Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on July 17, 2025, 02:45:11 pm
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I've posted about this amp before. All of the resistors and caps and tubes have been replaced. The field coil speaker has been replaced with a permanent magnet speaker using a choke. A schematic is attached.
It has an Instrument Channel with only 1 wired input jack. The other jack was disconnected for future removal and use of the chassis hole for something else. The other channel is the MIC channel. Each channel has its own Volume Pot, but there is only 1 Tone pot.
After working on this amp for some time, it has a 60hz and 120hz hum even with nothing plugged into the input jacks, and even if the jacks are disconnected from the circuit. If the 6SN7 tube is removed the hum goes away. Other pre-amp and PI tubes have been tried with no effect on the hum.
The amp has very little treble produced from the Tone pot.
The Instrument channel reacts normally, just low Treble range.
The MIC channel is another story. With the Volume pot at 2 (8 o'clock), and the Tone in any position, no problem except for the hum. After turning up the Volume from 2 through 7 (2 o'clock), low level treble hiss begins. At 7 thru 9, the signal is distorted greatly. But just past 9 it clears up. Still treble is minimal but it has increased.
I checked the signal frequency of the distortion and its around 21khz, and a high amplitude, so very noticeable. When I adjust the Volume below 7 or above 9, that signal practically disappears.
I hope someone can help in solving this signal distortion. Thanks.
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On the schematic, the input jacks don't shunt the input to ground when nothing is plugged in. This leaves the grid inputs floating, is also true if the input jacks are disconnected. What happens to the 60 Hz hum if you plug in a shorting plug on the input, or just short it to ground with a clip lead?
You didn't mention replacing the power tubes. Are they functional? How well balanced are they? If they are imbalanced, 120 Hz ripple from the B+ won't be fully canceled in the output stage. (But if removing the 6SN7 make the hum completely go away, this explanation is less likely.)
What is the signal you're using to measure the distortion? And how are you measuring it? I'll just note that "around 21 kHz" is just about the Nyquist frequency for the common 44.1 kHz sample rate. If you were, for example, sampling the signal with a computer sound card for distortion measurement, could that frequency be an artifact of the sampling?
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I replaced those jacks with shunting jacks. So they are grounded.
The power tubes are new and matched.
I've measured the signal 2 ways. One with an App on my iPhone called FFT Plot. I just open it up and put it in front of the speaker. No input signal being used. I also connected my scope to the Output jack with no signal injected and get the same results. So its getting the hum and the distortion even if no signal is injected into the amp's circuit, and even if the input jacks are physically disconnected from the circuit connection to the grids.
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If the 6SN7 tube is removed the hum goes away.
how much work do you want to do??
If it was a keeper, i'd make each triode it's own thing, no sharing cathodes or plates
I'd add grid-stop resistors at the tube socket.
did you increase PS filter capacitance?
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I also replaced the OT. The PT is original.
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I replaced the filter caps with individual caps of 20/20/10.
All the tubes are new.
Good idea Shooter on re-wiring the sharing of cathodes and plates. That shouldn't be too difficult.
I did add grid stoppers to the OT's but it didn't make any difference and I later removed them. Did you mean add grid stoppers to the grids of the 6SN7 pre-amp tube?
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For the 60hz hum, Have you tried elevating the 6v heaters?
If there is a separate wire from the 6v CT, remove it from chassis and jumper it to the 6v6 cathode
also without seeing the build are the heater wires away from signal path?
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for the 120hz hum
without seeing your build have you grounded your PS nodes properly?
Preamp B+ node ground to the preamp ground
Power Tube B+ node grounded to the Power Stage
there is a great grounding diagram posted here
https://el34world.com/charts/grounds.htm
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I never heard the amp before my buddy started working on it. So I don't know what it sounded like. I just know what it sounds like now.
All the grounding is original. Nothing has changed with that. The preamp ground is separate from the power supply grounds.
The only wire that was weird was a bare wire from the PT. I later learned that in these old amps, ie. 1947, they often grounded the PT chassis to a ground point with this wiring.
I haven't lifted the 6v grounding. It is original. Its wires are original. As far as separating the filament wires from the signal wiring, it is what it is because the space is so small and all the tubes are packed in like sardines.
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Did you mean add grid stoppers to the grids of the 6SN7 pre-amp tube?
If the 6SN7 tube is removed the hum goes away.
yep
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Sounds like you are making modern circuit changes from original.
As well modern ground techniques might help with vintage hums.
And the hum might be created with modern AC wall voltages that are higher than vintage AC wall voltages
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Grid Stoppers are for RFI right? not for 60/120hz hum right?
Nevermind - after rereading the original, you are addressing the signal distortion
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Have any pictures of the amp ?! The guts of it.
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sorry of you already addressed it and I missed
but is that .006 cap in the mic channel still good?
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It’s a .005, the reading isn’t the best to see. Yes it’s new and good.
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Do you have an as built schematic?
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The only real change was adding the choke. Everything else is wired as it was, just new parts. And the schematic attached is what I’m working with, and the wiring follows the schematic. I’ve checked it 5-6 times looking for a mistake.
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Shooter-Where should I install the grid stopper resistors for the 6SN7? One end at Pin 1/Pin 4 for the 2 grids, and then where does the other end of the resistors connect?
Is a 68K resistor the appropriate size?
Thanks.
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Should I disconnect the other components that are currently connecting to each of the grids, and install the resistor on each of those socket pins (ie. 1 and 4), and then connect the other end of the resistor to the components that were disconnected?
This amp is point-to-point wired, with very few terminal strips, so it gets tricky--at least for me- ie. adding a wire with no solid connection point.
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this is where I'd start, where you end is up to Hoyle, Roberts, or Murfree :icon_biggrin:
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Shooter-Great! Thanks for your help. Fingers crossed this will get rid of the oscillation, and maybe the hum too.
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Shooter-No luck with this mod. I still have 60 hz and 180 hz hum. The 120 hz seems gone based on the frequency analyzer app. But I still have the same issue with the 21khz oscillation/distortion. It's only in the MIC channel. It seems to begin appearing when the Tone pot begins to add some Treble, and if it's Vol. pot is above 2. Its really noticeable when the Vol pot gets up to 7, regardless of where the Tone pot is set as long as its adding some Treble to the circuit, BUT it cuts out when the Vol gets up to 9.
Could this be some kind of parasitic oscillation that's maybe coming from the PT? But only the MIC channel seems to be affected.
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120hz is a good indication of poor filtering on the DC, bad/sketchy grounds, sloppy sockets n connectors etal.
the 21K is a good indication of parasitic osc. or that channel picking up stray RF from the airwaves
try grounding the mic-channels grid to see if the 21K goes away. is the tone pot original?? you could temp-in a new TS/vol or bypass for testing
you can also "walk" a solid chassis ground wire to each "ground point" and see if the 120hz "changes" as you ground grounds
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Shooter-all of the pots have been replaced with new ones.
The 120 doesn't seem to be showing up much on the analyzer, but it showing 60 and 180 hz noise, and the 180's amplitude is much higher than the 60hz.
The amp pics up this 21khz wherever it's plugged in. It's not just one location but at locations miles apart. I don't think its RF from the airwaves for that reason, plus its "tunable" with the Vol/Tone pots but ONLY on the MIC channel where maybe the impedance is different than the Instrument channel.
