Since we're speaking about Princeton Reverbs

[Imntgrumpy]

Hey Guys,

It's been awhile.  Haven't had much to share. But I've been looking and reading the comments.  I finished my Princeton Reverb Clone about a year ago.  It's sound great and alot of the guys a play with are asking me to make them one.  The only real problem that I have is that I've yet to figure out is why I don't have any Vibrato.  I posed this question about a year ago and I've tried all the suggested solutions.  Unfortunately, I still don't have any "vibrato/Tremolo" to speak of.  It's virtually non-exisitent.

Any new suggestions would be greatly appreciated.

Thanks
Jim

BTW here's some photos of the project.

[Geezer]

I won't be of any help w/ trem stuff, but I bet the place to start to get help is by:

1 - posting some voltages, especially around the trem tube area.
2 - posting some pictures (hi res) of the board, again, especially around the trem tube area

[jjasilli]

I won't be of any help w/ trem stuff, but I bet the place to start to get help is by: 1 - posting some voltages, especially around the trem tube area.  2 - posting some pictures (hi res) of the board, again, especially around the trem tube area

Yes, and a schematic too.  (The dwell control reveals that you may be using the term "clone" a bit loosely.)

BTW:  in the photo those power tubes look like maybe 6L6's.  If so, that's an issue -- the big bottles have about 2X the bias voltage of 6V6's, which makes the stock trem voltage seem only 1/2 as powerfull.  A simple solution to that is to lower the value of the 1M series resistor in the trem circuit to 470K (more or less).

[phsyconoodler]

I build those with 6L6's all the time.I simply back off the bias a bit and the trem works fine.
   It's hard to diagnose a problem without having the amp right in front of you,but we can sure give it a shot.

[bnwitt]

It looks like he used the Hoffman Princeton Reverb layout with the long board version as a basis.

[tubenit]

Do you have a tremolo on/off switch pedal?

with respect, Tubenit

[Imntgrumpy]

It looks like he used the Hoffman Princeton Reverb layout with the long board version as a basis.

Bnwitt,

You're correct.  In fact, you and I shared ideas and drawings about that layout back in 2006/7. 

I'll post voltages tomorrow.  For now, gotta get some sleep.

Jim

[Imntgrumpy]

OK,

Here are the voltage readings.

http://www.el34world.com/forms/valvedata/Imntg51910.htm

Power Tube pins 3 & 4 look a little hot but other than that I don't see anything out of the norm.

 

Jim

[tommytornado]

Once you get the 6L6 bias stuff straight..  Replace the 1M resistor connected to the plate of the Trem triode (it goes to a .1uF cap that goes to the intensity pot) with a 470K. 

[Imntgrumpy]

Once you get the 6L6 bias stuff straight..  Replace the 1M resistor connected to the plate of the Trem triode (it goes to a .1uF cap that goes to the intensity pot) with a 470K. 

Tommy,

I'm using JJ 6V6s biases at 19ma.  I've already changed the 1M resistor.  I've tried 470k and currently have a 220k in there.  Still no Vibrato/Tremolo.

Jim

[HotBluePlates]

Are you able to measure the a.c. output voltage from the oscillator? You *may* have issues with getting a measurement, because the frequency is low enough to confuse DMM's. You might try setting it on the fastest speed you can just for the measurement.

[Danskman]

A scope would be of help, to be sure that the LFO is working... You could follow the signal across the circuit. With a MM, it's harder to see what's happening, IMHO.
are you sure about your footswitch? you can use a short-circuited RCA phono plug in lieu.
it's hard to help further without closer pics of the guts.
HTH a little...
Regards,
Danskman

[Imntgrumpy]

OK.

I have a tektronix scope.  Never used it.  Not quite sure how to.  Won't I need a tone generator?

Gut Shots.  I'll take some more of the Vibrato/Tremolo Tube and Circuit.

Thanks in advance for the help.

