Ghosting on high notes (recapped AB165)

Paul-McShartney

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So I just recently replaced the filter and bias caps for my ab165 (and resistors in the filter section) and I'm having some ghosting mostly on the higher notes of the G and B string. Could this be a cathode bypass cap issue since I've yet to replace those?

I really love the tone and feel of the amp as is and would rather not replace those if they are most likely not the culprit here. I've tried swapping the tubes without luck and both channels seem to do this even at 3 on the volumes.

Any help would be greatly appreciated
 

schmee

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Try plugging into the other channel, if it stops then yes, likely the cathode bypass cap. If not, I doubt you would have two with the same issue. Are you sure it's not just your guitar and you didn't notice it before?
If it is the one channel, take the cap you like out of the unused channel for that position if you want to retain the old cap style.
 

Paul-McShartney

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hmm its actually in both channels so I guess it's not the caps. I while note that there is some pretty audible hiss in the amp that goes away when taking v2 out (taking v1 and v3 tubes out will still produce the loud hissing). Think I've tracked it down to a few resistors in that section that were very out of spec. One of the 470k resistors after pin 7 drifted to around 250k which I found odd. Also the mixing resistors drifted way below spec. I've ordered CC replacements for those but is there a chance that could be causing some ghosting? Or maybe even a bad tube I somehow missed?

Also, its definitely not my guitar, I use it all the time either straight through the desk or many other amps and never had this issue. It really almost sounds like a lower octave that is out of tune when I play the higher notes.
 

schmee

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hmm its actually in both channels so I guess it's not the caps. I while note that there is some pretty audible hiss in the amp that goes away when taking v2 out (taking v1 and v3 tubes out will still produce the loud hissing). Think I've tracked it down to a few resistors in that section that were very out of spec. One of the 470k resistors after pin 7 drifted to around 250k which I found odd. Also the mixing resistors drifted way below spec. I've ordered CC replacements for those but is there a chance that could be causing some ghosting? Or maybe even a bad tube I somehow missed?

Also, its definitely not my guitar, I use it all the time either straight through the desk or many other amps and never had this issue. It really almost sounds like a lower octave that is out of tune when I play the higher notes.
Well, it's not impossible that the caps are bad in both channels, less likely though.
Hmmm, I'm a bit at a loss regarding ghosting. Power tubes can cause some of that type of issue as well as feedback. What power tubes are in it?
 

Paul-McShartney

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They seem to be some older, generic Russian tubes, maybe some NOS ones the previous owner put in. Though I'm not experiencing any feedback issues. I'll try swapping them with a matched pair of groove tubes I have laying around
 

Lowerleftcoast

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Check if it a speaker issue. 12 inch speaker cone cry? Sometimes it is as easy as the connections to the speaker.

Did you change the bias supply cap? What value did you use?

Triple check your soldering for the new caps... and everywhere else.

Have the sockets been cleaned and tightened? Any burn marks or cracks on the sockets?

What are the values of the replacement filter caps? Which manufacturer did you choose?

What are the values of the coupling caps in between the phase inverter and the power tubes?
 

Paul-McShartney

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So I tried the other power tubes that I know were 100% working and the ghost notes are still there, so definitely not the tubes.

also dont think its the cab since my buddy and I have been using it with a twin reverb head that doesn't exhibit these issues.

About the cap replacements, I used all F&Ts: two 100 ufs, three 22uf, and one 16uf. I replaced the bias cap with a sprague 50uf. Also replaced all the resistors under the doghouse with metal oxides. Soldering seems good, checked continuity on multimeter before turning the amp on. I'll definitely check for any cold solder joints and reflow the sketchy ones.

I'll check on value of the coupling caps between the phase inverter and power tubes to see if they have drifted (if you were wondering what they are, they're the original .22 uf blue molded caps.)

I'll also clean the tube sockets with some deoxit. I haven't noticed any burns or signs of extreme wear. Also the sockets seem tight still (previous tech might have re-tensioned them), tubes fit snuggly.
 

loudboy

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I had a Marshall combo that did this. It was the combination of the guitar, PU, and OD pedal causing cone cry in the V30 I had in there.

