Help Needed With 1966 BF Princeton Reverb Problem!!

[Pickergibson]

My 1966 blackface Princeton reverb went out on me Friday night on the last song. I had no output so I looked in the back of the amp and noticed the 6v6gt’s were pulsating as if in time with the tremolo so I shut it down. First let me say one month ago I serviced the amp. All the coupling caps were replaced with new 50uf@50v and I replaced the electrolytic can with a new 20/20/20/20 @ 500v and the amp sounded absolutely killer for an entire month. So far I’ve checked everything I know to check. I checked all the original blue Mallory caps and I checked the little disc cap associated with the tremolo circuit thinking this has something to do with the tremolo considering it’s pulsating. Of course these PRs are fixed bias with the cathodes grounded and the grid runs to the tremolo intensity pot so I checked the intensity pot and it checks perfect. My plate current was running 33 and 34 milliamps when I finished servicing the amp a month ago and now it’s cut almost exactly in half with my analog milliamp meter needle fluctuating up to 18 and down to about 13 and speeds up/slows down as I adjust the intensity control?! All tubes test excellent and I even switched the power tubes just to see if it changed anything and it didn’t. If some of you have experienced this problem and can set me on the right track it would be much appreciated! Thanks

 

[corliss1]

Can you clarify what you mean by "went out on me" - does the amp no longer produce sound?

Tubes glowing in time with the trem isn't necessarily a bad thing. Any tube that has a blue glow to it may do this, or you may just notice the brightness change a bit. Now...if the tubes were redplating, that's bad, but it doesn't sound like that is what you were seeing since you said this happened with multiple tubes.

 

[King Fan]

Welcome to TDPRI. If it helps, I always switch off the tremolo while measuring voltages. If your plate current has been cut roughly in half, that may be a clue.

Just WAGs, but 1966 is a long time back. You were smart to update the electrolytics; did you replace the bias board cap? Can you measure the bias voltage at the 220K junction? For that matter, how about voltages in general, and especially all the voltages on the 6V6s?

 

[jumpbluesdude]

I’m entertained by people who insist on changing the can filter caps when they are working fine. If the part is good, why does the age of a part matter? It doesn’t. The Cornell Dublier caps in 60’s Ampegs are almost always bad. The blue Ajax caps in Blackface Fenders are almost always good. Same age. I have a ’63 Fisher 800c with original Mallory can caps and it works perfectly. If you change them out when they are good, it’s like throwing out 30,000 rated tires with 3/8 inch of tread because you drove 30,0000 miles on them.

 

[corliss1]

@jumpbluesdude - that isn't how electrolytic caps work. 15-20 years max, as stated by the manufacturers. It's a maintenance item, no big deal, swap em out. There's no magic in old caps.

 

[jumpbluesdude]

Put them in a box and send them to me. I’ll keep using them and laughing. 60 years and still going in that Fisher. That’s 3 or 4 caps changes you do that I don’t. No hum, no noise, no loss in functionality, no lifting it onto the bench and risking dropping it or damaging tubes, no chance of electrocution, no wasted time changing a good part, and the thing I like best - about $200 in my pocket. You do you, I’ll do me.

 

[King Fan]

@Pickergibson , you're new to the forum; be aware you'll get a variety of opinions. I'd say you were smart to change the e-caps; it's why I mention the bias cap. The huge majority of folks I respect here and across the amp world would support you. Arguments citing non-electrolytic caps, like blue molded 'Ajax', aren't relevant.

The reasons to change old e-caps are numerous and well-documented; this article lists causes of e-cap failure and loss of function, noting age drives the most common ones.

The Causes of Electrolytic Capacitor Degradation

Learn what causes electrolytic capacitor degradation and how this degradation impacts the useful life of electrolytic capacitors.

resources.pcb.cadence.com

Their summary says, in part:

• planned maintenance, replacement, or swapping of electrolytic capacitors should be scheduled at regular intervals.
• The useful life of a capacitor is given in the electrolytic capacitor datasheet

 

[zambiland]

I’m entertained by people who insist on changing the can filter caps when they are working fine. If the part is good, why does the age of a part matter? It doesn’t. The Cornell Dublier caps in 60’s Ampegs are almost always bad. The blue Ajax caps in Blackface Fenders are almost always good. Same age. I have a ’63 Fisher 800c with original Mallory can caps and it works perfectly. If you change them out when they are good, it’s like throwing out 30,000 rated tires with 3/8 inch of tread because you drove 30,0000 miles on them.

