Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

[wsscott]

I working with a buddy who was brought this amp to fix its fault with blowing the main 3A, Slo-Blo, fuse. I say the "main fuse" because there's another 1/2A fuse which I think protects the OT, but I'm not sure.

We've tried to find a schematic, but no luck.  Have found some for other amps but they're not the same.  One that looks close is missing the Power Supply page.

So if anyone has a schematic to share, that would be appreciated.

We're just getting started, but as I said, it blows the main fuse when both 6L6's are installed. 

The only interesting history is that one of the 6L6's has a broken locator pin.  New tubes have been installed, and with a dim bulb tester connected, the tester (unfortunately) lights up nice and bright.  If the 6L6's are removed, no light up.  We thought maybe someone installed one of the tubes and broke the locator pin but plugged it into the socket anyway with a wrong orientation, and turned it on, and maybe that created a short and blew the OT.  But the OT seems to be okay.

The 2 6L6 screen resistors, 1K/5W, read fine.

One of the filter caps looks questionable, but it hasn't had its ESR tested yet.

The amp has a Parallel FX Loop circuit, and its Output jack is missing the nut to secure it to the chassis.  Maybe it's shorting out somehow.

Given the behavior of the amp when no Output Tubes are installed, we're thinking its in the Output section of the amp and not the Power Supply.

Any thoughts are appreciated.

[wsscott]

I did some work on it today, and I think that somehow the grid 1 and grid 2/screen wiring in both of the 6L6 sockets is reversed. How that occurred is a mystery.  I may be wrong on this since I don’t have a schematic, but it’s my only clue to the cause of the blowing fuse right now.

[wsscott]

I checked the wiring again, and it appears its wired correctly.

I cleaned the sockets, and hooked the amp up to power through a Variac and a dim bulb limiter, and no issues as long as amp was in Standby.  When that is turned off and power goes to the Output Tubes, it lights up starting at about 55 VAC, and gets brighter as I increase the voltage.  I stopped around 80VAC.

The screen resistors on the Output tubes are good, and the filter caps are within spec.

The only evidence of a short is the burned Pin 3 Plate pin on the socket where the tube with the broken locator pin was installed.  So I think the tube was installed improperly and caused a short.  But other than maybe blowing the tube and the fuse, why does there seem to be remaining damage?

I am getting equal voltages on the Output Tube Plates even at a low 55VAC level.  So I assume the Output Transformer is good and not damaged.

Any suggestions on how to trace a short? 

[acheld]

Check resistance in each leg of the OT primary.   

I've learned (the hard way) to assume nuttin. 

[wsscott]

Thanks.  Between the OT's CT and each leg, its 37ohms on one and 38ohms on the other, and between the 2 legs of the OT its 75 ohms.

So that seems ok--I think.

[wsscott]

I removed the Output Tubes from the amp, and it is unplugged and off.

I set my meter to read Ohms and checked for any resistance between the sockets for Pins 3 and 4, and on both tube sockets there is about 250K ohms of resistance.

Also if I set my meter for Continuity, there is a brief squeal when the sockets for Pins 3 and 2 are tested.  Pin 3 is the Plate and 2 is one of the Filaments.  This is true for both sockets.

Could a short exist within the socket between these pins?

[Adrien]

The only evidence of a short is the burned Pin 3 Plate pin on the socket where the tube with the broken locator pin was installed.

Based on this and your measurements, there is an arc path between pins on the burned socket.  The plate is probably shorting to the heater (i.e. ground).  Best fix is to replace the socket.

[wsscott]

Is this something that can be seen, like a crack or something, or is it "internal" to the material of the socket? 

[Adrien]

Hard to say for sure.  The arc will follow the easiest path, which is usually along the surface on any dust or whatever that happens to be there.  It would look like sort of a black carbon trace on the surface between two pins.  You may not be able to see it without a magnifying glass. 

But this is all a bit academic.  You say there is charring on the socket, and your meter is giving you a continuity reading where there shouldn't be any, so that's all the evidence you really need.  The short exists whether you can see it or not, and once that conductive path exists, it's really hard to get rid of.  A cleaning probably won't do it.  Just replace the socket.  Yeah, it's a pain...

