Need help troubleshooting 18 watt tremolo build

[Matt D]

This is a copy of a post I have up on the 18 Watt forum but I thought I might post it here as well since that forum has been closed to new members for a while:

I have a new 18 Watt trem build based on Ritchie's improved layout. I just fired it up in the last couple of days. The normal channel sounds fine but I'm having some issues with the tremolo channel.

With a guitar plugged into the either trem input I get a very low amount of signal passing through to the output section and out to the speaker. Over the next minute or so the volume gradually falls to nothing. If I then turn the amp off and turn it back on there is no sound. However, If I turn the amp off, discharge the cathode bypass capacitor on V3 and turn the amp on, the same small amount of signal passes again, then gradually falls to nothing.

As far as I can tell I have the cathode bypass cap wired correctly, negative side to the ground, postitive to the cathode.

The normal channel and output tubes are working and sound great.

I plugged a signal generator into the amp and tried to trace the signal with an oscilloscope. I got a nice sharp sine wave from the inputs, through the 68k grid stoppers and into the grid of the preamp triode of the trem channel. At the plate however what signal I get just looks like noise.

My voltages are a little low. The PT is from and old allied-knight kit amp and the voltages were initially way too high so I knocked them down by running the B+ into a large choke before the first filter cap. I still need to work with them a bit but here is what I have so far:

V1:
1- 134v
3- 1v
6- 134v
8- 1v

V2:
1- 209v
2- 42v
3- 61v
6- 209v
7- 43v
8- 61v

v3:
1- 259v
3- I have measured as high as 40 volts here but during measurement the voltage stedily drops.
6- 255v
8- 8v

v4:
3- 11v
7- 292v
9- 280v

v5:
3- 11v
7- 293v
9- 282v

Anyone have anything like this happen before? Obviously I suspect the 50v/50uf cathode bypass cap but perhaps I am on the wrong track. Why do I have this voltage on V3 pin 3? I've checked my layout over and over and reflowed questionable solder joints.

I'll get some pics up tomorrow.

Thanks

[selloutrr]

Has the trem ch ever worked properly?

checking if we are looking for the cause being a failing part or a misplaced connection.

If it's never operated properly when you post pics I'm sure one of us will see the problem.

Can you also post a schematic?

[Matt D]

It has never worked. Thanks for the quick reply. Attached are some pics.

This amp is slightly goofy for a couple of reasons. The chassis is a cast-off from another builder, the PT was free, the ot and choke (and most other parts) are salvaged, and the board was cut on the front stoop of my apartment using the crudest of tools.

The most ridiculous thing about it though is that it is wired up backwards. To make the best use of the holes already in the chassis I had to switch PT and preamp sides of the amp. I wired it up as a mirror image of the original layout, not thinking about the fact that the pots would work backwards!

I'll get around to changing them when I get both channels working!

With that caveat, here are the pics. This is my 4th build and I know I have a long way to go in the craftsmanship department.

[selloutrr]

Lets start with the easy

Do you know the Tube that feeds the Trem ch is good? Not just new but known to work. i.e. tested good or tried in another working circuit.

Have you gone over the circuit and tested continuity? wiring the amp as a mirror image could easily lead to over looking something.
Suggestion
print out a copy of the schematic and as you confirm each trace or contact is correct draw a red line over it until the entire schematic is traced in red. This will help confirm both the wiring and keep you from having to back track or miss anything during the test.

After that we can look for component failure.

You mentioned the Pots are wired backward.. any change to the tremelo when you fiddle with the knobs?

[xtian]

If you suspect the cathode bypass cap, take it out. This will boost your signal in a normally functioning amp.

[selloutrr]

If you suspect the cathode bypass cap, take it out. This will boost your signal in a normally functioning amp.

If the cathode bypass cap was part of the original circuit and it functioned for other builders wouldn't the actual problem be else where. It might be a fix but is it the right fix?

Then this brings up the question.. Is this a tested circuit or just a build and hope it works? If not troubleshoot it until something works? I've seen 100's of those types of builds and schematics posted around the web.

[Structo]

Can you post higher resolution pictures?

If you have a tripod that will help too in lower light situations.
Use the timer function for a blur free image.

If you have a lamp you can shine on the interior that will help in clarity.

[Richie]

even though you built it backwards or reversed, it can be confusing when wiring up the pots. And better pics would help as others posted, but the controls are wired incorrectly. Once that is corrected, then be sure to try a few different tubes in the trem channel. Lead dress is also important.
Another thing to do is put a grid stopper on the trem tube on the tube socket,like a 820ohm to 1k.

[Matt D]

Thanks for the replies. I'll put up some better pics shortly.

[Matt D]

Allright! I had a 1 meg resistor coming off the cathode instead of an 820 ohm resistor. Now the amp is passing signal beautifully except.......

The tremolo is sloooow. I'm going to double check my the rest of my values in the trem section and do some searching of past topics.

Thanks again!

[Matt D]

Problem solved. I had some values wrong. The amp sounds great and works as it should.

[Structo]

Congrats!