low volume/headroom in 1484 twin 12

[john_t]

I recently acquired a sears silvertone 1484 twin twelve amp. While it works, there is very little volume or headroom. I swapped out the Chinese 6V6 power tubes with new JJ tubes  and tried new 12ax7s. No change.  The op transformer is not original. Appears to be a 4 8 tap replacement. (22848 ULT) . Looking for a little direction as to what to close in on.

Thanks John T.

[john_t]

Correction The power Tubes are 6L6. Also all new caps. Very neat work inside the amp.

[Willabe]

Hi John T,

I'd check the voltages in the B+ PSU against the schemo to see if there correct.

Your also gonna want 1 of these if your working on the B+ PSU on an old amp or a new build.

It's a light bulb current limiter, is easy to build and won't cost much at all. It realy is a must have, must use tool to check for shorts. It can save your PT and more.

Here's the link from Sluckeys site with the build/use info;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


                    Brad      

[john_t]

Willabe, I have a variable auto transformer. If i cut back the voltage wont that throw off my B+ and PS readings

[Willabe]

Willabe, I have a variable auto transformer. If i cut back the voltage wont that throw off my B+ and PS readings?

Yes it will and so will the current limiter.

You might have a short in the B+ PSU. I'm NOT saying to do this yet, but have you fired up the amp at full voltage without the variac? If you have and nothing smoked you can check the B+ dcv nodes. If you've been trying to play through the amp while using the variac with the voltage turned down it wont play/sound right.

I have a variac too but I don't use it to check for shorts in the B+ PSU.

Have you looked at the current limiter yet?

If you use it, it will show you if you have a short in the B+ PSU without killing the PT. If it shows that you don't have a short then you can fire up the amp at full line voltage and test the B+ dcv nodes.


                   Brad      

[jjasilli]

I use flow charts & checklists for troubleshooting, like geofex

http://geofex.com/ > -Tube Amp Debug Page > Low power or volume, or volume drops off

[tubenit]

Have the original electrolytic filter caps been replaced yet?

[john_t]

OK, bad news to report. First all the caps have been replaced. I put together a current limiter. The bulb 60 watter comes to a good glow as the amp was fired up.  Maybe 50 % then started to decrease in light. I was thinking great its good. As the amp warmed up the light continued to increase. Back up to about 60-70% brightness. So I shut it down. I tried the limiter on two different good amps. The light only came on for perhaps a second total and then went right out altogether.So Does this indicate problem with the Power Tranny as opposed to the output T.

[Willabe]

I would say there's a short somewhere as indicated by the limiter. You saw how it acted on 2 other good amps. That was good that you did that to get a feel for how it's supposed to work when the amp is fine.

I think I'd pull all the tubes and try the limiter again. That should show if it's the caps,rectifier diodes and/or PT. Might not be the PT.

I think the B+ PSU is 2 half wave voltage doublers stacked 1 on top of the other?

A little more to go through to check for a problem, but not too bad. 


Here's the link to the schemo;

http://www.el34world.com/charts/Schematics/files/silvertone/silvertone1484.pdf


              Brad      

[john_t]

OK Willabe, I feel like I might be back in the game. Pulled the tubes, Amp on no light at all on the limiter. The only tubes I did not swap were v3 v6 v5. The 2 6gc7 tubes and V3 the 12ax7. I thought this was part of the vibrato unit.

[john_t]

correction v6 is the 12ax7. did not swap out

[jjasilli]

OK, bad news to report. First all the caps have been replaced.

It's not safe to assume that the new caps are good, or that they were replaced correctly. 

With no tubes in the amp, the light bulb limiter should not glow, or glow only very dimly.  Keeping an eye on the bulb:  If there is a tube rectifier install that first.  Then replace each of the other tubes one at a time.  If at any point in this procedure the light bulb goes form dim to bright, you have found a short.

[john_t]

JJasilli, the 1484 has no recto tube. (diodes) When replacing the tubes. Start with the preamp tubes ?

[Willabe]

OK, bad news to report. First all the caps have been replaced.

It's not safe to assume that the new caps are good, or that they were replaced correctly.  

With no tubes in the amp, the light bulb limiter should not glow, or glow only very dimly.

Hi jjasilli,

The amp has an SS stacked B+ PSU. If the guy before him wired it up wrong or it has a shorted filter cap/diode won't that draw current (to ground/short) to light up the limiters bulb even with no tubes in it?

I think it's 2 half wave doublers that are stacked 1 on top of the other. IIRC, PRR once said that the amp company probable got a deal on the PT's and went with them for cost reasons.

Have a look at the schemo;

http://www.el34world.com/charts/Schematics/files/silvertone/silvertone1484.pdf


            Brad      

[Willabe]

I'd start with the 6L6's (pair) first and go backwards to the input.

Did this amp ever work since you bought it?


            Brad     

[john_t]

Willabe the amp came as is. In fact This is a curb find with the cabinet intact and all the tubes. Missing the footswitch. The speakers are not stock. I believe they are eminence based on the codes #. The more I read about the amp the more I dont mind sinking a little time and $ into it.