[wsscott]

No luck-I installed a new socket and same result.  I could see a physical crack on the old socket after I removed it, but obviously that isn't the cause of the problem.

The resistors in that part of the circuit and for the bias all are good and read to spec.

I've got to believe it's in the OT.  What else could it be.

What's the best way to test the OT to see if it is having problems?  Disconnect the primary and secondary leads from the circuit, and using the Variac insert 1 VAC into the Secondary and read the VAC (or would it be VDC?) Output on the Primary/Center Tap for both Primary leads?

[wsscott]

Is it safe to remove both output tubes, and then install just 1 of the tubes, then power the amp on, and then see if the bulb limiter lights up in just one of the tube sockets when 1 tube is installed?

Or would this stress the OT and damage it?

[shooter]

is this a turret board, PCB or point2point????????

[wsscott]

PCB

[acheld]

Time for some detailed photographs.  And time to look underneath that PCB.

[wsscott]

I was afraid you'd say that.

Here are 2 photos and I'll send 2 more in another post.

[wsscott]

And 1 more.

[wsscott]

And last one.

[Adrien]

I removed the Output Tubes from the amp, and it is unplugged and off.

I set my meter to read Ohms and checked for any resistance between the sockets for Pins 3 and 4, and on both tube sockets there is about 250K ohms of resistance.

Also if I set my meter for Continuity, there is a brief squeal when the sockets for Pins 3 and 2 are tested.  Pin 3 is the Plate and 2 is one of the Filaments.  This is true for both sockets.

Did you re-check this stuff after changing the socket?

[wsscott]

I just checked, and each output socket reads 1.5K now vs. 250K before when the probe checks Pins 3 and 4, ie. the Plate and Screen.  There is a 1.0K screen resistor on each tube socket.

Now, there is no beep for continuity, not even a brief one, when I have my probes on Pins 3 and 2, ie. the Plate and Filament.

So changing the socket may have "fixed" those issues, if they were in fact "issues" to begin with.

[shooter]

there's a whole world waiting on the UNDER-side of that board also 

[wsscott]

That's what I'm afraid of!

[wsscott]

Any thoughts on my question in Response #10 above about just plugging in 1 of the output tubes to see if I can localize if just 1 side of the output is causing the problem?

[shooter]

your ohm values seemed ok, without a schematic i wouldn't tempt fate.  you can put a small VAC on the secondary n measure BIG VAC on the primary.
all PA tubes out and NO speaker connected.


one thing that will help, add 1 ohm R's from each cathode to ground... assuming... it's a PP amp.  ohm each cathode pin to ground, if they both read ZERO ohms, then you'll have to fin n cut the foil, then "bridge" that "gap" with a 1 ohm.

[wsscott]

Thanks Shooter. It already has 1 ohm resistors in place for biasing.

[shooter]

so it's fixed bias???
adjustable?
stable without PA tubes?



got a pair of bench-test PA tubes?
monitor the 1-ohms n see what happens

[wsscott]

It’s fixed bias with a trim pot, and with no tubes installed the bulb current limiter does not glow.  If I recall correctly, it’s grid biased.

Could the problem be with the PT? 

[wsscott]

This amp has 3 legs for the impedance for the speakers, 4, 8 and 16 ohms.  And it uses a rotary switch to change between the impedances. There is a wire which connects from one of the Input lugs on the switch and goes to the Tip lug on the Main Speaker Jack, and 3 signal wires (orange, brown, and red) come from the OT's Secondary into 3 input lugs on this switch.  There are also 3 other ground leads (smaller gauge but the same colors) going from separate lugs on the switch to separate ground points. Finally there is a purple ground wire coming from the Secondary that connects to the Speaker jack ground/shield lug.  Attached is a photo.

I don't know how to test this switch to see if it might be defective.  I'm getting a continuity beep on I believe every combination of connection points I've tried.  It's like every wire on that switch is going to ground.

Maybe this switch got burnt and that's the cause of the problem.

