Super Reverb Build Problems

[mjenki14]

Hi, I am new to the forum and relatively new to amp building. I successfully built a JTM-45 this past summer and so I thought I would attempt a super reverb. I got the kit from Marsh. I finished the build today and went through to make sure my connections were sound. I put the tubes in and had a bias adapter (don't know if thats what its called but its inbetween the tube and tube socket and allows you to check current and plate voltage; its kind of old and beatup though) under one of the power tubes. I hooked up the amp to the 4 10s but did not hook up the reverb. I turned on the amp and the tubes heated up fine. Everything appeared to be alright so I flipped the standby switch. Immediately a very loud hum/buzz came from the speakers for about two second and I immediately shut the power off but the fuse had already blown. Does anyone know what this could have been? My knowledge is somewhat limited on circuits and electronics so I have no idea where to begin trying to debug this problem. This kit was relatively expensive so I would rather not mess up a bunch of components like transformers and stuff.
Could the problem be with the bias meter between the tubes?
What could this problem be?
I feel I laid out the ground scheme fairly well and follwed the diagram from Hoffman's library.
I can post pictures if that helps anyone.
Advice????????

Michael

[vinman753]

Hi, Michael,
First, there are much better builders than me on this forum, but having built a few amps, and having had my share of problems, I know the feeling.
First thing I would do is to not only check the integrity of your connections but trace back each and every one that you don't have a crossed wire or an open ground anywhere or a short. Don't forget that a little piece of solder or a strand of wire can be your enemy. Take your time doing this even though you're anxious to try it out. Be sure to check the easy stuff like the power cord, the switch connections, a hot connection touching the chassis, etc.
After you've checked your connections and whether you've found a problem or not, pull the output tubes and the rectifier tube. You can leave the preamp tubes in. Make sure your bias adjust is turned down so you're not passing a too much current to the tubes. Turn the amp on and see if anything happens. If not, and no fireworks or blown fuse, shut it off and then just put in the rectifier tube.with NO output tubes in and check the voltages. That's my 2 cents and post what you have found or not found. If you're sure you found the problem then put the output tubes in and go for it.
Don't forget that there are HIGH voltages and if you are afraid or not sure, then don't do it and ask someone who is experienced for assistance.
I hope this helped, and be careful.

[ahamay79]

The first thing I would do is build a current limiter. Then you can "see" a short and speed up the detection process without damaging components and blowing a bunch of fuses.

[mjenki14]

Thanks for all the help! I have ordered some more fuses and am going to go through the entire amp today and see if I can find a short or anything. Is the limiter you were talking a light bulb limiter? I have seen diagrams for those but I don't have the materials to build them. Any more suggestions or insights on what the problem could have been? Is a short the most likely culprit?
Michael

[Diablo]

You can also use a multimeter to check the resistance of various parts of the circuit. They should match the schematic and component values. This can sometimes tell you if you made a mistake somewhere.

[txstrat]

Pull the power tubes (and recto tube?) and see if the fuse still blows.

1) If not, check the bias voltage on pin5 of the power tubes (lost bias causes the power tubes to draw too much current and blow fuses - with lots of HUM).
2) If so, check the wires to the power transformer and output transformer.

Report back.

Hope this helps

Matt

[ahamay79]

Yep, a light bulb limiter. Great bench tool.

[mjenki14]

Ok, I have some new developments. I went through and double checked the wiring of the build and didn't find anything. I adjust the bias pot opposite of how I had it and decided to give it another shot. I removed the power tubes and rectifier tube and turned the amp on. It started fine and no fuses blew. I checked the voltages on the heating wires and they were normal and all the preamp tubes were heating up. I flipped on the standby switch and measured the voltage on pin 5 of the power tubes. I was getting from -46V to -60V from how i turned the bias pot. I then adjusted so I was getting -46V on pin 5 and then put in the power tubes and rectifier. I turned it on, they heated properly and I hit the standby switch. I slight noise came from the speakers but sounded normal. I let it run for a while then tried to plug in a guitar. No sound at any volume level on either channel. I then tried tapping v1 and v2 and no sound from the speakers. I turned the amp off and checked my wiring for the input jacks and speaker jacks. All was normal. The speaker wiring was correct also, all four speakers wired in parallel. And now I'm stumped........
Does anyone have any ideas? Could I have ruined the OT during my first startup?? I really hope not.
Thanks again for everyones help and suggestions.
I can take some pictures of the build if that would be helpful to anyone (its pretty sloppy in there though)
Michael

