Finished Liverpool - Farty/Gated [PICS UP]

[soma_hero]

Finished wiring up my new Liverpool combo with VVR today!

Couple of things,

first it's not in its cab and I have it running into my 2X12 Sovtek cab. Next, when I was planning this out it was only going to be a 15W combo so I ordered only a matched pair of EL84s. Since I decided to go for the full 36W I didn't have enough tubes. I plugged in the matched pair I had in the two inside sockets. I believe this is how they should be plugged in right?

Ok, bulb limited fire-up and everything looked alright. B+ 330V, all the way down to B+5 255V. Plugged in and my immediate thought was: "Where's the juice?"

No singing distortion, no blooming, just moderately loud mild gain distortion. VVR works, goes down to no volume and back up. Played around for about two minutes and then the volume started to fade and everything got really blatty and farty.

I come from the stompbox world and I've heard this before when a transistor is mis-biased or a circuit is starved for voltage. Is this what's happening here? I have a misbiased tube? Why would it be louder at first and then a few minutes later get quiet?

What should I check first? I'll try and measure all the pins voltages for all the tubes tomorrow, it's time for a break here...

All in all, awhile of planning and one full week of construction after getting home from work. I'm real close!

[Richie]

VVR works, goes down to no volume and back up.

should not go to no volume.. it should still have sound at the lowest setting. Also did you install the cap and 1M resistor on the input grid etc..
thats what it sounds like is the problem..

[geetarpicker]

Yep time to check everything. Voltage checks might get you into the area of issue quicker. Yep, it should sing fine even with just two output tubes.

[Kregg]

This may sound like a dumb question, but, I've done this a few times. Is your input plugged in all the way?

[soma_hero]

Yeah I added .047uF (what i had) to the input and from the volume pot, with a 1M resistor after each cap to ground.

i'll measure voltages later today

[soma_hero]

Alright, well I measured these voltages last night and then thought "well I'll just plug in my strat and see...."

And it sounded great!! Beautiful! But it was late at night so I couldn't play. I turned the amp off and waited anxiously for work to end today.

Got home, fired it up and it was amazing. What an amp! And since my build is a weird match of Liverpool and combo in an 18W chassis, and it's my first ground up build, I was worried there might be some issues. No hum, no buzz! Everything sounds good, just the normal hiss.

I played around for 15 minutes, and then the symptoms returned . Power started draining, everything got gatted and farty again. This is so weird?! I measured the voltages while this was going on and they were not noticeably different.

Here's the voltages. Any ideas what would seemingly cause power to drain like this? Thanks all!

[Allynmey]

V1B plate is too high...220V should be 150V or so. Check the cathode resistor.
PI grids are too low...21V should be about 34-36V. check the PI grounding. Especially the tail to the presence pot.

[soma_hero]

Checked the cathode resistor, it checks out at 1.5K. I remelted the 4.7k presence resistor ground connection and it's good to go as well.

I had the amp on while I was measuring some voltages today and discovered that if my multimeter is on, with the black probe grounded, and I touch pin 8 of V1 the amp is restored to normal! It fades back to farty after a minute or so, sometimes longer, but if I touch pin 8 again it comes back in full strength. If the multimeter is off and i touch pin 8, there is no restoration of signal.

Any ideas? When I ordered parts I forgot the cathode bypass capacitors for V1A and V1B so I picked um up at Radio Shack. Maybe some problems with bad caps?

[soma_hero]

Correction, that's pin 7 on V1. If I touch pin 7 with the probe the amp comes to full life.

Remelted every solder joint, replaced the wire from pin 7 to input jack (capacitor for VVR actually). Still does this weird fading in and out. All tubes have been replaced and still no improvement. It has to be a component or wiring issue...

[fishy]

I think Allyn makes a good point though so a thorough check of component and component value would be a good place to start.

Did you figure out why your V1b voltage is so high? There is very little between the B5 voltage and plate suggesting a component value is not correct or something causing an elevated plate voltage.

You might want to consider the integrity of the tube socket if your looking at the input.

Any pics of the build?

[soma_hero]

I measured some voltages again last night, and didn't get the same voltages. They were more in the expected range. I'll go through it again tonight and measure every part.

I had my wonders about the tube socket as well, I might just try replacing it. Can't seem to find my digital camera, I'll try to borrow my roommates tonight and get some pics up.

[soma_hero]

Here's some pictures of the build. I'm sorry my camera sucks and it's so hard to get things to focus and have the flash right. Let me know if anything is unclear. The output tranny wires come through a hole under the turret board. The power tranny has a cutout under the power supply board.

http://smg.photobucket.com/albums/v368/ ... Liverpool/

[Jana]

It kinda sounds to me like you have a bad ground. When you say the farty goes away when you touch the probe to the grid of V1, well, that would indicate that the internal resistance of the multimeter is re-establishing a reference for the grid, hence the bias voltage. Is this chassis anodized and if so, did you scrape off the anodizing real good at any place where you are hoping to make a ground connection? A good anodize job is a very high resistance.

Check and double check all your grounds, especially in relation to V1. Reflow solder joints, check or maybe even replace the cathode and grid (1M) resistors.

[paulster]

Correction, that's pin 7 on V1. If I touch pin 7 with the probe the amp comes to full life.

You've got no DC grid reference on V1 because of that cap on the input jack, which is why you're getting the distortion you describe.

If you must keep the cap then connect the 1M resistor between ground and the pin 7 side of the cap. EDIT: Just noticed that you've got VVR so you will want to keep the cap to avoid getting DC on your guitar volume pot.

Once you fix that you might also want to check your NFB ground as that's relying on the riveted connection between the speaker jack panel and the chassis. That's a lesser problem than what you've already got though.

[soma_hero]

You've got no DC grid reference on V1 because of that cap on the input jack, which is why you're getting the distortion you describe.

Once you fix that you might also want to check your NFB ground as that's relying on the riveted connection between the speaker jack panel and the chassis. That's a lesser problem than what you've already got though.

Excellent. Thanks for that, I didn't even think about it. Also, what do you mean by NFB ground? Do you mean the speaker out jacks grounding, because the NFB loop isn't grounded anywhere....