How does this help the OP?

 

[Pickergibson]

@Pickergibson , you're new to the forum; be aware you'll get a varying opinions. I'd say you were smart to change the e-caps; it's why I mention the bias cap. The huge majority of folks I respect here and across the amp world would support you. Arguments citing non-electrolytic caps, like blue molded 'Ajax', aren't relevant.

The reasons to change old e-caps are numerous and well-documented; this article lists causes of e-cap failure and loss of function, noting age drives the most common ones.

The Causes of Electrolytic Capacitor Degradation

Learn what causes electrolytic capacitor degradation and how this degradation impacts the useful life of electrolytic capacitors.

resources.pcb.cadence.com

Their summary says, in part:

• planned maintenance, replacement, or swapping of electrolytic capacitors should be scheduled at regular intervals.
• The useful life of a capacitor is given in the electrolytic capacitor datasheet

Believe me this PR has been my go to amp for 12 years and I fought til the bitter end before I ever replaced a single capacitor but the original electrolytic finally gave out on it recently and I couldn’t stand the hum anymore. It idles nice and quiet now. Yes I did replace the bias cap along with all 7 of the 25uf25v coupling caps to 50uf50v Sprague Atoms. To the guy who believes the Ecaps should be left alone all I can say is you’ve apparently never had an amp die in the middle of a gig. Its best to do routine maintenance on these amps and replace the electrolytic and coupling caps at least every ten years unless you bought your amp just to sit back and drool over those original parts

 

[Pickergibson]

My 1966 blackface Princeton reverb went out on me Friday night on the last song. I had no output so I looked in the back of the amp and noticed the 6v6gt’s were pulsating as if in time with the tremolo so I shut it down. First let me say one month ago I serviced the amp. All the coupling caps were replaced with new 50uf@50v and I replaced the electrolytic can with a new 20/20/20/20 @ 500v and the amp sounded absolutely killer for an entire month. So far I’ve checked everything I know to check. I checked all the original blue Mallory caps and I checked the little disc cap associated with the tremolo circuit thinking this has something to do with the tremolo considering it’s pulsating. Of course these PRs are fixed bias with the cathodes grounded and the grid runs to the tremolo intensity pot so I checked the intensity pot and it checks perfect. My plate current was running 33 and 34 milliamps when I finished servicing the amp a month ago and now it’s cut almost exactly in half with my analog milliamp meter needle fluctuating up to 18 and down to about 13 and speeds up/slows down as I adjust the intensity control?! All tubes test excellent and I even switched the power tubes just to see if it changed anything and it didn’t. If some of you have experienced this problem and can set me on the right track it would be much appreciated! Thanks

Thanks for your attempts to help guys but I figured it out tonight. I found that my 22k bias resistor had drifted on me. I unsoldered it and found it was reading 29k. I went ahead and installed a nos matched pair of 1960s RCA 6v6gta power tubes and replaced the bias resistor with a new 22k. Afterwards the plate current was at 30 mA…a bit too cold for my liking so I settled in with a 20k resistor and now my bias is running 34 mA and I’m back in tone heaven! 34 mA seems to be the magic number for me. I like clean Tele sound with no distortion. It would be easy to install an adjustable bias pot but to be honest I’m in my mid 60s now so I’m not sure I’ll outlast this fresh nos pair of 6v6’s considering the original RCA power tubes lasted 57 years

 

[King Fan]

Glad you figured it out. Selfishly, I also feel better knowing my “attempt to help” included suggesting you measure the bias voltage.

 

[jumpbluesdude]

Believe me this PR has been my go to amp for 12 years and I fought til the bitter end before I ever replaced a single capacitor but the original electrolytic finally gave out on it recently and I couldn’t stand the hum anymore. It idles nice and quiet now. Yes I did replace the bias cap along with all 7 of the 25uf25v coupling caps to 50uf50v Sprague Atoms. To the guy who believes the Ecaps should be left alone all I can say is you’ve apparently never had an amp die in the middle of a gig. Its best to do routine maintenance on these amps and replace the electrolytic and coupling caps at least every ten years unless you bought your amp just to sit back and drool over those original parts

It failed, so you replace it. You needed to do that in a failed part situation, but I can tell you that as an USAF trained electronics technician who supported shuttle missions and rockets putting satellites into space, the Air Force changed parts when they failed and not before.

It absolutely works that way.

