New build, dead amp, frazzled builder.

[sjcasas]

Ok, techies, I have to throw myself on the mercy of the court, here.
I built a Hoffman inspired Princeton Reverb a few years ago (link to thread). It fired up the first time and has worked flawlessly ever since. I gave it to my oldest daughter (22 years old) and now I am building one for my youngest (20). She just got medically discharged from the US Army, but was a radio technician for two years. She helped me assemble the board and did a lot of soldering, but I am finishing it now. Or trying...


I fired it up the other day with no tubes and on a variac and light-bulb limiter. Measured some AC voltages, no shorts... All good so far. Put in Recto, all good still. Did a little ramping up with the variac for filter caps sake. Added preamp tubes. Still no smoke or shorts. Plugged in dummy load and added 6v6's, brought up slowly on variac just in case. No issues. Checked some DC voltages... all seems good. Plugged in speaker and guitar, fired up, and no sound. Dead quiet. No pops when I probe the grids with meter. Never made a sound.


The next day, after digging around and testing and tracing with highlighters and verifying components, swapping a few tubes, still dead.


Because I have the original working amp (named Miserlou) I can put them side by side on the bench and measure voltages, etc. knowing that one works for sure. They do have different power trannies so the voltages will be off a bit.


A few peculiarities: If I measure pin1 on V4 (PI) it is all over the place, like from 160 to 400 or so. It varies so fast I cant read it with my Fluke. Also, at one point I noticed that one of the spade connectors I had on my speaker fell off. I know, solder them on. Lesson learned. I don't know how long it had been off but it had to have happened within the hour and yes I had it powered up at some point. I de-soldered the OT and checked continuity... all seems good. 425Ohms primary, 0.8Ohms secondary, no shorts to ground or between windings. 6V6's test good in the working amp.


Also, I HATE those porcelain sockets. They are SO tight and the shield is so close that I worry about breaking a tube because I cant rotate while inserting/removing. And yes, I inserted an old tube during soldering.


Tubes are all new JJ's, and matched and burned-in 6v6's.


Yes, I have done the colored pencil tracing technique, checked all component values, grounds, swapped some tubes (6v6's, PI, Reverb driver) and always power down and drain caps before doing anything... All that being said, I really do hope someone see's something simple and I end up looking stupid but with a working amp.


Anyways, sorry so long, but everyone always asks for more info and I am trying to anticipate. Before I swap out the power tubes, and risk frying them, does any of the weirdness in the PI mean anything to anyone? Any other ideas, comments, criticisms? I'm really tired of tracing, highlighting, checking color codes on resistors, measuring, comparing with the other amp, etc. Any help is VERY much appreciated. Thanks in advance.

[SILVERGUN]

Please supply voltages for all tubes and all pins including negative voltage on pin 5 of the power tubes.

[sjcasas]

There is a voltage chart attached, right above the image of the chassis. It's a PDF file.

[sjcasas]

Heaters are all good, so you can assume 3.6VAC (and yes, I measured them) on those pins, and grids are 0v, depending on signal.

[mresistor]

concerning V4  pin one turn off the trem and it should settle down.  the phase inverter is pins 6,7 and 8


if you touch a meter lead to either pin 5 of the 6V6s with the volume up some  can you hear a pop?


schematic  https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf

[SILVERGUN]

There is a voltage chart attached, right above the image of the chassis. It's a PDF file.

I missed that.
Did a fly-bye on my phone.


It is common for the votage on the trem triode to fluctuate like that (as mresistor has eluded to)

[bmccowan]

Since you have DC voltage throughout, but no sound at all, a usual suspect would be that the signal chain is grounded somewhere. You used some shielded cable. Have you tried bypassing those? They are easy to get mangled up. And have you tried jumping out the tone stack? Shots in the dark here.
If continued frustration, a listening amp setup will let you find where the signal gets lost.

