Hot Rod Deluxe, what's that rustling noise?

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sixstringer
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Hot Rod Deluxe, what's that rustling noise?

Post by sixstringer »

After repairing a Hot Rod Deluxe that had a small portion of the PCB melted by an aching cap (I jumpered around the damaged traces, replaced the filter caps and changed out the problematic 470 ohm low voltage resistors), I'm left with a rustling noise when the amp is on. It works fine, all the controls and switches work as expected but after changing all the tubes (one at a time) the rustling noise still persists. Changing the control settings and plugging in a cord has no effect, the rustling remains, coming and going at random times. I chop-sticked the PCBs, the tube socket solder joints and could not get the rustling to get better or worse.l Anyone have a solution for it?
Thanks for your help.
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jaysg
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by jaysg »

Put a known good 20uF/500V across the power caps, one at a time. Sort of dangerous of course...you could tack it in each position
Firestorm
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by Firestorm »

It could just be the tubes. Too much voltage, too much current in that design. The ribbon cables are a known problem. They have to be folded just so. Better still is to cut between the leads in the last 2 inches and chopstick. It's one of those amps where the schematic is fine, but the physical execution sucks. Modern layout software doesn't seem to understand tubes. I've seen some with the B+ trace 1/8 of inch from a ground trace. 40V, no problem. 400V plus signal, Yikes.
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Post by Stevem »

These amps are very known for failing plate load resistors on the PI section, so just replace them and at least rule that out!
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sixstringer
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by sixstringer »

Adding a 22uf/500v cap to each power cap had not effect of the rustling noise at all. All the tubes were changed out with known-good new tubes, that did not effect the rustling either. The bias is set to 70mV, Fender recommends between 60 - 90, so that should be good. I'll check (change) the plate resistors next.

To be clear, the rustling sound is not the "hiss" that you get from old Fender amps with bad resistors. I've heard this sort of rustling from bad tubes, but these tubes are good.
Thanks for your advice.
Last edited by sixstringer on Sun Jan 31, 2016 9:41 pm, edited 1 time in total.
Firestorm
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by Firestorm »

Maybe not just in the PI, but any load R that sees high current. IIRC, Fender had an entire batch of resistors that was either defective or underspecced.
sixstringer
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by sixstringer »

All the 100k and the 82k (PI) plate resistors test good and in spec. Removing the preamp tubes (one at a time) did not change the noise. The only thing that seems to effect the noise is the "more drive" button. Engaging it reduces the noise by about 90%. Describing the noise more specifically; I'd say it sounds like a hiss the sound of a scratch pot combined.
Sometimes it's soft, other times a little louder but it never goes completely away.
Thanks for your help and advice
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xtian
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by xtian »

Up top you said your PCB had carbonized where it was burned. Sounds like it's conducting.
I build and repair tube amps. http://amps.monkeymatic.com
Stevem
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Post by Stevem »

Your amp may be in the process of failing one of the two 5 watt resistors, or one of the Zener diodes in the 16 volt switching supply, as this is a common failure too!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
sixstringer
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by sixstringer »

I checked the PCB after cleaning off as much carbon residue as I could and could find no conductivity around the board. But just because I could not find any does not mean that it's a possibility.

Regarding the 2 470 ohm/ 5w (16v circuit) resistors; this amp first came to me a couple of years ago with channel switching issues and I found a bad solder joint (and the telltale burn marks around the through-holes on the PCB. I replaced the 470/5 resistors at that time with the recommended 330/5 fix.

Now that I have the amp back, the area around those resistors looks a little more toasted than I remember it and the zen diode right next to those resistors looks likes it's been warm too. I replace the 330's with new high temp 470 resistors and left them off the board a bit for air circulation. I don't have a 5w zen diode replacement, I've not heard that zen diodes can cause noise, is that a possibility? Isn't that part of the PCB just for the 16V power supply?
Thanks for your help and advice.
sixstringer
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Re: Hot Rod Deluxe, what's that rustling noise?

Post by sixstringer »

I spoke to the amp owner and asked if he had ever heard a faint rustling sound. He simulated the sound with his mouth perfectly and said the amp has done that before. So, I guess the repaired PCB is not the issue. I suspect the 16v circuit but he has been using it that way without issue so I'm done with it for now.
Thank you all for your time, interest and advice.
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