Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gfarina55 on July 25, 2021, 03:22:54 pm
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Hello! Long time lurker, first post. After many hours of troubleshooting, I’m seeking help with my 5F6A build that has two issues:
1. The signal seems as if it’s bypassing the entire tone stack. (Is this even possible?)
2. When I change from Sovtek 5881s (not real 5881s I know) to Tung-Sol 5881s (tried two sets), I get a loud hum and blow screen resistors.
Details:
I’m more of a musician than a tech, but I’ve built a few solid amps over the years. The amp in question is a traditional 5F6A 2ohm 4x10 combo. With a few exceptions, it’s identical to Robinette’s “5F6A Suggested Pre-Build Mods” layout found here: https://robrobinette.com/images/Guitar/Bassman/Bassman_Pre-Build_Mods.pdf (https://robrobinette.com/images/Guitar/Bassman/Bassman_Pre-Build_Mods.pdf) The exceptions are:
* 1ohm resistors btwn. Pin 8 & ground on the power tubes for quick bias measurements
* Rob’s “3-way Neg. Feedback Switch” mod found here: https://robrobinette.com/5F6A_Modifications.htm (https://robrobinette.com/5F6A_Modifications.htm)
* Rob’s “Adjustable Bias” mod also found at the link above. (I used a 27K resistor, 50K bias pot, and the coldest bias is around -53 on pin 5 of the power tube sockets.)
* Hot side of power cord is connected to the center of the fuse, not the side
* 100K ohm, 2-watt resistor across the standby switch replaces death cap
I followed Rob’s startup procedure (https://robrobinette.com/Tube_Amp_Startup.htm (https://robrobinette.com/Tube_Amp_Startup.htm)) to the letter. The amp started up w/out issue and sounds fantastic at “at home” volumes, and seems adequately quiet. Then I noticed that the Mid, Bass, and Treble controls have absolutely no effect on the tone, as if the signal is somehow bypassing the tone stack. To be clear, I tried all four inputs, tested many combinations of settings, and the tone stack pots have zero effect on the tone no matter how I set the controls.
Here’s what I did for troubleshooting (in no particular order):
* Triple checked all wiring against the layout visually
* Continuity tested every connection in the amp, including wiring under the circuitboard
* Continuity tested every solder joint to lug
* “Chopsticked” the entire amp…no noises or intermittent connections
* Continuity tested all grounds (yes the mid-pot is correctly grounded)
* Checked all the wiring on the pots and tube sockets, made sure there are no tiny strands of wire touching something that they shouldn’t
* Measured all resistors (I did this before I started the build)
* Measured all pots (tested resistance between tabs 2 & 3). All pots are the correct value, and I didn’t mix up a 25K pot with a 250K pot.
* Double checked the value of every component against the layout, including all resistor color codes.
* Tried multiple sets of preamp tubes
* Replaced the coupling cap after the treble pot (Had a hunch, but no.)
* Measured all voltages against the original layout (https://robrobinette.com/images/Guitar/Bassman/5F6A_Annotated_Fender_Layout.gif (https://robrobinette.com/images/Guitar/Bassman/5F6A_Annotated_Fender_Layout.gif)), all are within 15%, most are very close to the original.
* Read lots of posts here and at TDPRI, but I haven't found a discussion of this issue.
I also tried two other sets of power tubes (Tung-sol 5881s) and this is where I ran in to the second problem. I used two sets of known-good Tung-sol 5881s. I set the bias as cold as it could go (-53v.) before installing. Both sets of Tung-sols produce a loud hum when the amp is powered up, and as soon as I flipped the standby switch to “on”, one of the screen resistors blew, and the 100K resistor across the standby switch started smoking. Do I need to reconfigure my bias supply so that it goes colder than -50v.? If not, what could be the issue with the power tubes?
Any help with these two issues would be greatly appreciated. Tried to post some photos here but they're too large, but there are some on my TDPRI thread: https://www.tdpri.com/threads/seeking-feedback-for-5f6-a-bassman-build.1076468/page-2 (https://www.tdpri.com/threads/seeking-feedback-for-5f6-a-bassman-build.1076468/page-2) (Scroll to the end of p.2.) It's difficult to see the wiring in the photos, but I'm 99% sure everything is as described above.
Thanks in advance for any guidance!
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Pin 8 & ground on the power tubes for quick bias measurements
what did you measure across the R's when you tested it?
I get a loud hum and blow screen resistors.
that looks like something mis-wired, or tube pins touching, solder blobs, stray wire strand....
did the original tubes blow the screen R's?
what voltage do you measure, with NO power tubes installed, at the grid pins of the tube socket (G1 control grid, NOT G2 screen)
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i'd sort out the catastrophic failure stuff before messing with the tone stack. above is good suggestions
if the tone stack ground were not connected to preamp ground, that would certainly make the tone stack not work as intended (some amps include a tone stack lift switch that achieves exactly that)
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Hi, thanks for the responses shooter & thetragichero! The tone stack is grounded as per Rob's layout, all tested for continuity. Regarding "something mis-wired, or tube pins touching, solder blobs, stray wire strand", I've scrutinized every connection as described above. I guess a lump of solder could have fallen into a pot or in between the circuit board and the insulator, but wouldn't continuity tests show errors? Again, the amp sounds great with the Sovtek 5881s, aside from the missing tone stack.
Regarding the power tube issue, the voltage across the 1ohm resistors on pin 8 of the installed 5881s reads 30v, roughly translating (as I understand it) into 30ma current. Before installing the Tung-sols, I set the bias supply on the control grids to -53 without the tube installed. This is as "cold" as my bias supply gets. (I'm wondering if I need to reconfigure the bias supply to give the Tung-sols a more negative bias voltage?)
Here are all the voltages with with the Sovtek 5881s biased to 30ma.
B+1 = 435
B+2 = 441
B+3= 392
V.1 Preamp 12AY7
1. Plate=160
2. Grid=0
3. Cathode=2.7
4. (Heater, all heaters show the proper 6-ish volts and 5-ish for the rectifier)
5. (Heater)
6. Plate=160
7. Grid=0
8. Cathode=2.7
9. (Heater)
V.2 Preamp & Tone Stack Driver 12AX7
1. Plate=192
2. Grid=0
3. Cathode=1
4. (Heater)
5. (Heater)
6. Plate=326
7. Grid=192
8. Cathode=203
9. (Heater)
V.3 LTP Phase Inverter 12AX7
1. Plate=235
2. Grid=22
3. Cathode=45
4. (Heater)
5. (Heater)
6. Plate=257
7. Grid=30
8. Cathode=45
9. (Heater)
V4+5 5881 (measured both tubes)
1. (None, 1 ohm resistors to ground for quick bias measurements)
2. (Heater)
3. Plate = 438
4. Screen Grid= 440
5. Control Grid: -44 (negative 44)
6. None=440
7. (Heater)
8. Cathode (tied to pin 1)
V6 5AR4 Rectifier = Checked all voltages at initial startup procedure.
Anything jump out at anyone? Thanks again!!
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1. (None, 1 ohm resistors
none what!!
1 ohm resistors
voltage
none of the above
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2. When I change from Sovtek 5881s (not real 5881s I know) to Tung-Sol 5881s (tried two sets), I get a loud hum and blow screen resistors.
Tell us if this is correct:
The amp worked with the set of Sovtek power tubes. Then you put one set of the "known good" TungSols in and it blew one of the screen resistors (and the standby switch resistor).
-This indicates that one of those "known good" tubes is now "known bad". Thats all you need to know. Move on.
NEXT - You replaced the screen and standby switch resistors, fired the amp back up and the power section is working OK now?
- You are done troubleshooting the power section because it is working with the Sovteks in and you have 30mA'ish through each power tube. We don't care about "real" 5881s here. If it says 5881 on the side of it then that's all we need to know. Those fake 5881s are working much better then the "real" ones. :icon_biggrin:
Your voltages look OK and you can move on to finding your wiring error in the signal path.
Say it with me:
"I wired something wrong"
Our fearless leader has been known to say:
"If it was wired correctly, it would be working correctly"
You WILL find it. You just have to stop believing that your triple-checking and continuity testing aren't possibly faulty.
There is no magic, no gremlins, and nothing that any of us here haven't gone through.
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Shooter, what are you asking about pin 8 of the power tubes? Isn't this pin normally grounded w/no voltage? I've installed 1ohm resistors to ground on pin 8 just like on my Victoria. Makes calculating a bias a snap. Am I doing something wrong?
Silvergun,
2. When I change from Sovtek 5881s (not real 5881s I know) to Tung-Sol 5881s (tried two sets), I get a loud hum and blow screen resistors.
Tell us if this is correct:
The amp worked with the set of Sovtek power tubes. Then you put one set of the "known good" TungSols in and it blew one of the screen resistors (and the standby switch resistor).
>>Correct, but I tried *two* sets of Tung Sol 5881s with the same results.