Jim

[phsyconoodler]

The grounds are important in the bias vary trem circuit.Recheck your work.Add a resistor from the trem pedal jack hot to ground,about 1meg and see if the trem starts working.If it does,then you have a poor ground somewhere in the circuit.
 

[sluckey]

are you sure about your footswitch? you can use a short-circuited RCA phono plug in lieu.

You don't need a footswitch to make the PR trem work. You need the footswitch to kill the trem.

The AB763 amps need a footswitch or a shorting plug to make the trem work. Not so with the PR though.

[HotBluePlates]

I have a tektronix scope.  Never used it.  Not quite sure how to.  Won't I need a tone generator?

The oscillator is a tone generator.  

The trem speed is probably somewhere between 1-6Hz, give or take. So you want to set the timebase to 1/6 = .167 seconds, but it is normally marked in a 1, 2, 5 series. So .2s or 200ms is probably ideal.

You can set the input to d.c. coupling *if* you measure the signal between one of the 3 caps that loops from the plate to the grid. In fact, just measure right at the grid. The idea is you don't want to see any d.c. offset, so you would normally use a.c. coupling, but the capacitor that's used inside the scope might form a voltage divider with the coupling caps returning to the grid. You might measure at the plate, but that's usually a bad idea in a guitar amp, because the voltage present may exceed the rating of the input cap. So use d.c. coupling and poke at the grid.

You'd like to be seeing a signal equal to the bias voltage or bigger (about 36v in your case), so start with a vertical amp setting of at least 10v per division. That would be ideally close to 3.5-4 divisions high. Make sure your probe is in the x1 setting if you have a x10 probe, or reset the vertical amp to a smaller volts/division setting and multiply the viewed result x10.

[Imntgrumpy]

OK Guys,

You can start laughing now.

I wasn't kidding when I said I didn't know how to use the scope.  I've just spent about two hours lookin all over Youtube and the internet to figure out how I'm supposed to hook this thing up.  I've downloaded a signal generator and can generate some sort of sinewave on the scope.  I'm not sure where to connect the 1x probe.  I don't want to fry the scope.

I guess I'll have to take a class or two.

OK -  you can stop laughing now.

Jim

[Fresh_Start]

OK Guys,

You can start laughing now.

I wasn't kidding when I said I didn't know how to use the scope.  I've just spent about two hours lookin all over Youtube and the internet to figure out how I'm supposed to hook this thing up.  I've downloaded a signal generator and can generate some sort of sinewave on the scope.  I'm not sure where to connect the 1x probe.  I don't want to fry the scope.

I guess I'll have to take a class or two.

OK -  you can stop laughing now.

Jim

No laughing here.  I still barely know how to use an oscilloscope.  Scroll down toward the bottom of the second post in the "Useful References" sticky here on the forum and you'll find plenty of o'scope info.  There also are a few useful threads in the "Tech Tools" section here too.

Hope this helps,

Chip

[jjasilli]

Ditto to the 'scope follies.  There's some helpful info here:  http://www.el34world.com/Forum/index.php?topic=9189.0  which I've been trying to get to. 

[bnwitt]

Ok, the most important thing with the scope is not to use a 1X probe on a high voltage tube amp node.  As HBP is suggesting, stay on low voltage signal points unless you have a high voltage probe.  Having said this, I have to invoke Hoffman's rule.  If it was wired right and all components were good it would be working.  Something is wired wrong, has an incorrect value or is bad.  Before scoping, I'd check out the schematic and layout to your build for errors.

V4A is the tube section which controls the tremolo.  So concentrate on that tube section and everything connected to it.  

On pin 1 of V4 you've got the 0.022uf cap, the 1 meg resistor and the 220K resistor.  the 0.022uf cap is connected to the speed control and the 0.01uf capacitor while the 220k resistor is the plate resistor for that tube section.  Check the values of these components and their connections to the other components of their terminals

On pin 2 of V4 you've got the 1 meg resistor and the 0.01uf capacitor.  Check their values and their connections to ground and the other points of their terminal connections

On pin 3 you've got the 3k3 reisistor the 25uf/25V capacitor and the 1 meg resistor to the jack (and via the underboard wire to the 0.01 uf capacitor).  check all of those connections with your ohm meter with power off and the amp discharged.  I would especially check the continuity of the underboard wire from the 1 meg resistor to the 0.01uf capacitor.

