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Constant noise from Silverotne Organ Amp


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6/20/2004 11:11 AM
Bill Gunter
Constant noise from Silverotne Organ Amp
NOTE: Posted originally in "Noise" forum; moved here at Wild Bill's suggestion.  
 
Hi,  
 
I am new to the tube amp world and need some help. I bought an old Silvertone chord organ that has a tube amp that I want to use for guitar. The amp is Model No. 4710 and has the following tubes: 12DW7, 12AX7, 6L6GT, and 5Y3GT. Total Power Consumption: 80 Watts. It has a pedal, which I assume controlled vibrato speed, and another pot in the back that adjusted with a screwdriver (controlling what?). I have no schematic and donít know what other Silvertone guitar amps this is similar to that I could compare.  
 
The amp itself works but I get a constant loud noise (motorboating maybe) whenever itís powered up. I removed the tubes one at a time and discovered that the sound goes away whenever I remove one of the power tubes. Any suggestions? Remember I am totally new to the tube amp world and need to be spoon-fed information.  
 
Also, before I read much about tube amps, I powered it up without a speaker load attached (less than a minute), which I now know is a no-no. Could that be the source of my problems and lead to a solution?  
 
Any help would be great!  
 
Thanks, Bill
 
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6/20/2004 10:12 PM
Enzo

I doubt you hurt it by running without a speaker for a little while.  
 
Pulling power tubes will make any amp go silent. If you pull the 5Y3, there will be no high voltage, so nothing comes out. If you pull the 6L6, there is nothing to drive the speaker so no sound.  
 
The pedal is almost certainly for volume. On an organ they call it an "expression" pedal. Ther may be a volume control on the panel somewhere, but there would be a pedal to allow expression via volume, just liek playing a piano louder and softer by how hard you strike the keys.  
 
The screwdriver control could be a number of things, but it may likely be a hum balance control.  
 
Motorboating is a low frequency oscillation, hum is the noise you get when you pull out a guitar cord and touch the end of it. Hum may be due to bad filter caps, or lack of grounding in the circuit. Motorboating can also be due to bad filters or from the output getting back into the input, or ungrounded circuit elements.  
 
This organ amp is not going to be similar to guitar amps. For a couple reasons. It is not a guitar amp, and serves a different purpose - it is designed different. Also, the SIlvertone name is SEars' marketing name. Sears does not make the products. Danelectro made many of their guitar amps for example. The chord organ was probably made by one of the main organ makers like Thomas or Lowrey or even Estey. SO the fact that the organ and the guitar amps are both called Silvertone, does not mean they have any relationship under the skin other than Sears sold them.  
 
ANy tube amp chassis can be converted to a guitar amp circuit by dint of the fact that you need a metal box with tube sockets and transformers for a guitar amp, and the other forms of amp include those things.  
 
Try Googling the model number of the organ.
 
6/21/2004 7:10 AM
Bill Gunter

Hi Enzo,  
 
Thanks for the information. I would say that my noise is motorboating. Information I have found on the web suggests my organ was made by Thomas.  
 
I also assume that the pedal is for volume. I forgot to mention that the power switch/pot also says "Vibrato" on the face of the organ.  
 
I have read a lot about the filter caps possibly causing noise problems. Where and what value are the filter caps?  
 
Thanks again, Bill
 
6/21/2004 11:38 AM
Wild Bill

Bill, the filter caps most often come in one of two forms - a large can sticking up from the chassis or a tubular "roll" inside the unit.  
 
Cans are usually multi-section, i.e. several caps inside one can with a common ground (-) terminal. Sometimes we see multi-section tubular style but not nearly as often in amplifiers.  
 
In either case, there will be a wire connection from the cathode of the 5Y3 rectifier directly to the first filter cap. This will also usually be the point that feeds the centretap of the OT and through that to the plates of the output tubes. From that initial point you will see dropping resistors (sometimes a small iron choke for the first drop) with another filter cap bypass in a chain, each R/C combo establishing a lower B+ tap for stages previous to the output until you reach the 1st preamp tube plate supply. The current drawn from each supply adds up through the stages to the OT tap and you get cascading voltage drops to establish the right voltage for the plates of each stage.  
 
The cap values are normally printed on the side of the can or the tube, along with the dc working voltage. It is assumed that the cap will handle a higher peak voltage, within reason.  
 