Is there a way to test the Power Transformer to see if its the source of parasitic oscillation?
I'll try grounding the grid and let you know.
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YES, grounding the Grid eliminates the 21khz noise!! So what do I do?
But there is a 60hz hum at -73db, then a slightly larger 120hz hum, but a very prominent 240hz at -45db.
The Tone control brings that in when it moves past 2 on the pot, just starting to let the Treble through.
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YES, grounding the Grid eliminates the 21khz noise!! So what do I do?
1. verify your new shorting jack is actually shorting (you can also just temp-short ring to sleeve)
1a. verify the ground is solid at the 1 meg grid-leak
2. "bypass" the pot n .006uF with a jumper, you want the 1MEG N grid-stop "in-circuit" BUT!!! don't add signal, just see if the 21Khz comes back.
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there is a 60hz hum at -73db, then a slightly larger 120hz hum, but a very prominent 240hz at -45db.
this sounds like your measuring device isn't very good a(t) discernment :icon_biggrin:
a scope with FFT/spectrum feature comes in real handy
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Shooter--#1 and #2 checked out fine. Ground continuity.
With no inputs in the Input Jack or any kind of input signal, and the Grid still jumped to a ground, I jumped the cap connecting the wiper on the Mic Vol pot to the 1 Meg resistor.
The 21khz remained gone.
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FYI-I grounded the Grid by connecting a jumper at Pin 4 Grid with the other end of the jumper to a ground point on the chassis.
Everything else remained connected in the circuit, but no Plug was inserted into the Input jack, and no Signal was introduced.
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Re-tension the faulty jack switch tip.
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The jack tip switch works fine. It goes to ground when nothing is inserted, and goes to circuit when a plug is inserted.
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and the Grid still jumped to a ground, I jumped the cap connecting the wiper on the Mic Vol pot to the 1 Meg resistor.
that proves nothing
once you proved the 21khz was "left" of the grid, then it's time to eliminate what's there, so UN-GROUND the grid, jump the cap, test, if still there, jump input jack-TIP to grid-stop/grid leak junction, "bypassing both cap and pot.
EDIT DO NOT use "continuity" for testing, use OHMS, continuity might show a "short" when in fact it could be 10-50 OHMS of resistance
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With the jumper to ground removed from the Grid Pin4, The 21khz signal is NOT present when either the cap is jumped or the Input tip to grid/stop/leak is jumped.
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When I jump the Tip of the Input Jack to the lead going into the 1 Meg resistor, not only is the 21khz signal NOT present, but the hum is much quieter.
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The jack tip switch works fine. It goes to ground when nothing is inserted, and goes to circuit when a plug is inserted.
How are you so certain? It might appear okay from a quick glance, but the contact could be slightly loose, causing microphonic vibration.
I suggest using a chopstick to mechanically press the tip against the tipswsitch to see if that cancels the noise
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A week ago I checked the jack just as you said, and it functioned properly. I just checked it again, as you suggested, and it is still functioning as designed.
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When I jump the Tip of the Input Jack to the lead going into the 1 Meg resistor, not only is the 21khz signal NOT present, but the hum is much quieter.
ok, jump or cut out the cap, still ^^^^^^^^^^^^^^^^^^?
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Shooter-As I said in #32, when I jump the cap there’s no 21khz noise.
Seems to me that maybe the 1 Meg resistor is the problem since a direct ground eliminates the oscillation, or whatever is going on. Is that resistor too big and blocking the path to ground? Should it be smaller or eliminated?
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I decided to check the voltage on the Plates of the 6SN7. It had previously read an actual voltage on each plate of 89VDC, with the schematic showing 75VDC. It now reads 31VDC! Now that's strange.
So I checked its B+ coming from the 10uf cap and it is now reading 190VDC at the cap, and it previously had an actual reading of 215VDC, with the schematic showing a value of 175 VDC. So that's probably ok.
Then I checked the voltage going into the 100K resistor between that B+ source and the Plates. It reads 190VDC going into the resistor, BUT 31VDC coming out and connecting to the Plates! Quite a voltage drop! I checked the 100K resistors value in circuit and it shows 99K, so its value is okay, but why the big voltage drop?
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E (volts) = I (current) * R (resistance)
so if E is low and R is constant.............
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Then current is very low.
So the tube is not working properly and not drawing a load?
Is that because the tube has a bias issue?
The caps I used at the cathodes are 25uf/50V vs. 10uf. I didn’t have 10’s.
Sorry but that’s the best I’ve got as a response without a solution.
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did you split the cathodes?
IF yes, what is the VDC reading at each cathode?
did you use 2 1.5k vs original 3K?
IF yes, did you verify they where actually 1.5k n not 15K or 150K or...
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Yes, I split the cathodes.
Cathode Pin 3 = 1.195VDC
Cathode Pin 6 = 1. 188VDC
I used two 1.5 K resistors which read 1.49K and 1.48K
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And you tried another tube in that input slot?
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At your suggestion, I put in another tube and the readings are the same, just around 30VDC. A voltage drop through the 100K resistor, which reads 100k in circuit, of about 160VDC.
The voltage is fine going into the resistor but drops dramatically for some reason when it comes out to the plates. Obviously the tube is not the problem.
Can the resistor read fine like this one does, but is defective in some other way that would cause this? The voltage read fine after I installed the new resistor a couple weeks ago.
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I went ahead and installed a new 100K resistor, and checked its value before installing, and the results are the same as with the old resistor. 190VDC voltage drop and 31VDC at the plates. The old resistor also tested fine out of circuit.
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The 2 plates at pins 2 & 6 are jumpered with a single source of voltage being supplied by the 100K resistor. Since the cathodes are now separated and no longer jumped and sharing a single resistor and cap, would that affect the voltage?
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Since the cathodes are now separated and no longer jumped and sharing a single resistor and cap, would that affect the voltage?
Absolutely! Your original circuit shows a 3K resistor shared by both cathodes. This resulted in 3V bias on the cathode. In reply 19 shooter suggested splitting the cathodes and using two 1.5K cathode resistors. The smaller resistors biased the tubes much hotter and this caused more tube current which caused a bigger voltage drop across your plate resistor. This is wrong! The correct thing to do when splitting shared cathode resistor is to DOUBLE the size of the individual resistors in order to maintain the same bias. So, use two 6.2K resistors and the bias will return to about 3V on each cathode and your plate voltages should increase to a more likeable number.
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Thanks. I'll give it a try.
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The closest I've got is 6.8K, so I'll give that a try.
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use 2 6.8K in || if you have 4 of 'em
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At your suggestion, I put in another tube and the readings are the same, …
And the noise?
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I replaced each 1.5k with one 6.8k resistor. The values look good. It now has a voltage drop of 120VDC, and Plate Voltage of 90 now vs. 89VDC before these changes. Remember the schematic showed 75VDC.
The hum seems to be much lower.
But, that 21.k oscillation/distortion still exists in the same positions on the Vol and Tone pot settings as before. When you play with the pots in those positions it's a very distorted sound. But turn the Volume all the way up to 9 and in cleans up.
So where we left off yesterday, the 21.k problem goes away with jumping the .006 cap to ground. So I still need to resolve that.
