Silvertone 1484 - low output

[scstill]

I have a few questions on my recently acquired Silvertone 1484 (schematic below).
It sounds pretty good although it does not get very loud, thinking its because the low voltage on V3 (#1 below)
Hopefully you can offer some advice this and a few other items:

1) V3 plate 6 measures 16v (should be 100v); I cannot figure why; meas 315v on the other side of R34 which meas 94kohm; I also measure 55v at V3 pin2 (is that right). Any thoughts?

2) V6 trem has fluctuating voltage on plate 6, is that expected? I have no tremelo experience, no footswitch either

3) could I jumper the trem jack chassis to simulate the switch?

4) How does the PS create DC at the front end of the choke 1? Seems that it just comes directly from secondary. I measure 325vdc at the output side of the choke.

5) Have not done much troubleshooting, but there is a hum when I plug guitar into Channel 2, not present in Channel 1. Any thougths?

6) Finally, before I plugged into 117vac wall, I used Variac at 20v and 50v. I measured the voltage at the convenience plug which is across the line and got essentially 0vac. Why would that be? Variac was outputting properly and the primary was continuity checked. And of course it works great at full 117.

[shooter]

V3 plate 6 measures 16v

what's pin 8 vdc measure?
If trem is "on" the plate will bounce


dinner calls 

[scstill]

I'm thinking because of the very low plate voltage on V3 pin 6 I'm not getting the gain it needs. It doesn't get very loud when cranked. Just can't figure why the low voltage.

BTW - reverb and tremelo seem to be working ok

[Latole]

".  1) V3 plate 6 measures 16v (should be 100v); I cannot figure why; meas 315v on the other side of R34 which meas 94kohm; I also measure 55v at V3 pin2 (is that right). Any thoughts "

-cstill

Look there is some short between pin 8 and 3.

Put know good tube in V3
Check C13
Or any mod on amp ? short circuit by bad solder around V3 socket

Did you know we like photo ?

[shooter]

what's pin 8 vdc measure?

[sluckey]

what's pin 8 vdc measure?

And pin 7 too.

[terminalgs]

1) V3 plate 6 measures 16v (should be 100v); I cannot figure why; meas 315v on the other side of R34 which meas 94kohm; I also measure 55v at V3 pin2 (is that right). Any thoughts?

the bottom half of that 6CG7 (1,2,3) is a cathodyne phase spliter.  The top  half (6,7,8) is a gain stage. voltages say something is seriously wrong there.  measure the resistors surrounding that triode  and -even easier- swap the 6CG7 from the reverb driver.  in the end, that triode should have 0VDC on grid, 100VDC on plate and (schematic says) 4VDC on cathode.

2) V6 trem has fluctuating voltage on plate 6, is that expected? I have no tremelo experience, no footswitch either

yes, thats normal. ground the trem jack to get steady VDC readings.

4) How does the PS create DC at the front end of the choke 1? Seems that it just comes directly from secondary. I measure 325vdc at the output side of the choke.

In that arrangement of caps C26-C29 and diodes you have two voltage doubler circuits, one stacked on top of the other.
There is a junction identical to the C26/C27 below it at C28/C29, but they didn't try to "use" that junction to supply voltage to the circuit like they do at C26/C27.    The choke passes VDC and blocks VAC,  so any VAC at C26/C27 doesn't make to the screen supply & B+ for triodes.



I think there were other threads in this forum in the past with much better explanations by gurus.  So I'd search for 1484 up above.


It's amazing that Danolectro found that all those components and the level of effort to assembly them cost less than a more traditional arrangement (that they used in smaller amps).


Also, I always mention this in 1484 threads:  the reverb driver 6CG7's two plate resistors 27K are under spec'ed at 1/4W.  the voltages on the schematic tell you that.  They need to be 1W.  On the one I had these two resistors failed open. 

[sluckey]

In that arrangement of caps C26-C29 and diodes you have two voltage doubler circuits, one stacked on top of the other.
There is a junction identical to the C26/C27 below it at C28/C29, but they didn't try to "use" that junction to supply voltage to the circuit like they do at C26/C27.

I like to think of this stacked circuit as four batteries connected in series. Label the batteries 1 through 4 with 1 being at the bottom of the stack. The choke is simply connected to the junction of batteries 3 and 4. This provides a lower B+ than the voltage present at the top of the stack.   

