Another Princeton Reverb Build w/ mods

[purpletele]

Nice work!

[EL34]

nice!

[Willabe]

It's fluctuating but when I turn the reverb and vibrato all the way down the fluctuations settle to within a range of about .8mv.

You have bias vari trem, it vari's the -bias. 

[rhodco]

I'm starting to think now that it's not such a good idea to replace the 12AT7 reverb driver in V2 with a 12AU7. That's just too much of a drop down in gain and I don't know how the difference would affect the rest of the circuit.

Instead, I'll use a JAN-Phillips 12AT7WC for V2 and try a 5751 in the V1 position. I also have another reverb tank to experiment with.

ETA on the Mather cab is Friday 

[Willabe]

I'm starting to think now that it's not such a good idea to replace the 12AT7 reverb driver in V2 with a 12AU7. That's just too much of a drop down in gain and I don't know how the difference would affect the rest of the circuit.

No it will be fine.     Guy's do it all the time, very common way to reduce/tame the verb's depth.

Instead, I'll use a JAN-Phillips 12AT7WC for V2 and try a 5751 in the V1 position. I also have another reverb tank to experiment with.

Try that too. 

ETA on the Mather cab is Friday 

            

[rhodco]

FedEx Home Delivery paid me a visit today...

[rhodco]

Looks like Mather did another great job. Quality is outstanding. Inside top looks well shielded. All I had to do was install my speaker and bottom velcro straps for the reverb tank. (Thumbtacks are there just until the glue dries.)

[rhodco]

Now that I've been burning it in, I've noted that:

A). The 12AU7 really is fine as a reverb driver in V2, but I still only turn Reverb up to about 2 or 3 at the most.

B). The MOD reverb tank I'm testing with seems more bouncy, the springs feel more firm. My friend Jim likes the sound but I'm not a big fan. I think I prefer the longer softer decay in the Accutronics reverb tank in my other PRRI. Will probably swap them.

C). The 470K resistor I substituted for the 1 meg in the tremolo circuit is just fine. Tremolo sounds really good. I only need intensity dialed up to about 3 or 4 at the most.

D). Most important of all, no popping, no hissing, no strange noises of any kind. Remarkably quiet when the volume is turned up with no input signal.

Will do some side-by-side testing next week and make video. Hoffman Hand-Wired Princeton Reverb vs. Fender '65 Princeton Reverb Re-Issue.

 

[uki]

Looks really good this amp, very good work !

[eleventeen]

Excellent news! A nice BF Princeton Reverb is pure fun, plain and simple.

" Power supply uses a 5AR4 but that circuit uses two diodes between the transformer and pins 4 and 6 where the Hoffman design doesn't. I thought the difference in design was the reason why a 5U4G is called for?"

Those diodes protect the amp against a very, very rare failure mode of the rectifier. For practical purposes, they have no particular function, nor does the amp know they are there. Leave them as is. (They have nothing to do with choosing a 5U4 over a 5AR4. Forget about them)

A 5U4 is a directly heated rectifier tube that uses 3 amps filament (properly called a filament) current and drop about 60 volts in use.
A 5AR4 is an indirectly heated rectifier (takes a bit longer to warm up and start conducting) that uses 2 amps heater current (properly called a heater-----but nobody except the very picky really cares about the distinction) and drops more like 25 volts in use.

Some people like the idea having the slow warmup time of a 5AR4, to allow the preamp tubes to warm up before full-on B+ is applied. There's some validity to that argument, nothing to go crazy about.

Their functionality is very close. A 5AR4 will raise your B+ by some 35 volts, you do not seem to need that if you are at +473. A 5U4 is probably quite a bit more rugged a tube and usually cheaper. I'd stay with the 5U4, you don't need 500+ volts on your 6V6 tubes.

Congratulations!

[eleventeen]

"A). The 12AU7 really is fine as a reverb driver in V2, but I still only turn Reverb up to about 2 or 3 at the most.B). The MOD reverb tank I'm testing with seems more bouncy, the springs feel more firm. My friend Jim likes the sound but I'm not a big fan. I think I prefer the longer softer decay in the Accutronics reverb tank in my other PRRI. Will probably swap them."


At your leisure, I would be interested in your report of 12AU7 driving the Accutronics (hopefully in this amp) and where you set the reverb knob  (differently) with that combo.


Continued congrats that you are working!

[rhodco]

Finished burn in and bias adjustments. I've noticed that the Hoffman Princeton Reverb design has a more lively tremolo and reverb circuit. I even swapped tubes between these two amps over the past few days to compare and it's not the tubes, it's the amp. The Fender PRRI seems a little weak and anemic in comparison. Fortunately, there is plenty of room to dial it up or down in either amp according to taste. Using the  12AU7 in place of the 12AT7 only tamed reverb slightly in the Hoffman PR, where the same tube exchange in the Fender PRRI made much more of a difference.