I think I asked whether that 1Meg resistor was the issue, and maybe it should be smaller. You guys know way more than I ever will.
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When you play with the pots in those positions it's a very distorted sound. But turn the Volume all the way up to 9 and in cleans up.
the 21.k problem goes away with jumping the .006 cap to ground.
the cap should have been "jumped out" not shorted to ground! so a wire from left side to right side, leaving ONLY the pot/input jack In-circuit
by jumping it to ground you have NO input signal, effectively "shorting the grid to ground" look at the schematic.
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Sorry, I jumped it , not to ground. My mistake in describing it.
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what's playing through this amp??
as long as the input device has a pot for gain/vol , the amp pot/cap can just be trashed
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Shooter, I acknowledge that you present a practical solution to the problem, however, it's not a real solution to the problem from the owner's perspective. Since this problem doesn't exist on the Instrument channel, it seems like there should be a solution. Since the problem disappears when the circuit is grounded at various points, I hope that a solution exists.
I appreciate everyone's help on this frustrating problem.
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Why does the MIC channel not have any resistor coming off the Input Jack, when the Instrument Input jacks have a 100K ohm resistor?
The rest of the preamp circuit is the same for both channels. However, only the MIC channel experiences the oscillation.
Is that the "canary in the coal mine"?
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The absence of 100k in MIC is likely for impedance matching, as MICs are different than guitars.
In case I missed it have you tried another 6SN7? in case the internal capacitance has changed in that triode causing a 21khz distortion only in MIC channel. Also since the distortion goes away when you jump the MIC .006 why don't you suspect that cap as suggested earlier?
BTW - can you post some internal pics of this amp
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why don't you suspect that cap as suggested earlier?
ya can lead them to water but....... :icon_biggrin:
I would replace both the cap n pot since the pot "appears" to have a bad spot, but that could be the freq-range of the cap/pot filter causing the "muddy til 9" distortion.
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Yes I tried another 6SN7 with the same symptoms.
The distortion goes away when I jump that cap or when I jump the 1M resistor. In fact the hum seems less when I jump the resistor vs. the cap. Both the resistor and cap are new and their values are in spec.
I'm attaching a couple of photos.
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Shooter-The pot has been replaced twice, thinking it was a bad spot on the pot. I even checked the "new" replacement that was replaced with another new pot, and there are no dead or weak spots on the pot. So I've ruled out the pot.
The cap is new and reads to spec.
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When I jump the 1M resistor there's a slight hum, and absolutely no 21khz signal or anything close to it.
Of course I can't play a guitar and get a sound because there's a jumper across the resistor.
If "removing" that resistor from the circuit by jumping it eliminates the oscillation, would increasing or decreasing the value of that resistor do the same thing? If so, which way do I go?
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Since the MIC channel isn’t going to be used for a mic but for a guitar with this sounding as a Bright channel, would adding a resistor at the input jack work, and would it be brighter than the other channel, or is some other mod needed?
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with the plates wired together there isn't really any "voicing" room in front of V1-lower
if the cathodes are split you can leave off the bypass cap on the mic channel, that will "attenuate" bass to some extent, otherwise an eq pedal before the mic-in might be the simplest
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Shooter- sorry but I don’t understand your first comment. Do you mean that because the plates of the preamp 6SN6 tube are jumped together, that the MIC channel’s tone can’t be changed?
If so, why is that?
Also, how did the circuit designer determine that no resistor is needed on the input jack of the MIC channel? How is impedance for the input jacks determined?
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Does the mic channel make noise only when something is plugged in?
Or all the time?
If when nothing is plugged in, when you jump the tip of the mic input to ground, there's still noise?
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The single tone control affects all three inputs.
The two 100ks on the instrument jacks are mixing resistors. They are used to isolate the two jacks from each other. If you had two guitars plugged into those jacks but did not have the mixing resistors, then turning the volume pot on one guitar would also affect the other guitar.
The mic channel only has one input. Therefore, a mixing resistor is not needed.
Make the two changes shown in the attached pic and I think your oscillation problem will be solved.
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he's already done those
you really want V1 for one purpose, to give the guitar signal some gain so you have "room" to add, subtract, multiply..... to the original guitar signal.
by trying to "add tone" to the input, you attenuate the guitar signal to the point where it's not much above the amps noise-floor
pretty much every E-guitar has a tone knob, use it, you might be surprised it actually works.
like I said, separate the cathodes, leave the bypass cap off, get rid of the cap .006 n 1 meg pot, verify the amp works making guitar noises, THEN we can tweak "tone"
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Those pictures show why you might be getting a 60/120/180hz hum
The original heater wiring could be the cause as it is poorly dressed.
With wall at 120vac (vs 110 in 1940) it could be a cause to more hum compared to running this amp in the 1940's
If you have a variac try running the amp at 110 and see if the hum is reduced.
If so, you might consider to apply modern techniques of heater wire dress and signal line separation as discussed earlier. Elevating the heater as discussed earlier might also help.
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Hey Shooter,
if the 21khz distortion is only in the MIC channel how would mods in the power tubes improve this?
Seems like it should be in the MIC preamp circuit.
Unless you are addressing something else that I missed...
Nevermind - sorry I see that the cathodes you are separating is the preamp
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Obviously, Sluckey's right, you can place a resistor at the grid and limit upper frequencies/oscillations. You could also wire in a capacitor to ground, to shunt those frequencies.
The circuit is the same at both sides, so you could also place a resistor between the tip of the input and the volume/tone pot to reduce RF frequencies.
What I don't understand is why in an amp which was designed with these features, there's an issue. Perhaps it's because grounded jacks were added? Maybe there's no shielding on the amp when testing, and your cell phone/router is in the area? Maybe the capacitor is out of tolerance? There's only 4 parts which could be an issue, compared to original design.
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What I don't understand is why in an amp which was designed with these features, there's an issue. Perhaps it's because grounded jacks were added? Maybe there's no shielding on the amp when testing, and your cell phone/router is in the area? Maybe the capacitor is out of tolerance? There's only 4 parts which could be an issue, compared to original design.
I had a noise issue when restoring the Silvertone 1484. No grounded input jacks but a noise in one channel with no input. It turned out that the PT was super close to the channel. When I added a shunted jack (just one) the noise was gone (thinking sluckey suggestion). Maybe different situation, but a change for improvement to the original design. I also removed the death cap but left the unconnected switch so the next guy can feel good when switching it. :-) I'm a big fan of leaving original as much as possible...
https://stillampd.com/silvertone-1484-restoration
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Thanks for all the comments. Here are my responses.
When I shunt the Mic input jack tip to ground, with nothing plugged in, the noise remains. It sounds the same even when and instrument is plugged into the jack. It only disappears when either the .006 cap or the 1M resistor are jumped across themselves. The only time there isn't 21khz noise is when the Vol pot is set at 2 or below, and Tone is about the same level. As Tone increases with Vol, it becomes more noticeable, and when Vol is 7-9 then it's distorted and very noticeable. With Vol past 9, it immediately clears up. But that's with the amp at full Vol.
I'm going to install a new Socket and see if that helps, and address the filament wire dress at that time. The old socket has some wobbly lugs.
Separating the cathodes and adding the caps and resistors substantially reduced the hum. However the 21khz distortion is still there.