Bottom line, if the end of a component is supposed to go to ground, check the continuity to ground from that point.  If one component is supposed to be connected to another, check continuity from one point to the other point.  Check all values with an ohm meter for resistors and a capacitance meter for capacitors if you have one.

This is a simple circuit that oscillates the bias voltage to ground varying the power level of the power tubes.  It's hanging off of the bias circuit and sucking electrons to ground at varying levels due to the oscillation set up by the charging and discharging of the three trem capacitors.

[Imntgrumpy]

I have a tektronix scope.  Never used it.  Not quite sure how to.  Won't I need a tone generator?

The oscillator is a tone generator.  

Maybe I should have asked if I'll need a Function Generator?  Can I use my PC to generate the Function?  If so, will the Sinewave be pure and distortion free?

Jim

[jjasilli]

You don't need a scope to test a trem circuit.  A volt meter should be helpful.

[sluckey]

You don't need a scope to test a trem circuit.  A volt meter should be helpful.

+1

An analog meter is very handy for this because you can see the plate voltage wobbling at the trem rate (slow is more pronounced) if the oscillator is working. Even a dvm can be useful because if the oscillator is working the plate voltage will appear to be randomly jumping all over the place, and if the oscillator is dead, the plate voltage will be very steady and probably close to the same voltage level for that B+ node.

[Imntgrumpy]

''....  Before scoping, I'd check out the schematic and layout to your build for errors....''

Barry,

I just notice a difference between the "Final Schematic" and the "Wiring Diagram."  The Wiring diagram includes a Master Volume with a SPST switch, the Schematic Does not.  Was I correct in just eliminating the connections from the Master Volume pot in my build?

Jim

[bnwitt]

Barry,

I just notice a difference between the "Final Schematic" and the "Wiring Diagram."  The Wiring diagram includes a Master Volume with a SPST switch, the Schematic Does not.  Was I correct in just eliminating the connections from the Master Volume pot in my build?

Jim

Yes.  Eliminating the wires and potentiometer of the MV leaves the circuit stock.

[Imntgrumpy]

OK,

I've got the Scope working.  Used my Yamaha keyboard as a sinewave generator.  Set the sinewave to 500hz, and fiddled with Volt/Div and Time/Div till I got a nice clean sinewave on the screen while the probe was connect to V1pin2.  Next, I moved the probe to V4pin2 to watch for anything that looked like vibrato.  No change.

Am I correct in assuming that the tube isn't properly oscillating and therefore I have no vibrato?  I'm running low on ideas, but I am determined to find the fix for this problem.

As Always,  I appreciate the guidance and/or suggestions that anyone may have.

Thanks,
Jim

[tubeswell]

All the caps and resistors in the LFO circuit need to be within spec and the tube you are using needs to be a good 12AX7. The tube itself only produces the first 180 degrees of the LFO cycle, and the remaining 180 is made up by the 3 x RC combinations (being the .02uF&3M+/-100k, the .01uF&1M+3k3, and the .01uF & 1M), which is one of those is out, will upset the applecart.

What are your idle voltages on the LFo stage? Have you got it (the 220k plate resistor) connected to the screen node on the power rail?

[sluckey]

Repeat...

You don't need a scope to test a trem circuit.  A volt meter should be helpful.

+1

An analog meter is very handy for this because you can see the plate voltage wobbling at the trem rate (slow is more pronounced) if the oscillator is working. Even a dvm can be useful because if the oscillator is working the plate voltage will appear to be randomly jumping all over the place, and if the oscillator is dead, the plate voltage will be very steady and probably close to the same voltage level for that B+ node.

