Princeton Build Chronology Complete and in Cab

[Ed_Chambley]

GOT IT!  I have independent swing on the control grid of both from -21 to -47.  Speed control on 0000000000000000000000.  Now I am not sure, but I think 7c5 tubes should bias around 25 with a plate of 417.  I have a resistor 2k2, 3 watt between the bias tap and diode.  Not sure of the change once a load is on it.

Anyone know what would be the best center position swing?  Right now it is about -35 with no load.  I am not going to put any tubes back in yet.  I will wait for one of the masters guidance.

Anyone want to guess what the problem was?




The answer.  The thing between the stool and the amp.
  Rectifier tube.  I have never seen this before.  It's an old JJ and it was putting out varying voltages.  I do not know how.  Put it in my tube tester and the needle would swing up and hold and then drop into the bad zone.  Wait a few minutes and back to really good.  Visually, you cannot see anything.  I have never seen this in my life and I have tested easily 10,000 tubes in my life.

That still doesn't remove the stupidity, but it helps.

[sluckey]

Any more progress? What's the state of the amp today?

[Ed_Chambley]

Some promising news and some not.  I checked the resistors which were in the path of the and all checked out.  I found one, the 470 ohm that the reverb pot connects to, when I touched it with my meter it would make an on and off distorted noise and continue when I removed the probe for 4 or 5 times.  I pulled it and it had drifted to 1k4.  I replaced it and this stopped. I have the tubes biased 22ma ant they are both independent.  To get to this range I had to install a bias range resistor of 7 ohms.  I am not sure of what the bias should be on the tubes, but going by weber's bias calculator and using a plate of 417 it shows 25.

So I need to know if 25ma is the number and then I can change the range resistor where this is close to center travel.

I checked out all the voltages and I am a little higher everywhere than the Fender schematic.  This is understandable.  Plug in guitar and hear a pop, but no sound.  Since the B+ voltage did get send to ground bus, the first thins in line would be the 1m resistor on the inputs.  Today, I plan on removing these as they are soldered as 1 piece and the shielded cable with the 68K resistors I cannot get to to check as they are under the board.  I usually make these by putting them on the outside of the amp and just flipping them over and connecting to the tubes.  Lot easier.

Other than that, I do not know.  I don't know what else would have been in the path.  I am not sure if it could have destroyed any of the bypass caps, especially the first one which connects to pin 3.

I am open to suggestion.

[tubeswell]

Some promising news and some not.  I checked the resistors which were in the path of the and all checked out.  I found one, the 470 ohm that the reverb pot connects to, when I touched it with my meter it would make an on and off distorted noise and continue when I removed the probe for 4 or 5 times.  I pulled it and it had drifted to 1k4.  I replaced it and this stopped. I have the tubes biased 22ma ant they are both independent.  To get to this range I had to install a bias range resistor of 7 ohms.  I am not sure of what the bias should be on the tubes, but going by weber's bias calculator and using a plate of 417 it shows 25.

So I need to know if 25ma is the number and then I can change the range resistor where this is close to center travel.

Whatever resistor gets you where you wanna go is good. You can bias the 6V6s down to 60% and it should still sound good.

I checked out all the voltages and I am a little higher everywhere than the Fender schematic.  This is understandable.  Plug in guitar and hear a pop, but no sound.  Since the B+ voltage did get send to ground bus, the first thins in line would be the 1m resistor on the inputs.  Today, I plan on removing these as they are soldered as 1 piece and the shielded cable with the 68K resistors I cannot get to to check as they are under the board.  I usually make these by putting them on the outside of the amp and just flipping them over and connecting to the tubes.  Lot easier.

Other than that, I do not know.  I don't know what else would have been in the path.  I am not sure if it could have destroyed any of the bypass caps, especially the first one which connects to pin 3.

I am open to suggestion.

When you (carefully) do the pop test on each stage, which ones pop and which one's don't?

[DummyLoad]

should bias around 25 with a plate of 417V

....should be similar to princeton reverb; around -35V, no?

--DL

[Ed_Chambley]

Some promising news and some not.  I checked the resistors which were in the path of the and all checked out.  I found one, the 470 ohm that the reverb pot connects to, when I touched it with my meter it would make an on and off distorted noise and continue when I removed the probe for 4 or 5 times.  I pulled it and it had drifted to 1k4.  I replaced it and this stopped. I have the tubes biased 22ma ant they are both independent.  To get to this range I had to install a bias range resistor of 7 ohms.  I am not sure of what the bias should be on the tubes, but going by weber's bias calculator and using a plate of 417 it shows 25.

