Guitar amp idea

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Naterbater

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Thanks for your response. I don't have a large assortment of higher voltage caps or high wattage resistors on hand. They're on the way, but I wanted to start experimenting with what I had laying around and those were it.

The meter I was using for the measurements is a fluke 117 true rms meter.

If it wasn't already obvious, I will state that this is my first foray into trying to design a circuit. I have built many paint by number kits and they're great. I just thought it would be (it will be) a fun way to learn more about how things work.
 

abbey road d enfer

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The meter I was using for the measurements is a fluke 117 true rms meter.
Interesting! I'll check how it fits.
If it wasn't already obvious, I will state that this is my first foray into trying to design a circuit. I have built many paint by number kits and they're great. I just thought it would be (it will be) a fun way to learn more about how things work.
Nothing wrong with that. We're happy to help.
 

Tubetec

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Simple cap smoothing after the rectifier will still be very noisey in a single ended amp as you dont get the benefit of hum cancelation in the op transformer like in push pull , adding a choke after the first filter cap , then another cap then resistance of a few hundred ohms followed by another cap would make things much quieter.
 

Naterbater

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My plan was to do something similar to an epiphone valve junior power supply with a few caps to ground and resistors in between them. If I find that it is too noisy I can add a choke later
0.1uF is not enough to smooth the rectified voltage. The 10k resistor draws about 15mA. You need at least 10uF for decent smoothing.
Will I see less DC voltage drop with the appropriately sized cap(s)?
 

abbey road d enfer

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My plan was to do something similar to an epiphone valve junior power supply with a few caps to ground and resistors in between them. If I find that it is too noisy I can add a choke later
It is absolutely doable.
In the 60's, a typical record player had no choke. Some had just one cap (32-50uF) to smooth the B+.
The VOX AC4 had two 32uF caps with a 470r resistor between them.
Will I see less DC voltage drop with the appropriately sized cap(s)?
Yes.
 

Naterbater

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I have been working on this project off and on in my free time. I decided to go DC on the heaters and they are working great. I am getting 12.7 volts drawing 900mA (that 12CU5 is a heater hog! It draws 600mA just for its heater). :cool:

After I got that sorted out, I moved on to the first preamp stage which is a 12av6. It is rated at .55 watts max dissipation. I was able to bias it to .39 watts (70% of max), but in order to do so I had to lower the value of the plate resistor to 15k in order to provide enough power to reach the desired dissipation. That's quite a bit lower than I have seen in other designs. Even with all that its only dropping .553 volts across the bias resistor which means that the grid is only that much different than the cathode. I think there should be more of a difference from the grid to the cathode.

Right now I am working on the 12au6 and I am not having much luck. It seems that no matter what I do, I can only get to about .12 watts of dissipation. I am at 7.5k on the plate. Is there a limit on how low you should go on the plate? I know that the plate resistor also acts as a shunt to send the ac signal onto the next stage. I am worried that if I set it too low it will degrade the signal.

Maybe it's a problem with the power supply? The transformer should be able to drive these tubes as there was one more tube in the original chassis than I have now. Bad tubes?

I have attached a schematic of my power supply and the first preamp stage to show you what I've done. Let me know where I went wrong. Thanks
 

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abbey road d enfer

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After I got that sorted out, I moved on to the first preamp stage which is a 12av6. It is rated at .55 watts max dissipation. I was able to bias it to .39 watts (70% of max), but in order to do so I had to lower the value of the plate resistor to 15k in order to provide enough power to reach the desired dissipation.
Why do you want to run the preamp tubes at 70%?
They usually run at 5-20%.
Typically, in a Fender amp, the 12AX7's are run at about 0.9mA for a dissipation of 135mW, so 13% of the 1W max dissipation.
That's quite a bit lower than I have seen in other designs.
Of course. The 70% target is for output tubes only, because it the the hottest that can be run with enough safety margin.
 