[wsscott]

I did one more test on the OT to see if it has problems.  I checked for resistance between the core/frame of the OT and each of the 3 Secondary leads going into the impedance switch.  I set my meter to Ohms, and connected 1 probe to the core plates and the other probe to each of the leads, and I got readings of about 1 ohm of resistance for each of those 3 leads.

So I think that would indicate that the wiring on the Secondary has some instances where its insulation is bear and it is touching the core.  That seems like it's a problem and would indicate the OT is bad.

Agreed?

[sluckey]

You're chasing rabbits! It's perfectly normal for everything attached to the secondary to read close to a dead short to chassis. You need to understand what the nature of the secondary winding is. As far as your ohm meter is concerned, it's just a length of pretty big wire. Stretched out straight, it may not even be 20 feet long. One end of this wire is connected to the chassis. If you connect one probe to chassis and touch the other probe anywhere along that length of that wire, the meter will correctly read nearly zero ohms. Doesn't matter if you touch the end of the wire, or a tap that's half way down the length, or a quarter of the way down the length, it's all gonna be nearly zero ohms. All you are doing is measuring the dc resistance of that wire. I hope this makes sense to you. Now go chase a different rabbit.   

[wsscott]

Why doesn’t the Primary read the same way?  When I touch a probe to the core of the OT and the other probe to a Primary lead it reads OL?

You used the word “chassis”.  I’m not talking about the chassis of the amp but the metal plates making up the transformer.

[shooter]

 


the OT is a step-Down so the in-side has lots o wire the out-side has 20 feet


OHMS is NOT inductance, you measure them different


the OT is bolted to the chassis, the ot has it own chassis

[sluckey]

Why doesn’t the Primary read the same way?  When I touch a probe to the core of the OT and the other probe to a Primary lead it reads OL?

Because the primary wire is insulated from the core. The secondary is also insulated from the core, ***BUT*** there is a fourth secondary lead called the common. It is not connected to the selector switch. It is connected to chassis ground somewhere. If you find this wire and disconnect it, your ohm meter will no longer read about zero ohms to the chassis/core.

You used the word “chassis”.  I’m not talking about the chassis of the amp but the metal plates making up the transformer.

The "core" is bolted to the OT frame and the OT frame is bolted to the chassis. I would expect the core and the chassis are also electrically connected together.

[wsscott]

Sluckey, is this the same for the Power Transformer?

I ask because I'm getting OL when I do this same type of measurement on the PT.

[wsscott]

Yea, the ground is the purple wire shown in the last photo I posted and it connects to the center lug on the stereo output jack.

I guess the OT is good.

[wsscott]

This amp has an adjustable bias pot.  The resistors connecting to it read according to their bands, so they're fine.

It also has 1 ohm resistors connecting the cathodes of the 6L6's to chassis ground.

But since this amp will blow a fuse unless its connected to the bulb current limiter, I don't know how I can check the bias.

I apologize in advance for not knowing a way to do this-but I'm guessing Sluckey or Shooter probably do.

[shooter]

since this amp will blow a fuse unless its connected to the bulb current limiter, I don't know how I can check the bias.

that's with ALL tubes out??

[wsscott]

No, it only lights up if the power tubes are installed.  If I remove the 2 of them, and even if I leave all the other tubes in, then no light up on the current limiter.

[sluckey]

No, it only lights up if the power tubes are installed.  If I remove the 2 of them, and even if I leave all the other tubes in, then no light up on the current limiter.

The next step I'd take is to substitute known good tubes. Then verify voltages are good on pins 3, 4, and 5. Check voltages with tubes pulled and amp plugged straight into the wall. (The dim lamp bulb limiter will cause voltages to read lower than normal). Finally, substitute a known good OT. Following these steps will probably fix the fuse blowing issue.

Part substitution is a valid troubleshooting method. If you're gonna work on tube amps you really need some good tubes and good OTs on hand.

[wsscott]

Thanks Sluckey-My buddy and I have been thinking of getting an output transformer to have on hand just for testing for instances like this.  We do this as a hobby to help other friends/players out, and we don't charge for our time.

Is there an OT that you could recommend we get to keep and use for stuff like this, that we could use for most applications?

