SOLVED: Stuck on this build

[munkeyboy]

Ok so I tried so many things.  On another thread I mentioned I'm having trouble in a lot of my most recent builds with a static/fuzzy breakup. But I want feedback on this most recent build and the troubles specifically with this one.

It is a Princeton 5B2 build.  Parts were from a SE tape deck with 5879, 6SL7, 6V6GT, 6E6 and 6X5GT.  At this point, the only parts I haven't tried swapped out for known working parts are the tube sockets and the PT.  I've re-soldered so many times trying to find a bad ground or bad solder, cleaned pins, re-tensioned pins, re-tighten bolts/nuts.   I cannot find the source of the bad breakup.  However, looking at the top of my chassis, I wondered if I got the placement of these components wrong.   

Could someone review this chassis layout and provide feedback on any ill placed components?   The tube farthest from the PT is the 6SL7.

To be clear, this thing sounds great up until breakup which is about halfway up the volume pot.  Grid stoppers don't help (much).   NFB does help a lot, but not completely.   Bypass cap makes it much worse (as expected).   Adjusting tone for less treble helps too.  Issue is there on all strings/notes.   

Edit:marked as solved

[munkeyboy]

My layout with voltages in red circles.

[shooter]

my guess, you've hit the limits.
It's SE, so pushing the PA into distortion results in the uglies, sorta confirmed by the NFB
by my math you're ~~9+W DC dissipation, so that yields ~~ 4W audio
the OT looks to be small enough to saturate, adding to the issue
most of the 5w'rs i've built were best used dimed as a distortion amp, mixed with a clean amp.


have you tried a really efficient speaker, something in the 98-103db_spl

[sluckey]

NFB does help a lot, but not completely.

Please explain how you have NFB connected?

Probably not related to your issue, but your OT is connected to the wrong B+ node. According to the schematic it should be connected to the middle cap on your board. This will be helpful with any hum issues that may pop up.

     https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5b2.pdf

[munkeyboy]

my guess, you've hit the limits.
It's SE, so pushing the PA into distortion results in the uglies, sorta confirmed by the NFB
by my math you're ~~9+W DC dissipation, so that yields ~~ 4W audio
the OT looks to be small enough to saturate, adding to the issue
most of the 5w'rs i've built were best used dimed as a distortion amp, mixed with a clean amp.


have you tried a really efficient speaker, something in the 98-103db_spl

Not sure on the speaker, I've tried a few... But also tried attenuator so I don't think the speaker is the issue.   I don't mind overdrive at all and actually want a little bit if dimed.  But this is just ugly noise on top of the distortion.  Kind of like a fuzz pedal and as notes trail off it sputters and fizzes.

I've tried 3 different OT... The latest is supposedly rated at 8w.

[munkeyboy]

NFB does help a lot, but not completely.

Please explain how you have NFB connected?

Probably not related to your issue, but your OT is connected to the wrong B+ node. According to the schematic it should be connected to the middle cap on your board. This will be helpful with any hum issues that may pop up.

     https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5b2.pdf

NFB is illustrated in my 2nd post.  Connected to the cathode of the 2nd stage.


Regarding the power supply, that was me trying different things.  But had it originally on the second node with the same issues.  You are right, it is quieter as wired in original schematic.

[munkeyboy]

I'm brand new to testing with a scope.   With a function generator app, the scope didn't really show anything that I could spot.   However with striking the guitar and the vol pot up 100%, there was some noise at the bottom of the wave form.   See attached.

I don't remember if this was off the 2nd stage or the ot.  I think off the 2nd stage.
 Not at home tonight so I'll rerun the scope tomorrow.

[munkeyboy]

have you tried a really efficient speaker, something in the 98-103db_spl

Ok, looked up my speakers.  I've tried a warehouse veteran 10 which is 93 spl and vintage 30 at 100 spl (I think).

[DummyLoad]

NFB does help a lot, but not completely.

Please explain how you have NFB connected?

