Issue With Bassman 5F6A Build

[Guitarnut]

Question:

As I turn the bias pot clockwise, I'm sending more voltage to ground...correct? Is this raising or lowering the bias current?

Also, when I added the second ground to the filter caps, I also added a second bleed resistor so each for the new ground. Could this be causing the volume issue?

Peace,
Mark

[martin manning]

Looking good, but the power tubes are still biased too cold. You need to get the idle current up to around 40mA. I can't tell how you have your bias pot wired, so put your meter on the junction of the two 220k resistors where the bias voltage is connected (near the bass and treble pots) and find out which direction you have to turn the pot to make the voltage there less negative. That is the direction you want to move, to higher idle current and "hotter" bias. Try raising it to -50V and then check the power tube current.

You only need one bleeder, maybe something around 200k and 2W. Your extra bleeder is not likely to be causing the volume drop.

[Guitarnut]

Well, I dialed in a hotter bias and the amp is slightly more tolerant of the input but stil dropping out. You can hear it in the sample. When it thins out, I just touch the guitar cable or one of the pots and it comes right back. Weird.

This was recorded with my iPhone just laying on the carpet so the quality is what it is.

http://www.crenshawweb.com/bassman/bassman.mp3

Any ideas what might cause this? A little more trouble shooting and I'll have an amp!!!

I just can't express how grateful I am to Martin, Miles and all the others who saw me through this. This is an amazing forum and I have learned so much along the way.

Peace,
Mark

[Reeltarded]

NICE!

I am jist a cheerleader. rah rah and such.

Umm. The drop out thing makes me think it is about controls that might take a little more tightening. The pots and jacks want to be cranked down and making excellent connections with the chassis. Ifnthe tiniest touch makes it pop back you might try to wiggle everything a little then crank it down a touch more?

Success sounds great, huh? Good job hanging in there!

[martin manning]

That's the right kind of noise! Congratulations, and likewise on sticking it out!

When it thins out, I just touch the guitar cable or one of the pots and it comes right back. Weird.

When you say "touch" do you mean nudge the guitar cable (plug?) or a knob physically? What if you just bump the chassis? A poor ground or signal-path solder joint could cause the loss of signal, or there could be some unwanted intermittent contact between a joint or wire and the chassis, grounding it out.

Another possibility is tube failure. In this circuit the cathode voltage on V2's pin 8 cathode is ~200V, and some tubes are rated to a maximum of 100V between the heater and the cathode. You had ~300V there at one point. If the insulation between the heater and the cathode breaks down the signal will be lost. Do you have more 12AX7 tubes to try? If not you could try swapping V2 and V3 and see if that makes any difference.

Lastly, are you sure it's not the cable?

[Guitarnut]

That's the right kind of noise! Congratulations, and likewise on sticking it out!

When it thins out, I just touch the guitar cable or one of the pots and it comes right back. Weird.

When you say "touch" do you mean nudge the guitar cable (plug?) or a knob physically? What if you just bump the chassis? A poor ground or signal-path solder joint could cause the loss of signal, or there could be some unwanted intermittent contact between a joint or wire and the chassis, grounding it out.

Another possibility is tube failure. In this circuit the cathode voltage on V2's pin 8 cathode is ~200V, and some tubes are rated to a maximum of 100V between the heater and the cathode. You had ~300V there at one point. If the insulation between the heater and the cathode breaks down the signal will be lost. Do you have more 12AX7 tubes to try? If not you could try swapping V2 and V3 and see if that makes any difference.

Lastly, are you sure it's not the cable?

HI Martin.

I've tried several known-good cables from my studio. One Hosa right out of the package

It's more like nudge the stem of the pot or wiggle the guitar cable plug.

This morning I checked all the soldered grounds, tightened all the bolted grounds and put one more twist on every jack and pot nut. I powered up, grabbed my guitar and plugged in. When I switched it out of standby, it blew the fuse. I replaced the fuse and it's playing again but with the same condition as before.

I have extra tubes. I'll try swapping out V2 and see what happens. But, to be honest, I may have had every 12ax7 I own in V2 at some point along the build. I didn't think to keep track of them. If we can't sort it out, I can pick up some from GC when the y open today,

Peace,
Mark

[Guitarnut]

I found one more 12ax7 that hadn't been on the bench. I tried it and no change.

Swapped power tubes no change, Swapped rectifier, no change. Seems like it was a cold start issue that blew the fuse. Now that everything is warmed up, it's holding fine.

I don't think is related to any one jack or pot. I can reach back and nudge a tube and it comes back. Or, just tap the chassis.

I metered it while it was in that "dropped out" state and nothing on the DC side seems to be off. I need to go over it with a magnifier and see if there's a blob or loose wire strand.

Peace,
Mark

[martin manning]

I don't think is related to any one jack or pot. I can reach back and nudge a tube and it comes back. Or, just tap the chassis.

OK this is what I was looking for. I think it is likely that something is shorting or grounding out. If the chassis is mounted in the cab it is of course getting shaken, and a loose bit of solder could be bouncing around in there somewhere. Any chance there are more pots with solder blobs inside?

[Guitarnut]

Any chance there are more pots with solder blobs inside?

Funny you should ask...I have my Bass pot in pieces right now. It didn't show any signs of shorting out but definitely needed cleaning up.

[Guitarnut]

Well, that bass pot was the problem. It freakin' screams now! All the punch and sparkle you could ever want. I was cranking along checking out the tone stack, different PU combos, several different guitars. I got it up to about 6 and it started breaking up nicely then...and the f***ing fuse blew again.

Come on carrot! I've been chasing you for 21 pages in this thread. I'm hungry!!!

Martin???

Peace,
Mark

[martin manning]

Getting really close now... it sounded pretty good before; low noise floor too.

What kind of fuse do you have in there? I think it should be a 3AT (3A, slow-blow).

[Guitarnut]

I wonder if the V4 & V5 grid stoppers are the issue. Several folks have commented on them being the wrong value. I may pull them and double check their R.

I have JJ rectifier and power tubes in now. May switch back to the Sovteks and see if it happens again.

Peace,
Mark

[martin manning]

Check the high voltage and filament wiring carefully for potential shorts, i.e. leads close to chassis-grounded screws, etc. Maybe playing it louder is shaking out more solder blobs.

[Guitarnut]

Mojotone! The layout says 2 amp. The schematic says 3 amp SB. I bought 2 amp replacements. Looks like I just need the right fuse. Radio Shack should be open by now.

Man, it felt good playing that amp!!!

Peace,
Mark

[martin manning]

I wonder if the V4 & V5 grid stoppers are the issue. Several folks have commented on them being the wrong value. I may pull them and double check their R.

Are these on the power tube sockets? The original had none, and some people add small ones to prevent oscillations and/or blocking distortion. I think if that is an issue you would hear a squeal, or the volume would drop out if it were an ultrasonic oscillation.