The choke passes VDC and blocks VAC,  so any VAC at C26/C27 doesn't make to the screen supply & B+ for triodes.

That's not quite right. A choke does not "block" AC. It does offer resistance (impedance) based on the frequency of the AC voltage. For example a 4H 100Ω DCR choke only has 100Ω resistance to DC, but has 3KΩ resistance to 120Hz ripple.

     X_L = 2*pi*F*L

[scstill]

Thanks guys, I added some attachments with the details, hopefully will help.
The V3 voltages are crazy, something is wrong somewhere.
Everything looks factory so I doubt its been modded.

I tried the V5 reverb tube in V3 and got same measurements (side by side in attachment); then returned to V5 and re-measured reverb voltages are fine

I noticed that the schematic is different than as built;
R30, R31, R32 are all connected in layout (see pic) but are not connected in schematic (see attachment)

[scstill]

here are a few full layout pics

[sluckey]

V3 pins 1,2,3 is an AC coupled cathodyne PI with bootstrapped bias. Voltages are all good. It's perfectly OK to measure 55V on the grid. Pin 2 is actually only a couple volts less than the voltage on pin 3, but your meter cannot measure voltage accurately on that grid because of the high impedance of the bootstrap bias.

V3 pin 6 is a problem. Since the resistor measures OK, I suspect a bad solder joint at the socket. Reflow the solder and add a little fresh solder to pin 6, or just replace R34.

The schematic has an error. Those 3 resistors and C2 must be connected together for the amp to work. Not the first error I've seen on that schematic.

[shooter]

+1

Since the resistor measures OK,

if you're bored, pull V3 and measure the vdc at the tube socket plate pin and see if you get the same volts as the PS side of the resistor

[scstill]

You guys rock.  Did all the suggested steps.
It was R34....   Roars like a monster now.  Appreciate the advice.

New voltages pin6 99v, pin8 3.6v, pin7 fluctuating 1mv (shouldn't it be negative to chassis??)
Original R34 measures at 995k - I read the meter wrong :-(

[sluckey]

New voltages pin6 99v, pin8 3.6v, pin7 fluctuating 1mv (shouldn't it be negative to chassis??)

Pin 7 should be zero. 1mV is close enough.

[shooter]

shouldn't it be negative to chassis?

negative in relation to the cathode

[Latole]

You guys rock.  Did all the suggested steps.
It was R34....   Roars like a monster now.  Appreciate the advice.

New voltages pin6 99v, pin8 3.6v, pin7 fluctuating 1mv (shouldn't it be negative to chassis??)
Original R34 measures at 995k - I read the meter wrong :-(

Congrat !

Your example illustrates the difficulty of helping without being there, in front of the amp. We cannot certify that the tests are carried out in the right way.

[DummyLoad]

that 100K 1/2W resistor is running at near spec and no wonder it was burned out. based on your telemetry, that system is flowing 2.16mA and dropping 216V across the 100K, so it's dissipating about 460mW. that part really should be a 1W part for long term reliability.

--pete

[scstill]

So now on to the safety issues
(in the future do you prefer to stay in same thread or start a new one since different subject?).

1) will be adding a three prong (see pic)
2) would you replace C31 with a Y2? or just remove it?
3) if C31 removed can SW4 remain in circuit without any issues?
4) should fuse be moved to the switched line side? Or leave on neutral side?

any other suggestions?

[shooter]

personally i would eliminate the cap n switch.  I would fuse the hot leg.  the convenience plug can be wired back in if needed, for now i'd make the 3 prong work.


it's the same amp so keep the same thread 

[scstill]

I guess I celebrated too soon.
The output power has gone down again.
The amp works but again at volumes that are lower than where it should be.
In fact the volume does not increase much past 25% on volume setting (both channels)
The  only thing I did was remove C31 (jumpering it back does nothing)
R34 is warm (DummyLoad mentioned at its limit) but v3 pin6 is still 100v
In fact all plate and screen voltages seem fine.
I  swapped the preamp tubes v1, v2, v3 (with  v4, v4, v5) same result. Do not have spare 6L6 or tester
Any thoughts?

[shooter]

couple;
verify your filament has a center ground - won't cause low volume, I just see you have a penciled ground.


what is your bias voltage at, schematic shows EDIT:-32 (-36), still there?

