Finished my first scratch build, problems-low output.

[Vanilla_Bean]

Hey everyone! I used to mod tube amps for a while, but I would pretty much just mess around with the preamp stage to change voicing. A few months ago I decided to do my first scratch build using a Marshall 18w lite schematic, with only mod being a post PI master volume. I am finally done, and when I went ahead to test the amp I found out that the output is extremely low, the only way I would hear it is if I put both preamp and master volumes on 10. At first I suspected it was the master volume mod, so I rewired it to bypass the MV, but it did not help. All the voltages are fine, I checked the cathode resistor and the cathode bypass cap on the preamp stage as well as the coupling caps. So I am completely confused now. One more thing, the preamp tube and the phase inverter barely glow and remain cool to touch for quite a while after the amp has been turned on. I thought it was pretty strange, but the voltage on the heaters is ok. I swapped the tubes around too, no difference.

[kwijabo]

pic's or it didn't happen.

did you swap tubes out of the amp, or just move them around?
when you bypassed the MV mod, did you re-insert the 220k load resistors?

i can usually touch pre-amp tubes as soon as i turn the amp off. they are warm but not usually hot. power tubes get really hot to the touch. so i'm not too concerned that the preamp tubes are not hot.

post some pics and people may see something.

[Vanilla_Bean]

pic's or it didn't happen.

did you swap tubes out of the amp, or just move them around?
when you bypassed the MV mod, did you re-insert the 220k load resistors?

i can usually touch pre-amp tubes as soon as i turn the amp off. they are warm but not usually hot. power tubes get really hot to the touch. so i'm not too concerned that the preamp tubes are not hot.

post some pics and people may see something.

Thank you for the reply! Yes I did reinsert the resistors. I swapped the tubes within the amp. I am going to stop by our practice spot tonight and get the 12ax7s from my other amp and try them out in the clone. Ok, I will post pics when I get home tonight or tomorrow morning. Any areas that I should do a close up of?

[vibratoking]

Any areas that I should do a close up of?

Yes

[Vanilla_Bean]

Any areas that I should do a close up of?

Yes

Which ones? Preamp sockets? Preamp wiring on the turret board?

[vibratoking]

There's not that much area to cover. Just take some well focused and well lit shots of the whole thing.

[Stevem]

What is the cathode voltage on all of your preamp tubes?

[M Fowler]

Has to be a wiring issue.

Is the PI tail resistor going to ground?

Is input jack and tone stack grounded?

Are the EL84 power tube sockets wired correctly?

Mark

[matt h]

(deleted)

[Vanilla_Bean]

Useful homework to do before and during your reunion with your amp:

Before: read up on the "divide and conquer" method of troubleshooting. If you've got another amp kicking around, that let's you further troubleshoot.

During: take a full set of voltage readings and make a chart. Fun fact: you can do a less-fun "buzz" or "pop" test version of divide and conquer while doing this. hooray multitasking.

In the future--build in stages and check as you go. Power supply first. Confirm no shorts. Power amp next. Confirm it works. Then your preamp. No surprises.

Thank you for your advices! I will especially keep in mind the last one next time. I have just got home from work, and I didn't have time to upload the pictures yet. I will do the homework you suggested, and I will try to see what I can do this weekend...sorry it seems like this will be a busy week for me with non amp related things, but I will post my findings in this thread if I discover anything about my build. Thank you everyone for the replies!

Added:

Ok, I have just powered the amp again and noticed that one of the el84s is red plating. I switched the tubes around and red plating follows the tube regardless which socket it is in. I'm going to investigate.

[Phil_S]

When the problem follows the tube, it's a pretty sure indication there is a problem with the tube. The question is whether the tube was bad to begin with or if there is some other problem that caused the tube to go bad. You may find after a time that the other tube red plates if there is a problem at the socket that affects the tube. It's a simple point. Don't be sucked in unwittingly by the problem following the tube. In any case, you are going to need a new tube.

The tube problem aside (I don't think red plating explains low output in this instance but facts are still thin), my reaction to low output when all voltages seem to check out OK is that the problem is with the output jack wiring or the OT. We have to reserve judgement until we see a voltage chart, which should include all tube pins (filaments AC, others DC) and all B+ filter cap nodes.

How did you wire the output jack? Is it grounded? What type of jack was used?

[Vanilla_Bean]

Sorry, still no photo but here is the voltage chart:

V1 12ax7
Pin#
1: 145
2: 0
3: 0.9 (this seems super low)
4&5 heaters 6.2 ac
6: 143
7: 0
8: 0.9
9 heater

V2 12ax7
Pin#
1: 221
2: 35.5
3: 72.8
4&5 heaters
6: 218
7: 35.1
8: 72.8
9 heater

V3 el84
Pin#
3: 11.3
4&5 heaters
7: 360
9: 330

V4 el84
Pin#
3: 11.3
4&5 heaters
7: 363
9: 329

All heaters measure 6.2-6.1 ac

[Vanilla_Bean]

When the problem follows the tube, it's a pretty sure indication there is a problem with the tube. The question is whether the tube was bad to begin with or if there is some other problem that caused the tube to go bad. You may find after a time that the other tube red plates if there is a problem at the socket that affects the tube. It's a simple point. Don't be sucked in unwittingly by the problem following the tube. In any case, you are going to need a new tube.

The tube problem aside (I don't think red plating explains low output in this instance but facts are still thin), my reaction to low output when all voltages seem to check out OK is that the problem is with the output jack wiring or the OT. We have to reserve judgement until we see a voltage chart, which should include all tube pins (filaments AC, others DC) and all B+ filter cap nodes.

How did you wire the output jack? Is it grounded? What type of jack was used?

All the jacks are grounded. The speaker jacks are grounded at the chassis and they are switchcraft type, the input jack is cliff and it has separate ground.

[martin manning]

Your voltages look reasonable except the plate voltage is a bit high for EL34's. That says the problem is in the signal path. I agree with Mark, there is probably a wiring error, a missing ground, or perhaps a bad solder joint. Matt referred to a pop test... Did you take notice of which points produced a pop in the speaker when you were measuring voltages? That will often give a clue as to where the sugnal is being dropped.

[Stevem]

Set your meter for low ac volts and follow the signal thru the amp, if you crank the amp wide open and play your axe with the ac volt meter hooked across the input grids of the output tubes do you read at least 12 volts ac? If not than keep going back up stream to the input jack until you do find audio signal..

Even the weakest single coil pickup will output .150 volts when you smack and A cord!

If you can pump a I pod or such into the amp you can keep your two hands free for testing.