Mojo tone super reverb new build causing ghost notes

[eggbert]

Kinda new to forum, i built a super reverb mojo tone amp, single channel, with verb and vibrato, i have built 5 of these with no issues, now (ghost notes appear on d, g and b strings 10th fret and higher) on this build. Amp is dead quite, all voltages great, Turning volume past 2 o’clock causes the ghosts to appear, more appearant using neck pickup and increasing the bass.  I have moved the grounds around thinking loop. Reflowed all, clipped in new caps and choke, and nothing helps.... rebiased hot and cold no change, tried different speakers, guitars, cords etc. no change. Changed tubes as well, could it be the o.t. ???   I am running 4 ohm into 8 ohm speaker on my bench then it goes into a 2x10 at 4 ohms... Any help much appreciated

[eggbert]

Update, ran a ts9 od in front of amp, volume @ 11 o clock, without od on, beautiful full clean, not much breakup using 16k output pickups. With od on nice sustain, great tone yet ghost notes appear. Could this be preamp causing it?  At first thought the p.i  yet if not pushing the power tubes why would it still ghost note.  I am using 1 of the 2 220k mix resistors to p.i tried reducing, increasing and removing nub. No change.

[eggbert]

This was built to the schematic by mojotone, just deleted the first channel and changed the 2nd stage cathode from 820 ohm to 1.5k.   Using sozo caps .1 and .022 tone stack, and 3 sozo .1 at p.i.   Remaining couple caps are orang drops,  didnt try strapping a small cap v1 plate to grid yet. And help much appreciated. Like i said earlier i have built 5 of these the same way without issues.

[pdf64]

What exactly do you mean by ghost notes? Consider recording an example and upload to youtube etc, so we're all on the same page / referring to the same issue. As different types of ghosting will have different root causes.

[brewdude]

Are the ghost note in tune with the intended note? Or, dissonant?

[eggbert]

The notes i am hearing are octave sounding lower and out of tune with the root note, it decays before the root note, and more noticeable the harder you dig into the note. I will try to upload a clip as soon as i can. Thanks for the reply’s

[eggbert]

So i ran the amp again, the notes sound like there a step octave pedal on stepping down in 3 notes out of tune with original note. The ghost notes step down every 2 seconds after hitting original note and sounds like 3 steps it takes before fading until it fades to original note, also sounds like it makes the notes die out instead of sustaining, i cant get a clip up on YouTube yet, still working on that......

[eggbert]

Ok had a brain storm, maybe this doesn’t even matter but? I ran a bass guitar into the amp, amp cranked to 8 or 3 o clock and no ghost notes,  the pickups are @ 5 k, 1/3 less than my wolfgang but i am hopeless and had to try something.  Embarrassed to say i have a scope and signal generator i just dont know how to use it. Help?

[pdf64]

So i ran the amp again, the notes sound like there a step octave pedal on stepping down in 3 notes out of tune with original note. The ghost notes step down every 2 seconds after hitting original note and sounds like 3 steps it takes before fading until it fades to original note, also sounds like it makes the notes die out instead of sustaining, i cant get a clip up on YouTube yet, still working on that......

I guess a bad HT cap, either at the power amp’s OT or screen grid nodes.

[eggbert]

Thanks for the reply, trying to get video up with no luck,  uugggg
I did clip in temporary caps with no change, it didnt even reduce it.

[eggbert]

Update... so i got my scope running, i dont know if i used it right but, i traced a clean signal thru the amp, yet at the screen node where the choke and screens and 1st cap join, signal looked like 3 bat wings flying all over the place, is this the ghost in the machine ? And what do i do? I traced the bats thru the b rail to the vibrato and then to the reverb transformer.  New tube sockets, no wires touching at the sockets, screen and grid resistor new and right on resistance. I changed the choke and no difference. So maybe o.t. Is shorting? I do have a twin reverb 8 ohm o.t. Can i drop it in to just test for 10 mins.   Thanks in advance for any help..

[SILVERGUN]

I'm not sure this would cause your complaint but double check that you have the OT wired correctly.
i.e...make sure the center tap is connected to B+

The other thought would be to make sure that your sockets are wired in pairs together correctly.
This was wrong. I was picturing 4 output tubes with the pairs being mixed up. Super Reverb only has 2.


Just throwing some darts at it for you.

[eggbert]

i also pulled the vibrato tube and reverb driver tube, thinking the signal would have cross talk ?
Didnt appear in those areas after pulling the tubes. Changed the filter caps 2 times, ( this is a new build too )  heaters are at 3vac each, 440 plate and -42 dc and biased @ 32 to 36. This is suppose to be a custom build for a client and i am stumped,

[pdf64]

Update... so i got my scope running, i dont know if i used it right but, i traced a clean signal thru the amp, yet at the screen node where the choke and screens and 1st cap join, signal looked like 3 bat wings flying all over the place ...

