debugging adding reverb to deluxe

[mikepukmel]

I understand that you are talking about frequencies above the audio range causing oscillation but what exactly would you call the crap picked up well within the audio range that for example can be heard plugging in a guitar that is not properly grounded or shielded? There may be 60/120hz hum but there can also higher frequency crap well within the audio range, often coming from the electrical wiring in my condo. (When neutrals and grounds are used interchangeably you can get a LOT of noise coming from the electrical lines, even the circuit that your amp is plugged into.)

From lack of the proper term I just lump all of that together as "noise from the ether"...

BTW my Jordan Boss Tone from the late 60s would pick up a radio station from Sacramento... you could almost make out what they were saying. What would you call THAT?

Thanks

Steve A.




I can't even bear to think of the possible damage to tubes caused by running them at very low filament voltages. EVH ran his original Marshall at around 80-85VAC using a variac so depending on the wattage of the light bulb what kind of voltages do you get with a guitar amp plugged into a light bulb limiter? (I read that his EL34s lasted about 15 or 20 hours but there is a lot of false information on the internet.)

For mikepukel: the light bulb limiter should be used only when turning on an amp for the first time, or when you have made changes that could change the amp draw or possibly blow a fuse. When the light gets brighter than usual (this will depend on bulb wattage as well as amp draw of device plugged in) it usually indicates excessive current draw and there is a problem that needs to be corrected.

I guess it would be okay to use it on initial startup after ANY changes have been made to the circuit, just to be safe. As long as the bulb does not glow brighter than it usually does it should be okay to connect your amp to full power.

Once an amp has been initially cleared using a light bulb limiter I will use a clamp-on digital ammeter to monitor current without lowering voltages whenever I think it appropriate.

Good luck!

Oh yeah, read all sorts of stuff in Eddie Van H's 'brown tone'!

Thanks Steve, left the light bulb limiter on more out of paranoia than anything else. Worried that I left a teeny scrap of wire from a clipping that is sneaking its way to bridge a HV connection to ground and short something, despite how careful I was wiring and soldering.

All of the voltages I measured before a day or so ago, were through the limiter. Posted the table of voltages, mostly looked OK, not far out of range. B+'s were a little on the high side, but not a heck of a lot different without the limiter. Didn't use the amp all that long with the limiter, but now that I know it's not a bundle of sparks and smoke, the limiter is in the closet in a box.

[mikepukmel]

And, if you don't have a scope, bring your dog into the room. He'll let you know if it's oscillating.

Geez, so THAT's why the pack of wild and neighborhood dogs were out racing around the house when I crank up the amp. I thought they liked my guitar playing.

[Chuck H]

Got a hum that increases when I turn up the reverb control. Checked the wiring around the reverb tubes, to the reverb control. I twisted the wires to the reverb control, and those on top of the eyelet board. Ran the wires to and from the RCA connectors to/from the tank close to the chassis. Grounds look ok.

There's a lot to be improved about the grounding in many reverb circuits. Especially WRT hum abatement. Conceptually it's really simple, and the same as what you do for preamp circuits. That is, treat the input and output as completely separate circuits!!! Why? because electronically they are. Nothing but vibration passes through the tank, there is no electronic relationship between the input and output. Grounding both ends in any sort of shared way can create ground loops. And the reverb tank puts out very little so the reverb recovery must amplify a lot. Any loops are amplified as much as the tiny reverb signal. So...

The preamp circuit sending to the driver tube grid, the driver tube cathode and the send cable shield should be grounded in one place. Typically with the filter for those circuits. The recovery circuit should be grounded with it's filter. It is fine if you have only one preamp ground location, but grounds for circuits on the input side of the tank should never be daisy chained with any circuit grounds on the recovery end of the tank. Recovery circuit ground and the cable shield should have their own ground lead. Finally, there should only be one ground for the tank. It will be connected to one of the cable shields. I don't know if it maters which, input or output, but only one shield should be connected to the tank chassis. The other shield should not be connected to the reverb tank.

