Robinette Micro Bassman - no audio. A couple questions…

[camdenh]

Hello!

I just finished up the Robinette Micro Bassman LTP and fired it up for the fist time - no shorts (as indicated by lightbulb limiter) and all looks good aside from the fact that I have no guitar audio coming out - I am positive the output jack is wired correctly, and I tested the guitar input and got the appropriate 1m ohm resistance so all should be good there…

I don’t yet have a 12AU7 for the power tube so I put a 12AX7 in its place - would that have anything to do with why there is no guitar audio coming out? I wouldn’t think so, but not sure.

thanks!

[The Dude]

A schematic would help.

[camdenh]

A schematic would help.

Yessir you’re right! Just added it - I thought I did before but it was the layout instead. You should be able to see the link now. Thanks!

[The Dude]

A 12AX7 should make sound come out of the amp, so that's not the issue (unless it's bad). If you have a scope, I would inject a signal and see where it's getting lost. If not, you can use ACV on your DVM to do similar. Otherwise, start at the beginning- are all of your plate voltages/supply nodes present? Are the filaments lit/Do you have filament voltage?

[camdenh]

A 12AX7 should make sound come out of the amp, so that's not the issue (unless it's bad). If you have a scope, I would inject a signal and see where it's getting lost. If not, you can use ACV on your DVM to do similar. Otherwise, start at the beginning- are all of your plate voltages/supply nodes present? Are the filaments lit/Do you have filament voltage?

Yes so I’m working through them all now, but everything I’ve checked thus far seems fine — I’m getting 3.3v on each individual heater wire — all filaments are lit — plates seem to be okay to my relatively “green” mind. I’m not sure about a couple readings:

V1 (12ay7)
pin 1: 149v
pin 6: 141v — not sure about this one

V2 (12ax7)
pin 1: 157v
pin 6: 280v

V3 (12ax7)
pin 1: 191v
pin 6: 192v

V4 (12au7 on schematic, but I have a 12ax7 installed here)
pin 1: 325v
pin 6: 325v — this is also where the blue OT wire connects and I do get speaker pop

V5 (EZ81)
pin 1: 120v
pin 6: only 10mv here — not sure about this one either

**edit** I tested AC voltage on pins 1 and 6 of the rectifier and got 260v on each.

I’d love any input you may have!

[The Dude]

I see nothing there that would prevent the amp from making sound. I would start tracing signal through the amp.

[camdenh]

I see nothing there that would prevent the amp from making sound. I would start tracing signal through the amp.

Thank you for your help! Any thoughts on the best way to go about that? I’m not entirely sure how to trace the signal.

one thing I do notice is that I get “pops” from the plates on V4, so that seems to rule out the possibility of the OT being bad or something like that.

[The Dude]

Inject a test tone at the amp's input. If you don't have a tone generator, there are plenty of test tones on the web. You can use a phone, tablet, computer, etc. Here's one for example:

https://www.youtube.com/watch?v=3FBi...el=SoundNation

Then use a scope to follow signal through the amp if you have one. If you don't, use a voltmeter set to read ACV. Be mindful not to exceed DC voltage ratings of test equipment if measuring a tube plate. It's best to measure at grids to avoid high DC content. Look for the spot where you lose the signal. There are various tutorials on the web if you'd like more info that can be found with a Google search.

[Helmholtz]

What is the voltage at the output tube cathodes (pins 3 and 8)?

[camdenh]

Inject a test tone at the amp's input. If you don't have a tone generator, there are plenty of test tones on the web. You can use a phone, tablet, computer, etc. Here's one for example:

https://www.youtube.com/watch?v=3FBi...el=SoundNation

Then use a scope to follow signal through the amp if you have one. If you don't, use a voltmeter set to read ACV. Be mindful not to exceed DC voltage ratings of test equipment if measuring a tube plate. It's best to measure at grids to avoid high DC content. Look for the spot where you lose the signal. There are various tutorials on the web if you'd like more info that can be found with a Google search.

thank you for the input (haha) - I’ll do some research!

[camdenh]

What is the voltage at the output tube cathodes (pins 3 and 8)?

Power tube (I assume that’s what you mean by output tube) is V4, where I have the 12ax7 instead of the 12au7.

pins 3 and 8 both show a hair over 3v DC.

[The Dude]

It might also help if you could upload a gutshot picture of your amp. We may be able to spot something.

[Helmholtz]

Power tube (I assume that’s what you mean by output tube) is V4, where I have the 12ax7 instead of the 12au7.

pins 3 and 8 both show a hair over 3v DC.

I said output tube because the amp doesn't use real power tubes.

3V at cathodes looks ok.

[camdenh]

The Dude can do - fair warning - I’ve been untangling wires and etc to see if I can work out the issue and I’ve resoldered a few without worrying about lead dress just to hone in on the issue so it’s kind of a mess right now… but here it is.

[camdenh]

I said output tube because the amp doesn't use real power tubes.

3V at cathodes looks ok.

great thank you - I’m still very much a rookie at this so the terminology gets me sometimes.