I finally pulled my project aa-50 to take a look at it.
It ran fine all day.
I shut it down.
I pulled each tube and cleaned, tested and replaced it in its socket.
I came back the next day and powered it back on.
No sound came out.
I looked over and the rectifier tube was glowing bright red and something was burning.
I shut it down. The fuse is still good.
I pulled the rectifier tube and turned it back on.
The heaters all lit up.
I checked the ac voltage on the rectifier socket and there is 5v on one set and 400 on the other.
I put a new 5ar4 in and turned it on. Instant fireworks in the tube.
How do i go about diagnosing this problem?
It ran fine all day.
I shut it down.
I pulled each tube and cleaned, tested and replaced it in its socket.
I came back the next day and powered it back on.
No sound came out.
I looked over and the rectifier tube was glowing bright red and something was burning.
I shut it down. The fuse is still good.
I pulled the rectifier tube and turned it back on.
The heaters all lit up.
I checked the ac voltage on the rectifier socket and there is 5v on one set and 400 on the other.
I put a new 5ar4 in and turned it on. Instant fireworks in the tube.
How do i go about diagnosing this problem?
Could it be that the heater supply for the non-rectifier tubes is fused, and that fuse blew so that the heater voltage of the rectifier was increased because there was less current being drawn? Sounds like it has to be linked to the work that was done in some way.
Could just be that it was dust on contacts. Do you have a variac or dim bulb tester? Occam's Razor points to the maintenance that was done as the likely cause, since it worked when nothing was disturbed.
At this point I'd be grabbing a variable HV supply, they typically have heater voltage available (6.3 VAC) also. Allow the heaters to warm up and increase your HT supply. With the B+ rectifier pulled, you can use the existing supply to fire up the heaters. You can see when you begin drawing current and can then trace where it is going without damaging anything else.
A dim bulb tester is not recommended in this case especially.
The filter cap is a prime suspect, so is an output without bias. Clearly it is a high voltage current draw issue. I have seen output transformer shorts do this. The transformer then needs replacement or rewinding.
A dim bulb tester is not recommended in this case especially.
The filter cap is a prime suspect, so is an output without bias. Clearly it is a high voltage current draw issue. I have seen output transformer shorts do this. The transformer then needs replacement or rewinding.
I found that a extra cap has been added to one of the cap cans.
I will take it back to stock and recap it.
Thanks.
I will take it back to stock and recap it.
Thanks.
Many times, poor service techs will tack a new capacitor across a bad section in a multi-section can capacitor. You can't do that! The bad section may well short, it has already proved defective. It is in the same can as the other capacitors, so what affected it also affects the other sections. The only correct service is to replace the entire can capacitor. Anything else invites trouble.
If I find even one bad section and the client refuses to allow me to replace the entire capacitor, I return the unit to that client unrepaired. I will not do an improper repair, and should that capacitor remain and later fails, the client will often blame the guy who worked on it last.
When you "recap" the unit, use the exact same style capacitors (axial, not radial like "orange drops capacitors). Remove the leads from terminals, don't just cut the leads and "tack" the new capacitor in. That is hack work. Old bad TV guys did that. Measure the resistors too. Replace out of tolerance ones, and a few are normally matched. Buying new resistors, look at the voltage rating! Old 1/2 watt carbon composition were rated at 500V.
If I find even one bad section and the client refuses to allow me to replace the entire capacitor, I return the unit to that client unrepaired. I will not do an improper repair, and should that capacitor remain and later fails, the client will often blame the guy who worked on it last.
When you "recap" the unit, use the exact same style capacitors (axial, not radial like "orange drops capacitors). Remove the leads from terminals, don't just cut the leads and "tack" the new capacitor in. That is hack work. Old bad TV guys did that. Measure the resistors too. Replace out of tolerance ones, and a few are normally matched. Buying new resistors, look at the voltage rating! Old 1/2 watt carbon composition were rated at 500V.
Many times a multi-section is not available and the client wishes to keep the original look of the unit. Then the old FP needs to be completely disconnected and proper new caps mounted up under the chassis. If changing the look is ok, then the one section that sees the most ripple current should be replaced with a good snap-in, mounted with the appropriate clamp where the old one was. Mount the other one or two up under the chassis.
I am not familiar with this particular amp, so I'm making some generic suggestions. The problem appeared when the tubes were pulled, then replaced. It is not stated whether the amp was tested after this was done, only that it did not work the next day. Let's start there.
