Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on September 08, 2014, 05:19:59 pm
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So I finished my breadboard. Wanted to try an experiment that would be sort of quick. I loaded up the gretsch 5 watt schematic (my last build) and since I eliminated the tremolo, I had an extra preamp tube to use. So I though I would get some experience with adding a gain stage.
http://www.in2guitar.com/gretschamp/gretsch6151schem.pdf (http://www.in2guitar.com/gretschamp/gretsch6151schem.pdf)
I came out of the second half of the first triode (pin 6) passed through a .01 coupling cap like that is on the design but instead of hitting pin 5 of the 6v6, I connected to a 68k resistor and hit pin 7 of my unused tube. I have a 470k resistor going to ground between the 68k and pin 7.
Pin 8 has 2.7k resister and a 33uf bypass cap.
Pin 6 is connected to the same b+ as all other tubes via its own 270k plate resister just like the rest of tubes. Also has a .01 coupling cap and then travels onto pin 5 of 6v6. Ive tried multiple value coupling caps here.
For some reason, I get a voltage on the grid of my added gain stage. Its around 35vac and -23vdc (this is just what my meter says on both settings.) And lots of weird noise. No voltage on that side of the coupling cap when I wire up amp per schematic and eliminate the added stage but as soon as I add it...I get voltage there
I dont know if this matters but even though the second half of that tube is not used, the heater is hooked up. I used a pre wired socket tht has pin 4 and 5 jumpered and pin 1, 2, and 3, wires just dangle unused.
One thing to note. The tone knob has been moved to other side of coupling cap to keep B+ off of tone knob.
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> around 35vac and -23vdc
It is oscillating. Oscillation builds-up to infinity, or whatever upsets the gain enough to limit the oscillation. In tube grids you see a large AC voltage and also the large unexpected Negative DC voltage (because positive swings are caught by the grid).
3-stage amps are very tricky. The least sneakage from 3rd stage to 1st stage will oscillate. This includes sneakage through the B+ filters, which should usually only have two consecutive gain stages per B+ stage.
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Just like anything else I try.....starting completely over. My entire day was ate up from this project. Going to disassemble my board and pay more attention to grounding this time.
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Just like anything else I try.....starting completely over. My entire day was ate up from this project. Going to disassemble my board and pay more attention to grounding this time.
No, your NOT that's what your breadboard's for! :icon_biggrin:
You said it's your 1st go with your - NEW - breadboard, it will go faster to set up/tear down circuit ideas as you get more used to it. :icon_biggrin:
Sooooooo much easier than if you had;
1. Drawn the amp out on paper/computer.
2. Then cut/punched a chassis.
3. Made an eyelet/turret board.
4. -AND- soldered it all up!
Here read this on grounding;
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
Brad :icon_biggrin:
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Ok Guys. I drew the schematic. Can anyone tell me why this isnt working. I get lots of gain (when my signal is audible) ...maybe too much....and tons of noise. Do I need to re configure values from previous stages (which ive tried)?
http://rdkelectricalservice.com/amps/5%20watt.pdf (http://rdkelectricalservice.com/amps/5%20watt.pdf)
Original schematic that I edited http://www.in2guitar.com/gretschamp/gretsch6151schem.pdf (http://www.in2guitar.com/gretschamp/gretsch6151schem.pdf)
I tell you what though. Its just noisy. But it has AWESOME tone. Just like I was hoping. Swapped out plate resistor on new gain stage. Helped a little. But still noisy.
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Just like anything else I try.....starting completely over. My entire day was ate up from this project. Going to disassemble my board and pay more attention to grounding this time.
No, your NOT that's what your breadboard's for! :icon_biggrin:
You said it's your 1st go with your - NEW - breadboard, it will go faster to set up/tear down circuit ideas as you get more used to it. :icon_biggrin:
Sooooooo much easier than if you had;
1. Drawn the amp out on paper/computer.
2. Then cut/punched a chassis.
3. Made an eyelet/turret board.
4. -AND- soldered it all up!
Here read this on grounding;
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
Brad :icon_biggrin:
Thanks for the Willabe. I read that. I ended up re laying it all out. I laid it on my board from input all way to power supply. Grounded like I read on Bus Bar grounding on that article.
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Ok Guys. I drew the schematic. Can anyone tell me why this isnt working. I get lots of gain (when my signal is audible) ...maybe too much....and tons of noise.
Is 3.9K resistor grounded? or are those triodes sharing the same cathode resistor?
I'd get the gain to a manageable place, and then evaluate the level of the noise.