It had this problem even with the original non-switching jacks. Changing them to switching made no difference. No cellphone or router nearby. Does this in different houses. All of the components except the 5M resistor (whose value is about 6M and in a different part of the circuit) are new and test within specs, including the subject cap. Yes I agree, those 4 parts are all there is, or so it seems, and their values are fine.
AINewman, what values for those caps and resistors should I try?
Shooter, I'm leaving the cathodes separated. I think I'll leave in the bypass cap since the change improved the hum even if it didn't affect the 21khz distortion. I'll try jumping at the same time both the .006 cap and the related 1M resistor. The result would then be a direct connection from the grid, through the new 50K grid stopper to ground.
Could there be anything going on internally with the PT that is creating this 21khz oscillation?
Why does the 21khz oscillation stop when I jump the Tip of the Input Jack to the lead going into the 1 Meg resistor?
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Why does the 21khz oscillation stop when I jump the Tip of the Input Jack to the lead going into the 1 Meg resistor?
1. the original wire is close to the 21Khz "source" and coupling it into the signal path
2. the ground for your new shorting jack is sketchy not shunting the 21k to ground, so it sneaks' in
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Shooter-the 1M resistor is wired directly to the Ground lug on the MIC Volume pot on one end of the resistor, and the other end of the resistor is wired directly to the Grid Pin 4.
So I don't know what it could be. It is close to the 100K resistor connecting the Tip on the Instr. Input Jack to its Volume Pot, and also close to the coupling wire connecting the Tip of the MIC Input Jack to its Volume Pot. BUT I have previously disconnected both of those wires at their Volume Pot connections, and the 21khz is still there and distorts.
It only goes away when I jump the MIC Tip to the 1M resistor.
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It sounds like the input volume pots are causing more problems than they solve (naturally, they don’t solve any problems since almost anything you plug into this will have its own volume pot), and one may be just plain broken. Everything I’m reading here points to that volume pot as the problem.
If this were my amp, I’d wire the input jacks straight to the grids with grid stop resistors on the tube sockets. Then I’d put the two volume pots after the first stage, ala 5E3.
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Good point-They are new pots and I've checked them for dead spots, but no issues.
I did think it was unusual to have the Volume pots before the Grids. I've not seen that before, but that doesn't mean much. But of course, this is the way it was designed and sold, so one would assume the circuit worked.
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It was designed for lap steel guitars at a time when many lap steel guitars didn’t have volume knobs.
And just because it worked doesn’t mean it remains a good design.
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True-things were pretty basic back then.
I just did the grounding test on the 1M resistor again. This resistor goes from the Grid Pin on the socket to the Input Jack Ground lug. When I jump the resistor on its lead side connecting to the Grid Pin on the socket, and the other end of the jumper to ground, there is a very low background hum and the 21khz oscillation is gone. When I connect the lead on the resistor to its lead to the MIC Input Jack Ground the oscillation is back. Of course, the jumper is just another ground lead in this configuration.
If I remove the 1M entirely from the circuit with only the .006 cap in circuit, then I get the same oscillation/distortion. Nothing is connecting the Grid to Ground.
If I connect one lead of the 47k grid stopper to the Grid Pin on the socket, and the other lead to the lead coming from the .006 cap that connects to the Wiper of the MIC volume pot, then same problem as above.
Finally, if I connect the Grid stopper directly to the Wiper, then the same problem remains.
It doesn't matter if a guitar is plugged in or not. The hum and distortion are present in all configurations described.
I've ordered a new socket, and maybe that will be a solution. I don't see what's left to be the cause.
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what happens when you wire like this???
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I'll give it a try tomorrow. The cocktail hour has started out here, and I stay away from electricity!!
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It was designed for lap steel guitars at a time when many lap steel guitars didn’t have volume knobs.
And just because it worked doesn’t mean it remains a good design.
True, but this side is a microphone input, and most microphones don't have a dedicated pot for volume.
Again, I wonder if everything is shielded and/or electronics are out of reach when testing.
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The oscillation is present regardless if my cellphone is on the workbench, or upstairs in my bedroom to floors up.
Any suggestion on shielding? I don't think that's the source since the problem exists in different locations miles apart.
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Maybe a cookie sheet on top of the chassis when testing?
The jack has been mentioned, but it's been tested. Maybe if the routing of wires has been changed, shielded wire may be necessary, or chopsticking/rerouting while testing.
Again, there's only a handful of parts that could be faulty or installed improperly. If you're sure everything is good, a modification like the grid resistor in series with the signal should solve your problem.
Look to Merlin's explanation on Miller capacitance.
https://www.valvewizard.co.uk/gridstopper.html
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I'll give it a try tomorrow.
Any suggestion on shielding?
quit following all the rabbits at once, shoot one, move on to next!
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Shooter-When I wire it like your last suggestion with the MIC Vol Pot removed from the circuit by disconnecting the lead from the Mic Input Jack tip from the pot and connecting it in-line with the 50k resistor and the 1M resistor that goes to Ground, I get a clean sound. No distortion. Only a very slight hum. Of course the Mic Vol pot and the Tone pot do not function, and the Volume is all the way up from the amp, but controllable from the guitar.
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Shooter-When I wire it like your last suggestion with the MIC Vol Pot removed from the circuit by disconnecting the lead from the Mic Input Jack tip from the pot and connecting it in-line with the 50k resistor and the 1M resistor that goes to Ground, I get a clean sound. No distortion. Only a very slight hum. Of course the Mic Vol pot and the Tone pot do not function, and the Volume is all the way up from the amp, but controllable from the guitar.
This is how it should be wired IMO, and how 99% of amps are designed for a reason. The grid stopper makes a low pass using the tubes internal capacitance and cuts out your HF noise. Just put the volume pot after the first stage and move on.
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So how would you wire the Volume Pot back into the circuit?
Insert it between the 50K and the Grid, by connecting the 50K's grid side lead to the Input lug on the Volume pot, and the Wiper to the Grid? What about the .006 cap? Leave it out of the circuit?
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Like I said above, wire it like a 5E3 with vol/vol/tone in between the first and second stages.
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Would the attached wiring work if I wanted to keep the Instrument channel as is?
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the plates are TIED TOGETHER, what is the 500k labeled that has a cap to ground??? it should act both as tone and volume to some extent
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I got no response from either the Mic Vol or the Tone pots when I tested it today. It was putting out volume at full level with no reaction to turning the Mic Vol pot, and also no reaction when I turned the Tone pot.
The oscillation/distortion is gone in this configuration, and minimal hum.
So I'm just trying to see how I can control the Mic Channel's volume. Tone may be an entirely different issue that can be dealt with later.
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You'd have to separate the plates. Give each one its own load resistor (double the shared value). Send the signal out from the mic channel plate through a coupling cap (say 0.05uf) to the top of its volume pot. Then send the signal from the wiper to the grid of the PI through a 220k mixing resistor.
On the instrument channel, go from the plate through a coupling cap (0.05uf) through a 220k mixing resistor to the PI grid.
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What does the amp have THIS pot labeled AS??????
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Tone
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so make it look like this then call it MV for testing since earlier you said it had little to no effect on signal.
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I'll give it a try. Thanks.
I remembered just last night, that when the Mic Vol pot gets past 9, the distortion cleans up and of course the amp is at full Volume. That's the way it is with the Vol Pot removed from the circuit. So I don't think the underlying issue has been resolved.