Overall, the Fender PRRI sounds good... more jangle, more chimey. But, the Hoffman PR design seems to have more of a strong and tight low end. Classic Princeton Reverb tone, but with more clean power in reserve... at least it seems that way. Frankly, they both sound great. It's just that they have different voices, like two great singers. Each has characteristics in its tone that are appealing.

Most importantly, I paid about $900 for the Fender two years ago and I've had to fix it twice. The Hoffman cost me almost as much to build myself but I used good quality parts and had fun doing it. Hopefully, this hand-wired amp will prove more reliable than the Fender in the years to come. I plan to use both in a stereo rig for a while and see how they hold up.

[rhodco]

" At your leisure, I would be interested in your report of 12AU7 driving the Accutronics (hopefully in this amp) and where you set the reverb knob  (differently) with that combo. "

In this amp, the Accutronics reverb tank is my favorite. The decay is a little softer and slower to fade away than the MOD tank. The 12AT7WC that I used at first seemed a bit strong since I found myself setting the reverb level between 2 and 2.5. After switching to the 12AU7 for a while I was surprised to find that I'm still in that range of between 2 and 3, only I can tolerate 3 better now.

[EL34]

The amp looks good

[mresistor]

You could use a 5R4 rectifier in the princeton, it drops a few more volts than a 5U4 and filament current is lower at 2 amps.


http://www.fourwater.com/files/fullrect.txt

[rhodco]

Interesting idea. So, it's more energy efficient? What's the downside?

[Willabe]

So, it's more energy efficient?

No, it's less efficient, it drops/losses more volts.

What's the downside?

It's less efficient, it drops/losses more volts.

But that's for max dcv, in your case if you want to drop a few more dcv's it's good.

[eleventeen]

"You could use a 5R4 rectifier in the princeton, it drops a few more volts than a 5U4 and filament current is lower at 2 amps."


Excellent idea. More for the lower fil current than the measly 13 volts. It will supposedly drop 13 more volts than a 5U4. Worth the trouble?

[mresistor]

5R4GA drops 17-23 volts more than a 5U4,   and to me that is considerable when running 20-30 volts over what 6V6's like to see. I'm concerned with the power tube longevity. Some people aren't.

[SILVERGUN]

A). The 12AU7 really is fine as a reverb driver in V2, but I still only turn Reverb up to about 2 or 3 at the most.

One change that I've made and have now stuck with through the last 2 builds that had reverb was to eliminate the cathode bypass cap on the 12AT7 reverb driver tube.
I've found it to achieve a "softer" reverb sound with a much more usable range throughout the pot.
Thanks again PRR.
 
Keep up the great work rhodco.

[rhodco]

Thanks, but I'm still confused about the choice for rectifier tube. 
So I've done some research....
The Hoffman PR design calls for a 5U4G,
The Fender 1964 design for the AA1164 called for a 5U4GB,
but then they replaced it with a GZ34/5AR4 using the same power transformer. 

Would you guys agree with this quote? - " Any amp that uses a 5U4GB as OEM, can easily go to the 5AR4/GZ34. It's a drop-in replacement. Now, which rectifier is "better," depends on amp, speaker(s), and what tone you are after. The GZ34/5AR4 has a higher B+ potential and less forward voltage drop (10-15VDC) = stiffer tone. Better bass, better transients.

The 5U4GB usually has 10-20VDC less final B+ in the same amp. Much more forward voltage drop (30-40VDC). Makes for more sag in the tone. A bit more "slow quality" to the transients. Some ppl like this in certain amps. Esp those with very efficient speakers, who are looking of a more Bluesy tone.

Then, there is this guy who says the GZ34 is better and should not be swapped with a 5U4 ...

https://www.youtube.com/watch?v=0f8OiTrnnUA



 

[mresistor]

As Uncle Doug says, the AA764 (july 1964) had a GZ34 in it. That predates the AA1164 circuit. And according to Doug and many others I've read the PR used either tube until '69' or '70.  My 1979 Fender PR uses a 5U4GB in it from the factory.


I think it's important to pay attention to plate voltages on the 6V6's. If you pop a GZ34 in there and the result is 500V of plate voltage it is not a good thing.
A lot of people will tell you that modern tubes can take that abuse, but is it really a good idea? That's why I was talking about tube longevity. Even though a modern 6V6 , or a vintage one for that matter, can take the high plate voltage, it comes at a cost. And that cost is how long it lasts.
Of course cork sniffers might say that the tube will sound better and have much more headroom for certain types of music genres. Well that's OK. Run it like that and replace your tubes more often, if you got the bux it ain't a biggie.


