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Thread: Safe to put V1a coupling capacitor on a switch for variety?

  1. #106
    Lifetime Member Enzo's Avatar
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    Hum comes from many sources, it is not generic. You elevation will only reduce hum if that hum was coming from heater/cathode leakage.
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    This is neat and I think worth mentioning. With the volume halfway up, I can crank the gain to a sweet spot where hum is quite diminished. I thought it was neat.

    Now I have a stupid question. I was probing the AC coming off the coupling caps of each stage with the gain off and the volume way up. When I probe V1b (remember I'm calling the added stage V0) the amp goes haywire with super loud hum. Is this normal for some reason, because no other spot in the circuit does this.

  3. #108
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    The fact that the hum goes in and out with various changes and adjustments of gain and volume controls means there is more than one source of hum, they are out of phase with one another, and are phase-cancelling as various levels of the hum are reached or passed. If it steadily increased as the gain or volume was advanced, it would mean there is only one source of hum. As far as how to find them without a scope, I know nothing more than trial and error... :P

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    That's what I figured. It's a new behavior with this particular preamp tube in V2. And you're right, I'll be getting my own scope in the coming weeks. It's difficult carting things to school and back when the lab is open.

    The fact that probing between V1b and V2a producing an insane amount of hum when probing any other part of the path ever so slightly reduces the noise seems like a major clue to me, but I could be missing an obvious reason why that would be so. I'm going to retrace my grounding and layouts today and check all my joints just to make sure I didn't miss something silly. It's happ

  5. #110
    Lifetime Member Enzo's Avatar
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    If you have an amp that hums most at mid-setting on volume control, start at latest point, say the master, that does this. It it is the only control doing it, fine, if the gain also does it, start at the later one anyway. It hums least at mid, meaning it hums more at zero. SO set it to zero and fix THAT hum. After that, THEN you can turn up that control and deal with hum in the earlier circuits.
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    Thanks Enzo. Maybe I wasn't clear on that, I apologize. With gain off, hum steadily increases as volume is raised. No hum at zero volume. However, with volume set to around noon, increasing the gain decreases the hum until about 9 o'clock, in which case it begins to increase again.

  7. #112
    Lifetime Member Enzo's Avatar
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    Clear enough, I was speaking in general to expand on Justin's thought.

    Wherever in the control settings it nulls, it still points to two separate sources of hum that are out of phase. One needs to cure one at a time.
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    I've been reading some of these good grounding threads that Chuck mentioned, and......well........I've always followed the old Marshall style grounding schemes which couldn't be further from what you guys recommend for quiet operation. On a stock 2203 layout, I count 13 or 14 possible paths to ground not including safety earth. Looks like this could be done much cleaner to begin with. So.... after studying a bit more, I may be ripping the whole thing out. On one of those threads I dug up, a guy said he managed to get the output voltage on an amp he was working on with gain off / master wide open....down from 400mV to 4mV just by changing the grounding scheme. That's NUTS! Learning is fun.

  9. #114
    Supporting Member Chuck H's Avatar
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    Rip it all out if you REALLY want to idealize and have the time and $$$ for wasted components, etc. The thing we didn't know from the start was just exactly what this amp was, other than a sort of 800 type build. Which can be done with typical Marshall grounding and a "normal" amount of noise. The 800 models that stack the channels are a little noisier. Then you kept adding switch options for circuit parameters. Now have stacked an additional gain stage. Some switches are possibly grounded for convenience of location. So, yeah, you basically complicated the potential for ground loops, less ideal ground location and amplified the problem by a gain of, what, 40 to 60? Whatever your added stages gain is. In fact, had I known you were building a four stage cascade I might have made some different suggestions earlier. And we can't forget that you had an orientation problem with your OT and it's lams are parallel to the PT. But anyway...