So I need to know if 25ma is the number and then I can change the range resistor where this is close to center travel.

Whatever resistor gets you where you wanna go is good. You can bias the 6V6s down to 60% and it should still sound good.

I checked out all the voltages and I am a little higher everywhere than the Fender schematic.  This is understandable.  Plug in guitar and hear a pop, but no sound.  Since the B+ voltage did get send to ground bus, the first thins in line would be the 1m resistor on the inputs.  Today, I plan on removing these as they are soldered as 1 piece and the shielded cable with the 68K resistors I cannot get to to check as they are under the board.  I usually make these by putting them on the outside of the amp and just flipping them over and connecting to the tubes.  Lot easier.

Other than that, I do not know.  I don't know what else would have been in the path.  I am not sure if it could have destroyed any of the bypass caps, especially the first one which connects to pin 3.

I am open to suggestion.

When you (carefully) do the pop test on each stage, which ones pop and which one's don't?

I will check and post.

[Ed_Chambley]

should bias around 25 with a plate of 417V

....should be similar to princeton reverb; around -35V, no?

--DL

Not sure, that is why I am asking.  I want to know so I can get the proper resistor so the "optimum" bias will be in mid travel of the pot.

[tubeswell]

Ed - getting that resistor value is a bit of science and a bit of suck it and see. You just have to take the plunge

[sluckey]

Treat the 7C5s just like 6V6s. Set the bias to -35v and forget about it until you fix the amp.

[Ed_Chambley]

Treat the 7C5s just like 6V6s. Set the bias to -35v and forget about it until you fix the amp.

That was my plan.  I just needed the number, I can get all the little tweaks done later.

[Ed_Chambley]

Its playing.  It sounds great and I don't even have it in a cab, just my test cab.  I have a problem which is not a big deal to me, but my wife will hate it.  The volume knob does not work.  It is at full volume all the time.  I will check it and insure it is working.  It is fairly straight forward tho.  Wiper direct to pin 7 ov v1b, lug 1 to treble and 3 to ground.

The nfb pot will squeal if above 8k ohm with the tremolo running.  the tremolo is great.  I have not hooked up the reverb yet.  Gotta work out this volume pot.  Is it possible the volume pot is acting this way if I added a mid control incorrectly?

Anyway, thanks for the help guys.  Still got some bugs, but I've got sound.

[sluckey]

I found one, the 470 ohm that the reverb pot connects to, when I touched it with my meter it would make an on and off distorted noise and continue when I removed the probe for 4 or 5 times.  I pulled it and it had drifted to 1k4.  I replaced it and this stopped.

I have found an issue.  V3b, pin 7.  Touch it with the meter and I get a fairly large negative reading and it sets a helicopter in motion.

I'm quoting from two different posts because they're probably related. First, there is no 470Ω resistor connected to the reverb pot. That resistor should be 470 thousand ohms. Check to be sure you have a 470K connected to the pot now. If not, make it so.

Now notice that that resistor also connects to V3 pin7? Helicopter when you touch pin 7 could mean the grid does not have a good ground path. Turn the amp off and check the grid resistance. Put one meter lead on the chassis (not your pot ground buss). Put the other meter lead on V3 pin 7 (directly at the socket). With the reverb pot set to zero you should measure 470KΩ. With the reverb pot turned all the way up you should measure 570KΩ. What two numbers do you get?

[sluckey]

Mistake, reverb pot not grounded 3rd lug.  Now high pitched squeal.

Seems I write too slowly! 

The squeal could simply be NFB is wrong phase. Disconnect the NFB loop (lift one leg of the 2700Ω) from the speaker jack. If this fixes the squeal, reverse the OT primary plate leads. If not, keep lookin'.

[Ed_Chambley]

I found one, the 470 ohm that the reverb pot connects to, when I touched it with my meter it would make an on and off distorted noise and continue when I removed the probe for 4 or 5 times.  I pulled it and it had drifted to 1k4.  I replaced it and this stopped.

I have found an issue.  V3b, pin 7.  Touch it with the meter and I get a fairly large negative reading and it sets a helicopter in motion.

I'm quoting from two different posts because they're probably related. First, there is no 470Ω resistor connected to the reverb pot. That resistor should be 470 thousand ohms. Check to be sure you have a 470K connected to the pot now. If not, make it so.