I'd start by attenuating the gain from V1A: you have the .0047uf cap connected directly to the 500K volume pot. A resistor in between will lower available gain to V1B's grid. Start with 1M. 470K might be too aggressive, but its worth a shot as well.
You also might want to divide up the B+ for all the triodes into two power supply nodes instead of one, but I'd consider that after the gain is under control....
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Ok Guys. I drew the schematic. Can anyone tell me why this isnt working. I get lots of gain (when my signal is audible) ...maybe too much....and tons of noise.
Is 3.9K resistor grounded? or are those triodes sharing the same cathode resistor?
I'd get the gain to a manageable place, and then evaluate the level of the noise.
I'd start by attenuating the gain from V1A: you have the .0047uf cap connected directly to the 500K volume pot. A resistor in between will lower available gain to V1B's grid. Start with 1M. 470K might be too aggressive, but its worth a shot as well.
You also might want to divide up the B+ for all the triodes into two power supply nodes instead of one, but I'd consider that after the gain is under control....
Ooops. Yea 3.9k is grounded.
All triodes have independent cathode resistors.
Should I replace the .047 cap with the mentioned resistor or add the resistor in series with the volume pot.
How would I devide up the B+.
Thanks for your help
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Should I replace the .047 cap with the mentioned resistor or add the resistor in series with the volume pot.
I'd start by adding the resistor in series. Leave the .0047's and the 470K in between as-is, since you liked the sound of the amp before the new gain stage...
you can even use a 1M pot wired as a variable resistor to dial-in the ultimate value for that resistor if you like...
How would I devide up the B+.
Take the most sensitive stage (V1A), and give it its own 10uf filter cap with a 10K between it and the other two triode's B+.
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Should I replace the .047 cap with the mentioned resistor or add the resistor in series with the volume pot.
I'd start by adding the resistor in series. Leave the .0047's and the 470K in between as-is, since you liked the sound of the amp before the new gain stage...
you can even use a 1M pot wired as a variable resistor to dial-in the ultimate value for that resistor if you like...
How would I devide up the B+.
Take the most sensitive stage (V1A), and give it its own 10uf filter cap with a 10K between it and the other two triode's B+.
SO FAR SO GOOD. Had it die on me once but I think I bumped the preamp tube. Ill be back shortly :)
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I ended up having to put a much larger resistor on the first Cathode resistor. I think the value was something around the 6.8 K when the original was a 1.5 K and in my additional gain stage I ended up having to put a 33K on the cathode. With putting a resistor in series with the volume pot and giving V1 it's own power source separate from the other tubes the noise stopped. There was a point in which I think the first preamp tube was failing. And that's when I ended up raising the resistor value to slow down some of the gain going into that second half. Does anyone have any guesses as to why my preamp tubes might have failed? Would it have been too much of a signal going into them? The amp played for about an hour with my new tubes and updated values on cathodes and it worked fine.
When the amp had the noise problem, it almost sounded like a solid state crate combo with the gain cranked to 11. It was super distorted so I knew it had to be over doing it. So I played with raising values at the cathodes of the first triode and lowering values to the grid resistors to ground.
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It is oscillating. ... 3-stage amps are very tricky. The least sneakage from 3rd stage to 1st stage will oscillate. This includes sneakage through the B+ filters, which should usually only have two consecutive gain stages per B+ stage.
PRR already told you what the problem was. Do you really have all 3 preamp tubes fed by the same filter cap, as shown in your schematic? If so, that's what's causing the oscillation. You need a decoupling resistor (some call this a B+ dropping resistor) and a 2nd filter cap. Don't feed more than 2 consecutive inverting stages from a single filter cap.
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It is oscillating. ... 3-stage amps are very tricky. The least sneakage from 3rd stage to 1st stage will oscillate. This includes sneakage through the B+ filters, which should usually only have two consecutive gain stages per B+ stage.
PRR already told you what the problem was. Do you really have all 3 preamp tubes fed by the same filter cap, as shown in your schematic? If so, that's what's causing the oscillation. You need a decoupling resistor (some call this a B+ dropping resistor) and a 2nd filter cap. Don't feed more than 2 consecutive inverting stages from a single filter cap.
Thanks for the input HBP. I COMPLETELY missed PRR's comment there. But I ended up doing the above mentioned and it took care of it. But Still had to get my gain levels under control.
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I'd like to be able to use my tone knob as a mid control. Like the "filter" knob on the Rat distortion pedal. My goal is to create a great lead tone. I like to crank the mids to my tone. Anyone see a way I can re configure tone control to do so?