-This indicates that one of those "known good" tubes is now "known bad". Thats all you need to know. Move on.
>>I'd love to, but both sets work perfectly in my Victoria Tweed Super. Maybe I missed something and will try again tomorrow, but after two sets I wonder if something might be different in the specs of each brand?
NEXT - You replaced the screen and standby switch resistors, fired the amp back up and the power section is working OK now?
>>Yep, although I have not replaced the pop resistor on the standby switch. Need to order one.
- You are done troubleshooting the power section because it is working with the Sovteks in and you have 30mA'ish through each power tube. We don't care about "real" 5881s here. If it says 5881 on the side of it then that's all we need to know. Those fake 5881s are working much better then the "real" ones. :icon_biggrin:
>>Can you confirm if -53v. is enough negative bias current for any 5881? This is what I'm wondering. After two attempts today it still appears that Sovtek's work and the Tung Sol's don't, and the Tung Sols work fine in another amp. I may have bungled something and I'll try again more methodically tomorrow, but I'm pretty sure this is the case.
Your voltages look OK and you can move on to finding your wiring error in the signal path.
Say it with me:
"I wired something wrong"
>>I get it, but I've done this: https://el34world.com/Forum/index.php?topic=5.0 (https://el34world.com/Forum/index.php?topic=5.0) twice now, in addition to idiot checks after every step, so I'm skeptical. That said, I'll check again!
Our fearless leader has been known to say:
"If it was wired correctly, it would be working correctly"
You WILL find it. You just have to stop believing that your triple-checking and continuity testing aren't possibly faulty.
There is no magic, no gremlins, and nothing that any of us here haven't gone through.
I get it, Thx everyone!
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Do your tungsols have a metal base? I’ve read how their metal base on some of their tubes is tied to pin 1 and causes problems. Not sure if that’s relevant here.
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so where's the math??
what did the calculations say?
you have NONE, what is NONE??
Voltage, continuity, current????
None is a big space
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so where's the math??
what did the calculations say?
you have NONE, what is NONE??
Voltage, continuity, current????
None is a big space
In this case NONE means zero volts. He has pin 1 tied to pin 8 with a 1Ω resistor to ground. So the voltage on pin 1/8 is actually 30mV (close to NONE)
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The OP wrote
the voltage across the 1ohm resistors on pin 8 of the installed 5881s reads 30v,
guessing that's wrong 30v/1ohm = 30A
If that's wrong, like SG pointed out, there's other things are wrong, but the OP won't find them by rounding 30V, 30mV, to NONE
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Of course that's wrong, but you're in the wrong rabbit hole. :icon_biggrin:
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:laugh:
I'm with the masses, there's a problem in the pre with wiring error, etc
just trying to make sure "that" rabbit hole is properly buried 1st
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>>Can you confirm if -53v. is enough negative bias current for any 5881?
Yes. Definitely. (based off of your reported plate voltage)
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Jordan86, the Tung-sol's don't have the metal base, but thanks for the tip. Silvergun, thanks for the confirmation. Good to know the problem's not in the bias supply. I'll give the wiring another check this eve and post again.
Slucky and Shooter, can you clarify your point about the 1ohm resistors for measuring bias? Isn't this an acceptable and widely used method for rough bias measurements? My electronics knowledge doesn't go much further than Ohm's law, but doesn't a 30mv drop across that resistor translate into roughly 30ma?
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Slucky and Shooter, can you clarify your point about the 1ohm resistors for measuring bias? Isn't this an acceptable and widely used method for rough, quick bias measurements?
You posted 30v (as in 30 VOLTS) when you meant to say 30 Millivolts or 30mV
This caused shooters brain to backfire and sluckey had to help him out. :icon_biggrin:
ALL m's and V's matter!
Regarding the power tube issue, the voltage across the 1ohm resistors on pin 8 of the installed 5881s reads 30v
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30mv is .030 Volts - imagine how many electrons are flying around in that tube.. when 1A = Quintrillions of electrons
gfarina Make sure the ground conection on the mid pot left side terminal (as viewed from the back) is physically sound and has continuity to ground. Did you solder that left side terminals ground to the back of the pot? You can take a small jumper and jump the left terminal to a ground point to check it.
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Sorry silvergun, I'll pay more attention to the "m's & v's"!
Mresistor, I've checked that mid-pot ground connection thoroughly. If you look at my OP and the link to Rob's layout, I use his preamp ground bus. My grounds are exactly like Rob's layout.
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You're at an interesting point in amp troubleshooting. You can now have the fun of jumpering the signal to different points in the circuit to see if you get expected results.
We use jumper wires of varying lengths that have alligator clips on both ends to clip in the signal where we need it to go.
Obviously, you have to be very careful, but this can be a useful exercise.
That's one of five things I would do to try and find out why your tonestack isn't in the path.
Check out Rob's signal path example here: 5F6A Mods (robrobinette.com) (https://robrobinette.com/5F6A_Modifications.htm)
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More specifically:
1) Pull V2 and leave it out of the amp (the tube with the cathode follower)
2) Jump the signal from after the coupling cap on either side of V1 directly to the tone stack input (the cathode connection of V2)
* if you don't have clips you can tack in a temporary jumper wire
3) the amp will be less loud but your signal will be where you want it and you can test the controls this way
4) If your tone controls work then you have a wiring issue "pre" stack
5) If the controls still don't work then your tone stack is obviously mis-wired
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Any chance the mid pot ground lug is internally disconnected? what happens if you temporarily ground the wiper?
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ST, How would I know if the mid pot internal ground is disconnected? On TDPRI, someone also asked if tab 3 to ground measured 25K, but I can't get a resistance measurement between tab 3 and ground of the mid pot. Is this a clue?
Silvergun, thanks for this procedure! I don't have banana clips but I'm going to order a few parts this week and I'll order them and try this. I'm learning a lot through this process!
So it appears I've solved the power tubes mystery. What I *think* happened is that when I initially put in the new set of Tung-sols, they slightly reoriented the power tube socket pins and there was an arc btwn. the heater and one of the high voltage pins. The pins weren't touching, but some solder was very close to a heater pin and I can see evidence of an arc. This is also the socket that blew the screen resistor. I have no idea why the Sovtek's worked and the Tung-sol's didn't, but to make a long story short, after multiple sets of 5881s in and out and a third go around with Rob's startup procedure, I have the Tung-sol's up and running. Does it matter that the "matched set" are about 6ma apart in plate current? If I set one to 33, the other is at 27.
Spent an hour rechecking the preamp/board/tube wiring again against the layout this morning and it still appears to me that everything is wired correctly.
Thx!!
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I looked at Robs layout and the picture of the back of your tone stack pots.. I see the left terminal of the mid pot has a ground wire going to the preamp ground buss. The solder joint is a little questionable but may be ok as I'm going by the picture. There is another black wire on the mid pot left terminal that is not shown on Robs layout. What is that for? what is it connected to? Also just looking at the ground wire attached to the mid pot, it looks like there is a good length of exposed conductor that is very close to the pot case and the wiper terminal.
(https://www.tdpri.com/media/img_4782.47207/full?d=1627155643)
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OK I see your next pic and the exposed part of the ground wire is sufficiently away from the case and middle terminal. Also I see that the other black wire is part of a mod and goes to the presence pot. (https://www.tdpri.com/attachments/upload_2021-7-26_15-23-39-png.882486/)
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another view (https://www.tdpri.com/attachments/upload_2021-7-26_15-21-45-png.882485/)
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Hi MResistor,
You can't tell much from my terrible pics. I assure you that the solder joint is good and that tab 1 of the mid pot is grounded. That ground wire isn't near any other metal, although the photos make it look like it's touching tab 2. You're right...the other black wire to the left is in Rob's layout and it's connected to tab 1 of the presence pot. Again, I've checked the wiring methodically and can't find an error.
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...someone also asked if tab 3 to ground measured 25K, but I can't get a resistance measurement between tab 3 and ground of the mid pot. Is this a clue?
That's more than a clue! That's the problem. The mid pot is open. Replace it.
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...someone also asked if tab 3 to ground measured 25K, but I can't get a resistance measurement between tab 3 and ground of the mid pot. Is this a clue?
That's more than a clue! That's the problem. The mid pot is open. Replace it.
OK! Just to confirm, tab 3 (farthest to the right, looking from the back of the pot) should measure 25K to ground, and NOT have infinite resistance, correct? If that's the issue, I'll order a new pot today and get back to you. Thx everyone!
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for extra fun while you wait;
ohm middle tab to each end tab with pot set at 1/2
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New mid pot installed,everything works perfectly! Thanks everyone for your thoughtful ideas and willingness to help. The amp sounds fantastic at "apartment" volumes, and tomorrow I'm going into the studio for a few hours to crank it.
