Changing Plate resistor question.

[Shawnobi]

Hi,
I've picked up a chinese amp as a project. Looks like it may be losely based on a Fender Bassman.

I wish to give it a bit more drive and I understand that I can change the plate resistor on the first 12AX7. It's currently at 100K, I'm looking to push it to 220K in the first instance.

Just to confirm, I am supposed to be changing the resistor that is connectoed to Pin 1 on the tube and not pin 6. Can't seem to get clarification on this with any of the tube documentation I've have read.

Thanks.

[pdf64]

Probably but it depends on how the amp has been laid out. If the input jacks were in pin 2's circuit it would be a good indicator.
Pete

[Jazz P Bass]

Pin 1 & pin 6 are both plate pins (12AX_ type tube).

[Shawnobi]

Ah, I see. Yes the input is via pin 2. Many thanks.

[Shawnobi]

Thanks Jazz.

So would I change the value of resistors on both plates? ie a value of 220K for both 1 & 6?

[Jazz P Bass]

Without some kind of documentation on the circuit involved, I wouldn't recommend anything.

How about drawing the first tube circuit.
It doesn't have to be fancy.
Plate resistors, cathode resistors, coupling caps for a start.

[Shawnobi]

Good idea! I believe you've seen my other post noting the lack of schematics. I'll get on it. Many thanks.

[Shawnobi]

First draft. Its PCB so a bit of a bitch!

[Dave H]

Changing the plate resistor from 100k to 220k will only increase the gain a little (about 20%) but it also changes the bias which may result in the bottom peaks of the signal being clipped. You should also change the cathode resistor to correct the bias.

[Dave H]

I’ve just seen your schematic and it’s biased way cold (probably to clip off the tops of the peaks for distortion. Do you want that?) As that’s the case R8 could be changed to 220k for a bit more gain and a more normal bias without changing the cathode resistor (assuming the cathode RC is grounded).

[g1]

Are R1,R2, R67 going to input jacks? What is the value of R4?
I suppose it is also possible that this is not the "first" 12AX7. The reference to "first" is in terms of the circuitry, not the physical location.
So what you need to determine is which is the first grid (pin 2 or 7) that the input jacks are routed to. Then you need to draw the circuit around that particular grid.

[Shawnobi]

I’ve just seen your schematic and it’s biased way cold (probably to clip off the tops of the peaks for distortion. Do you want that?) As that’s the case R8 could be changed to 220k for a bit more gain and a more normal bias without changing the cathode resistor (assuming the cathode RC is grounded).

Yes I'm basically looking to give it a bit more gain and distortion.

I'm a little confused after some early morning checks. It seems the High & Low inputs come in via R2 & R67. I'd kind of presumed that half of the triode dealt with the high channel and the other half dealt with the Low. So affecting the plate resistor on Pin one would only affect 1 channel.

Looking at it again. The cathode resistors (R4 & R10) are 1.5K. (Very fenderlike). It seems more common practice to change the these from what I am reading. So I think I will start by changing these to 820 Ohm values (More Marshall like).

[dwmorrin]

Does your chinese amp have a name? Maybe there's already a schematic out there we can find for you.
Why do you say it's loosely based on a Bassman? Which Bassman? The first Marshalls were themselves based on the late 50's Bassman, with 820Ω cathode resistors, so 820Ω cathode resistors are not brand specific, in my experience.

If you do try the higher value plate resistor, you may, in addition to the biasing issue, find that you'll increase the noise substantially, and the higher impedance may or may not upset the operation of the next stage.

[Shawnobi]

Does your chinese amp have a name? Maybe there's already a schematic out there we can find for you.
Why do you say it's loosely based on a Bassman? Which Bassman? The first Marshalls were themselves based on the late 50's Bassman, with 820Ω cathode resistors, so 820Ω cathode resistors are not brand specific, in my experience.

If you do try the higher value plate resistor, you may, in addition to the biasing issue, find that you'll increase the noise substantially, and the higher impedance may or may not upset the operation of the next stage.

The amp is a Subzero H50R. Also known as the Belcat H50R or the Strauss SVT-H50R. The reason I believe it is developed from the Bassman is that it is a 50W 3x12AX7 & 2x6L6 configuration. Also the use of Cathode resistor value of 1.5K with a 0.47uF bypass seems very Fender like.

I've changed the cathode resistor value (coming in on Pin 8 of the 12ax7 - Input feed) to 820 ohms from the 1.5K. It's better but still too clean when driven. So 2 questions.
1. Should I also change the cathode resistor on the other side of the 12ax7 (Pin 3) which goes off to the gain knob to 820 ohm also to achieve more 'crunch'?
2. Should I change the cathode bypass capacitor on the input side to something larger (250uF for example) for more 'crunch'.

Crunchy thanks!

[Dave H]

I've changed the cathode resistor value (coming in on Pin 8 of the 12ax7 - Input feed) to 820 ohms from the 1.5K. It's better but still too clean when driven. So 2 questions.
1. Should I also change the cathode resistor on the other side of the 12ax7 (Pin 3) which goes off to the gain knob to 820 ohm also to achieve more 'crunch'?
2. Should I change the cathode bypass capacitor on the input side to something larger (250uF for example) for more 'crunch'.

1. You could end up with less crunch by doing that. The low frequency gain will be reduced because of the 820R and 0.47u bypass capacitor.

2. No, 47u is already more than enough with a either a 1k5 or 820R resistor.

I think the 5F6A and early Marshal first cathode resistor is 820 ohms because it is common to both 12AX7 triodes so the effective value is 1.64k for each triode and I think they used 820 ohms for the next triode to make sure the plate voltage was below 180V so that the heater to cathode voltage of the directly coupled cathode follower was below its 180V max rating. I’m just saying that 820 ohms isn’t a ’magic’ value for crunch.