how to disable princeton reverb vibrato

[texwest]

I'm wanting to make my princeton reverb have a bigger sound for low volume jazz playing. I will be installing a new 12inch baffle and speaker. And will be putting in an adjustable bias.

i'm thinking that disabling the vibrato might make it sound bigger. How do you do this? Do you guys have any other ideas for getting a bigger clean sound?

Thanks

Wes

[sloblues]

disabling vibrato

you can replace the 50K RA Intensity Potentiometer with a 50K RA that includes a switch (hoffman amps carries these pots), this wiull allow you to disconnect the Vibrato circuit or have it on. The simple method is to unsolder the yellow wire connected to the right most terminal ( looking at the back of the pot) of the 50K RA intensity pot. leave the wire in place but tape off the bare end with electrical tape, that way very easy to restore the vibrato circuit.
This will certainly add gain and richness and a bit of rawness to the amps sound.

[Bruce / Mission Amps]

you can replace the 50K RA Intensity Potentiometer with a 50K RA that includes a switch (hoffman amps carries these pots), this wiull allow you to disconnect the Vibrato circuit or have it on. The simple method is to unsolder the yellow wire connected to the right most terminal ( looking at the back of the pot) of the 50K RA intensity pot. leave the wire in place but tape off the bare end with electrical tape, that way very easy to restore the vibrato circuit.
This will certainly add gain and richness and a bit of rawness to the amps sound.

Uh oh... Wrong.
This is amp does not use an LDR vibrato circuit and disconnecting the vibrato from the circuit will not do anything to the gain of the amp.
If you want a little more gain in this amp with out having to rewire the whole thing, pull off the wire from the output jack that goes to the NFB resistor on the eyelet board and insert a 4k7 to 10k resistor in series with the wire.
If you want a little more mid range punch pull the 6800 ohm resistor off the bass pot and use a 10K to 15K.

[texwest]

Uh oh... Wrong.
This is amp does not use an LDR vibrato circuit and disconnecting the vibrato from the circuit will not do anything to the gain of the amp.
If you want a little more gain in this amp with out having to rewire the whole thing, pull off the wire from the output jack that goes to the NFB resistor on the eyelet board and insert a 4k7 to 10k resistor in series with the wire.
If you want a little more mid range punch pull the 6800 ohm resistor off the bass pot and use a 10K to 15K.

Thanks alot Bruce. I was wondering if the vibrato was like the bigger blackface amps and was sucking tone off the signal. I guess not.

I am going for a big round jazz tone while playing a strat. I want bigger fatter sound not distortion. Would anything like the stokes mod or the paul e. mod help with this? Or would the simple resistor changes you mentioned work better?

thanks

[texwest]

you can replace the 50K RA Intensity Potentiometer with a 50K RA that includes a switch (hoffman amps carries these pots), this wiull allow you to disconnect the Vibrato circuit or have it on. The simple method is to unsolder the yellow wire connected to the right most terminal ( looking at the back of the pot) of the 50K RA intensity pot. leave the wire in place but tape off the bare end with electrical tape, that way very easy to restore the vibrato circuit.
This will certainly add gain and richness and a bit of rawness to the amps sound.

Is this the correct way of doing this in an AB763 circuit? I hear that Stevie ray disabled his vibrato to get a bigger tone. Is it really this simple of a mod to get the bigger tone?

[Bruce / Mission Amps]

Is this the correct way of doing this in an AB763 circuit? I hear that Stevie ray disabled his vibrato to get a bigger tone. Is it really this simple of a mod to get the bigger tone?

Yes the 50K level pot in the vibrato circuit dumps a nice chunk of the raw audio signal to ground so lifting it from ground will give it all back for more drive into the phase inverter.
A nice trick is to use a 50K or 100K pot with a push pull on-off switch.
The pot is held above ground with a 33K to 56K resistor and the switch shorts the resistor back to ground on both ends so you loose the extra gain. Simple.

[Bruce / Mission Amps]

Thanks alot Bruce. I was wondering if the vibrato was like the bigger blackface amps and was sucking tone off the signal. I guess not.

I am going for a big round jazz tone while playing a strat. I want bigger fatter sound not distortion. Would anything like the stokes mod or the paul e. mod help with this? Or would the simple resistor changes you mentioned work better?

thanks

Yes increasing the B+ voltage to the phase inverter and then using DC bias on it (the P C MOD) instead of cathode bias will slightly increase the balance and subsequently decrease distortion a little bit.
The best thing for you is to use a 12" baffle board and clean 12" speaker with an adjustable bias supply and a larger 6K6 to 8K 20 watt output transformer, instead of the little flat sounding 10" speaker and 12 watt OT in the stock amp.

[texwest]

Bruce thanks for all the great info. I'm even more curious about all this because I read another thread about lifting the ground of the intensity pot. A few more questions:

1--I've always noticed that the normal channel of my blackface amps always sound better to me. It has a slightly fuller richer sound. I was told that it was because of the vibrato circuit sucking some of the signal to ground. Is this the reason?