[AlNewman]

I would think you should have some sign of life at your speaker.  Have you tried plugging it in to a different amp?  Maybe your cable is shorted.  Maybe your nfb is shorted.  Maybe your external jack is open...?  I'd start at the back and work my way up.
Also, you tried it without the current limiter, correct?

[tubeswell]

If it fires up with no sound and the fuse hasn’t blown, it’s probably not the power supply. But check the B+ voltage at the reservoir cap anyway (and also at each power supply filter cap node -you’re looking for the highest voltage at the reservoir cap and the lowest voltage at the preamp node, with the others all in between). Also check the heater AC voltages across each respective set of socket heater pins. Pins 9 and 4-5 on the preamp tubes and 2 and 7 on the 6V6 pins.


Then, before you plug the output tubes in, check that you’ve got a decent negative voltage at Pin 5 on each 6V6 socket. (You want around -20V or so to stop the tubes red-plating).


If that’s all okay, then plug the 6V6s in and fire it up and do a pop test at each grin pin, working backwards through the signal chain from the 6V6s to V1. The 6V6s should pop the quietest, and V1 should pop the loudest. If you don’t get a pop at one of the grid pins, then that’s a clue and you can start looking for the missing wire etc at that point. A common beginner mistake is wiring up the preamp tube socket plate and cathode pins the wrong way around. Pins 1 and 6 are plate pins. Pins 3 and 8 are cathode pins.

[acheld]

Take a look at your shielded cable -- good chance the solder has burned through the insulator of the inner conductor, and it is grounded.

(I can only see the the cable coming off the input jack, but there is a LOT of solder on the braiding implying lots of heat). 

Frequent problem.   Consider using RG316, which has a PTFE inner insulator (it don't melt!)

I should add -- your build looks great!

[PRR]

Dead quiet. No pops when I probe the grids with meter. Never made a sound.

So you have heater and plate voltages?

Ear in the speaker: Not a HINT of hiss or hum?

If a good speaker is connected to a live amp there should be "something".(*)

Bad connection from OT to speaker. May be joints, jacks, even bad speaker. May be open, may be short.

(* Allowances for your hearing. I thought my PC speakers were dying but turns out I could not hear hiss in my right era. Fixed that: both ears are pretty hiss-free now.)

[sjcasas]

concerning V4  pin one turn off the trem and it should settle down.  the phase inverter is pins 6,7 and 8


if you touch a meter lead to either pin 5 of the 6V6s with the volume up some  can you hear a pop?


schematic  https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf

Good call on the tremolo... made a shorting plug and it got stable at 237V. I shoulda known that. No pops at ANY of the grids...

[sjcasas]

I would think you should have some sign of life at your speaker.  Have you tried plugging it in to a different amp?  Maybe your cable is shorted.  Maybe your nfb is shorted.  Maybe your external jack is open...?  I'd start at the back and work my way up.
Also, you tried it without the current limiter, correct?

Speaker is good... checked with the other amp. Cable is good, speaker measures 5.2 ohms through the 1/4" cable. I will check NFB in the morning. And yes, after I know there is no smoke or short to ground and voltages are close I get rid of the limiter.

[sjcasas]

Since you have DC voltage throughout, but no sound at all, a usual suspect would be that the signal chain is grounded somewhere. You used some shielded cable. Have you tried bypassing those? They are easy to get mangled up. And have you tried jumping out the tone stack? Shots in the dark here.
If continued frustration, a listening amp setup will let you find where the signal gets lost.

Good call... I will check that in the morning... I hate soldering that shielded cable!

[sjcasas]

If it fires up with no sound and the fuse hasn’t blown, it’s probably not the power supply. But check the B+ voltage at the reservoir cap anyway (and also at each power supply filter cap node -you’re looking for the highest voltage at the reservoir cap and the lowest voltage at the preamp node, with the others all in between). Also check the heater AC voltages across each respective set of socket heater pins. Pins 9 and 4-5 on the preamp tubes and 2 and 7 on the 6V6 pins.


