• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

pilot 264

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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 :)
 
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.
 
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?
 
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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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
 
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. :)
 
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.
 
As a matter of course, you should replace all electrolytic capacitors outright. They are probably the most unreliable devices once old. They dry out and then...:bigeyes:

Paper capacitors also dry out. If they are used as coupling capacitors, they can allow DC on to the grid of the next stage. This may give the result you are seeing (I have seen this too). Best to replace them with modern film capacitors but don't throw them out just yet. If they happen to still be serviceable, some will agree they sound very good.

So, in conclusion, just replace them.
 
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Actually, the easiest rule is to replace all coupling caps and turf (throw out) the old ones. They will fail and they are now over the shelf life. Why risk it?

Lukas,
The coupling capacitors don't dry out as much as they allow DC current to flow. Stick a couple hundred voltas across on with a 100K load on the other side (a forgiving test, most might try a 1 M load). Measure the DC voltage on the loaded side with a high impedance meter. Leakage allows a voltage to be measured on the "other" side.

Jaron,
Testing capacitors properly is time consuming and expensive. My meter cost about $6,500 CDN a few years ago, the kelvin lead set was over $500. Mine doesn't give you the whole story, but it's a good indicator. Cheaper products tell you even less. So to test, measure the voltage on the grid side. Any positive voltage is a fail, lack of voltage does not mean the cap is good. Follow? Just change them.

Selenium rectifiers, those things with the plates, just replace them. If they have not failed, don't worry - they will for sure. Rectifiers are cheap these days.

-Chris
 
Okay. I'm convinced. Change out the capacitors and rectifiers. So, any recomemdations on what to get and where to get them? I'm not on a super low budget, but not looking to spend a fortune. I've found a schematic and am attaching it. Sorry for the low quality. OOPS! FILES TOO BIG. If you're interested let me know and I'll email it as well as any images of the amp you want. The local library copied it and sent it to my door free of charge. Nice. Here's what's printed on the largest cap which I suspect may be the culprit due to it's card board casing that doesn't seem to have any kind of seal like other paper caps I've seen:

24-188
(sideways D symbol) 1000mfd 35vdc
(square symbol) 100mfd 35vdc
(triangle symbol) 100mfd 35vdc
-compos 1081 6023
CAN NO CONNECTION

And there's two more large ones with metal casings with identical values:

24-184
(sideways D symbol) 40mfd 450vdc
(triangle symbol) 80mfd 450vdc
CAN COM NEG
1081 6021

I'm still not certain what the rectifiers are but I'm researching that. Thanks.
Jaron
 
The old ones have multiple sections in a can. I usually like the sound of a Rubycon or an Elna electrolytic. Look at Black Gate (by Rubycon) too.

_The_ way to go on a budget IMO, is motor start capacitors. They are paper or polypropylene, they are foil, they are available in 30uF give or take a couple dozen uF. Able to handle more than 500VDC, sound better than electrolytics and cost $10 or so apiece.

You may not have any selenium rectifiers in your amp. They are usually brown, look like an elongated 2W carbon comp resistor but don't have colour bands.
 
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Hi Jaron,
The cardboard is actually over the metal can. They do that if the capacitor's negative terminal is biased to a voltage other than ground. If the manufacturer has stock of these, they will use them rather than order others if the case is grounded. You can replace them with normal electrolytics (they will fit under the chassis unlike motor start capacitors).

Selenium rectifiers can be any colour, green is common too. They normally have plates.

-Chris
 
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