Probably not related to your issue, but your OT is connected to the wrong B+ node. According to the schematic it should be connected to the middle cap on your board. This will be helpful with any hum issues that may pop up.

     https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5b2.pdf

NFB is illustrated in my 2nd post.  Connected to the cathode of the 2nd stage.


Regarding the power supply, that was me trying different things.  But had it originally on the second node with the same issues.  You are right, it is quieter as wired in original schematic.

NFB is useless inserted on top of 25uF cathode bypass. get rid of 25uF bypass or insert 100R under 4.7K/25uF and insert there (with 100R insertion point, 22K will be overkill - e.g., very little NFB - use 3.3K or thereabouts). 

[shooter]

as notes trail off it sputters and fizzes.

that ringing can produce the sound you describe
after you fix the B+ and NFB;
do a scope shot at the speaker with your sig-gen as source signal - NOT guitar
use a signal ~~~~100mV

[munkeyboy]

that ringing can produce the sound you describe
after you fix the B+ and NFB;
do a scope shot at the speaker with your sig-gen as source signal - NOT guitar
use a signal ~~~~100mV

I had a cheap DIY sig-gen that doesn't work as it turns out, so I used apps.  But, I haven't seen an app yet that allows me to set mV.  Is there an estimated equivalent of phone vol to mV?

I took the attached two readings yesterday at the output jack (does it have to be at the speaker if the cables tested good?).   This was at 1000hz with the phone vol up 100%.   one with the amp volume pot at 50%, one at 100%.

Other things I've noticed that maybe normal (i don't usually play at 100% vol): I did notice heavy feedback with the volume pot at 100% with guitar.  Character of the sound changes drastically between 95 and 100% volume.    Putting a metal plate between the preamp tube and output tube drastically changes the sound for the worse (drops volume, sounds worse like signal cancelation?).

Note: any of these tests were done without NFB and without cathode bypass, just as the 5B2 schematic is.  Only difference (i think) is the 0.002mfd coupling caps instead of 0.01mfd and the 1k screen grid.

[munkeyboy]

Here is a link to sound clip.   NFB/Cathode switch off (until later in the video).   Using attenuator at first, removed later (100%).  if you see the screws on the pots, that is 50% if they are straight up.   Tone is closest to camera, volume furthest.

edit: the way the phone captures the sound is slightly different than it sounds in person.   In person it sounds more static/fuzzy/sputtering.  Playing a solo sounds nasally or like a honking duck (like a really bad fuzz pedal).

https://youtu.be/GkmyMxunH_E

[shooter]

you're fine sampling the output jack
make the phone app do a sinewave, set amp vol to 10 use phone vol to increase till the scoped sinewave at output jack just starts to get bent.
screenshot the scope so I can see the data/setting to tell signal amplitude

[munkeyboy]

@shooter, here it is

[shooter]

thanks, can you figure out the signal amplitude?  I see the scale set to 5V, but I don't see the graticules that show actual signal amplitude.
the signal is clean, so the amp works. if you take the signal amplitude, square it, then divide by speaker impedance you have the audio power out in watts (ya, I know P2P vs rms :)


listen to the signal for a few minutes, increase phone volume slowly til the sound gets crappy and screen shot that point.

[munkeyboy]

thanks, can you figure out the signal amplitude?  I see the scale set to 5V, but I don't see the graticules that show actual signal amplitude.
the signal is clean, so the amp works. if you take the signal amplitude, square it, then divide by speaker impedance you have the audio power out in watts (ya, I know P2P vs rms :)


listen to the signal for a few minutes, increase phone volume slowly til the sound gets crappy and screen shot that point.

I have no idea how to read this, much less figure out how to do the rest.  I'll need to read/watch more tutorials then get back to you.

As far as listening to this signal thru the amp, it is ear piecing.  Not sure I want a few seconds of that in my ear, much less minutes.   However, I have done that thru the attenuator, but I think that colors some of the results.   Even so, I can't really tell when it gets bad with a steady signal from the generator.  I can hear it change, but it isn't the same as notes falling off after striking strings.