Then, before you plug the output tubes in, check that you’ve got a decent negative voltage at Pin 5 on each 6V6 socket. (You want around -20V or so to stop the tubes red-plating).


If that’s all okay, then plug the 6V6s in and fire it up and do a pop test at each grin pin, working backwards through the signal chain from the 6V6s to V1. The 6V6s should pop the quietest, and V1 should pop the loudest. If you don’t get a pop at one of the grid pins, then that’s a clue and you can start looking for the missing wire etc at that point. A common beginner mistake is wiring up the preamp tube socket plate and cathode pins the wrong way around. Pins 1 and 6 are plate pins. Pins 3 and 8 are cathode pins.

All of that was done and is good.

[sjcasas]

Take a look at your shielded cable -- good chance the solder has burned through the insulator of the inner conductor, and it is grounded.

(I can only see the the cable coming off the input jack, but there is a LOT of solder on the braiding implying lots of heat). 

Frequent problem.   Consider using RG316, which has a PTFE inner insulator (it don't melt!)

I should add -- your build looks great!

I will check that in the morning, as another poster said. I have a hunch that is the culprit. And I will get some RG316 next time! Thanks!

[sjcasas]

Dead quiet. No pops when I probe the grids with meter. Never made a sound.

So you have heater and plate voltages?

Ear in the speaker: Not a HINT of hiss or hum?

If a good speaker is connected to a live amp there should be "something".(*)

Bad connection from OT to speaker. May be joints, jacks, even bad speaker. May be open, may be short.

(* Allowances for your hearing. I thought my PC speakers were dying but turns out I could not hear hiss in my right era. Fixed that: both ears are pretty hiss-free now.)

hah! I definitely have some hearing issues, but I heard the pops just fine, loud even, when I was checking voltages on the known good amp today. I will check everything AGAIN in the morning and report back.

[AlNewman]

Is it possible your output jack was built on a Friday and they forgot an insulated washer?  I wish I was your daughter by the way, nice amp.  I especially dig the General Lee colour scheme.

[sjcasas]

Is it possible your output jack was built on a Friday and they forgot an insulated washer?  I wish I was your daughter by the way, nice amp.  I especially dig the General Lee colour scheme.

Hah! General Lee! I have a bit of a fetish for Orange stuff... some day I'll get my hands on an orange JBL D 130F, then my life will be complete.

Man, that would be frustrating if that ended up being the problem... I will check that, as soon as my shop warms up!

[sjcasas]

Ok, so, the shop is warm. I checked the speaker jack for manufacturing defects. All good, tip open to ground, sleeve grounded. I checked the shielded cable to V1Pin3, all good, and V1Pin8 and... SHORT TO GROUND. I replaced it with plain wire for now... still no sound. Ordered some RG-316 Coax.


So next, I decided to de-solder the NFB. Has anyone else sat there wondering why the solder won't melt only to discover you are holding your multimeter probe and not the soldering iron? No? Just me? Ok, moving on.


Once I disconnected the NFB I was going to check some voltages and heard a pop! It happens at pins 1 and 6 (plates) not grids??? And it goes away when I reconnect the NFB. Here is a video I made to show that the speed and intensity pots affect it.
https://rumble.com/v28jzxw-princeton-build-problems.html

At that point I turned everything off and am gong to make breakfast and contemplate why I didn't just stay in the military 35 years ago...

[bmccowan]

Has anyone else sat there wondering why the solder won't melt only to discover you are holding your multimeter probe and not the soldering iron? No? Just me? Ok, moving on.

No, but I have tried to melt some buss wire, thinking I had picked up the solder. When the buss wire burned my fingers I figured it out.

[sjcasas]

Has anyone else sat there wondering why the solder won't melt only to discover you are holding your multimeter probe and not the soldering iron? No? Just me? Ok, moving on.