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I should have mentioned it sooner but the one time I had a tonestack issue with a CF stack...it was a bad treble pot. Everyone I talked to swore up and down that’s uncommon and would be the last thing to worry about. I fussed with so many other things before swapping it. Swapped it and problem was solved.
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Everyone I talked to swore up and down ...
In this case, I felt so bad that I sent a PM apologizing for insinuating that it must have been something he did wrong.
Nobody really expects a newly installed pot to be open.
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Nobody really expects a newly installed pot to be open.
I have received some that had "open spots", gritty. but bad outta the box is a fluke.
I've found "splatter and heat" being the main pot killers
another reason I check 'em before I solder 'em
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...of course I may have cooked the pot myself, who knows! I'm almost sure the wiring is perfect, but there's certainly other things for me to mess up. In any case, it's working now and that's what's important.
I played the amp for about an hour today and it was noise-free. I carefully cleaned up a few solder joints and did some tube swapping, and somewhere along the way I noticed that the treble, bass, and (to a lesser extent) mid controls make a lot of noise when I turn them. The treble makes a "trebly" scratch noise, the bass makes a "base" whoosh, etc. This is new; Could I have heated up a cap that is now passing DC current onto the tone controls? I read this: https://el34world.com/charts/fenderservice6.htm (https://el34world.com/charts/fenderservice6.htm). Since the noise is on all three tone pots, it would be unlikely that all three tone caps are passing DC, correct? Should I suspect one of the two .02uf coupling caps from pins 1&6 ov V. 1?
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There are four caps that could possibly put DC on the tone pots, C5, C6, C7, and C8. Lift the side of each cap (one at a time) that connects to a pot. Check for dc volts on the dangling cap lead. Repeat for all four caps.
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OK, so these are the three tone caps and the cap coming off the treble pot wiper? I'll check...
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Found it, fixed! Learned something new today.
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Which one?
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It was the little 250pf treble cap. Tone controls are completely quiet now, and the amp is nearly silent at idle.
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It was the little 250pf treble cap. Tone controls are completely quiet now, and the amp is nearly silent at idle.
That's the one that's most likely to fail, usually heat damaged while soldering. Using a heat sink on the lead between the body and the solder joint is a good idea with these small caps. Good idea for small diodes too.
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Good to know, thanks! I think I also fried a rectifier diode I was working with. Need to up my soldering game.
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I had the same issue on a new Fender PR build. Sluckey pointd me the exact 500pF cap to swap out and using a hetat sink, I successfully replace it. Good lesson learned.
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...I think I also fried a rectifier diode I was working with....
Modern parts are not that fragile. Don't use this iron. Use good for-electronics solder and flux.
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Anyone within a block of Experimental Sound Studio in Chicago heard my Bassman yesterday afternoon. I played it at full overdriven volume for 2+ hours, and it sounds heavenly and has zero issues. I can really hear the JTM 45 in this amp, and I was quite pleasantly surprised at the range of the tone controls. Unlike my other tweeds there are no bad tone control settings. Here it is with my '57 Pro, Victoria 35210, and the 5E3 I built at the beginning of the pandemic. The cab is by John Mergili, and the speakers are Weber 10A125s. Thanks everyone!
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Nice line!
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So, I've had some time to crank the amp, use it with pedals, etc., and it appears that I have a "ghost note" problem. When I play the amp cranked and play lead notes up the neck, I get slightly-out-of-tune notes that are lower than the note that I'm playing. They are very noticeable and very annoying, but they are notes, not random noises. It seems to happen on higher notes when I play overdriven leads with an OD pedal or when the amp is cranked. I don't think it's "cone cry" b/c it comes out of every speaker. I'm thinking of adding grid stop resistors? Should I bother to change the filter caps, as they are new? Other ideas?
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So, I've had some time to crank the amp, use it with pedals, etc., and it appears that I have a "ghost note" problem. When I play the amp cranked and play lead notes up the neck, I get slightly-out-of-tune notes that are lower than the note that I'm playing. They are very noticeable and very annoying, but they are notes, not random noises. It seems to happen on higher notes when I play overdriven leads with an OD pedal or when the amp is cranked. I don't think it's "cone cry" b/c it comes out of every speaker. I'm thinking of adding grid stop resistors? Should I bother to change the filter caps, as they are new? Other ideas?
What choke did you use?
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I used a Hammond 194B. Grid stoppers had no effect on this issue, and they may have even degraded the clean tone. Thx for any advice you can provide!
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That is the "right" choke but it could have been damaged during the initial screen resistor incident. (over-current)
You could patch in a power resistor in it's place to see if that makes a difference.
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This topic got me reading more today and I stumbled across this interesting article:
Class AB Power Supply Ripple (ampbooks.com) (https://www.ampbooks.com/mobile/classic-circuits/class-AB-ripple/)
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Interesting.
Well, according to that article, any true push/pull class A power amp will be missing a lot in the tone we like from what is really happening when the amp is turned up enough to strain the power supply enough to sag.
(And, an AC30 does come out of class A and into class A/B turned up enough.)
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does come out of class A and into class A/B turned up enough.
^^^^^^^
that's one of the infuriatingly fun things to "get right"
I've had the most luck with SE Kt88 or PP EL34's
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That article is a bit over my head, but it reminded me of something else I noticed: A pronounced 60 cycle hum when I turn the amp up to around 7 and beyond. (I believe it's 60 and not 120). As a player not really an issue b/c at 7 the amp takes paint off the walls, but it's definitely there. Might the two symptoms be related?
Silvergun, I don't have a power resistor but I'll order one to try. A bad choke could cause these ghost notes?
Also, I'm also using new Weber 10A125 30w speakers, light dope, and Weber says it could quite possibly be "cone cry" from all four new speakers. Even so, the amp sounds kind of brash and sharp in overdrive and I'm wondering if there's something in the power section that could be improved.
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That article is a bit over my head, but it reminded me of something else I noticed: A pronounced 60 cycle hum when I turn the amp up to around 7 and beyond. (I believe it's 60 and not 120). As a player not really an issue b/c at 7 the amp takes paint off the walls, but it's definitely there. Might the two symptoms be related?
Silvergun, I don't have a power resistor but I'll order one to try. A bad choke could cause these ghost notes?
Also, I'm also using new Weber 10A125 30w speakers, light dope, and Weber says it could quite possibly be "cone cry" from all four new speakers. Even so, the amp sounds kind of brash and sharp in overdrive and I'm wondering if there's something in the power section that could be improved.
Yes the choke could contribute...and yes the hum could be related.
If you PM me your address I can drop a resistor in the mail for you tomorrow so you won't have to pay more shipping than resistor cost.
I'm not suggesting that I am right but there are different ways to troubleshoot power supply issues and this is one of them. Once you rule out the choke you can cross it off the list. (and I don't want you to have to overpay to try it)
After reading more about power supplies today I'm also assuming that there is a balance between an amp that sounds great due to SOME power supply ripple and an amp that can deal with extreme overdrive.
SOME ripple can manifest into a more harmonically rich tone whereas an over-filtered (less ripple riddled) power supply will handle overdrive better.
So, what I'm suggesting is that your amp might be sounding great until YOU push it too far into overdrive and CAUSE the nonlinear distortion effect of adding new frequencies (harmonic distortion). You could fix it with better filtering, but by fixing it you could sacrifice TONE.
Keep in mind, I am a welding machine guy and I'm learning right beside you, but this is pretty cool stuff. You might be able to "tune" out the issue with a different value filter cap, which will be a compromise between retaining some good ripple but enabling your brand of full tilt overdrive.
A great man once said "Very low level hum is not offensive; it is "the sound of electricity". An utterly hum-free amp sounds artificial, phony. You are free to disagree."
I agree.
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On a separate note:
Let's go back to your original post...
You said that you incorporated Robrob's NFB switch.
Does your OT have a 2 ohm tap?
Did you use it for your NFB tap?
Does the ghost noting change at all if you select a different NFB switch setting?
You will also want to confirm if that is 60hz or 120hz that you are hearing.
Use your phone to generate those test tones and match it up. That's about as simple as it gets.
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Thanks Silvergun! Yes, the OT has a 2-ohm tap only. I have used Rob's NFB mod and the three settings (Bassman/none/Marshall) have no impact on the ghost notes. (I also did all the standard trouble shooting: swapped every tube, tried all inputs, settings, etc.) I've also listened to 60vs.120 and I'm convinced this is 60. Again, the hum isn't an issue from a player's perspective as it's not very loud, but I wondered if it might be a clue.
At this point I think the ghost notes and the "brashness" of the overdrive might have different causes. I think the ghost notes may be cone cry, as it seems to be common to 10A125 speakers. (I have to lug my C-30 over to the studio to test the 5f6a speaker cab at some point.) The "brashness" might be related to what you are describing.
