Mods for princeton reverb

[proaudioguy]

I just finished scratch building a PR from the mojo kit. I ordered the parts from them because I am a dealer (my company) and because I am not qualified to pick and choose individual components and There was not yet a parts list here.  The amp electronics were assembled over the Summer, the cabinet last week.  I fired it up through a 100watt bulb2 days ago.  I fixed a few minor issues quickly.  I have moved on to tube rolling and have what I think is a good set.  Amp sounds good.  I want more.  I want more treble, deeper bass, and more gain overall.  I do not want more clean headroom , just more gain.  I want to be able to distort it a bit more.  It's very clean.  It's a. It dark but I'm used to a twin reverb.  Bass is almost all the way up.  I'm using a 12" EV so the speaker is not the issue here.  Cabinet is standard size but that's not likely the culprit either.  I suspect the tone stack of coupling between stages are to blame for the limited bass.  If I was using a loose 10" with low damping. I think this would be perfect to keep it from getting flabby but I have a high power very stiff EV that can take a ton of bass.  I like I run with the bright switch on.  I may even consider putting a pull bright switch in this tone stack but even with the treble all the way up its darker than any of my blackface a or Rivera era amps.  The tone overall falls between a blackface and a tweed.  I think with more gain it will be nice with hi buckers but I also want that fender clean chime.  That sparkle is what is missing.   This is a Christmas gift for my step father.  I'm trying to get it dialed in before the 25th.  The is guys.  This was my first scratch build but I rebuilt my twin almost from scratch so this was not intimidating by comparison.  Mojo includes no instructions FYI.  If you can't read a layout and schematic then good luck.  I was a bit out off that they don't even tell you to keep the grid wires under all the others and close to the chassis.  I would have no way to know that if I didn't have friends and a few photos on the Internet.

I am trying to figure out how to out a custom graphic on the black blank space on the faceplate.  I want it to have his last name.  I designed it on the computer and have it as a png.  Open to suggestions here.  If it were larger I think I could cut the letters out of sticjy paper and use it as a template to trace then hand paint it, but it's not that big and my hands are t that steady.

[proaudioguy]

And that my friends is what autocorrect does!  I should not post from my phone.

[tubeswell]

I'm thinking that without faffing around changing the dropping resistors, you could try a few easy tube substitutions such as


12AY7 in V1 maybe (to get a touch more more harmonic distortion at start of signal chain)?


Up the B+ voltage maybe (with a GZ34?)

[proaudioguy]

I thought using a 5ar4 or GZ34 would make it cleaner.  I want it dirtier.  More than that I want the chime.  I have 5ar4 and the voltage is about 20-30 volts higher on the power tubes.  I was looking for the sag of the 5u4GB. I have no experience with this.  Perhaps these old RCA 6V6GTs are to blame for the darkness.  I will experiment. I have about 8 or 10 6v6s to try as well as a set of JJs which aren't 6V6s.  These old tubes are not NOS so no telling. 

[proaudioguy]

Swapped to the JJs and it got way louder.  Probably over biased but the JJs can take it.  Louder is not what I'm looking for though.  I'm looking for more gain before the power tubes and more chime.  The chime didn't come up with the JJs.

[proaudioguy]

Update, swapping around 6V6s and a set of GEs my friend salvaged from a side of the road rain soaked cheap organ from the 70s did the trick to really make this amp sing.  About the same volume as the JJs, but more life.  Still looking for chime and bass.  I think more bass will give more gain by default.  i.e. more drive for more distortion.  I could live with it as is but it just sounds like a twin with a set of dark tubes and the bright switch turned off.  I don't know if it's related but the treble pot seems to do nothing from 1-4 or so.  Also when I first was using it, it would pop from 9.5 to 10, I figured there was a piece of debris in the pot perhaps.  It's been sitting around here for months waiting for an amp.


Anyway if you have any suggestions for adding that BRIGHT switch chime from say 0-5 (like the opposite of a loud circuit in the car radio), that would be great.  I think it takes a center tapped POT to do that though and I don't have one of those.


If anyone needs to see the circuit or layout let me know.  It's supposedly the 1164, but Mojotone changed the bias resister to 22K so they may have made other changes here and there that I don't know about.


Any suggestions on what to change to increase the low frequency response?  Coupling caps bigger perhaps?  Which ones and how big?  I'd like a more "Deluxe Reverb" low end (and high end).

[tubeswell]

Oh the bright switch chime! Put a 100pF cap between the vol pot's input and wiper lugs.

[tubeswell]

To make the amp break up sooner drop the HT supply for the V1 stage to about 150-180V, to get the plate voltage down to around 100V or so.

