5f6a Build - 60Hz and 120Hz in Signal - Any ideas why?

[Mr. Mojo]

Hi All —

Long time listener, first time caller here.  Thank you for the collective wisdom shared so far!

Okay, as long as I am stuck at home, I just finished my 2nd build — a 5f6a.  Started with a 1990 ’59 Reissue — kept original trannies, but replaced all else (included all pots and sockets).  My build is based on the Mojotone schematic.  Only mod is added bias pot per Rob Ronette’s design.

Amp is sounding pretty good, but I am having trouble with noise in the signal.  60Hz gets louder with both volume controls.  Adding treble or presence introduces 120Hz.  From reading various trouble shooting guides, clearly something is getting in my signal before getting to the volume pots — but that is far as I am getting. 

I would appreciate any insights!

Here are the clues:

- Problem gets quieter with V2 removed; problem disappears with V3 removed.
- I feel pretty confident about my lead dressing — things are tidy.
- Heater wires ARE wired correctly to reduce hum.
- Chopsticking wires or joints does not change sound at all anywhere.
- Chopstick does reveal that my AC Coupling Cap (a Sozo .1uf 500v) is microphonic!
- Sometimes tapping this cap with Chopstick can produce an arc to nearby pot leads.

One other possible factor:

For pre-amp grounds, rather than use a naked bus wire at the top of my turret board, I used 18-gauge solid-core wire to connect turrets beneath the board, and then attached ground leads from pots directly to the tops of nearest filter cap turrets.  Otherwise my ground scheme follows the Mojotone schematic.  I did things this way on my 1st build, a 5e3, and that amp is dead quiet. 

Thank you in advance for any insights!

Best,

Charlie

[shooter]

Sometimes tapping this cap with Chopstick can produce an arc to nearby pot leads

there's one down 

Problem gets quieter with V2 removed; problem disappears with V3 removed.

that appears to have it in the preamp, so you're 1/2 split.

what happens when you move the amp to the neighbors (assuming they won't shoot 1st:)  maybe use the garage if they're real cranky 

[aabbs20]

Definitely change that cap out and maybe try shielded wire going to the grids and V1 and V2a.

I just went through a similar thing with a Princeton Reverb type build. Here are some good resources to check out. Could be a lot of things and difficult to hunt down but these will help.

http://el34world.com/Forum/index.php?topic=21999.0

https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Hum_and_Buzz

https://el34world.com/Forum/index.php?topic=13955.0

http://www.geofex.com/ampdbug/hum.htm

[pdf64]

Have you checked that the hum isn’t due to a bad tube in V1-3?
How have you balanced the heaters (with respect to 0V)? eg with a winding CT or 100ohm resistors.

[Mr. Mojo]

Thank you so much for the replies --

I have combed and re-combed these guides looking for ideas --

To speak to my suggestions, my PT has a CT so I am grounding the heaters this way, and I have tried swapping tubes.

I did discover a poor solder joint behind the microphonic cap -- repairing this stopped the cap from being microphone, but did nothing to quell any buzzing.

In Robronette' guide, he says that if the buzz / hum is effected by volume, then it must be entering the circuit before there.  As near as I can tell, this means it is going:

Jacks = (Rewired this twice, substituted 1M resistors w/ different ones, no effect.)
v
v
68K Resitors  = (Triple checked my values are correct.)
v
v
V1 Tube = (Tripple checked my wiring looks good; swapping tubes w/ good ones has no effect.)
v
v
100K Resistor Pair / .022UF Capacitors  = (Triple checked my values are correct.)
v
v
Volume Pots = RECEIVING BUZZING

Am I thinking about this correctly?  It just does not seem like there is that much to go wrong between input jacks and volume pots?

I found another forum discussion about problems with 5f6a builds being noisy, and so they adopted the Ceritone ground system.  As near as I can tell, the only big difference is that Mojotone has Pre-Amp grounds all terminate at the bright input jack ground, whereas Ceritone grounds the jack to the volume pot, and then grounds the pot to the preamp bus.

ALTERNATELY, since I did add a brass plate to my amp, what if I just soldered all Pre-Amp ground leads to that?  I believe this is how the originals were made.  I could maybe just do that, unless someone here can tell me specifically why another method would be better?

Thanks again!!

Charlie 

[Mr. Mojo]

Hold on!  It turns out my PT does not seem to have CT for the 6.3v heaters -- 

Now I just need to source a couple 100 Ohm resistors -- hopefully I will be in good shape then.

In the mean time -- I do have a (2) 250 Ohm resistors handy -- if I wired them in parallel, that should get me to 125 Ohms -- which is close.  What would happen if I ran both green leads >>> 2 parallel 250 Ohm resistors >> single ground?

Thanks in advance --     

[PRR]

Two 100 Ohms is not magic. Two 250 Ohms will make a real difference. (IF this is heater related.)