Thanks for the offer about the power resistor; I just ordered an Ohmite "Dividohm" 8-ohm 225W Power Resistor to use as a dummy load. Would this work? Also, what exactly do you mean by "better power filtering"? I read this phrase in a lot of posts/articles and I'm not sure about the specifics. Is there a "better" scenario than the 4x22/500V filter cap setup?
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Thanks Silvergun! Yes, the OT has a 2-ohm tap only. I have used Rob's NFB mod and the three settings (Bassman/none/Marshall) have no impact on the ghost notes. (I also did all the standard trouble shooting: swapped every tube, tried all inputs, settings, etc.) I've also listened to 60vs.120 and I'm convinced this is 60. Again, the hum isn't an issue from a player's perspective as it's not very loud, but I wondered if it might be a clue.
Since ghost notes can commonly be caused by a faulty power supply any clues are helpful. It can be difficult to differentiate between 60 and 120hz because they are directly related harmonics. A slight 60HZ hum is pretty common in amps. My new favorite saying is "it's the sound of electricity".
At this point I think the ghost notes and the "brashness" of the overdrive might have different causes. I think the ghost notes may be cone cry, as it seems to be common to 10A125 speakers. (I have to lug my C-30 over to the studio to test the 5f6a speaker cab at some point.) The "brashness" might be related to what you are describing.
Yeah, that could be the case. The "brashness" is subjective and could just be the inherent quality of the amp fully overdriving. Are you driving the input with a boost or distortion pedal? Ghost notes (or harmonic distortion) can often come from a defective power section.
Thanks for the offer about the power resistor; I just ordered an Ohmite "Dividohm" 8-ohm 225W Power Resistor to use as a dummy load. Would this work? Also, what exactly do you mean by "better power filtering"? I read this phrase in a lot of posts/articles and I'm not sure about the specifics. Is there a "better" scenario than the 4x22/500V filter cap setup?
The offer still stands. The resistor you ordered will work as a dummy load for testing, however you would have wanted to order something closer to 2 ohms since that is your OTs output impedance.
My suggestion was to temporarily use a power resistor in place of the choke to see how or if it affects your ghost note issue. It is common for a power supply to either have a resistor OR a choke in this position, so if you are troubleshooting a possibly faulty choke you can do so by tacking in a resistor (of a similar resistance as your choke) to see if it clears up or changes the ghost note issue.
Before my reading yesterday I would have told you to go ahead and replace the filter caps with higher values and if your choke was Ok then this would probably fix the issue. I didn't think you could over-filter an amp to a fault (within reason).
But after reading that article I am convinced that "over-filtering" can sterilize your amp. And we don't want that.
And yes (to a previous question you brought up), you could still have a damaged filter cap from the initial screen resistor incident. I was just giving you a simple way to eliminate the choke as a possible suspect. Disconnect one wire from choke, tack a resistor in it's place and test.
PM me your address and I can still get one out today. I haven't seen the over-heated postal person yet today.
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Thx Silvergun! The power resistor I bought is variable, so I can "set it" to 2 ohms. I believe I saw one of these on Slucky's page so I found one on Ebay. I'll use it and also patch in some other filter caps to troubleshoot sometime soon.
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Thanks for inspiring me to continue reading about the subject.
Here's more brain food so you understand where I'm coming from:
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Glad you're learning faster than I am Silvergun!
So, I've been reading about grounding all night (I've read Aiken and the Hoffman info multiple times) and I'm wondering if the 60 cycle hum might be reduced w/a better grounding scheme. See my setup in the photos below. I used a Rob-style preamp ground bus connected to one of the input jacks, and I have the AC earth bolted to the side of the chassis. I have another bolt to the right of the PT that is my power amp ground. I've connected the PT high voltage CT, the filament CT, the long ground wire from the filter cap doghouse, and a wire from pin 8 of the power tubes (through resistors for quick bias measurements) to this ground. The OT is grounded to the speaker jack.
I found this thread on TDPRI that cites Rob and Hoffman and suggests some improvements:
https://www.tdpri.com/threads/try-robs-pt-ct-trick-one-wire-to-quiet-them-all.1015082/
It suggests that a better way to ground the PT CT is to connect it to the negative side of the "first" filter cap. (Which cap is "first"?) However, the example used in the thread is a Champ in which the filter cap is in the chassis close to the PT. In a 5f6A, the caps are in the doghouse way over on the other side. Is it advisable to run the PT CT the entire way to the doghouse, and then the ground wire back to the power amp ground? Is there a better way to accomplish this?
In the same thread, a TDPRI member wrote: "a trick I learned from Hoffman years ago is to ground the heater CT at the input jack grounding point. In the amps I've done this on, 60Hz hum is gone." Would it be advantageous to actually run the filament CT wire the entire way across the chassis to the termination of the pre-ground bus at the input jack?
Finally, is there a better grounding scheme that might reduce 60 cycle hum? I know that 120 cycle hum is more often a symptom of ground loops, but is my grounding somehow allowing some ripple into the signal? (Silvergun, I'm still considering the caps themselves and the choke.)
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It suggests that a better way to ground the PT CT is to connect it to the negative side of the "first" filter cap. (Which cap is "first"?) However, the example used in the thread is a Champ in which the filter cap is in the chassis close to the PT. In a 5f6A, the caps are in the doghouse way over on the other side. Is it advisable to run the PT CT the entire way to the doghouse, and then the ground wire back to the power amp ground?
Yes.
The 1st B+ filter cap is the cap that supplies the power tube plate. The nosiest ground is that cap because it has the most current going through it and has the most ripple on that cap. So you ground the CT to that caps ground lead, then run a wire to the chassis power amp ground or to the far end of the ground buss.
Read this;
http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
That wire nut is not safe to use.
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More good info here:
Grounds (el34world.com) (https://el34world.com/charts/grounds.htm)
Current Flow (el34world.com) (https://el34world.com/charts/currentflow.htm)
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OK, thank you, good to know! I may rethink my grounding scheme.
A bit of troubleshooting this morning: the hum goes away when I pull V.1, so I think the heaters are responsible. Has anyone elevated the heaters in a 5F6A (Fender layout)? Where exactly do you tap the voltage from?
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Thanks for inspiring me to continue reading about the subject.
Here's more brain food so you understand where I'm coming from:
Totally OT here but that information was written before 1953 when the ISO made us go to 440 tuning. Reading here about the summation and differential tones, It mentions 256 as C and 320 as E. The differential tone is C two octaves below. With A=432, C=256, D=144, E=320, etc. What a simple concept music was until they had us go to 440 as standard, where all other tones are irrational numbers.
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OK, thank you, good to know! I may rethink my grounding scheme.
A bit of troubleshooting this morning: the hum goes away when I pull V.1, so I think the heaters are responsible. Has anyone elevated the heaters in a 5F6A (Fender layout)? Where exactly do you tap the voltage from?
Before you elevate. Try a different V1.
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Thanks for inspiring me to continue reading about the subject.
Here's more brain food so you understand where I'm coming from:
Totally OT here but that information was written before 1953 when the ISO made us go to 440 tuning. Reading here about the summation and differential tones, It mentions 256 as C and 320 as E. The differential tone is C two octaves below. With A=432, C=256, D=144, E=320, etc. What a simple concept music was until they had us go to 440 as standard, where all other tones are irrational numbers.
It's still simple for most guys who don't think about frequency numbers. :icon_biggrin:
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Yeah, I tried different tubes in every position. No reduction in 60cycle hum.
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A bit of troubleshooting this morning: the hum goes away when I pull V.1, so I think the heaters are responsible.
That doesn't mean it's the heaters. You can check the heaters by using a large 6v lantern battery. My $$'s on the grounding.
Has anyone elevated the heaters in a 5F6A (Fender layout)? Where exactly do you tap the voltage from?
Yes. I like it.
I'd make sure your grounding correct before you chase elevating the heaters.
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Willabe, could you look at my grounding in the photos I posted yesterday (a few threads above) and see if anything looks suspect? I used the preamp ground bus in Rob's layout, my OT is grounded at the speaker jack, the AC cord is "earthed" to a dedicated bolt on the side of the chassis, and my power amp ground has the following connections:
>Grounds from filter doghouse
>PT CT
>Heater CT
>Power tubes pin 8 (through a resistor for easy bias measurements)
Just to be clear, you suggest that I run a long wire from the PT CT to the doghouse and connect it to negative lead of the first filter cap, even though that negative lead is running through another wire back to the power amp ground point (where the PT CT is "currently" connected...pun intended.)
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> Is it advisable to run the PT CT the entire way to the doghouse, and then the ground wire back to the power amp ground?
Yes.
> Is there a better way to accomplish this?
No. "Better" how? Less wire cost?
In a HIGH current (transistor) amp we'd put PT Rect and Caps all real close and strap them with copper busbars. But in a tube amp the current ultimately flows through a vacuum, not very well, and even long skinny wires conduct better than that.
The "first cap" is the one fed directly by the rectifier. (In a 2-Diode CT rectifier, one leg is the PT HV CT.)
