Got this sucker fixed. It was a bad gain/overdrive pot.
First I tried a 12AU7 tube. Colored the sound a little but no real overdrive sound. Next tried a 12AT7 tube. A bit more gain so some overdrive punch but still not enough. Finally plugged in a 12AX7 tube. Now were talking! But very noisy (hiss and hum). That won't fly for me and not sure a metal case and better wiring techniques can completely fix that. No joke, if I touch the foot switch with my finger I can hear a local AM radio station.
My plan is to stick with the 12AT7 tube and play around with different gain stage plate resistors and maybe even change the gain pot value.
Also seems like I'm getting unneeded gain from the output plate follower stage. I have the output stage volume pot at only 10%. Might try rewiring it so that it's a cathode follower topology (no gain). I seem to remember Marshall amp's gain stage drive into a cathode follower stage. Need to google some schematics to verify.
Good to hear you found the problem! Does this use a grid stopper on either of the stages? That and maybe a very small cap to ground might help keep FR junk from creeping in. You could also play around with the load resistor on the second stage. Not sure what that might do to the tone, but you could clip lead something in there just to try it.
Not sure on what value cap would work on that design. The phono stage I'm playing with doesn't have a cap on the input, but he did use 1K grid stoppers.
It's not clean. There I saved you the time of getting out the scope.
I have been using the Mean-Well DDR15 on many projects that require a clean dc voltage. these will give you 75mvpp ripple. My measurements say that they are much better than the spec sheet says. This will also open up some securities in that the power supply can now be a broad range of wall wort and the DDR15 will make sure that is it a consistent voltage. https://www.mouser.com/ProductDetail/MEAN-WELL/DDR-15G-12?qs=sGAEpiMZZMv0DJfhVcWlK0%2BNBIYn4O1XIOt%2Bj1Qupp447ikeOa2LzQ==
Just as a heads up for those unaware, there is a filter for smps at diyaudio called amyalice, and while its only rated at 3a and won't work for most amplifier projects, it's excellent for line level builds.
I added the grid stoppers. That fixed the issue with picking up RF but the overall noise is still there. It's got to be the smps. I have all the parts I need to build a linear power supply with good filtration and a TS2812 voltage regulator.
Had time yesterday and this morning before all the sleepy heads got up lol. I built the linear power supply. It was only slightly better noise wise. Then just for S&G's I powered it from a 9V battery. I still don't like it one bit. No balls or crunch so to speak.
I printed out the 12AX7 curves and tried to draw a load line. It became clear a device (vacuum tube) designed for high voltage operation doesn't really work well operating down in the weeds. A plate voltage of 7.5 V is quite different than the 180+ V I see on Fender and Marshall tube amp designs.
I'm not done with this but it's no longer going to be a very affordable guitar stomp box project powered by a cheap wall wart. I'm going to build a proper high voltage power supply and put the tube in its sweet operating spot. Unfortunately the plate/filament transformer alone is almost $40! And not only the cost, now there are dangerous voltages present. Luckily I have had all the parts needed for this so no money out of pocket and this has been a fun experiment for me.
Comments
Just plugged my Fender strat in. Obviously something wrong. Nothing but noise. Maybe a bad pot, tube, or solder joint. I'll get it sorted out.
Thinking my next build- for the 3rd order HP on the tweeter, I just have to use a stack of Orange Drops. I saw a good video on orientation:
Naw... watching the Steelers play the Ravens right now.
Got this sucker fixed. It was a bad gain/overdrive pot.
First I tried a 12AU7 tube. Colored the sound a little but no real overdrive sound. Next tried a 12AT7 tube. A bit more gain so some overdrive punch but still not enough. Finally plugged in a 12AX7 tube. Now were talking! But very noisy (hiss and hum). That won't fly for me and not sure a metal case and better wiring techniques can completely fix that. No joke, if I touch the foot switch with my finger I can hear a local AM radio station.
My plan is to stick with the 12AT7 tube and play around with different gain stage plate resistors and maybe even change the gain pot value.
