Hi,
I have finished this amp build and everything works great except the reverb circuit has too much noise as I turn up the reverb control.
The issue...The hum increases from normal (very little) to a good bit as the reverb control is increased.
The reverb tank is disconnected.
V3 A and B plate voltage=425VDC
V3 A and B cathode voltage=7.7VDC
With no signal there is 5.5 mv AC ripple at the grids of V3
B voltage=427VDC with 116 mvAC ripple
V4 A Plate=180VDC
V4 A Cathode=1.3VDC
I need help to fix the issue or redesign the circuit.
I have tried several things such as changing the tube to a 12Au7, install a .003 bypass cap, checked and measured component values.
I have not run braided wire from the reverb control to the board yet.
Any ideas would be appreciated.
Cheers,
Billy
I have finished this amp build and everything works great except the reverb circuit has too much noise as I turn up the reverb control.
The issue...The hum increases from normal (very little) to a good bit as the reverb control is increased.
The reverb tank is disconnected.
V3 A and B plate voltage=425VDC
V3 A and B cathode voltage=7.7VDC
With no signal there is 5.5 mv AC ripple at the grids of V3
B voltage=427VDC with 116 mvAC ripple
V4 A Plate=180VDC
V4 A Cathode=1.3VDC
I need help to fix the issue or redesign the circuit.
I have tried several things such as changing the tube to a 12Au7, install a .003 bypass cap, checked and measured component values.
I have not run braided wire from the reverb control to the board yet.
Any ideas would be appreciated.
Cheers,
Billy
Thousands of AB763 circuit amps have been soldiering along now for over 50 years, they do not require redesign.
Does it hum WITh the reverb pan installed? Assuming the pan transducers are not open?
V3 is the drive side, and won't be involved, especially with the reverb pan gone.
Do you have the footswitch plugged in? Unplug it, if so. Are the cables down to the pan still plugged into the chassis? get rid of them too. Still hum?
Changed WHICH tube to 12AU7? And WHERE did you install a 0.003uf bypass cap? The to tubes involved both have 25uf bypass caps already, so adding 0.003uf to 25 ought not make any difference.
V4 clearly works, as it amplifies the hum.
Since the hum is controled by the pot, it is in the recovery stage. Ther is a 220k grid return resistor. Tack a short wire directly across it. Hum gone or stays?
It looks like you have made substantial changes to the grounding scheme of the amp, which MAY be involved.
Does it hum WITh the reverb pan installed? Assuming the pan transducers are not open?
V3 is the drive side, and won't be involved, especially with the reverb pan gone.
Do you have the footswitch plugged in? Unplug it, if so. Are the cables down to the pan still plugged into the chassis? get rid of them too. Still hum?
Changed WHICH tube to 12AU7? And WHERE did you install a 0.003uf bypass cap? The to tubes involved both have 25uf bypass caps already, so adding 0.003uf to 25 ought not make any difference.
V4 clearly works, as it amplifies the hum.
Since the hum is controled by the pot, it is in the recovery stage. Ther is a 220k grid return resistor. Tack a short wire directly across it. Hum gone or stays?
It looks like you have made substantial changes to the grounding scheme of the amp, which MAY be involved.
Hi Enzo,
Hum is the same with or with the pan and pan wires connected.
No footswitch connected.
The tube chance was V3 from a 12AT7 to a 12AU7 which changed the way the reverb control worked a bit but no effect on the hum.
The 220K V4 grid leak resistor goes to ground at the footswitch and the .002 cap jumps that resistor with no change. Jumping that resistor has no effect on the hum.
I have in mind to redo the heater wiring and place it in a vertical position over the tubes and also reduce it's length plus tighten up the twist . From the photo you can see the heater wiring is a bit too long and currently running across the back of the chassis.
There is very little hum in the amp with the reverb driver tube removed. The hum/hiss that exist starts at about 7 on the volume controls and is not a problem....always could be better...lol
All jacks (Cliff jacks) are isolated and all ground buss terminate at the star.