Forget the scope and keyboard. Step one is to determine if the oscillator is working. Use your meter. Set your voltmeter to measure 400vdc. Stick a meter probe on V4 pin 1 with the ground lead on chassis. If the voltage is wobbling or jumping all over the place the osc is working. If the voltage is very steady, the osc is not working.

Is the oscillator working?

[HotBluePlates]

What Sluckey said. My only reason to ask for a measurement of the oscillator's output is to determine if it is big enough to have an effect on the bias of the output tubes and therefore create a noticeable effect. That assumes a working oscillator, so sort that out first. You also need no external tone generator, because we're only concerned with what the oscillator is doing and how well it's doing it at this point.

[sluckey]

Here's a pic of Barry's layout that shows only the trem components. It may help in troubleshooting. Since your trem has never worked I suspect a wiring error or wrong component value (Hoffman's First Law). This is a proven circuit and proven layout. It will work if wired according to the layout.

I looked at your voltage chart. Looks like V4 pin 1 is a steady voltage, indicating the oscillator is not working. Barry posted some good tips for checking the circuit. One more tip,,, verify that the 100K resistor on the speed pot is grounded.

Get the oscillator working and it should produce a trem effect on the output tubes. You may have to tweak the bias for a compromise between optimum trem and output tube current.

[Imntgrumpy]

Sluckey,

Thanks.  I'll check this circuit again when I get home.  You don't think cap type has anything to do with this?  I use polypro (715p) type in this circuit but have noticed that the layout called for mallory 150s.

Logic says the issue should be value not what the cap is made of, but hey what do I know?

Thanks again,
Jim

[Fresh_Start]

I use polypro (715p) type in this circuit but have noticed that the layout called for mallory 150s.

Actually, bnwitt & company originally did the layout with Orange Drops.  Then one or more annoying people (including me) wanted to use Mallory 150s.  You say tomato, I say tomato (guess that doesn't work when typed )

Chip

[phsyconoodler]

I've used mallory's in many trem circuits.They work fine.

[bnwitt]

Don't forget that underboard wire in the trem area

[P Batty]

It would really drive me crazy if I found out that I had left a solder trace grounding the footswitch jack

[Imntgrumpy]

Don't forget that underboard wire in the trem area

No I haven't forgotten that one. Just trying to figure the best way to do it without lifting the entire board out.

[bnwitt]

No I haven't forgotten that one. Just trying to figure the best way to do it without lifting the entire board out.

All you have to do is unplug the amp, drain it by shorting V1 pin 1 to ground for about 30 seconds.  Then take your ohm meter set to continuity and check from the top lead of the 1 meg resistor to the bottom lead of the 0.01uf capacitor.  You should get zero ohms between those points if the wire is good underneath.  The ohm meter is your friend here.  Use it to check every component's continuity to ground that is supposed to be grounded.  Use it to check the value of every resistor in the circuit by checking across them.  V4 pin 2 should have approximately 1 meg to ground.  See if it does.  V4 pin 3 shold have approximately 3300 ohms to ground.  See if it does.  Check to see if the center pin of the tremolo pedal jack is grounded without a pedal connected.  It shouldn't be.  Check from the tremolo pedal center pin to the end of the underboard wire at the 0.01uf capacitor for continuity.  You should have 0 resistance between those points.  Check from the common terminal of the 0.01uf cap and the  0.022uf cap to the left terminal of the speed pot for continuity.  Should be 0 ohms there.  Then check from that common cap terminal to ground while turning the 3 meg reverse audio.  You should get 3.1 megohms to ground in one position and 100kohms to ground in the other.  You do have a jumper between the middle terminal and the end terminal with the 100k resistor don't you?

[sluckey]

I've just been comparing the layout to some of your pics from your original thread and I definitely see a 220K where a 1meg should be. There are a couple other suspect resistors also, but I don't have a clear view to be able to say for sure.