On the other hand, I feel that the 6V6 tube is more suited for plate voltages of 390-430. I like the sound and feel of them at this level. Especially in my PRs.


Will any of these rectifiers work, and work well. Yes.   So it really is personal preference.   

[p2pAmps]

As Uncle Doug says, the AA764 (july 1964) had a GZ34 in it. That predates the AA1164 circuit. And according to Doug and many others I've read the PR used either tube until '69' or '70.  My 1979 Fender PR uses a 5U4GB in it from the factory.


I think it's important to pay attention to plate voltages on the 6V6's. If you pop a GZ34 in there and the result is 500V of plate voltage it is not a good thing.
A lot of people will tell you that modern tubes can take that abuse, but is it really a good idea? That's why I was talking about tube longevity. Even though a modern 6V6 , or a vintage one for that matter, can take the high plate voltage, it comes at a cost. And that cost is how long it lasts.
Of course cork sniffers might say that the tube will sound better and have much more headroom for certain types of music genres. Well that's OK. Run it like that and replace your tubes more often, if you got the bux it ain't a biggie.


On the other hand, I feel that the 6V6 tune is more suited for plate voltages of 390-420. I like the sound and feel of them at this level. Especially in my PRs.


Will any of these rectifiers work, and work well. Yes.   So it really is personal preference.

I am running an old Amprex 5AR4 in my PR (Hoffman).... 410v on the plates, no issues what so ever.

[mresistor]

I'd like to try one of those Amperex rectifiers. However, my wallet isn't that deep. But it's nice to learn that it's not causing a huge increase in HV.

[p2pAmps]

I'd like to try one of those Amperex rectifiers. However, my wallet isn't that deep. But it's nice to learn that it's not causing a huge increase in HV.

My 5AR4 was a freebie and been sitting in the stash for 12 years.  I finally decided to pull it out.

[sluckey]

Some PR AA1164s call for a 5U4...
http://el34world.com/charts/Schematics/files/fender/Fender_PRINCETON_REVERB_AA1164.pdf

Some PR AA1164s called for 5AR4/GZ34...
http://el34world.com/charts/Schematics/files/fender/Fender_princeton_rev_gz34_aa1164.pdf

If your PT 5VAC winding is rated for 3A (or more) you can use either a 5U4 or a 5AR4 (or any other 5V rectifier tube).

If your PT 5VAC winding is only rated for 2A you cannot safely use a 5U4.

The difference in B+ voltage between the two types is not harmful to your PR. Use whichever you prefer as long as you honor the current rating of your PT's 5V winding. My preference is the 5AR4 for all the reasons Uncle Doug stated on that video.

[rhodco]

OK... then someone please correct me if I'm reading this wrong. It looks like the 125P1B power transformer is only rated at 2 amps, and therefore should NOT be used with the 5U4G ...?

[DummyLoad]

correct. you should not use a 5U4Gx with that transformer. it will work, but will reduce transformer life, it will run hotter than it already does, it will sag, and IMO sounds like crap. use a GZ34.

if you must use a 5U4Gx, then use one of these. it's beefed up a bit and has a recto winding rated at 3A.

personally, i prefer SSR and better filtering, IMO sounds more like a twin reverb in smaller, manageable package. < flame on :p >

--pete

[mresistor]

All the schematics I see show 410V on the plates of the PR.  If a person wants his PR playing and sounding like an original, shouldn't one strive to get as close to that as possible? Pointing out a slight difference in the circuits, the AA764  PT secondary shows 330V whereas the AA1164 and AB1270 shows 340V. 
The 6V6, especially EH and JJ new production are touted as being able to handle higher plate voltages and supposedly won't harm them, true, they'll take it,  but also true is they may suffer a reduction in service life.  Another thing to consider is some of the caps in older gear, and even some newer ones,  aren't rated over 450V.

Has anyone used any of the new production 5AR4/GZ34 for a significant length of time and have experienced problems with them? Which ones are good and which ones are not so good IYO? I've read some mixed reviews on them. If you're using one, what brand is it? 

DL that transformer is great; and the 40-18019 also has a 3A rated rectifier winding. But it's not as beefy, probably more like an early original? I wish they would make one with a higher rated 6.3vac filament winding. 3 to 4 amps would be nice for using 6L6, EL34, etc.

[sluckey]

All the schematics I see show 410V on the plates of the PR.  If a person wants his PR playing and sounding like an original, shouldn't one strive to get as close to that as possible?

Read Note 1 at the top of the schematics.