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By "better" I meant more elegant (less wire), but I'll try this and report back. Thanks!
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....could you look at my grounding in the photos I posted yesterday (a few threads above) and see if anything looks suspect?
>Grounds from filter doghouse
>PT CT
>Heater CT
>Power tubes pin 8 (through a resistor for easy bias measurements)
I can't tell be looking at a picture. Read the link on grounding I posted for you.
In it you will see you can't ground all the filter caps at the power ground and use a ground buss for all the preamp and PI circuitry.
Just to be clear, you suggest that I run a long wire from the PT CT to the doghouse and connect it to negative lead of the first filter cap, even though that negative lead is running through another wire back to the power amp ground point (where the PT CT is "currently" connected...pun intended.)
Read the grounding link. Then we can talk if you still need a little more clarity.
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Thx Willabe.
Regarding the PT CT, I ran a wire from it to the leg of the filter caps (where the white wire is connected in the photo below) and it made absolutely no difference in the hum. (Again, there is a wire running right back to the power amp ground, so I don't fully understand this concept.)
I've read the valvewizard info to which you linked (also before I built the amp), and also the Aiken Amps grounding page, and some others, and I've followed all I can understand. I get the concepts, but I'm still trying to understand "where the wires go". When I built the amp, I tried to stick to this: https://el34world.com/charts/grounds.htm
But mine has some differences:
1) I followed Rob's layout which only has 4 filter caps (why?). My filter cap board (photo below) is exactly like Rob's. In my photo, the white wire goes to the power amp ground point, and the black to the preamp ground bus, just as valvewizard states. Correct, yes?
2) I have no *wire* that connects the power amp ground to the preamp ground bus, and no *wire* from the OT secondary (grounded at the speaker jack) to the power amp ground. Should I, or does the chassis suffice?
3) Hoffman's grounding scheme doesn't show a PT heater CT. My transformer has this, and I was told to connect it to the power amp ground. Is this correct?
The other thing is that I notice the hum more when I turn the bright channel vol up. I assumed this is b/c it "brightened up" the hum, but could something be awry only in one Chanel?
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Maybe this helps!
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That's basically the same ground scheme that Hoffman and I use.
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I looked at robrob's layout ground wiring. And found your link to robrobs updated 1st B+ filter cap ground wiring that's on a web forum. What robrob did in the updated ground wiring is what I'm talking about.
He moved the PT B+ CT from the chassis power amp ground lug to the 1st filter cap negative lead. Then he ran a wire from that junction, ie; filter cap negative lead/B+ CT, over to the chassis power amp ground lug/bolt. That's the same as PRR, Merlin, Kevin O'Connor/London Power and others recommend.
Here's why;
Short answer = create a short ground loop for the 1st filter cap/CT isolated from the other grounds.
Long answer;
Fender and many others used a random ground scheme. They used the chassis as the ground buss where it was convenient to make a ground chassis connection. Fender did separate the power amp ground, ie; power tubes and PI grounds from the preamp grounds. Look at the Fender Tweed, brown face and black face amps layout drawings, it's there. Although on the Tweeds, the power amp filter cap chassis ground is inside the dog house on the back of the chassis, so it's not shown on the layout. Our host Doug uses a grounding scheme very close to Fenders. He separates the power amp chassis ground from the preamp ground but he uses a ground buss for the preamp. It works.
KOC, Merlin and others use a wired ground scheme. They put the B+ filter cap close to the circuitry it feeds and wire the grounds from those circuits directly to the filter caps ground lead that feed that circuit. This keeps the loop short and isolated.
Doing that they do not use the chassis as a ground buss. Random ground currents in the chassis can cross each other in the chassis and impose a signal that will modulate a different circuit, that causes the buzz.
They also make individual stars on the buss that are then wired together. They don't just run each ground wire to the closest buss point. A preamp tube would have the grounds for the K, G1 return and filter cap that feeds that tube tied together as a star then run a wire (buss) to the next star. Merlin has drawings showing this.
What this does is creates individual ground loops for each filter cap that are short/small and isolated from each other. Current for the tube comes up from ground, thru the K R, thru the tube, out the plate, thru the plate R, to the + end of the filter cap, thru the B+ filter cap, out of the B+ filter cap thru it's ground lead and back to the K. And round and round it goes. But that current is isolated from the other loops so it doesn't mess with them.
With the power tubes, current up thru the K, out the plate to 1 side of the OT primary, thru that 1/2 of the OT wind to it's CT, out thru the CT, to the + of the B+ filter cap, thru that filter cap, out the negative lead and back into the power tubes K. And round and round it goes.
The 1st B+ filter cap is the cap that supplies the power tube plate thru the OT CT. The nosiest ground is that cap because it has the most current going through it and has the most ripple on that cap. So that's why this ground should be wired up correctly to minimize noise.
That's why I'm telling you as PRR, KOC, Meriln, and robrob, to make that B+ filter cap loop isolated, ie; B+ filter cap negative lead/OT CT, then add a wire from there to your tube power tube ground star, that wire isolates the loop from the power tube star ground. It's the purple arrow in Merlin's drawing.
Then, if your using 2 chassis B+ filter grounds, ie; power amp ground/preamp ground, run a wire from the CT /red arrow wire, power tube ground star/purple arrow wire and the B+ filter cap negative lead to your chassis ground. In Merlin's drawing, it would be the preamp daisy chain ground wire. He uses 1 chassis ground, not 2 chassis grounds for the B+ filter caps.
Look at this; the red arrow points to the OT CT, the purple arrow to the power tube star ground.
All said, you want to keep the filter cap/circuitry ground loops short, ie; star and place the filter cap close to the circuit id supplies, and use less/least amount of chassis ground connections as is reasonable/possible, ie; star buss. This works well. Ground stars forming a galaxy = galactic grounding scheme.
And I attached robrobs updated power amp OT CT ground connection.
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Look at this, B+ CT/1st B+ filter cap move.
From PRR;
https://el34world.com/Forum/index.php?topic=27590.msg305790#msg305790 (https://el34world.com/Forum/index.php?topic=27590.msg305790#msg305790)
Then his result after moving wire from chassis to where it should be;
https://el34world.com/Forum/index.php?topic=27590.msg306339#msg306339
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Hi Willabe,
Thanks for this detailed explanation! I didn't understand from your previous posts that you were suggesting that I move one of the filter caps, but creating isolated "individual ground loops" for each filter cap/cap set is genius. B/c I'm building a "traditional" 5F6A I think I'll keep the caps in the doghouse on this build, but I'm definitely going to try the wired grounding strategy on my next build. Thank you again for such a clear and detailed explanation. With a "wired ground scheme", is it advantageous to isolate the inputs?
Robrob's layout does indicate the first 3 filter caps grounded at the power amp ground point, and the fourth at the preamp ground bus, so that's what I did. I know the old school way is to ground them right to the doghouse, but I can't imagine this is safe or quiet. I spent a couple hours in the studio with the amp yesterday, and I have to say the 60 cycle hum that I'm getting is negligible when I'm playing the amp at proper volume. I'm just trying to get it as quiet as possible w/in certain limitations, and this exploration of grounding has been instructive.
Regarding the "ghost notes" that I was encountering earlier in this thread, I believe the issue is that I switched from a Tele to a Les Paul recently. After some troubleshooting, I've found that the Les Paul also creates ghost notes on other amps, and I think the 5F6A just doesn't have the right power filtering to handle overdriven humbuckers. When I played my Tele through the 5F6A the amp began to make a lot more sense.
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Thank you again for such a clear and detailed explanation.
Seems not clear enough. :w2: :laugh:
I didn't understand from your previous posts that you were suggesting that I move one of the filter caps, but creating isolated "individual ground loops" for each filter cap/cap set is genius. B/c I'm building a "traditional" 5F6A I think I'll keep the caps in the doghouse on this build, but I'm definitely going to try the wired grounding strategy on my next build.
I'm not telling you to move any filter caps. Leave them where they are in the dog house.
I'm saying to move the PT CT wire to the 1st filter cap negative lead.
Just to give a name and description of some ground schemes to help sort them out, let's say there's;
1. Standard traditional ground scheme.
2. Better ground scheme(s). (Separate power ground/preamp ground using a ground buss with 2 chassis grounds or a full ground buss with 1 chassis ground. Either of these are normally very quite.)
3. Better+ ground scheme. (Galactic grounding. You can have this with out moving some/all of the filter caps, just that the wired loops will be longer. At some point it doesn't make sense with long wire loops running around the chassis.)
4. Best ground scheme. (Galactic grounding, all caps moved, distributed in the chassis, stars made, small loops, and all jacks isolated from the chassis and wired to a ground star point, input, speaker, verb, etc. KOC favors this. Often can be over kill.)