[munkeyboy]

Ok, is this better i hope.  I assume each grid line is the scale value. I also found the Autoset feature.  The average doesn't seem to read less than 400mV, but the peak to peak look right.   I set the sig-gen to 147Hz so I can hear the noise better and with less blood boiling out of my ears.

I don't know if the calculation is correct.   Where I think the bad noise starts is about 12v peak to peak, into an 8ohm speaker (8ohm OT as well if that matters).  so 144 (12 squared) / 8ohm = 18.  Doesn't seem right.

[shooter]

lol, ya I use 880hz, it's more musical but still annoying.
I'm back to my OP, the amp is working, the design/parts are adding ugly color.
you can try and swap in more 6V6's, raise the B+ to the 6V6, "tap" the pre signal into another PA and test.


I would run your guitar setup through another rig since  you can see the ringing when you had it in, and the sinewave is clean.
the issue appears to be way to much drive slamming the 6V6, (based on the squarewave/sawtooth images)
if you can figure out amplitude on the scope;
you're biased ~~~~15vdc, you want about 20VAC driving the 6V6 max

[shooter]

P-P should be in RMS, but I have a calculator 
so 12vacp-p =  4.24Vac rms ^2/8 ~~= to 2.25W audio

[munkeyboy]

you can try and swap in more 6V6's, raise the B+ to the 6V6, "tap" the pre signal into another PA and test.

Yeah, I've tried other 6v6s.  Also tried the 6K6.  I think I'm at my limit with this PT and Rectifier.   I can try a SS instead of that 6x5/6ax5.   I have a weber copper cap that emulates 5ar4 (have to change out pin wiring).

if you can figure out amplitude on the scope;
you're biased ~~~~15vdc, you want about 20VAC driving the 6V6 max

Not sure I understand this last comment.   I am biased at 14.5vdc, but I don't understand the 20VAC.  What is that in relation to the bias?

[shooter]

It's my basic rule of thumb on SE amps
the cathode 14.5vdc is -14.5vdc below the grid.  you don't want the grid going positive, drawing current.


so draw a straight line( 14.5vdc), now "add on" a sinewave to the line. once the amplitude (top peak) of the sinewave is 14.5Vac you hit a limit, more signal causes the grid to conduct current, the more signal the more current.
the same is true for the bottom peak, the future down it goes the closer you get to tube cutoff


you can see it in your scope shots where both upper and lower lobes of the  sinewave are squared of or saw-toothed.  sometimes that sounds ok to good, most of the time it's ugly sound

[munkeyboy]

Awesome. That makes sense!

[munkeyboy]

It's an old tape deck that i can't make out the current rating on the PT.   I assumed because of the number of tubes it had, this PT would be enough.  Voltages matched, for the most part, the 5C1 (300 plate, 16 cathode).   

I've tried getting that cathode voltage up to 20v+ (1k = 21v k), but doing so brings the dissipation way down to 44% pdiss and still has the same issues.  I also tried the opposite, (130R = 6v k) which brings it up to almost 100% pdiss with the same issues.   Simple rect diodes did not help aside from bringing voltages up a bit.   Watching the scope, and no matter the kv, the peak to peak amplitude is going past that kv line and that is when it sounds bad.  (i.e. at 6v k, peak to peak is max is greater at like 8v and past 5v or so is when it craps out).


Not sure my next move.  I think now I just need to tear down and built back from scratch... for the 3rd time.   Maybe I'll catch a grounding issue somewhere I missed. Or new sockets could help, I don't see arching but could be.   Or just give this project up.

[DummyLoad]

before you dismantle maybe consider the following?

ditch the grid-leak bias:
reconfigure the input of V1a to a conventional grounded cathode amp with a cathode bias resistor & bypass cap. IME, grid-leak bias sounds horrible when overdriven, hot single coils can overdrive a grid-leak amp. remove the two 47KΩ resistors for the grid stoppers from the turret board and relocate to the input jacks - that'll allow room for the parts. the 4.7MΩ grid-leak becomes a 1M grid-leak for V1a at the input jack. it's a single channel amp, why do you need 2 input jacks?