Now notice that that resistor also connects to V3 pin7? Helicopter when you touch pin 7 could mean the grid does not have a good ground path. Turn the amp off and check the grid resistance. Put one meter lead on the chassis (not your pot ground buss). Put the other meter lead on V3 pin 7 (directly at the socket). With the reverb pot set to zero you should measure 470KΩ. With the reverb pot turned all the way up you should measure 570KΩ. What two numbers do you get?

Rverb pot 470 to 578.  the resistor is correct, my typing, not so much.

[Ed_Chambley]

Mistake, reverb pot not grounded 3rd lug.  Now high pitched squeal.

Seems I write too slowly! 

The squeal could simply be NFB is wrong phase. Disconnect the NFB loop (lift one leg of the 2700Ω) from the speaker jack. If this fixes the squeal, reverse the OT primary plate leads. If not, keep lookin'.

Found the squeal. Fixed.  The squeal was very faint, not like an ot reversal.  I just limited the pot, no problem.  8K is plenty of nfb.  I can go from 0 to 8k.

[Ed_Chambley]

Here is where I am.  The squeal is coming from increasing the reverb pot.  The high, the louder.  I am looking for the problem with the volume pot.  I don't know if they could be related, but it needs to be working.

The ohms checked out on the reverb.  Don't know why it wound cause a squeal.  Without the reverb pan connected, I would think it would do nothing.

[sluckey]

DISCONNECT THE NFB LOOP AND POSITIVELY RULE IT OUT. NFB issues do not always have screaming/howling symptoms. It's a new build. You must rule it out.

[Ed_Chambley]

DISCONNECT THE NFB LOOP AND POSITIVELY RULE IT OUT. NFB issues do not always have screaming/howling symptoms. It's a new build. You must rule it out.

No squeal with NFB disconnected.  Change the PT primarys and squeal loud.  I have no resistor at the NFB, just a jumper going to a 10lk pot.  Lead to wiper, lug 1 to Secondary 8 ohm and ground lg 3.  Should I reverse the pot wiring?

Also the footswitch will not turn off the reverb.  Tremolo, yes.  I have the the red red yellow 220k resistor from tip to ground.

[Ed_Chambley]

Another alpha pot got me.  Checked the pot and nothing.  I had just gotten rid of all those things right after I put this one under the board.   Removed it and put in the 2k7 resistor.  The amp squealed. Changed the ot wires.  Working fine.  All I have left, other than getting a 10kl adjustable resistor mini I can put on the board (the amp sounds better with a smaller value NFB resistor, but I want to be able to clean it up as well) is putting in the MV.  Already have it hooked to the cab and it sounds great.  I think I need to install a dwell and get a large tank.  The tank can be done later.  I have it hooked up to a small 3 spring tank and the reverb is a little thin, but the tremolo is great.  What a wonderful lower power amp these are.  I have never had a princeton.

It is very quiet with all dale 1 watt resistors. 

The 7C5 are a good choice as well.  The amp is very versatile.  I've got some real 7025 for it.  Bugle Boys, NEW.  The rca 12at7 seems fine in the reverb.

The control grid is sitting at -35 on both and I am reading 25ma from the cathode.

Thanks Steve, with your help and the encouragement from Brad and John, I really have something special.  I will post some completed photos in a week or so.  I've got some playing to catch up on.

I really don't know how to express my appreciation.  Brad, man you are a cool dude.  Thanks for the push.  Hell everyone, Thank You!

[sluckey]

Glad you got it going. What about the reverb footswitch? That's got to be a simple fix. Remember though, you must use shielded cable for the reverb footswitch. Tremolo does not have to be shielded.

[Ed_Chambley]

Glad you got it going. What about the reverb footswitch? That's got to be a simple fix. Remember though, you must use shielded cable for the reverb footswitch. Tremolo does not have to be shielded.

I have a 2 button footswitch, not a fender type.  If I remember correctly it grounds on, lift the ground and the reverb is off.  With my Supers, I don't even use a footswitch.  I have a a rotary switch on the intensity and just turn it on an off from the front with a jumper RCA on the back.  I use some reverb all the time, sometimes I turn it up or down, but it is on all the time.

Should be a simple fix, but it wasn't last night.  Probably, all I have to do is use Fender type footswitch.  The one I had connected is the old Marshall type with RCA.  I am quite sure it is not shielded.  I've got a box full of the fender style, but I never really liked them.  The 2 buttons are too close together and they do not weigh enough.  I will get it going, but will probably end up with a single button for the tremolo.