What you have now for the grounding scheme is somewhere between standard and better, robrob used 2 chassis grounds and a ground buss for the preamp circuit grounds, instead of random chassis grounding. But he missed the PT B+ CT ground, which he now has changed over to like many others. (And he missed the PI ground, see below.)
....I'm just trying to get it as quiet as possible w/in certain limitations, .....
I'm suggesting a slightly better ground scheme. The difference in wiring is subtle, but can make a large difference.
Look at the hi-lighted robrob drawing below.
1. Move PT B+ CT wire, red/yellow, now hi-lighted red, to the 1st B+ filter cap negative lead.
2. Move your white power ground wire, gray in robrob drawing, now hi-lighted orange, to the 2nd filter cap negative lead, it's the screen cap. The hi-lighted yellow wire is now isolating that 1st B+ filter caps current loop from the rest of the filter cap loops.
3. I just saw this; robrob should have grounded the filter PI cap with the power ground because this amp has a negative feed back loop. That -FB loop is feed from the OT secondary. So it gets grounded with the power amp.
Disconnect the PI caps ground, gray wire in robrob's drawing, from the preamp buss, connect a jumper, hi-lighted purple from that PI cap over to the screen cap.
So now all 4 dog house filter caps are grounded together but the 1st filter caps loop is isolated.
Robrob's layout does indicate the first 3 filter caps grounded at the power amp ground point, and the fourth at the preamp ground bus, so that's what I did. I know the old school way is to ground them right to the doghouse, but I can't imagine this is safe or quiet.
There's a subtle difference in wiring up the ground that can make a big difference in noise levels. He now does this, it would be nice if he went back and up-dated his layouts to reflect his new change in wiring that 1st filter cap ground. :dontknow:
It's just as safe to ground it in the dog house as grounding it in the chassis. Both are soldered to the chassis.
Fender did move that ground after the tweed amps to the B+ CT ground, instead of having the dog house ground go through the chassis over to the PT B+ CT chassis connection. We don't know why they did it, might have been they found it quieter, might have been easier to build, might have been both? :dontknow:
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I've found that the Les Paul also creates ghost notes on other amps, and I think the 5F6A just doesn't have the right power filtering to handle overdriven humbuckers.
Could be the PUP's, the guitars wiring, grounding matters there too.
Grounding schemes can cause all sorts on weird, strange sounds, noises. That's why you want to eliminate as much as you can to rule that out as the possible problem.
With a "wired ground scheme", is it advantageous to isolate the inputs?
Some do, some don't, it's the extra mile. The higher the gain of an amp and the louder you play, the more these things seem to get noticed.
But layout and lead dress can make problems too.
Getting the layout, lead dress and grounding as good as you can, goes a long way to eliminating noise, hum, buzz and other assorted bugs.
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He moved the PT B+ CT from the chassis power amp ground lug to the 1st filter cap negative lead. Then he ran a wire from that junction, ie; filter cap negative lead/B+ CT, over to the chassis power amp ground lug/bolt. That's the same as PRR, Merlin, Kevin O'Connor/London Power and others recommend.
Yes, I understand this. As I mentioned above, I tried exactly this and there was no difference in the 60 cycle hum or other noise. However, my 4th filter cap is going to pre-amp ground as in Rob's layout. Your annotated version now shows all 4 filter caps together and to run the wire back to the power amp ground? Isn't this contrary to the Vavlewizard info you cited?
In it you will see you can't ground all the filter caps at the power ground and use a ground buss for all the preamp and PI circuitry.
In any case, I will try it....
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Yes, I understand this. As I mentioned above, I tried exactly this and there was no difference in the 60 cycle hum or other noise.
So you moved the PT B+ CT wire to the 1st filter cap negative lead?
But did you move the power ground wire from the 1st filter cap negative lead to the screen cap negative lead?
Your white wire, I show it as orange.(I explained why this is done. It's subtle but it maters.)
I just found that robrob also has the -bias ground grounded to the ground buss, can't do that. That -bias comes from a tap on the B+ wind and it's fed into the power tube grids. It has to be grounded at the power ground.
And he has the Presence control pot grounded to the mid pots ground that goes to the ground buss. But the Presence circuit is fed from the PI's tail and OT. So he ended up tying the preamp buss through the mid pot ground to the OT ground presence pot/PI and the -bias ground. That defeats the separation of the 2 chassis grounds.
That's all part of the power ground. Not supposed to do that. (No disrespect to robrob, he has a great web site with lots of very good info on it. :icon_biggrin: )
However, my 4th filter cap is going to pre-amp ground as in Rob's layout. Your annotated version now shows all 4 filter caps together and to run the wire back to the power amp ground? Isn't this contrary to the Vavlewizard info you cited?
In it you will see you can't ground all the filter caps at the power ground and use a ground buss for all the preamp and PI circuitry.
I wrote that wrong, sorry.
The PI is the splitting point for the ground scheme if the amp has a -FB loop and is using 2 chassis ground points. With 2 chassis grounds and a -FB loop the loop just made the PI part of the power amp, so it goes with the power amp ground. No -FB loop, PI goes with the preamp buss chassis ground. That's the split and it falls to 1 side or the other. With a single chassis ground, like Merlin and KOC use, they have a full ground buss going from the power amp to the input jack where the chassis ground is mounted. Although, KOC used to favor the ground buss chassis connection around the PI section of the buss, don't know if he still does?
It's hard figuring out what's going on from the layout without a schematic. I don't think robrob posts a schematic, just a layout. :BangHead:
As I said If you have a -FB loop/Presence control, and this amp does, that's all part of the power amp, so it all gets grounded with the power amp. The -FB is tapped off the OT secondary. If you didn't have a -FB loop then you would ground the PI with the preamp buss going to the input jack ground. (You said you read the Merlin link, he's very clear in there about this.)
Look at this;
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This topic got me reading more today and I stumbled across this interesting article:
Class AB Power Supply Ripple (ampbooks.com) (https://www.ampbooks.com/mobile/classic-circuits/class-AB-ripple/)
Thanks for the link. Motto of the story, if it ain't broke...
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I did not rearrange the grounding on the filter caps when I connected the PT CT to the negative leg of the first filter cap, but I will. Your annotations to Rob's layout is incredibly helpful to a layperson like me, and I'm surprised there's not a 5F6A groundling layout on one of these forums. I would have loved to have seen this before I built the amp!
So you are saying the PI ground should also go to the power amp ground? What/where exactly is the PI ground? Is it the 10K "tail" resistor currently connected to the NFB resistor and the Presence & Mid grounds?
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Filter caps layout = from the left in the drawing;
1st 2 caps = PT B+ CT , 3rd cap = screen, 4th cap = PI.
I re posted the hi-lighted drawing below and here's step by step instructions;
1. You disconnect the black wire from the presence pot lug 1 that's going to the mid pot lug 1. Red line cut.
2. Leave the black wire from the mid pot lug 1 that's there in place going to the ground buss.
3. Then run a wire from presence pot lug 1 to the 4th filter cap negative lead, PI cap, hi-lighted purple wire.
4. Disconnect the ground wire that's there now, that goes to the closest end of the ground buss. In rob's drawing it's gray, I have 3 red lines cut through it.
5. Now disconnect the short gray wire, it's the ground, that goes from the -bias caps/R/new red ground wire to the ground buss. I have 1 red line cut through it.
6. Then run a wire from the -bias supply ground, junction of 2 @ 8uF -bias caps and 1 resistor, back to the power chassis ground, I have a red wire there.
7. Then connect that 4th PI filter cap's negative lead to the 3rd filter cap, screen, red wire.
8.Last, run a wire from that 3rd filter cap, screen cap, I show it as orange, back to the chassis power ground.
That should get the ground straightened out.
There's 1 last thing, the -bias wire, it's probably a red/blue on your PT, in rob's layout it's dark blue, should not be wrapped around the heater wires. There's very little current going through that wire, only the -bias caps charging current, but, that -bias SS rectifiers pulses produce a very nasty strange wave that could get on/into those heater wires. Move it away from the heater wires.
Here's the hi-lighted drawing again;
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Good call on the filament wire and the bias wire. Mine are intertwined b/c the two wires are intertwined on my Victoria 35210, so I copied that, but I'll untangle them.
Does this diagram represent everything you're saying? The only ground we didn't talk about is the filament CT. Can this go directly to the power amp ground as I have it in my diagram?
Also, both OT wires go only to the speaker jack, and there's no actual wire that goes from the speaker jack to the power amp ground, correct?
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Does this diagram represent everything you're saying?
Yes! :icon_biggrin: :blob8: :happy1: :laugh:
The only ground we didn't talk about is the filament CT. Can this go directly to the power amp ground as I have it in my diagram?
Yes, leave it where it is, that's why I didn't mention it.
Also, both OT wires go only to the speaker jack, and there's no actual wire that goes from the speaker jack to the power amp ground, correct?
