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Old February 22nd, 2012, 02:29 PM   #1 (permalink)
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old class A amp, power issues

I've posted here before for help maintaining a '57 Rickenbacker M8-E (it's basically a Champ copy). About a year ago I replaced all the elctrolytic capacitors. The amp has not seen much use since then, but lately I've been practicing with it a lot. It got through several practice sessions fine, then one day the fuse blew. It was original, so I dropped in another and that blew. I thought perhaps the tubes were the culprit. I put in a known good 5Y3 and 6V6, turned the amp on and it seemed to be happy. I suspected the original 5Y3 was the one to blame, simply because it's first in the power chain, so I put back the original 6V6 and turned the amp on once more. Everything seemed fine for a couple minutes, then I started to hear this crackling or melting sound. I quickly turned the amp off. The sound seemed to be coming from the power transformer. THe PT was also quite warm to the touch. No other signs of damage are visible anywhere.

So, before I totally destroy this thing, does anyone have some suggestions on troubleshooting this issue?

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Old February 22nd, 2012, 02:57 PM   #2 (permalink)
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There is a short either in the PT or in the power supply.

Do you have an ammeter? If so, you can measure the current draw in the following tests. This is when it's nice to have an ammeter and a variac in line - you can get an indication quickly as you bring up the voltage that something is wrong.

If not, you can still judge by the seat of your pants - if the PT is getting hot, and/or blowing fuses.

Take all the tubes out of the amp and then turn it on. If the PT draws a lot of current, blows a fuse, or gets hot then, you likely have a PT issue.

If the PT doesn't get hot or draw a lot of current, the problem is downstream. Shorted or miswired filter caps, shorts in the supply, shorted rectifier could all be at fault. At that point, you have to be methodical in tracking it down.

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Old February 22nd, 2012, 02:59 PM   #3 (permalink)
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If it were mine, I would pull all the tubes, open the chassis where circuit is exposed. Turn on the power and let it run a short while and check the AC voltage going to the rectifier socket. If the transfomer is bad it will blow a fuse without a load. If it seems to be happy with that, turn the amp off and install the 5Y3, turn it on and let it run a short while. If no fuse blows, check DC voltage to OT and filter caps--should be in the 400 range without a load.
Look for any possible shorts where the high voltage DC may be touching anything it shouldn't in the total circuit and shorting out. Old burn marks is a good sign something is a-miss, You can also use a chop stick to probe the live circuit to see if you see any bad connections/sparks. If it blows a fuse at this point, it's either the 5Y3 or a short in your power supply.

If this proves to be OK, turn amp off and install the 6V6, turn it on and let it run a short while. If everything seems OK, at this point, turn amp off and install your preamp tube and re-power amp. Hopefully by this point you've found the problem by process of ellimination by identifing a falty tube or short. Platefire
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Old February 22nd, 2012, 05:29 PM   #4 (permalink)
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Crawfish, I actually do have a variac, as well as a multimeter. Should I run at lower voltage while testing to avoid damaging the PT or other components?

Platefire, that seems like a good method for elimination. Thanks.
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Old February 22nd, 2012, 11:15 PM   #5 (permalink)
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OK, so I pulled all the tubes and ran 100v through the amp for a few minutes. I was getting low voltage readings going to pin 8 of the rectifier, something like 17v. Probably a worse sign, the PT starting making that melting/crackling sound again. The fuse did not blow, so I quickly turned the amp off.

Does this mean my PT is shot?
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Old February 23rd, 2012, 12:37 AM   #6 (permalink)
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Don't sound good! Without a load, the PT shouldn't be having a problem. You might double check to make sure the secondarys from the PT--heater wiring/rectifier wiring/ground wire is not shorting out on anything. Even though you haven't got a load on the PT, if one of those wires were shorting out on something, it could make your PT moan and groan---but seems like it would blow a fuse too. Also you need to make sure the PT ground is a good solid connection. You can use your multimeter continuity setting to check for shorts and proper ground connections.

