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Old 8th March 2006, 09:58 PM   #1
jaron is offline jaron  United States
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Default pilot 264

Hi. First time ever posting on a forum. Here goes. I just got an old Pilot 264 tube stereo power amp that uses 4x 7591's and 4x 12ax7's. It was rated at 30 wpc. I brought the voltage up slowly on a variac when I powered it up and the power transformer got very hot and all the 7591's except one got very hot and the plates started glowing before I shut it off. The 12ax7's didn't glow as far as I saw. I know very little about electronics. Any suggestions for a non-techie? The main question I have is this: Does anyone know someone in Portland, OR that could look at my amp for me? Also, does anyone have a schematic for a Pilot 264 that they could email or send a copy of or know where I can get one? Thanks very much.

Jaron
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Old 8th March 2006, 11:12 PM   #2
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I don't have it, but the schematic can be found in Sams Photofact folder 583-9. Your library may be able to get it, or you can get it from many online vendors. A few:

Bob at findatube.com
Walt at infotronix.us
Michelle at Michelletroutman.com

Sounds like a pile of bad coupling caps or a failed bias supply.
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Old 9th March 2006, 12:47 AM   #3
lndm is offline lndm  Australia
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Yes, or perhaps the mains setting is incorrect. Measure the heaters as you bring it up to find out.
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Old 9th March 2006, 12:49 AM   #4
lndm is offline lndm  Australia
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Oh, and about those 12ax7's, if they have failed, they may be upsetting the power stage somehow.
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Old 9th March 2006, 07:36 AM   #5
jaron is offline jaron  United States
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Thanks for the replies. I will try the library then the online sources. I'll slowly try to check the things suggested, but I'm learning everything as I go. When Indm says that it could be that the 12ax7's have failed, does that mean that it's possible that all four are not working correctly or just some? And could that be the fundamental problem? In other words should I swap them all out and see what happens? Or could this be caused by a single bad power tube? Thanks again.
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Old 9th March 2006, 08:21 AM   #6
lndm is offline lndm  Australia
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Hard to say without looking myself.

What I will say is that the glow of 12ax7's is not as easy to see as most output devices. You should be able to see the top and bottom of the two heaters.

Yes, usually when something fails, there is a reason. Sometimes you can get lucky and just replace the faulty devices. It's a good thing you have a variac. How are the supply rails?
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Old 9th March 2006, 08:44 AM   #7
SHiFTY is offline SHiFTY  New Zealand
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You must make sure to connect a pair of speakers when warming up a tube amp, they don't like being run without speakers.

It sounds like you have lost the bias voltage on the output stage. This may require replacing some capacitors and rectifiers, however its a pretty easy fix. Simply replace all the coupling capacitors (usually around 0.1uF 600V or so) and any selenium recitifiers.

Also you might want to get a schematic, makes it much easier to see what is going on.
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Old 10th March 2006, 01:17 AM   #8
anatech is offline anatech  Canada
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Hi Jaron,
A failed 12AX7 will seldom create any more problems than not passing a signal.

Look for coupling caps leaking electrically, bad bias supply for the outputs and lastly, any selenium rectifiers (finned square structures).

As SHiFTY pointed out, there must always be a load on a tube power amp when it is powered up.

If you are not comfortable or use to working on tube equipment, have someone do this for you. Do not fail to read the safety thread at the top of this forum (sticky).

Lastly, welcome to our club! Enjoy.

-Chris
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Old 10th March 2006, 01:25 AM   #9
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Hey! A fellow Portlander! Welcome

I could take a look at the amp for you if you like,but I'm going to be out of town for a while,probably untill April 5th-ish.

It's probably the coupling caps,it's a pretty easy fix,you could probably do it yourself if you're handy with a soldering iron.
DO be aware of the high voltages present,and discharge the filter capacitors before you put your hands inside to do anything.
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Old 10th March 2006, 04:42 AM   #10
jaron is offline jaron  United States
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This is cool. Didn't realize how quickly and how much people would take interest. THANKS. Luckily I did have speakers connected to the two channels. This amp has a center speaker output. Should this have a load on it as well? Now I need to learn how to test the capacitors (I read the safety bit) and the rectifiers. I think I'm able to get a schematic at the local library. Can anyone recomend a site that has tutorials on how to test capacitors and/or rectifiers? Thanks for the warm welcome.
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