Yes, if your not using plastic Cliff jacks, Marshall amps. A Switchcraft metal jack, Fender amps, by itself makes a chassis ground connection, just like the input jacks. It goes through the chassis, from the speaker jacks ground lug, through the jacks mounting shell that's bolted to the chassis with a star washer to get a good bite into the chassis. No extra wire needed.
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Ok, fantastic! Yes, I'm using switch craft jacks and star washers.
And just to clarify, this "power amp ground" is just a point on the chassis, separate from the earth ground and the PT Faraday cage ground, correct? (Mine is a dedicated bolt as you can see in the photos).
I'm going to try this "Better +" ground setup, and then when I have everything exactly as I want it, I'm going to rewire the amp from scratch. (Just for fun, and b/c I want to have cleaner solder joints, use different wire, and some different components.)
Thank you for your time and knowledge Willabe!
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And just to clarify, this "power amp ground" is just a point on the chassis, separate from the earth ground and the PT Faraday cage ground, correct? (Mine is a dedicated bolt as you can see in the photos).
Yes.
Almost forgot, the -bias wire and it's ground wire, you can run that as a nice twisted pair at least for part of the way.
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OK, so I'm rewiring the entire amp in part to implement this new grounding scheme. Willabe, as per your advice I now have 6 wires running to/from the doghouse (see photo). The three red wires are B+, and the three white wires are as follows:
1) Neg side of the 2 x "Plates" filter caps that will connect to the HV CT
2) Neg side of "Screens" filter cap running back to the power amp ground
3) Neg side of the "PI" filter cap that will run to the Presence ground
My question is this: Can I twist the three white wires together? They are quite long, and I'd love to twist them together and tuck them into the corner before installing the board.
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Yes.
But it's better when you can run twisted pairs, +/-.
Edit; Twisting those 3 ground wires together will defeat the purpose of running separate grounds. Any current induced in/on 1 of those ground wires will be induced into the other 2 ground wires.
Your amp was not laid out for what you want to do. But since you've gutted the amp, you would have to move the 1st B+ node, the 2 stacked B+ OT filter caps inside the chassis next to the PT.
Running tightly twisted pairs of B+/ground greatly helps to nullify any/all magnetic field noise from being radiated into anything else near those wires. They cancel each other because their out of phase from each other.
I don't know why you gutted that amp. :dontknow:
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Here's the problem I mentioned with layout.
3. Better+ ground scheme. (Galactic grounding. You can have this with out moving some/all of the filter caps, just that the wired loops will be longer. At some point it doesn't make sense with long wire loops running around the chassis.)
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Can you post a full length pic of the back side of the chassis?
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Yes.
Edit; Twisting those 3 ground wires together will defeat the purpose of running separate grounds. Any current induced in/on 1 of those ground wires will be induced into the other 2 ground wires.
Your amp was not laid out for what you want to do. But since you've gutted the amp, you would have to move the 1st B+ node, the 2 stacked B+ OT filter caps inside the chassis next to the PT.
Running tightly twisted pairs of B+/ground greatly helps to nullify any/all magnetic field noise from being radiated into anything else near those wires. They cancel each other because their out of phase from each other.
Ugh....I went ahead after you said it was OK to twist the wires together, and the amp is (was) just about finished. At this point I have no way to separate the white ground wires and run them under the board, and there are ground connections missing under the board.
A simple question: Is there any way to improve the grounding of Rob’s layout without moving the filter caps? (Moving the caps is no longer an option.) Or, is Rob's layout the best grounding scheme for a "traditional" Bassman layout?
I don't know why you gutted that amp. :dontknow:
I gutted the amp b/c I wanted neater wiring and better solder joints, a more elegant bias supply, CC resistors and pushback wire, and to implement this ground scheme.
I've annotated the photos. The 3 white wires are the ground wires running to the doghouse. The 3 red wires are the three B+ wires from the doghouse, and I've tried to keep them separated. (They do run parallel for a few inches, but they are about a half-inch apart.) The yellow wire running from the top left of the board to the power amp ground is the + side of the bias filter caps.
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So, how to proceed? Should I simply revert back to Rob’s grounding scheme? Or is there some middle ground? For example:
-I can easy connect the Presence pot ground (now connected to the neg side of the PI filter cap via a long wire) directly to the power amp ground, and this will eliminate one wire running to the doghouse. Would this be an improvement, and if I do it, should I “re-separate” the neg side of the PI and Screens filter caps?
-Should I keep the power amp ground and pre amp ground separate, or should I re-connect the pre-amp ground to the neg side of the PI filter cap as Rob does?
-Should I keep the bias ground connected directly to the power amp ground (yellow wire in photo)?
-If I take the above steps, there will only be two ground wires running to the doghouse: the HV CT to the Plates cap and the return from the Screens cap. Would it be an improvement to keep these, or should I just reconnect the HV CT directly to the power amp ground?
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You should simply quit obsessing about this. If there was only one good/successful grounding scheme, every amp would be using it.
Follow one basic rule... Keep power amp grounds close to the PT and keep preamp grounds away from power amp grounds.
There are many different successful grounding schemes that share this basic rule.
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You should simply quit obsessing about this. If there was only one good/successful grounding scheme, every amp would be using it.
Follow one basic rule... Keep power amp grounds close to the PT and keep preamp grounds away from power amp grounds.
There are many different successful grounding schemes that share this basic rule.
Sluckey's right about this.
So, how to proceed? Should I simply revert back to Rob’s grounding scheme?
No I wouldn't. I explained to you why I would change what I showed you to change.
Or is there some middle ground?
Yes. I already showed you the middle ground.
Now all you need to do is add 3 twisted pairs of wire to what I already showed you;
There's rules of thumb for lead dress. One of them is to use twisted pairs of wire when/where you can. It's just good practice when doing a build. We know that 2 wires of opposite polarity or opposite phase if -tightly and evenly- twisted together will greatly help cancel any noise and/or magnetic fields from being radiated/induced into other wires and other adjacent circuitry. That's why you see PT and OT wires twisted together.
You have 3 B+ filter cap node wires twisted together and their 3 negative return wires twisted together. You should have twisted them together in their respected pairs, A node, B node, and C node. That should help keep the B+ wires from inducing any magnetic field from current and noise into each other, just good lead dress practice.
I would have also twisted the -bias tap and it's ground together too. (You don't have to do that, you should be fine, very low current. And the yellow push back wire wont twist up well with the PT -bias tap wires insulation.)
Also, in the pic you posted of the dog house eyelet board, it looks like you have the caps in backwards? The B+ A node, stacked caps for the OT CT should be closest to the PT. Looks like you have them on the input jack/preamp side? If so that lengthened the filter cap wires that have the most current going through them, B+ A node; OT CT and B node; screen. That should be fixed, turned around, if that's the way you wired it up.
There's reasons why the power ac cord, PT, and heaviest current is on 1 end of the chassis far away from the sensitive low signal input/preamp side of the chassis. Part of good layout and good lead dress.
You had the amp gutted, you should have moved the stacked B+ filter caps for node A, OT CT. They have the most current going through them and their +/- leads. They could have gone right in between the PT and the eyelet board, inside the chassis. Would have been pretty easy. You could have left the B node, screens and C node, PI in the dog house.
-I can easy connect the Presence pot ground (now connected to the neg side of the PI filter cap via a long wire) directly to the power amp ground, and this will eliminate one wire running to the doghouse. Would this be an improvement, and if I do it, should I “re-separate” the neg side of the PI and Screens filter caps?
No, leave it like I showed you.
-Should I keep the power amp ground and pre amp ground separate, or should I re-connect the pre-amp ground to the neg side of the PI filter cap as Rob does?
I'd leave it separate.
-Should I keep the bias ground connected directly to the power amp ground (yellow wire in photo)?
Yes.
-If I take the above steps, there will only be two ground wires running to the doghouse: the HV CT to the Plates cap and the return from the Screens cap. Would it be an improvement to keep these, or should I just reconnect the HV CT directly to the power amp ground?
No, I wouldn't. Do it like I showed you. Just add the 3 twisted pairs for the B+; A/B/C nodes.
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And the wiper wire and ground wire on the Mid pot should be twisted together and moved away from and/or cross the B+ wires at 90 degrees.
You don't have to do this, just another example;
Same with the volume pot wires. Twist them together, use 3 wires on each pot. Add a separate ground wire to the pot that doesn't have 1 going to the ground buss. Twist the 3 wires together, solder the ground wire to the buss right at where the wiper wire/center lug is on the eyelet board. The signal is still pretty weak here, twisting the wires helps protect them from noise being injected.
You will have to put a small mark on the wires with a marker so you know which wire is which. I use several different colors of push back or any wire for this reason. And just to be able to see what wires going where.
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Ah, the filter caps are indeed backwards. Working from Rob’s layout and it’s difficult to understand the orientation of the cap board in relation to the rest of the amp.
