Deluxe OPT in Princeton

[casey73]

A little backstory before I get to my question. In 2007 I built what I call my "hotrod Princeton". The original SF Princeton was a sawed off combo made into a head with a non working reverb I picked up at a pawn shop around 1990. It was my first tube amp. I learned about tube amps fixing and modifying that amp many times before tube amp info became readily available on the internet. Since then I've built many amps, most of which I sold and learned much along the way and the old Princeton "head" sat in a corner in my shop gathering dust.  In 07 I rebuilt the Princeton chassis close to the original circuit with a few mods, purchased a reproduction cabinet with a 12" speaker baffle and blackface faceplate.

Two major changes I made were putting in an Allen Amps drop in upgraded PT with original Princeton voltages but higher secondary and filament amp capacity allowing the use of 6L6 or EL34 output tubes. I also installed a Mercury Magnetics Deluxe Reverb OPT with 4, 8 and 16 ohm output taps. And I installed a lockable bias pot and test points on the back of the chassis, all without drilling the original chassis. I've had several speakers in it over the years and right now I have a Jensen 12" Electric Lightning.

Since I've put this amp together there is very little of the clean headroom that Princeton's are known for. The amp doesn't sound bad, it actually sounds pretty good and even with a strat starts to crunch up around 5 or 6 and with humbuckers is breaks up even earlier. Most amps I've built I just followed standard SE or PP designs for the output section and they've behaved as expected. My question is, could the fact that the primary impedance of a Princeton OPT is 8.5K and the primary impedance of the Deluxe OPT is 6.6K account for the earlier breakup? I have 6V6s in it now and it's been a while since I've run 6L6s in it. IIRC there seemed to be a bit more headroom using 6L6s. Of all parts of the amp, the relationship between the output tubes and the primary OPT impedance is an area I'm not well versed in. Curious how those number translate into the sound of the output.

[jordan86]

I believe it’s the opposite, that the 6.6k primary will give your more power, which isn’t exactly the same as headroom but prob would be perceived that way. Have you checked out the amp recently? Tested voltages, connections, etc? Or maybe try swapping some tubes. If it had more headroom before something changed, but would strongly doubt it has anything to do with the OT.  May want to try a grid stopper on the phase inverter. I just built a Princeton and Rob’s mods were very helpful to me. I did these:

pI grid stopper (470k-1M)
Power section mods (ala deluxe)
Trimmed low end with bypass caps
Stokes mod
20w OT from Allen

Has plenty of headroom. More than my other 18 watt amps. Hangs with a deluxe easy.

https://robrobinette.com/AA1164_Princeton_Reverb.htm

I’d also give a few other disclaimers/comments fwiw...

1. my Princeton starts to break up at 5-6 with single coils. Most do in my experience. That would include other blackface fender models too.

2. Per the above, I’ve never heard Princetons being known for clean headroom. Usually the opposite. If 5-6 on the volume dial isn’t enough, speaker and transformer upgrades are the way to go. Which you’ve done.

3. Assuming all is healthy with your amp, you may just need more headroom than an upgraded 6V6 Princeton can provide. Would try 6L6’s since you have the iron to support it.

[mresistor]

I don't think the Deluxe OT is the cause of your early breakup. I have one in my HPR and also an Allen TP25 PT and with the Weber Deluxe OTpretty good clean headroom. I ran it on an attenuator for a long time and at 7 - 8 on the dial it was still pretty clean, so I used pedals for distortion.  Also  what speaker are you using? 
Any mods other than the Allen TP25 and and MM Deluxe OT? Do you have the feedback disconnected? What brand of 6V6 are you using?  What output bias are you set at?

[tubeswell]

All other things being equal, a 6k6 reflected impedance should be cleaner than 8k. There is probably (an)other factor(s) at work.


What plate and screen voltages have you got on the output tubes?


How hot are the tubes biased?


What are the supply voltages on each B+ node?


etc

[jordan86]

Just to clarify...this is a Princeton Reverb, correct? Not a Princeton (non-reverb). The non reverb model will stay clean up the entire dial, but the Princeton Reverb breaks up at 5.