[sluckey]

Swap tubes first.

[scstill]

couple;
verify your filament has a center ground - won't cause low volume, I just see you have a penciled ground.
what is your bias voltage at, schematic shows EDIT:-32 (-36), still there?

The pencil ground is just to remind me that it is chassis. Won't be earth ground until I get the 3 prong.
Both grids on 6L6 pin5 have -30v

[scstill]

Swap tubes first.

I swapped all Preamp/PI tubes. No change. I do not have spare 6L6's

[scstill]

I have only had the amp for a couple of days and it has been upside down in the bench.

I decided to try it right side up.
It started to fluctuate loud and soft, like it wanted to behave as a 60watter. But unusual...
And it also started with unusual noise not heard before, even with the guitar unplugged and volume at zero...

[shooter]

It started to fluctuate loud and soft,

suspect a bad solder joint

i'd power up with meter monitoring the PA tube tap;
now chop-stick around, "see" if you get crackles, pops;
stick a signal in (something with volume) or a sig-gen :)
poke around again "seeing" if you get loud - quiet, play with knobs, wiggle tubes
DO IT SAFELY 

[sluckey]

What's the deal with the tiny fonts?

[scstill]

So I went chop sticking around and got alot of static noise on V2 Pin6, 8 and 3 and the resistors connected.
But before I started reflow and replacing resistors, I thought to swap the tube. Sure enough the noise went away and the output volume returned.

Then after about 15min of playing through it (with a smile), the volume went low again.
I moved the original speaker wire and discovered that the volume crackled back.
As I moved the cable it seemed to affect the volume.  So that may be the culprit, deeper look tomorrow.
I plan to install 1/4" speaker jack at both amp and speaker cab.

[shooter]

tiny fonts?

I didn't have tiny bubbles?


MS forced me from exploring to the EDGE 
now I have to click

then paste the words, click

(and add a fwd /) IF i forget to do a CR, tiny txt

[scstill]

Works good now that I added jacks at speaker and amp.
Next up the 3 prong.

Does anyone know what the 3rd secondary wire on the OT is for? Its just floating. Red/yellow physically between the yellow and black. Could it be for an 8ohm load, but its not on the schematic and measures same as the 4ohm secondary.

[sluckey]

Does anyone know what the 3rd secondary wire on the OT is for? Its just floating. Red/yellow physically between the yellow and black. Could it be for an 8ohm load, but its not on the schematic and measures same as the 4ohm secondary.

Not 8Ω. That same OT is used in the 1485 model which has a 2.67Ω speaker load (3 X 8 ohm spkrs in parallel).

     https://el34world.com/charts/Schematics/files/Silvertone/Silvertone1485.pdf

[scstill]

Another question....
When adding the 3 prong, I would like to route the cables clean and out of the way.
But that would route them right next to the speaker out
Would you expect any issues if these are close?

[sluckey]

no

[scstill]

So back to my first post #5 - there is a hum on Channel 2, not on channel 1

The hum is significantly reduced when guitar plugged in

The hum is also gone with a simulated shunt jack.
If I add one shunt jack its gone but with two I would wire like a Deluxe (see pics)
Wo0uld you go this way?

OR... Could this hum be expected becasue of reverb or trem circuit (if so what to look at)? Both are turned down. OR is there another suspected area to look at in the main circuit?

[scstill]

BTW - here is the new AC input layout with 3-prong and fuse on the hot switched side.
Interesting that the original design had switch on the Neutral side and fuse on Hot side

Also added a jack for the new speaker wire which is hard-wired at Cab.

Left the convenience outlet wired
But "death cap" is gone and its switch is unwired left to fill the hole.

[SoundmasterG]

Do a search for 1484 and look for posts I made about modding my brother's 1484. They're probably from around 10 years ago. I did a bunch of mods to it and you might be able to get some ideas from that thread. I swapped over the 3 prong ground, upped the screen voltage by moving it up one node in the doubler, added a double to the bias and made it adjustable, added a Fender reverb circuit and re-did the layout, increased gain, put trem and reverb on both channels, etc.


Also these amps are around 25 watts RMS for their power output. Not 60 watts.