2--In another thread you said that lifting the ground of the intensity pot often results in a sound that blackface amp lovers don't always like. Did I understand this correctly? Can you explain this. Does it give you too much gain in comparison to the normal channel? If it gives back enough gain to make it like the normal channel I'm sure I would like it.

3--Would it be better to connect the normal channel to the reverb? All I really want is the sound of the normal channel but I don't want to give up the reverb. does it effect the sound of the normal channel to connect it up to the reverb?

Thanks
Wes

[Bruce / Mission Amps]

Bruce thanks for all the great info. I'm even more curious about all this because I read another thread about lifting the ground of the intensity pot. A few more questions:
1--I've always noticed that the normal channel of my blackface amps always sound better to me. It has a slightly fuller richer sound. I was told that it was because of the vibrato circuit sucking some of the signal to ground. Is this the reason?

Well there is a pair of 220K resistors that try to isolate the two channels from each other and the 50K pot is under the vibrato one.... but that channel also has one extra gain stage.
The dry signal from the reverb channel is mixed with the pure reverberated signal across a 3m resistor and then reamplified... so that channel sounds different anyhow.

2--In another thread you said that lifting the ground of the intensity pot often results in a sound that blackface amp lovers don't always like. Did I understand this correctly? Can you explain this. Does it give you too much gain in comparison to the normal channel? If it gives back enough gain to make it like the normal channel I'm sure I would like it.

Yes the gain difference is enough that the vibrato channel is much hotter and distorts way easier... classic Fender tone player don't really like that much.

3--Would it be better to connect the normal channel to the reverb? All I really want is the sound of the normal channel but I don't want to give up the reverb. does it effect the sound of the normal channel to connect it up to the reverb?

Thanks
Wes

I mix the two channels together all the time internally by putting reverb and vibrato on both channels... but I also change the tone stack in the vibrato channel to different values so the two channels still sound different.
I think it is the extra gain stage and the attenuation through the mixing resistor and it's tiny, parallel 10pF cap, that you are hearing.
So, I'm not sure you can get "your normal tone" by doing that mod.

[texwest]

Bruce thanks for all the help. I have now put in the 12 inch speaker and adjustable bias in my princeton. sounds great. I will still have to try a deluxe OT. Again questions about the vibrato vs normal channel on the ab763 circuit.

1--It's sounding like there isn't much to be done to make the vibrato channel sound like the normal channel. Is this correct?

2--But I do remember you mentioning a mod (in a different link) where you use a pot to vary the resistance of the signal going to ground at the intensity knob. Can this do the trick?

3--How do you do this mod?

Thanks

Wes

[Bruce / Mission Amps]

Bruce thanks for all the help. I have now put in the 12 inch speaker and adjustable bias in my princeton. sounds great. I will still have to try a deluxe OT. Again questions about the vibrato vs normal channel on the ab763 circuit.

1--It's sounding like there isn't much to be done to make the vibrato channel sound like the normal channel. Is this correct?

2--But I do remember you mentioning a mod (in a different link) where you use a pot to vary the resistance of the signal going to ground at the intensity knob. Can this do the trick?

3--How do you do this mod?

Thanks

Wes

Well...
1. the normal channel's last triode goes directly to it's 220K mixing/isolation resistor... and the output of the vibrato channel goes to a divider and the reverb is paralleled off and then the dry signal has to go through a large value 3.3m to 4.7m resistor with a little 10pF cap across it... and is then mixed with the raw reverb signal.
There is an additional 470K isolation resistor to the reverb pot so the level of the reverb pot doesn't effect the mixed signal too much.
But all that is fairly low level now so a whole new triode is needed to boost it all back up to a level that is similar to the normal channel.
That output is sent to the other 220K mixing/isolation resistor and then to a tiny 500pF to 1000pF coupling cap that drives the phase inverter tube.
The reverb mixed channel always seems to a very slight increase in signal compared to the normal because of this stage and the fact that it went through another triode that the normal stage does not have, the tone is very slightly different too.

2. That trick is to make the vibrato reverb channel have even more gain compared to the normal channel.

3. Lift the grounded lug of the intensity pot from chassis ground.
Use a 33K to 47K resistor under the grounded lug of the intensity pot and ground the resistor on the opposite leg.
Use an SPST switch to short the resistor on both ends back to ground.

[truth_hz]

whoops, sorry.

[garyk]

I have a Silverface Vibrolux Reverb.

I have used a switched pot across for the intensity pot, so at minimum, it switches out the yellow lead.

When it is in the offposition, the power output (well, at that setting) does go up considerably, but as time passes, the channel gets noisier ...any thoughts on this?


Thanks
Gary

[Enzo]

Hi Gary, welcome to the forum.

May I suggest you start a new thread for your amp instead of tacking onto the end of a thread that has been dormant for a month or two. You will get more response, and no one will confuse your Vibrolux with the Princeton this thread is about.

I am sure we will respond to it.