Tube bias calculator help

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gkwallace
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Tube bias calculator help

Post by gkwallace »

I'm having trouble understanding Rob's tube bias calculator.
When I do my EL84's it comes up with reasonable numbers.
For some reason my pre-amp tube numbers come out nonsensical.
I've attached my sheet here.
Could someone please help me understand how to use the sheet with the calculator for preamp tubes.
Which numbers go where in the calculator.

Thanks!
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Re: Tube bias calculator help

Post by JMPGuitars »

What value resistor do you have between points C and D? Please verify.

Also verify your cathode resistor values on V1 ( measure Resistance to ground on pins 3 and 8 ).

Thanks,
Josh
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Re: Tube bias calculator help

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JMPGuitars wrote:
Wed 12/18/24 10:08 pm
What value resistor do you have between points C and D? Please verify.

Also verify your cathode resistor values on V1 ( measure Resistance to ground on pins 3 and 8 ).

Thanks,
Josh
Will do.
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Re: Tube bias calculator help

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JMPGuitars wrote:
Wed 12/18/24 10:08 pm
What value resistor do you have between points C and D? Please verify.
27k 3 watt
JMPGuitars wrote:
Wed 12/18/24 10:08 pm
Also verify your cathode resistor values on V1 ( measure Resistance to ground on pins 3 and 8 ).
1.47k and .668k
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Re: Tube bias calculator help

Post by JMPGuitars »

Try swapping V1 and V2. If the voltages generally look the same in each position, then try increasing the value of the 27k 2W resistor. 33k to 39k range, and take new voltages.

ETA: This isn't necessary unless you want to lower the voltages of V1, which are fine. Lowering the voltages will change the characteristics of the distortion/headroom from the preamp.

The more important thing for your amp is the Paul Ruby mod, which I would do before making any other changes.

Thanks,
Josh
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Re: Tube bias calculator help

Post by gkwallace »

JMPGuitars wrote:
Thu 12/19/24 10:25 pm
Try swapping V1 and V2. If the voltages generally look the same in each position, then try increasing the value of the 27k 2W resistor. 33k to 39k range, and take new voltages.

ETA: This isn't necessary unless you want to lower the voltages of V1, which are fine. Lowering the voltages will change the characteristics of the distortion/headroom from the preamp.

The more important thing for your amp is the Paul Ruby mod, which I would do before making any other changes.

Thanks,
Josh
Thank you very much Obi Wan
I want to know how to use the calculator with the info I posted.
For some reason my numbers in you voltage chart work for my power tubes, but measuring the right points for my preamp tubes has me confused.
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Re: Tube bias calculator help

Post by JMPGuitars »

gkwallace wrote:
Fri 12/20/24 10:38 am
I want to know how to use the calculator with the info I posted.
For some reason my numbers in you voltage chart work for my power tubes, but measuring the right points for my preamp tubes has me confused.
Without a screenshot of what you tried it's hard to tell, but if I had to guess... Check where it asks how many tubes share the cathode resistor.
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Re: Tube bias calculator help

Post by gkwallace »

JMPGuitars wrote:
Fri 12/20/24 6:31 pm
gkwallace wrote:
Fri 12/20/24 10:38 am
I want to know how to use the calculator with the info I posted.
For some reason my numbers in you voltage chart work for my power tubes, but measuring the right points for my preamp tubes has me confused.
Without a screenshot of what you tried it's hard to tell, but if I had to guess... Check where it asks how many tubes share the cathode resistor.
I measure pin 3 to cathode ground it 1.7vdc
here's what I put in the calculator
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Last edited by gkwallace on Fri 12/20/24 8:20 pm, edited 1 time in total.
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Re: Tube bias calculator help

Post by JMPGuitars »

Scroll down the page and read the part where it says "Triode Tubes." You have to take into account the fact that your V1 preamp is two triodes, not in parallel. So it's like two tubes in series.
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Re: Tube bias calculator help

Post by gkwallace »

thanks for trying to help, but I'm not getting it.
at most I can come up with 20% dissipation, more like 5%
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Re: Tube bias calculator help