Second Curb find. The first a Ampeg G-15. Really nice sound. smooth reverb.

[Willabe]

the amp came as is. In fact This is a curb find with the cabinet intact and all the tubes.

So... it never worked? From the start it had low volume output? We need more info to try and help you track down what's wrong.

Do not turn on that amp without the limiter until it's light dims back down and stays dimmed down.

Your PT might not be bad. So you want to be very careful not to kill it before you can find/fix what's wrong.

Did you change the B+ filter caps? If you did you might have made a mistake in the wiring. That's OK, we all make mistakes in wiring. We don't know how much experience you have working on/repairing amps. That PSU is a little more complicated than normal.

Could you take some good pics of the amps insides, mainly the B+ diodes/filter caps area and post them?


              Brad      

[john_t]

If I can figure out how to post some pics I am sure that would be more helpful. All of the caps seem to be pretty new. At least not original.  The cap can While in place has been replaced with caps in the chassis. Every cap seems to have been replaced. The workmanship is very neat and clean. all the solder joints look really good. It would appear to have been professionally done to me. The ot has also been replaced at some point. I have successfully built several amps, but that is not the same as diagnosing a problem. I will start replacing the tubes one at a time tomorrow with limiter in place and post again.

Thanks all

[jjasilli]

The amp has an SS stacked B+ PSU. Good grief!  It sure looks easy to rewire that incorrectly! 

If the guy before him wired it up wrong or it has a shorted filter cap/diode won't that draw current (to ground/short) to light up the limiters bulb even with no tubes in it? With SS diodes in place, the PS should draw some current with no tubes in, causing the limiter bulb to glow very faintly.  If it glows bright, there's an issue.  Next. . .

I'd start with the 6L6's (pair) first and go backwards to the input. I don't think it matters which end you start with; but replace one tube at a time. 

[Willabe]

Good grief!  It sure looks easy to rewire that incorrectly!  

Oh yeah, lots of fun.    

I don't think it matters which end you start with; but replace one tube at a time.

I only said to start with the power tubes end because the PSU is shortest in run starting there.  

Now that I think about it I think your right about replacing 1 tube at a time. Sorry John.
        
                
                   Brad      

[john_t]

I believe were closing in on the problem section. Here are My test results.

No tubes Limiter-No light

V-7 or V-8 OK

v-7 + V-8 Light in limiter

No power tubes v-6 v-3 v-2 v-1 v-4 v-5 (one tube at a time) ok

V-6 + V-3=Light in limiter

So problem valves V-8 V-3

Is v-3 the phase inverter.

thanks john

[jjasilli]

Light in Limiter: how much light?!?   

V3 is the phase inverter

[john_t]

I dont know how clear my post was to understand but.

I could insert a power tube in v7 or v8 =ok

 with 6l6 tube in v7 when i inserted a tube in v8 the limiter light came on.

I could install any tube in v1 through v6. one tube at a time. =ok

with the power tubes out I started with the 12ax7 in v-6 =ok when I inserted the 6cg7 into v-3 the limiter light came.

so I can not install both power tubes or v6 and v3 at same time.

I did try different tubes in those test to rule out a bad tube.

[jjasilli]

It is not OK to just say that the limiter bulb comes ON.  What matters is how MUCH it comes on.  Glowing dimly might be ok.  Full bright is never ok.  So, for each situation, is it full bright or just dim?

Sometimes the limiter bulb is so dim that it is hard to tell that there is any glow.  A darkened room is helpful.

[john_t]

I used a 60 watt bulb for the limiter. 1484 test. The bulb (when lit) comes on as a warm glow maybe 20% light then continues to increase in intensity. Not full on though. I also used the limiter on two separate working amplifiers. The light came on for a brief second and went out. So in large part I am comparing those results to the 1484.

[jjasilli]

If the limiter bulb goes to full intensity and stays there, that indicates a short in the amp.   

Often, at turn-on, the bulb will flash to full intensity for a moment, then go dim.  It will not go completely out, but may be so dim that it is hard to see unless the room is darkened. 

If the limiter bulb is starting dim, but then increasing in intensity, maybe there is a partial short in the amp.  Usually tubes should be suspect.  Maybe try different tubes if you have them available. 

Also, there could very well be a wiring error in the re-built High Voltage power supply.  A good way to verifyy the wiring is to print-out the schematic and put it near the amp or on the wall overhead.  Then, starting from the PT secondaries, use a chopstick to physically trace each leg of the PS circuit, one section at a time.  Use a yellow highlight marker on the schematic to highlight each leg of the circuit on the schematic as you check it it in the amp.   

[sluckey]

I recently acquired a sears silvertone 1484 twin twelve amp. While it works, there is very little volume or headroom.

A lightbulb limiter has no value troubleshooting these symptoms. Put it under the table. Swap tubes. Measure voltages. Use jjasilli's flow charts.

[john_t]

Wanted to thank all for the Help.  Bad output transformer.

Thanks john T