Coming up short on plate voltage.

[illuminatiNPS]

So I'm building another one of these Bigger Muff pedals that has a 12ax7 in the circuit. I am using a SMPS to power the tube. the problem I am having is the efficiency of this SMPS. I can get 300v B+ out of it, but the plate resistor calls for 220k and cathode is 1.5k. The spec on the needed plate voltage is 125vdc. i just can't seem to get past 70vdc or so with a 220k resistor, and in order to get the voltage needed, I have to drop the plate resistor to 100k. This gives me the usually plate voltage being half the B+ however the opearting point of the tube is now different and am concerned that the tone will be off. Now granted the tube gain stage sits between two diode clipping stages and it is a very high gain distortion so being that is an odd place for the tube Im not sure if there would be a tonal difference.

The question is: How can i get up to 125vdc with a 220k?? and if not does dropping to 100k plate resistor really affect anything in such a high gain circuit??

[PRR]

I would not aim for a specific plate voltage.

Are you monitoring your "+300V"? How much does it sag with 220K and 100K?

[illuminatiNPS]

By "sag" do you mean drop in b+ voltage with each resistor??

[merlinb]

At anode voltage of ~70V with those values makes me think your cathode resistor is shorted out somehow. Have you got a cathode bypass cap installed backwards maybe?

[amptramp]

I have a Trio KW-55 receiver where the stereo preamp section is a pair of 12AX7's with the first stage plate at 65 volts and the second stage at 85 volts.  The tone control driver stage runs at 105 volts and the gain stage driving the inverter runs at 93 volts.  These are all 12AX7 stages and they are all intended to be linear with these voltages measured on the plate.

Many AC/DC tube radios used a 12AV6 stage with the plate running at a nominal 82 volts and linearity was not a problem.  A 12AV6 is a double-diode triode with the triode section identical to one side of a 12AX7.

I would stick with 220 K for the plate load and let the plate voltage fall where it may.  125 volts on the plate is a typical operating point used in tube manuals as a test voltage for data characteristics but the device is rarely run that high and is quite linear below that.

[illuminatiNPS]

Ok I guess I will leave it as is. This circuit is a modified Muff that was used on those early Korn records. The tube gain stage sitting between the two diodes make me believe the tube is being used to just drive the second clipping stage, and being that it is a high gain circuit maybe the difference in tone would be indistinguishable.

[illuminatiNPS]

A quick update. I have a friend who built the same circuit. He was able to get 125vdc at the anode with 220k plate res and 1.5k cathode and 300vdc B+. He did mention he is using 1 watt resistor for anode and cathode. I noticed mine are 1/4watt. Not sure if that matters, however most 1/4 watts are rated at 250vdc. I also swapped in a 2.2k cathode and 100k anode and achieved 125vdc with ease. In all honesty I hear no difference between 70 and 125. especially with the placement of the tube sitting smack between two diode clipping stages in a Muff like Circuit. If it were more of an overdrive circuit that realys more on picking dynamics, maybe so. But with an extreme fuzz, not hearing it. Please let me know if upping the wattage of anode and cathode resistors are worth it.

[tubegeek]

Georg Ohm can help you:

Say your bias is about 1.5V on that 1.5K resistor.

I = V/R
I = 1 mA

Power formula: P = I^2 x R

.001 x .001 x 1500 = .0015 W

The 1/4 Watt resistor is overspec'ed by over 100 times. Worry about global warming, not this.

[mth5044]

Let's get this straight - you're asking for a help on a pedal that you're selling commercially? A pedal the community helped you build, with the assumption (since it's a DIY website) that you'd share the project?

[vigilante397]

Let's get this straight - you're asking for a help on a pedal that you're selling commercially? A pedal the community helped you build, with the assumption (since it's a DIY website) that you'd share the project?

Not sure I see where you're getting that. I see no mention of selling it commercially.

[Cozybuilder]

"Quote from: vigilante397 on yesterday at 07:53:31 PM

"Quote from: mth5044 on Yesterday at 06:21:01 PM
"Let's get this straight - you're asking for a help on a pedal that you're selling commercially? A pedal the community helped you build, with the assumption (since it's a DIY website) that you'd share the project?"

Not sure I see where you're getting that. I see no mention of selling it commercially."

http://www.diystompboxes.com/smfforum/index.php?topic=112183.msg1081173#msg1081173

http://www.diystompboxes.com/smfforum/index.php?topic=110776.0

[merlinb]

Please let me know if upping the wattage of anode and cathode resistors are worth it.

A 1/4W anode resistor will more than likely fail under a 300V HT. Not immediately, but prematurely.

[vigilante397]

http://www.diystompboxes.com/smfforum/index.php?topic=112183.msg1081173#msg1081173

http://www.diystompboxes.com/smfforum/index.php?topic=110776.0