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Post subject: Squealing reverb
Posted: Tue Jan 26, 2010 4:11 pm
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in order to cast a wider net, I'm cutting and pasting a post I started on HarmonyCentral:

re-furb my Reverb / squealing sound

I have a '69 Fender Bandmaster Reverb TFL5005D. I've taken it out of storage and am addressing a problem with the Reverb.

I can hear the Reverb working, but when i dial anything past '4' all i am left with is a piercing, high pitch squeal.

So far, I have:

Cleaned all pots and switches.
Checked the innards and solder joins for obvious burn marks.
Visually inspected the inside of the tank. Looks new.
Replaced the tube that is most directly tied to the reverb pot, according to the schematic i found (link below)

I've had the amp since 1993 and I remember the reverb sounding like a heavenly (& springy) choir, and would like to restore the amp to peak performance.

TIA for advice as well as hints or performance tweaks

cheers

schematic:
http://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=177571



Reply ; Even though you replaced a tube (the 4th position I assume), that still sounds like it's almost certainly a microphonic tube. Try turning the reverb up to just below the squealing point, and then try GENTLY tapping each preamp tube with the tip of a pencil. If you can hear the tapping through the speaker, on a given tube, that tube may be your culprit. Some types of tubes are more prone to microphonics that others, and on my Rivera the reverb driver tube is by far the most sensitive position for microphonics. A new tube can be microphonic, so don't assume your new tube is good.

still squealin'
did the pencil trick, and yes, found a microphonic tube that would klank through the speaker.

However, the squeal did not improve after replacing the tube.

I would best describe the sound i am hearing, not as a feedback squeal, but more like a Theremin / Moog / sci-fi / whale song pitch. It starts abrubtly around '4' on the dial and then drops in pitch about a full step higher up the dial


update:

tried a different reverb tank, this one from a 70's electronic organ. Same result, identical noise. This rules out a problem with the tank , at least.


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Post subject: Microphonic Tube
Posted: Tue Jan 26, 2010 4:23 pm
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I agree with the previous reply that said you had a microphonic tube. The reverb driver is the 12AT7 that is the 3rd tube from the right looking at the back of your amp. Replace that one and give it a try. If that didn't do the trick, then try replacing reverb recovery tube, which is the 4th tube from the right and is going to be a 7025/12AX7A.


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Post subject:
Posted: Tue Jan 26, 2010 7:12 pm
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Hi Cool, If you're sure that V2, V3, and V4 are not causing the squeal, then I'd try putting the tube shields on and also check the reverb cables. Leaky filter caps can also cause low and high frequency oscillation. Unfortunately, if the amp was in storage for 6months or more, the electrolytics may not have had time to reform, unless the power was brought up very slowly. This includes all of the electrolytic capacitors. I would also check the 10pf and 3.3meg isolator circuit on V4b input grid. There is a possibility the lead dress may need some adjustment. If this doesn't help, I guess you'll have to go through the reverb circuit. Art

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Post subject: video of my amp
Posted: Wed Jan 27, 2010 12:46 pm
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this may help:

http://www.youtube.com/watch?v=YRvfqKcRoOQ

the sound is much louder and more vivid in person, but it's good enough representation of the theremin- like squeal


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Post subject:
Posted: Wed Jan 27, 2010 10:47 pm
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Yeah Cool, you've got a sweet oscillation going on there. Are you familiar with working on tube amps? Art

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Post subject:
Posted: Thu Jan 28, 2010 6:50 am
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nope. I'm learning as I go.

I can Desolder & solder and check for continuity, the bare basics. Any stunt wiring or stuff that requires intuition through experience, wont be done by me.

However, if we're just talking about replacing a handful of caps & diodes, I'm all in.


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Post subject:
Posted: Sat Jan 30, 2010 12:15 pm
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Hi Cool, just wondering if you've tried cleaning the jacks and new cables? I'd also check all the ground connections, including the output jack. It makes the squeal with or without the pedal, right? Art

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Post subject:
Posted: Sun Jan 31, 2010 10:57 am
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Check the coupling cap off of V4A. It's .003 I believe. Voltages are dangerously high in all tube amps, so don't get electrocuted. There is around 225 VDC in the area you will be measuring. Put your meter on DC voltage, black probe in the common jack, red probe in the V-Ohm jack. Measure with the black probe to chassis and red probe to one side of the reverb pot as you turn it up and then measure the other side. If you have a DC content in your measurement, you have a bad coupling cap.

Take it to a tech if unsure how to do it.