To clarify your advice Willabe, the three B+ nodes correspond to screens caps, plates cap, and PI caps, correct? You are saying that I should keep all 6 wires running to/from the doghouse, but I should just twist the white (-) and the red (+) wires together into three pairs for each cap (or set of caps). So after I reorient the filter cap board, I will twist together:
* The red PI/B+3 wire and the white wire to the Presence ground
* The red screens/B+2 and the white wire from the neg side of the screens/B+2 cap that goes to the power amp ground
* The red plates/B+1 wire and and the white HV CT wire
So three twisted pairs of red/white wires, each one for a single (or set of) cap(s) running horizontally to/from the doghouse to the power amp ground, in the corner above the circuit board, correct? And this will be better than the "trad" layout even though these 3 pairs are running parallel to each other?
"You had the amp gutted, you should have moved the stacked B+ filter caps for node A, OT CT. They have the most current going through them and their +/- leads. They could have gone right in between the PT and the eyelet board, inside the chassis. Would have been pretty easy. You could have left the B node, screens and C node, PI in the dog house."
In your first sentence, you mean “PT CT”, correct? I can still move that B+1 cap set. I’m trying to keep a somewhat traditional layout, but I’ll think about this. Regarding twisting the wires on the pots, maybe I’ll do everything above and go through the power up procedure, and if there’s still some hum I’ll attack the mid and vol. pots.
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* The red PI/B+3 wire and the white wire to the Presence ground
* The red screens/B+2 and the white wire from the neg side of the screens/B+2 cap that goes to the power amp ground
* The red plates/B+1 wire and and the white HV CT wire
Yes, yes and yes.
So three twisted pairs of red/white wires, each one for a single (or set of) cap(s) running horizontally to/from the doghouse to the power amp ground, in the corner above the circuit board, correct? And this will be better than the "trad" layout even though these 3 pairs are running parallel to each other?
Yes they can stay where they are, just twist them up nice and evenly/tightly. Start twisting them right from the caps in the dog house and push the twisted pairs through the chassis grommet.
It's not better than the Fender layout, you haven't changed the layout. It is a better way to wire up the grounds than robrobs. I've already explained why I think this. And just better lead dress with twisted pairs, which I explained too.
In your first sentence, you mean “PT CT”, correct?
No, I mean OT CT. Those 2 stacked caps are node A (B+1). They feed the OT CT that feeds the power tubes.
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I can still move that B+1 cap set. I’m trying to keep a somewhat traditional layout, but I’ll think about this.
If you do move the 1st filter cap B+ node A, I'd try and place the caps left/right, parallel with the chassis. With the ground towards the PT and the + towards the eyelet board. Shorter wiring that way.
If they wont fit that way, I'd place them so the ground end is towards the power amp chassis ground, again, for shorter wiring.
You don't have to move them. If it were my amp, I think I probably would. But twisting those 3 pairs of B+ wires will probably take care of it. I don't know if robrob wired up his amp the way Fender did. :dontknow: It's just a couple things he did with the grounds are not what several authors/builders say not to do.
I normally use a blank chassis when I build an amp, so I can do some things like laying out the circuits/parts/PT/OT/B+ filter caps so I can use twisted pairs and wire up the grounds like I want to. But most guys don't bother with the time it takes to do this.
To be fair, most of them end up with very quiet amps.
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Quick troubleshooting question: Would using 18ga stranded speaker wire (https://www.angela.com/clothcoveredwire18gaugeblackperfoot.aspx) cause a pretty significant lack of high end and volume on this 5f6A?
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How long is it?
I'd run 18ga all over a livingroom no concern. 50 feet may be just audible. Lack of highs is unlikely.
If you don't believe me, cut the cord off a power tool and try it. But my 2 cents is on something else.
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Not long. It's in a 4 x10 combo cab.
I finished rebuilding the amp (and did end up moving one of the filter cap nodes...more on that soon). All voltages are correct and everything seems in order, except the amp produces about 2/3 of the volume that it should produce and sounds muffled with little high end. The speaker wire gauge was my last best theory....
I went through the obvious troubleshooting (double checked speaker wiring, jacks, tubes, chopstick, etc.) but I can't figure it out. I thought it might have had something to do with the negative feedback b/c the Presence pot doesn't seem to work (or I can't hear it working), but I disconnected the neg. feedback resistor and got the same results. Here's a pic of the "vintage style" speaker wiring. Am I making some obvious mistake?
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What is the value of the cap in this snip?
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Aha, it's .047 mfd....I had thought this was the same as 47pf but now I see that I was WAYYY off! Good eye Silvergun!
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Good eye SG indeed! :icon_biggrin:
If you don't have the correct value, for now, just take that cap out and see if that fixes the volume. That cap is just for keeping the amp stable, it should work fine without it for now.
The value you have in there now is causing to much signal to get canceled across the PI's output. The small 47pF cap only cancels a small amount of very high end frequency.
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Aha, it's .047 mfd....I had thought this was the same as 47pf but now I see that I was WAYYY off! Good eye Silvergun!
:thumbsup:
Yeah, that's where all your highs went.
It should be .000047uf, or just 47pf, but it really doesn't even have to be there. You can snip it out clean and fire away.
I'm glad you posted that pic!
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Has that .047 cap always been in there?
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Aha, it's .047 mfd....I had thought this was the same as 47pf but now I see that I was WAYYY off! Good eye Silvergun!
:thumbsup:
Yeah, that's where all your highs went.
It should be .000047uf, or just 47pf, but it really doesn't even have to be there. You can snip it out clean and fire away.
I'm glad you posted that pic!
Good eye. :thumbsup:
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No, I replaced all the caps and resistors in the rebuild. That cap wasn't in the other working version of the amp.
I do have some 100pf caps. I'll throw one in there and report back!
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The cap swap did the trick! The amp sounds perfect (at least at "apartment volume"), all voltages are correct, and aside from a bit of hiss (CC resistors throughout), there is virtually no hum or any other noise when the amp is cranked. I’ll take the amp to the studio on Friday and give it a good workout at full volume.
Willabe, I did end up moving the B+1 node into the chassis with the neg. side right on the power amp ground terminal as you suggested. Photos below. You can see the PT CT & Filament CT go right to the power amp ground, as do the bias ground (yellow wire) and the neg side of the other two B+ nodes from the doghouse (white wire). At the top of the amp, you can see 2 pairs of red/white wires from B+2 and B+3 on the board, and each pair goes to a corresponding filter cap in the doghouse. I twisted these really tight and into the doghouse. In these red/white pairs, one of the white wires returns from the doghouse to the power amp ground (as stated above), and the other goes from the doghouse to the Presence ground. In other words, I've wired the grounding exactly like the chart I drew in my earlier post based on the info you provided.
The top/left side of the amp isn’t as neat is it could be with the additional wiring, but the hum is virtually gone so I think it’s a good tradeoff. I did spend some time moving wires around so that "different" wires only cross at right angles. I’m also going to put a bit of silicone under the B+1 caps so that they’re supported by more than their leads.
I’m really glad I rebuilt the amp from scratch. The CC resistors are quieter than I expected, and I doubt I’ll ever work with PVC stranded wire again. The cloth pushback wire is so much easier to use, looks better, and allows for neater layout IMO.
So, thank you all, and if I don't find any issues when I crank the amp this Friday, I'll start the rebuild of my 5E3!
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.... there is virtually no hum or any other noise when the amp is cranked.
Just to be clear, redoing the grounds like I showed you got rid of the last bit of hum? Or it made no difference at all?
Looks yery nice.
Only 1 thing, get rid of that wire nut, that's not safe.
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Oh, and the presence control works again?
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.... there is virtually no hum or any other noise when the amp is cranked.
Just to be clear, redoing the grounds like I showed you got rid of the last bit of hum? Or it made no difference at all?
Looks yery nice.
Only 1 thing, get rid of that wire nut, that's not safe.
Yes, it made a difference. The 60-cycle hum wasn't so bad in the old version, but yes, your advice completely eliminated the hum AND you taught me a ton about grounding, so thank you!! I'll get rid of the wire nut. The Presence control works, and I'm wondering if it was ever broken, or if I just couldn't hear it working when there was no high-end.
I managed to sneak into ESS this afternoon and crank the amp for a full hour and it's perfect. FWIW, the new version with all CC resistors and fancy caps sounds/feels significantly better than the old version with modern resistors, and keep in mind the transformers, speakers, and cab are the same. The voltages are nearly identical in both versions. (I measured every resistor before installing them both times, so this is not unexpected.) The old version sounded great but had a harsher distortion ("crossover distortion" I believe) when cranked, and and comparatively it felt a bit more sterile and unforgiving. The new CC version just sings, and the overdrive is noticeably smoother and more musical. I'm a player and not a builder (clearly), but my assessment is that the CC resistors and fancy caps made the difference between a good amp and an inspiring amp. My Centaur just sat in my case today as the amp's natural overdrive sounded so sweet.