[Willabe]

The 3.3M R with the 10pF cap across it, try changing it to a 2.2 or 1M. You'll loose some reverb but it will give you some more gain for the dry signal.

Some guys like to add another 20uF cap in parallel with the 1st 20uF cap in the B+ power supply (PSU) so you then have 40uF for the 1st B+ filter node, to tighten up the bass some.


                      Brad   


   

[DummyLoad]

leaving the circuit mostly stock, the most effective boost mod is in the tone stack: make the 6.8k a 150k with a switch. good for about 7dB of boost in low and high bands, and about 16dB mid band.


--pete

[jjasilli]

I had the same experience with my SF Princeton.  I modded it as shown, and loved the results for for tone and sparkle.  It also overdrives.  Gain is enhanced with the mods to the preamp stages, along with the Raw control also suggested by Dummyload. 

[proaudioguy]

Thanks guys I'll check all this out.

[proaudioguy]

Mojo is telling me it should sounds like a deluxe.  I don't have a deluxe but that leads me to believe I have missed something in the circuit.  https://www.dropbox.com/s/oatmo4fr2ky569d/2014-12-23%2000.00.28%20HDR.jpg?dl=0

[proaudioguy]

Oh the bright switch chime! Put a 100pF cap between the vol pot's input and wiper lugs.

I had this in mind for another amp a long time ago and already had a 120pf (2 left over) from Doug and as you suggested that worked well.  Thanks!  That is indeed te chime I am looking for.  Still don't know why the tone controls are not more aggressive.  There is a coupling cap between the v1 tone stack side and the reverb mixer and anoter one between the v3 reverb mixer and the PI. They are both .022. I have a .1 of the same type and voltage here.  I may have 2.  Is it advisable to change one or both of those for increased depth of low end?

[proaudioguy]

https://www.dropbox.com/s/w64zyufo3bwl593/2014-12-23%2002.43.09.jpg?dl=0
https://www.dropbox.com/s/kzwpux1ijbxgyy0/2014-12-23%2002.43.03.jpg?dl=0
https://www.dropbox.com/s/jhb6s3htua8rpn3/2014-12-23%2002.42.57.jpg?dl=0

I can't find anything connect wrong.  I could use a second set of eyes.  Don't mind my mess.
If you see anything questionable let me know and I'll look close.

[tubeswell]

More distortion sooner = decreasing the headroom in the pre-amp. In which case try lowering the plate supply for V1 like I suggested earlier. (e.g.; try a 100k dropper for the pre-amp supply node)

[proaudioguy]

My bigger issue right now is I feel like the tone controls are not fully functional and that there is a gain loss there.  With the treble turned all the way up it should be shrill, but it's not.  With the bass up all the way and the treble down, it should have a massive amount of low end but it doesn't.  I can't find anything visually wrong.  I think if this issue is fixed the amp will be what it should be.  I attached pictures above if anyone wants to take a stab at it.

[Jim Coash]

Greetings:  My Princeton Reverb re-build is coming along nicely.  I now have replaced every cap and resistor with new, cleaned the controls and switches, built a plug in solid state rectifier inside of an old tube and installed the adjustable bias pot as directed by EL34.  My question today is this; when I re-built my Twin Reverb I did as suggested by many and replaced the original bias resistors with new, better ones and I mounted them outside the the base of the tube (between pins 4 and 6).  My plan was to do the same with the PR but there are no resistors on the bases of the two 6V6 output tubes.  My reading of the pages regarding general upgrades is that if there are none, install them.  Is that correct?  I also replaced the old original  bias board diode with a shiny new IN4007 and the original fixed bias resistor with a new one.  What about the large 3 watt resistor on that board?  Any other suggestions?  I am sticking with the Celestion AT-10 speaker.  It is a four ohm unit and I really like the way it sounds.  I am also building a box the same size as the PR for an E/V SRO-12 that I can use instead.  Jim

[tubeswell]

My bigger issue right now is I feel like the tone controls are not fully functional and that there is a gain loss there.  With the treble turned all the way up it should be shrill, but it's not.  With the bass up all the way and the treble down, it should have a massive amount of low end but it doesn't.  I can't find anything visually wrong.  I think if this issue is fixed the amp will be what it should be.  I attached pictures above if anyone wants to take a stab at it.

Did you put a wrong cap value in the tone stack maybe?

[jjasilli]

I'm not sure which specific resistors you mean. 