Can you use a 1 meg switchable dwell pot to turn on and off the reverb?  I am going to order a large tank 3 springer from Doug today, so I will need to add a dwell.  If a switchable dwell will turn off the reverb, I may go that way.  Either way, Like I said, I always have some reverb running with Fender type amps.  It just seems like it is part of the Fender sound to me.

I still can't get over the difference in the tremolo and how smooth it is compared even to my Bluesbreaker and I never use the roach tremolo.  I made a pedal years ago.  The click has always driven me crazy.

Next will be the Master Volume.  I have the one you posted wired and ready to install, but I have a feeling it is going to be a non-master amp and the hole I have in the back will have a dwell in it.  I have room for both, but I have not drilled a hole for a dwell.  Reason being is when I had the NFB disconnected the amp has breakup at very low volumes.  I may be wrong, usually am, but I don't know if adding a MV will benefit.

Also, I read where the 7C5 is just a 6V6 in a different socket, but I have never heard such a strong and tight bottom end from a 6V6.  Maybe that is just what a Princeton does.  If it is just the AA1164 I question why my Blackface Deluxe won't do it and why this circuit was not used in anything other than the Princeton Reverb.  I don't know the differences in the AB763 and the AA1164 are as I have not compared the schematics of both.  I know right off hand they are filtered differently.

[sluckey]

I have a 2 button footswitch, not a fender type.  If I remember correctly it grounds on, lift the ground and the reverb is off.

All the footswitches for Fender tube reverb circuits operate the same.

Switch contacts closed = ground = reverb OFF
Switch contacts open = ground lift = reverb ON

Can you use a 1 meg switchable dwell pot to turn on and off the reverb?

sure.

Next will be the Master Volume.  I have the one you posted wired and ready to install

I don't recall which circuit you're referring to. I hope it doesn't tie into the bias for the output tubes. That circuit is already busy enough with dual trem intensity and dual bias. That's 4 pots that could fail and kill the bias. I'd be nervous adding two more pots that could possibly fail and kill the bias. I'm not convinced that a MV on a PR has any benefits.

Switching the NFB might be useful. And a raw control in the tonestack could be useful.

[Ed_Chambley]

I have a 2 button footswitch, not a fender type.  If I remember correctly it grounds on, lift the ground and the reverb is off.

All the footswitches for Fender tube reverb circuits operate the same.

Switch contacts closed = ground = reverb OFF
Switch contacts open = ground lift = reverb ON

Can you use a 1 meg switchable dwell pot to turn on and off the reverb?

sure.

Next will be the Master Volume.  I have the one you posted wired and ready to install

I don't recall which circuit you're referring to. I hope it doesn't tie into the bias for the output tubes. That circuit is already busy enough with dual trem intensity and dual bias. That's 4 pots that could fail and kill the bias. I'd be nervous adding two more pots that could possibly fail and kill the bias. I'm not convinced that a MV on a PR has any benefits.

Without a doubt the added mid control and sweepable NFB is good.  I put them on the bottom of the chassis under the board.  I am going to have to saw the shaft off the alpha pot I had for the nfb to get it out.  I am going to replace it with a fender bias adjustment.

The alpha pot only has 1 more chance to bite me in the butt.  The only one I have left is the mid control on this amp.  I must be snakebit with them.  I have lots of trouble from them.  I found a guy who has the Allen Bradley NOS.  I bought a supply of them.  The CTS I have had good service out of as well.  PEC's are the best, but I only use them in certain places as they are expensive.  I have a dual in this amp for the intensity.  Since that location is so important I thought it was worth it.

Switching the NFB might be useful. And a raw control in the tonestack could be useful.

You remember the MV, it was the one that had the 33k resistors you redrew them out.   Basically a .1 to a .1 and back, Dual pot, 1 meg.  I'm with you, I am not convinced it needs a master.  After I wrote that, I reconsidered.  It is so nice the way it is. Really!
I will use the hole for a dwell and sew me a reverb bag out of some of the tweed I have left.

Got a schematic for a raw control?

[Ed_Chambley]

The proper footswitch took care of the on and off of the reverb.  Installed a NFB switch this evening.  Next thing is deciding on the reverb tank.  I gotta PM Doug for the specs on his large 3 spring tank.

Anyone using one?  If so, does it have that deep undertone or is it more sparkley?  I prefer the deep, like you are playing over it if you know what I mean.