    In the end this could be a good thing. If you do rebuild it you'll have the option to omit some of the switches you probably find you aren't going to use. I imagine you must have a favorite position that, in practical use, you would never change for your variable preamp cathode circuits. I mean. really? You have some hundred possible combinations? You're probably not really going to use them all. In fact I'll wager there are two of those preamp cathodes you already have a favorite position for and only switch them for the novelty.?. Of course, the holes are there now so you'll want to use them because a plug is aesthetically displeasing on a build you went to some cosmetic trouble with. It's a very ambitious build for anyone. It would be for me and I've designed several amps and mods. FWIW you've already done better, faster than I expected.

    You can still change a lot about the grounding, likely enough, without ripping everything out. You could probably even change to DC filaments without undoing most of the filament wiring. I'd probably start with these two things before a tear down.
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  10. #115
    Lifetime Member Enzo's Avatar
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    I am not going to go find the 2203 drawings, but on many models, GROUND, as in chassis, is separate from circuit common. Circuit common, what we usually call ground, is separated from chassis by a small grounding network, typically a pair of diodes, a cap and a 10 ohm resistor.

    The number of paths to ground isn't the issue, the issue is sharing ground paths where it doesn't belong. MArshall put a lot more care into that than it may seem.
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    Can someone confirm something for me. I redid that lamp battery test, which ends up slightly under powering the heaters by a volt or so. Should you be able to play with the heaters on a 6 volt battery? Would the tone be thin with reduced output if the heaters were only showing 5 volts or 4.8, etc?

    Edit: I guess I just answered my own question by using a different battery that was fully charged. I think I'm doing this test more correctly now, which is leaving the guitar unplugged to rule out other interference outside the amp. Gain off, volume max, and measuring actual voltage off of the speaker output.

    Leaving filaments AC I'm getting 185 mV off the speaker jack. Still don't have it on a scope, but the fluke set to measure frequency is showing it as 60 Hz predominant. My best guess is since the cab can't reproduce that very well, the mic is showing the strong harmonic.

    Switching out to the lantern battery (hooked up right this time) is giving me almost dead silence in the room. Voltage on speaker jack is 11 mV. SPL at 1 inch from the speaker is 48 dBA.
    Last edited by Mr. Bill; 02-11-2018 at 06:43 AM. Reason: Add info.

  12. #117
    Supporting Member Chuck H's Avatar
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    Well then... I hate to say it, but I may have led you wrong in suggesting that you could follow the standard Marshall filament lead dress. With the extra gain in your circuit and the extra lead wiring it could be that a tightly twisted pair right to the pins is what you need. Also, make certain that all your grid leads meet at right angles to any filament leads and are elevated high above those leads and even the chassis they sit near. This has worked for other builders using AC filaments in high gain amps before DC filaments were all the rage. A PITA I know, but not undo-able.

    But we also can't neglect that there is probably another hum issue. as evidenced by your previous observations with phase cancellation at specific settings. Still... Solve for one first as identified, then tackle the other.

    Going to DC filaments could still be an option and you wouldn't need to redo your filament leads. This would tax your winding even a bit more than it is now, but I think it would be ok. I think the winding could handle it and the tax on the voltage may not be enough to drop voltage below spec. The only issue might be heat. You could build the DC circuit on a separate board and just zip tie it in somewhere if it works out, or skip it if it doesn't work out.
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  13. #118
    Lifetime Member Enzo's Avatar
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    So steal the circuit MArshall did use, where they ran the first couple tubes on DC. REctifier, couple caps.
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    I have a theory that the noise is there despite the twisting because I've located all of those cathode rotary switches on the rear panels right above the twisted heater wires.

    At any rate, for the sake of learning, I'd like to reattempt elevation to see if that helps, and possible the hum pot idea. Then whether I use it or not would like to try and build a DC supply.

    Can you point me in a direction of how to actually do the elevation correctly? Ditto for DC, I have found a bit of conflicting information as to the necessity of regulation, etc.

  15. #120
    Supporting Member Chuck H's Avatar
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    The elevation circuit is easy. See schematic below. The first figure would simply be connected to the screen supply. The second figure incorporates the screen supply balance resistor on the totem so there is no additional load on the HV rail and uses one less resistor (total count for screen circuit and elevation) R2 and R3 should have a series value close to R1 and create a roughly 10:1 voltage divider for the elevation circuit. I don't think either is better, but I like the eloquence of figure 2.
    Attached Thumbnails Attached Thumbnails elevate1.png  
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    Thanks Chuck. I have a couple questions for clarity.

