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My current favorite amp, a SF Princeton Reverb, is exhibiting some very bad problems after about 4~5 years of perfect service. I use it on 3~4 gigs per week on average. The symptoms are a bad (and very unpleasant) distortion sound which develops after about 40~50 minutes of continuous playing time. The first thing I did was run my amp at home for over an hour and then I played thru it for about 20 minutes and it sounded great, with no problems. I took it to another gig after that but after about 40~50 minutes of continuous play, the same distortion developed. So I deduced that it wasn't just running the amp but it probably had to be played thru for the problem to present itself.
I then benched the amp. I tested all tubes on my B&K transconductance tester and ended up replacing the 12AX7 V4 PI tube as Fender amps are hard on PI tubes (that tube was only marginally low on one side of the dual triode). I last had the amp on my bench in August (2 months ago) to replace a switch. When it's on the bench, I measure all relevant voltages and re-set the bias to the power tubes. I keep a voltage chart for every one of my amps so I have a good database of numbers to compare. That time, 2 months ago, everything looked (and measured) fine and the amp has worked great in the subsequent 25 or 30 gigs that I used the amp on.
This time, the first thing I noticed was low B+ voltage. This amp normally has about 425 Vdc at B+ and I noticed it was low, about 382 Vdc. First, I replaced the rectifier tube (5AR4A/GZ34) with my brand new bench test tube and the resulting voltage was the same. I figured that the low voltage situation would generate some excess heat somewhere in the circuit. I did check all the heavy-lifting resistors in the amp (plate load, screen grid, filter) and all checked out ok. I ran the amp with my bench load for about an hour and the stock L010020 Fender power transformer did get hot, maybe a little too hot, I'm not quite sure - a gray area. The filter capacitor can, stock again, has not been changed out since this amp's mid-70's manufacturing date, probably time to do so. Next I went to R.G. Keen's excellent power transformer troubleshooting checklist and the power transformer passed every test. It might have an unusual internal short. I ran the amp without any tubes for about a half hour/40 minutes and the power transformer did not even get warm or have any vibration or have any audible hum. It seemed perfectly normal.
I guess my next step is to replace the cap can (luckily, I have another one in stock) and re-test after that, I guess I'll substitute another L010020 Fender power transformer into the amp - yes, I've got a spare one here also. These power transformers were used with quite a few Fender amps in the 70's - Champs, Vibro Champs, Broncos, and Princetons as well as Princeton Reverb amps. The Princeton Reverb put the greatest demand on this power transformer due to its higher tube compliment so these transformers have always run on the hot side in a PR.
I welcome any other informed comments, suggestions or help. Thanks for your time.
Bob M.
I then benched the amp. I tested all tubes on my B&K transconductance tester and ended up replacing the 12AX7 V4 PI tube as Fender amps are hard on PI tubes (that tube was only marginally low on one side of the dual triode). I last had the amp on my bench in August (2 months ago) to replace a switch. When it's on the bench, I measure all relevant voltages and re-set the bias to the power tubes. I keep a voltage chart for every one of my amps so I have a good database of numbers to compare. That time, 2 months ago, everything looked (and measured) fine and the amp has worked great in the subsequent 25 or 30 gigs that I used the amp on.
This time, the first thing I noticed was low B+ voltage. This amp normally has about 425 Vdc at B+ and I noticed it was low, about 382 Vdc. First, I replaced the rectifier tube (5AR4A/GZ34) with my brand new bench test tube and the resulting voltage was the same. I figured that the low voltage situation would generate some excess heat somewhere in the circuit. I did check all the heavy-lifting resistors in the amp (plate load, screen grid, filter) and all checked out ok. I ran the amp with my bench load for about an hour and the stock L010020 Fender power transformer did get hot, maybe a little too hot, I'm not quite sure - a gray area. The filter capacitor can, stock again, has not been changed out since this amp's mid-70's manufacturing date, probably time to do so. Next I went to R.G. Keen's excellent power transformer troubleshooting checklist and the power transformer passed every test. It might have an unusual internal short. I ran the amp without any tubes for about a half hour/40 minutes and the power transformer did not even get warm or have any vibration or have any audible hum. It seemed perfectly normal.
I guess my next step is to replace the cap can (luckily, I have another one in stock) and re-test after that, I guess I'll substitute another L010020 Fender power transformer into the amp - yes, I've got a spare one here also. These power transformers were used with quite a few Fender amps in the 70's - Champs, Vibro Champs, Broncos, and Princetons as well as Princeton Reverb amps. The Princeton Reverb put the greatest demand on this power transformer due to its higher tube compliment so these transformers have always run on the hot side in a PR.
I welcome any other informed comments, suggestions or help. Thanks for your time.
Bob M.
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