We need some feedback from you to be able to troubleshoot this thing. Tell us what you're doing. And when we ask you to do something, report your findings.

[bnwitt]

I did a search for the old thread but didn't see any pics of Jim's amp.  I must be missing something.  I did notice that it goes back to 2006.  Dang!  Time is just flying by in my old age.  I did read that Jim did some re-layout of the board in Visio so some good pictures of the board would really help and maybe even sharing the visio board layout in pdf would be useful

[sluckey]

Here you go Barry. The thread is only a year old.

http://www.el34world.com/Forum/index.php?topic=6982.msg61685#msg61685

[bnwitt]

Yep, definitely have some incorrect resistor values on that board.  Jim compare all of the color bands on your resistors to the ones in the layout.

[HotBluePlates]

You may also need to unsolder a leg and measure resistance. All Xicon 100 ohm resistors are easy to confuse with 100k. In fact, any place that calls for yellow or brown color bands are easy to mix up with Xicon carbon comps. Just the way it is with the colors they use.

[Imntgrumpy]

Eureka!

First,  I want to thank everyone who offered ideas and suggestions.  You guys have always been very helpful to this noobie.

Now,  What was it?




W A I T     F O R     IT   !!!








It was the lousy underboard jumper.  I actually had  it connected between .01uf cap and the 1meg resistor going to pin1 of V4. 

When I checked for continuity and got none, I knew I had to lift the board.  I couldn't just put a surface jumper without ensuring that the underboard was properly removed.

I can't believe that I've been playing with that problem all year.

Again,  Thanks guys
Now onto the next project - an 18 watt clone. Maybe I'll leave the tremolo out.

Jim

[tubeswell]

Glad you found it.

I would definitely build trem into the next build, because now you are the master.  and amps go better with trem

[HotBluePlates]

Those underboard wires can be killers!

Back when I made Hoffman-style boards, I checked every board 2-3 times after the lugs were laced and underboard wires were installed, and prior to loading resistors and caps. I learned the hard way you have to check, verify and recheck those hidden wires.

Glad you got it solved!

[Fresh_Start]

FWIW here's my solution to the under-board wire problem:  I drill a hole next to each turret where an under-board wire is supposed to be attached.  Wire comes up through hole and gets soldered to the base of the turret. 

Makes it much harder to make a mistake about which turret the wire is connected to.  You can see every solder connection.  Plus I don't have to worry about an under-board wire dropping out of the bottom of a turret I've overheated.  (Figuring out that had happened where I couldn't see it was the PITA that lead me to this approach.)

I really like the look of builds where all of the off-board leads come from under the board - they're squeaky clean in appearance.  I just don't trust myself to get things right the first time, every time.

2CW

Chip

[Imntgrumpy]

FWIW here's my solution to the under-board wire problem:  I drill a hole next to each turret where an under-board wire is supposed to be attached.  Wire comes up through hole and gets soldered to the base of the turret. 

Chip,

This sounds like a very good approach.  Will have to keep it in my during my next project.

Thanks
Jim

[bnwitt]

I had a feeling it was that wire.  And like I said, the ohm meter is your friend.

So how do you like the sound of the tremolo?

[bnwitt]

FWIW here's my solution to the under-board wire problem:  I drill a hole next to each turret where an under-board wire is supposed to be attached.  Wire comes up through hole and gets soldered to the base of the turret. 

Chip

Kind of like the method used on Marshall boards.

[Fresh_Start]

Unfortunately, I've never had the opportunity to look inside a Marshall.  A Blues Jr. and two original Blackface Fenders are the only amps I've seen inside myself (other than the 4 I've built).

Obviously, this idea wasn't original - it's just what works for me.  Here's a pic showing "access hole" for both under-board and some off-board leads:
http://i222.photobucket.com/albums/dd298/Chipster457/Super%20Reverb/GutsPreamp.jpg?1274892429

There's also a "tie down" turret for the reverb transformer's B+ if you look for red wires on the upper edge of the board.

Hope this helps,

Chip