Cheers,
Billy
Hum is the same with or with the pan and pan wires connected.
No footswitch connected.
The tube chance was V3 from a 12AT7 to a 12AU7 which changed the way the reverb control worked a bit but no effect on the hum.
The 220K V4 grid leak resistor goes to ground at the footswitch and the .002 cap jumps that resistor with no change. Jumping that resistor has no effect on the hum.
I have in mind to redo the heater wiring and place it in a vertical position over the tubes and also reduce it's length plus tighten up the twist . From the photo you can see the heater wiring is a bit too long and currently running across the back of the chassis.
There is very little hum in the amp with the reverb driver tube removed. The hum/hiss that exist starts at about 7 on the volume controls and is not a problem....always could be better...lol
All jacks (Cliff jacks) are isolated and all ground buss terminate at the star.
Cheers,
Billy
Hi Jon,
Yep, the heater wires suck. I am going to change them.
Perhaps, I should do that next and see what effect it has on both the circuits in general and the reverb.
Note: The hum as it stands now is not acceptable to me but I have seen a lot worse in production amps.
Cheers,
Billy
Yep, the heater wires suck. I am going to change them.
Perhaps, I should do that next and see what effect it has on both the circuits in general and the reverb.
Note: The hum as it stands now is not acceptable to me but I have seen a lot worse in production amps.
Cheers,
Billy
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Obviously, There are several changes I have made to the original design. The tremolo circuit does not use an opti coupler (roach). The cathode resistor on V4 is 1.5K not 820 ohms. The filter caps are inside the chassis. The caps are all orange drop 5% Polypropylene 600V. The power transformer produces a little higher voltage than the original, about 15 VDC higher B+. The current output transformer has 4,8 and 16 ohm taps and I don't really know the exact impedance of the original or how it was interleaved plus I assume wound with paper. I don't know exactly how the current Hammond transformers are constructed.
The grounding buss along the pots is isolated from the chassis and not a brass bar soldered to the chassis like the original.
Also the ripple on the B voltage of 116mv could I assume be reduced with a larger value cap.
Other that the heater wires do you guys see any obvious issues with the grounding scheme?
Thanks,
Billy
The grounding buss along the pots is isolated from the chassis and not a brass bar soldered to the chassis like the original.
Also the ripple on the B voltage of 116mv could I assume be reduced with a larger value cap.
Other that the heater wires do you guys see any obvious issues with the grounding scheme?
Thanks,
Billy
I redid the heater wiring which only slightly reduced the overall hum at max channel volume. It looks a bit better but not much change.
It had no effect on the hum in the reverb control.
I am not sure if the photo is clear enough anyone to look at it and say if it is twisted correctly.
Is there any reason that would prevent me from adding another filter cap to the B voltage to reduce the ripple?
Thanks,
Billy
It had no effect on the hum in the reverb control.
I am not sure if the photo is clear enough anyone to look at it and say if it is twisted correctly.
Is there any reason that would prevent me from adding another filter cap to the B voltage to reduce the ripple?
Thanks,
Billy
Perhaps because the D node the reverb return uses is the same one used by all the preamp circuits, and they do not hum. And if in doubt, measure the ripple there. I bet you find none.
turk, the roach is in the trem circuit, not the reverb, so I might expect it to be uninvolved.
Hum comes from many sources, each having its own cure. You need to DETERMINE what sources of hum you are facing. Don't just go down a list of possible hums and toss their solutions at the amp. heater wiring does nothing about ripple, and filter caps do nothing about heater hum. Grounding woes do nothing about PT fields. and so on.
Ripple will be 120Hz. Don't rely on your ears, use your scope to determine whether your hum is 60 or 120Hz. The waveform is usually distorted, so 60 can have higher overtones that confuse the ear. One small exception is the bias, which is half wave rectified, so any ripple there will be 60Hz. But bias ripple would always be present, and independent of the reverb control.