Naterbater

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Thanks for the reply. I will go back redo the first stage. Hopefully that will give me more available power for the 12au6. With the way it is now I was only able to reach 3.5% of max dissipation. I will work on it tonight and see what I can figure out. Thanks again.
 

abbey road d enfer

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Thanks for the reply. I will go back redo the first stage. Hopefully that will give me more available power for the 12au6. With the way it is now I was only able to reach 3.5% of max dissipation. I will work on it tonight and see what I can figure out. Thanks again.
What do you use the 12AU6 for? Is it the power tube?
 

abbey road d enfer

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The 12au6 is the 2nd preamp stage. The signal goes guitar - 12av6 - 12au6 - 12cu5 - output.
So, that's what I thought, you don't need to run it at 70%.
It is spec'd at 3W, so it's a medium-power tube, but if you use it as a preamp, you can run it at a small fraction, no more than 10%, even much less.
 

musipol

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Agree with Abbey. Preamp tubes in a guitar amp are biased for gain and distortion considerations(at least that's what I do). Choose cathode and plate resistors to et the q point for each stage where you want it ( max clean swing or where it sounds best to you). There is no point in trying to maximize dissipation.
 

Naterbater

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I had searched for info on biasing preamp tubes, but all I found was people asking if they needed to rebias their amp after swapping preamp tubes. lol There were some old links to tube preamp design articles posted in a few forums, but the links were all dead. Since I couldn't find anything stating otherwise, I assumed they should be biased similarly to the output tubes. Glad I found out before I went too much further.

Are there any rules to follow when biasing preamp tubes? Should I be looking for a certain range of negative voltage on the grid? Higher plate voltage? More current?
 

Tubetec

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Take a 12ax7 as an example , you'll amost never find anode load less than 100K , or more than 220k , cathode resistor tends to be between 1k and 2.7k bypassed , you can't do any damage by experimenting with up to a couple of hundred volts at the anode with those values.
 

Naterbater

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That link helped a lot! I re-worked the first stage and I am much happier with the results. Plate resistor is 120k plate voltage is 169v and the current is flowing at .5mA. The cathode resistor is 2.7k dropping 1.336v. Those numbers all seem to be in the range of the "typical operation" examples on the back of the data sheet.

The next question I have is about biasing the 12au6. Do their cathodes go straight to ground? Looking at other schematics, I cannot find one that has a biasing resistor.
 

abbey road d enfer

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The next question I have is about biasing the 12au6. Do their cathodes go straight to ground? Looking at other schematics, I cannot find one that has a biasing resistor.
Really? The datasheet clearly presents many possible variations with Rk ranging from 480 to 5300 ohms.
 

Naterbater

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Thanks for all your help. Forgive me for being ignorant to the ways of the tube data sheet, but I have been trying to get the 12au6 set up somewhere within the typical operation guides. On the sheet it shows that the grid 1 should have a negative bias in the -4 to -6 range, but even with a relatively low (82k) plate resistor and a relatively large (5.1k) bias resistor I am only getting about -1.5 volts on the first grid. The second grid is tied to the B+ with a 2M resistor and to ground through a .1uf cap and the screen is tied to the cathode. What should I be looking at?
 

abbey road d enfer

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On the sheet it shows that the grid 1 should have a negative bias in the -4 to -6 range
It exactly mans that tehegrid must be negative compared to the cathode. If the cathode is at e.g. +4V and the grid at zero (as it should be when "leaked" to ground via a high-value resistor) it's exactly what is needed.
, but even with a relatively low (82k) plate resistor and a relatively large (5.1k) bias resistor I am only getting about -1.5 volts on the first grid.
You should have zero at the grid. If you measure different, you made in mistake either in your implementation or in your measurement.
The second grid is tied to the B+ with a 2M resistor and to ground through a .1uf cap and the screen is tied to the cathode. What should I be looking at?
Think it over.
 
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