Another point of mystery, is that the socket for the 6SN7 preamp tube appears to be original to the amp; however, it appears to have been removed from its riveted original orientation with the other tubes, and turned 90 degrees and then bolted with machine screws and lock nuts.
Originally all the tube sockets had their locating pin at 6 o'clock. This one is now at 9 o'clock! Maybe someone was trying to solve this same problem by changing its orientation?
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I remembered just last night, that when the Mic Vol pot gets past 9, the distortion cleans up and of course the amp is at full Volume. That's the way it is with the Vol Pot removed from the circuit. So I don't think the underlying issue has been resolved.
can you clarify this?? there is no mic-vol now, what happens sound-wise as you roll the guitar volume from 1 to 10???
my sense is you are running into clipping/compression, which causes distortion, not a "bug in the system"
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Shooter, nothing happens now when I roll the Mic Vol or Tone pot. It puts out full volume.
Before this change when the Vol pot was still in the circuit, it would start getting the distortion around a setting of 7 on the volume pot which continued through about 9. Then from 9-10 full volume, it clears up and sounds fine, just like now with the Vol pot out of the circuit.
These numbers are dial number positions, and not referenced to the position on the dial of a clock.
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Shooter-I made the changes shown in your last schematic, and SIGNIFICANT PROGRESS!
I can control the MIC Vol. with the Tone Pot. Turning the Tone Pot for the MIC channel as it is now wired, adjusts the Volume of the Channel and the Treble. It seems to have a lot more Treble output.
The MIC Vol pot seems to have no effect, I assume since it no longer has any Input source.
Is there a way that I can connect that pot so that it is functional?
I ask since it will probably be confusing to the player when he wants to increase Volume when he's using the Mic channel and nothing happens when he turns the Vol. pot.
I also can control the Instrument channel as before with its Vol Pot and also the Tone Pot. And it too seems to have a lot more Treble.
The hum is very low, but does have a higher pitch, ie. its "brighter", as the Tone pot level is increased.
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Is the tone pot now wired like reply 96????
is the MIC-Vol pot and .006 cap still disconnected???
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Yes, its wire like the schematic in 96.
Yes, the MIC-VOl pot is disconnected from the Mic Input Jack, and the .006 cap that was part of that circuit has been removed.
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you could move that pot to Mic-VOL and just eliminate "MIC" to help future guitarists not get confused.
if you still want a tone knob build this TS in place of the one that became "Amp volume" (reply 96)
it's from a Fender 5E3
NOTE: caps are real sensitive to hot solder pencils, you over-heat them, they do weird things like you've just experienced
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So putting that Mic Vol pot back into the circuit to control the amount of signal coming out of its Plate won't work?
Not even if the Pot is placed between the 50K grid stopper resistor and the grid, or between the plate, through a coupling cap, then to the Pot, and from there to the jumper connecting the 2 plates?
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you have a vol knob on whatever is plugged into Mic, this is no longer the 1940's!!!!
put a NEW pot there and see what happens:)
find any guitar amp built since 1960 and copy it's volume before 1st gain stage, I'll wait :icon_biggrin:
or simply re-name the holes and call it a day, it's 18:20 here so it's herbal-tea time
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Fair comment. Thanks so much for your help. Much appreciated! Best.
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If we find that this MIC channel configuration is just not what the owner wants/likes, etc., and since the Instrument Channel functions properly, is there an easy way to wire the circuit to just use the one active Instrument channel through both 1/2's of the tube, and maybe get more gain than it has when only just 1/2 the tube is used for the Instrument channel?
I assume I couldn't just jump the grids?
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I guess a Fender 5F1 Champ pre-amp circuit is really all this would be. Pretty basic.
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I took the amp over to my buddy's today so he could see and hear the finished product. He was really excited with it and loves the "crank" its got through the MIC channel. He said to pass on his appreciation to you guys for all of your help in helping me get this amp "cranking" again. Best.
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Passaloutre: In post #93 you said:
You'd have to separate the plates. Give each one its own load resistor (double the shared value). Send the signal out from the mic channel plate through a coupling cap (say 0.05uf) to the top of its volume pot. Then send the signal from the wiper to the grid of the PI through a 220k mixing resistor.
On the instrument channel, go from the plate through a coupling cap (0.05uf) through a 220k mixing resistor to the PI grid.
If I wanted to make this Mod to the Mic channel, which grid of the PI should I connect it to, Grid A, Pin 4, or Grid B, Pin1? Before the plates were separated they were jumped and connected to the Tone pot which connect to Grid A, Pin 4.
Thanks.
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Definitely pin 4. You will want the gain provided by that stage. This would also be a good opportunity to put the instrument channel volume control after the first stage. Again, if this were my project, I would just copy the control circuit from a 5E3 Deluxe.
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I think the 220K mixing resistor may be too big. I'm not getting any volume out of that MIC Volume pot until it's around a value of 9 out of 10. Before that really none. And there's still some influence coming from the Tone pot. Remember that Tone Pot is also connected to Pin 4. Much less gain than the Instrument Channel and it's volume pot.
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Could you show us a diagram of how you have it wired? From the discussion, it's hard to tell exactly what's what anymore. Does the tone pot still ahve the cap to ground, or is it wired as a volume pot as earlier suggested? Please draw the circuit
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It's not pretty, but here's my drawing of the circuit as it now exists. The cathodes were separated and the original single 3K resistor was replaced with two 6K resistors to get the proper voltages. The Plates on the 6SN7 preamp tube have also been separated and wired separately to the B+. The single 100K resistor was replaced with a 168K (ie. 100k in series with 68K) resistor coming from each lead off the B+ to the Plates to get the proper voltages. Then your Mod was added. A .05uf coupling cap coming from the Plate on Pin 5, then to the top end of the 1M MIC Vol Pot, then wafer to 220K resistor to Pin 4 on PI, and finally bottom lead of this pot to Ground.
Hope this makes sense--at least my explanation.
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6k seems a cold bias for the input tube
Does the instrument channel have its own mixing resistor?
Looking at the original schematic, does pin 4 on the PI still have a 500k grid leak to ground? You can remove that, as it makes a 2/3 voltage divider with the mixing resistor. Your volume pot now provides a grid leak.
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Here's a copy of the original schematic.
Is the Mixing resistor the 500K between the Tone Pot's wafer's connection and the Grid Pin 4 of the PI?
Remember, the bias resistor was originally a 3K shared between the two Cathodes. And when we split the cathodes, we needed to double the value of the new resistors. Originally Shooter said it should be two 1.5K resistors but I wasn't getting any real Volume, and I think you said it needed to be doubled in value not halved since the two cathodes are no longer sharing the resistor.
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Agree about the bias.
The datasheet suggests about 3k for each triode of the 6SN7.
What is the cathode voltage with 6k?
What are you measuring at the plate?
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The cathodes on the 6SN7 are reading 4.06 VDC, and the Plates 88 VDC.
The Schematic show 3 VDC for the cathodes, and 75 VDC for the Plates.
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here's the quick-e as I know it. maybe color in what you've done since it was working and the owner happy
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Attached file "WES Revisions" shows the revisions as they exist now.
