Reducing hiss from a fender tube amp

Tom Wolverton · From: Carpinteria, CA

I have a Fender tube amp (Pro Reverb) that has significant hiss coming out of it. It is smooth. Not the frying eggs sound. Just steady white noise. That amp sound great otherwise. The hiss is still there even when I turn the vol, tone and reverb all to zero.

So, my guess is that it is the power stage. Should I try replacing the power tubes? Or can V6 be noisy? I think this is called the phase inverter, right? Any suggestions would be greatly appreciated. Maybe the solution is to not worry about it. The amp sounds great otherwise. But it is more hiss than my Showman or Fox vintage amp, which both have 2 more 6L6 tubes in them.
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Tom Wolverton · From: Carpinteria, CA

Maybe I should search better. I'll try these suggestions.

http://bb.steelguitarforum.com/viewtopic.php?t=239488&highlight=hiss
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Craig Baker · From: Eatonton, Georgia, USA - R.I.P.

Tom,
Not only might the plate resistors be to blame, but cathode resistors have the tube current passing through them also. The phase inverter cathode circuit operates from a 2-resistor voltage divider, followed by four more resistors feeding the cathode. Not just the immediate resistors, close to the tube. . . but ANY resistor feeding voltage to the circuit could be the culprit. If you have a .1 cap at 650 volts or greater, you can try shorting it across various plate and grid connections. This will almost certainly take the hiss to ground indicating which stage is troublesome. Please let us know what you find.

Sincerely,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024
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James Morehead · From: Prague, Oklahoma, USA - R.I.P.

Tom, Wouldn't the first thing suspect be filter caps, then go from there?
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Tom Wolverton · From: Carpinteria, CA

The filter caps are on the suspect list. But maybe not at the top. I'll just call them "persons of interest" for now. They are brand new.

I replaced V1 & V4 with fresh JJ 12AX7 tubes and that knocked it down by about 50%.
I'll just play it as is for a while and see how it goes.

Thanks for that help James and Craig.

Just goes to show, when you find a tube amp that is super quiet without background hum and/or hiss, you have found a real gem.
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Craig Baker · From: Eatonton, Georgia, USA - R.I.P.

James, Tom,
Usually, when filter caps take the rest of the day off, it results in a low frequency hum, rather than hiss. (either 60 or 120 Hz hum depending on the power supply) The hum rides on all power supply lines and is usually consistent regardless of any control settings.

Sincerely,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024
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James Morehead · From: Prague, Oklahoma, USA - R.I.P.

Tom, "persons of interest"--cool. Laughing

Craig, thanx for clarifing. I have a twin, and I leave the volumes on "10", as I control the volume with my pot pedal. Eat your heart out Tom, It is so quiet I forget it's on, and have left it on for hours, getting distracted. Quiet as a deceased church mouse.

I had it gone through and updated all the caps etc.. I never have trouble or burn out my old tubes, but had burned out a new JJ 6L6GC--cost me nearly $400 to fix THAT melt-down. No more of these junky new tubes for me!. I put my old tubes back in. I have a stash of Jan Philips 6L6 GC's and Jan Philips 7581-A's.
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Brad Sarno · From: St. Louis, MO USA

It's sort of a tradeoff with old Fenders. They're loaded with carbon comp resistors and those, by nature, make hiss. But not all of the carbon comps impart much hiss. The hiss is largely proportional to the voltage biased across them. In a Fender, the high voltage situation is mostly at the plate resistors. You can significantly reduce the hiss by simply replacing the preamp tube plate resistors, all those 100k's, with a quiet RN70 Dale metal film type. That will definitely reduce the hiss, but it will also change the sound a bit. Pedal steel players actually seem to like the cleaner, more refined tone of the Dales. But the carbon comps are "warmer". Another trick is to replace the 1/2 watt carbon comp's that are likely in the amp with 1 or 2 watt carbon comps. By upping the power rating you decrease the hiss. That's another approach.

While the cathode resistors have current passing thru them, the voltage is very low so the noise from them is quite a bit lower than at the plate resistors.

Like Craig said, the capacitors don't really have anything to do with hiss, only hum. Resistors are the culprit and then the tubes too can get hissy if they're aging.

Fender Twins are set up so the power amp stage is wide open cranked and also a few stages of the preamp are also wide open cranked. The gain/volume control falls fairly early in the circuit, so at low volumes when you've got the guitar turned down, you still have this monster amp idling and ready for 80 watts or so. When you're on stage and using that power, the noise is insignificant. But sitting in front of a Twin at home at low volume, there's always just gonna be some idling noise. The louder you play, the less the noise is part of the picture. It's that old signal-to-noise ratio at work. The more you raise the guitar signal above the noise floor, the noise virtually disappears.

B

Tim Marcus · From: San Francisco, CA

you can also replace the grid stop resistors on the input jacks of the amp with a less noisy resistor - or move them closer to the to the socket. They are generally mounted on the input jacks on fender amps.

Fender switched to 1W plate load resistors in the 70's so your amp may already have those. I use them for high power amps - anything over 40W.

Tubes themselves can become noisy - some pick up heater noise more than others. Putting DC on the heaters of the preamp is a big job, but it will really quiet the amp down. Especially that crucial V4 in a twin which almost always adds more hiss and noise than V1.

Or swap V4 with a 12AT7 - you will still get the full power of the amp, slightly less reverb (not always a bad thing) and take the noise floor of the preamp down a few notches.
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Tom Wolverton · From: Carpinteria, CA

Thanks mucho to everyone. James, i 'm green with envy. : )

I put a Weber copper cap in there for the GZ34. Maybe I'll try switching it back.
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Tom Wolverton · From: Carpinteria, CA

Just to update you, I changed all the 100k carbon comp resistors with RN70 Dale metal film resistors. Also changed out the 3M3 and a few others. I replaced power tubes with short bottle TADs. re-biased. Also replaced a 12AX7 with NOS RCA. Cleaned all tube socket pins and re-tensioned them. That beat the hiss dragon back a bit. Not perfect, but good enough. Perfect is my Ken Fox steel amp, which is dead quiet. I'm done for now. Thanks to all for the suggestions.
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George Seymour · From: Notown, Vermont, USA

Tom Gorr · From: Three Hills, Alberta

Don't know if its been mentioned, but 12AX7A or 7025 tubes are the low noise spec version of the standard 12AX7.

Tom Wolverton · From: Carpinteria, CA

Good point Tom. I'll give that a try. NOS 7025's are not cheap.
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Tom Gorr · From: Three Hills, Alberta

there's used ones that guys test and sell for about 20-40 bucks a piece...make sure they are tested for microphonics...I've accumulated enough microphonic ones over the years, but they still work pretty fair in tube positions other than v1, but v1 is by far the most important for noise.

There's also some pretty decent new production tubes available...I'd get Brad Sarno's input on those...most of my tube experience is for voicing for thick and dynamic Marshall crunch tones...haha....totally different than what we're after on steel...well, most of the time.

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