1969 SFPR - Tremolo Issue

Blues_N_Rock

Member
Messages
221
I have a 1969 Silverface Princeton Reverb. It sat unused for about a year. I recently plugged the amp into the outlet, without a signal going into the amp, and you can hear a faint sound of the tremolo. If I adjust the intensity higher I can clearly hear it through the amp with no guitar plugged in. If I adjust the volume it follows. The reverb has no effect if adjusted. With the volume set to 0, I cannot hear the tremolo, but there is slight hum. It is not a 60 hertz cycle hum. This was not the case a year ago. By the way, the bias cap and the cap can were replaced about 5 years ago. Anyone has an idea what is going on with the amp and the tremolo?
 

HotBluePlates

Member
Messages
10,276
... without a signal going into the amp, and you can hear a faint sound of the tremolo. If I adjust the intensity higher I can clearly hear it through the amp with no guitar plugged in. If I adjust the volume it follows. ... With the volume set to 0, I cannot hear the tremolo, but there is slight hum. ... Anyone has an idea what is going on with the amp and the tremolo?
Your description is somewhat unclear; it sounds like you are saying the tremolo works, but I don't follow why you're talking about what it does/doesn't do with the Volume at 0.

If the tremolo is not strong when you play a guitar through the amp with the Volume above 0, try installing a known strong 12AX7 in the socket next to the 6V6s. If that is not enough to have strong trem, the amp needs further tech-work.

... the amp with no guitar plugged in. If I adjust the volume it follows. The reverb has no effect if adjusted. ...
I don't know if the above means there is no reverb, or only that the control does nothing with no guitar plugged in. Just focus on whether the reverb works with a guitar plugging in and the Volume turned up.

... there is slight hum. It is not a 60 hertz cycle hum. ...
Hum could be caused by about 5 different things. If you don't have a scope or signal tracer, best to leave the hum-chasing to a tech.
 

slider313

Gold Supporting Member
Messages
8,210
You will hear the tremolo if the footswitch is in the "on" position. The Princeton has bias variable tremolo. The hum may be from a mismatched set of power tubes.
 

Blues_N_Rock

Member
Messages
221
Sorry for the confusion and thank you for your quick responses. I solved one issue. The tremolo can be heard without any guitar plugged into the amp. It is not loud as in when a guitar is plugged in and strummed. This is without any instrument. The issue was that I did not have the vibrato/reverb pedal plugged in. I connected my pedal and the tremolo is no longer heard unless I engaged it with the pedal. This is solved.

My other issue is still there. Continuous humming sound. With my guitar plugged in you can also hear what appears to be a microphonic tube. Just ringing. Can that be the cause of the humming?
 

slider313

Gold Supporting Member
Messages
8,210
Yes, a tube can cause hum also. You can try another tube, if you have one, or switch the microphonic tube with one of the other 12ax7's in the amp to see if that helps.
 

Blue Strat

Member
Messages
30,152
Does the volume control (or any control) affect the loudness of the hum? Does it hum with the guitar plugged in, unplugged, both? If it hums with the guitar plugged in does turning the guitar volume down help?
 

Blues_N_Rock

Member
Messages
221
I swapped out V1 with a known good 12AX7 and still the same issue.

Nothing plugged in except the vibrato/reverb pedal.
(1) Reverb at 1 & volume at 1 and the hum is faint.
(2) Increase volume to 10 with the reverb at 1 and the hum is louder.
(3) Set reverb to 10 and volume to 1 and the hum is louder. About the same as (2).

Plugged in my guitar without strumming. Setting volume to 1 and reverb at 1. Increased slowly the volume and once it hits around 6 their is a self-induced hum. It begins to get increasingly louder without adjusting the volume. Shut down.
 

pdf64

Member
Messages
7,251
My guess is that issue is oscillation, facilitated by the ‘D’ section of the can cap going bad. It’s not a good idea to allow ecaps to completely deform, ie by not charging them up in 12 months. Usually will get away with it, or it will reform ok, but perhaps not always.
 
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Blues_N_Rock

Member
Messages
221
Thanks pdf. Is there anyway of testing to verify the can cap is going bad? I can borrow an ESR meter from my brother-in-law. Would that work for this application?
 

pdf64

Member
Messages
7,251
Maybe, if it tested bad then it would be bad, but ‘good’ at 9V doesn’t equate to good at >250V.
The sure test is to tack another 10-22uF of suitable voltage rating in parallel with the D section, reassess, then C section and reassess etc.
 




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