• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Anyone ever made the EL84 P-P circuit referenced on the Tubecad Journal site?

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cjkpkg

Member
Joined 2008
Yes, there is an error in the schematic. 200 would get you closer but I ended up with an LM317 CCS biased to ~82mA per pair. Highly recommend this mod... YH's Lazy Man Constant Current Source

My PSU is the Tubecad PS-4. Its a CRC with 47uF at each C. R is 150ohms (2x300 parallel) B+ is spot on 340V. Heater voltage comes by a voltage doubler rectified to 12.6VDC - the 6n1p's are in series and each el84 pair is wired in series.

The Edcor transformers perform flawlessly.

I just started messing around with LED cathode biasing at the front end where you only need 5-10mA of current...I may convert this amp next once the 300b is "done"...
 
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jeffe

Member
Joined 2010
Yeah I guess for one extra component, the CCS is the way to go, I'll read up on that.

The cap I'm wondering about is the one on the B+ at the top of the schematic. I was thinking of a PS4 as well.

Just had a look at Edcor's site, good prices on their stuff...

Thanks,

Jeff
 
6L6

6L6

Moderator
Joined 2010
Paid Member
Can you get some voltage measurements on the Vin and Vout pins of the 317? I'm curious about the differential...

I have an amp that I want to try this on, but it has individual cathode resistors, so I would need the YHLMCCS set to 40ma (requiring 4 in total and probably 30ohm resistors)

Would the Vin to Vout be higher or lower with the current set lower?

Do you still have the cathode bypass caps in the circuit with the YHLMCCS ? Or are they removed?

Would it be possible for you to draw a schematic of what you have right now? That would be really quite helpful.

Looking forward to your LED bias on the front end. (I think you should do that before your 300B..) :)

Thanks!!
 
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cjkpkg

Member
Joined 2008
I will dig in tonight and post my findings...I forget how I last had it set up.

The beauty of adding pots and sharing one CCS for the EL84 pair is it makes tube matching not a major issue as you can tweak so each tube draws the same and reduces complexity and cost.

You can always count on the LM317 to drop 1.25V so a 31.25 ohm will give you your 40mA. I guess dedicated CCS per cathode would also address tube matching...
 
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cjkpkg

Member
Joined 2008
Here is how I implemented this...
el84pp_0001.jpg
 
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cjkpkg

Member
Joined 2008
I finally got around to trying out red LED's on the first section cathode of the 6n1p. The bass appears "cleaner" and I think gain has increased a little most likely due to there being no bypass cap on the cathode on the original schematic and I never tried one...

Definitely worth a try!
 
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cjkpkg

Member
Joined 2008
The schematic calls for 1.7V across that resistor so I just replaced the resistor with a single red LED. While 10mA might be a little "hot" for a 6n1p it ran all weekend with no red plating - and sounded GREAT.

I did not use a bypass cap either. I have tried with and without on my 300b experiment and it seemed to make little if any difference with and without a cap.

Also there are many tubes out there that like to run at 10mA so I think this makes tube rolling a little easier without having to change the cathode resistor all the time...
 
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