Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bohema79 on January 29, 2014, 06:29:24 pm
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Hi guys,
I have a 1966 Fender Princeton Reverb and seem to have an issue with the vibrato speed range.
Nothing seems to happen until around 4 on the knob.
From 1-4 the speed is that standard slow one.
:(
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Quite possibly leaky/borderline caps in the phase-shift oscillator. That would be my guess. A phase shift oscillator does not start robustly. If you have the vibrato pedal, you can sort of test this by turning the vibrato off and on and just see how quickly it starts, even at the knob rotation that does work. Or you can do this with a shorting RCA plug in the footswitch jack. I suppose it could also be a very weak tube. You could also test this by swapping a different 12AX7 in the trem tube (V1) oscillator socket and see if acts differently.
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Nothing seems to happen until around 4 on the knob.
From 1-4 the speed is that standard slow one.
What does that mean??? Are you saying you have no tremolo with the speed set below 4? Or you have tremolo at some slow speed but it doesn't change speed until about 4 on the dial? Please clarify.
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As he's talking about the speed control I believe he means the latter
He has the slowest tremelo speed up till 4 on his speed pot and then it starts increasing
Or so I understand it
It could be a dirty or worn pot too
Have you tried some cleaner in the pot?
My Father says always start with the simple things first...strangely I was his last child :l2:
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Hi Guys,
Thanks for the reply.
Yes, I mean that up until 4 on the dial I have the slowest speed of the vibrato, after 4 i it seems to be working fine.
The tube is fine (Ive tested a few) and the pot is cleaned.
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You may need to replace 3 capacitors or tremolo circuit . Two 0.01 mfd and one 0.02 mfd . These are cheap and don't waist time to find wich one is defect , replace them all.
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Change the pot.
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Hi.
Thank you I will replace them.
Can someone tell me what for are responsible the large "blob" .1 and the other ceramic .02 capacitors?
Thanks.
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I sounds like you have an audio log pot instead of a linear one...
Philippe
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Do you realy think thay might have made the mistake at the factory back in 1966?
The pot is stock vintage. Did they put any markings on pots to confirm that?
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The speed pot should have a reverse audio taper. It's not a linear pot.
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The speed pot should have a reverse audio taper. It's not a linear pot.
my bad, I was thinking intensity pot...
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Pots are relatively cheap and easy to change
I'd 2nd Sluckey and suggest you swap it out
You can always put back the stock one if it doesn't make a difference if you're at all keen on stock parts
The caps sound like a relatively easy thing to try too.
Is it an abrupt change to a faster speed at 4? (missing speeds 1-4)
Or does it start speeding up in the normal gradual fashion past 4 ?
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I will try a new pot.
Im missing speed from 1-4, from then it speeds up normaly.
Thanks