Whatever your hum issue might be, it only seems to affect the reverb circuit recovery stage. If you think it might be the lack of shielded cable, lift the 100k resistor from the plate. That will stop the tube from conducting, so any remaining reverb related hum would be from the wiring.
That is a pretty good explanation Enzo.
If you go building another one of these amps PlanoBilly,
I'd also try to find some different pre amp sockets...these are
below or almost flush with the chassis. Personally I'd like 1mm
above the chassis (thru - as you see in your pics here)...
but many of the vintage sockets used are about 1/2 mm.
Also and this is a error that even experienced builders and
manufacturer do, is they don't leave enough wire in the amp for service.
It is commonly called a service loop, that is when servicing the amp
it is sometimes necessary to trim the ends of wires that had to be
desoldered and moved for service, testing etc. So you want the
wires to be long enough for trimming down the road.
Generally your work is pretty nice and tidy. Good luck
with your trouble shooting, you are in good hands.
Post Script - Also double check and ensure you have the correct reverb pan.
If you go building another one of these amps PlanoBilly,
I'd also try to find some different pre amp sockets...these are
below or almost flush with the chassis. Personally I'd like 1mm
above the chassis (thru - as you see in your pics here)...
but many of the vintage sockets used are about 1/2 mm.
Also and this is a error that even experienced builders and
manufacturer do, is they don't leave enough wire in the amp for service.
It is commonly called a service loop, that is when servicing the amp
it is sometimes necessary to trim the ends of wires that had to be
desoldered and moved for service, testing etc. So you want the
wires to be long enough for trimming down the road.
Generally your work is pretty nice and tidy. Good luck
with your trouble shooting, you are in good hands.
Post Script - Also double check and ensure you have the correct reverb pan.
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I'm looking at the schematic around V4B, and its drawn in a way that could potentially cause grounding issues. (I assume Fender Corp. drew it this way, but they also knew what layout they had in mind.)
To minimise the potential for grounding problems, V4B grid leak (220k) and the cathode resistor of V4B (other end from point E) should both connect to ground near each other.
The area around V4B grid is at very high impedance, and will be prone to pick up hum. The schematic is laid out so that it looks as though long wires are connected to this point; in the actual physical layout, they should be as short as possible.
That means the 3.3M and 470k mix resistors should be physically close to V4B grid. Same thing for the 470k/220k (marked attenuator/V4B grid leak).
Leo was a bean-counter first, and there are places where his penny-pinching isn't a particularly good thing to emulate. Using shielded cable for any and all wire runs carrying small audio signals at high impedance is a good idea, whether or not Leo and his assemblers did it that way. Using unshielded wire hugely increases the chance of noise, hum, buzz, and outright instability (oscillation, frequently at a frequency too high to hear.)
-Gnobuddy
To minimise the potential for grounding problems, V4B grid leak (220k) and the cathode resistor of V4B (other end from point E) should both connect to ground near each other.
The area around V4B grid is at very high impedance, and will be prone to pick up hum. The schematic is laid out so that it looks as though long wires are connected to this point; in the actual physical layout, they should be as short as possible.
That means the 3.3M and 470k mix resistors should be physically close to V4B grid. Same thing for the 470k/220k (marked attenuator/V4B grid leak).
Leo was a bean-counter first, and there are places where his penny-pinching isn't a particularly good thing to emulate. Using shielded cable for any and all wire runs carrying small audio signals at high impedance is a good idea, whether or not Leo and his assemblers did it that way. Using unshielded wire hugely increases the chance of noise, hum, buzz, and outright instability (oscillation, frequently at a frequency too high to hear.)
-Gnobuddy
I have always been concerned that adding capacitance to the B+ filter would reduce the nice "compression" effect that these tubes amps are known for. I am wondering if using a higher-value choke for TR2 (standard is 4 H, I think) would be a better solution to reduce ripple.
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