Questions about my Princeton Reverb

[Bruce / Mission Amps]

>>I think it's the only one that is biased quite like that. I wonder if turning the Intensity pot has any effect on the bias ?<<
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I don't think so Kerry, because the intensity pot is only coupling very low frequency AC from the LFO.
It is an AC voltage divider not a DC voltage divider.
Yes the intensity pot is connected to a DC supply but the pot is not grounded on the wiper end nor the the oscillator end, so the "voltage divider" action normally seen with a pot can not send the DC anywhere, just AC.
It is still just has a static negative DC voltage sitting on it.

Sliding the wiper of the intensity pot closer to the LFO couples more low freq AC to the grids of the power tubes. Lots of vibrato.
Sliding it closer to the bias voltage supply couples less AC voltage.
The AC now has to go through the enitre 250K resistance and any residual oscillator AC is shunted to ground through the bias filter cap, thus having virtually very little or no effect on the average bias voltage. No vibrato.

So, with the wiper over at the LFO end, the coupled AC creates the vibrato effect by varying the average bias voltage with the AC.
A great tubey sounding vibrato as the power tubes go from cold class AB (maybe if deep enough class B) to normal class AB and into very high idle current class AB.
By the way, since this low freq AC is sub-audible and coupled right to the grids of the power tubes, if the oscillator went any faster then the RC time constant set by Fender (higher frequency), it would become and audio signal!

[Bruce / Mission Amps]

Yes the 27K resistor is replaced with a 25K trim pot, wired with only two lugs as a variable resistor and a 6K8 1/2 watter in series with it.
Total DCR would be around 6K8 ohms to about 31K.
But the bias voltage will now be variable.

[Amp Kat]

>>I think it's the only one that is biased quite like that. I wonder if turning the Intensity pot has any effect on the bias ?<<
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I don't think so Kerry, because the intensity pot is only coupling very low frequency AC from the LFO.
It is an AC voltage divider not a DC voltage divider.
Yes the intensity pot is connected to a DC supply but the pot is not grounded on the wiper end nor the the oscillator end, so the "voltage divider" action normally seen with a pot can not send the DC anywhere, just AC.

By the way, since this low freq AC is sub-audible and coupled right to the grids of the power tubes, if the oscillator went any faster then the RC time constant set by Fender (higher frequency), it would become and audio signal!

When I saw the pot wasn't grounded that's what I though but wasn't really sure as far as the DC was concerned. FWIW I don't think you could have explained that any better than you did Bruce. Excellent !

[sparky]

Thanks, Bruce...

Now, I've got another question.... Hoffman's site shows two ways of approaching the bias pot.

One is to use a 10KL pot with the 27K resistor in series with it -- very similar to the standard Fender bias circuit.

The other way is to use a trim pot with a smaller resistor in series, as in your example (he suggests a 50K pot and 10K resistor).

Which way would you recommend? The 10K pot and resistor looks more "Fender-y" and is easier to adjust (don't have to remove the chassis, though I would have to drill a hole), but it means that I wouldn't have the ability to go lower than the stock 27K resistance.

The trim pot, while a PITA to adjust, allows a much greater range of resistance values.

In your experience, which is better?

[Bruce / Mission Amps]

Well in those amp there is no bias pot mounting location without drilling a hole.
99.99% of the time I don't own these amps so in order to save the vintage value, I always just pull the 22K-27K bias setting resistor out and replace it on the same little eyelet board with the small trim pot and 6K8 1/2 watter. Sometimes I have to use a 10K but that is rare.
Using a stock 10KL bias pot with the 27K resistor is OK too, but with that series setup, the range is limited to the same negative voltage or more negative voltage (colder tubes) because the total resistance is always higher when the 10K pot is at zero resistance.
Regardless, most of the time more resistance is the correct way to go (deeper negative for cooler running power tubes).
I prefer to use the 25K pot and 6K8 because I can still get cooler but I frequently run into modern tubes that are different then vintage and this combo allows a little more variable settings and a higher idle current bias setting too.

