(Fixed)Liverpool Issues...volume pot has no effect, squeals

[gtrcollectr]

I just finished this Liverpool Combo and it seems to work pretty well except the volume pot doesn't seem to have any effect on the volume....it seems like the amp is dimed all of the time. I can control the volume from my guitar and at very low volumes the amp sounds great.

Also some oscillation and squeals to diagnose after the volume thing gets corrected....

Anything you guys can recommend? Link to the photo below.

Thanks,
Todd

http://www.flickr.com/photos/toddrjackson/7162405686/

[M Fowler]

The pot is wired wrong. (EDIT) wiring looks good on second thought.



Mark

[passfan]

I'm not so sure that the mirror wouldn't make the volume pot work backwards Mark. Even though the amp is built "backwards" , so to speak, the pot would still need to be wired as per the original to work correctly. The numbers on all the sockets are wrong as well. A lost or missing ground would cause this problem as well, Do any of your tone pots work ? Have you tied your pots ground buss to the chassis ? It sure sounds like everything could be floating without a ground reference. Your other problems could take care of themselves after you back off the gain a bit.

[gtrcollectr]

Yes the tone pots do seem to work....small adjustments to the tone controls do change the tone. I tested the ground reference from the back of the pot to the chassis and I do have a good ground. I can try swapping the pot wiring but it seems like that would make the pot work backwards...A clockwise turn of the pot would still be the same flipped into a combo chassis. I will recheck some of the wiring against the schematic and run an actual ground from the buss bar. I was counting on the the pots grounding via the chassis.

This chassis was prepunched this way so I had to flip the board orientation when I made it. It has caused a bit of confusion for me at times when working from a standard layout.

Thanks,
Todd

[M Fowler]

I looked again and your wiring does appear to be correct. Yeah you wouldn't want to wire it flipped like I showed.

Usually a grounding problem with the volume control.

Mark

[gtrcollectr]

I tried the pot wired like you said and it did make the pot function backwards.......I added a ground from the buss bar to the chassis direct and also noticed that I was missing the wire from the speaker terminal to ground and added that. This may have helped the ground a bit but it really didn't change much as far as the volume thing.

Even with the pot wired backwards it seems like all of the volume happens in the 1st 1/8th of the pot rotation.....after that it seems to only add more drive. My pot is an audio taper so it shouldn't ramp up that quick normally.

The amp sounds great but is just untamable because the volume goes from zero to hero with almost no movement in the pot.

Would it be overkill to use shielded wire to all of the pots from the board? It may help with some of the noise.

Thanks,
Todd

[passfan]

Could you take some better pics ? Check also to make sure your pot is an audio taper and that it's correct despite what it is marked. A voltage chart would help as well.

[M Fowler]

You could have easily ruined that pot when soldering to the back.

[gtrcollectr]

I measured the pot and it is a audio and it still works when my voltmeter is hooked to it....I found an old post that described my problem exactly and something was wired wrong on the tube. I am going back over my connections and hopefully I will find something. I reflowed all of my grounds and checked everything else I could think of.

Not to derail this to much but if my pt has 5v Tap for a rectifier tube and also another set for 300 v. Should I ground those center taps as well? Since I am not using these taps they are simply unhooked right now.

I will get some better pics tomorrow if I cannot figure it out on my own.

[hans-jörg]

Hi, if you dont use the 5 V tap there is no need to ground this CT.

Hans-Jörg

[gtrcollectr]

Just to catch this post up .....I got this fixed.

I had a grounded connection to the presence from the board. After I fixed this the tone controls got more responsive. Volume pot was hosed as well. One side worked ok but pot had to be wired backwards to get it working. I swapped in a PEC 1m to test and it works great.


Still some ground hum but I can work on this. If I only use the first 1/2 of the volume pot anyhow would there be any reason I couldn't sub in a 500ka pot?

Also I find this amp overly bright with the 100 PF for the bright cap. Can I just remove this cap altogether and not run any "bright" cap on the volume?

Thanks,
Todd

[Colossal]

Just to catch this post up...I got this fixed...Also I find this amp overly bright with the 100 PF for the bright cap. Can I just remove this cap altogether and not run any "bright" cap on the volume?

Thanks,
Todd

Glad to hear you got it straightened out Todd. These kinds of things can be really trying on the patience. Yes, you can simply remove the 100pF bright cap if it doesn't do it for you.

[gtrcollectr]

I swapped back in the 500pF to try and it is still very bright. I am going to pull this cap altogether and see how it goes. I also swapped in a A500K volume pot because I had one handy and it really seemed to make the volume a bit more usable. With the A1M when you barely turned it on you had what seemed to be full volume right outta the gate.

With the A500K it comes on a bit more gradual and reaches 100% grind around 40% of the pot.....I may even try a A250k to really get some adjustabilty next time I open it up. My thinking might be off but if I never go more than 1/4 of the pot on a 1M than a A250k would be just about perfect for my ears.

One thing I cannot get over is how loud this amp is......I built the 2x84 version and it peals the paint off the walls in my house. I stepped away from the trainwrecks for a while after I built my Plexi 18 and I guess I am just use to a master volume.

I know the consensus on the Trainwreck camp is to not use a Master volume but is this because it doesn't work well in this circuit or mostly because you are cloning the original that did not have one?

I admit that I am primarily a build-from-layout kinda guy and some of the theory eludes me but these amps are just really difficult to get right. I have built 2 liver pools and they have always been a bear to debug and troubleshoot.

Next time I am starting from the correct chassis and using the approved layout to minimize my chances of failure. On both of my builds I had to build them into a chassis that required some compromises.

Thanks for all of the help and this thing may get some vvr like my last liverpool if I decide to hang on to it.

Todd