No, but I have tried to melt some buss wire, thinking I had picked up the solder. When the buss wire burned my fingers I figured it out.

  I'm gonna have to try that next!

[Platefire]

Just wondering if you checked those main grounds for continuity. When I was building in a pre-painted chassis,I always scraped the paint off to bare metal where the nut was touching the chassis. I'm sure you already checked that, but if you haven't, a possibility.

[sjcasas]

Just wondering if you checked those main grounds for continuity. When I was building in a pre-painted chassis,I always scraped the paint off to bare metal where the nut was touching the chassis. I'm sure you already checked that, but if you haven't, a possibility.

Yes, I did.
On the first amp, before I painted, I used nuts/bolts/washers in the holes where I knew I needed a ground. I forgot on this one so I had to scrape.

[sjcasas]

So, last night my daughter and I meticulously verified all the resistors and caps in the whole amp. All was good. I have also verified and highlighted, wire by wire, every connection. I know this circuit works. Something is off. Today I will replace ALL the tubes, and test the old set in the known-working amp. I have not tested the reverb tranny yet.
Really hoping someone has an "A-HA!" moment, because I got nothing...

[AlNewman]

I hear ya.  It's funny because just yesterday I was having the exact same issue...  So I spent an hour scratching my head, retested the OT, all the voltages, and I knew everything was working...  I tested AC voltage at the output jack, and it was good, so then the heavens opened up and I realized....  I use an AB switch to my speaker cabinet so I have a male to male connection....turns out I needed to switch it... :)  I blame my dog, mainly because she can't defend herself.

You can try building a signal tracer, here's one I found online, and have seen on these forums as well.  I built it exactly the same, except I used a 630v cap instead of 50v.  Then you plug into another amp and start tracing your signal from the input and see where you lose it.  You'll want to adjust your volumes each time you change the placement, it can get pretty loud.  Good luck!

http://musicfromouterspace.com/analogsynth_new/HOT_TIPS/images/amp_signal_tracer.gif

[bmccowan]

The signal tracer AlNewman mentions is oft called a listening amp, or stethoscope around these parts. Doug has good instructions on this page:https://el34world.com/Hoffman/tools.htm
I use one when I get into a situation like you are in. Before I start, I remind myself that I am in a live chassis with high voltage.
Replacing that cap as Al mentioned is a must as you will likely touch HV DC at some point.
To each his own, but I do not trust an Alligator clip with a wire lead on the hot end of the tool. Much safer to do as Doug suggest, or what I did was to remove the guts from a plastic cheap ballpoint pen and insert a cap with the lead sticking out the end. I use an iPad with a signal generator APP, but a guitar will do, especially if you have a helper (that trusts you)

[bmccowan]

Further thoughts - you may want to replace the remaining shielded cable with a standard hook-up wire, just to rule it out for sure. And if you have not yet, you may want to jumper out the tone stack. And take a resistance reading on each resistor that is connected to ground.

[mresistor]

When using my signal tracer (listening amp) I used to use a small late 70's Japanese battery powered transistor radio.. I used the appropriate adapters to input it in the quarter inch jack on the vicitm amp.  Sadly that little transistor radio fell off a shelf while operating in my garage and splattered all over the floor.  I have found a replacement that should work. It's helpful to use as a signal input for listening.

[bmccowan]

I have a cheap little Dan Electro mini amp with a speaker and a headphone output - basically a transistor radio without the radio. Agree that its helpful to input a signal into the sick amp - tone generator app for a tablet or phone works well unless you are trying to fine tube a guitar signal. That simple little listening amp has saved me a lot of time and even cut down on my foul language a bit.

[sjcasas]

I blame my dog, mainly because she can't defend herself.

You can try building a signal tracer,

It was totally the dogs fault!
I will try all that next. Thanks.