    1. Can I take figure 1 straight off the screens cap positive end?
    2. Is the capacitance of the bypass cap critical to the noise in this instance? Any harm in it being larger than it needs to be?
    3. The center tap doesn't need to be an artificial one to be effective, correct? I can use the actual center tap on the 6.3V winding? I don't see why not, but just clarifying since I think I did it wrong the first time.

    Figure 2 is more eloquent indeed. I don't have the values for it in house though.
    Last edited by Mr. Bill; 02-11-2018 at 06:06 PM.

  17. #122
    Lifetime Member Enzo's Avatar
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    Don't guess, find out. If you think your switches are picking up hum via the wires, take the switches off the panel, and stick them straight up in the air on their wires, getting them as far from the heater wires as possible. Does this reduce the hum or not. If not, then the switches were not at fault. If it does reduce the hum, then you are correct.
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  18. #123
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Mr. Bill View Post
    1. Can I take figure 1 straight off the screens cap positive end?
    Yes.

    Quote Originally Posted by Mr. Bill View Post
    2. Is the capacitance of the bypass cap critical to the noise in this instance? Any harm in it being larger than it needs to be?
    Yes, the cap needs to be large enough to offer a low impedance to 60Hz. No harm in a larger cap to some extent. In fact 22uf is larger than needed. Knowing the actual tech is beyond me, but going too large may cause issues due to charge time or inrush voltage spike on the filaments? Maybe that's not an issue with DC filaments because the caps in those circuits are comparably enormous.

    Quote Originally Posted by Mr. Bill View Post
    3. The center tap doesn't need to be an artificial one to be effective, correct? I can use the actual center tap on the 6.3V winding?
    Correct.

    Quote Originally Posted by Mr. Bill View Post
    Figure 2 is more eloquent indeed. I don't have the values for it in house though.
    NBD really. I did it as per figure 1 in my last build. It only occurred to me later to incorporate the already existing totem. Next time.
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    Sorry to double post, but..... Something went amiss the first time I tried elevating the ground reference. I'm not sure what I did wrong, but this time I used the 220k/27k with a 330uf/50volt bypass on the 27k, and it helped A LOT. Not quite as good as the battery heating, but the noise is down from 186mV to 75mV. I'm dancing around the table, ha. That's with the washer and dryer running too, which always adds a little noise in the garage. I'll check it again tonight and see if it's actually quieter than that.


    Also, thanks for confirming those question Chuck.

    Also, Enzo, I will try doing that later tonight as well. Those 3 switches are fairly heavy, so I'll have to find a way to hoist them up. Maybe one at a time and I can physically hold the knobs.
    Last edited by Mr. Bill; 02-11-2018 at 07:00 PM. Reason: Add thanks without double posting.
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  20. #125
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Enzo View Post
    Don't guess, find out. If you think your switches are picking up hum via the wires, take the switches off the panel, and stick them straight up in the air on their wires, getting them as far from the heater wires as possible. Does this reduce the hum or not. If not, then the switches were not at fault. If it does reduce the hum, then you are correct.
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  21. #126
    Lifetime Member Enzo's Avatar
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    I use a towel. If I want to lift a board from a chassis and power it, I fold a towel under it in the chassis to prevent it touching anything. Your switches? Pull them up while stuffing a towel around them. Or get some string and tie to them and anchor it off something overhead. It isn't permanent, we just need a moment to see.

    I also sometimes use a magazine or a newspaper folded. Be creative.
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    Well I broke something. Amp has no sound now and I'm getting 400+ volts on all preamp plates.

  23. #128
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    If all your preamp cathodes are tied to the same ground point, it's possible that ground is faulted open so no current is flowing through the tubes. That's about all I can think of that would raise preamp plate voltages that much and shut the signal down.