[mjenki14]

Here are some pics

http://gallery.me.com/mjenki14#100054

[bnwitt]

I can take some pictures of the build if that would be helpful to anyone (its pretty sloppy in there though)
Michael

Well you need to start with that. If it is a sloppy build, how do you ever hope to troubleshoot it? Go through the whole amp and clean up the wiring. Do it right before you even flip the on switch. A high voltage holy grail amplifier is not a place to do sloppy work.

Look, there is an old saying in the amp building world. If it were wired right and all things correct, it would work. You keep saying you checked this or that and all was correct. Think about it, if all were correct, it would work. Obviously you have made one or more mistakes in the amp or it would be working. Sit down with the layout you received with the amp kit and go from one end to the other checking every single connection and every single component value. Did you use a 100K resistor where is 1 meg resistor is supposed to be, do you have a wire grounded that isn't supposed to be grounded, etc, etc. Triple check every single detail. Something is definitely not done correctly. If after that you haven't found anything wrong, post some high quality digital pictures of the guts of the amp and folks in here with more skilled eyeballs can take a look. But, do your homework first.

[octal]

It's probably time for some signal tracing and voltage measurements. I'll assume that you have a multi-meter, but don't have an oscilloscope, sinewave generator, etc. If you are familiar with the safety procedures needed to do high voltage measurements, you could start by checking that you have appropriate voltages on the screens (pin 4) and plates (pin 3) of each of the 6v6 output tubes. I would be expecting 400V+ on both screens and plates.

The loud buzz and fuse blow on your initial power up makes me think that some part drew enough current to smoke and open up, so I would tend to focus my suspicion on the output stage as a short in the preamp isn't going to blow the mains fuse.

Good luck, and report back with your findings.

[mjenki14]

I checked the plate voltage and am getting around 460v on screen and plates. On pin 5, I'm getting -51V. I'm in the process of checking the continuity of lead. I don't know if you can see it well, but would someone mind reviewing my input jack and speaker jack wirings? I had an issue with the jacks on my last build from Mojo and am wondering if the layout is wrong. I checked it against a weber layout, and they were the same. Any other info i can give to help? Thanks to everyone for helping with this
Michael

[bnwitt]

Something is still wrong with your layout.

[loudthud]

There are two types of input jacks available these days and the terminals are not in the same order. Since we don't know what jacks you have or what jacks the layout used, we'll just try resistance checks.

With power off and no cable plugged in, the grid of the tube (pin 2 or 7) should measure 34K to ground. With an open ended cable plugged into the #1 input you should measure 1.034Meg. With the cable in the #2 input you should measure 68K.

Next measure from the end of the cable where it plugs into the guitar. The #1 input should measure 1Meg between tip and ground. The #2 input should measure 136K.

Now measure between the tip of the cable and the grid pin on the tube socket. 34K on the #1 input and 68K on the #2 input. Measure both channels.

[martin manning]

I checked the plate voltage and am getting around 460v on screen and plates. On pin 5, I'm getting -51V.

Your voltages look normal now (and since you have voltage on the output tube plates, your OT primary is not open), but you didn't say that you corrected anything.

I'm wondering if you had a short caused by a stray strand of wire or some other lead dress issue that has been opened by moving things around. If so, it could easily happen again. I suggest that you inspect closely for such a possibility by looking for signs of arcing between terminals in the power supply and output tube areas.

MPM