With cans you may see various symbols stamped or marked by the positive terminals of each section. This relates to a legend on the side of the can. So you may see that a square refers to a 40 mfd 450 volt section and a triangle refers to a 20 mfd section.  
 
Voltages only make sense with tubes in the amp lit up and drawing current - with the tubes pulled for testing you get no current draws and of course no voltage drops along the filter string.  
 
As I mentioned in the original forum each filter section does 3 things. It establishs a lower B+ tap for an earlier tube stage, it smooths out the rectified DC ripple and it bypasses the B+ point of each stage to ground. This is often forgotten but is very important. With no bypassing the output current swing can "leak" from one stage to another causing all sorts of problems, like motorboating! :(  
 
Hope this helps...  
 
---Wild Bill
 
6/21/2004 12:02 PM
Bill Gunter

OK, my amp has the the large can sticking up. What should I be looking for? Low voltage readings. No voltage readings. Can you replace the entire can? If so, any recommended places to get one?  
 
I will go home tonight and try to locate all the items you have been talking about from the rectifier cathode and so on.  
 
Thanks again, Bill
 
6/21/2004 6:15 PM
Wild Bill

Cans are not popularly made anymore, Bill. In the golden days of tubes they were a standard part and came in a multitude of flavours. Nowadays there are a few sources of equally few combinations. You can try http:///www.thetubestore.com or a bunch of other online sources. Antique Electronics out of Phoenix Arizona is another. There's quite a few with good graphics to show you what is available. (I should shamelessly plug thetubestore.com for running some great summer sales on tubes! At today's prices we need every break we can get...)  
 
Since today they are a low volume specialty product can caps are kinda pricey...  
 
You may not find the exact combination of your amp's can. Tell us what you've got! The good thing is that you don't have to be exact. As long as the working voltage is as high or higher and the mfd's are no lower and not too much higher you're ok. What do I mean by not too much higher in mfds? Going from 20 to 40 is not a big deal. The first cap should not be larger than 80 mfd or so 'cuz too big a filter is a strain on some tube rectifiers.  
 
When replacing in an existing guitar amp too big a jump will start to change the tone of the amp, by reducing the "sag" in B+ under high current peaks. The sound becomes "tighter" or "snappier". With your amp - who cares? It never was a guitar amp in the first place! It's your project to modify it to give a great sound - you aren't worried about your existing design.  
 
A cheaper alternative is to use individual tubular capacitors. The same sources as for cans will have offerings. It means a bit of bother to shoehorn them inside the amp but usually there's room.  
 
Unless you are incredibly lucky and live in a town where such caps are made I wouldn't even bother trying to find them at any modern electronic parts distributor. They might get two requests a year for $10 worth from hobbyists like us. In the industrial market such volume in sales is too trivial to support. I speak with some experience - I worked for such sources for 25 years. It's like looking for leather tack for a horse - there's still specialty sources around but in great-grandpa's day you never had to walk more than a block or two!  
 
As for testing - rarely does a cap just short and make the problem obvious. They just slowly dry up inside over the years and the effective capacitance fades away. When they're really gone you should hear some loud hum when the amp is turned on. The caps can't smooth out the rectifier pulses into reasonable DC.  
 
Get something rated at 20 or more mfd at 450 wvdc or higher and gator or tack solder it across the suspected filter, taking care not to have the polarity reversed. If the cap is bad you'll immediately hear an improvement, even if you've jumpered only one cap in the whole filter string. Replace 'em all - if you had 4 tires on your car that were 50 years old would you replace only one if it went flat? Don't give Murphy's Law any advantage!  
 
REMEMBER what I told you about the cap holding a charge! Discharge it or it WILL bite you!  
 
Are we having fun yet? :)  
 
---Wild Bill
 
6/23/2004 7:00 AM
Bill Gunter

OK, I checked the cap can and got these values:  
 
-20 mfd 450 VDC and a half circle  
-10 mfd 450 VDC and a square  
-10 mfd 450 VDC and a triangle  
-10 mfd 450 VDC  
 
The can had these numbers as well:  
DO120  
UPT21145  
 
I can probably come up with some caps to gator across and test soon. I'll let you know the results.  
 
Bill
 

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