Separate names with a comma.
Discussion in 'Amp Tech Center' started by Achysklic, Jun 16, 2020.
Why does the time matter? 5 minutes? 3 hours? Doesn't matter - still needs to get fixed
When I do a recap, I replace all electrolytics. I find that often times odd problems disappear with fresh e-caps....and I like to work on the circuit freely. Pull the old filters and there is NO more ions to bite. So, I do everything that is needed before putting ions into those caps. Get it done and I don’t have to go back and replace a 50 year old electrolytic cap. The amp works...it doesn’t develop problems in another year or whenever...because it is ready to run for another good while. People who have problems with old tube amps are, ime, those who don’t want to make the amp fresh again.
You need to check the bias on that amp, run the amp for a while, then check again.
The tremolo on a Princeton Reverb is bias vary, and if the bias is too high you will not get much effect.
If the bias does move, I'd suspect the one or both output coupling caps may be leaking DC dependng on the temperature. If the bias is not rock steady, I'd change those caps and rebias. If you're using a tech, it is a good time to have a bias potentiometer added.
I’m looking at the diagram looks like there are 2 flat .01 caps and a reg blue .022 does that sound right? You referred to it as a .02 so I didn’t know if you meant .022
It can be either. Not significant.
As said early on, and PaulG said, bias adjustment may do it. It's working, just doesn't come on all the time.
It might help to see some pics of your amp's board. I don't think it's usually 2 discs and a blue cap, but...
There are quite a few caps in a PR. Also, by the way, a 'complete cap job' by a good tech might legitimately not replace these little caps. The caps you're looking for are these...
These are usually all 'flat' (disc) caps, but you're right, the .02 may sometimes be a 'regular' cap. Here's a '69 with a brown blob in that spot.
I've had trems on other bigger fender amps be intermittent for sure. It's hard to decipher what's going on.... you're playing and suddenly the Trem comes on! Then you realize it's been switched on the whole time. On those amps (not bias driven trem) it seems often it's that .02 cap or the opti oscillator
It seems like a "blob" .02uF cap and two "disc" .01uF caps were the rule from 1968 on. Not always, though. Here is a 1969 Vibro Champ with doubled .01uF caps for the .02uF. They may have bought a truckload of ceramic .01's and only a pickup load of ceramic .02uF caps.
I had an issue with my PR where the tremolo was intermittent. It was quirky some times it worked and some times it didn’t to make it work I would use a rca shorting plug and use it in my trem pedal input and pull it out and it would work. But I knew something was wrong. After several attempts inside I finally discovered that one leg of one of the disk caps had broke between the leads and were ever so close that the electrons would jump. I used the chop stick method and tapped each connection in the trem circuit and finally found it and fixed it
Thought it over.
I would start by replacing the tremolo V4 12AX7. If that doesn't do it then I'd replace the tremolo cathode bypass cap (25uF 25v), and then the oscillator caps--I'd use two .02 and one .01 for extra slow tremolo.
That’s what I did after I required my 68PR as the tremolo didn’t work. I found the tremolo bypass cap was bad but while in there I changed all the necessary electrolytic caps including the can and of course changed out the 2 prong for a 3 prong ac plug and sniped the death cap. That’s when I found the tremolo to be intermittent and found the broken leg on one of the tremolo disk caps. Now I’m not sure but I assume the broken leg took out the tremolo bypass cap
I tried several other 12ax7’s in V4 no change. I just replaced all the bypass caps no change. Now I will replace the oscillator caps. Question should I use all flat disc caps? What’s in it now is 2 flat disc and 1 blue drop cap. Thanks for all the help
Hey @Achysklic - that was your 1000th post - congrats!
The amp doesn't care physically what the caps look like. Stick in whatever you've got that fits.
Finally got my caps in I’m going to replace the .01 with a .022 as you suggested. Which .01 do I replace? The top one or middle one?
Doesn't matter - whichever is most convenient to get to.
I’m replacing all 3 not want yo be one and done.
On the first page, I inquired as to whether or not a voltage chart had been made. Along with that, one would be interested in the bias numbers.
If the trem does not start working correctly after you replace those three capacitors, I am going to suggest that you start measuring voltages and the bias numbers. You also want to know the power tube plate dissipation. It would be interesting to note if there is any difference in certain voltages and the plate dissipation when the trem is functioning compared to when it is not.
Yes, IIRC the suggestion to measure bias/dissipation seems to have been made a number of times in this thread. Do you need more info on how to do that?
The odd thing here is it takes a long time (an hour?), but the trem starts to work eventually. Nothing much in the amp takes an hour to warm up, but when warming up cures or causes a problem, I tend to think of solder joints (especially on tube sockets but also other hot or expansile bits). As @Paul G. said, it's also possible the bias is changing as the amp heats up.
An echo in my brain was telling me that trem needs a certain imbalance in the LFO circuit to *start* it oscillating -- but is it even conceivable that would take as long as an hour, or that warming up some component could have an effect? Naaah, don't think so, Merlin says it may take several seconds...
The circuit for the tremolo is dependant upon the bias voltage. If too warm, no tremolo. As the amp warms up and the 6V6 gets tired, the bias will adjust again.
One question, did it work before you changed the components you changed?