Two-Stroke Amp loses volume intermittently

[DavidL]

I need some help with an intermittent problem I am having with an amp I build from scratch. The design is the Two-Stroke Amplifier as described in the Guitar Amp Handbook by Dave Hunter. The amp works great most of the time and sounds fantastic. However, after what seems like a random amount of time, the amp stops producing any sound. The filaments and pilot remain lit. The problem is remedied by switching the power switch off and immediately back on. Trouble shooting to date: Have re-soldered all connections, checked the voltages, and checked for a loose connection with the “chop stick” method. Any ideas would be appreciated.
Thanks

[MWJB]

Please furnish us with the voltages that you have checked, any red plating from the power tube?

Some pics of your build (not a schem, not a reference to Hunter's book) would be useful.

[DavidL]

Please furnish us with the voltages that you have checked, any red plating from the power tube?

Some pics of your build (not a schem, not a reference to Hunter's book) would be useful.

I did not record the voltages and measured them some time ago so I do not recall what they were. They were within the tolerances that Hunter lists in his book. I am not familiar with the term "red plating".

[MWJB]

Redplating is when the power tube plates turn from black or grey because they are dissipating too much current, however you say that your voltages are within spec, so with 500ohm cathode resistors on the power tubes, I'll assume that your plate voltage is no more than a few volts over 400v?

Those plate wires from 12AX7 pins 1 & 6 should be well away from the grid wires at pins 2 & 7, the plate wires should be long & run flat to the chassis floor. You look like you have shielded the wire from input jack to 12AX7 pin 2, but for some reason you have threaded through the board material & made it muchlonger tha it needs to be, run it straight to the tube.

I can't see the wiring around the power tube sockets...looks a bit haphazard from what I can see, any more pics looking down on the sockets?

If voltages are good, it looks like you have a layout issue.

[MWJB]

What's the switch at the far right?

[DavidL]

Redplating is when the power tube plates turn from black or grey because they are dissipating too much current, however you say that your voltages are within spec, so with 500ohm cathode resistors on the power tubes, I'll assume that your plate voltage is no more than a few volts over 400v?

Turns out I had recorded the voltages when I checked them and the poer tube plate voltage was 345.

Those plate wires from 12AX7 pins 1 & 6 should be well away from the grid wires at pins 2 & 7, the plate wires should be long & run flat to the chassis floor. You look like you have shielded the wire from input jack to 12AX7 pin 2, but for some reason you have threaded through the board material & made it muchlonger tha it needs to be, run it straight to the tube.

The layout for the shielded wire comes right from the book.

I can't see the wiring around the power tube sockets...looks a bit haphazard from what I can see, any more pics looking down on the sockets?

I have a better view, but don't seem to be able to attach it to this reply. This is my first time posting on this forum.

If voltages are good, it looks like you have a layout issue.

You also asked about the switch on the right...it changes the voicing by stitching the cathode bypass cap.

[DavidL]

The switch on the right on the front panel is a voicing switch that changes the cathode bypass cap. The switch on the right on the back panel changes the output transformer tap for different speaker configurations.

[Bruce / Mission Amps]

First thing I would do is run the piss out of it until it fails and then check voltages without shutting it off. Especially since you say it comes back on when you shut it off and then back on.
Other then that, you'll have to do some signal tracing to find out where the signal "is" and where it stops. This should be very simple to figure out.

[MWJB]

The switch on the right, at the bottom of the chassis is the one that concerns me...if this has the OT secondaries running to it (I can only see the OT common wire running directly to the speaker jacks) then this is not a good location for this switch. Any reason why you cannot rewire the speakers so that the both feed from one output jack, remount the OT secondary switch next to the speaker jack?

[Steve Conner]

Yes, having the OT tap selector switch right next to the preamp tube seems like a bad layout issue. It could easily make the amp unstable. But, if it's unstable it should be unstable ALL the time, not just at random times.

When it stops making sound, does whacking it bring the sound back? Give it a good thump. If that makes a difference, then there's a loose connection, a wire you forgot to solder, etc. Yes, even if you "resoldered them all": I've had problems where I missed an unsoldered joint three times.

What are the two speakers, a 10 and an 8? And what is the chassis from? Looks like a very nice build.

[DavidL]

Yes, having the OT tap selector switch right next to the preamp tube seems like a bad layout issue. It could easily make the amp unstable. But, if it's unstable it should be unstable ALL the time, not just at random times.

I will consider making some changes in the layout, but I do not understand the electronic basis for this causing the problem.

When it stops making sound, does whacking it bring the sound back? Give it a good thump. If that makes a difference, then there's a loose connection, a wire you forgot to solder, etc. Yes, even if you "resoldered them all": I've had problems where I missed an unsoldered joint three times.

Tried the whack method repeatedly, with no success.

What are the two speakers, a 10 and an 8? And what is the chassis from? Looks like a very nice build.

Speakers are Jensen C8R and C10Q. Chassis is a Weber 5F2.

[DavidL]

First thing I would do is run the piss out of it until it fails and then check voltages without shutting it off. Especially since you say it comes back on when you shut it off and then back on.
Other then that, you'll have to do some signal tracing to find out where the signal "is" and where it stops. This should be very simple to figure out.

I will check the voltages after it fails. What should I be looking for? By signal tracing, are you referring to using an audio signal generator and an oscilloscope? This is equipment I do not have. This is my first electronic build since ham radio equipment in the early sixties! Is there an alternative method? Thanks.

[DavidL]

I think i could do this, but this is the layout described in Dave Hunter's book, and I do not understand how a layout problem could electronically be creating the problem I have.

[MWJB]

Layout problems are very common in novice amp builds, even in some commercially available amps. Many folks embark on a fruitless cycle of changing parts, unaware that the parts are OK, just in the wrong place.

You have all your wires running over the power tube sockets, your heater wires should either be suspended high over the sockets, or flat to the chassis running around the sockets, right up close to the lip at the front of the chassis. Plate & screen wires should run flat to the chassis floor & away frm the heaters. Grid wires feeding 6V6 pin 5 should be kept away from B+ & heater wires. You don't need to ground out the pin 6 connections.

12AX7 plate wires I covered in the previous post.

Speaker tap switch - you don't want your signal criss-crossing back & forth, up & down the chassis.