Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on August 23, 2015, 09:28:40 am
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Sorry for the bombarding of topics. I deleted my previous thread to try and focus on figuring this out and hopefully learning something along the way. This has been bugging me so bad. I know this circuit like the back of my hand. I have the entire schematic memorized. After going over my build for probably 6 hours straight and not finding anything it really played a toll on my confidence ha ha.
So the only thing I can think is, somewhere, somehow, i wasnt getting a very good connection. Maybe I didnt crimp a wire around a tube socket. Maybe it was on of my wires coming off my board, which were soldered underneath. Using 18 awg solid i can only put the wire in the bottom of the turret so far and solder it.
Ok so even with a bad connection, could that explain the TREMOLO I was getting. Master volume after 9 oclock. Pre amp volume close to max. It was dependent on how hard I strum. If I strum the guitar light, it wasnt so noticeable. If I hit the strings harder, It was an actual tremolo. Just like the ones in my little 5 watt with trem. Ok so leaky coupling caps or filter caps cause motorboating. Is what Im getting a form of motorboating? I changed ALL caps. However...heres another scenario.
My filtering has a dual cap can for the choke. In and out. This cap can was used. Not very though. It was even banged up. from getting dropped I think. Tested ok with my cap meter. Now when I originally had the amp problem, this was the first thing I changed. I just temporarily soldered two 16uf caps in place of the cap can. It didn't seem to change anything so then I put it back. Went on to tinker with the amp. Had a ground issue. Got sound. Got full volume. Still lacking gain and then the tremolo problem occurred. I changed every cap on the board except here i am with the dual cap that I once changed but put back. I figured when I changed it the first time I didnt get any change in the amp so I put it back...but that was before I uncovered what I think was a different problem.
So after changing all these caps on board and leaving this dual cap, could it be the culprit for the tremolo? Filter caps are the only thing I can think of. At this point I dont even care about the amp. I already took it apart. Going to re wire it. I just cant sit still until I understand what the heck is happening.
Maybe I need to look past the "but this is a new part" way of thinking. Maybe my "new" choke is going bad. Im using a 20 hy Classic tone choke. I dont know maybe thats going bad somehow. Beats me. But I have a Hammond 5 hy that works fine on my breadboard. Maybe I'll put that in and see.
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I have the entire schematic memorized.
We need to see that schematic. (Don't just say it's a 2204 preamp into an AC-30 PA.) And pics of the actual build.
Maybe it was on of my wires coming off my board, which were soldered underneath. Using 18 awg solid i can only put the wire in the bottom of the turret so far and solder it.
IMO that is inviting problems. First, why 18AWG? 22AWG should be good enough for any interconnections to a board. I don't solder interconnecting wires to the bottom. Instead, I drill a hole next to the turret so the wire can pass thru and wrap around the turret. See this pic...
http://home.comcast.net/~seluckey/amps/6v6plexi/P-6V6_05_big.jpg (http://home.comcast.net/~seluckey/amps/6v6plexi/P-6V6_05_big.jpg)
When I have underboard jumpers that connect one turret to another turret I use 22AWG solid and feed the wire from the bottom and all the way out the top of the turret. Then I bend the wire back down over the outside of the turret. I've never had to wonder if the jumper is making a good connection. See this pic...
http://home.comcast.net/~seluckey/amps/VAC15/underboard_jumper.jpg (http://home.comcast.net/~seluckey/amps/VAC15/underboard_jumper.jpg)
tremolo problem
I suspect something is not wired as you think. Again, need to see your actual schematic and hi-rez pics.
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need to see your actual schematic and hi-rez pics.
+1
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Follow Sluceys leads
I've had that effect on 2 amps, the 1st wasn't a *problem* since the player liked it :icon_biggrin:
This last amp I did some extensive scoping since it's mine, the only "oddity" I found was a low freq osc riding below the grass on my main B+ (noise floor about 7-mV, osc about 5hz, covering about 60% of the noise amplitude), which makes sense to me, since trem is a low freq modulation of the main signal. I re-did the ground a couple times, then gave up, because, it just doesn't bother me enough. :icon_biggrin:
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Ok so even with a bad connection, could that explain the TREMOLO I was getting. Master volume after 9 oclock. Pre amp volume close to max. It was dependent on how hard I strum. If I strum the guitar light, it wasnt so noticeable. If I hit the strings harder, It was an actual tremolo. Just like the ones in my little 5 watt with trem.
Something like this could be caused by insufficient decoupling between the power supply nodes, which manifests itself when you strum hard.
I had a similar problem in a two-channel (6G6B) build where putting extra blocking caps in the signal path cured it.
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I completely rebuilt the amp on Sunday. Used 22awg wire instead of 18awg that I have an abundance of. I did use the 18awg for switches and for some B+ runs. I changed the dual cap can and my choke. This was a good thing really as I did some re routing and made it look a bit neater. It's not as neat as Id liked since Its been re wired and messed with so much.
Turned amp on. Great news finally. Super quiet. Good sound. My layout worked. Everything operates as it should. I did forget an LED resistor. It burnt up quick. I threw a 330ohm resistor and a new LED.
I tried a pre phase inverter master volume on it this time. The difference that I can notice is very subtle. The gain seems to have a slightly different color if you will. My PPIMV was a 500k pot. My MV now is a 1m. I feel like with the 1m you have to turn it up more before you get a usable tone. IM sure it's due to the larger value. 0 to 1 will have more resistance than 0 to 1 on a 500k. But this isn't a big deal.
I've done all solid state rectifiers on my builds so far. Thinking of putting a tube rectifier on this to see what that's like. I have no idea what to expect. Is it possible to use a double throw switch to have the option to switch from tube to SS rectifier?
Here are some photos of the build. I decided to not use a pot bus. I just daisy chained all of my grounded pot terminals from left to right and terminated at pre amp buss. My mini ELC Lava Cable came in very handy for my shielded inputs. Took Sluckeys advise on standoff turrets for grids of el84 and of 1st 12ax7. Actually this whole build is pretty much per Sluckeys advice. Ha. Smaller wire. Holes in board. Wires terminated through holes and on lug.
I'm most proud of this build since the layout worked. I laid it all out per all of my research on grounding. The b+ rail is unusual but it works and made for a clean organized layout.
Also my heater wiring is per merlins book. I like this method.
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Looks good! :icon_biggrin:
One thing, the speaker jack ground, blue wire, since you have an isolated jack, if you have NFB, could/should go to the PI ground, no NFB, could/should go to the power tube cathodes (K's) ground. Look at Merlins grounding drawings. There's a lot of current flowing in that wire, good to keep it short.
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Looks good! :icon_biggrin:
One thing, the speaker jack ground, blue wire, since you have an isolated jack, if you have NFB, could/should go to the PI ground, no NFB, could/should go to the power tube cathodes (K's) ground. Look at Merlins grounding drawings. There's a lot of current flowing in that wire, good to keep it short.
Thanks a ton Willabe! Ill move that now! You know speaking of NFB, I do not have it but wanted to try it and see what kind of difference it makes. Can you make a suggestion or recommend a schematic for a NFB in an AC30 style build?
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Can you make a suggestion or recommend a schematic for a NFB in an AC30 style build?
No, but someone else will know. :icon_biggrin:
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Just copy Hoffman's Plexi 50 NFB circuit into your AC30. One pot, resistor and cap.