Issue With Bassman 5F6A Build

[martin manning]

All right, another step forward! I guess the V2 ground was bad. I think that ground might have to be moved back to the buss with the volume pot grounds eventually.

You should be getting a pop from the speaker as you are measuring the voltages. That is what Reeltard is referring to.

The V3 voltages look good, but there is still a big difference between the V1 plates, and one is quite a bit lower than expected (they should be ~200V).

Still don't know why the voltage drop across the power tube screen resistors is reversed. If you are going to instal current sensing resistors on the power tube cathodes you can verify that they are conducting current and that they are biased correctly. Alternatively you can measure the DC resistance across the OT primaries (red to blue, and red to brown with the power off) and also get a measurement of the voltage drop on each side with the power on (Vred -Vblue and Vred - Vbrown). Then you can calculate plate current for each power tube as V/R.

[alvarezh]

Martin, if the plate resistors are the correct values, and the cathode resistors are correct also (on V1), then maybe one of the bypass caps could be shorted, giving the umbalanced voltage reading.

Right or wrong?

All the best.

[martin manning]

Hi Horacio, in this circuit V1a and V1b have a common cathode resistor and bypass cap so the cathodes are tied together. The grids should both be at ground, and the plate loads are at least close. Maybe try another tube, a 12AX7 even, and see if the balance is better.

[alvarezh]

Thanks Martin. I see, yes, bad tube or bad socket (it happens!).

I would clamp the tester's leads and wiggle the tube to see if there is any noticeable change on the reading.

Working (reading and educating myself) on that SE amp I am working on with you guy's help. Will post some information (schematic included) this week end.

Cheers!

[TUBEDUDE]

Be sure to include another clip. Nice job on the strat. As good as it sounds, It will sound worlds better on the Bassman.

[vibratoking]

... in this circuit V1a and V1b have a common cathode resistor and bypass cap so the cathodes are tied together. The grids should both be at ground, and the plate loads are at least close. Maybe try another tube, a 12AX7 even, and see if the balance is better.

Or pull the tube and measure the plate voltage. That should tell you if the problem is related to tube or socket or something else vs a basic problem with the B+ supply and it's wiring.

[JazzGuitarGimp]

A little bit of DC on one of the grids would cause a mismatch on the plates.

[Guitarnut]

Thanks for the suggestions guys.

Last night before wrapping up for the day, I moved the pot and second stage grounds to the same point as the first stage ground. The amp is very quiet now with about 1/4 the hum as before. Still no guitar sound though.

I poked around with one probe like Miles suggested and didn't find any components that were causing pops or crackles. I did notice that when I touched some of the wires on V2 with the probe that they made significant cracking sounds. I removed the sheild and it made a similar crackle. On further inspection after powering down, I noticed that the tube was not seating fully like V1 and V3. I think there's a bit of solder remaining in some of the pin holes...I had a slight issue with solder running early on in the project. I ordered new ceramic sockets to match the octals but they sent the wrong size...too large for the chassis holes.

I've checked continuity between the blades and lugs on the socket and they're fine, the tube is just not getting to that "pop into place" point and it leaves it sort of wobbly. I hadn't noticed before since I had the shields on. Any suggestions for cleaning this out and getting the tube to seat properly? I've tried the solder sucker but it only does so much. I have replacement sockets of the right size on the way but not due until Monday.

Peace,
Mark

[Reeltarded]

So the signal is killed before V2. Make sense? Noise from the last point back.. The problem with signal is between the input and V2. If you use a clip wire from input + to the grid resistor of V2 you should hear guitar.

If you have a way to blow compressed air into the socket pins while it's boiling you can make some work again.. but that is a dirty way to go..

I would never do that!

[Guitarnut]

If you have a way to blow compressed air into the socket pins while it's boiling you can make some work again.. but that is a dirty way to go.

I would never do that!

Thanks Miles! I think that's a great "outdoors" idea. I'll give it a shot...literally.

Peace,
Mark

[vibratoking]

I've been able to work solder braid into the socket and clean them out nicely.

[JazzGuitarGimp]

I've had good luck with heating the socket terminal with the iron (on its highest temperature setting) and then using the solder sucker from the other side of the socket (the tube insertion side). Get the solder sucker tip as tight-in to the pin opening as you can. Keep the iron on the terminal while sucking out the solder from the other side.

[Reeltarded]

Better idea there, at least you don't get 1000 700° impact craters.

[Structo]

Not that this is the problem but looking at your pictures of V4 & V5, the screen resistor appears to be 47K.

Yellow, violet, orange.

Double check your shielded cable from the input jack to pin 2 on V1.

Common mistake to damage the shielded cable when soldering, shorting out the input.

Measure resistance from pin 2 to ground, should not be zero.

[Firestorm]

The amp is very quiet now with about 1/4 the hum as before.

A very secondary issue at this point, but it looks like you may have your output tube heaters wired out of phase (awful hard to tell with all green wire). Pin 7 on one socket has to connect to Pin 7 on the other (and 2 goes to 2). Otherwise the 60Hz hum won't phase cancel. That might be part of the residual hum you have.