SOLVED Seeking 5F6A Build Help: Tests and sounds great, but bypasses tone stack?

[gfarina55]

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 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
* 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) 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), 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 (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!

[shooter]

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)

[thetragichero]

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)

[gfarina55]

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!!

[shooter]

1. (None, 1 ohm resistors

none what!!
1 ohm resistors
voltage
none of the above

[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).

-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. 

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.

[gfarina55]

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. 

>>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 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!

[jordan86]

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.

[shooter]

so where's the math??
what did the calculations say?
you have NONE, what is NONE??
Voltage, continuity, current????
None is a big space

[sluckey]

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)

[shooter]

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

[sluckey]

Of course that's wrong, but you're in the wrong rabbit hole.   

[shooter]

 
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

[SILVERGUN]

>>Can you confirm if -53v. is enough negative bias current for any 5881?

Yes. Definitely. (based off of your reported plate voltage)

[gfarina55]

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?

[SILVERGUN]

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. 

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

[mresistor]

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.

[gfarina55]

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.

[SILVERGUN]

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)

[SILVERGUN]

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

[st]

Any chance the mid pot ground lug is internally disconnected? what happens if you temporarily ground the wiper?

[gfarina55]

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!!

[mresistor]

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.

[mresistor]

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.

[mresistor]

another view

[gfarina55]

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.

[sluckey]

...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.

[gfarina55]

...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!

[shooter]

for extra fun while you wait;
ohm middle tab to each end tab with pot set at 1/2

[gfarina55]

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.

[jordan86]

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.

[SILVERGUN]

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.

[shooter]

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

[gfarina55]

...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. 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?

[sluckey]

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.

[gfarina55]

OK, so these are the three tone caps and the cap coming off the treble pot wiper? I'll check...

[gfarina55]

Found it, fixed! Learned something new today.

[sluckey]

Which one?

[gfarina55]

It was the little 250pf treble cap. Tone controls are completely quiet now, and the amp is nearly silent at idle.

[sluckey]

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.

[gfarina55]

Good to know, thanks! I think I also fried a rectifier diode I was working with. Need to up my soldering game.

[72Blazer]

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.

[PRR]

...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.

[gfarina55]

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!

[sluckey]

Nice line!

[gfarina55]

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?

[SILVERGUN]

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?

[gfarina55]

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!

[SILVERGUN]

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.

[SILVERGUN]

This topic got me reading more today and I stumbled across this interesting article:
Class AB Power Supply Ripple (ampbooks.com)