Princeton Reverb Tremolo Peculiarity

Princeton Reverb Tremolo Peculiarity

The blackface Princeton Reverb Tremolo employs the standard oscillation circuit of three Hi-Pass RC filters in series, that act in 60˚ increments to shift the phase of the inverted plate signal 180˚, thus reinforcing the oscillation.

In the AA1164 Fender circuit schematic, the three capacitors of the tremolo circuit are .01 µF, .02 µF, and .02 µF.  This is different from other Fender tremolo circuits which use .01, .01, and .02 capacitors.  The .01-.01-.02 scheme is used by the higher-power amps like the Twin Reverb, which use the opto-isolator tremolo, as well as the lower-power amps like the Deluxe Reverb and Vibrolux, which use the direct bias tremolo, like the Princeton Reverb.

The interesting thing is that in every Princeton Reverb I have seen, the capacitors in the tremolo are .01, .01, and .02;   not .01, .02, .02 as shown in the AA1164 schematic.

From the schematic, here are the Princeton Reverb capacitors, numbered 1, 2, 3:

And here is the “standard” Fender tremolo circuit, also with capacitors numbered:

Finally, this is a typical Princeton Reverb tremolo section on the eyelet board, with the caps numbered.  Note that you can read the values, and they are .01, .01, .02.  They don’t agree with the schematic:

What does this mean in terms of the operation of the tremolo?

The change of the cap value from .01 to .02µF would not affect the phase shift of the section – it’s still 60˚.  However, the change to a higher cap value here will change the speed of the tremolo effect.  This will lower the speed of the tremolo.  With a .01, .01, .02 cap series, the lowest speed will be about 5 Hz.  With the .01, .02, .02 cap series, the lowest speed will be significantly lower than 5 Hz.

I would guess that Fender wanted the Princeton Reverb to have a slower tremolo than other Fender amps, but for some reason the schematic values never made it into production.

Don Hayward

1967 Fender Princeton Reverb

This 1967 Fender Princeton Reverb was in great shape, but the owner had issues with the bass “bottoming-out” at certain settings.

There are a number of ways to approach the solution to the problem, but the goal of the repair was to not change the great tone of the amp in any way, other than improving the bass.

The fix was simple and quick – change the V1 first gain stage cathode bypass capacitor from the original 25 µF to 4.7 µF.

This completely solved the problem, and the customer is happy.

The modification changes the frequency 3dB point from the original 4.3 Hz to a more reasonable 23 Hz.  Humans can’t hear 4.3Hz, and the amp can’t reproduce it, so there is no sense in wasting amp power trying to amplify anything under about 20 Hz.

 

The Open Voice Coil Problem

We all know that it is a Bad Idea to run an amplifier with no speakers connected to the output.  The reason is that the output transformer secondary winding will try to reflect the impedance of nothing to the primary winding and the output tubes.  The output tubes respond by increasing the voltage, to extreme levels.

This voltage can and will arc to ground, or arc inside the transformer, or inside the tube itself – all of which are Bad.

Fender (and other manufacturers) usually employ a grounding switch on the speaker output jack which shorts when there is no plug inserted in the speaker output jack.  This provides some protection, at least, in the event you forget to plug your speakers into the amp.  Unfortunately, this does not offer any protection if the speaker voice coil fails open.

This Traynor YGM-3 had the original Marsland speaker fail open, which tripped the YGM’s circuit breaker.  The user tried several times to reset the circuit breaker to no avail.  A look inside the chassis revealed the reason:

The EL84 tube socket pin #7 (plate) has arced to ground rather spectacularly; melting the plate wire and even scorching the nearby cotton-covered wire to the grid.

Here is the amp with the damage repaired:

In this instance the output transformer survived, since the arc of the plate to ground ate most of the excess electrons.

How do we prevent this from happening?

We could put a 270 Ω, 5W resistor across the secondary of the output transformer. This should prevent catastrophic failure if the speaker should fail open.  There are those who insist that this will change the tone of the amp.  There are also those who insist that this will not change the tone of the amp.  So it goes…

 

Best Regards,

Don

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McIntyre Bluesmaker

John McIntyre ran Musitech from Calgary in the 80s-90s, and was the developer of the Bluesmaker amplifier.  He wrote an article in Feb 1993 issue of Guitar Player describing how to build the amp.  The article was one of the most popular ever to appear in GP.

McIntyre also developed the Very Low Gain stereo direct amplifier for studio work, which is now known as the Lexicon.  In addition, he developed the Musitech on-board mid boost for guitar called the “Smooth and Creamy”, which is still highly regarded by guitar players.  Also an interesting tube Wah.

I am currently researching an amp the John McIntyre built in 1995 for Don Cameron.  The donor amp was a 1965 Fender Bandmaster. This amp may be unique, in that in addition to the standard Bluesmaker modifications, McIntyre also fitted the amp with EL34 tubes, replacing the stock 6L6GCs, and a Pentode/Triode switch.  The amp also has reverb, master volume, and two channels that are VLG.  This was right about the time that John was developing his VLG stereo amp, and this may be an early mono version.

