SEX 2.0 B+ voltages too high [resolved]

[Bill S]

I  have a SEX 2.0 that I have modified with Magnequest iron, C4S kit (I adapted a 2.1 C4S kit) and upgraded coupling and parafeed caps. The amp was working great and sounded fabulous. Not a lot of hours since I did the upgrades, maybe 15 hours or so but with zero problems.
I then replaced the RCA 6DN7 tubes with a couple of Sylvania coin base 6DN7s I had purchased as spares. I just wanted to see if they were OK as the RCA's were still just fine. Everything was OK but about 2 hours after replacing the tubes I suddenly lost both channels at the same time.
I verified it wasn't a source problem so I assumed it had to be a power supply problem somewhere before the B+ circuits since the SEX 2.0 has dual B+ power supplies and I had lost both at the same time. I checked and touched up connections around the power inlet and power transformer just to try to eliminate any issue there. I have slightly high voltages at terminals T6 and T9 of the power transformer. They showed 177 VAC. When I measure any other voltages in the B+ circuits they measure way too high. Both channels are the same.  For instance I measured 477 VDC at tube terminals A2 and B2. They should be more like 345 VDC. When I did my upgrades everything was within tolerance although on the high sides. Even when I built the amp years ago I recall voltages were on the high sides but within tolerance.

Any ideas of what might have happened and what I should look for?

Thanks, Bill

 

[Paul Birkeland]

Are the tubes still glowing?  I would suspect they are not, and that the issue you have is in the 6.3V DC supply feeding pins 7 and 8 of each octal socket.  This is about the only thing that's shared between channels on a SEX 2.0 and would explain the high voltages you're seeing.

 

[Bill S]

I can't check this until I get home but I'm curious if it could have been a tube failure that caused my problem.  I didn't notice any bad connections around the power transformer or the heater circuit when I checked. The two 3 volt terminals of the power transformer had correct voltage. Would heater failure of one tube have caused a failure of the entire heater circuit or should I be looking for a problem somewhere between the power transformer and pins 7 & 8 of the tube sockets?

 

[Doc B]

Can we assume you tried with the original tubes and ran the voltage measurements again? I'd put those back in and then do as PB suggests, check the voltage between 7 and 8 on the tube sockets.

 

[Bill S]

I replaced tubes with the older RCA tubes that had been working previously. I still have the same problem. I find that at terminal C2 I have 0 voltage (which should be 6 VDC) and resistance of 456K (which should be less than 1 ohm). I have -5.8 volt at terminal C4.
I did a resistance check from C5 to tube sockets A8 and B8 which is measuring the resistance through the .1 ohm heater resistor and through the connections at terminal C2. That resistance is .3 ohm which leads me to believe it's not a bad solder joint at C2.
Should I go over solder joints again or might it be a component failure? I don't see anything visually that looks bad. I'd already reflowed these solder joints.

Thanks,  Bill

 

[Doc B]

It sounds like you have 5.8VDC across the heater supply and your probes are referencing ground in such a way that you read it as a negative voltage.  It would really be helpful if you would check the voltage across pins 7 and 8 of each tube socket.

 

[Bill S]

Voltage is 0 across pins 7 & 8 of each tube socket (with older RCA tubes installed)

 

[Paul Birkeland]

Well, that is your problem.  Are you able to post build photos?  Specifically seeing the area where the 1N5820 diodes mount to the power transformer would be helpful.

 

[Bill S]

Paul,  Thanks for the help and advice. I certainly can send photos although it will be a day or two until I can do so.
My amp has many hundreds of hours on it since I first built it. I did do the upgrades recently as mentioned in my initial post regarding this problem but I did not change anything in the DC heater supply circuit during the upgrades and it worked great for probably about 17 hours when suddenly both channels failed at exactly the same time. When I did my first tests after this failure I noticed the B+ voltages were very high so I was looking at what might of caused the high voltages (not knowing that the 0 voltage to the tube heaters could cause this.)
Now that I know I appear to have a problem in the DC heater supply rectifier section I'm trying to diagnose what failed.  Is it just a bad solder joint or connection or is it a diode failure or capacitor failure?  I'm afraid I don't have enough knowledge of circuit theory to diagnose the problem from the voltage and resistance measurements I've taken that are incorrect.

Bill

 

[Paul Birkeland]

Looking at photos of that area would help to diagnose what's going on.  Voltage measurements at the C terminals would also be useful.

Don't be to worried about a problem like this, as this is one of the easier issues to remedy.

-PB

 

[Bill S]

I got a chance to get a couple of pictures. I have no voltage from tube sockets #7 to #8

I took resistance readings which are:
Terminal C1  .3 ohm  (my meter shows .3 ohm on any/all ground connections I checked)
Terminal C2  997.6 K ohm
Terminal C4  100.8 K ohm
Terminal C5  98.1 K ohm

Voltages are:
Transformer  T1  123 VAC
Transformer T4  3.5 VAC
Transformer T5  3.5 VAC
Terminal C1  0  VDC
Terminal C2  0  VDC
Terminal C4  -5.8 VDC
Terminal C5  0 VDC

 

[Paul Birkeland]

C2 and C5 connect with a 0.1 ohm resistor.  Your voltage measurements show 5.8V on C4 and 0V on C2.  IF those two terminals are actually connected by the 0.1 ohm resistor and you have those voltages, the resistor would burn up very quickly, so I suspect that resistor is loose.

 

[Doc B]

It sounds most likely to be a bad solder joint. The solder joint at terminal 4 of the power transformer looks a little suspect. I would be inclined to reflow PT terminal 4, C1 and C4. Then recheck the voltages at C1,C2,C4,C5.

 

[Bill S]

Fixed!    Thanks guys!  You are a great help.
It did turn out to be a bad solder joint.  It was a bad connection at terminal C1 which is a ground connection. The black jumper wire that comes from C4 had a poor connection at C1. I had reflowed all the connections around that part of the circuit at least once while troubleshooting. This evening I did a much more thorough redoing of the power transformer connections and terminals C2, 4 and 5. After I was done my resistances at C2 and C5 were normal but C4 still had high resistance. C4 connects to the C1 ground connection with a short jumper wire. I used  a jumper with alligator clips to connect C1 and C4 and then my resistance at C4 was fine. I then heated and pulled the black jumper wire connection loose from C1, resoldered and everything's fine. The amp sounds awesome again.

My lesson from this is that even after 11 years and many many hundreds of hours of use a solder connection can still fail.

When I built this kit in 2011 I did not have a whole lot of soldering experience. It was the first kit I built. I think I had a bad solder joint when I built it but no more problems cropped up for years.
I was also surprised to learn that failure of the 6 volt tube heater circuit would drastically increase the B+ voltages which get their current from a different winding of the power transformer than the 6 volt heater circuit.

Thanks again,  Bill

 

[Doc B]

You are welcome. And yup, a cold joint can fail at any time.