[sjcasas]

Lots of good ideas about a listening amp. I used to have a little battery powered Radio Shack headphone amp and a chopstick with a capacitor taped to it and a guitar looper pedal that I used a few times for that exact purpose. The headphone amp died a horrible death and I still have the chopstick and looper pedal... I guess now I have to build another one...


This time I will use a Vibro Champ I have for the listening amp, a Looper pedal with some simple guitar chords playing for the input signal, and a Hammond box with a few resistors and a fat 600V capacitor, banana jacks for input and a 1/4" output jack. I'll upload pics when I'm done... I had to order banana jacks... 

[sjcasas]

Y'all really ought to think about using a looper pedal for input. You can play some simple chords, loop it, and save it for later... any time you want you can plug it in and it will play the same chords, just how you remember it sounding, so you can compare. If you're troubleshooting a guitar amp, it should have a guitar signal for input.

Here's one for $45 on Amazon.

[bmccowan]

Seems like you are well schooled on using a listening amp. Armed with such, you should find the issue quickly - good luck. Is that looper how you got that repeating popping sound? Sorry

[thetragichero]

ya'll's ears must not be as sensitive as mine... even the buzz from my resistive load box while probing with my scope irritates the hell out of me

[AlNewman]

If you get really bored, you can build one of these. 

Pretty awesome unit.

[sjcasas]

Seems like you are well schooled on using a listening amp. Armed with such, you should find the issue quickly - good luck. Is that looper how you got that repeating popping sound? Sorry

No, not well-schooled... I made it to try to find a noise problem in a Bassman about 15 years ago. Never used it again. I am a carpenter, by trade. But I was a radio tech in the Army in the late 80's and have a degree in Electronics from a community college. I am disabled now and have much more time to play with electronics...

I imagine I will use this one I am building much more. I have two Vibro-champs I am going to re-build, and a Bassman, and a Princeton that need some heavy troubleshooting.

[bmccowan]

Sounds good. Many here on the forum (many more expert than me) use listening amps and can provide much help. Don't worry about arguments about which setup is best - if its safe and will pick up a signal - that's what counts. Too nice a build to not have it working.

[PRR]

By happenstance I came across a cheap ugly radio kit which I think is a listening-amp project. https://el34world.com/Forum/index.php?topic=29919.msg329516#msg329516

Yes, that shade of aqua went-out in 1997, but you don't need any of the oriental inscriptions, you can use the 1957 Cadillac Red touchup spray like Fender did on guitars.

[bmccowan]

That radio kit is cool. I recall converting a little Radio Shack telephone speaker to a mini-guitar amp. Trashiest distortion ever. Its probably in a box somewhere in my basement.

[SILVERGUN]

I got nothing...

Throwing darts here, but...Is there any chance that there is a stray strand of wire or blob of solder here that is shorting the OT secondary.And while you're there, please verify that the tip of that speaker jack isn't touching the black wire under it.


EDITED - missed replies above regarding this

[sjcasas]

Ok, back at it.
I had to order some parts so I took the time to build a nice chassis stand, a listening amp, and almost finished the next amp build!

I de-soldered some wires so I could lift the board to look for problems... all good. Put it all back together, fired it up and probed around with the listening amp. Some observations:

The WHOLE CHASSIS is microphonic! Everywhere I tap with a chopstick comes through the speaker.
V3B plate has great signal. Probably means everything is good up to there, no?
V4A plate has hum and very weak signal.
6V6 grids have very weak signal.
6V6 plates have loud low freq oscillation.


I will get back to this later today, but if anyone has any idea what's going on here I'd lOVE to hear it!

[sjcasas]

Ok. Disregard... nothing to see here. Move along.


I left out an underboard wire. Yep. I can't even blame it on my daughter. I pulled the board up and STILL didn't see it. And I didn't check the underboard wires when I highlighted the circuit... TWICE!


Everything works, now. Except the reverb is weak. I will swap in a 12ax7 instead of the 12at7 that's in there now.

