[Videoshielded]

Mr. Lau:

I'm considering replacing the plate load resistors in two of your amps with constant current sources. I need to know the plate current for these tubes. But my calculations (based on the schematics you supplied with these amps when I bought them) don't seem right to me. They seem too low. So, I thought I would ask you if you could provide the correct figures.

On the "original" MG Head (without the output transformerless option), what is the plate voltage and plate current on each plate of the 12AX7 tube?

On the AV-8, what is the plate voltage and plate current on the triode of the first 6BM8 (the one used for the input signal, not the second one used for the phase splitter)?

Thank you!

 

[Joe Lau]

MG-Head Plate Voltage is 50V , Plate Voltage before 100K Loading resistor is 123V. Plate current is 0.73ma.

AV-8


Plate Voltage is 43V , Plate current is 0.43ma

 

[Nick Dangerous]

Videoshielded... are you going to implement Doc Bottlehead's C4S loads? I've always wanted to try that on the MG Head!!

 

[Videoshielded]

Joe, thanks for the data. I had the same results but I thought the current was too low.

Nick, how's it going? I see you have the drivers in the Oris' now. Wish I had the floorspace and SAF. Thing is, I love my spouse a lot more than I love audio, so there you have it.

C4S? You bet! I have to pull the board from the MG Head anyway, because I want to pull out the 2.2uf Auricaps I put in as coupling caps (I know, that's huge, but I had them lying around and ppl talked me in to trying it, and it worked out nicely) so that I can use them as parafeed coupling caps for at 2A3 SET I'm planning to build later this year. The 'Head is going to get .47uf Dayton film-and-foils from Parts Express, which are the best cheap coupling cap deal going, as far as I know. I put a pair in each AV8 and have been very pleased. If you want to try it, my math says R1 should be around 1.37K and R2 should be around 124K, but I've noticed Doc bumps up R2 higher than the formula to run less bias current through the LEDs for low-current applications like this.

I'm a little more dubious how the constant current source will work in the AV8 -- I'm not sure how it will interact with the phase splitter triode following it. There's a couple hundred volts before the plate load resistor (two resistors actually, a 10k and 390k in series), and 43 volts after. The plate of the first triode is direct-connected to the grid of the second triode.

The way I understand the C4S, when I pull the plate load resistor and put in the C4S instead, the cathode load resistor of the first triode will control the plate voltage of first triode. But I'm worried there might be way too much voltage going to the grid of the second triode? You have any clues whether the C4S will take care of it? Maybe I need to leave the plate load resistor in place and bias the LEDs based on the lower 43V? Ugg.

 

[Nick Dangerous]

If you have the necessary specs from the MG Head, why not post a question to the Bottlehead forum? Voltsecond, Paul Joppa, Doc, or maybe some of the other tweakers can help.

 

[Videoshielded]

Thanks, but ever since the Decware fiasco between Steve and Dan, I've tried to avoid asking Doc's group for advice on how to modify commercial designs!

The more I look over the C4S circuit theory and the AV8 schematic, the more comfortable I get that the grid of triode 2 will get the same voltage as the plate of triode 1, so long as the grid bias resistor is the right size. I'm going to order the parts and give it a try.

 

[Nick Dangerous]

Keep us updated!