AB764 Vibro Champ Build - Voltage Questions

[Chuck H]

Turning the heaters to DC will ONLY affect the hum caused by heater wiring. There are many other sources of hum, and that will not affect them.

Duly (and often) noted

It's important to practice good layout and grounding practices. Daisy chained grounds are bad. Preamp grounds sharing nodes with power amp grounds at some distance from the input is bad (with star grounding being a potential exception). Grounding the same circuit in two different places is bad. failing to orient the transformers laminates at 180* respective to each other is bad. Having any power supply parts or leads near the input circuitry is bad (this does not include the preamp HV supply).

Mmkay

[bridgford]

The rectified heater question was meant to be more of a broad question, as in "is it bad", is it a "staying true to the theme" type thing. PT is horizontal, OT is vertical, by the way. Grounds all go to a common point on the chassis. I do have a ground buss on the circuit board that also ties into the same chassis ground. The electrolytic filter capacitors do ground to the chassis about 1.5 inches from the chassis ground, but I also ran a jumper between the two.

This it is my first tube build, and I made use of an existing chassis from an organ, so layout is not optimal. It was done as a proof of concept, in the spirit of repurposing and experimentation.

And, to my voltage questions that I measured and posted on in #11 above, I was kind of hoping for some DC voltage coming out of the secondaries in the OT, because I have a Princeton OT and I was going to rewire it (adding the second octal socket, 4th filter capacitor, etc.) to be an AA964 Princeton. Now I don't "have" to. Maybe later.

[Chuck H]

Interpreting (or misinterpreting) your post...

If all grounds go to the same point on the chassis then is it only the pot casings on the ground bus and nothing else? Any redundant grounds should be eliminated. The ideal star ground point should be nearer the input than the power supply or main filter circuit. You don't need a jumper to the chassis ground. The chassis ground is for safety. If the star ground is nearer the input any voltage on the chassis (small as it would be between the chassis ground and the input across something like a chassis) becomes a sort of floating 0V reference. The 0V reference would be the circuit ground and the chassis ground would be "earth" ground. Even though they are almost always virtually shared in guitar amp circuits they should be treated exclusively.

[bridgford]

Thank you for helping me work through this.

So, I have attached a picture of my amp. I had a shielded lead from the organ that donated itself to the project. I ran the shielded lead from the input to pin 2 on V1, then from pin 7 to the volume control. The ground to the shielded lead only connects to the ground point on the volume pot. It then picks up the ground off of the bass, speed, and intensity pots, loops around to the filter cap grounds then on to the earth ground at the transformer mounting point.



Now that I have talked my way through this, I don't remember where I connected the circuit board bus ground at, so I will have to check that. Also, the speaker ground is just at the chassis.

Thanks,

Dale

[bridgford]

It turns out I never actually ran the bus to ground, it was as long as the perf board and made contact with the standoffs the board sits on. I trimmed the ends so they don't touch the standoffs and connected a ground lead from the center of the board and connected it to where all the other grounds meet. No discernable difference. It still sounds good, though.

[Chuck H]

It still sounds good, though.

That's what I was after

All stock Champs hum a bit due to the filament winding and/or the ground scheme. Idealizing both those things does yield good results, but most players don't object to the stock amp anyway. It's not a gain monster and the tone is there. So a little hum is acceptable.

[bridgford]

Chuck H and g1, I appreciate your input, guidance and suggestions. I think I have taken this one as far as it will likely go, electronically. I still have to finish the cabinet and the exposed chassis on the front. Sound is quality is more than acceptable, the extraneous noises are likely due to my less than optimum layout. Attached are a picture of the chassis as I am going to leave it and the face of the cabinet with knob that I had on hand. I just need to figure out how to finish it. At least I can practice on it and have my future son-in-law play music through it.



Thanks again.

Dale

[Chuck H]

That's just begging for a vented wood front panel a la late 40's early 50's style.

[bridgford]

I am not familiar with those. Got any examples you can point me to?

And now that I look at the chassis photo, I have the 120VAC leads untwisted going right past the filter capacitors. Last night I swore that I was done tinkering with it chasing down potential hum sources, but I lied to myself.

[Chuck H]

Like this:

[Leo_Gnardo]

I have the 120VAC leads untwisted going right past the filter capacitors. Last night I swore that I was done tinkering with it chasing down potential hum sources

Seriously doubt that would induce any hum.

If you decide go try the front panel cutout designs like Chuck posted, keep in mind it's a good idea to stand off the grill cloth from both baffle and deco-grill. One bugaboo of many old amps (and some newer ones) is hearing grill cloth rapping up against either when playing loud low notes. Building your own, you can design & build that problem out of your product.

[bridgford]

Seriously doubt that would induce any hum.

Good call. I tried twisting up the power leads and had no change. Its good for now.

I am not sure I want to go the wood face route. I will have to look at more of them to see if it grows on me. It's got a ways to go to get me heading that way.

Thanks.

[Chuck H]

I am not sure I want to go the wood face route.

Aw... C'mon.?. Built in "beam blocker"

I've done it. A drill, a cheap jigsaw and some sandpaper. Wood finishes can be as easy as a rubbed oil. I can honestly say that I think a frame and grill cloth is MORE work.

[bridgford]

So, I tried a rectifier on the heater leads no additional success and the voltage dropped to 5.2V. After further reading, I decided to redo my heater leads and add psuedo center tap. It looked better, but had no improvement. After further reading, I heard about using a shorting plug and read more on ambient emf.

The forehead slapping result was that I have about 15 compact fluorescent lights in my basement where I was working on it. So once I had the amp upstairs and the lights off downstairs, I repurposed a headphone extension cable by cutting off the 1/4 jack, stripped all three leads, twisted them all together and found a sound pressure level app for my phone. I ended up with a reading of 68.2 dB, but better yet, it sounded acceptably quiet to me.

Thank you to everyone who had offered up their time and suggestions. I appreciate all the help.

Dale

[Chuck H]

If you shield your cabinet you may actually be able to use the amp in a space like that (lots of EMF) without too much annoyance when it's not an open chassis. If it helps, you should have seen me trying to figure out what was "ticking" in the amp I was working on... While I had a cell phone in my pocket. This was a long time ago when I was a new cell phone owner.