-
I replaced a reverb tank in my Mini Brute IV and had annoying hum and buzz after that. Then I read somewhere – here in fact! – that hum and buzz can sometimes be eliminated by turning the tank so that the input jack is not near the power amp's transformers.
So I turned the tank 180 degrees and voila!, now my reverb is silent! ... I mean not totally silent, it makes the reverberation, but no hum and buzz.
If You find the reverb tank You can try this. Remember to mark which cord (in and out, only two it is) goes where.
Most difficult part in my MB was not electrical but mechanical: the cabin is not very big and to get the screws unscrewed was a bit struggle. But I took it as a excercise in patience and spiritual growth!
-
04-28-2016 08:11 AM
-
Lawson,
Never owned one of the open backs...but aim to. On all of the other models the reverb tank is on the side, mounted vertically, because the preamp is up top and the power amp is on the bottom of the cabinet. keeping the reverb tank away from the power supply down below is important for a high signal-to-noise ratio.
Find the reverb tank and make sure that the wires are plugged in firmly, both at the tank and at the other end.
A reverb tank, by the way, is basically a speaker magnet/coil at one end that is attached to a spring instead of a paper cone. It excites the spring. At the other end of the spring is a microphone coil/magnet that is attached to the spring instead of a mic diaphragm. The "sound" runs down the spring, is picked up, runs back up the spring and back, is picked up again...and so on. Hammond invented this technology for their organs back in the 40s.
The two transducers are hooked up to the spring(s) with whisker-thin wires. Both the transducers and wires are fairly easy to mechanically damage by dropping an amplifier, alas. If one of the transducers is degraded (usually the receiving unit, IIRC) then they get noisier or even quit working. Tanks _can_ be repaired, but they are much more easily replaced.
The deal is, you need to use the same type of tank. Here is a URL that explains the "code." IIRC, the code on the Baby Brute reverb tank is 1db2c1c. That's a short tank, 2 springs, medium delay, 250-ohm input impedance, 2250-ohm output impedance, vertical mount, inputs out, IIRC. These can be purchased, but--of course--they are not as stocked as the standard "type 4" Fender tank. You can practically get those at the local convenience store.
-
I'm happy with a reverb pedal.
-
So all you amplifier tech fans, I just pulled the reverb tank on my Polytone Baby Brute with the open back and crooked control panel. For reference, the amp works nicely, but when i turn up the reverb I get a nasty hum. The control panel also sits in the frame crooked, which bugs me to no end.
So privately, but also here, I'm asking a question. The tank comes off cleanly, connecting to the unit merely by input/output RCA plugs.
My question is whether I can just connect the two RCA plugs (via an adaptor) and run the amp normally without the reverb? I ask because if that works and there is no hum, then I know my hum is a problem with the reverb tank.
Here's the pics for fun and any observations or advice you folks might have to offer:
Last edited by lawson-stone; 04-28-2016 at 04:59 PM.
-
K. Yep. The tank is the correct, 1db2c1c tank. It is probably in process of garfing up, however. With a Fender amp you could just disconnect the tank by pulling the two cables from the amp chassis. With the Polytone, however, the chassis end of the reverb cables are soldered to a Molex plug. So, if all you want to do is d/c the reverb, disconnect the cables, as you have done, at the tank end and tape the cable ends off with insulated tape--to ensure that they do not ground out against the tank or amp chassis.
-
So now with the tank out and the leads taped off, the amp works great but when I turn up the reverb pot the hum comes in, increasing as I turn up the pot.
So am I right in concluding the pot has a problem? Interestingly, that's the end of the control panel that is misaligned, as if something happened to it. Maybe the tank's fine but I need to replace that pot?
-
Could be mechanical damage from the injury to the amp that knocked the preamp cockeyed. The hum sounds like a ground issue...even a damaged neg lead on a capacitor near the reverb pot, or a broken solder lead on the neg side of the pot. Got a picture of the guts on that side?
-
I haven't tried to pull the pre-amp out. I worry that some parts might retain a charge and treat me to a nasty shock or cause damage. I could try tomorrow to see what I can see in there.
Pretty nice, though, I bought this totally "for parts" as it were, the guy telling me the reverb definitively did not work. But in fact, the whole amp works except for the hum in the reverb pot.
So even if I just make it reverb-less, still a nice deal!
-
So somehow I started unscrewing stuff and now I have the cover off the pre-amp... here's the reverb pot.
-
I've seen quite a few polytones with that problem - according to my tech the reverb tank is too close to the power transformer. He solved it with shielding.
-
It can't be the reverb tank. I get this hum when I turn the pot even with the reverb tank removed and the leads taped off so as not to short.
That leads me to the somewhat inexperienced but seemingly logical conclusion that the hum is related to the pot, not the tank.
-
Lawson - I don't know a thing about electronics and the guts of amps, but I'm glad you are cautious about capacitors or things that could deliver a nasty shock. Wonder if we have any amp guys here who can tell us if it is safe to poke around. I can tell you that my old Ampeg blew a tube once that could knock an eye out if you were in the wrong spot at the wrong time. Be careful, man.
-
Actually, with the cables still connected at the Molex plugs, when you turn up the reverb knob it _would_ start to hum because it is trying to load the tank but is seeing a cable that is just acting as an antenna. With the cables not present you wouldn't have this problem, IMO. Still, in the photo you provided the solder lug to the farthest right looks suspiciously like it might be mechanically loose, no? try wiggling it by flexing the green PC board in that location with the pot still connected to the front panel. Do you see any movement of the solder connection? If so, resoldering there would be in order. The knock that the board received when the preamp got pushed back could have fractured the joint there.
