Oops. It would appear that I put a blown fuse in my amp (I've got quit a few of them laying around at this point) and then turned it on. I tried it with a new fuse and it did not blow the fuse. I have 7 volts at the heater. I'll connect the 5V rectifier heater now and and see if I blow the fuse.
Re: chassis ground blowing fuse
It sounds more like you've done something 'silly' when you changed the transformers - really it should be simple to find the problem, valve amps are simple anyway, and this particular problem is really a VERY simple one.
Might be simple, but it's hard to diagnose by remote
Could you post pictures of it?.
krzanik said:
It sounds more like you've done something 'silly' when you changed the transformers - really it should be simple to find the problem, valve amps are simple anyway, and this particular problem is really a VERY simple one.
Might be simple, but it's hard to diagnose by remote
Could you post pictures of it?.
This would be a good time to summarize what secondaries are connected, and where. The high voltage secondary is connected to the power supply, but not to the amp? The filament secondaries are both connected to the tube sockets, but are the tubes plugged in?
Have you got a schematic diagram of your power supply?
Have you got a schematic diagram of your power supply?
If you cut and paste this address: http://www.angelfire.com/vt/audio/se6aq5schem.gif into your browser you shoud see the schematic I'm using. It calls for 6AQ5's but I'm using 6V6GTs. I had to change the value of the capacitors a little to get the voltages specified because my power transformer is 300-0-300 rather than the 290-0-290 in the schematic. I'm trying to find someone in Austin who messes around with tube amplifiers as a hobby and might be able to help me out. I tried looking at the "members" section but couldn't search by location. I suspect someone with experience would have this problem sorted out in no time. I've tried to post pictures in the past but always get the square with a ? in it (maybe because I'm on an Mac?).
It has to be something to do with the new output transformer. I would disconnect it, then check again that there is no DC resistance from either of its windings to chassis ground. I bet there is. It could be an intermittent short, so jar it around a bit while checking, or loosen it a bit and wiggle it around.
You should be able to determine the problem without blowing another fuse. Throw away the blown fuses, by the way.
Please be careful with the high voltage. You don't necessarily need a tube amp person to help you, any competent electronics person would be able to help you find the problem.
You should be able to determine the problem without blowing another fuse. Throw away the blown fuses, by the way.
Please be careful with the high voltage. You don't necessarily need a tube amp person to help you, any competent electronics person would be able to help you find the problem.
Ok, This is what I found. I checked for continuity between the chassis ground and the 8ohm output (speaker binding post) and they are connected. I disconnected both the common and 8 ohm secondaries (primaries still connected) from the output xformer and checked for continuity between the common secondary to chassis and the 80hm secondary to chassis. There is no connection between each individual secondary (common, 8 Ohm) wire and chassis ground. I then clipped the leads onto the common and 8 ohm secondaries of the output xformers and they are connected on both output transformers. I checked my old output transformers and they are the same way (secondaries have continuity). Does this help?
Would it be practical for you to wire an incandescent lamp in series with the primary, or (as I have done) make a fixture to do it? I always power up first with the lamp in series. If there's a direct short, the lamp will glow brightly but no harm. There was a recent thread on this, I believe.
Sounds like a short on the B+ to me. Try the amp without the rectifier tube. If it doesn't blow the fuse there may be a B+ short. Since the problem appeared when you changed the transformers, I would look there. The new transformers could be miswired or bad. The Edcors are wound on a plastic bobbin, so a short from the windings to ground is unlikely, but a short from primary to secondary is possible. If the amp does not blow the fuse without the rectifier tube, but does with it installed, try disconnecting the output transformer primaries. If this stops the fuse blowing, reconnect one at a time to find out which one is causing the problem.
I tried the amp without the rectifier tube and with a 1.5 amp fuse. I had no other tubes in (is this correct?) and the output transformers wired up as I have all along. The fuse blew instantly. I'm not sure if this is usefull information. I think with the larger (1.5 vs 1.0 amp) fuse in and it blowing instantly I must have a short to ground. I don't see were it can be though. With no tubes in (including recitifier) how would power even get to the curcuit? Wouldn't this have to be a problem with the power transformer?
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