the 6X5 rectifier gets you no tone advantage with SE but perhaps you used it to tame B+?  with a vacuum rectifier we get all the deficiencies of the technology: limited filter size, the ensuing buzz bomb due to that lack of filtering, added unnecessary heat, and diminished reliability. also, perhaps PS buzz may be a contributor to the distortion issue you are having?

improve the PS filtering:
relocate the bypass cap and bias resistor for the 6V6 to the chassis wall; add 47uF+47Ω RC filter (if it'll fit) in front of the 6V6 plate supply 16uF cap in the holes vacated by the 6V6 bypass cap and bias resistor. strap a pair of uF4007 diodes on the 6X5 socket and ditch the 6X5 altogether. you'll need to rebias the 6V6 since the SSR will bump up the B+ 40 to 50 volts. if you have room for it, a hammond 156L choke would work 1000% better than the 47Ω resistor.   

respectfully,

--pete

[munkeyboy]

Good ideas, thanks.   I did put in 1n4007 diodes in this last try, not uf4007.     I've also tried normal conv cathode bias preamp the first build (based on the valco/supro spectator) as well as the normal fender hi/low input with 1m gridleak (which is where the 2 input jacks came from).  Same fizzy issue there, which was my frustration with this project.

I used the 6x5 because it is what I had from the donor build and was going to make it switchable.  Then I realized the cap limit of the 6*x5, but by that time I had already cut the holes out for the sockets. :)

But you all have great points on all this.  I won't throw this out, but may dismantle components and rebuild based on your feedback.  I have a suspicion that crappy ultra thin .1mm solder wire didn't help me create solid solder joints.  Got the nice kester 44 0.062in now.

So to be clear, you recommend 47uf+16uf+10uf?   Or do you think a 4th cap (10uf) is needed?  I already have a choke on order, but stuck in shipping limbo.

[shooter]

fwiw;
if it's still together working;
hunt up a cap or many caps that add up to ~~~~100uF 50vdc or more then gator clip across the cathode cap on the 6V6 and evaluate

[munkeyboy]

fwiw;
if it's still together working;
hunt up a cap or many caps that add up to ~~~~100uF 50vdc or more then gator clip across the cathode cap on the 6V6 and evaluate

Ahh, no, Already took it apart.   What might that do?

[shooter]

It "stiffens" up the signal, sometimes helps with frequency response, usually bass

[munkeyboy]

Ok, so the rebuild is done... an it is a failure still.  I did follow DummyLoad's ideas for replacing preamp cathode scheme, input, solid state.  I installed new octal sockets, all R and Cs removed, turrets cleaned and resoldered.    And remember, I've tried multiple different tubes, multiple different OTs with the latest OT being a new 8w.   The 100uf on the bias made it worse actually, but not by much.   I'm at 68% pdiss (300v plate to k, 15.6v k).  I have not yet tried adjusting the k, but I can't see  how it would help based on past trials.

So I'm guessing what's left in order of suspicion:
Bad cables.  I don't have issues with my main amps which use these cables (1 SE, 2 PP) and both speaker x2 and instrument x2 cables tested good on the CT100.  However like I said, I do have at least two other projects in the backlog with similar fizz issues, but  I assumed old resistors were to blame.
Bad Caps. All are new(ish), but I guess one or more could be bad.  I have reused the 16uf (and now the 47uf) from my last failed project. But the 10uf are brand new to this build. And at one point when the 6x5 was in there, I only had 3 new 10uf.  The 16uf came later when trying to solve the issue.
Bad 6SL7.  I have three, find it hard to believe all are bad with the same issue.  I thought at least one of them was verified good, but I have no way to test at moment. 
Bad pots (i have new ones on order, but had same issue with other older pots i have laying around)
Bad PT.  Not sure how this can be an issue.
Bad wall power. Again, not sure how this could be the issue.

[munkeyboy]

BTW, I'm being overly dramatic, when I say it was a failure.   It works almost like the last one, but a little less crunch and thus a tiny bit less fizz. 

QQ.  Since the 100uf across the 6v6 cathode made things marginally worse, could that be an indicator of anything?   