[sjcasas]

A few issues I'm still having:



Replaced V2 with a 12ax7, Reverb is still weak. Turn it up to 10 and it's like it's on 2.


Amp is weak with NFB connected, like VERY weak. When I disconnect NFB it sounds great. it has a 2.7k feedback resistor.


With shorting plug in tremolo jack I get a loud vibrating low frequency click when I turn up intensity past about 2. Even with volume all the way down. Very loud. Remove shorting plug And tremolo works just fine.


.047 cap in tone stack is microphonic, but only as bass is increased.

Any help will be greatly appreciated!

[sluckey]

Replaced V2 with a 12ax7, Reverb is still weak. Turn it up to 10 and it's like it's on 2.

Put the 12AT7 back. Examine the cables and tank. Good cables connected correctly? Do you have a proper tank? Is there a Styrofoam block inside the tank to keep the springs from rattling during shipping?

Amp is weak with NFB connected, like VERY weak. When I disconnect NFB it sounds great. it has a 2.7k feedback resistor.

Sounds like way too much NFB. It's a very simple circuit. Verify 2.7K is really 2.7K. Be sure the other part of the NFB voltage divider is really 47Ω (yel-vio-blk). Measure the resistance between V3 pin 8 and ground. Should measure 1,547Ω. Look for a wiring error.

With shorting plug in tremolo jack I get a loud vibrating low frequency click when I turn up intensity past about 2. Even with volume all the way down. Very loud. Remove shorting plug And tremolo works just fine.

Probably a bad or missing ground to the trem footswitch jack.

.047 cap in tone stack is microphonic, but only as bass is increased.

Resolder and/or replace the cap.

[sjcasas]

Thanks for the reply, Sluckey!


Yep, it was a bad reverb tank cable... Works great now...


Re: the NFB... pulled both resistors, tested within tolerance, correct values, V3 pin 8 to ground measures 1,523 Ohms. Maybe I should just leave it disconnected? I just hate having an unresolved issue. The other amp is wired up EXACTLY like this one, just different output trannies, and NFB works fine. OT is a Hammond 1750H. Maybe the different OT is to blame?
Picture is the NFB circuit. The dead end of that 2.7K (red/violet/black/brown/brown) is where the wire is supposed to go to the speaker tip. The black wire goes to V3 pin 8.


Re: Trem... I verified everything, ground to the trem footswitch jack is good... still happening. I want to ignore this one also because trem is rarely used and shorting plug will always be in, but someone is going to turn up the intensity out of curiosity and that can't be good for the speaker if it's turned up loud.

[acheld]

I have limited knowledge about the interaction between the output tubes and the OT, but I think that if the design impedance differs between your two OT's, that could very well make the difference.

My latest build incorporates a pot to vary NFB, primarily so that I could experiment . . .  and the level really does make a huge difference (not a surprise).   What I will probably do in the future is  build the amp, and then tune the NFB to the specific amp, install a resistor and call it done.   The pot is too fiddly to be really useful while playing.

[Platefire]

On NFB, if your trying to maintain loudest clean headroom for all situations, you need two finely tuned NFB loops. One for weaker SC pickups and another for humbuckers made switchable, with a Center off position switch, you could also have no NFB

[bmccowan]

I would think the next step on the NFB is to try to find where/why the resistance from pin 8 to ground is not the number that Sluckey suggested. That number is simply the sum of the two resistors. And the number you are reading is off by 1/2 of the resistance of that 47R resistor. I'd measure resistance to ground at each one of those connections.

[bmccowan]

I'm not changing my mind that you should check the resistance of each of those connections to ground. But two other things:
First amp I ever rebuilt was a PR. I disconnected the NFB. It made the bass flubby, so I connected it back up. Stick with it, you'll get it.
Also - have you tried swapping the OT leads from the power tubes? Squealing is not the only symptom, I have heard of all kinds of funky stuff going on, and NFB is in the mix.