Also seems like I'm getting unneeded gain from the output plate follower stage. I have the output stage volume pot at only 10%. Might try rewiring it so that it's a cathode follower topology (no gain). I seem to remember Marshall amp's gain stage drive into a cathode follower stage. Need to google some schematics to verify.
Good to hear you found the problem! Does this use a grid stopper on either of the stages? That and maybe a very small cap to ground might help keep FR junk from creeping in. You could also play around with the load resistor on the second stage. Not sure what that might do to the tone, but you could clip lead something in there just to try it.
Great suggestions Tom! No, no grid stoppers resistors on either stage. I didn't even think about that!
Maybe a 10 nF cap to ground on the first stage grid?
Not sure on what value cap would work on that design. The phono stage I'm playing with doesn't have a cap on the input, but he did use 1K grid stoppers.
Yeah for me usually 1k on preamp grids and 100-200 on power tubes. That'll be my first attempted fix. Thanks again for reminding me about that.
Also power supply filters. Your taking the signal from the plates.
True. Hard to say how clean that smps wall wart is without busting out my scope.
It's not clean. There I saved you the time of getting out the scope.
I have been using the Mean-Well DDR15 on many projects that require a clean dc voltage. these will give you 75mvpp ripple. My measurements say that they are much better than the spec sheet says. This will also open up some securities in that the power supply can now be a broad range of wall wort and the DDR15 will make sure that is it a consistent voltage.
https://www.mouser.com/ProductDetail/MEAN-WELL/DDR-15G-12?qs=sGAEpiMZZMv0DJfhVcWlK0%2BNBIYn4O1XIOt%2Bj1Qupp447ikeOa2LzQ==
https://www.meanwell.com/webapp/product/search.aspx?prod=DDR-15#1
Just as a heads up for those unaware, there is a filter for smps at diyaudio called amyalice, and while its only rated at 3a and won't work for most amplifier projects, it's excellent for line level builds.
I added the grid stoppers. That fixed the issue with picking up RF but the overall noise is still there. It's got to be the smps. I have all the parts I need to build a linear power supply with good filtration and a TS2812 voltage regulator.
Had time yesterday and this morning before all the sleepy heads got up lol. I built the linear power supply. It was only slightly better noise wise. Then just for S&G's I powered it from a 9V battery. I still don't like it one bit. No balls or crunch so to speak.
I printed out the 12AX7 curves and tried to draw a load line. It became clear a device (vacuum tube) designed for high voltage operation doesn't really work well operating down in the weeds. A plate voltage of 7.5 V is quite different than the 180+ V I see on Fender and Marshall tube amp designs.
I'm not done with this but it's no longer going to be a very affordable guitar stomp box project powered by a cheap wall wart. I'm going to build a proper high voltage power supply and put the tube in its sweet operating spot. Unfortunately the plate/filament transformer alone is almost $40! And not only the cost, now there are dangerous voltages present. Luckily I have had all the parts needed for this so no money out of pocket and this has been a fun experiment for me.
Try a "starved plate" or "valvecaster" sesign and see how you like it.
That's the design that I built. The "valvecaster" and I didn't like it at all.
How about a DC to DC converter Recom has some that go from 24v to over 100v for about $50.
Yup maybe something like this
https://www.aliexpress.com/item/DC12V-to-DC-200-450V-70W-High-Voltage-Converter-Boost-Step-Up-Power-Supply-Breadboard/32892978438.html?spm=2114.search0104.3.1.4ec0762drp89Le&ws_ab_test=searchweb0_0,searchweb201602_1_10065_10130_10068_10890_10547_319_10546_317_10548_10545_10696_453_10084_454_10083_10618_10307_537_536_10059_10884_10887_321_322_10103,searchweb201603_52,ppcSwitch_0&algo_expid=cfa4aec2-ed44-416f-95ae-1221c478f311-0&algo_pvid=cfa4aec2-ed44-416f-95ae-1221c478f311
This project was fun while it lasted. Not happy with the results and have totally lost interest. Project closed.
throw it on the shelf and come back to it in a couple of years.