It would be helpful to take a photo and upload it to a hosting site, eg imgbb. And provide details of the scope settings.

[eggbert]

I tried pics, didnt work out, believe it or not i have a flip phone(old school) lol and a computer at home, digital camera shit the bed and i didnt win the lottery yet. I dont know if i am running the scope right,  1k signal in input jack, scope set to ac, and by turn time/div and volt/div,i can see a nice sine wave with the volume at 2 to 3 with tone controls straight up, turning volume up it increases, (the sine wave ) yet at that node point its fuzzy/waves then using the time/div and volts/div it zooms in and it turns too sharp jaggered bat wings( like 3 or 4 of them )

[eggbert]

My kid is going to try to take pics with her phone hopefully soon.... she works 2 jobs and i work 3.  No joke.......

[eggbert]

Pics while building, just to see if i was able to upload.

[eggbert]

Finished, after changing multiple pieces, uggg

[eggbert]

These 4 pics are as follows, input at 68k volume on 3, then  at slope resistor volume at 3, then 4vb  volume at 3, and at the 220k mix to p.i volume at 3.  Settings were changed pic to pic on scope

[eggbert]

These pics are at the screen/choke node, 1st pic volume at 5, then volume at 2, then volume at 3 then volume at 4.  Hope these help and sorry if i dont know how to use the scope correctly.

[eggbert]

Update,  so scoping the amp, which i dont know if i did it right but,  i measured the plate voltage at pin 3 it reads 440vdc, with volume on 2 on amp. Turning the volume up to 4 and then 6 the plate voltage drops to 398 ?????  Could this be a draw off the ot shorting out ? Help please..

[pdf64]

Don’t scope output valve anodes.
The voltage spikes there can be enormous, way higher than your scope or probes are rated for.

[eggbert]

I scoped at the screen resistors pin6, and measured voltage drop at pin 3, when increasing the volume pot from 4 to 6.  I dont know if the pics can show setting good enough to see if i am using it correctly, but that’s what i see and changing almost everything other than ot and pt i am stumped.  I do have a used 50 watt bassman ot, would it make sense to drop it in just to see if anything changes ?  Thanks pdf64

[pdf64]

... when increasing the volume pot from 4 to 6 ...

The above isn’t really helpful or meaningful.
It would be much better to refer things with regard to power output levels, ie across a resistive load.

[eggbert]

would the resistive load be running the amp into 2 8 ohm speakers parallel for 4 ohm load and amp is wired to 4? I am a newbie so sorry if i misunderstand.

[pdf64]

No, it would be beefy resistor/s that put a resistance value equal to the nominal intended load impedance on to the output of the amp.
So in your example, for an amp designed to deliver 50W into a 4ohm load, a 100W 4ohm resistor or resistor network.

[eggbert]

Thanks pdf64 for the reply, 

[pdf64]

It’s not feasible to assess power output with a real speaker as the load, due to its highly reactive nature. ie its impedance, and the phase relationship between the voltage across it and the current flowing through it, will vary significantly according to the frequency used.

[PRR]

> with a real speaker

This "can work" if you hunt around 200-400Hz for the impedance minimum. And measure it closely. And again as the VC heats up. (Modern materials hold-together while copper resistance doubles in the heat.)

Of course people who have to ask won't know how to do this.

As for maximum power: the speakers used with most higher power bass/guitar amplifiers will not actually take full power long enough for careful meter reading.

[eggbert]

Thanks pdf64 and PRR,  again i am a newbie and wont ask how to, i am still learning, reading everyday, and process of elimination has worked for me in the past. Maybe a class for me real soon to up my knowledge would be in my favor. I respect everyone here and i am thankful for everyone’s knowledge. 

[pdf64]

Any progress with solving the ghost note issue?
I suggest to scope the output waveform of a high power 400Hz signal. Looking for it being modulated, eg by HT ripple.

[eggbert]

No luck yet, ordered up some 100w and 200 watt resistors for a dummy load,  i rewired, regrounded, moved wire placements, reduced coupling caps, reduced 1st cathode cap to 10uf to reduce some lows,  nothing worked. Put everything back ( of course i did 1 change at a time).  Only thing i noticed was checking bias w/ bias probe biased @ 36. I played the guitar and hit the notes where the ghost notes appear and the bias meter hit 130. The notes faded with the decreasing ma.  Thanks for checking in pdf64, keep you posted.

[thetragichero]

just a bunch, but does it do this with any speaker or just one in particular?
reason i ask: i have had the same symptoms (aaaaalmost sounding like an analog octave up above the 12th fret on the skinny strings) when i torqued the speaker mounting screws too tight, slightly deforming the basket. could be why you're not tracking down the issue on the scope (I've also had a buzzy kind of octave after the note with a damaged voice coil)

[eggbert]

Yes it does it with any speaker, tried 4 to 5 different cabs/speakers. Noticeable when pushing the amp @ 5 to 6 on volume,  lower octave out of tune note follows the original note< more pronounced g and b strings above 9 th fret.  Thanks.