    EDIT: Assuming the tubes are glowing then
    Last edited by Chuck H; 02-11-2018 at 08:35 PM.
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    I fixed it, sorry for the false alarm. I had temporarily alligator clipped the beginning of the preamp filament string together since I was going back and forth between the battery and the ac testing noise levels. The clip either came loose or shorted to another pin. Re-soldered in the AC heaters and all is well. Heaters being off would also cause no current to flow through the tubes, correct?

  25. #130
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    Correct.
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    Would elevating the heaters cause mains fuse to blow when playing loudly? Here's how I have it tacked in for now.
    elevated-heaters.jpg

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    Supporting Member Chuck H's Avatar
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    That looks correct. And no, the elevation and the elevation circuit should not cause the fuse to blow. Did you get to play the amp loud before installing the heater elevation? And the fuse didn't blow? My guess would be that excess drive to the power tubes, due to all your extra gain, may be over dissipating the screens. Modern EL34's can be tender about this. On the only two EL34 amps I've built in the last five years BOTH were prone to excess screen current with too much grid drive voltage. I solved the problem in both builds by reducing grid drive and increasing screen grid circuit resistance. It's also possible to increase screen grid impedance (same thing for this purpose) with a higher Henry choke, but that shouldn't be necessary. Since your amp derives most of it's gain from the preamp design it's unnecessary to drive the power tubes deep into clipping. Simply reducing grid drive, but not such that the amp won't produce full power, should be alright. I'm not able to positively identify this as your problem based on the data, but it's a good possibility based on my experience. And most modern high gain preamp design amps DON'T overdrive the power tubes much at all.
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    Thanks Chuck. Yes, the circuit hasn't changed in a while besides messing with heaters. It's got hours of playing on the high gain channel. The fuse actually blew while playing loudly on the straight JCM800 circuit settings bypassing the first stage. Hasn't done it again. Maybe vibrations for the heater elevation just being clipped in with gaters. I'll solder it in tonight and try and make it look neater and see if it happens again.

    For the sake of learning. Is there a reason you chose 220k/22k in your example? Wouldn't a larger 1mg/100k or some such combination steal less current? Also, where is the line of diminishing returns on elevation? I'm currently at 50V round about, would 60 be quieter? 40?

  29. #134
    Supporting Member Chuck H's Avatar
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    Since the main filament hum problem is with your earliest gain stage you actually only NEED about 15V of elevation to be sure that the filament is positive WRT the cathode. This should keep electrons from going the wrong direction and allowing hum from the filaments to head that way. But since your amp also uses a cathode follower the higher elevation can only help, since cathode followers commonly stress the heater to cathode voltage differential allowance.

    I chose 220k/22k arbitrarily because at that resistance it's not taxing the HV supply significantly, they are common values that you are likely to have on hand, they've been used for this circuit by others with good success and, for some reason that MAY have technical bearing I'm not aware of, very high resistances may be of some consequence to a circuit that must both elevate AND decouple AC relative to amplifier signals. I mean, the CT is commonly grounded! How high then should we be comfortable elevating it? Unfortunately I'm not electronically educated enough to answer this one better.

    I will say that, sans my knowledge of the circuit beyond rote implementation, 330uf is much larger than typical. If that's causing any problems I can't say. 10uf would reduce hum volume by some 45+dB (that's a lot) so 330uf would be a lot more. It takes time to charge that cap. Some modern tubes "talk" to their filaments more than the golden age tubes did, so it may make a difference. I had one build where the pilot lamp (run off the filament supply) would modulate in brightness with the trem function. There's no explanation for this I can find, but there it is. Maybe the 330uf cap is taxing the circuit and this interaction is causing a problem? Not sure. But 22uf is PLENTY for the elevation circuit.
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  30. #135
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    Is there a schematic of this build yet? Is it the HT fuse blowing or mains?
    I may be grasping at straws but I don't think I've seen cap bleeders as low as 56K before. Maybe at high volumes you were right at the edge of the fuse value and this extra elevation string made a difference? Not likely but thought I'd throw that out there about the 56K bleeder value anyway.
    Edit: digging around a bit, I guess the 56K bleeders were stock in some Marshalls for the screen supply?