Swapped it out, fixed it.
 

Paul-McShartney

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hmm maybe it could be the speakers then...I currently don't have another cab to try it out with but I'll keep that in mind. Holding out for a Marshall 1965 cab so won't know for sure till I find one.

Also ill definitely post the voltages here, looking like I won't be able to work on it till thursday though.
 

Paul-McShartney

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So I finally got some 1965a and b cabs to try out and the ghosting is unfortunately still there with these new cabs.

Actually, before I tried the amp with both cabs (4 ohms total) I ran it just into one of them (8ohms) because many people seem to run this amp into once of these cabs with issues but I noticed a hum that was new. I readjusted the hum balance pot and that fixed it. However, every once in a while, I noticed that I need to adjust the hum balance again. Maybe something isn't biasing right? I'll note that the 10k and the 15k resistors at the hum balance pot are a bit out of spec as well so will definitely replace those.

Also, when I opened up the amp for the first time, I noticed that one of the 220k resistors that connects to the power tubes (think it's part of the negative feedback circuit?) has a burn mark on the chassis right under it. Don't know why I forgot to mention this but maybe this is contributing to the problem here?

Screen Shot 2022-05-27 at 9.32.07 PM.png


After measuring both of these resistors, they are way out of spec (Both at around 85k). Also, they are seeing 430V there. I checked the bias resistor and the two screen grids and all three test within range so I'm not sure there's a problem with those. However, the grid stoppers have drifted outside tolerance though no burn marks are present.

I took voltages of all the new filter caps and I'm not sure if these voltages look healthy:
Screen Shot 2022-05-27 at 9.29.26 PM.png


The new bias cap read 56V on the negative side and again, I'm not sure if this is normal. I also measured the resistance of coupling caps between the phase inverter and power tubes and the value kept increasing so I think they're good? (don't have a capacitance meter so not sure how else to check). Voltages after these caps and before the nfb 220k resistors read 271V and 263V. I'm new to all this stuff but I love learning about it so any help is greatly appreciated!

I'll update this thread once I get the new resistors in (once I get them I will also replace the cathode bypass caps while I'm there). But if you guys have any ideas what it could be before I get to that, please do let me know. Also, not sure what you meant exactly by voltage chart @Lowerleftcost but let me know where else I should probe to narrow down underlying issues.
 

Paul-McShartney

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Since I figured measuring resistance of the coupling caps isn't going to tell me anything (saw a video demonstrating that but after more reading, realized that's not telling me anything), I just ordered a multimeter that also measures capacitance, and an esr meter.

Also saw on here that screen grid resistors might test well and look visibly okay, but occasionally can cause these types of issues so ordered some 3 watt wire wounds.

List of things to do goes as follows:
-test coupling caps after PI (replace if bad)
-replace 220k resistors after coupling caps
-change cathode bypass caps and resistors
-replace screen grid (and maybe bias) resistors
-replace grid stoppers
-change rest of resistors that are severely out of spec
-add bias balance pot on top of hum balance pot (figured I might as well)

Again, if there's something else I should be looking at or testing please let me know! Otherwise I'll report back once I do all these things
 
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Lowerleftcoast

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You can use something like this excel type voltage chart or you can list each voltage for a tube like this...

V1 12ax7
p1 265
p3 1.6
p6 263
p8 1.8

V2 12ax7...
-----------------------------------------------------------------------
upload_2021-2-17_8-14-1.png
if there's something else I should be looking at or testing please let me know!
I am not convinced this is typical ghost note behavior. It is possible the circuit is oscillating so perhaps attempts to quell oscillation might be a way to proceed before changing out every component.
 

Paul-McShartney

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You can use something like this excel type voltage chart or you can list each voltage for a tube like this...

V1 12ax7
p1 265
p3 1.6
p6 263
p8 1.8

V2 12ax7...
-----------------------------------------------------------------------
View attachment 988553

I am not convinced this is typical ghost note behavior. It is possible the circuit is oscillating so perhaps attempts to quell oscillation might be a way to proceed before changing out every component.