Anyway, tube pinouts are specified on the tube charts.  For 6L6's & 6V6's, pins 1 & 6 are not used by the tube.  Sometimes the tube base has a pin there, sometimes not; but it doesn't matter.  Hence those socket lugs are conveniently available as mounting lugs.  This is especially useful for resistors that are best placed as near as possible to their tube pin, such as grid stoppers and screen resistors.  It's also a good idea to stand-off such components a bit away from the tube socket, to distance them from the heat of the tube.

[proaudioguy]

Greetings:  My Princeton Reverb re-build is coming along nicely.  I now have replaced every cap and resistor with new, cleaned the controls and switches, built a plug in solid state rectifier inside of an old tube and installed the adjustable bias pot as directed by EL34.  My question today is this; when I re-built my Twin Reverb I did as suggested by many and replaced the original bias resistors with new, better ones and I mounted them outside the the base of the tube (between pins 4 and 6).  My plan was to do the same with the PR but there are no resistors on the bases of the two 6V6 output tubes.  My reading of the pages regarding general upgrades is that if there are none, install them.  Is that correct?  I also replaced the old original  bias board diode with a shiny new IN4007 and the original fixed bias resistor with a new one.  What about the large 3 watt resistor on that board?  Any other suggestions?  I am sticking with the Celestion AT-10 speaker.  It is a four ohm unit and I really like the way it sounds.  I am also building a box the same size as the PR for an E/V SRO-12 that I can use instead.  Jim

You should start a new thread.  Folks that have already looked here may never see your post.


Interesting question about your Twin and those 470Ohm resisters, not bias resisters but rather screen resisters.  The PR doesn't come with them.  I do not know if that would be a valuable upgrade in this amp.
Taken from the PRII unofficial page... John Philips writes....
[size=-1]The screen-grid resistors prevent the tube grids drawing as much current under heavy load, and also make sure they remain at lower potential than the plates. Under very heavy tube loads, the plate voltage can actually drop below the screen grid if no resistors are fitted, which results in a massive increase in screen grid current (because it begins to act as a secondary plate) and very shortened tube life, or even sudden failure. I don't know why Fender didn't fit them to all their amps - the larger amps all have them......[/size]
The PRII is a totally different amp btw.  That doesn't mean this wouldn't be a good thing to have.  I have not located a page of PR upgrades that show this.


Not sure which bias resister you changed.  Was it the 27k or the other one?  The 27k should have moved off that board if you were adding adjustable bias.  It should have been moved to the bias POT.  You can see what I did in the picture linked in the above post. https://www.dropbox.com/s/w64zyufo3bwl593/2014-12-23%2002.43.09.jpg?dl=0  Directly to the right of the bias board you can see the bias pot and the resister that used to be next to that Cap on the bias board is on the back of the bias pot.  You should upgrade the CAP on the bias board to at least 70V.  Mine is 100mfd 100v, but the stock value should work with a higher voltage.  What I've read said that Cap at 50 volts is right on the edge.

[Willabe]

I don't know if you know this, so look at this;    (it's only 1 paragraph.)

http://el34world.com/Forum/index.php?topic=17807.msg182509#msg182509



                     Brad     

[HotBluePlates]

... With the treble turned all the way up it should be shrill, but it's not.  With the bass up all the way and the treble down, it should have a massive amount of low end but it doesn't. ...

Do you have non-EV speakers in your Twin (or any other amp)? If so, try plugging them into the Princeton for a quick test. I'm wondering if the speaker is a bit much for that amp to move.

As a gauge, I used to have a '67 Princeton Reverb (and a few later Princetons as well); I typically played with the Bass control around 7-8 and the Treble around 4-5. Not a bright sound, but the Princeton didn't have "booming bass" either. The Jensen 10" speaker may have accounted for at least some of that.

[proaudioguy]

I have traced the entire preamp section and could find nothing wrong.  I do have a quick question I forgot to ask. Should the Reverb Out (send to reverb) jack be ground isolated from the chassis?  I have it grounded and I do have a ground loop with the reverb on.  It's a low hum just like in my world of PA systems.  I thought it was isolated on the reverb tank side so I grounded the whiled on the chassis side.  Perhaps I need to investigate the reverb and see if it's indeed isolated.


I am going ahead and tracing down the rest of the amp post preamp....

[proaudioguy]

I have now traced the entire circuit.  I can find no mistakes.  I am going to try disconnecting the 10pf cap and see if that makes a difference.  If it doesn't that could be damaged or defective and the cause of a loss of HF, overall gain is subjective.
If that doesn't do it I'm going to lift the 6800 Ohm resister on the tone stack.  I was told to expect only a small gain increase over maxed out tone controls.  If there is a large difference then I'm going to say I'm having a gain suck in the tone stack.