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Post subject: Amp Noob questions / picture of my amp
Posted: Mon Feb 01, 2010 9:38 am
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i"m excited. I think I can do this; or at least not screw it up so badly that a tech cant reverse what i have done:D

a few questions first,
-will i need anything hotter than a 40watt soldering iron? I have used 60/40 rosin core solder.
- Are melting points for solder different in Amps than in other electronics?
- how do I safely discharge the caps?
- Does anyone have a favorite supplier for the parts I'll need.

I took a look under the hood this weekend and took a photograph. I also made a composite from the photo and the schematic.
http://picasaweb.google.com/theorangeme ... andmaster#

Thanks


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Post subject:
Posted: Tue Feb 02, 2010 7:00 pm
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Hey Cool, what did you decide to try? Soldering iron and solder would be correct, melting point should be the same( I use a 700degreeF tip). Do you have a decent multi-meter? I've used Antique Electronic Supply, WWW.tubesandmore.com for over 20yrs, for most things. I'll give you a discription of how to discharge the filter caps, but you might want to Google the subject, so that you're perfectly clear on different methods. You'll need to make a jumper, using a (2' x 16awg) with insulated alligator clips. Cut,strip, and install a 470-1000ohm 2watt metal oxide resistor in line and heat shrink the entire installation. To discharge the caps: 1) connect one clip to a good ground point on the chassis. 2) connect other end clip to the plate pin of V1( pin 1) for a couple minutes. 3) remove from pre amp tube and connect to the plate pin of a power tube(pin 3) for a minute. Check these pins and any other filter cap terminal with a meter,to verify the discharge. This amp has bleeder resistors on the caps that are designed to do this task over sometime, so you don't really need to leave the jumper on while you work. Hope this helps. Art

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Post subject: thanks.
Posted: Wed Feb 03, 2010 2:02 pm
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thanks, All

The General consensus is that one or more of my caps have seen better days.

I think I will just replace all of them. I don't want to diagnose & repair one problem then have another one pop up a month or two later.

I'll gather the parts I need and keep the thread updated.

any faves for suppliers?


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Post subject:
Posted: Wed Feb 03, 2010 3:01 pm
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Don't know about price, but Tube Depot has the best selection. If I can't find them local, I rely on TD.


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Post subject: Re: Squealing reverb
Posted: Sun Mar 16, 2014 1:35 pm
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Well today or the edification of all - i will show the innards of a reverb tank from;

a newer vibroking - no squeal :mrgreen:
a 6g15 (RI - no squeal (but a naughty 60hz annoyance i shall tack down and eradicate) :?
deluxe reverb ri new one - squeals :roll:
supersonic 22 squeals :roll:

mesa boogie .50 cal no squeal :mrgreen:

All of my squealers have had both sides of the tanks V's changed NOS 12at7's, Tung sol RI, Mullard RI 6201 RI and CV 4 something's - squeals....12au7 squeals :twisted: :twisted: :twisted:

knowing the inheRENT ISSUE WITH THE CURRENT STUFF I DECIDED LAST NIGHT AT 11PM WHEN I HAD FINALLY fixed THE EASIEST GUITAR IN THE WORLD TO REPAIR MY TELE)...LONG STORY i decided to test it out :D

damn caps lock..i thought i'd pick a grin a bit...squeal :o

so i ripped it (ss22) apart -and i will document to show what i found, missing parts (brand spanking new ss22)...so now i dissect the others then i will post... :wink:

the problem (when not tube oriented 99% is the damn vibration isolation and i was missing two dampners, and then it lays on a HARD corrugated cardboard base)

that is getting dealt with post haste...stay tuned

Rollo has a plan and is doesn't involve hammers, incantations, pee extract from an extinct dinosaur...but common freakin sense.. :shock:

i shall hire maidens to hold my tanks gently using longer cables...and if one of them drops one :oops:

i shall immediately play Pipeline 8)

stay tuned..there are materials out there that will solve our problems (with squeals not bat crap crazy EEs) :mrgreen:

Ah, wa state the haven of geekdom and blackberry cush!!! :wink:

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Post subject: Re: Squealing reverb
Posted: Sun Mar 16, 2014 3:07 pm
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" a 6g15 (RI - no squeal (but a naughty 60hz annoyance i shall tack down and eradicate) "

Prolly 120Hz hum. It's important to be able to distinguish between 60 and 120Hz hum noise. The only sources of 60Hz hum (which is lower than an open low-E string, standard tuning) is the 1.) power transformer (primary and the secondary, right before the rectifier). 2.) heater lines.

120Hz hum (just above an open A-string) source is after the rectifier. Usually hum leaked from the wiring or poor filtering in the PSU. This 120HZ hum more common, as a poor grounding scheme in the amp may lead to 120Hz being injected into the signal. Because the PSU voltage taps are used throughout the amp.

So you can narrow down sources of hum by being able to tell 60Hz from 120Hz hum.


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