    47048d1518379352-elevated-heaters.jpg
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    That picture is from the "stock 2203 layout" and schematic. They had 56k bleeders. I based it loosely off of that amp with an added gain stage.

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    Some updates.

    I've managed to get 77mV (down from 185 post elevation) of hum down to 44mV by messing with the routing of the heater wires around V2, the cathode follower. I theorized that moving the negative loop around to the low impedance side of the cathode follower would reduce the noise and it did. I'm going to try rewiring this section as tight as the others and see if there are any further benefits. At this point I find the filament noise totally acceptable, but I love to learn and tinker, plus I'm still obsessing over the battery giving me noise levels of around 11mV. If you don't mind, I would like to ask about a few things that I "think" I understand just to check my logic as I hunt down further noise.

    My circuit chain is currently as follows. Keep in mind V0a is unused with the pins tied together.

    V0b>V1a> gain knob > V1b> V2a>V2b(cathode follower)> tone stack > master volume >V3a>V3b(PI)> NFB and Output.

    So questions...

    1. With the gain shut off and the MV at max, logic tells me I should not be hearing the hum induced into V0b or V1a since they are sent to ground by the gain knob being off. Is this correct? My experiments hunting the hum would seem to back his up. Pulling the preamp tubes with a meter hooked to the output shows that pulling V0 made no difference in hum levels; pulling V1 brought the level down from 77mV to 66mV ( a little change since half of that tube is not sent to ground ); pulling V2 drastically dropped the noise down to 10mV, and pulling V3 further reduced that small amount to 3mV. This is what prompted me to attack the cathode follower to try and reduce noise first.

    2. As I understand it, running high current out of phase wires close together focuses the fields emitted by the current into each other, sort of trapping it as they approach null. My question is, does twisting in phase high current wires have the opposite effect? Does it strengthen the emission field or make no difference at all?

    3. I asked once already, but I'm not sure if anyone noticed because it didn't seem relevant. Going back to question 1 about V0 and V1a being shunted to ground with the gain knob off..... I have noticed that probing the voltage on V1a's plate and coupling cap on either side causes a very loud hum to come from he amp. How is this possible if that part of the circuit is already shunted to ground by the gain knob? I've noticed this on a few builds, not just this one, and the logic of why has always escaped me.

    4. Can heating an unused triode, like the V0a in my amp, induce noise in V0b or is that filament contained to that half of the tube with no effect at all on noise levels?

    5. Last one. Noticing that the cathode follower seems to be my prime inducer of hum in the circuit has me slightly baffled. I'm sure I'm missing the obvious, but as I understand it a cathode follower has theoretical gain that approaches 1, but does not exceed it. Since there is no "amplification" taking place there, how can it induce so much noise?

    I have learned more in the past few months here than I have in forever. I sincerely thank you guys for all your help in my journey to better understand these things.


    Edit: Thinking harder about #5, I believe it is because both triodes in a cathode follower are in phase, so you get no help from cancellations.
    Last edited by Mr. Bill; 02-12-2018 at 10:13 PM.

  33. #138
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    Quote Originally Posted by Mr. Bill View Post
    1. With the gain shut off and the MV at max, logic tells me I should not be hearing the hum induced into V0b or V1a since they are sent to ground by the gain knob being off. Is this correct? My experiments hunting the hum would seem to back his up. Pulling the preamp tubes with a meter hooked to the output shows that pulling V0 made no difference in hum levels; pulling V1 brought the level down from 77mV to 66mV ( a little change since half of that tube is not sent to ground ); pulling V2 drastically dropped the noise down to 10mV, and pulling V3 further reduced that small amount to 3mV. This is what prompted me to attack the cathode follower to try and reduce noise first.
    You are correct-ish. If the hum is due to grounding and the first or second stage have any circuit grounded where there is heavy current fluctuation (near, shared or in between power amp or power supply grounds) this can be read as AC on the signal chain. Even though it's coming straight from ground! The first couple of stages in a cascade amp are EXTREMELY sensitive. Since you're chasing filament hum right now, this may not be your problem. As your testing and reasoning has proved out. Since much of the hum left seems now to be with the cathode follower I would suggest trying yet another tube in that position. And NOT a Rusky made tube. Some of them are especially sensitive to the filament/cathode voltage differential and have been known to fail as cathode followers. Some reports say the manufacturers have corrected for this, other reports say they haven't. Why take a chance? When I build an amp I order five preamp tubes for every three I need. I'm buying from vendors, not bulk in any way. I do this because I have, indeed, had two noisy tubes, new out of the box in a purchase of five. This is a relatively recent phenomenon that is the new "normal".

