Blackface deluxe again - weird symptom.

[kgreene]

So, I've been overhauling this blackface Fender Deluxe Reverb, and one customer complaint I'm looking to is this:
the Vib channel is noisy (hissy, slight hum), but it decreases significantly as you turn UP the normal channel volume control.

Any thoughts out there??

I'm going to remove the vol pot and input jacks from the norm channel and re-establish the grounds. Might replace the vol pot.

[jjman]

Whatever noise enters both channels might be reduced by increasing the volume of one channel. This because they are out of phase. Acting like a humbucking pickup. PS hum or heater current related hum could enter both channels.

[rsi]

I remember seeing this problem discussed somewhere...maybe one of my books. If i remember correctly, the problem's root cause was the grounding of the filter caps, or rather where the caps were grounded.

[kgreene]

Whatever noise enters both channels might be reduced by increasing the volume of one channel. This because they are out of phase. Acting like a humbucking pickup. PS hum or heater current related hum could enter both channels.

Wow, I didn't think of that - thanks. This effect is very pronounced on this amp. I replaced the pot and sured up the grounds, which seems to have helped quite a bit.

[kgreene]

I remember seeing this problem discussed somewhere...maybe one of my books. If i remember correctly, the problem's root cause was the grounding of the filter caps, or rather where the caps were grounded.

Thanks. When you say "where" they were grounded, do you mean the location was a problem, or there was a poor connection at that spot?

[rsi]

The location of the ground connections. Again, If my memory is not completely shot, it was that one of the filter caps further down the line was grounded at the same spot as the reservoir filter cap.

[Firestorm]

The location of the ground connections. Again, If my memory is not completely shot, it was that one of the filter caps further down the line was grounded at the same spot as the reservoir filter cap.

I think the problem being referred to here is that in reverb Fenders, the screen supply is grounded with the PI and preamp grounds. Fender supposedly did this to cure problems in the reverb circuit, which is supplied from the screen node. If that's the problem, you can separate the grounds (move the screen ground to the main filter ground along with the 12AT7 cathode ground. That said, the two biggest problems I've found with AB763 circuits is the huge length of wire connecting the tone stack and the reverb pot. Those leads run right past the B+ leads coming from the doghouse. The reverb pot wires need to be dressed as far away from B+1 as possible. As to the tone stack, I have sometimes replaced those leads with a shielded three conductor cable to great effect, but in a vintage piece, it might be considered a "hack."

[kgreene]

I hate to sound dumb here, but I need to know what some abbreviations are that you guys are using. PS hum? PI ground?

[rsi]

Firestone hit the nail on the head. That's exactly what I read (about the screen supply/PI/preamp grounds).

I still can't remember where i saw this, though.

[kgreene]

Could somebody explain this more specifically?
What difference does it make where the grounds are, if they are common?

[rdjones]

Could somebody explain this more specifically?
What difference does it make where the grounds are, if they are common?

The main thing is not to have current flowing through the chassis.
Grounds to the chassis are to provide a shield not a current path.
This includes speaker current where the speaker ground should connect directly to the output transformer secondary 'low side'.

Next, you want to have a group of circuits say for example the preamps all grounded to their filter caps ground, but kept separate from the power amps ground which is connected to it's filter cap's ground.

There are several theories of grounding each with it's own merit, but the most widely accepted modern approach is to use a "star" ground to single point which avoids any chassis current.

reddog Steve

[rdjones]

I hate to sound dumb here, but I need to know what some abbreviations are that you guys are using. PS hum? PI ground?

PS = power supply
PI = phase inverter (more accurately polarity inverter)

Every field has it's 'alphabet soup'

rd

[kgreene]

So the "star" point is isolated from the chassis?

I don't really understand the difference in these ground points. Lets assume that you have a blackface Fender amp. The chassis is ground for multiple points in both the preamp and power amp. When one upgrades an old model to a 3 prong chord, then the green wire (typically clamped to the chassis) is common to the many parts of the circuit which are grounded to the chassis, as are any transformer center taps. It seems to me that under these conditions, the chassis is a primary source of electrons.

Can anybody school me on this?

[kgreene]

Every field has it's 'alphabet soup'

rd[/quote]

I know what you mean....

[rdjones]

So the "star" point is isolated from the chassis?

It can be. But it doesn't have to be to gain the benefit of 'star grounding'.
You choose the star point connected to the chassis based on the best shielding/ground path compromise.
Again, different schools here but one modern thought is that it's best connected near the input jack.

I don't really understand the difference in these ground points. Lets assume that you have a blackface Fender amp. The chassis is ground for multiple points in both the preamp and power amp. When one upgrades an old model to a 3 prong chord, then the green wire (typically clamped to the chassis) is common to the many parts of the circuit which are grounded to the chassis, as are any transformer center taps. It seems to me that under these conditions, the chassis is a primary source of electrons.

Can anybody school me on this?

The green power cord wire is an Earth ground and references the chassis to earth potential. You really want this to separate from any other chassis connection and it should be as close as possible to the AC line entrance. This is dictated by safety codes to minimize shock hazards.
No current should ever flow through this path, AC or DC.

Most of the Fender grounding schemes have been obsoleted by modern safety codes. This includes the 3 wire grounded AC supply and eliminating any connection of either the line or neutral to the chassis.
("Death cap" and ground switch)

The Blackface preamp controls and inputs are connected to a brass strip along the front of the control panel which is then connected to the preamp power supply filter caps negative. The chassis is used to physically support the controls and jacks. The 'star' connection in a typical Blackface is on or near the power transformer, which is where the centertaps and the filter negatives connect. The chassis really doesn't conduct a whole lot of current even though it's electrically connected. The ground returns are made through wires.

Some of this may not make perfect sense, another contributor will be able to explain things a bit more eloquently ...

Here's Merlin's grounding page:

http://www.freewebs.com/valvewizard2/Grounding.html


rd