5e9-a build - sounds great - no trem

[mort]

I've been trying to find my error without asking for help and at long last the problem still hasn't been found. I'm sure it's right in front of me but I can't see it.

I will continue to look over the wiring for mistakes, unmade connections, etc. and here's what I've got so far.

general amp function is fine, sounds great, etc. the intensity control damps the overall volume of the amp when dimed(which I'm assuming is normal even when the effect is switched to off), but there is no trem effect at all. I've lifted the leads off the pots and switch and checked them and all operation reads normal with a DMM. Swapped out the v3 tube with no improvement. The only thing I can find to go on is the cathode voltage on v3a.

Here's V3

[(voltages from layout in parentheses) actual voltage]

1. (370) 341
2. (235) 256
3. (205) 279
6. (235) 256
7. (00) .1
8. (2.1-4.5) 3.6

http://www.thevintagesound.com/ffg/s...9-a_layout.gif

I used the lower B+ version of the tweed tremolux PT offered by Mercury so the lower plate voltage looks normal to me, but the grid and cathode voltages(2&3) look wonky but I'm not sure where to go from here...

[mort]

Luckily I just happen to have an identical amp that was built at the same time and it's fully working normally. I've done some comparing and on the one that is working, I'm getting a small amount of varying voltage to the speed pot, and there is no voltage at all on the one with non-working trem. I see that the B+ goes through a 10m resistor to feed power to the trem circuit. I have appropriate voltage on one side of the signal caps that feed the speed pot (.02 and .01 caps), but on the pot side of the caps I have no voltage. I have .02 caps on hand so I swapped it out with no improvement. Could it be the .01 cap bad? I thought signal caps blocked dc but there's clearly DC on the speed pot of the one that's working. I'm confused...

[mort]

more things I've checked:

made sure footswitch jack wasn't shorted to ground(even lifted lead off of jack)

read continuity with DMM through each pin of the socket of v3

verified heater voltage and that both heaters are actually lighting up

subbed in replacement .02 and .01 caps (3 total) associated with the trem circuit.

read resistance across sections of a circuit (no anomalies found)


Looks like everything is correct but it's not oscillating the signal..

[mort]

here's the voltages right now. with the vibe switch engaged, no footswitch attached, etc.

I'd be more than happy to update it with any readings requested to aid in troubleshooting.

[nickb]

A low gain sample for V3 could stop it oscillating - try the one from the working amp.
Also low gain as could be caused by a bad bypass bad cap on V3 cathode pin 8.
Too big a value of bypass cap on the PI would also do it.
If the 10meg were the wrong value, that would do it.

With it set to slowest, you should get a varying reading on V2 pins 2 and 3.

Failing that, how about a gut shot and maybe some eagle-eyed guru ( i.e. not me ) will spot something.

[lowell]

Something is odd about this layout. Maybe there's a note I'm missing. The footswitch jack is correct. Why is the on/off switch on the speed pot connected to a different part of the circuit? Try grounding the point where the footswitch jack is connected and making sure the speed switch (.01/.02uf junction) is NOT grounded.

[mort]

Seems like the first triad in V3 is conducting but no current being drawn and that's why the voltage is elevated on pin 3?

I'm about to sub in a replacement cathode cap from V2 to see if that cures it.

One of these amps was for a customer/friend and was promised to ship today so I just got back from shipping off the good one, so it's gone.

And as for the layout, this is my 3rd and 4th build of this exact circuit and it's always worked fine. Still can't find a wiring error or bad connection either. I've shorted the jack to ground, and also disconnected the lead from the jack, also I've put a jumper on the vibrato switch just to make sure the switch wasn't an issue.

[Enzo]

Lowell, look at the schematic, it clearly shows the on switch and speed pot connected to the junction of the 0.01 and 0.02, just as the layout. It also shows the jack connected to the junction of the 10m, 1m and two caps, just as the layout does.

I wish folks would stop posting layouts instead of schematics. A schematic only is fine, and a schematic with layout is fine, but a layout only means we have to trace out the circuit in our heads. The schematic shows the electrical relationship between parts, while the layout shows the physical relationship between parts.