[casey73]

Thanks for all the input. Just to be clear, I don't think the amp is malfunctioning, and I don't hate the way it sounds. I am just curious about the relationship between the output tubes and primary impedance of the OT. In reality I don't think I've ever heard this particular amp as an unmodified Princeton Reverb through a 10 inch speaker. But a few years ago a guy I worked with had an original blackface Princeton Reverb that he wanted me to look at for him so I got to compare a real one with mine and there were some differences but it was so long ago I can't recall the details of how they compared.

Been a while since I opened it up and took some readings so I'll do that and check back in. I have several amps that I play with and the Princeton has been ignored for quite a while.

[pdf64]

It would be best to avoid using the terminology 'headroom', unless a definition is also provided.
As guitarists use 'headroom' to describe both gain, max clean power output, max available SPL, eg 'I fitted a 12AY7 to V1 and got and nice increase in headroom', 'an EVM12L will give you more headroom'.
For a given amp, using 6L6 rather than 6V6 may result in a change in both gain and power output.

[ALBATROS1234]

Like the other said that it will not cause earlier breakup. If you had a smaller cheaper one with less iron you would get more core saturation. I used to do that deliberately for more lo fi raunch. I built a supro 25 with a beefy ot from a late 50s fisher hi fi and it has tons of headroom and chime. My guess is a beefed up mercury would give more headroom than the original ot.

[HotBluePlates]

... there is very little of the clean headroom that Princeton's are known for. ... with a strat starts to crunch up around 5 or 6 ...

My first decent amp was a 1967 Princeton Reverb.

"... that Princeton's are known for" made me laugh a bit.  They have all the clean headroom of any other 12-15w amp.  My Princeton distorted at ~4 when played with a Strat.

... putting in an Allen Amps drop in upgraded PT ... I also installed a Mercury Magnetics Deluxe Reverb OPT ... I have a Jensen 12" Electric Lightning. ...

6.6kΩ allows more power output than 8kΩ.  While a measurement is called for, 22w vs 15w is a safe guess.  The 12" Jensen Electric Lightning is a 99dB speaker, very much more than the 10" Jensen in my old amp.

     10 log (22w/15w) = 1.66dB SPL increase

So the difference going from Princeton Reverb to Deluxe Reverb is very modest; you got most of it with the speaker change.

     Princeton with a Jensen C10Q:  10 log (15w) + 94.7dB = 106.5dB SPL

     Princeton with a Jensen 12" Lightning:  10 log (15w) + 99dB = 110.8dB SPL   (+4.3dB)

     Deluxe with a Jensen 12" Lightning:  10 log (22w) + 99dB = 112.4dB SPL   (+1.6dB)

... IIRC there seemed to be a bit more headroom using 6L6s. ...

If there is/was, it's due to the difference of bias voltage.  You'll need a ~4kΩ OT to actually get the 6L6s to make more output power in that amp.

[casey73]

Since my OP on my Princeton Reverb, I've read through the entire Rob Robinette section on the Princeton and suggested mods. I now remember reading through his info in the past when working on the amp. The one mod I missed was the PI grid resistor. I don't play in a band and haven't for decades, so my amps get played at home and some, like the Princeton tend to sit ignored for long periods of time. I recently started playing through it again.

In the past I recall a nasty overtone at higher volume when the amp was overdriven and remember it was something I'd "get to later". It was still there when I started playing with it recently. I tried different speaker cabs, checked sockets pins, cold solder joints, etc. and it was still there. Yesterday I installed the PI grid stopper as suggested in Robinette mods, and that rasp is now gone.

My comments about Princeton's reputation for being "clean" were from reviews I'd read on the reissue Princeton Reverb. The PR I have was never a typical PR, so I'd never actually played through a stock PR.

Experimenting with it yesterday I plugged in a set of Sino 6L6WGS (short) in it (The Sino 6L6WGS are the same tube as the TAD 6L6WGC). Initially I had the solid state rectifier in and watched the start up voltage get close to 500 VDC before it dropped into normal range. I decided it was stressing my CE FP cap that is rated 475 V. I put the JJ GZ34 back in to allow for slow warm up. It now idles at 440 on the plates and 30ma. Lots of smooth overdrive available. Preamp tubes are V1 Amperex 12AX7, V2 Amperex 12AT7, and RCA 12AX7s for V3 &4. This morning I replaced the V1 with an RCA 5751 to clean it up a little. I'm liking it now. Played a strat and LP through it and as expected it cleans up with the strat.