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Old 23rd March 2009, 08:42 PM   #1
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Default 12B4 Line Stage Amp

There has been a lot of interest in a 12B4 line stage amp, and I thought the time had come to take a second look, post some findings, get the help of the group, and put together a really good version we could all use.

I really like the 12B4A--it has a lot going for it.
- Very low plate resistance means
. Low output impedance w/o extra CF stage (which I hate)
. High current capability--could double as headphone amp
. Choke fed drive available for high voltage SET
- Cheap and available: rolling to select won't break the bank
- Lo mu
. You don't need more than 12db gain in a line stage
. Low distortion
. Good for Grounded cathode design

It aslo has a few problems to work through
- Not as linear as say a 5687 or 6DJ8
. Means careful attention to working region
. Can't over-drive without consequences
. Runs best in high current regions

I'll start posting results and design thoughts--the final version will show up on my website: www.galaxySETlabs.com

Cheers
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Old 23rd March 2009, 08:43 PM   #2
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Default A little about me and design interests

http://www.enjoythemusic.com/magazine/manufacture/1108/
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Old 23rd March 2009, 11:14 PM   #3
mach1 is offline mach1  Australia
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coffeedj, re the following issues:

Quote:
It aslo has a few problems to work through
Quote:
Not as linear as say a 5687 or 6DJ8
Quote:
Runs best in high current regions
At around 100V – 200V and 20mA they are as linear as any 5687.
I reckon they sound best around 20mA, sounding rather ‘thick’ and slow at 30mA.
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Old 23rd March 2009, 11:50 PM   #4
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Default 1st results

The first test I did was to look at performance in a basic minimum grounded cathode design. Parameters are:

Load = 10K
Input R = 220
Cathode bypass = 100uF

WHat I've discovered using various values of B+ is that the 12B4 operates best at high voltage and high current. That means that the grid bias ranges from -12 to -22V, which seems excessive for a line stage, but it works well and is not a problem.

In the basic GC design you get about 9b of 2nd harmonic distortion reduction moving from 100V B+ to 200V, and another 2 db moving to 255V, and viturally no change if B+ is increased beyond that. The third harmonic is independent of B+ voltage and relates only directly to input voltage. These results are fairly independent of the cathode resistance as long as you keep the cathode current above 10ma.

The other thing that is noted for the basic GC design (using optimum parameters from above) is that as long as the output signal is limited to approximately 0 db (1 V RMS or 2.4V P-P), the distortion is not measureable for any harmonic (which is >65db down on my Techtronics TPS 2024). With the 12B4 giving gain of about 5--assuming a anode load of 5K or greater--that means an input signal of .250V RMS, or -6db will drive to full output.

Since most power amp designs require somewhere between .25 and 1.0V RMS to drive to full power, and most CD's are mastered to 0 db for full power, and most phono amps will drive to 0 db or greater, that means that a basic GC design will work quite well for a pre-amp. This is simplicity in the max--one tube, two resistors, two capacitors and an input VC.

However, most commercial pre-amps are made to drive more output. THe Cary SLP-05, for example, will output about 20V P-P of clean signal. Is this necessary? The simple answer is no. In fact means that you are always running the Volume Control at minimum on the Cary. The longer answer is that you would like to be able to drive up to about 2V RMS with no distortion out of the line stage in order to meet overload requirements from the phono stage. The CD will never overload past 0 db (1.0V RMS) because that is how the CD output is defined. So if you only use CD's you could quit here. You can build a very cheap and simple line stage.

For Vinyl enthusiasts, we need to work a little harder, unless your power stage requires .5 RMS or less. Then you are there already, but it is no fun to quit the design at this point so let's keep going.

Next thing to look at is voltage regulation and current sources. The guru's on this forum will hopefully weigh in. I'm going to look at a simplest solution first.

One final note on dynamic range. It is interesting that although CD's have theoretically more dynamic range than Vinyl, the actually listening requirements seem reversed. This is because most CDs are mastered with the music compressed. This especially true for POP where a total dynamic range of 15 db is not unusual for a recording! Jazz is better, and classical usually has the highest dynamic range, but people don't listen to classical at ear splitting levels so it kind of all balances out from the design perspective.
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Old 24th March 2009, 12:04 AM   #5
Salas is online now Salas  Greece
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A notice. 12B4 needs 3.3k CC resistors on both pins 2&7 so to work oscillation free. At least it happened to all of those I have seen.
Also people must know that 12B4 is microphonic upon slight impact on glass or chassis when giving gain. It will ''ping'' but it is nothing like a DHT. I don't care about that, but it is enough reason for some to pass it.
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Old 24th March 2009, 12:16 AM   #6
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Default Microphonic 12B4

I've definately had to select 12B4's for microphonics.
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Old 24th March 2009, 12:25 AM   #7
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What make gave you the least? RCA's and Sylvanias are sounding good, but I haven't come across one that could pass the fingernail knock test unless configured as a follower.
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Old 24th March 2009, 12:41 AM   #8
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Quote:
Originally posted by salas
What make gave you the least? RCA's and Sylvanias are sounding good, but I haven't come across one that could pass the fingernail knock test unless configured as a follower.

Salas,

Have you tried silicone rubber "O" rings mounted over the micas?


---------------------------------------------------------------------------------------


All,

I'm attaching my version of a 12B4 line stage. I used CCS loading, which allows the NASTY 10 KOhm IHF "standard" load to be driven.
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File Type: jpg 12b4_preamp_eliduttman5.jpg (64.6 KB, 3261 views)
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Old 24th March 2009, 12:50 AM   #9
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Eli, 90 - 100V @ 20mA is exactly where I reckon they sound best, and where I run mine.
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Old 24th March 2009, 12:53 AM   #10
Salas is online now Salas  Greece
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Quote:
Originally posted by Eli Duttman



Salas,

Have you tried silicone rubber "O" rings mounted over the micas?


No, I like the sound as it is since it does not have a microphonic echo. Just to inform those who haven't experienced a 12B4 and may think its ''dead'' due to low mu. The big hot cathodes are sensitive.
I have used a cascode Mosfet CCS (enhancement mode) with plate at 160V to ground, 1k2 cathode resistor BGNX bypassed, with 18V across (15mA). Up to 20mA is OK. Also tried Jung's 317 10M45S cascoded. But best sound is with constant voltage enhancement cascoded Mosfet Gyrator based on a Wavebourn's idea. But with a 12B4 care is needed to PSU decoupling unless the whole active thing reacts and blows serious hiss. With resistor load is easier to keep tame. Active loading pushes the Mu and if there are hidden gremlins they come out. FFT scrutiny is advised during building.
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