[wsscott]

I think I have eliminated the OT as the problem.

Here's what I did:
I installed all the tubes, including the output tubes, and hooked the amp up to the current limiter.

Then I disconnected the leads from the OT Primary to the Plates on their sockets, and the bulb brightly illuminated!  The OT is out of the circuit.
So I then reconnected the OT with the Plates.

Then I disconnected the both Screen connections on the sockets.  NO GLOW.
I then separately connected the screens to see if it would indicate which Screen was the problem, but the limiter still Glowed even with just 1 of the 2 screens wired into the socket, regardless of which one was wired into the circuit.

So it seems that the problem lies with the Screen part of the circuit, and not the OT.

The screens should be connected directly to one of the main filter caps.  So hopefully it's just one of those that's bad.  I don't have a schematic that's even close that shows the power supply stage and this is a PCB, so it may be more difficult than it otherwise should be.  It could be a trace on the PCB.

[wsscott]

I think I've found it.

The 100 ohm resistor coming off the rectifier that connects to the B+ that goes to the Screens tested to be 10K ohms.

I'm going to install a new one tomorrow.  I'm burnt out tonight.

I'll confirm the results.

[shooter]

there's a whole world waiting on the UNDER-side of that board

It could be a trace on the PCB.

did you ever look?
when you fix PCB stuff for bread money ya just accept the fact that a complete visual inspection is simply "the cost of doing business"

[wsscott]

The PCB is difficult to remove because of a base for a shield that’s attached to the V1 socket that restricts removal of the PCB. So I tried but no luck.

[shooter]

well, do what i've done the past 10 years when someone asks about repairing a PCB amp...say no 

[wsscott]

Shooter, I have a question about voltages in the filter caps that are connected in series.  Hope you, or someone can answer this.

This amp has 2 220uf/350V caps in series.  They are the A+ caps coming off the full wave rectifier.  The stored Voltage on the first cap to get voltage from the rectifier is about 230 VDC, but the voltage on the second cap in series is only about 115 VDC.

There's also another set of caps is series at the B+ node that are 100uf caps and they have similar readings.

Is this correct?  Ie. that the voltage being stored in the second cap in the series is about 1/2 what the voltage being stored in the first cap of the series is storing?  Or is something amiss?

Thanks.

[sluckey]

The voltage ***ACROSS*** each cap is the same and they add to provide a higher voltage. Just like putting two batteries in series.

[wsscott]

Well NO luck with replacing the out of spec resistor.

I just cannot find the short, and there's nothing much to check-that I know of.

I've removed the Filter Caps serving the Screen with B+ and they read in spec and perfect ESR.

The screen resistors read in spec.

The B+ at the Screens is a little over 250VDC and is just a couple of volts lower than the Plate voltage.

If I have all the tubes in, and turn the amp on the light bulb limiter glows.
If I disconnect the leads at the tube socket that connects the screens to the PCB, the bulb does not glow.

What's left to check?

[dogburn]

Did you ever substitute a pair of known good power tubes (as Sluckey advised)? Reading through the thread it seems you never mentioned that, and a bad tube (even new ones can be bad) is usually the first suspect.

[shooter]

it only lights up if the power tubes are installed.  If I remove the 2 of them, and even if I leave all the other tubes in, then no light up on the current limiter.

NO luck with replacing the out of spec resistor.I just cannot find the short, and there's nothing much to check-that I know of.

i'm missing something;
with the PA tubes out you have NO B+, bias volts, any volts???

[wsscott]

Sorry, Yes I replaced the tubes with a set from another amp that works fine, and experience the same symptoms.

With the power tubes removed, I have 465 VDC at A+, B+, Plates and Screens of the Power Tubes.  And NO glow from the bulb.

If I install the power tubes, it GLOWS.  The Voltages with the Power Tubes installed is now around 250 VDC.

If I then disconnect the Plate leads at the Sockets, it GLOWS.

If I re-connect the Plate leads at the Sockets, and DISCONNECT the Screen Leads at Pins 4 of the Sockets, it DOES NOT GLOW.

Sorry if my descriptions have been confusing.