in part...

any thoughts, insights, ideas, practical suggestions ??

Thanks in advance...


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PostPosted: 15 Mar 2018, 23:21 
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Joined: 08 Sep 2013, 18:43
Posts: 8
Hi TS

Tubes with higher current capability than 12AX7s are usually preferred in cathode follower applications, but as your circuit parallels them it should be ok.

I wouldn't recommend a DC coupled cathode follower for hifi applications
But I love them and always use them in in guitar amplifiers.

In the DC cathode follower, the CF's grid is connected to the previous stages plate, so is sitting at well over 100V
This creates grid conduction at the CF's grid (it becomes forward biased like a diode) and this pulls current down through the previous stages plate.
The CF ends up biassed very close to cutoff - but it settles to a balance point.
When the signal voltage at the CF grid swings negative it all works ok.
When it swings positive, the CF grid conducts again and pulls additional current through the previous stages plate resistor which creates distortion.

If you implement an AC follower it will be better for hifi

Merlin has some excellent on AC & DC coupled cathode followers here:

http://www.valvewizard.co.uk/accf.html

http://www.valvewizard.co.uk/dccf.html


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PostPosted: 27 Mar 2018, 05:25 
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Joined: 09 Dec 2011, 07:04
Posts: 12
Location: Australia
reubster wrote:
Hi TS

Tubes with higher current capability than 12AX7s are usually preferred in cathode follower applications, but as your circuit parallels them it should be ok.

I wouldn't recommend a DC coupled cathode follower for hifi applications
But I love them and always use them in in guitar amplifiers.

In the DC cathode follower, the CF's grid is connected to the previous stages plate, so is sitting at well over 100V
This creates grid conduction at the CF's grid (it becomes forward biased like a diode) and this pulls current down through the previous stages plate.
The CF ends up biassed very close to cutoff - but it settles to a balance point.
When the signal voltage at the CF grid swings negative it all works ok.
When it swings positive, the CF grid conducts again and pulls additional current through the previous stages plate resistor which creates distortion.

If you implement an AC follower it will be better for hifi

Merlin has some excellent on AC & DC coupled cathode followers here:

http://www.valvewizard.co.uk/accf.html

http://www.valvewizard.co.uk/dccf.html


Thanks Reubster, however I don't have as much knowledge as yourself about valve design etc...

A person I emailed came up with these modifications to the JP200 schematic (attached)

He used 2 x 12AT7's for the cathode follower positions (T3A and T3B) to lower output impedance...

....he also changed a few resistor values...
- Change R7A and B to 1kOhm
- Change R10A and B to 1.5 kOhm
- Change R14A and B to 100 Ohm

Lowering gain from 20 to 12dB:
- Change R5A and B to 33 kOhm
- Change R3A and B to 1 kOhm

- Replace C4A and B with silver mica 8 pF / 500 V and place a silver mica 22 pF / 500 V capacitor between grid and cathode of T1A and B to fight self oscillation

What do you think of this mod with regards to the attached schematic?


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PostPosted: 27 Mar 2018, 18:27 
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Joined: 04 Jun 2008, 20:59
Posts: 4024
Location: Arizona, USA
Hi, I use cathode followers in phono preamps as the output stage. I do directly couple them to the preceding stage. There are no problems. Using 12AU7s this way will give an output impedance of about 200 ohms (varies on B+ and other factors). This is not low enough for your 20 ohm Z in. In parallel two tubes would only get you to 100 ohms. Still not enough. I would consider using a power tube instead. Possibly something like a 6AQ5. An EL84 might work as well. Either in triode mode. It ought to get you down to about what you need. Personally I would put a stage in between the preamp and power amp. Any number of IC based buffers would be suitable. Many ICs can source and sink enough to drive 20 ohms. Tap the amps for power for it.

Good listening
Bruce

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