Thirty years working on amps, vacuum tube military TACANs and other electronics, I’ll use the best quality working part at my disposal. Treating all electrolytics the same is like treating all tubes the same. Mullard 12AX7s last a lot longer than the 2000 hours they are rated. It’s not just the sound, it’s the quality and longevity. Funny how people have no problem using those until they fail. You can still find 60s Marshalls with the original Mullards in them. The Mallory can caps are like the Mullard tubes. People who know amps well, know this. Most other electrolytics are approaching or past end of life and should be changed.

There is a difference between best practice and practical knowledge.

The FACT is some products are made better quality and last longer than others. They transcend best practices.

 

[Jasonpatrick]

 

[Mexitele Blues]

It failed, so you replace it. You needed to do that in a failed part situation, but I can tell you that as an USAF trained electronics technician who supported shuttle missions and rockets putting satellites into space, the Air Force changed parts when they failed and not before.

It absolutely works that way.

Thirty years working on amps, vacuum tube military TACANs and other electronics, I’ll use the best quality working part at my disposal. Treating all electrolytics the same is like treating all tubes the same. Mullard 12AX7s last a lot longer than the 2000 hours they are rated. It’s not just the sound, it’s the quality and longevity. Funny how people have no problem using those until they fail. You can still find 60s Marshalls with the original Mullards in them. The Mallory can caps are like the Mullard tubes. People who know amps well, know this. Most other electrolytics are approaching or past end of life and should be changed.

There is a difference between best practice and practical knowledge.

The FACT is some products are made better quality and last longer than others. They transcend best practices.

Totally agree with this. In fact, my 98 Ranger is on the original tires at 190,000 miles. Everybody says I'm going to kill someone but I know better.

 

[Dacious]

It failed, so you replace it. You needed to do that in a failed part situation, but I can tell you that as an USAF trained electronics technician who supported shuttle missions and rockets putting satellites into space, the Air Force changed parts when they failed and not before.

It absolutely works that way.

Thirty years working on amps, vacuum tube military TACANs and other electronics, I’ll use the best quality working part at my disposal. Treating all electrolytics the same is like treating all tubes the same. Mullard 12AX7s last a lot longer than the 2000 hours they are rated. It’s not just the sound, it’s the quality and longevity. Funny how people have no problem using those until they fail. You can still find 60s Marshalls with the original Mullards in them. The Mallory can caps are like the Mullard tubes. People who know amps well, know this. Most other electrolytics are approaching or past end of life and should be changed.

There is a difference between best practice and practical knowledge.

The FACT is some products are made better quality and last longer than others. They transcend best practices.

So that doesn't explain Challenger which was the Morton-Thiokol lowest-bid 50c o-rings - bean counters literally dollar and centing to death.

But maybe it explains Columbia to some degree, given it was on its 28th mission.and so were some of the reusable parts. Maybe some of them should have been swapped out before lethal pyrotechnics.

So if it's a fact the manufacturer of the e-caps suggests you replace them by a designated expiry date, you should ignore it?

'There is a difference between best practice and practical knowledge.'

This is true. And you may have the knowledge and experience to take the risk for yourself, but what would you say if someone without those things blithely takes your advice and loses a vintage power trans to a shorted e-cap like I've seen?

 

[Jasonpatrick]

Just gonna set this here…

 

[King Fan]

the Air Force changed parts when they failed and not before

I don't do that with my car. Heck, I don't do it with guitar strings, razor blades, or milk in the fridge. And you're saying they do it on airplanes?

 

[SnidelyWhiplash]

How does this help the OP?

This isn't the first time that jbd has argued his point on this forum. Maybe the cap can will last, maybe it won't. If it does fail, one risks taking out the transformer. I'd rather not risk such, especially on a BFPR. if the bias resistor & or e cap fails, the output tubes will likely go kerflooey. The parts in ? are cheap & will provide extra insurance against failure.

Win, win...

 

[Michael Smith]

A couple years ago I had a cap fail (or it could have been the adjacent resistor first) in a 1976 Super Twin Reverb that had not been played in over 5 years. Based on the values of the caps and the presence of a diode nearby, I assume this is the bias supply. The Super Twin Reverb schematic is very complicated, and I haven't found a layout diagram. Luckily I was able to quickly shut the amp off without any damage to the matched quad of Svetlana 6L6GC's.

Since that happened, I make it a point to change out ALL of the electrolytic caps in any older Fender amp I purchase.

 

[corliss1]

We know JBD is a troll - best not to feed them