[shooter]

could that be an indicator of anything?   

my understanding, the bigger the cap the less effect NFB will have 

fizz.

You should be able to see fizz on a scope
I had one amp PP 6V6 that once I hit clipping/saturation G1 would have DC, the more I cranked, the higher the DCV (Up to 8vdc).
rebuilt twice, same problem.  never proved it was my testing,, never found problem.  Rebuilt again to PSE 6V6 problem solved   


EDIT spelling


sometimes the electrons win! 

[munkeyboy]

Btw, PT is not the issue (swapped to another), PT current/voltage not issue (more current, more voltage, but definitely hot at 115%pdiss), speaker cable not the issue (wired directly to speaker), speakers not the issue (wired in new speaker), instrument cable not the issue.   Interestingly, speaker cables make terrible instrument cables.  Introduces tons of buzz/hum and attenuated the signal.   With speaker cables used at instrument cable, the fizz was less, but audio in general was horrid.

I did see -15vdc or more on the grid when playing (not at idle).  I could not find much noise on the scope with a sig-gen.  I get noise at the bottom of the wave at the OT when playing the guitar (it is in one of my earlier screen shots). 

I don't know why I haven't tried a listening amp yet.

[munkeyboy]

Just logging this for myself and anyone else who might find it useful.    Moving over to the vintage 30 speaker, the fizz is less by a little.   this is a 16ohm speaker instead of 8 if that plays a part (8ohm ot).    Also found that different 6v6 make different sounds.   So one of them (westinghouse that was stock with the PT) sounds like more like fizz, the other two (nos RCA and new tungsol) sound more electrical the higher the volume.  Almost like really fast arcing or ringing.  Swapping 6sl7 didn't make a difference in the sound character.

[shooter]

put a signal in, scope the 6V6 grid,(make it a sinewave), then scope speaker, see if there's anomies.   

[shaun]

Heyo. Sounds like you're in the sort of hell I find myself in quite often.

If I'm not mistaken, the 5879 is somewhat similar to the EF86. I just built an amp with an EF86 in V1, and in my experience, they don't stay clean for too long. Granted, I could be doing something wrong, but they don't don't tolerate a high signal input as well as a 12AX7. The break up in mine is quite pleasant, so I don't mind it, but I have resigned myself to getting considerably less headroom with a pentode in V1.

I found a sorta fix by running global NFB from the speaker output to the cathode of the EF86. That helped a lot (used a very large resistor for minimal NFB - 680k), but only because I also made the 22uF bypass cap switchable. With the bypass cap in, I get max overdrive - kinda cool. With the cap out and the NFB in, I get more headroom and a tamer tone. It is an on-off-on switch, so in the off position, I just get the 2.2k cathode resistor.

Now, I claim no expertise, especially in this crowd who are so highly knowledgeable, but I have learned to lower my self-imposed expectations and experiment to see what I can learn, rather than what I can make. The above results gave me a blast. It's not the amp I was hoping for, but many guitarists would find it - at the very least - interesting, and quite playable.

Hope that is some help. Oh, and speakers do have different break-up levels. I like old alnico speakers, but I believe the magnets can go flat, resulting in early breakup and a lesser tone.

[munkeyboy]

So using a diy listening probe into another amp, it seams like the noise is on the 6v6 grid/6SL7 preamp plate.  The Preamp grids are fine from what I can hear.  So "seems" like it's a problem in 2nd half preamp tube amplification.   I don't know if I'm unlucky enough to have 3 faulty 6SL7's or it is something else with the tube placements or just to much gain.   

I clipped across a second 3.9k to the 2nd half cutting the cathode R in half to bring up headroom.  That reduced some noise.  With NFB on, it is almost usable at 75% volume, but NFB is like a wet blanket.

Still learning how preamp overdrive works.  Do you seen any issues with changing the preamp like this to try to increase headroom:

• 100k on preamp plate load (vs 220k)
• 1.8 on both cathodes (vs 1.8k and 3.9k)
• up the grid stoppers from 47k to 68k or 72k
• Or up the voltage divider from 270k

I also have one 6SJ7 that I tried in one of the first attempts, I think it is microphonic so I went back to the 6SL7 circuit.  But I just realized I have a 6SK7 in my tube drawer.  So if I can not solve this without loosing that good kind of breakup (i don't want a 100% clean amp), maybe I'll [re]wire a 5C1 circuit with that 6SK7 and try to eliminate tubes as the issue.