[eggbert]

So further testing/and messing with the amp, (while waiting for resistors for dummy load. I can measure up to .31 vdc  on most parts of the fiber board, i believe this would cause parasitic  oscillation although with my ears i can not hear a church mouse. The amp is dead quite yet, i get static crackle running my multimeter probe across board and slightly pushing down on board, i have pushed this amp to the limits with volume dimed on 10 slamming rock chords and its tight with no farting out, i  ( reduced p.i. Coupling caps to .022 and using 2 (100k) and the norm 82k &100k), i have replaced again the filter caps in the power supply screen, p.i. And preamp with a 40 uf ( 1 at a time ) and no change with my main issue in this amp. I will conquer this,  ( prr stated to look @200 to 400 hz)  saying this the problem is more noticeable with the neck pickup on all guitars i use.  ( i did change the ot with a twin reverb just to test ( total 3 mins) ran into an 8 ohm speaker and the problem was still there.   Keep you all updated. And thank you for the replies i have received..

[eggbert]

If i scoped it right? 1khz input, 4ohm 200watt dummy load(4ohm secondary off ot) scope set at time/div 1 volt/div .2 probe at x10 scope at output jack. Using 200hz/400hz and 1 k input results the same. Clean signal thru volume 0-10 but t-m-b controls set under 2...(pic 1)if i turn controls up to 3  the sine wave appears to square with distortion (3 pic)  if tmb controls are at 3 i can reduce volume to 3 and it cleans up? (Last pic) Any thoughts or am i scoping wrong.

[eggbert]

I also changed 470ohm screens to 1k 5w and changed grids 1.5k to 5.6k
And should i disconnect nfb from speaker jack when testing ?  Also reverb tank disconnected. Foot switch disconnected and verb/ speed and intensity knobs at 0.  If that helps.

[eggbert]

Also with volume at 3 i can turn all tone controls and it keeps a clean signal, then increasing the volume ( to 4 ) produces the same effect as above, is this because the peak to peak increased due to the signal increasing from the volume pot?

[PRR]

> 1khz input

What Voltage are you putting in?

And what actual Voltage at the output? I don't trust myself multiplying all the 'scope dial and probe multipliers, I put a dumb meter on the output. (Some meters go deaf above 400Hz.)

Basically from here it looks like normal distortion. There is always some input level which will distort the output, and it changes with knob settings.

[eggbert]

At 1khz input i have .580v a/c and at output i have 10v a/c with volume on 3 no clipping, turn volume up to 8-10 and reads 17v a/c. Hope this helps. Thanks PRR.

[acheld]

Is your input voltage measured RMS, or peak to peak?   0.6 volts AC (RMS) would be the equivalent of really mashing on a hot Humbucker.  And, the signal would taper off quickly.   Even 600mV PtP is a substantive input signal.

[eggbert]

I just measured the signal from generator at 1/4” jack and same on output with multimeter. Is this wrong ?  I am a newbie so excuse my ignorance.  Thanks

[sluckey]

Set your input signal amplitude to 100mV_pp using your scope.

[PRR]

Guitar can be approximately 20mV to 200mV (0.020V to 0.2V).

0.580V is really hot. I would expect to turn-down to "3" if I wanted to avoid distortion.

[eggbert]

Thanks for the replies, sluckey and prr ...Funny thing is the amp is dead quite, no hum at all. Sounds great...i am trying to track down that ghost note. The transition from clean to dirty is smooth and distortion sounds great, no flub/flutter, holds together great. Using 16k output pickups in my peavey wolfgang, cleans up very nice with amp on 8 and rolling guitar down to 7.  Just that damn ghost note.  I will drop the scope down see what i can find. Thanks again.

[SILVERGUN]

What is the value of this resistor?

[sluckey]

I see 22K.

[eggbert]

Yes 22k to 100 ohm nfb is 820ohm.  I set scope to .1v (100mv) tested with volume on 3 and probe set to 10x the signal went up to 8v peak to peak, at this point the bottom peak went more negative and sharper, the top was a curve with a notch in it. Then i had to switch to .2 (200mv ) to see the range in the scope. The signal was the same yet 2 peaks up 2 down, so this would be 8v peak to peak at this point turning volume up to 4 on amp squared the notches off and 1 top side of square notch went up to a sharp fuzzy point. Try to get pics up tomorrow. Is my math correct? And no matter what i set on the generator or scope i am still reading .580vac. 

[eggbert]

No news and still bad news. Still haven’t figured this out yet.... i keep trying everyday.

[eggbert]

Update, new pt and ot installed and still ghost notes are there. Also swapped choke, no difference. Swapped coupling caps, cathodes and several types of tubes, no luck.  Tried reflowing solder joints and moving grounds, and rewiring tone stack and other wires, no luck…. The only thing left is to rebuild on epoxy board instead of fiber board( maybe board is conductive??????