Just finished a 6G5-A Pro copy -- inconsistency between layout and schematic --

[mort]

What should the grid voltage be on v6, pins 2 & 7 ??

[Enzo]

You can't read it to ground with a meter. You can read pins 3,8 for the cathode voltage, then you have to measure voltage directly from pin 3 to pin 2 or 7. Try it. If there is 32v on the cathode, you might read 20v to ground from 2,7. That would be 12v of bias on the tube. Now read that 32v on pin 3, then read from 3 to 2, and get about 1.5v. Subtract that from pin 3's 32v for a 30.5 result. More or less. If you measure to ground from pin 2.7, your meter impedance will skew the reading.

Note the schematic shows the cathode voltage, and then the voltage at the bottom of the cathode resistor. They do that instead of a grid voltage, because you can measure it down there. Whatever is at the junction of the 820 and 6800 is what is really on your grids, unless the couplers are real leaky.

Mort, I do believe you that you have checked and rechecked this stuff. But something is escaping your view. And to me, circuits are more important than parts, that is why I didn;t ask to measure each resistor, I asked to measure the total circuit from A to B, in this case tube pins to ground. Each individual part might be fine, but that doesn;t make the circuit fine. All it takes is one little connection being off by one eyelet. Everything looks great, and the more often we look at it, the more normal it seems, but something is wrong.

I had a guy pulling his hair out for weeks on some Fender clone he built. sent to me, it took me a while to spot, but it was all down to a small resistor and cap that had swapped places on the eyelet board. Nothing looked out of place, but there it was.


See below

[Enzo]

Try an active method.

We have signal tracing and we have signal injection. I signal trace with a scope, but audio tracing works very well. To make one, all you need is another amp of some sort and a probe wired to its input. The only thing we need remember is there are DC voltages in our amps that we need top block. SO we add a cap, something like a 0.047uf 600v cap in series with the input to our extra amp. Now we can probe points of the test amp and hear what is there on our tracer amp. You can also build one out of any little headphone amp circuit, or some guys reuse old little powered computer speakers.


To use it, you input a steady signal - I use a line from my stereo - to the test amp, then follow it with the tracer, looking for the point the signal drops away. Start at the start and work through the amp.

The other method is injection, and there we start at the end and work back. Really, it is the same thing just looking at it different. Injection means we inject a signal into the amp and see what comes out. Again remember the DC voltages in the amp, so a blockinbg cap is important. You can use a signal source, but I usually just use the hum that resulkts when I touch a point with a screwdriver blade. The resulting hum tells me. For injection, we start at the power tubes, pin 5, touch them, and get some sound out the speaker? Now move back to the input of the phase inverter, whichever pin sees that 0.001uf. Louder sound? Keep working backwards, until we find the point we no longer can inject and hear it.

[mort]

I have just discovered what I wasn't seeing.

Jeeze *facepalm*


It had nothing to do with the amp


See I have this 2 x 12 speaker cabinet that I use for testing purposes. Not long ago I had stuck a speaker in there to test it and disconnected the other speaker in there. Well the one I was testing sounded like crap with no output and I'm discovering right now that the faulty speaker was still hooked up


Ive had finer moments

Anyway now I have full signal, everything works, and I just need to clean up some idle hum

[gui_tarzan]

Damn, I wish mine was that easy.

Glad you found it though.

[Tom Phillips]

I have just discovered what I wasn't seeing...

This is a good lesson in troubleshooting and the requirement to consider all possibilities. The original complaint of "The problem I'm having is really low output..." turned out to be that the amp was putting out full power but the speakers were producing low sound level. That's quite a different thing. During all of our 33 post discussion no one suggested trying a different set of speakers. *Facepalm* squared.
Tom

[Jazz P Bass]

During all of our 33 post discussion no one suggested trying a different set of speakers.

Or actually measuring the Vac grid input signal as compared to the Vac output signal at the speaker terminal.
P= V x V / R.

[mort]

OK I'm having a tough time finding the source of some hum...

Here's how it's grounded.. All pot grounds and board grounds go to a buss that is grounded to the chassis about midway down the length of the chassis. The main ground from the power cord, the CT's, and the filter pack grounds are grounded on a PT lug lab. All the filter cap grounds are connected together and grounded to the chassis via 1 wire. The main power grounding point and the grounding point midway up the chassis are the only two grounding points, except for where the jacks/pots ground themselves...

The hum is constant from the time the amp warms up, even with nothing plugged in. As I increase the volume the hum lessens just a tad (?).

I haven't found anything to follow up on while moving wires around, etc.

All my B+ wires from the filter pack are more or less twisted together as a bundle. Should this be a suspect?


Thoughts?


Also, there is a small issue with teh vibrato. The effect works and sounds good but the speed pot maxes the speed when placed at noon and any higher it just sounds like the effect goes away. Granted, I still need to get the bias just right. With the resistors I have I'm either getting 13W per tube or 25 watts per tube(just need to pick up a resistor that will land me in between). Bias does not seem to have an effect on the speed though. How do I dial in that speed pot in order to use its full swing? And it would be nice if I could get it just a touch slower.

[loudthud]

... and the filter pack grounds are grounded on a PT lug lab. All the filter cap grounds are connected together and grounded to the chassis via 1 wire.

The ground of the filter cap for the preamp must be separated from the other grounds and connected via a separate wire to the ground buss for the preamp. This should ground to the chassis at the input jacks. If that isn't quiet enough, move up the chain to the next filter cap, separate it's ground and connect it to the ground buss near the phase inverter. Move the preamp ground wire to the input stages of the preamp.

[Enzo]

When hum lessens as the volume is turned up, that tells me you have more than one source of hum involved. These are out of phase, so as you advance the volume it increases the one hum to the point it cancels the other.

Hum is not generic, and is not all caused by your grounding.

Important to determine if the hum is 60Hz or 120Hz or both.

And by pulling tubes, you can determine what stages are involved with this hum.

[Chuck H]

I agree with Enzo (there's a shocker). And I'll add that to fix multiple hum problems you'll need to tackle them one at a time as opposed to trying any shotgun hum fixes like separating filter grounds or isolating jacks. Instead, find the specific source for hum in a given instance. Turn the amp down to where it produces the loudest hum and leave it that way until the cause of the hum is located. Then you can turn the amp up to locate any hum that increases with volume. Toggling between the two conditions will confuse troubleshooting. So...

Turn the amp down and pull the first preamp tube. Does the hum change? if not, pull the next. Repeat until a change or elimination of the hum is found. This helps isolate the afflicted circuit/s so that the number of circumstances to be evaluated is reduced and the possibility of understanding and success is increased.

[mort]

I got around to workin on this again this morning. I set the bias with 3K (down from 3.9k) to bring me to 18 watts per tube (down from 25w). And I wen ahead and separated the reservoir and first filter cap from the rest. Now the amp is real quiet and stable. Also, the issue with the speed control cleared up and I have use of the full swing of the pot.

This is a real nice sounding circuit btw. I've owned several brown Fenders but this one is a rare bird. The vibe is super smooth as expected. The way the effect rolls out as you turn it up is on the more subtle side so it sounds REALLY natural even as you get into the upper half of the speed and/or intensity.