heater-cathode voltage question

[bpasok]

I am aware of heater-cathode specs and my PV11 had an open 910k resistor causing the heaters to float to ground. With the cathode followers at 170v, that left a pretty high voltage difference. When I replaced the resistor, it clearly muddied up the sound. I removed it and the magic returned. I tried researching this and found numerous others indicating similar experience with many other preamps. what I could not find is why this affected the sound. with the resistor, the voltage was raised to about 34 volts. I thought of experimenting with different resistors, but thought I would throw this question out to others on this forum.

How does this affect the sound and is there an 'optimum' voltage (I could try various resistors or even put a zener in the ckt)?

[AnotherJohnson]

Did you try a new tube after replacing the open resistor?

[bpasok]

I tried several different tubes and I did notice a consistent change in sound with the resistor in place or not.... sounding much more open and airier with the resistor out of the circuit. I did further testing today and increased the value of the resistor (to increase the h-k voltage) and I am now running around 11 volts instead of the 34 volts and I put a switch in to cut it in or out.... I can not hear a difference at that voltage and hopefully it will help protect my tubes from any possible flashover.

I found with the different tubes that the cathode voltage varied by around 6-7 volts... they are running around 165V.

do I figure the h-k voltage as the ground reference difference or the 12.6VDC filament ckt?

is it 165-22 (filament ckt)= 143 V H-K
or
is it 165-10 (filament ckt gnd ref) = 155V H-K

[roberto]

Hola bpasok,

By any chance, do you have the schematic of your PV-11 with the voltages?

If not, I think that you should address all these questions directly to Conrad Johnson´s service dept. They will guide you with the right voltage values at certain points of the circuit. service@conradjohnson.com

I don't have on hand those important values that you are seeking. but perhaps a member might have them.

Happy listening!

[roberto]

Hola,
When I need to go back to school, I use this as a remainder. http://www.valvewizard.co.uk/accf.html

I can solve minor problems remembering.

This is what I have for the PV11.

pv11 schematic.pdf

(82.72 KiB) Downloaded 91 times

Happy listening!

[bpasok]

I have used the same schematic.... it doesn't list voltages.

The B+ voltage is about 370VDC and it uses a voltage divider of R7 (910k) and R8 (92k) that makes the ground reference of approx. 34VDC. it works well, but doesn't sound as good as when the circuit is open and the ground reference is at 0vdc. I have done some experimenting and changed the value of R8 and dropped the reference voltage to 11vdc and can't hear the difference when that ckt is opened. it does improve the H-K voltage by that 11v and will certainly help, but it appears less than ideal.

my primary question is why the unit sounds audibly different with the ground ref at 0vdc vs. 34vdc. I understand the H-K spec benefit, but not the audible difference.

thanks

[bpasok]

by the way Roberto, I should have prefaced the earlier email with a thank you for your effort to assist in answering my questions.

[roberto]

Thanks for the kind words.

Honestly speaking, I can't answer that question. It looks like a bias thing. Also, when you do that, you might vary the operating point of that particular tube that you have. Just thinking loud.
https://i0.wp.com/www.vtadiy.com/wp-con ... =424&ssl=1

I hope that you keep us posted as soon as you solved the problem.

Happy listening!

[Big Dog RJ]

Yes, definitely sounds like a bias operation, as Robbo pointed out.

I remember back in the day one of our techs had to piece back together a pv12 and PFR due to a naughty customer fiddling around with mods... He managed to get both back to their original specs with the help of Ed. We learnt that along the circuit at every point which includes resistors etc., there are certain voltages/ values that do alter the sound. CJ's circuits are designed in a particular way to create a specific sound signature. Alter those values and the sound is altered... not so much the performance though.

Just thought I'd share that past experience.

As advised, best to contact CJ service for the correct values/ voltage points. The only issue is being a vintage unit, I wonder even if they would have these values & specs on file... too long ago to back up I would think.

Hope it goes well, I'm sure you'll somehow find the correct specs for the unit.
Best, RJ

[bpasok]

thank you for your help.

I have information (schematic and parts list) to make it work as intended. what I found out as a result of a failed resistor was that I liked the sound better when it was not in the original factory configuration. what I am hoping to find out is a technical explanation of why the h-k voltage makes a difference in the sound quality. If I understand the physics of why it sounds different, I might be able to make an better decision about how I want to leave it configured.

if is always difficult to put words to sound, but I would say that in the original configuration (raising the heaters to 34v) made it sound sharper and more detailed.... when the 910k resistor was open, the sound was a bit more 'airy' and I liked the effect. The problem was that with the resistor open, I periodically had the tube flash over and cause a loud pop over my speakers, so that isn't an option. Right now, I have a 2M resistor in place and it puts the voltage at around 17v.

I actually have a new DAC coming in next week and that may change my opinion about which value I have in place....

the component values around the PV11 linestage cathode follower are identical to those on the PV12 schematic.