Finally I will plug into another speaker.  My Twin has EVs in it.  It is NOT dark.  I have an EV in my SUperchamp.  Also not dark.  I have a JBL K120 in my Concert.  Def not dark.


Anything else I should be checking?


I'll post back.

[proaudioguy]

I can tell no difference in sound with or without the 10pF before the reverb mixer.


The difference between tone stack maxed to 10 and lifting the 6800 resister taking the tone stack out of circuit is HUGE.  That's 2 strikes.  I mean if you tell me I should get a huge gain from lifting the tone stack, cool but my friend the amp tech says it should not be a huge difference between all controls on 10 and lifting the tone stack.

[sluckey]

The difference between tone stack maxed to 10 and lifting the 6800 resister taking the tone stack out of circuit is HUGE.  That's 2 strikes.  I mean if you tell me I should get a huge gain from lifting the tone stack, cool but my friend the amp tech says it should not be a huge difference between all controls on 10 and lifting the tone stack.

I wonder if your friend has ever made that comparison?

That Fender tone stack has a lot of loss, so when you break the ground path the signal will shoot way up, like you just witnessed. There is a better way...

I would put the 6800Ω back in but rather than connect it to ground, connect it to the wiper of a 100K or 250K pot and connect one outer leg of the pot to ground. Call the pot a RAW control. I bet you'll like it. Heck, you don't even need the 6800Ω, but it allows you to get back to stock just by dialing the RAW control to zero.

[proaudioguy]

Yea for sure.  For now I just want to get it fully functional.  Looking for another 10pF.  I could swear I have a spare.  I could not hear the difference with or without it.  That alone could be the cause of the HF loss I hear.  My Concert was very dark and that cap was missing.  A forum member sent me a package and I think there was more than one cap in there.  It did the trick so I'm trying got find the spare.  My shop does not define "organized"!

[proaudioguy]

Gentlemen, I believe we have solved the problem.  The sneaky 10pF cap which in this case was present but appears to be nonfunctional.  I would love your advise here.  I put a 120 cap code in there temporarily because everything I can find online says that is a 12pF.  Can you guys confirm if you agree with that?  If it's actually a much larger value then it would explain the increase in top end but it "sounds" the way I expect now.  I need to locate that 10pF I have and install it.  Past experience tells me this cap should be able to be heard when inserted and the one that came with the parts kit did not make a difference in tone so I can only assume it is bad.  Any input is welcomed.  Now assuming I fix this back to the next question.  Can i increase overall bass response by swapping out 1 or both of the .022 coupling caps between stages with .1

[proaudioguy]

Should I add the 47 ohm grid stoppers?  I have the resisters for this.  When I added them to my SuperChamp I ordered extras.

[proaudioguy]

Found the spare 10pF SILVERGUN sent me as a spare to the one he sent me to use in my Concert.  Thanks Silvergun!  I am inclined to install the grid stoppers.  I think I will try subbing a .1 and a .047 orange drops in place of the .022 Orange drop between one of the stages and see if the low end increases.  Unless of course someone here tells me a better idea for that.  Thanks guys!

[sluckey]

Should I add the 47 ohm grid stoppers?

Do you mean screen grid resistors? If so, use 470Ω/1W or 470Ω/3W.

[Jim Coash]

I am pleased you seem to have your PR working properly.  The tone stack on mine is all new but I used the same values, just new, close tolerance, modern parts.  For the bias board I found that my fixed bias resistor was not a 27K but a 23K so I did use a new one soldered to the back of the new bias pot as recommended.  I also went with a new 100mf/100V bias cap and a new IN4007 diode.  The only part I have not replaced is the 3 watt resistor on the board, mainly because I remounted the board to the chassis and that resistor is so tight to the front panel it will be tricky to replace.  In a few days I will be done and ready to either hook it up and hope I did everything right or contact my tech friend and see when he wants to get together.  I also have a Sovtek Mig-50 that is now ready for him.  IT has new JJ filter caps and pots.  Jim

[proaudioguy]

Should I add the 47 ohm grid stoppers?

Do you mean screen grid resistors? If so, use 470Ω/1W or 470Ω/3W.

Yes but turns out the resisters I had were 47Ohm so I didn't do it.


Found a set of tubes that needed the highest negative bias voltage to be at the correct current draw.  I took that to mean these were the strongest of the bunch.  Measured the voltage ended up being -31.5V.  The 3-8 voltage is at 408V iirc.  measured 23mV across my 1 Ohm resisters.  These are old RCA relabeled 6V6GTA.


