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Old 20th March 2011, 05:17 PM   #1
Minion is offline Minion  Canada
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Default Cathode/plate resistors ??

Hi guys , Im building a simple 12ax7 Guitar preamp and need to figure out the optimal Plate and cathode resistors based on my supply voltage ..... I am doing a single tube with a 250k pot in between the triodes .....

My PSU consists of a 100uF Cap with a 10h Choke and another 100uf cap (CLC) , I have about 127vDC coming off of the PSU under a load and have DC heaters with 10,000uF capacitance ......

I have tried it with 100k Plate and 1.5k cathode (and 220k/2.2k and some other values I have seen in guitar amp schematics) but the sound is just a screamin mess , when you aren"t playing a note the sound devolves into feed back and hum even with the pot at a very low setting , but when your playing there is no hum or feedback but is still way too distorted .....

I"m thinking that maybe because I"m running this tube at a bit lower voltage than most tube preamps , that maybe the values I am using aren"t biasing the tube correctly ??.......

Is there a way to calculate the correct Resistor values to get the tube biased correctly ?? (Disclaimer: I have about pre-school abilities in math)
Or maybe an exel sheet or chart showing it ??


Thanx a lot guys .....
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Old 20th March 2011, 06:00 PM   #2
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Look in the resistance coupled amplifier section of the RCA tube manual for good values to use.
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Old 20th March 2011, 06:19 PM   #3
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If you take some voltage measurements at the tube, you will have numbers for your bias question, which should help. e.g. voltage across the cathode resistor, plate, grid voltages.

This is a preamp which is being used like an effects pedal? ie between your guitar and guitar amp? Is it modeled/copied from an existing design?

Do you have a picture of your wiring, and a schematic? Both would help readers to give you some more suggestions.

What test gear do you have?

Two 12AX7 gain stages will add a lot to your signal, BTW.
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Old 20th March 2011, 06:51 PM   #4
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Though your bias point may or may not be suboptimal it sounds to me like you have some other problem going on since a poor bias point should not cause oscillation or hum.

If you can supply more information (schematic) of what you have done especially in the area of power supply and layout. What is this preamp stage feeding? Are you using a grid stopper? Are heater wires twisted? Are you using shielded or twisted pair wiring from the input to the first grid? Are the shields grounded only at one end? Do all grounds go back to the same point (star ground)? Is the power supply properly decoupled? Are you using any feedback? Did you take care to route input and output or power leads away from each other and not parallel to each other?
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Old 20th March 2011, 07:04 PM   #5
Minion is offline Minion  Canada
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Well the design is exactly the same as allmost every single tube pre-amp for guitar ever made only some of the Values are different ....... the same pretty much as any marshal , fender . orange ect ......

pretty much the same as this :

DIY 12AU7 Tube Preamplifier Project

But with a 12ax7
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Old 20th March 2011, 07:26 PM   #6
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Hmm... I admit to not being a guitar amp specialist (I have only built one) but I don't remember seeing too many that have a LTP at the input. Does yours have an input pot like that? If so I hope it is larger that 100K as I don't think your guitar will do its best running into such a small load. It is traditional to have a 68K grid stopper at the input at least on common cathode stages. I would presume that it would be advisable on a LTP also.

Do you have that switchable feedback tone control on yours or do you just use the signal as is from one or both of the outputs? Single ended output or balanced?

For comparison this is what I built. In retrospect it probably would have been better had I decoupled the power supply for U6 and U7 as they can be switched to cascaded mode but it worked pretty well. Note that I use straight common cathode until the final phase inverter stage which is LTP.

http://www.diyaudio.com/forums/attac...ght-wesamp.pdf
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Old 20th March 2011, 08:34 PM   #7
Minion is offline Minion  Canada
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What is this preamp stage feeding?

A high impedance input on a Power amp ....

Are you using a grid stopper?

Yes , 27K .....


Are heater wires twisted ??

Yes ...

Are you using shielded or twisted pair wiring from the input to the first grid?

I"m useing shielded wire (connected at one end) for all audio inputs and outputs and twisted pair for all PSU leads .....

Is the power supply properly decoupled?

What makes a PSU properly decoupled and not properly decoupled ?

Did you take care to route input and output or power leads away from each other and not parallel to each other?

Not sure cuz I don"t really understand the question .....

-------------------------------------------------------------

I don"t really have a schematic cuz Like I said it is like hundreds of others ....

I guess I"ll probably be going back to the drawing board .....

Last edited by Minion; 20th March 2011 at 08:39 PM.
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Old 20th March 2011, 08:53 PM   #8
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PS decoupling is the RC filter on the B+ between stages. I like to have one between each inverting stage if possible. For layouts you want to be sure to keep low level signals (inputs) away from high level signals (outputs, power section etc) and AC power lines. Where they do come close you don't want them running parallel to each other.

For the hum it may help to elevate the heaters using a voltage divider on the B+ so that it is at a DC level above that of the cathodes of the preamp tubes. I presume that you have the heater center tap connected to ground right now correct? You didn't leave them floating did you?

Is that a solid state power amp? If so it might not be all that high of impedance (10 or 20k). I don't think that will cause the problems your describing but it might result in high frequency roll off. Putting a cathode follower between the gain stage and the power amp would fix that.

One thing you can try is to take a non-conductive probe (wood or bamboo chopstick is traditional ) and gently move some of the wires around while the unit is powered on and see if you can find a wire or component that is causing the problem. Sometimes you find the noise and oscillation changes when certain parts are moved or touched and that can give you a clue to the problem. These high gain circuits can be tricky.
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Old 20th March 2011, 09:13 PM   #9
Minion is offline Minion  Canada
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Well the transformer I have for the heaters has a center tapped 6.3v (3.15v 0v 3.15v) so I didn"t use the center tap , I just used the other wires to get 6.3v AC , then rectified it and used a 4r 5w resistor to get the voltage to 6.3dc .....

Is this the wrong way to do it ?? also am I supposed to connect the heaters DC ground to the Plates DC ground ??

Thanx
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Old 21st March 2011, 12:59 AM   #10
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Quote:
Originally Posted by Minion View Post
I guess I"ll probably be going back to the drawing board .....
If you are headed to the drawing board....
One thing to consider would be upping your B+ voltage - you don't need much current to run preamp tubes, so you could easily convert your PS to voltage doubler with a couple more caps.
With a more common B+ voltage you could just lift the preamp section component values from a known (eg Fender) design...
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