The left triode is a cathode follower, so we expect high cathode voltage. You report 279v on pin 3, and there is 250k plus 1.5k resistance to ground with depth control at zero. (highest resistance) So I=V/R=279/251500=0.001A=1ma. Current is flowing. At max depth I get about 71k for the pot and resistor in parallel, so 72.5k total. 279/72500=3.8ma it is conducting.


You have two triodes, the right one is the oscillator and the left the interface to the signal path. if the oscillator doesn't work, it doesn't much matter what the other triode does, so get that going. turn the amp around the other way, work on it 180 degrees. It will be like working on a different amp. You stare at something long enough everything looks like it belongs whether it does or not.

This is a phase shift oscillator with three series caps as the feedback path from plate to grid. Something is wrong. But it isn't about the parts, it is about the circuit. A bad part can screw up a circuit, sure, but good parts in a bad circuit won't work either. So don't test the parts, test the circuit. In other words measure resistance from pin 7 to ground, do you get 1 meg? Is there 1 meg from the 10m to ground? From the jack tip to ground? That measures the part but also all the wires that connect it to somewhere. From the junction of the 0.01 and 0.02 to ground do you get from 2 meg down to about 200k as you rotate the control? I can assume pin 3 to ground has a path because you have cathode voltage, but does it measure about 3300 ohms? Is there about 220k between pins 1 and 6? I don't expect to see any DC voltage normally, but this 10meg deal negates that. So we have 265 where 290 was, and the 10m and 1m mean we should see about 265/11=24v, you have half that. So is the 1m half what it should be? Or is the 10m way high? Did you try disconnecting one end of the 10m? Ultimately we want to take voltage reading at pin 6 and see the meter bobbing up and down.

[lowell]

My bad I had the wrong schematic.

[Tom Phillips]

...I wish folks would stop posting layouts instead of schematics. A schematic only is fine, and a schematic with layout is fine, but a layout only means we have to trace out the circuit in our heads. The schematic shows the electrical relationship between parts, while the layout shows the physical relationship between parts...

++1. The schematic tells us what is really going on. The actual wiring dress is important but a layout drawing doesn't really even show the exact placement of the components and routing of the wires. Looking at a layout is like listening to someone describe a concert. Seeing the schematic is like listening to the actual music.

[DrGonz78]

Should be the schematic in question.

[lowell]

Apologies in advance for side stepping the repair. Not understanding the osc switch. Does the dotted line mean the sweeper gets shorted to the caps when switched?

Nevermind the layout answered my question. The top of the speed pot gets disconnected altogether from the caps.

[Enzo]

The dotted line is not electrical, it is mechanical. it is a very common drawing convention. All it means is the pot and switch are linked mechanically.

[mort]

Before making this thread I thought of you, enzo. And I could hear you saying to "check sections the circuit" and I had done so on a few paths. I'll go through your suggested test points as well. Something else I frequently think of is "look for horses, not zebras". Thanks for those, btw

[mort]

This is a phase shift oscillator with three series caps as the feedback path from plate to grid. Something is wrong. But it isn't about the parts, it is about the circuit. A bad part can screw up a circuit, sure, but good parts in a bad circuit won't work either. So don't test the parts, test the circuit. In other words measure resistance from pin 7 to ground, do you get 1 meg? Is there 1 meg from the 10m to ground? From the jack tip to ground? That measures the part but also all the wires that connect it to somewhere. From the junction of the 0.01 and 0.02 to ground do you get from 2 meg down to about 200k as you rotate the control? I can assume pin 3 to ground has a path because you have cathode voltage, but does it measure about 3300 ohms? Is there about 220k between pins 1 and 6? I don't expect to see any DC voltage normally, but this 10meg deal negates that. So we have 265 where 290 was, and the 10m and 1m mean we should see about 265/11=24v, you have half that. So is the 1m half what it should be? Or is the 10m way high? Did you try disconnecting one end of the 10m? Ultimately we want to take voltage reading at pin 6 and see the meter bobbing up and down.

I'm reading 69k to 210k from pin 3 to ground, depending on where the intensity pot is set.

And I'm only reading ~4M across the 10M resistor in circuit... but when I lift one end it actually reads 10.00M on the dot. (shouldn't it read 10m in both cases?) edit: I see it now. I'm reading through the power filter resistors, the PT CT, ground and one more 1M resistor when I attempt to read the 10m in circuit.