[Richard]

Sparky, how are you installing the filter caps. I considered cetting some Sprague caps for mine but I wasn't sure how to mount them so I went with a can from AES. As far as extenally adjustable bias, are you planning on biasing by ear or are you also planning on installing test points?

[sparky]

I'm getting the can from AES. It's always a PITA to get the old one off -- even with a 90-watt iron.

I'm not setting the bias by ear. I have one of the bias meters that Ted Weber sells.

[stokes]

I wouldnt use an "externally adjustable bias pot",it is possible that someone could mess with it when you arent looking or it could get moved accidentally.I like to use those small rectangular multi turn cermet pots,a drop of glue on the end and the pot is affixed to the circuit board and the screw slot adjuster is in position to be adjusted.These pots also can stand the dc you are putting across it.I have seen the volume type pots burn the trace from the dc, if you ever had a cap leak dc onto a pot you will know these pots dont like dc voltage whereas the cermet type is perfect for this.

[Amp Kat]

I agree with Stokes and unfortunately there are assholes out there that will intentionally turn that pot and then theres some that will turn it unintentionally but theres always a change it will get turned.I'd put it where Bruce said and you'll be fine.

[sparky]

One more question....

While I'm in there changing the electrolytics, should I replace those chocolate drop caps? I've got some Mallory's lying around...

[Amp Kat]

I actually like the old coupling caps however you can't go wrong replacing them. If they have bubbles on them or if they are leaking DC on the opposite side of the high voltage then I'd replace them.

[sparky]

Thanks, guys...

I worked on the amp yesterday...

I ended up leaving the old coupling caps in there; I can always change them later....

I replaced all of the 25/25 electrolytics with 25/50s and replaced the cap can with a new CE unit.

I replaced the two-pronged cord with a three-pronged cord, then removed the ground switch and replaced it with a standby switch. I also rewired the outlet properly and put the fuse on the hot side. While I was in there, I swapped the heater wiring on one of the 6V6s so that it would be consistent.

The amp sounds better, now, but still a little flabby on the bottom end when playing on the neck pickup. That's just the old Oxford that's in there, as it doesn't do that through other speakers. I've got a JBL D110F that needs reconed, but I may just get a Weber and stick in there. I've also got a Weber 12A125A lying around, and the amp sounds wonderful through it. Perhaps I should just get a 12" baffle and run that speaker, instead.

The only complaint that I have with the amp is that the vibrato is not nearly as strong as the vibrato in my Vibro Champ. The circuits appear to be a bit different, though, as it seems the VC uses the entire tube, rather than just half of it....

I've been experimenting with different tubes. Other than the old RCA rectifier, I've swapped the originals out (they were the ones that came in the amp, originally). Strangely enough, for me, I've got all JJs in there at the moment. I've tried JJ, Shuguang, and EH preamp tubes and the JJs seems to work best of these (the others are a bit shrill, IMHO). I've also tried JJ and EH 6V6s; the difference was not as noticeable, but, again, the JJs seemed to have a bit more clarity in the bottom end.

I've got some old smoked glass RCA 6V6s that I want to try in this amp. They're great-sounding tubes with a lot of midrange honk in them. In fact, I decided that the midrange was a bit too much for me in my 5E3, so I swapped them for a set of RCA 6V6-GTAs that were a bit more balanced. I've also got some Shuguang 6V6s that I haven't tried.

I may try some old blackplate RCA 12AX7s that I have lying around, too...

I'm really not so much into the brand of the tubes, but what sounds best to my ears.

[Bruce / Mission Amps]

I would say that the cap issue is a bit subjective and most of us can't really hear that much difference between the brown caps and other cheap signal caps but the cost of high quality coupling caps, such as O'drops, Mallory 150s, or other well made modern plastic caps is so low that I think you should change them just to satisfy yourself.
Personally I like the Mallory 150s, but the inexpensive items like Xicon MPP caps sound great too.
When I have an older Fender in for a refurb, I always ask to replace "all" the brown and ceramic coupling caps with higher quality caps as I think the upgraded ones do lend a certain congruency and clarity to the refurb project.