    Quote Originally Posted by Mr. Bill View Post
    2. As I understand it, running high current out of phase wires close together focuses the fields emitted by the current into each other, sort of trapping it as they approach null. My question is, does twisting in phase high current wires have the opposite effect? Does it strengthen the emission field or make no difference at all?
    Interaction is interaction. Proximity and current determine the amount. The result is (at least partly) determined by phase. Two identical, out of phase currents will cancel. And two like phase currents will feed back. That is, other limiting factors not withstanding they will amplify each other indefinitely until the universe is consumed in a seething plasma. Fortunately there are always limiting factors Determining phase is sometimes tricky though because it changes with frequency and shifts are complicated and exacerbated by clipping. It's not entirely a crap shoot though. You can usually count on two concurrent gain stages being sufficiently out of phase and this can help with determining safe lead dress and grounding for high gain circuits.

    Quote Originally Posted by Mr. Bill View Post
    3. I asked once already, but I'm not sure if anyone noticed because it didn't seem relevant. Going back to question 1 about V0 and V1a being shunted to ground with the gain knob off..... I have noticed that probing the voltage on V1a's plate and coupling cap on either side causes a very loud hum to come from he amp. How is this possible if that part of the circuit is already shunted to ground by the gain knob? I've noticed this on a few builds, not just this one, and the logic of why has always escaped me.
    Touching tube pins with your meter does two things. It adds a significant length of unshielded lead hanging off the pin and it impresses whatever circumstances your meter uses for testing into the circuit. It's common to hear SOMETHING. Sometimes a just pop, sometimes hum or buzz. It's possible that there is a fault in your decoupling or grounding if probing the plate prior to a volume control is causing significant noise. Hard to know though. Like I said, it's common enough that probably everyone here (including me) read that and figured "Yeah, well..." And went on, distracted, to answer about another question that might have seemed more significant.

    Quote Originally Posted by Mr. Bill View Post
    4. Can heating an unused triode, like the V0a in my amp, induce noise in V0b or is that filament contained to that half of the tube with no effect at all on noise levels?
    Many amps incorporate an unused triode in their design. Typically the pins are left floating. I'm not even sure what tying them together would do. Nothing I would think since there is no voltage on any of the elements.

    Quote Originally Posted by Mr. Bill View Post
    5. Last one. Noticing that the cathode follower seems to be my prime inducer of hum in the circuit has me slightly baffled. I'm sure I'm missing the obvious, but as I understand it a cathode follower has theoretical gain that approaches 1, but does not exceed it. Since there is no "amplification" taking place there, how can it induce so much noise?
    Again... Grounding could be an issue. If any part of the CF or it's buddy triode is grounded with, near, or between high current grounds this can inject fluctuation in current as AC into the signal path. And read above where I mention noisy tubes too. And since the CF is direct coupled it's possible that differing ground points for any circuit on either triode could form a loop. So maybe check for that. Use individual leads for each ground associated with circuits from either triode (rather than daisy chain) and run them to the same ground point nearer the preamp end of the buss.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "Shut up, you big dumb poopy-head!" Justin Thomas

  34. #139
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    Thanks again Chuck!

    27mV and counting. Tried 3 different tubes in position. All were within a mV or 3 of each other. Got reduction further reduction by other means of trial and error.
    Chuck H likes this.

  35. #140
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    I think I'm getting excess noise with the gain up because of how I have my switches wired. From cathode, all RC loops are wired in and getting voltage and the ground is switched. It was easier to wire this way, but I'm pretty sure I'm creating little antennas on every tube. If I get time tonight I'm going to change that and see if it helps with the noise.

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