[rhodco]

 

The more pressing and pertinent question for me right now is...

Why does this Hoffman Princeton Reverb kit, design, and parts list, specify a 5AUG rectifier with a 125P1B power transformer when the PT is only rated at 2 amps on the 5V winding?

[EL34]

See the PR layout link below
That is what I use to design all the boards

http://el34world.com/charts/Schematics/files/fender/Fender_PRINCETON_REVERB_AA1164.pdf


It shows a 5U4GB rectifier tube

[p2pAmps]

Some PR AA1164s called for 5AR4/GZ34...
http://el34world.com/charts/Schematics/files/fender/Fender_princeton_rev_gz34_aa1164.pdf

If your PT 5VAC winding is rated for 3A (or more) you can use either a 5U4 or a 5AR4 (or any other 5V rectifier tube).

If your PT 5VAC winding is only rated for 2A you cannot safely use a 5U4.

The difference in B+ voltage between the two types is not harmful to your PR. Use whichever you prefer as long as you honor the current rating of your PT's 5V winding. My preference is the 5AR4 for all the reasons Uncle Doug stated on that video.

I say read this again and again...

[EL34]

I have sold 100's of those 125P1B trannies over the last 15+ years I have been stocking them


No issues so far


I would get so wrapped up in all this
Is the amp working?

[sluckey]

See the PR layout link below
That is what I use to design all the boards

http://el34world.com/charts/Schematics/files/fender/Fender_PRINCETON_REVERB_AA1164.pdf


It shows a 5U4GB rectifier tube

A 5U4 wants 5V@3A. The 125P1B transformer you have listed in the PR BOM is only rated for 5V@2A (according to your transformer data table). Maybe the 125P1B that Fender used in the '60s was really rated for 3A. Probably was. Classic Tones and AES both list their 125P1B rated for 5V@3A. Maybe yours is rated for 3A also. Could you check?

With the PR becoming so popular lately this issue/non-issue? is bound to pop up again.

Maybe you could just reference this schematic instead...
     http://el34world.com/charts/Schematics/files/fender/Fender_princeton_rev_gz34_aa1164.pdf

[EL34]

I can change my parts list to a GZ34 no problem


Maybe someone can actually measure the current draw from the rectifier and see what it is in real life?

[Willabe]

I have sold 100's of those 125P1B trannies over the last 15+ years I have been stocking them

No issues so far

I would say the evidence (real world, in use) shows that the PT you sell is underrated and really has a 3A/5v rectifier heater wind.   

[sluckey]

I can change my parts list to a GZ34 no problem

Perfect solution. Maybe change your Visio/PDF schematic too? Everybody be happy! Until someone comes along and says "but I want to use a 5U4".     

[EL34]

I would think that if a winding is rated right at 2 amps exactly and you run it at 3amps, there would be a voltage drop?

Is there a voltage drop running a 5U4GB?

[Willabe]

Everybody be happy! Until someone comes along and says "but I want to use a 5U4".     

But I want to use a 5U4 because it has more sag, BUT I want to use 2/5U4's for even more sag!   

[sluckey]

I would say the evidence (real world, in use) shows that the PT you sell is underrated and really has a 3A/5v rectifier heater wind.

I wonder. That same PT is used in the blackface Champ with a 5Y3.

Just out of curiosity I'd like to know for sure what the manufacturer's rating of Doug's transformer is. It could be that Doug's spec table is wrong.

But it's a non issue for me. The 5U4 belongs in B&W TVs and cheap Silvertone amps.    There are just too many better choices IMO.

[Willabe]

I wonder. That same PT is used in the blackface Champ with a 5Y3.

Oh, Leo, where are you when we need you? 

[EL34]

I have a amp here with the 125P1B


I'll see if I can get some real life figures from it

[EL34]

Ok, I connected a meter in line with the 5 volt winding on the 125P1B transformer and tried 4 different rectifier tubes

I measured AC amps and AC voltage on the 5 volt winding

5V4 - 1.94 amps - 5.26 vac
5Y3 - 1.99 amps - 5.25 vac
5AR4 - 2 amps - 5.24 vac
5U4GB - 3 amps - 5.07 vac

I would say the 5 volt winding is not rated at 2 amps max
It looks to me like there is some cush built into that winding

[rhodco]

Thanks guys for all the helpful input. I decided to go with a Tung Sol 5AR4 and biased the amp at 25mv after warm up.
Just out of curiosity, I measured the temp of the power transformer after running it for a while to see how hot it would get. It rose slowly and leveled off at just under 118 degrees F. This was on the bench, not mounted in the cab.

I didn't want to go back to the 5U4G to see how hot it would run the PT. Maybe someone else would like to try that sometime.