[Mr. Mojo]

FOLLOWUP: Part 1

Because I searched this and other forums like crazy looking for answers to solve my problem, I circling back to log what finally cured my woes:

When I last left off, I mentioned that I had overlooked how the stock Output Transformer on my 1990 '59 Bassman Reissue did not have a green and yellow center tap wire to ground for the 6.3v heaters, so I needed to create an artificial tap using (2) 100 Ohm resistors.  I did this.  And it eliminated the 60mz hum I was experiencing!  But it did nothing for the 120mz hum, which was as loud as ever.

I read in several forums that others who experienced this identical symptom eliminated it by abandoning the ground scheme shown in the Mojotone diagram, and instead using the one in the Ceriatone layout.  I studied the Ceriatone layout a lot -- near as I could tell, it switched how a few things were orientated on the pots and jacks, and sent all grounds to a star ground.  A lot of other sources I encountered reported mixed results using star grounds, and almost all sources I encountered emphasized grounding the power section at 1 point at or near the output transformer, and grounding the pre-amp section at another 1 point at or near the output jack.  Since a star ground is really just a way to consolidate everything being grounded at 1 point, and that is more-or-less what the Mojotone layout already did, I only tried copying the Ceriatone layout at the pots and jacks, but it did nothing.

Finally, at wits end, I decided to ground my amp the way the original '59 bassman had been grounded: by using a brass plate (available via Tim Weber) behind the pots and jacks.  The holes in the brass plate I got did not line up perfectly with my genuine 1990 Fender 59 Bassman Reissue chassis, but this was easy to solve by gently using an auger.  I soldered individual wires running from each eletrolytic cap on my board directly to the brass plate.  For the (4) big 22uf caps, I drilled (4) small holes into the side of "dog house" cover, and soldered the (-) negative end of each cap directly to the chassis this way.  The result?  My amp was now DEAD quiet!  YAY!  Except...

Only the Bright channel now worked.  Not sure what the Normal channel went dead.  I ended up rewiring the jacks and replacing the (4) 68k resistors they lead into.  And the problem was solved!  Yay!!  Except now...

After getting warmed up, the amp would crackle a bit.  Static would trail notes, especially on the low strings.  You could tell the amp was trying to sound great, but this artifact was very yucky sounding.  According to more forum research, a faulty solder joint was most likely the culprit.  I chop-sticked around.  My whole board was acting microphonic!  But it seemed especially bad at the 56K resistor in the tone stack.  I replaced this resistor and this did seem to make it less sensitive to chopstick tapping, but otherwise the crackle / static symptoms continued unabated.

Now it seemed the 8uf 450v electrolytic in the tone stack was most sensitive to chopsticking.  Per suggestions in another thread I began in this forum, I swapped in a 22uf power cap here, but that did not cure things, and to my ear, it changed the breakup of my amp in a way I did not like, so I put the 8uf cap back in.  Crackle and static were still happening, and various parts of my board still seemed microphonic when tapped with a chopstick.  At one point during testing, after leaving the amp on a while, I saw some lightning in the rectifier tube and my 2amp slow blow fuse blew.

According yet more forum research, after a cold solder joint, the next most likely culprit was a bad tube.  I tried swapping all tubes, one at a time, but this did nothing.  On a hunch however, I began to suspect that my issue may be power-tube related.  I tried using a small screw driver to re-crimp pin hole on the socket.  And voila!  This seems to have fixed it!! 

[Mr. Mojo]

FOLLOWUP:  Part 2

My Bassman sounds glorious now!  I bought this amp on Craigslist a little less than a year ago for $450.00.  Such a deal!  It was in pretty great shape, but just felt sterile to me... it almost behaved more like a solid state amp!  The original owner had already swapped the stock solid state rectifier for a GZ34 before I bought it, so that was not the issue.  The first thing I did, I was get a solid pine cabinet made for it vs. the stiff plywood it came born with, and this gave the amp a little more vibe, but it still felt very stifled.  Replacing the pots with CTS versions, linear and audio as specified in the schematic, and replacing the board with all Blue Sozos and NOS carbon comp resistors has made a big difference. 

One thing I did not change were the transformers.  But I wanted to!  There is no love on the internet for the transformers Fender used in this series.  In fact, there are sources dedicated to improving this particular reissue amp, who would have one believe that upgrading transformers is more pertinent to improving the sound and feel of this amp than changing any other components.  I was all set to invest in a new set of Classictone transformers for my amp, but I was confused by all the options Classictone offered, so I wrote them asking for advice.  I heard back from their rep named Mark, who told me that in his opinion, the original transformers in my amp were as good as anything out there.  Wow!  Now that is honesty.  They may have lost a single sale, but they gained a customer.  As it happens, I am already onto my next build - a Tweedle Dee 5E3 following Rob Ronette's simplified diagram, and based on my interaction with Classictone, I am using their transformers for this project. 

Lastly, as a finishing touch for my Bassman?  Genuine polished Bakelite NOS 1950s chicken head knobs.  The amp finally sounds and feels like it belongs in the tweed family!

Moral of the story, is that sometimes its not one single problem, it's layers of problems.  Frustrating as it may be, it comes down to how being analytical, tackling one issue at a time is the only thing that can untie them.  Thanks to all those who weighed in to help me here.