If it's not shorting out on somethin and the sound is coming from the PT, it would have to be some kind of internal PT short. Platefire
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Old February 23rd, 2012, 01:00 AM   #7 (permalink)
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Hey Calvin hope you can get your amp working good again. My little 5 watt is an M-10 model, it's got round white Bakelight knobs-most likely from the 30's/40's and originally sold as a Hawaiian steel amp. I play guitar through it a lot and also use it to amplify Uke in the Hawaiian duo my friend and I do. Mine has the tone knob wired backwards. Old Ric amps are cool and worth keeping alive.
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Old February 23rd, 2012, 08:48 AM   #8 (permalink)
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I looked at the schematic for that, its transformer can be replaced with a new one for a Champ amp.
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Old February 23rd, 2012, 11:40 AM   #9 (permalink)
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Alright, I used the continuity setting (with one probe on the chasis) to check for proper grounding. There are two PT wires that go to ground, and they both tested fine. The PT wires going to pin 4 and 6 of the rectifier also test for continuity to ground, as well as the wire going to the lamp. Does this sound normal? How would I test for a short in the wire going to pin 8? I don't have another side of the wire to test with. Is there anything else I'm missing in my testing?

Yes, I understand a Champ transformer would be very similar. Only problem is my PT is mounted vertically with the wires coming out the bottom, while Champ PTs are a horizontal configuration with wires coming out the sides.

This is a cool little amp, and I think this one was also originally sold as a bundled set with a lap steel. I like the amp a lot, so I wouldn't mind spending $60 for a new PT, assuming I can find one that fits.
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Old February 24th, 2012, 08:37 PM   #10 (permalink)
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Calvin, you have two circuits going to the rectiifer from the PT and one circuit which is the 6.3V heater filament circuit going to the pilot light and on to the 6V6 and the 12AX7.
ON Fender amps and all other 'modern' American PTs, the circuits going to the rectiifer are red and yellow while the heater filament is green. Some old amps do not have this color code...or the colors are made unreadable by dirt and oil, ime. The center tap for the red will be red/yellow, and the green heater filament center tap is green/yellow.

Lift the wires from their contacts to the socket pins and from the pilot light.
Or...pull the light bulb and the 5Y3 from their sockets to eliminate them from the equation. .
The 'red' winding, which connects to pins 4 and 6, has a center tap as does the heater filament. The heater filament winding goes to the pilot light...and on to the 6V6 and 12AX7. These center taps go to ground and therefore you will see continuity to ground when testing those two windings. This continuity will be revealed by a reading of some resistance....I just read 62 ohms on each end of a 'red' winding in a PT sitting on the bench here. If there is no resistance between the winding and ground, you can assume that winding is shorted to ground, right? YOu can test for continuity from end to end and from each end to the center tap. These resistances should be close to equal from each end to the center tap. IF there is no resistance from any part of the winding to ground, the PT is shot.

The wires that connect to pins 2 and 8 should show a certain resistance from end to end. IF there is no resistance between that winding and ground, the PT is shot.
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Old March 2nd, 2012, 12:21 AM   #11 (permalink)
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Thanks for your reply, Wally. I now understand how those grounded wires relate to the heater and rectifier wires. Got a much better picture of what's going on.

I had difficulty checking for resistance between the windings and ground. I would get a quick reading on my multimeter (sometimes ~60 ohms, sometimes ~170) but it would ultimately settle on zero. This was true for the heater and both rectifier windings. Yet when I test using the continuity setting, I get a beep in all the right places. Thoughts?

Strangely, I just put a whole different set of tubes in and have been running the amp for the last 2 hours with no issues. What is going on here? I don't see why different tubes should make a difference, since when I ran the amp without any tubes the PT still seemed to overheat. Yet now it's fine. I guess this is good, but I want to make sure the issue is truly resolved so I can use the amp reliably. I'm just totally confused now as to what the problem is/was.

Last edited by calvin006; March 2nd, 2012 at 01:20 PM.
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Old March 6th, 2012, 02:25 PM   #12 (permalink)
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Calvin, I've been out of the shop for a rest. INteresting that the amp is 'working'. What are your voltages throughout the amp?? Whenever an amp is acting up, one should be taking note of votlages both when it is 'wroking' and when it is not working. I would suggest that you make a chart to log in your voltages for comparison purposes. An amp cna fucntion at voltages that are not 'correct' for the design of the components and the circuit. It is always good to know what is going on with respect to voltages.
Shorts can oddly occur...and then cease in some radical situations. REst assured that if there is a problem in that PT, it will finally come to be obvious. Run the amp until then, I suppose.
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Old March 8th, 2012, 05:39 PM   #13 (permalink)
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OK, thanks Wally. I put the chasis back in, but I'll open it up again and measure the voltages some time. When I play it now, I just keep a close eye on the PT. So far so good! I'm confused as to why it resolved itself, but happy that it's working.
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