Note that when the cathodes were separated early on in this process, the single 3K resistor was initially changed to two 1.5K resistors, and then when the voltage wasn't correct, they were both increased to the current 6.6K. Also, at the time that the cathodes were separated, another cap was installed with the new resistor as shown. Both of these caps are actually 25uf/50V vs. 10uf/25V. So each leg of the cathodes has a cap and resistor bridged.
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Put a 220k resistor in seriews with the 0.05 cap coming from the instrument channel anode, and remove the 500k resistor to ground.
If that doesn't do it for you then I'll repeat my prior advice: copy the entire pre-PI circuit from the 5E3. You're already halfway there. It is a known good circuit and a better design than the Oahu or the mishmash presented below... Personally I would also use a 6SL7 in place of the 6SN7, but that's player's choice.
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Do you have a recommendation on where to put the 220K, ie. between the .05 cap and the wiper connection, or further on down the line?
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Put it exactly where I've shown it on the diagram: between the 0.05 cap and the grid of the PI.
One end of the resistor should connect to the 0.05 cap and nothing else. The other end is connected to the node sharing the other 220k mixing resistor, the wiper of the tone pot, and the grid of the PI.
One more suggestion: the tone cap (the one between the tone pot and ground), change to 0.005.
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Thanks. Sorry, I didn't realize you had made the insert on the schematic.
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Sounds the best yet. Much quieter on the "hum" too. The Gain in the Mic channel is back where I think it should be, and there is much more control of the Tone in that Channel than before. So this is definitely a major improvement.
What got me started with this change after my buddy heard it and liked it, was his desire to see if the MIC channel could be made just a Volume only channel, with no Tone control on it.
He plays in a group, and plays lap steel. He also plays a standard electric guitar on some of the songs. So he was hoping he could use the Instrument Channel for his standard guitar and use the Tone Pot with it to adjust the Volume and Tone of the guitar, BUT when he's playing the Lap Steel he didn't want to have to be changing the Tone settings that he had setup and was using for his guitar and would just be using the Lap Steel's tone controls to adjust it. It would be easier switching between songs so he wouldn't have to fool around with changing the Tone pot settings.
Is there any way that can be achieved with this amp?
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Connect the wiper of the tone pot to the wiper of the instrument channel volume pot
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Sorry, but that doesn't work.
I didn't change anything else, just added the jumper between the wipers of the Tone and the Instrument Vol pots.
Only can get Volume on Instrument channel when the Vol is turned almost all the way up, and nothing at all when the MIC channel is being used.
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Not a jumper. Disconnect the tone pot from the grid of the PI and then connect it to the volume pot. See attached.
I'd strongly recommend using a 0.015uf for the tone cap here. The original 0.05uf may be way too dark depending on the guitar pickups.
For the record, these controls would still work better if they were after the first stage, not before. Where they are is simply duplicating the knobs *already on the guitar*
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This would be a better design and fulfill the needs you have described without changing the tone of the amp.. LTSpice doesn't have a potentiometer symbol, but those doubled up 500k resistors represent 1meg pots.
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Got your MODS working!! Thanks so much.
We had talked about replacing the 6SN7 with a 6SL7, so that may be something my buddy tries. Should have a lot more gain than the 6SN7.
We'll play with this circuit for awhile, and see if he wants to go with the other schematic you included or is happy with this setup.
Best.
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Glad you got it sorted out!
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I checked the voltages on the amp and also checked the bias for the 6V6S output tubes. I think it's running a bit cold on the bias, but I thought I would post the numbers and get your input.
6V6S is a 14 Watt Tube.
I'm reading 36mA for each tube.
The Plate to Cathode voltage is 250.1VDC.
The Plate Voltage is 268VDC.
The Screen Voltage is 274VDC.
The Bias resistor is 200.8 ohms, with a voltage drop of 16VDC.
Rob Robinette's Bias calculator says that for a Cathode Biased Amp the safe max is 56mA. And the plate dissipation per tube is 9 Watts at about 66%.
So do you think I should adjust the Cathode 200 ohm resistor to increase the bias?
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You can try something like a 180 or 150 ohm resistor, but a lot of times the tube "self-adjusts" to the new conditions and ends up in the same place. You might think it's simple as a smaller resistor will drop less voltage, but it also increases the current through the tube so the smaller resistor drops more voltage and the dissipation ends up being not very different.
Personally, I'd leave it as is. 200 ohm is already smaller than what most 2x6V6 amps use. The power tubes should sound good and happily last a very long time.
If you're looking for more volume, there are better ways than trying to squeeze out a couple more watts from the power amp. Try a more efficient speaker.
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Thanks. I was concerned it was way too cool, but I'll take your advice and leave it as is.
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Just curious, do you have any more photos of the amp? I'm a big fan of these ancient beasts
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I'll have my buddy take a picture when its installed in the cabinet.
Here are a couple at the beginning before anything major was changed.
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I was wondering if the bias current is so small because the 6SN7 preamp tube maybe doesn't draw much current and have much gain. If we put in the 6SL7 which has more than 3 times the gain that the N7 has, would this affect the bias current? (Sorry if my terminology is not correct).
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this comes in handy when setting up a tube, usually you can find a datasheet that has one for whatever tube you're interested in.
Gain by itself means little;
how much signal can the "next" stage take before it starts clipping?
how clean/dirty do you want the amp?
is the tube more or less "sensitive" to next-stage loading/impedance matching?
those are question you want to have BEFORE you just throw gain at the next stage
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Still fiddling with the amp, and a 60hz hum at low level.
I've changed some of the grounding and that improved the hum, which is dramatically better than when I started with this. I would just like to get it a little lower. The hum starts after the tubes warm up.
It goes dead silent when I remove V1, the 6SL7 pre-amp tube. So does this tell me that the hum is originating somewhere between that tube and the Input jacks? I've actually unwired the Input jacks from their pots and the hum remains.
The PT has a center tap for the rectifier tube. I haven't added any 100 ohm resistors to the filament supply.
Any suggestions on what to try? Thanks.
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addressed this in reply #6 and #7
did you do these things yet?
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So does this tell me that the hum is originating somewhere between that tube and the Input jacks?
yes.
If you know it's 60hz for sure, that is most likely filament induced hum. so ANY signal wire that gets "close" to the fil. wires will couple the 60hz into the signal path. the closer to the input, the more sensitive the signal path.
it can be loose tube sockets, poor solder connections, sketchy coupling caps besides just wire positioning.
are the filament wires 2 wires n a CT? or just 2 wires no CT?
If 2 wire, do BOTH wires run to all the sockets? or is only 1 wire run to the sockets with the other grounded??
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The amp is currently wired as original. No changes to that wiring have been made.
No input signal is being injected into the "Switched" Input jacks. The jacks have been wired between their respective ground and switch lugs.
Both the main 300VDC secondary windings and the 6.3 VAC filament windings have Center Taps.
Both of the CT's connect at the same ungrounded lug on a terminal strip, and then go from there to the ground lug on the Filter Cap Can.
The 120hz hum is substantially reduced and based on a frequency meter it shows less amplitude than the 60hz hum which is at about -72db.
If I have the amp upside down so the inside is open and facing up, and if I move my hand around over it, when my hand is over the area where the Input jacks, Volume pots, and Tone pot are located, the hum is reduced. When I move my hand away it increases.
