When the amp is first fired up, the fidelity is very good and loud, however as the heat in the amplifier builds, the output volume will decrease - not by much, but I will end up haveing to bump the volume up another 15% or so.

While this is not too severe of an issue, the 2nd issue has become very apparent recently and is more important. That is, the fidelity of the audio really goes to crap after 2 or 3 reels. The best way to describe it would be if a noise reduction circuit was too strong. only loud passages are amplified and discernable. Dialogue gets lost.. what comes out of the speaker is only the loud part of a performer's speech. the soft portions fall off completly into the noise floor. Another symptom would be that the audio that does make it sounds 'fuzzy' or 'snowy' I have taken compressed air and cleaned the sound drum area and exciter lens very well. I don't think it is an alignment issue because this happens with both variable density and variable area soundtracks. Yesterday I tried switching out some tubes with ones I happened to have in a box, but the problem did not change at all. I am trying to determine if I have a failing PE cell or a bad tube or cap in the amplifier. I don't know much about vacuum tube electronics so any help would be appreciated.

some of the tubes in the amp that I can remember off the top of my head are:

5U6 (tried changing with NOS)
5881 (two of them.. haven't tried changing)

6N7 tried changing with NOS
6J7 havent changed

I am sorry that I don't have the amp model number. I can get that if it would help someone help me.. as well as verify all of the tubes.

I guess what I am asking is if anyone has seen this symptom and knows right off what it might be.

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Or use a voltmeter to monitor the bias voltages on the tubes as the amplifier heats up.

I suspect capacitors that are over 40 years old are pretty deteriorated, and should be replaced anyway.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

As for getting it repaired, I would seek out someone who does tube Hi-Fi repairs/rebuilding, and avoid the guitar/Hammond techs as they are more into building amplifiers that will sound good with guitars and tonewheel organs and may not have the 'ear' for what a film projection amp should sound like (even in mono). If you are interested in bringing the amp up north, I might be able to have a look at it myself.

Good Luck!

Aaron

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It could also be you have a coupling capacitor that is leaking, which will drive the next stage nuts. If you know a little about tube theory, I would suggest checking the plate voltage, cathode bias voltage and control grid voltage when you fire the system up, and do the same when it starts deteriorating. You will probably notice a change somewhere.

Shorting or leaking of the screen by-pass capacitors will also cause some major problems with distortion and gain, so I suggest you check those voltages as well.

A significant change of screen grid voltage could be caused by the capacitor leaking, or the screen dropping resistor changing its value.

Unless the amplifier employs direct coupling, the DC voltage on the control grid in the quiescent state should be ZERO, but if the output is push-pull, the control grid of the phase inverter may ride at a positive voltage. It should never be more positive as the cathode in a class "A" condition.

In vacuum tube applications, bias is the DC difference in potential between the grid and the cathode, using the cathode as a reference.

Again, in class "A" applications, the control grid should never be positive or driven positive with respect to the cathode. If that condition occurs, it'll draw grid current and you'll have distortion up the gazoo.

If it is, you more than likely have a leaky coupling capacitor.

Many vacuum tube amps have a degenerative feedback loop that uses resistors and capacitors. Beware of those. If those capacitors start shorting out, they, too, can feed a positive voltage back to the control grid and cause some major problems.

I might add that your PE cell may be going south. Have you replaced it yet? They do get gassy.

Have fun....and good luck.

Get their catalog! it's GREAT!

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I remember you used to just go buy a regular vacumn tube and be done.. now, some are sold as a "special" matched set in a fancy wooden case.