FWIW, the gut shots of the reverb tank look like the tank is okay.
A second way to check the lugs on the pot, without disassembling things, is to take needle nose pliers and grip the lugs, one at a time, and wiggle them. If you see movement at the solder joint there is a fracture. I had a Peavey amp where this occurred after a drop (oops). A quick re-solder and everything was good as new.
-
I have a general idea of what holds current. Mainly I've been simply unscrewing obvious mechanical connections and looking at everything. Honestly everything looks in order here, so I am wondering if something inside the pot itself is amiss. Then again, I can live without reverb on this little amp.
Originally Posted by targuit
-
Herbie,
I have been able to replace the 15" speakers with the originals. I have some on hand. I did use a 4-ohm JBL PA speaker in a smaller mini brute. It sounded very good. I think those CTS and Eminence 4-ohm OEM speakers are important to the sound of the Polytone.
-
What's the impact of the 4 ohm speaker as opposed to 8 ohms? Originally Posted by Greentone
-
@4 ohms the amp is working harder, dissipating more power...twice, in fact. My mentor, who owned a store, was a Polytone dealer, and did all the service work, said the 101, 101, 103, 104 models ran at two ohms...really hot. He claimed that the Polytone warmth was largely due to low impedance CTS and Eminence speakers.
That said, I put a 8-ohm JBL into a MiniBrute once. It sounded beautiful.
-
BTW folks, after checking over everything, I re-installed the reverb tank and the main thing is I have not destroyed the amplifier by my fumbling efforts!
Reverb pot at zero it still sounds great. I might be wrong, but the hum when I turn up the pot seems a bit less now. I have no idea why that would be unless the shot of contact cleaner I squirted into the pot made a difference?
But for my maiden voyage deep into the interior of a Polytone, I think i've done alright!
Thanks for the guidance and encouragement!
And Greenstone, I have totally hijacked your thread... sorry about that!
-
Lawson,
It is your thread now. :-)
Yep. Reverbs are inherently noisy affairs. They are so delicate that the tanks are basically in process of degrading as the amp is used, er, abused. Each time the amp is pushed on rollers over a sidewalk--bump, bump, bump--the springs are doing the mambo against their delicate connection points with the transducers. The contacts will get weaker and things will get noisier.
The great surf guitarist, Dick Dale, takes the high road. He suspends his stand alone Fender Reverb unit from a mic boom stand from a piece of rope...complete isolation.
That said, I suspect that the vertical mount on the side of the cabinet that Polytone used is rather more susceptible to damage, judging from all of the reverb complaints that Polytone folks have lodged through the years.
Me? I have never had one fail...but I take very, very good care of all of my equipment. Short of a vehicle accident, my guitars and amps aren't going to get banged up. (famous last words, eh?)
Given that reverb tanks cost about as much as a vacuum tube and are just plug in, why don't people keep them on hand like tubes? (I do, by the way.)
-
Part of the Accutronics code tells if the reverb is designed for vertical or horizontal right side up or upside down mounting. They compensate for spring sag and pull on the transformers which are set up to maintain a certain air gap. Vertical mounting is the most durable because the springs pull in a straight line.
-
Cavalier,
That is, of course, true--the way the reverb tank is designed. However, consider the way the tank is used in practice. On virtually all of the Polytones sold after the 1980s the amp handle isn't on the top--it's on the side. The vertical-coded reverb tank is on the side, too. The amp is going to be carried, and transported/stored on its side. (The amp feet are on the side opposite the handle, as well as on the bottom.) Thus, when the amp is being jostled the reverb tank, which is designed to be _used_ in a vertical position, is being banged about horizontally. Lots of the Mini Brutes I have seen that folks have complained about with busted reverbs have been the metal grill, side handle series. Not all, but many.
FWIW, I have always thought that Polytone amps are well made amps. Maybe there are some heat issues in the smallest ones (BabyBrute)...a one cubic foot cabinet with a 75-watt amp inside gets hot in there. Otherwise, these amps were made to just keep going and going.
Here's a picture of a side-handled, side-reverbed MiniBrute:
-
you probably already know about this, but you can get Polytone grill cloth, that diamond stuff that was on the old brutes.
my grill cloth fell off and ended up somewhere stupid many moons ago and I found people selling the original stuff on EBay. I don't know if that link is to original stuff or not, its just to show that there is replacements for the old diamond grill cloth.
I got lucky and found somebody with a consignment of warehouse backstock of the original stuff they were unloading on EBay
but the sound of a brute with the diamond speaker cloth is so much better than without and its not very expensive if you do manage to find it
-
So I noticed something unusual about this reverb hum on my amp. First, the hum does not change if I alter the volume setting on the amp. It's directly tied to the reverb pot. Second, it is actually TWO sounds. There is a background hum, and then there is a kind of pulse!
Check this out:
-
I used to take those cabinets apart and nab the speakers. By the time I realized that I _like_ the sound of a B3 with a Hammond tone cab and a 122 Leslie, there were hardly any tone cabinets around anymore. They used to be ready sources of 10", 12" and 15" Jensen alnico magnet speakers.
Pretty fancy circuits. LOTS of tubes...not that many that go into guitar amps, though.
-
Make sure your cell phone and computer are away from the amp and try again.
Reply With Quote
Playing live and getting the best sound from the...
Today, 02:08 PM in Guitar, Amps & Gizmos