Also I notice that the V1 Grid pin socket is loose in the socket but the tube is firmly seated in that socket.
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If I have the amp upside down so the inside is open and facing up, and if I move my hand around over it, when my hand is over the area where the Input jacks, Volume pots, and Tone pot are located, the hum is reduced. When I move my hand away it increases.
good indication of a crappy ground^^^
Also I notice that the V1 Grid pin socket is loose in the socket but the tube is firmly seated in that socket.
ground the pin for testing, hum change???
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I grounded the grid on V1 Pin1 by connecting a jumper from it to the chassis, but no difference in hum.
I also ran a shielded cable from the wiper on the Volume Pot to the Wiper on the Tone Pot, with the shield grounded on the GND lug on the Volume Pot. That resulted in a slightly lower hum with no reaction to my moving my hand in the area.
I'm about out of grounds to test.
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I injected a 1Khz, 150mVrms signal into the amp, connected to a dummy load, and then connected to the scope.
From the input jack to the grid of the Output tubes it was a clean sine wave signal for both Inputs.
I found that moving around the shielded cable that I just installed between the Vol. Pot Wiper and the Tone Pot Wiper seems to clear up the hum. There's still some background noise, but that may just be because it's "a tube amp". I don't know.
I'll keep working on it a bit more.
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I forgot to mention that I also hooked the scope up to the speaker output jack, and it also has a very clean sine wave at 1Khz. It is oscillating slowly, ie. sort of like a wave. But the peaks don't change in amplitude, just the whole wave form rolls a bit.
If I can hear the noise/hum through the speaker, why isn't the output signal showing a sine wave at a frequency higher than the 1Khz signal that's being injected?
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the whole wave form rolls a bit.
that's an indication of your "60hz" riding on the signal. change your time-base and you should be able to see the lower frequency. If your scope does FFT ..spectrum, then you should be able to see both the 1K and lower frequency together
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Yes the scope has FFT. I really don't know how to use it, but here is a photo of the signal at the output speaker jack and it shows 60hz.
Now is there a way to trace it to the source of the problem?
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yep, "walk it back", normally we follow the signal back to the input. the problem, I suspect you'll find it "everywhere", not just along the signal path.
start by grounding your scope to the amp chassis, then probe all the grounds at there "destination" so probe the NEG terminals of your E-caps, neg wires at pots, any ground wires at tube sockets, input jacks.
In an ideal world there will be NO AC - 60hz on any of those points.
IS this amp a 3-prong power cord????
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Thanks. Yes 3 prong.
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Yes it’s 3 prong.
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The only place I get any 60hz signal is at the Output Jack.
The spec sheet for the OT is attached. I have the brown, 8 ohm, wire lead from the OT connected to the Tip of the output jack, and the Black common lead connected to the GND lug of the jack. I also have a wire connecting that jack lug to the chassis to ground the OT.
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The Output Jack is not physically attached/mounted to the chassis. The owner didn't want to drill another hole in the chassis. So it just gets stabilized with a plastic support mounted to the cabinet. That's why there is a separate ground wire running from the output jack sleeve lug to the chassis ground point.
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There’s no 1kHz signal on that scope
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The sine wave is the 1khz signal.
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your scope thinks otherwise!
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I don't know why it did that. But if you look at the top of the screen where the FFT is running, it shows a frequency of 60 hz.
Also on the screen that you point out, it shows VRMS of 159mv, which is the voltage of the input signal from my signal generator app.
So I'll check it out again today. But when I probe the circuit's ground points for an AC signal, with no input signal injected since the amp has this hum even if there is no input signal, the only place in the circuit that the scope in FFT shows any AC frequency is this 60hz frequency that is at the Output Jack.
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decrease your time/base to expand the FFT trace
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I was probing the circuit with no input signal, and have found that I first pickup the 60hz signal on V1, Pin 2 which is the Plate. And it next shows up on to V2, and I guess continues until it gets to the Output Jack.
So why does it start at V1's Plate? It's not on V1's Grid. We did separate the Plates and also the Cathodes on the circuit.
Maybe it has something to do with that change to the Plates of V1?
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I first pickup the 60hz signal on V1, Pin 2
repeat that only probe BOTH sides the plate R
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I had just done that.
The B+ to the Plate is coming from the 8uf cap. There is no 60hz signal between the 8uf filter cap the connection to the 170K resistor that connects to the plate. However once after the voltage goes through that resistor (actually its a 67K in series with a 100K since I didn't have anything close) then the 60hz frequency appears, and its taken to V1, P2 Plate.
So it would seem that its something about that connection with the resistor that brings in the 60hz to the circuit.
The Plates had originally been linked, but they were separated as part of dealing with the MIC channel's oscillation.
The resistors read properly.
So what's going on?
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I also should add that this 60hz only appears on the connection between the P2 Plate and the B+, and it does not appear on the P5 Plate even thought the resistors going to P2 and also to P5 are wired the same for each of their paths, and they originate at the same point on Pin 6, which is an unused lug on one of the 6V6 tubes.
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So what's going on?
how close are those R's to the filament wires? are they "running" parallel to filament wires?
got a spare tube?
for fun, tack, gator-clip a smallish cap ~~~ .001uf from the plate to ground, any change??
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No spare tube available.
The R's are running sort of perpendicular and they are elevated "in the air" above them. I wouldn't say close. The Pin 2 resistors start at Pin 6 and the Heater is at Pin 7 on the 6V6. The resistor with the signal ends up at Pin 2 and the Heaters are at 7 and 8. These resistors were originally 100K, but when the plates were split I had to increase them to 168K to get the plate voltage lowered.
I added a .001 cap, and it did reduce the hum some.
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60hz on the plate is either "induced/coupled" by its surroundings, generated by the tube, or amplified by the tube.
me, I would simply re-build (all new) that stage then go play outside, snows coming.
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Sounds like good advice! Thanks so much for your help!
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I've decided to replace the V1 socket and re-wire it.
I noticed that the filament wiring between Pins 7 and Pins 8 on V1 and V2, the pre-amp and PI tubes, is not wired like I've always done it. In this case Pin 7 on V1 connects with Pin 8 on V2, and Pin 8 on V1 connects with Pin 7 on V2.
Does it make any difference if they're not connected Pin 7 to Pin 7 and Pin 8 to Pin 8?
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7to7 8to8 would be "best practice" in my book, although it shouldn't make a difference
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Shooter-I took your advice and replaced the V1 socket, and then wired only the V1A side according to the original schematic. The cathodes and plates are connected as in the original with the original component values. This 1/2 of the tube receives the signal from the instrument.
So the result is a really low volume. I checked voltages and found something odd at the cap can. Nodes A and B read normal high DCV. However Node C that drives the V1 plates reads 4 VDC! Is that the sign that that cap is bad?
Remember I haven't wired in the V1B side of the tube yet, ie. nothing going into its grid, plate or cathode.
The rest of the circuit from the PI on is wired per the original schematic.
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I tested that cap by connecting my meter to it and to ground with the power off. When I turned the power on the DC voltage gradually increased to 23VDC, and then started falling off and finally stopped at 5 VDC. The other 2 Nodes were reading normal.
I guess somethings wrong inside the cap?
