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FDP Forum / Fender Amps: Vintage (before 1985) / Deluxe Reverb - White Noise on normal channel only

Gobo

USA

Dec 7th, 2017 02:02 PM   Edit   Profile   Print Topic   Search Topic

Hi, I am trying to diagnose an issue on the normal channel of my 70's DR.

- It only occurs on the normal channel
- It is a white noise "shhhh" type sound that increases with volume
- The noise does not happen right away on the normal channel, but it will very predictably start after 3-4 seconds, after turning up the volume pot.
- I have swapped in a known good preamp tube from the normal channel (where I can not reproduce the issue) and it is still happening.

Since the issue takes a couple of seconds to present itself, does this suggest a bad capacitor?

Any input would be much appreciated!

Gobo

USA

Dec 7th, 2017 02:05 PM   Edit   Profile   Print Topic   Search Topic

I just realized I should have posted this in the "Amp Mods, Repairs, and Projects" forum. Is there any way to move my post? I'm not seeing the option...

Leftee
Contributing Member
**********
*******

VA

Life makes a man tired.
Dec 7th, 2017 02:43 PM   Edit   Profile   Print Topic   Search Topic

It should get answered here as well. No worries.

nuke

USA

Dec 7th, 2017 04:06 PM   Edit   Profile   Print Topic   Search Topic

Usually white noise is from a bad tube. But the next most likely source of trouble would be the plate resistors.

Does the hiss go up and down with the volume or stay the same? How about if the plug is in or out?

Gobo

USA

Dec 7th, 2017 04:26 PM   Edit   Profile   Print Topic   Search Topic

Thanks for the replies. I was swapping preamp tubes around today when I noticed the issue with the normal channel. I swapped a couple of tubes that were working well in the vibrato channel into V1 and the issue is consistently reproducible there. The issue does increase in volume as you turn up the volume of the normal channel, and I would describe is as an "ocean" type sound with the occasional crackle.

Gobo

USA

Dec 7th, 2017 05:09 PM   Edit   Profile   Print Topic   Search Topic

I've had the amp for several years, and the caps were reworked before I bought it. I just opened it up and it looks like there is one old component in the normal channel tone stack (attached image). I see a Cornell Dubliner .1 uf cap on the very right side there.

DR Chassis

nuke

USA

Dec 8th, 2017 01:21 PM   Edit   Profile   Print Topic   Search Topic

So I hate seeing things like that.

That board has been majorly gone over and there's rosin all over the place and lots and lots of parts have been changed. I cannot fathom a reason to do wholesale replacement of caps in a SF era amp. They just don't go bad.

The workmanship is where I'm going to criticize.

I prefer axial leads because the orange drops are meant to be PC-board mount and the leads will crack if they are not carefully bent to fit the eyelet board. There's nothing wrong per-se with the orange drop caps, they work and sound fine, but if the epoxy is broken at the leads where they get bent, humidity can enter over time and make bad things happen.

I'll give it a pass on wrapping the cathode resistors on the bypass caps - I've had to do that because long-leaded resistors have been very hard to find at times. Electrically and mechanically acceptable if done properly.

While it is fine to carefully solder a few components for repairs on the eyelet boards, doing a lot of parts like this one has had requires removing the eyelet board so that the soldering can be done properly and the rosin cleaned up.

Worse - what you can't see is the underside where there will be gobs of rosin on the eyelet board and all over the insulating board underneath it. Probably solder slag too.

Rosin attracts moisture over time and can absolutely create crackling noises, hiss and all kinds of fun stuff, especially in high-impedance and high-voltage circuits - like tube amps.

All through the electronics industry, rosin flux is always cleaned off boards, except maybe in the very cheapest stuff. You can use "no-clean" flux for minor repairs, but it is best practice to clean it off.

You got a lot of flux going there. Like I said, over time - it degrades. And you probably can't even get to the worst of it.

Also I see the 100k plate resistors have been replaced with 1/2w carbon films. They work fine at first, but over time, they tend to get noisy as well. I prefer 1-watt or 2-watt carbon comp or film resistors. The silverface amps usually had the 1-watt resistors in these positions to reduce hissing noises. Not sure why they were changed. Curiously - the 100k resistor to the tone stack is a 1-watt flame proof - which would have been better in the plate circuits.

Honestly, I'd recommend pulling the board out and doing a full restoration on it. That's messy and there's more mess you can't see.

On the tubes, there are 4 kinds of preamp tubes. The 12AT7 that drives the reverb, a 12AX7 that is the tremolo oscillator and the three marked "7025" on the preamps and the reverb recovery stages.

The one marked 12AX7 can be any crappy microphonic, noisy tube as long as it makes the tremolo blink, it is good. The first two 7025's, you want your very best tubes. I've often gone through my tube stash finding low-microphonic and low-hiss tubes for the preamp positions. the reverb recovery stage likewise needs to be low noise, but isn't as critical as the first two stages.

Good luck - I'd recommend a board-out clean up of that amp based on the photo.

Gobo

USA

Dec 8th, 2017 02:04 PM   Edit   Profile   Print Topic   Search Topic

Hey Nuke, thanks for the feedback, and yeah I did notice the huge amount of rosin around the eyelets as well. Ugh, what a mess. I'll consider a full on board out clean up at some point, but for now may just change out those plate resistors with what you recommended, as well as the old .1uf in the tone stack. If the problem stays, well I know what needs to be done :)

nuke

USA

Dec 8th, 2017 03:00 PM   Edit   Profile   Print Topic   Search Topic

Here's a pretty good example of a Silverface that has been recapped. The electrolytics in this amp have all been replaced. Pretty much the rest of it is original.

Note that the factory eyelets are clean of rosin, where the re-worked ones have a bit of rosin on them. Pretty decent job. Also the note the large 100k plate resistors. These do help reduce hiss and noise.

SF DR chassis

Gobo

USA

Dec 8th, 2017 03:45 PM   Edit   Profile   Print Topic   Search Topic

Oh yeah, that's a nice example to have. Very clean in comparison!

FDP Forum / Fender Amps: Vintage (before 1985) / Deluxe Reverb - White Noise on normal channel only




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