Curious, is there any harm is using tubes that are different but matched current draw?


I would like to get a Pull Treble pot and switch the Bright cap with it.  If there were space I would have mounted a sliding bright switch.  I like the sound both ways depending on what guitar, or volume setting.


https://www.dropbox.com/s/f9ng95ncld4utgp/2014-12-24%2004.43.51.jpg?dl=0


I ordered the chassis from Mojo with the small parts.  I bought the EV from ebay.  I used parts I already had from other projects.  I had those tilt back legs for about 17 or 18 years waiting for a project.  I had left over tolex from when I recovered by Twin Reverb several years ago, same with the grill cloth.  I ordered the corners a few days ago.  Disappointed they are 1/2" radius instead of 3/4" radius, but they'l do.  Reverb tank is a MOD 3 spring (what they called a 6 spring back in the day).  Speaker is an EV Force 12.  Same basket and cone as the 12L but the magnet weight (and mass) is less, and the power handling is limited to 150 or 200 watts AES.  It is a little darker than my other EVs.  I may have a bunch of small things stacking up against me to make for a less bright amp.  It's not broken in yet so perhaps that will make a difference.  A D120 would be nice here.  I have read this forum for hours on end and that really helped with the project so thank you guys and Doug for hosting.

[proaudioguy]

I am pleased you seem to have your PR working properly.  The tone stack on mine is all new but I used the same values, just new, close tolerance, modern parts.  For the bias board I found that my fixed bias resistor was not a 27K but a 23K so I did use a new one soldered to the back of the new bias pot as recommended.  I also went with a new 100mf/100V bias cap and a new IN4007 diode.  The only part I have not replaced is the 3 watt resistor on the board, mainly because I remounted the board to the chassis and that resistor is so tight to the front panel it will be tricky to replace.  In a few days I will be done and ready to either hook it up and hope I did everything right or contact my tech friend and see when he wants to get together.  I also have a Sovtek Mig-50 that is now ready for him.  IT has new JJ filter caps and pots.  Jim

IF you make changes or try unknown poser tubes I highly recommend starting with the bulb limiter in line.
http://www4.picturepush.com/photo/a/10715222/1024/Anonymous/AmpCurrentLimiter1.jpg
It will save you from letting the smoke out.

[Jim Coash]

Good advice.  I do have a Variac and a meter to monitor amp draw but I have always been a little reluctant to delve into an area where my experience is minimal.  My past success, not withstanding.  Of all those Dynakits and Heathkits, all but 3 or 4 worked perfectly when done.  That number is almost 100 units.  On my question about screen grid/grid stopper resistors, there are no resistors at all on any of the pins of my two 6V6 output tube sockets.  EL34 suggests that I should add them and he shows 470/3W resistors between pins 4 & 6, hung outside the sockets.  Also shown are 1.5K/1/2w resistors between pjns 1 and 5.  This is exactly what I did on my Fender Twin Reverb as recommended by Torres, EL34 and others.  Those were 6L6 X 4, however.  Doug notes that all of the larger Fender amps use this arrangement and seems to say that using them is an improvement to the smaller amps too.  BTW, I have purchased many E/V speakers on eBay.  The last was a classic SRO-15 in an Ampeg cabinet for $50.  What a deal!  The Ampeg amp chassis had been removed and blocked off with cardboard.  I cut the cabinet down to eliminate the space, completely re-furbed it and it now sounds fantastic.  Once I bought 4 E/V 15s for .99/each.  Sure they were all fried but I paid to have them re-built at The Circuit Shop in GR.  They made the two EV-12Bs into EV-12Gs and the two EVM-15s as new.  Sure it cost about $75 bucks each but where can you find E/V speakers like those for well under $100 all in?  I'm always on the hunt for deals like that on Altecs, JBLs and E/Vs.  If any of you are interested in some of the original stuff I have removed from my amps, I will trade.  I have the Oxford 12" speakers from the Twin Reverb and the baffle with grill cover, the original Jensen for the PR and the original dolly and baffle from my Ampeg B-15.  Jim

[proaudioguy]

Bob's Amp


This is a video of the amp build pictures and the unveiling.  Please forgive my use of plywood.  Has no affect on the tone as far as I can tell.  Only straight wood I could get other than MDF.


Jim Coash, the resisters on the tube sockets are there for protection as I understand it and affect the sound in no way, or at least no way most of us would ever notice.

[Willabe]

Very Nice!!!!!!! What a wonderful present!!!!!!

And yes, Merry Christmas Bob!!!!!! hehehe


             Brad