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However Node C that drives the V1 plates reads 4 VDC!
pull the tube, does the VDC go back to normal??
If So;
I suspect a wiring error or a component value error. I ALWAYS use my meter to verify resistance before installing, old eyes n CCP colors are not compatible.
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Nope-I took out V1 from the socket I was working on, and checked the voltage from cold start. It did the same thing. Went a little higher this time, and then started going back down and stopped at 5VDC. The other Nodes are reading properly.
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NO POWER
disconnect the + lead from the cap, use a couple gator-clips ...clip....100K R...clip put one free end of a clip to the + , the other free end to ground. This should discharge the cap without a snap. leave connected
put your meter in ohms mode, + to + of cap, - to ground. you should read near ZERO ohms with the bleeder in place.
now disconnect the bleeder, leave meter connected. the meter should charge up the cap to ~~~~>500k out to infinity.
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I checked again this morning and realized that I had not installed a resistor between the B and C nodes. So I installed that and get voltage, but with a lot of feedback. I’m guessing that’s because the other half of the circuit isn’t installed yet. I was hoping I could just wire up half of the V1 tube for the Instrument Input, and get that working, and then deal with the MIC Input for the other half of the V1 tube. Maybe that plan doesn't work since both halves of the V1 tube go into the PI.
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Until I hook up the V1B side of the 6LSN7 tube to the MIC Input section, should I ground its grid to prevent the feedback I'm getting at this point in the rebuild?
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Yes, grounding the grid cleared that up. So the 1st stage for the Instrument Input is running fine but with a little hum that does not vary with adjusting its Volume Pot.
The plate voltage is about 143VDC and should be around 85 or so. I've got a 100K coming off the Node C B+ to the plate, so I think maybe I need to increase that to 150-200K?
I've also wired in the second stage for the MIC Input. It is extremely loud at its lowest volume, but its a hotter input which I think will drop when I increase the resistor mentioned above since the two plates, 2 and 5, are jumped with each other and are each receiving the 143VDC.
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The plate voltage is about 143VDC and should be around 85 or so.
does V1 share a cathode R or does each 1/2 have it's own?
IF.. they have individual R's, measure the VDC at the cathode of each, you'll probably find the one with a grounded grid is not drawing current which will affect the plate VDC some.
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It appears that there's not enough voltage drop across the 50K resistor connecting Nodes B and C to bring down Node C voltage to around 175-185 VDC. Node B's voltage is fine, but it's reading 210VDC at Node C after the resistor. So I guess I should increase that resistor?
The cathodes are sharing a 3K across a 10uf cap to ground.
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wait til both haves of V1 are connected so the tube is properly biased. then measure the VDC of the cathode to determine the actual current in the tube, remember to divide by 2 since it's shared.
typical biasing would be ~~~~ 1-2mA each 1/2, no signal.
higher plate VDC allows for a bigger "swing" before the tube goes into clipping/compression, if biased close to "center", so don't sweat a larger plate voltage than the amp had 80 years ago, you're playing it now, go by the sound/performance the amp gives you within an acceptable range the tubes operate.
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When I increase the nodal resistor by as much as doubling it, it brings the VDC down, but not enough to be in proper range. And if I go farther and increase the resistor from 100K to 220K which connects the B+ to the plate, its still over 111VDC vs. 85. and the cathode voltage which has been about 4VDC is down to about 2VDC. The schematic shows 3V.
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I = E / R
4 / 3000 = 1.3mA /2(tubes) = .65mA which is about 1/2 a typical fender runs with No signal.
again, you can't set up the VDC until the whole system is operational, so get the other 1/2 V1 wired n tested. then we can have fun with math :icon_biggrin:
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Sorry, I wasn't clear. It is wired up fully--both sides. And these are the measurements I get. Even when only 1/2 was wired up I was still getting the same numbers.
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1st give up on trying to get 1950's voltages with 21st century grid. the tube can handle 300VDC at the tap and outlast all of us. your goal is "how does the amp sound".
the math says the tube wants more current, since you decided to go back to a shared plate n cathode, that make the math a 'lil more complicated since you have to divide or multiply by 2 :icon_biggrin:
here's a chart for the tube with typical 6SN7 triode values for each half.
Ebb I believe is the Tap VDC, not VDC at plate, but I might be wrong there
Rp = plate R
Rk = cathode R
Ck = cathode bypass cap
VG = tubes Voltage gain
start by getting the tube to draw ~~ 2-4mA at cathode R
using I = E/R of the cathode
Changing either Rp or Rk will have an effect on the VDC at both plate n cathode. once you get the bias set to 2-4mA then play it, or sig-gen n scope it.
post what you settled in on for component values.
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Have you got that chart for a 6SL7? That’s what he’s using in it now.
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I think the 6SN7 draws more plate current than the 6SL7, and maybe that's why the plate voltage is higher with the 6SL7 tube?
Maybe I should just leave the components as is per the schematic in case my buddy decides to go back to the 6SN7 tube.
I don't know.
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Ok, I've got everything wired in, and am getting good sound. I think I'll leave the voltage issue alone.
I returned the wiring of the Plates to the original schematic where they are jumped together, and not separated like I did in the Mod.
The only problem I've got is with the MIC Volume pot. It's putting out full volume even when the pot is turned full CCW. Remember this pot has been disconnected from the Tone pot, and this Volume pot is currently connecting off the plate of V1 which are jumped. Originally this pot connected after the Mic Input jack and then to the Grid.
I think I may need to put it back to its original location before the grid.
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I checked the voltages on the amp and also checked the bias for the 6V6S output tubes. I think it's running a bit cold on the bias, but I thought I would post the numbers and get your input.
6V6S is a 14 Watt Tube.
I'm reading 36mA for each tube.
The Plate to Cathode voltage is 250.1VDC.
The Plate Voltage is 268VDC.
The Screen Voltage is 274VDC.
The Bias resistor is 200.8 ohms, with a voltage drop of 16VDC.
Rob Robinette's Bias calculator says that for a Cathode Biased Amp the safe max is 56mA. And the plate dissipation per tube is 9 Watts at about 66%.
So do you think I should adjust the Cathode 200 ohm resistor to increase the bias?
No, it's push pull - leave it be, it was designed as such to work with that reflected load - If you must, for hotter bias, reduce the value of the cathode R.
Was thinking perhaps g2 of the output stage is injecting +FB into gain stages leaking through PS rial, since phase inverter and 6V6 G2 share same PS node, perhaps PS could use some minor tweaks, decoupling power stage G2 and PI. See attached.
Regards,
--Pete
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Is this mod intended to deal with the hum?
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DummyLoad-What is the purpose of the 500K resistor just before the PI?
As most can tell, I'm really just a "mechanic" when it comes to working on amps, and not knowledgeable of the theory. And at 76, I'm not really wanting to start learning theory, but I do appreciate learning how and why things work.
Thanks.
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The 500k is grid leak for the first stage of the PI.
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Is this mod intended to deal with the hum?
Was thinking perhaps g2 of the output stage is injecting +FB into gain stages leaking through PS rial, since phase inverter and 6V6 G2 share same PS node
yep
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Found the fix for the hum. In doing the poke test I found that the .05uf cap connecting the Tone Pot to Ground had a bad lead. I replaced the cap, and the hum is gone!!
Thanks for your help.