Princeton help

[496dart]

I have a silver face princeton here . Its seems to me it doesnt have a lot of volume--even on 10. SHouldnt this amp be somewhat loud turned up all the way?
It also has very faint distortion to it. you can hear it as a note decays . Heres what ive tried so far:

new speaker
swapped all tubes out
checked all resistors
checked all caps and paralelled a .1 across the .1 and .022 's in there. Paralelled a 25uf across the 25uf caps
cleaned the pots
moved all the wires around

Any ideas ??

possibly the OT ?

[Enzo]

You did all the consumer stuff, now start troubleshooting the circuit. What is the B+ voltage: at the output plates, at the output screens, at the other nodes for the preamp and phase splitter. What is the bias on the power tubes?

Apply a signal and trace it with a scope or an AC meter. Where does it diminish rather than grow?

is there reasonable plate voltage on all the triodes? How about the cathode voltages, all within reason? Any DC voltage on any grids? REmember that cathode followers and some phase splitters will have substantial DC on the grids, but at the same time there will be higher DC on the associated cathodes.

If the DCs are all OK, then trace the signal, a bad coupling cap could hang you up.

The amp ought to get loud, of course nothing like a Twin Reverb or something

[496dart]

i dont have any way to apply a signal to trace it--ill try to come up with something.

the voltages look ok to me
B+ was about 378v i believe
367/368v at the power tube plates
-29 at pin 5
Bias current is 20/26ma (6V6)

preamp:
V1 pin1=156v
V1 pin6 =151v
V2 pin1 =2.2v
V2 pin6 =179v

no voltage at grids
0 cathode volts

i mentioned before that i paralleled all the .1 and .022 caps,etc.. with a good cap. Wouldnt the amp come to life and get loud as soon as i applied that cap across a leaky or shorted cap ?

[496dart]

ok--i rigged up a small cd player to run some signal thru the amp. I probed everything inside of it. The ONLY place you hear any signal is when i touch the plates of the output tubes--BUT this signal is about 75% quieter than the normal signal directly to the speaker. That it---no where else i can hear anything .

[Rob Mercure]

Dart,

So when you apply the signal to pin 5 of the 6V6s you get nothing? Then your output stage isn't working. Enzo asked but got no reply so I'll reitterate - do you have any voltage/adequate voltage at your screen grid (pin 4). And is the bias current number you supplied for two tubes? If so the 10-13 ma/tube is only 3.5-4.5 watts per tube static dissipation - about 28-35%- and I suspect that the -29 bias voltage may have your output stage pretty well cut off. What you want is about 25 ma per tube or so to get in the 70% ball park (or at least not much more - some like as low as 50%).

A "QAD" (quick and dirty) signal source if you need one is the amps own filament tranny. Just clip a 1K or greater pot across the leads and use the wiper as a signal source - you can use your DMM to set the level to ground because you'll need the entire 6 volts to drive either output tube sufficiently enough to check out the stage (the voltage swing from the PI should be something around 30-40 V P to P so again the -29 seems way too high).

Rob

[496dart]

what im saying is i get about 20ma on one tube and 24ma on the other(bias).

NO signal when i apply the cd signal to pin 5. only at the plates.But very low

pin 4 voltage is 373vdc at each output tube

pin 5 is -29.6vdc at each 6V6

[Rob Mercure]

Well your static dissipation is a little low but not bad - you could probably drop the bias voltage a bit but this doesn't seem to be your problem. And your screen voltage is OK. So I assume that your signal source is capable of putting out a few volts? What happens if you - with on hand in your pocket - touch a screwdriver to pin 5 while touching the metal shaft. Any hum or pops?

Since you got a signal - as best I can tell - from the plates to the speaker then out OT should be good and your output tubes seem to be conducting.

Looking back at your post - no, paralleling a good cap across a leaky or shorted one - which is essentially resistor - should make little change. But paralleling one that was open would work. The leaky cap would need to be taken out of circuit and another cap substituted. But with the -29 on the outputs grids you probably don't have an leaky cap on this stage. Is there any way that the signal is getting shorted to ground? What are the values of your grid return ("grind leak") resistors? And how about the bias feed resistors? Your symptoms seem to be those of a signal short - AC - from pin 5 to ground.

