Blown screen resistors and more

[titser_marco]

I'm trying to see if my amp actually is close to original Fender-style schematics and I found that it doesn't seem to have these resistors (highlighted in red).

What are these resistors for? Could them being missing be part of the reason that my B+ is so high?

[JazzGuitarGimp]

Those resistors serve two porpoises: first, they balance the B+ so that each cap sees 1/2 of the B+ voltage (without them, the voltage at the center point can wander which could result in too much voltage across one of the caps); and, they are bleeder resistors which bleed off the residual charge the caps would otherwise retain after the power switch is switched off.

They pose such a small load that they won't appreciably reduce the B+ voltage.

[Stevem]

Look very close at the main filter stack ( filter one and two)as those two resistors have those caps sitting right on top of them and all you can see are the leads at each end.

[titser_marco]

Look very close at the main filter stack ( filter one and two)as those two resistors have those caps sitting right on top of them and all you can see are the leads at each end.

No, there are no resistors under.

Another question: what is the difference between power supplies that use 4 diodes and those that use 6?

[sluckey]

In a full wave conventional rectifier circuit the diode voltage rating needs to be twice the B+ voltage.

When Fender first started using 6 diodes, 400V PIV was the affordable choice. 1000V PIV was expensive and not very common for consumer electronics. An amp that produces 450VDC would have 900V across the diodes half the time. Two 400V diodes is not enough. So, you would need at least three 400V PIV diodes per side to offer 1200V PIV rating which gives some extra headroom.

Today, 1000V diodes are common and very cheap. Two are plenty to offer good voltage headroom for most any amp.

[titser_marco]

In a full wave conventional rectifier circuit the diode voltage rating needs to be twice the B+ voltage.

When Fender first started using 6 diodes, 400V PIV was the affordable choice. 1000V PIV was expensive and not very common for consumer electronics. An amp that produces 450VDC would have 900V across the diodes half the time. Two 400V diodes is not enough. So, you would need at least three 400V PIV diodes per side to offer 1200V PIV rating which gives some extra headroom.

Today, 1000V diodes are common and very cheap. Two are plenty to offer good voltage headroom for most any amp.

I see. OK then, I guess I'll just use these 4x1N4007 diodes I got last weekend as I rebuild this amp. Thanks again!

[titser_marco]

Hey guys!

So the amp I've been working on rebuilding is finally working. Here's a summary of what had transpired.

1. After the choke died, I decided to call my tech (who's been doing coaching via text as well as I was troubleshooting prior to the choke dying) and have him rewind the choke and rewind the power transformer to spit out the right voltage. Turns out that my secondary was firing about 450-475 AC and that was the major reason I was getting close to 600vDC on my B+

2. While the tech was working on it, I reinstalled new sockets and put in 1K screen resistors instead. I also changed the diodes from the 70s era DS1K (?)to 1N4007s.

3. After two weeks, the PT and choke came back. I installed it in and when I turned it on, one of the dropping resistors burned out. I changed it and it still did that which made me think that at least one of the electrolytics (which were at least a decade old, even two may be) was shorted.

4. I replaced all electrolytics and now the amp is working!

So now, the next thing to do is to eliminate all possible sources of hum. I still get some without anything plugged but all controls to max, and I figured I'd just need to make sure that everything is in the right place.

Thanks everyone for all your help!

[Stevem]

Do not forget that the condition of the bias circuit filters can add hum and since it's in the output stage it will always be heard !

[titser_marco]

Do not forget that the condition of the bias circuit filters can add hum and since it's in the output stage it will always be heard !

I see. So is that something that I should suspect as a cause of hum?

More information: the amp has a pre-PI master volume and there is no hum when the control is zero'd. Perhaps the cause of hum is in PI stage all the way back to the preamp?

[xtian]

You said there's hum, "with all controls to max." What about with controls at zero? This will help us identify where the hum originates.

[titser_marco]

You said there's hum, "with all controls to max." What about with controls at zero? This will help us identify where the hum originates.

All zero'd out, including master: a very low frequency hum, 60hz perhaps.

Preamp all zero'd out, master from 1-to full: a higher pitched hum compared to the one with all controls zero'd out.

I forgot to mention that I didn't replace the bias supply electrolytic when I replaced the other power filter caps.

[xtian]

Fix up the bias first. Then reevaluate.

[titser_marco]

Will do that once I get the parts!

I tried some troubleshooting and removed the PI tube. The hum disappeared when I did that. I also tried disconnecting the preamp from the PI and the hum also disappeared when I did that. Thoughts?

[xtian]

Yeah, you've got ground loop hum in the preamp or PI. Time to evaluate your grounding scheme. General principle is the reservoir and screen caps should be grounded near the PT, same spot as the HT CT and heater CT. PI and preamp grounds should be grounded near the input jack. Merlin is a good reference on this. Let me know if I've abbreviated anything too hard!

[titser_marco]

Yeah, you've got ground loop hum in the preamp or PI. Time to evaluate your grounding scheme. General principle is the reservoir and screen caps should be grounded near the PT, same spot as the HT CT and heater CT. PI and preamp grounds should be grounded near the input jack. Merlin is a good reference on this. Let me know if I've abbreviated anything too hard!

I see. Currently here is how it's grounded:

1) B+ caps grounded to chassis (via terminal strip mounting screws.

2) Screen B+, PI Plate, Preamp Plate cap grounds connected to each other, then terminates at another terminal strip mounting lug.

3) My heater taps do not have a center tap, so my tech advised me to ground one of the heater taps. Currently, it's grounded to the chassis. Should I use the 2x100ohm grounding resistors on heater taps, and then ground it to where the PT ground is attached?


4) All signal grounds connected to another terminal lug. on the component side of the chassis.

Question: Should I use shielded wire for all signal connections in the preamp?