The amp sounds really wonderful.  You can read all about it (at least what I know so far), on the HardWay website.

 

Vintage Voltage

I am making a “Vintage Voltage” unit that supplies drop-down voltage through a bucking transformer to supply the user with three optional voltages to an amp:  Line voltage (125 VAC), minus 6 V (119 VAC) and minus 12 V (113 VAC).

Because our vintage amps were designed for voltages from 110 to 117 VAC, it is important to supply it with the correct voltage.  Voltage that is too high can shorten tube and capacitor life, and also change the tone of the amp.

This device also provides EMI filtration and spike protection, as well as setting an alarm if the ground is missing or the polarity is reversed, an all-too-common occurrence with wall power in questionable venues.

It is ground issues and reversed polarity that has shocked many musicians, and in some cases has resulted in death.

vv-website

Best Regards,

 

e

 

 

 

 

Fender Champion 600 Reissue

IMG_0945

I really like the little 5 Watt Champion 600.  It’s best attribute is being able to turn it up to 12 for nice sustain and distortion without alarming the neighbors.  It’s also very amenable to modification, and there are dozens of sites out there which detail potential modification; everything from more clean headroom to massive gritty distortion.

I have 4 of them at the moment, for some reason, all in different states of modification.  In the photo, from right to left:

1.  Bone stock

2.  Better tubes and a Jensen Mod 6″ speaker.

3.  Gain stage and tone stack mods, no feedback loop, 1959 Jensen  AlNiCo 5  8″ speaker,  I replaced the fuzzy grill cloth with Fender Oxblood grill cloth – it is more transparent and looks better, in my opinion.

4.  Original circuit board replaced with the 1958 Fender 5F1 circuit; New transformer, needed for rectifier output, 5Y3GT rectifier, New Jensen AlNiCo 5  6″ speaker ( this is a new speaker offering from Jensen, and it’s quite good).  Changed the grill cloth.

Notes on the modifications:

1.  The stock unit sounds pretty good out of the box.  You can buy used stock Champion 600s on e-Bay for about $100 or less.  You will be hard-pressed to find a better tube amp value for this kind of money.  Yes, it’s a little clanky and muddy, but overall a nice tone, especially when cranked.

2.  The Jensen Mod speaker cleans up the amp a bit, and better quality tubes than the stock Chinese tubes is a good idea.  The gain is reduced a bit, probably by the inefficiency of the speaker, and the amp seems to have more clean headroom.  Smoother distortion at volume.

3.  The biggest contributor to the tone here is the 8″ Jensen AlNiCo 5 speaker.  Combined with the gain improvements to the circuit, and changing tone capacitors so the amp doesn’t try to reproduce low frequencies that are under its range, the amp is louder, with more presence, smoother distortion, and less muddy.

4.  The Fender 5F1 circuit is well established as one of the best designs from the early Fender days.  With some effort, the 5F1 bits can be stuffed into the (empty) chassis.  It requires a new power transformer for the rectifier, and of course a new hole and socket in the chassis for the rectifier.

The bottom line is that the 5F1  it is not a particularly difficult modification for an experienced builder, but be prepared to spend several times what you paid for the amp for all the new components.  The Jensen 6″ AlNiCo is expensive (about $70) and a very good speaker:  it is hard to distinguish it from the vintage 8″ Jensen in amp #3.  The tone is superb.

The tweed Champ is arguably the quintessential studio amp;  It has been used in the studio for many years by guitarists like Joe Walsh, Joe Perry, Billy Gibbons, Jeff Beck, Eric Clapton, etc., etc., etc.

This is the amp that Clapton used to record the studio version of Layla.  It is “The Tone”.  (By the way, Duane Allman used the same amp for his slide parts on Layla.)

Early Fender Champion Amps, Information

1948:  The first little amp was the Champion 800, which had an 8″ speaker pushing 3 Watts.

1949:  The Champion 600, a 6″ speaker, and the circuit was designated 5B1.

1953:  I was one year old, and Fender introduced the tweed “wide-panel” amp. circuit 5C1.  Tubes were a 6SJ7, 6V6, and a 5Y3.

1955:  The “narrow-panel” tweed amp.  Circuit 5E1, and the amp name was officially the “Champ”.  It had a 12AX7 as the pre-amp, and two stages of gain increased power to 5 Watts.

1958:  The 5F1 circuit.  same as 5E1, except the circuit lost the choke of the 5E1, and went to an 8″ speaker.

1964:  The end of the 5F1 tweed Champs.

 

If you are a guitar player and you don’t have one of these amps, you need to get one.  Right now.  Hurry up.  I’m timing you!

 

Best Regards,

 

e

 

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