Greg

[scstill]

I did find a 1484 thread where there was a hum on channel 1 isolated to the volume pot.
I checked and my vol pots are at 260k and 390k (sb 1m)
I will replace these first and see if that remedies.

I did see the post mentioned about 1484 mods.
Thanks for all the good advice.
The 3spring reverb mod I found seems nice.

Even at 25w its way loud. Absolutely love bringing it back to life
My preference is to keep it a 1484 with only necessary mods to run right.
BTW - all the electrolytics seem well as there is no noise and voltages are solid.
Thinking of keeping them. Love their look.

[SoundmasterG]

I did see the post mentioned about 1484 mods.
Thanks for all the good advice.
The 3spring reverb mod I found seems nice.

Even at 25w its way loud. Absolutely love bringing it back to life
My preference is to keep it a 1484 with only necessary mods to run right.
BTW - all the electrolytics seem well as there is no noise and voltages are solid.
Thinking of keeping them. Love their look.

Are those the Planet brand 'guaranteed for one year' electrolytic caps you're talking about? Replace them. They do last  along time but any new cap will be better quality and F & T's look pretty nice in there too.


A worthwhile upgrade would be adding a doubler to the bias circuit along with a pot and cap so it can be adjustable. The tubes run pretty hot with the stock bias.

Greg

[scstill]

Hey Everyone, Just an update....
Thanks for all the help, the 1484 is working perfectly now.

I only replaced a couple of resistors (1/2 to 1watt), three prong/removed the death cap, replaced a 12ax7 and added a shunted input jack. Also ran a plugged speaker wire. One speaker was reconed. The reverb was repaired. All other parts are original. I also harvested some tolex from inside the cab to repair the corners and then used ebony wood stain to help it match the patina.

I love this amp it sounds great.
And it is super quiet at idle. Amazing considering the filter caps are original.
Last night I bridged the channels and that is nice touch.

Any thoughts about how I might source or build an original footswitch?

[tubenit]

Left a PM for you about a footswitch.

Tubenit

[Johnnypgood]

Hey everybody! Sorry to highjack this thread, but I am repairing a 1484 and have a similar question. Using the schematic in the OP, would I be correct in assuming that the center wire off of the OT is the RED wire? If so, does anyone have a clear picture of how V5 is wired? I replaced the socket that was zapped, so it wasn't clear where everything connected. I'm assuming the Red 480v goes tot he power caps and nowhere else. Thanks in advance

[shooter]

You should be able to ohm the OT primary to figure out if red is the ct.  The CT will ohm ~~~ = to the other 2 wires, so 230ohm to one wire, 205 ohm to the other wire.  Ohm the "other 2" and they should equal 205+230 in my example
as for V5, the schematic is pretty clear where everyone hooks up.  what "line/pin" can't you find?

[scstill]

...would I be correct in assuming that the center wire off of the OT is the RED wire?
If so, does anyone have a clear picture of how V5 is wired?

Yes the center tap of the OT is red; blue goes to pin 3 plate of 6L6 V8 and brown goes to pin 3 plate of 6L6 V7. My noted schematic is attached, hope you can zoom in and see all my references. Glad I wrote the wire colors so I did not need to pull the chassis :-)

I do not understand why you are asking about V5 in the "same breath" as OT? Was that a typo?? V5 is the reverb drive tube I believe. I found that the V5 is wired as the schematic, but others have stated that its an odd reverb circuit. I also found that my reverb tank needed to be completely rebuilt; I think I posted pics of this is another 1484 reverb thread. Now it works excellent. I think it sounds really good.

BTW - after my 1484 rebuild including one speaker recone, and the footswitch from Tubenit, this is my absolute favorite amp. Now I want a four 6L6 1485.

[Johnnypgood]

Thanks so much! The V5 reference was from a different schematic. On this schematic it would be v7. Thanks again. I'll keep you updated.

[scstill]

Left a PM for you about a footswitch.
Tubenit

BTW - this footswitch is awesome thanks again.
Just added a stereo plug and it works great.
The tremelo and restored reverb of this 1484 sound amazing to me.
Still can't get over that this thing idles so quiet with all all of its original filter caps.
Its my favorite amp of them all....

[scstill]

BTW - Here is a photo essay of the Silvertone 1484 restoration process
http://stillampd.com/silvertone-1484-restoration