Thanks so much for elaborating! It's my first time working on an amp so sorry if I seem a little lost. I just took the voltages of each tube here:
Screen Shot 2022-05-30 at 8.13.23 PM.png


Note I took the voltage of p7 on V2 after .01 Cap and 470k resistors. looking at the charts, does anything seem abnormal here?



Also I took some of the bypass capacitors. The only thing that might be questionable are voltage loss readings but everything else should be pretty accurate. Looks like V2 and V3 are way out of spec.

Screen Shot 2022-05-30 at 8.24.06 PM.png


Lastly, the coupling caps tested fine as for capacitance (21 nf each) but their voltage loss is around 6% (again, may not be too accurate)

I'll definitely look into investing if it is oscillations I'm dealing with before I replace anything that doesn't need to be (though I'm still leaning towards changing the severely out of spec resistors (about 4 of them) and the out of spec bypass caps).

I did notice that the wires from pins 2 and 7 to the volume and input jacks are very microphonic. also a few more wires in V1 are a bit microphonic as well.
 

Lowerleftcoast

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Note I took the voltage of p7 on V2 after .01 Cap and 470k resistors. looking at the charts, does anything seem abnormal here?
It appears that .01uF cap is not blocking DC. Replace it.
I am also concerned about the 470k local negative feedback resistor that terminates with the .01uF cap and the 220k resistor. You will need to lift one leg of the 470k to measure the resistance.
Hopefully you have found the culprit!
the bypass capacitors
The measured voltages of the tubes do not indicate a problem with the bypass caps but the measurement of the caps looks questionable. Imo, they are e-caps... I would replace them.
the coupling caps tested fine as for capacitance (21 nf each) but their voltage loss is around 6% (again, may not be too accurate)
I too, question the accuracy of the meter. If the pots are not scratchy, I would leave the caps in (except the .01uF above).
I'm still leaning towards changing the severely out of spec resistors (about 4 of them)
You have a good case for changing these out.
I did notice that the wires from pins 2 and 7 to the volume and input jacks are very microphonic. also a few more wires in V1 are a bit microphonic as well.
These pins are the beginning of the gain stages. They will be microphonic because of the amplification factor.
 
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Lowerleftcoast

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I usually don't pay too much attention to each leg of the heater voltage. I just check across the pins for the total ~6.3VAC. I find your measurements curious.

It seems one leg is 2.6VAC and the other leg is 3.1VAC. Does your PT have a heater center tap, or are there two resistors for an artificial tap? If it has the artificial tap, one would like to know if the resistors have the same resistance. Do you notice a 60Hz hum?

Just curious...
 

Paul-McShartney

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Ah sure, I got a replacement coming in for the .01 uf cap (was microphonic). Yeah that 470k resistor was measuring around 250k but I didn't;t take the leg out so I will check that out later tonight! That 220k did measure 150k as well. I'll replace all three (if 470k is indeed out of spec) once my package comes in and hopefully that'll fix it! By the way, these two were one of the 4 resistors that really drifted. The other two are the 220k resistors on the 6l6s.

And yeah I guess I really should replace the bypass caps, I've been hesitant because I love the way this amp sounds as is (despite the ghosting haha) but they are really out of spec. I guess the high capacitance of the latter three are making the amp retain more bass? I guess it could use a little tightening with the stock values.
 

Paul-McShartney

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I usually don't pay too much attention to each leg of the heater voltage. I just check across the pins for the total ~6.3VAC. I find your measurements curious.

It seems one leg is 2.6VAC and the other leg is 3.1VAC. Does your PT have a heater center tap, or are there two resistors for an artificial tap? If it has the artificial tap, one would like to know if the resistors have the same resistance. Do you notice a 60Hz hum?

Just curious...

Yeah I remeasured last night across the pins and was getting 6V but I was curious as well about why the difference in voltage. My PT has the heater center tap. I don't really notice any 60hz hum, when there has been hum, I've dialed it out by adjusting the hum balance pot. When I turn up the volume though, there is some hum, but I'm thinking it's normal?
 




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