Hey Shaun thanks for the feedback.  BTW, I'm not using the 5879.  I'm saving that for something else.   I'm just using the octal 6SL7 and 6V6.

[brewdude]

At the risk of derailing this thread...
I found that the headroom on the pentode preamp tubes I played with (mostly 6SJ7) varies substantially with the screen voltage.  If you find the pentode to be too dirty too soon try a smaller (say, half as big) screen resistor. 

[munkeyboy]

BTW, attached is current layout.  I think it is current at least 

At the risk of derailing this thread...
I found that the headroom on the pentode preamp tubes I played with (mostly 6SJ7) varies substantially with the screen voltage.  If you find the pentode to be too dirty too soon try a smaller (say, half as big) screen resistor. 

Thanks brewdude.   Yeah that was one idea I was toying with, but I sure don't want a 100% clean amp.  The fizz and breakup seem to be one and the same, even the a small amount of breakup introduces fizz.

[shaun]

At the risk of derailing this thread...
I found that the headroom on the pentode preamp tubes I played with (mostly 6SJ7) varies substantially with the screen voltage.  If you find the pentode to be too dirty too soon try a smaller (say, half as big) screen resistor.

Good to know! I have been trying to follow the Valve Wizard's advice. He says emphatically that the grid voltage should never be greater than the anode voltage. Lowering the screen resistor increases the screen voltage. I'd like to understand that balance a little better.

[PRR]

> He says emphatically that the grid voltage should never be greater than the anode voltage.

Where?

[shaun]

I'm sorry. I misspoke. He says the screen grid voltage should never be greater than the anode voltage. It seems a bit weird, but I can't find it online any longer. I printed it out a year ago, so here's an attachment. It was titled Small-Signal Pentode, but that online doc is not the same as this. 2nd paragraph down. I may have misunderstood, but he seems to say the screen must have lower voltage than the anode.

[munkeyboy]

Well, all three of my 6SL7s are bad... at least that is my conclusion after changing to an 5C1/6SJ7 circuit.   The 5C1/6SJ7 circuit sounds ok so far, tho I worry about the 5vdc on pin 6 vs the 20vdc I expected.   Here is the latest layout.

[PRR]

> He says the screen grid voltage should never be greater than the anode voltage. It seems a bit weird, but I can't find it online any longer.

Thanks.

He says "for normal operation". Whatever that is.

I wonder if it vanished from the site because he realized it was not a good statement. These are not True Tetrodes. A Pentode's G3 breaks-up the current-stealing until the anode gets "way below" G2, like 1/3rd. That IS the reason G3 exists. See 6AU6 data, bottom of page 3. At Vg2=150V, Ig2 does not rise much until Va is below 50V.

(The "beam power tetrode" is a pentode where G3 is implemented skeletally, as side-plates, to avoid a patent fight, and IS a pentode in all but name and patent.)

[shaun]

Aha! Very good to know. Thanks PRR. I will study the datasheet and learn something new.

[munkeyboy]

Just following up.  Calling this solved for now.  This was an perfect storm of 2 bad OTs and 3 possible bad 6sl7 tubes.

I have not verified the 3 6sl7 tubes yet as I had already change the circuit over to the Champ 5C1 6sj7 circuit.   But I did test the 3 OTs in the new 5C1 circuit and verified two bad OTs (old 1950-1960's iron) which were causing static/fizz.

The new build sounds perfect.  Surprisingly bedroom friendly with that 10" WGS.

[PRR]

...perfect storm of 2 bad OTs and 3 possible bad 6sl7 tubes....

I got a refurbished laptop. The USB ports on the side did not work. Well, after an hour of frustration I discovered I had two bad USB mice and three bad USB thumb-drives. Angry because one of the mice and two of the thumbs were nearly new.