Before doing anything pull each tube one at a time. Make sure that each tube is in the proper socket. Look at the bottom to see if any pins got damaged. Pay special attention to the key in the center of an octal tube. They often break off allowing the tube to be installed incorrectly. A tube that is rotated can put the heater across the high voltage supply in some cases making the rectifier tube very unhappy. It is possible for the current flow through the HV circuit to light the heater of an octal tube. I have seen it happen only once in my 60+ years of messing with tube stuff though. If this is the case continued operation, even for short periods of time can fry the power transformer.
If I were looking at this amp, I would remove all tubes, making sure that you can put them back in the right place (I use blue painters' tape to make numbered stickers). Install only the rectifier tube and briefly test again. If the tube gets angry, it is not a tube or bias current issue. A shorted cap on the B+ line or an OPT is likely to be the culprit. Look carefully at the point where the OPT wires pass through the chassis for a worn spot on the wire. This can often be fixed with heat shrink tubing.
Before doing anything pull each tube one at a time. Make sure that each tube is in the proper socket. Look at the bottom to see if any pins got damaged. Pay special attention to the key in the center of an octal tube. They often break off allowing the tube to be installed incorrectly. A tube that is rotated can put the heater across the high voltage supply in some cases making the rectifier tube very unhappy. It is possible for the current flow through the HV circuit to light the heater of an octal tube. I have seen it happen only once in my 60+ years of messing with tube stuff though. If this is the case continued operation, even for short periods of time can fry the power transformer.
If I were looking at this amp, I would remove all tubes, making sure that you can put them back in the right place (I use blue painters' tape to make numbered stickers). Install only the rectifier tube and briefly test again. If the tube gets angry, it is not a tube or bias current issue. A shorted cap on the B+ line or an OPT is likely to be the culprit. Look carefully at the point where the OPT wires pass through the chassis for a worn spot on the wire. This can often be fixed with heat shrink tubing.
Hi wg_ski,
FP (twist-loc) caps are manufactured by CE Manufacturing, www.tubesandmore.com. They may not have the correct value exactly, but normally they have something that will work. I have had to enlarge the hole (clearing the sides of the can) and mount the capacitor below chassis level before so the cover fits. A pain, but now the unit operates as it should.
Mounting capacitors under the chassis can block air movement for the associated dropping resistors that tend to run very warm. You have got to be aware of all the factors at play.
Hi George,
Great suggestions and observations. It is hard to pick up where someone else is working. We all have our ways of doing things in a safe manner.
FP (twist-loc) caps are manufactured by CE Manufacturing, www.tubesandmore.com. They may not have the correct value exactly, but normally they have something that will work. I have had to enlarge the hole (clearing the sides of the can) and mount the capacitor below chassis level before so the cover fits. A pain, but now the unit operates as it should.
Mounting capacitors under the chassis can block air movement for the associated dropping resistors that tend to run very warm. You have got to be aware of all the factors at play.
Hi George,
Great suggestions and observations. It is hard to pick up where someone else is working. We all have our ways of doing things in a safe manner.
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Another method to progress fault-finding is to use a variac and a psuedo valve rectifier (either an octal plug with ss diodes, or ss diodes temporarily soldered to the existing valve rectifier socket), when all valves are out of the amp. That way the dc B+ rail can be raised from zero to allow shorts or shorting parts to be identified within the DC supply circuitry. If a variac is not available then even a 12Vac plug-pack or a transformer with low voltage secondary can be used to clip onto the mains input of the amp under test.
Yup. Those are small capacitors inside a can. Not original construction and I would never use them.
Well, I'll never use CE can caps again. They have a tendency to arc when their max voltage is surpassed and will not tolerate any over-voltage. I've had much better luck with Authenticaps, which are similar in construction to Hayseed Hamfest cap. Lots of restorations have been done with these brands and I've never heard of any complaints.
Also, CE can caps have a very poor temperature rating (55 degrees) compared to Hayseed and Authenticaps (105 degrees), and they advertise +50% tolerance, which is not really acceptable. The temp rating can be very important for caps mounted under tha chassis (as in some Sherwood and Fisher amps) or close to the rectifier or output tubes.
Original FP cans never had decent temp ratings either. 55 or maybe 65C. I thought they were long obsolete and long gone.
I also had bad experience of the CE caps, but it was 25 years ago and they were just getting their old "original Mallory" equipment going. Maybe the early issues have been resolved, but I'm snake-bit. Hayseed Hamfest parts are expensive, but they're 105C Nichicons mounted above the chassis, so worth it to me. My current order is taking quite a while, so don't expect speedy delivery.
All good fortune,
Chris
All good fortune,
Chris