[mikepukmel]

There's a lot to be improved about the grounding in many reverb circuits. Especially WRT hum abatement. Conceptually it's really simple, and the same as what you do for preamp circuits. That is, treat the input and output as completely separate circuits!!! Why? because electronically they are. Nothing but vibration passes through the tank, there is no electronic relationship between the input and output. Grounding both ends in any sort of shared way can create ground loops. And the reverb tank puts out very little so the reverb recovery must amplify a lot. Any loops are amplified as much as the tiny reverb signal. So...

The preamp circuit sending to the driver tube grid, the driver tube cathode and the send cable shield should be grounded in one place. Typically with the filter for those circuits. The recovery circuit should be grounded with it's filter. It is fine if you have only one preamp ground location, but grounds for circuits on the input side of the tank should never be daisy chained with any circuit grounds on the recovery end of the tank. Recovery circuit ground and the cable shield should have their own ground lead. Finally, there should only be one ground for the tank. It will be connected to one of the cable shields. I don't know if it maters which, input or output, but only one shield should be connected to the tank chassis. The other shield should not be connected to the reverb tank.

Thanks Chuck! I have my work cut out for me when I get home tonight.

[Leo_Gnardo]

Geez, so THAT's why the pack of wild and neighborhood dogs were out racing around the house when I crank up the amp. I thought they liked my guitar playing.

Because they haven't figured out how to stick their fingers in their ears. When my neighbors had their last, dumbest dog, he would howl pitifully and try to bury his head in the backyard dirt every time the fire siren went off 50 yards away. Also every time I fired up an amp for a real world guitar crankin' test Awoooooooooo!!!!!!

[Enzo]

Werewolves of London.


(I don't know, it just came to mind.)

Isolate the hum. Lift the reverb pan out of the cab and get it as far away from the power transformer as possible. ANy difference? Always mount the reverb pan with the OUTPUT end as far from the Power Transformer as possible. Also, putting some sheet metal over the open side at that end of the pan can help.

Does shorting the retirn cable jack on the chassis stop the hum?

[Chuck H]

Oh! And THAT^^^^^^^^^^^^^^^^^^^

It's very often possible to minimize hum by shifting the pan around. Enzo's suggestion of removing it from and significant proximity is a good way to discover how much hum this part of the equation is responsible for. I once had to mount a designated horizontal mount pan on it's side to get rid of hum. It just wouldn't skirt picking up hum from the PT any other way. And this was in the cabinet of a large combo!?! I had to get jiggy with a few physical aspects but I made it work. First time I ever saw that. Regardless of orientation or position, if the pan was horizontal it would hum. Tilted on it's side the hum would stop. I guess I can suspect that the wrong transducers were installed or the wrong pan markings used? But since well behaved and good sounding pans are hard to come by I made it work the only way I could.

[mikepukmel]

Thanks Enzo, and Chuck. Leo had suggested that on another post as well, but can't find it, said something about noticing the difference if the pan was in "backwards". So, like a dope, I reversed both cables, and it had no effect. Go figure.

The second mistake: what happens when you have reverb on 10, and bump the pan? The neighbors think that the invaders from planet Zolton have landed.

Seriously: the pan is about 6" narrower than the chassis. I shifted the pan to the right, so that the red jack side is not directly under the OT, and HUM went down by 90%. Eeek. So THAT"S what Leo meant by 'pan in backwards'? Should I reinstall the pan, with the jacks facing to the front of the cabinet?

I'll try your other suggestions (with reverb on 0 this time),
- moving the pan outside the cab
- shorting the input wiring (wiring to the reverb recovery tube)
and making sure that the grounds don't make a loop (chassis, out through shielding, across pan, then back through shielding to chassis)

Regarding the other suggestion:
Its going to be harder than I thought to remove the cathode cap and resistor that is attached to the reverb driver tube and ground it separately. Working on that.

[Chuck H]

I hope by "reversed both cables" you mean that you turned the pan? Never run the pan backwards, into the output and out of the input.

[Enzo]

Yes, mount the pan so the OUTPUT end is farthest from the power transformer. it doesn't matter which way the jacks face. MAke them face the way that satisfies keeping the sensitive end away from the transformer.

[mikepukmel]

I hope by "reversed both cables" you mean that you turned the pan? Never run the pan backwards, into the output and out of the input.

even more stupid than that: take the output cable, move it to the input, take the input cable move it to the output. Just swapped the cables. The short cable is now where the long cable was, the long cable is now where the short one was. Zero sum game.