Rob

[496dart]

Well your static dissipation is a little low but not bad - you could probably drop the bias voltage a bit but this doesn't seem to be your problem. And your screen voltage is OK. So I assume that your signal source is capable of putting out a few volts? What happens if you - with on hand in your pocket - touch a screwdriver to pin 5 while touching the metal shaft. Any hum or pops? NO--NO POPS OR HUMS

Since you got a signal - as best I can tell - from the plates to the speaker then out OT should be good and your output tubes seem to be conducting.

Looking back at your post - no, paralleling a good cap across a leaky or shorted one - which is essentially resistor - should make little change. But paralleling one that was open would work. The leaky cap would need to be taken out of circuit and another cap substituted. But with the -29 on the outputs grids you probably don't have an leaky cap on this stage. Is there any way that the signal is getting shorted to ground? What are the values of your grid return ("grind leak") resistors? And how about the bias feed resistors? Your symptoms seem to be those of a signal short - AC - from pin 5 to ground.

Rob


There is a VERY--very faint clik" when i touch pins 3 and 4 on the 6v6's. This is with the vol on 10.

ALL resistors in the amp test good.

[Enzo]

Did we verify the output tube cathodes are connected to ground? From pin 8 or the socket to chassis should measure zero ohms, or 1 ohm if there is a sense resistor.

Disconnect the OT secondary wires from the jacks and everything else. Now connect them directly to a speaker with clip wires. I am bypassing potentially shorted output jacks.

Touching pin 5 of the 6V6 ought to make a good hum.

[496dart]

Did we verify the output tube cathodes are connected to ground? From pin 8 or the socket to chassis should measure zero ohms, or 1 ohm if there is a sense resistor.

Disconnect the OT secondary wires from the jacks and everything else. Now connect them directly to a speaker with clip wires. I am bypassing potentially shorted output jacks.

Touching pin 5 of the 6V6 ought to make a good hum.


cathodes are grounded good

connected sec wires directly to speaker = still doesnt seem more than a 2 watt amp

touching pin 5 --NO sound

touching pins 3 and 4 gives a barely audible click

[496dart]

this sounds like a Laney 50w amp i had a while back. It did the same thing--very low output and i couldnt find anything wrong. Turns out it was the OT....

[Amp Kat]

You can do a simple resistance check on your output tranny with your meter. Check from center tap to each primary winding. These wires are almost always red,brown and blue. Red is the B+ lead and center tap. From red to blue and brown you should get around 30 to 70 ohms and from blue to brown you should get double that or 60 to 140 ohms. If your output tranny is bad it is shorted not open. If it were open you would get nothing. You may also go back to the Enzo approach and insert your signal to the input. Take a jack with two wires coming off of it. One for tip (signal) and one for ground (sleeve) and make sure they are not shorted. You can connect that jack to any point in the circuit where signal is and send it to the input of another amp or the return of another amp using it as a power amp. If it is strong on the first preamp tube go to the next. Right after the coupling caps are a good place because hopefully the voltage is low there and the risk of shock is reduced. You can hook it up with test leads and just flip the switch. Trace it until you get to the output and find out where it dissapears and you'll most likely find the problem.

[496dart]

Thanks so far for your help guys! Heres what i found today.

dark blue to red 154 ohms

dk blue to brown 308 ohms

red to brown 153.5 ohms

I ran a lead out of the princeton , connected to the cathode of V2. This is right before the 6v6 tubes. Ran that into the return of a 100w amp and then into a 4x12 cab. Everything seemed fine--nice and loud.

I reversed that and ran the fx send out of the 100w amp into the power section of the princeton. Very low output . sounded like 1 or 2 watts

[Amp Kat]

Then your loop jacks are corroded are not making good contact. You can try using a deburring tool or something to clean them off but it will most likely return in the future. Changing the loop jacks would be the way to go.

[496dart]

loop jacks ?? fx loop ? the princeton doesnt have one. The laney 100w has a loop but that amp works perfect.

:confused:

[Enzo]

OK, so we are dancing around a bad OT. Disconnect the OT promary wires and tack solder some other OT in its place. Just run the secondary right to a bench speaker. If the volume restores, then it is indeed a bad OT. It doesn't have to be the right OT, most anything will work well enough to tell us what we want to know. I have an old Fender Bassman 50 watt OT on the bench that I use for this test REGARDLESS of what the amp is. ALl it has to do is kinda work for 10 seconds, and I know the answer.

