Fender Reverb Tank Grounding

CJ

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anybody know why the Blues Jr amp uses this grounding scheme?

in other words, why not go straight to ground ?

Thanks!

 

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squarewave

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CJ said:
anybody know why the Blues Jr amp uses this grounding scheme?

in other words, why not go straight to ground ?

Thanks!
I think the tank output shield is grounded. So it does go straight to ground. The network you highlighted is just setting the gain and response of the driver. The tank input shield isn't connected to anything. Not sure why it wasn't just grounded. Some sort of bootstrapping maybe.
 

JohnRoberts

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CJ said:
anybody know why the Blues Jr amp uses this grounding scheme?

in other words, why not go straight to ground ?

Thanks!
The specific location of R45 and c21 ground connection is important for ground noise pickup. Any local ground noise errors coming into that - input will be amplified by 4.7k/47 or +40 dB.

If you are talking about the P2 connection, hopefully the schematic is wrong (as often happens with old hand drawn schematics). I see no reason to hang anything off an op amp - input other than a feedback network.  Perhaps if there is significant capacitance between P1 and P2 that would look like a LPF negative feedback capacitance, but any stray capacitance from P2 to anything else would inject noise and degrade stability.

Maybe somebody experimentally determined that the specific connections work, or the schematic is just a mistake.

JR
 

abbey road d enfer

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CJ said:
anybody know why the Blues Jr amp uses this grounding scheme?

in other words, why not go straight to ground ?
Because it's current-driven. The 47r resistor senses the current in the drive-coil. The capacitor makes sure there's 100% NFB at DC, so the coil does not see any significant DC current.
Accutronics always specified that their tanks should be current-driven for best response; however many designers have used voltage drive, which makes it necessary to pre-emphasize the signal because of the increasing impedance due to the essentially inductive nature of the drive-coil.
In tube-driven circuits, the output impedance of the drive circuit does part of the job, so less emphasis is needed.
BTW, if the connection was direct to ground, there would be no NFB and the opamp would go wild.
Some designers chose using the Hewland pump, but it's not as efficient as the current FB circuit.
 

abbey road d enfer

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squarewave said:
I think the tank output shield is grounded. So it does go straight to ground.
No, it's not! And it shouldn't be. It is essential for the coil to be floating in order to make the current-drive work.
And that is why grounded/insulated input is one of the many options when ordering an Accutronics tank.
 

squarewave

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abbey road d enfer said:
No, it's not! And it shouldn't be. It is essential for the coil to be floating in order to make the current-drive work.
And that is why grounded/insulated input is one of the many options when ordering an Accutronics tank.
I said the output shield is grounded (as in connected to the shell) and that the input shield was not. So the input is isolated but the output is not. Near as I can tell this is still correct and consistent with your claims about NFB drive which make perfect sense to me as well.
 

JohnRoberts

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What Abbey said (+1) but the 4.7k feedback R in parallel with the winding degrades the pure current source topology.

JR
 

abbey road d enfer

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squarewave said:
I said the output shield is grounded (as in connected to the shell) and that the input shield was not. So the input is isolated but the output is not. Near as I can tell this is still correct and consistent with your claims about NFB drive which make perfect sense to me as well.
I stand corrected.
 

CJ

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thanks you guys!

one more question, would a 5532 be better to drive that tanks rather than the 072?
 

abbey road d enfer

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CJ said:
thanks you guys!

one more question, would a 5532 be better to drive that tanks rather than the 072?
5532 would have a little more drive capability (not much since the TL072 is not really stretched), and could be a tad lesss noisy on the recovery side, though the 10k resistor (R48) in series would need to be lowered to take real advantage of the 5532's lower input noise voltage. There may be an issue with stability, requiring an additional cap to ground at the non-inverting input.
 

PRR

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> would a 5532 be better to drive that tanks rather than the 072?

What do you want, Hi-Fi spring reverb?

Leo used the cheapest transformer and an  obsolete power tube.

> 5532 would have a little more drive capability

Yeahbut: the drawing shows 27mV applied across 47 Ohms. Nominal current is 0.6mA. Even allowing 20dB peaks (for a spring??) we don't need 6mA, far short of a '072's grunt.

The 4.7K is there because a True Current Source would rise to infinity, which would be shrill. It is the tissue over the tweeter. (May also damp the inevitable high frequency resonance, which may be supersonic, but might upset the op-amp.)
 

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