Post by JMPGuitars »

So what? First, it's 10%, not 5%. Like it says on the page, you need to double it. But more importantly, we normally only look at the bias on output tubes. I'm not sure what you hope to learn from doing that on the preamp tube?
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Re: Tube bias calculator help

Post by gkwallace »

JMPGuitars wrote:
Fri 12/20/24 8:45 pm
So what? First, it's 10%, not 5%. Like it says on the page, you need to double it. But more importantly, we normally only look at the bias on output tubes. I'm not sure what you hope to learn from doing that on the preamp tube?
I did get 10%, so I was doing it correctly.
I thought I should be shooting for 85% like output tubes.
Had no idea that a tube would operate with so little dissipation.
I looked at other peoples 12ax7 values that were calculated mathematically and their numbers were close to mine.

The whole reason for this is to understand why my amp doesn't sound good without using a tube screamer to cut the distorted bass notes.
My PEQ works to accomplish this too. I don't cut anything on the PEQ, just boost 850k area.
My tmb mv had perfect tone and response and it was almost identical component values to the ultralite. (except and extra stage of gain) I don't know how to explain it, but the amp does not sound good turned up. MV on 5-10 and Gain anywhere above 5 and it sounds like a complete distorted mess, like a blown speaker mess. My tmb mv only got better and better the higher you went on MV or Preamp.
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Re: Tube bias calculator help

Post by JMPGuitars »

Have you done the Paul Ruby mod yet?
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Re: Tube bias calculator help

Post by gkwallace »

JMPGuitars wrote:
Fri 12/20/24 9:14 pm
Have you done the Paul Ruby mod yet?
I CNC'd my board remember?
I'd have to take everything out, re-drill and rewire from scratch.
Remember how you told me you did't like the cloth covered wire?
You are right. It sucks. The slightest nick in the wire from stripping it will cause it to fail. If you move it more than once or twice it will fail.
The turrets I used suck, they are pretty, but under that gold I'd swear they are aluminum.

If I can't get this sounding good by swapping some components, I'm just going to start over with more robust materials.


PS: I was randomly looking at your fx loop layout and noticed the ruby mod on that and that it's only a point to point mod. I'll order the parts.
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Re: Tube bias calculator help

Post by JMPGuitars »

gkwallace wrote:
Fri 12/20/24 9:22 pm
I'd have to take everything out, re-drill and rewire from scratch.
Incorrect. It's a mod. It was designed to go into the common 18W setup. All you need to do is solder in 4 cute little components, that are already detailed in the build docs.

Here's one I installed in an older build:
PaulRubyMod.jpg

I understand you're having some fun learning pains, but think past that. You can do this, and it's the correct thing to do. A common mistake less experienced builders make is thinking "I'd have to take everything out, re-drill and rewire from scratch." Nah, slap that stuff in there, solder it well, and it should hopefully solve your problem.

Research the mod and read about it a bit and see if it makes sense.
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Re: Tube bias calculator help

Post by gkwallace »

funny you say that Obi Wan, I just read the original paper on it.
I also see it's a simple point to point.

I'm on the case.

Reading the original paper, he describes my issue almost exactly, as far as wave form and distortion goes.
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Re: Tube bias calculator help

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JMPGuitars wrote:
Fri 12/20/24 9:14 pm
Have you done the Paul Ruby mod yet?
Mod done. It sounds better, more like what I wanted. I also used 1.5k cathode resistors to replace the existing. That tightened it up too.

On a side note, when I touch the chassis it hums, not awful, but more than I think it should. Also when I touch my shielded wires.
I have a very old house and a lot of my receptacles don't have real ground wires. They are grounded to the box and who knows where else.
My thinking is the amp is not truly grounded. It's dead quiet at half volume and slight hum at full volume, which I would say is pretty good.
Any thoughts?
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Re: Tube bias calculator help

Post by JMPGuitars »

Grounding is important. Get yourself an outlet tester and make sure any outlet you're using your amp in is properly grounded and correctly wired.

Glad you got your amp sounding better though! Now don't electrocute yourself.
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