[496dart]

thats my next move enzo. i will post the results

[Amp Kat]

Sorry bout that dude but you said you sent it back to the power amp so I was thinking return. The ohmage value you gave is a tad high but seems somewhat legit. Let us know what you find on Enzo's test.

[Rob Mercure]

Dart,

If you've got a spare OT Enzo's got the test - if not -since you seem to have good continuity/resistance measures on your OT you might cobble up the shorted turns tester that Doc Horner provided on Ampage about 8 years ago - as best I know the info is available on R.G. Keene's site. 'Tis a simple device consisting of little more than a neon light bulb, a switch and a 9 volt battery. Basically you DC pulse a winding of the unloaded tranny and determine if you get a flyback ring by seeing if the neon bulb flashes. If not you've got a shorted turn which absorbed the induced pulse. A simple but elegant circuit.

Rob

[Rob Mercure]

Try this link:

http://www.geofex.com/

Look under Tech Tips.

Rob

[Rich M]

May I ask what you found out to be the problem with the Princeton? I know this was back in Aug of 2006 but I seen this here and really am running into the same problem with the faint volume and the fact all the voltages tested fine. Was it your OT or something else? All help is greatly appreciated. Thanks

[Valvehead]

May I ask what you found out to be the problem with the Princeton? I know this was back in Aug of 2006 but I seen this here and really am running into the same problem with the faint volume and the fact all the voltages tested fine. Was it your OT or something else? All help is greatly appreciated. Thanks


man...ive worked on so many amps since then, i honestly dont remember what the fix was ! sorry !

[Amp Kat]

Hey Rich ,post your exact problem,type of amp (model AB763 ?) theres several people here who may have run across it if everything here doesn't work. Theres only a few things that will cause loss of volume if everything checks out. Loose or corroded tube sockets,jacks, bad tubes,pots or a solder joint. One good way to troubleshoot it is isolate the preamp from power amp and cut your time in half. Since the SF doesn't have loop jacks you could make one off of the coupling cap after the peak inverter and send that to another amps input or return jack. If it's still low in volume it's back in the preamp somewhere.

[Rich M]

Thanks for the reply Amp Kat, The amp I built is from the original 5F2 schematic and layout from Fender. I also used Angela Instruments as a reference site also. This amp is wired 100% to the original layout. The only deviation was I added a standby switch. The voltages check out good . The cathode (pin 8) on the 6v6 is somewhat high around 27.5v and it should be about 18v. But this really shouldn't effect my problem. The amp has very faint volume. The tone control works good. No freaky things going on. Overall in my opinion I believe I have a bad OT. The 8 ohm or 4 ohm connection has the same low volume. I have around 410v on the plate and grid of the 6v6. The voltages except for the cathode one seem to be in great shape. I guarantee there are no wires crossed. It's built solid and no different that the othe rbuilds I've done. This time it just sorta went a little south with this low volume thing going on. Anyone have any suggesstions? Help!

[Rich M]

I briefly only mentioned the most important thing. The main problem with this 5F2 build is the volume is veryt very low and everything else checks out fine, voltages, etc. This goes with the last post I put here. Help!!!

[Enzo]

Without reading through the whole thread, before you replace the OT as a guess, what signal level is at the output tube grids? FOr that matter, have you traced the signal through the amp? A scope would be the best, but a signal tracer will work, or just set your meter to AC volts and use ut. APply a steady signal to the input and follow stage by stage and see where the signal goes away. If you have full signal at the output grids, then the OT is suspect, but if not much signal is there, then the amp has nothing to put out.

And just for science, disconnect the OT output wires entirely from the amp and connect them to a speaker with clip wires. Does that make any difference? A faulty output jack could short across your OT secondary. I had to diagnose that exact failure on a friend's home build.

[TD_Madden]

maybe I'm not understanding...what are "loop jacks" doing in a silverface princteon?

[Princeton]

I have a 64 princeton reverb and it has this crunch to it all the time... the clean tone is going down the drain. I just biased it and put in new tubes... Any ideas anyone?

[Don Moose]

Idea #1 - start a new thread. Link to this old one, sure, but it's almost 2 years old and kinda long to wade through for the new stuff.

(I don't mean to come off like a tool here - just trying to help your problem get the attenetion it needs)