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**The Dude** · 04-10-2014, 06:12 AM

Do you have a scope?

**Chuck H** · 04-10-2014, 01:56 PM

You mention that "Everything in amp is new." When did this hum become apparent? Did it happen spontaneously after everything was replaced? Was everything replaced in efforts to abate the hum without success? Did the hum start immediately after everything was replaced? Did it start after any one thing was replaced?

More info please. How come "Everything in amp is new."? Were all the old parts broken? You do mention caps. I want to be sure that includes the large electrolytic capacitors in the power supply and not only the capacitors on the circuit board. Info?

Does the hum change if you unplug the guitar from the input?

I have had many hummy preamp tubes. No harm in subbing another tube in there on the off chance that's all it is.

**R.G.** · 04-10-2014, 02:40 PM

The bad thing about generic hum is that there are so very many ways you can get it.

showman has hum/buzz. more like a 120 cycle hum than 60 cycle.

120Hz buzz is always a rectifier/filter issue. It is difficult for me to separate these by ear, because of the octave relationship. The question about whether you have a scope would help here. Look at the hum and see if the peaks are 8.66mS or 16.7mS apart.

If it is 120Hz and not 60Hz, then it may be coming from:
- main filter cap ripple; your scope would help figure out how big this is
- rectifier buzz; snub your rectifier diodes if you're using solid state diodes
- incorrect wiring of the PT center tap; the PT CT has to go to the minus terminal of the first filter cap, nowhere else, period

If it's 60Hz, it could be many, many things. With as extensive a rebuild as you've done (it sounds like you only kept the chassis and knobs) lots of things can go wrong. For instance, is the filament supply still connected to signal ground? Did you convert to three-wire AC cord? This is a huge list.

sound stops at PI tube.

Sound stops, or hum stops? Does this mean that when you pull tubes, starting with the input tube, that it makes no difference until you pull the PI tube?

Everything in amp is new. trannies, pots, caps, resistors, new turret board, diodes, all tubes and sockets (ceramic), you name it.

With such an extensive rebuild, there's a good chance you may have missed one ground point. I would be tempted to hook up my ohmmeter with one lead on the input jack bushing, and then go probe every single component lead on the schemo that was supposed to be hooked to ground, checking for continuity.

already checked all according to geofex.com. still has hum.

I do desperately wish I could have thought of everything in that write-up, but I keep learning stuff as I go.

I adjust the bias to 45mv/415vdc at pin 8 (cathode) of output tubes (through a 1 ohm 1 watt resistor on each tube) and get great hum and -36 volts on tubes.It hums like crazy.

Adjusting it to -55(on schematic) on tubes takes bias to 8mv./400vdc and hum goes almost all the way, away.

-55 is taking the output tubes almost to cutoff/Class B, and so their gains are very small for small hum signals. That says the hum is being fed to them, not picked up at the output tubes most likely.

Do tone and volume control settings make any difference in the sound at all?

g1 · 04-10-2014, 05:11 PM

Originally posted by ampcabinets View Post

I adjust the bias to 45mv/415vdc at pin 8 (cathode) of output tubes (through a 1 ohm 1 watt resistor on each tube) and get great hum and -36 volts on tubes.It hums like crazy.

Adjusting it to -55(on schematic) on tubes takes bias to 8mv./400vdc and hum goes almost all the way, away.

Do you have those voltage readings reversed (plate voltage)? With the lower idle current (8mV) the plate voltage should go up, not down.
If decreasing the idle current reduces the hum, to me that implies a power supply ripple issue (or bias supply).
And keep in mind that new parts can be defective, so you can't just rule them out because they are new.

**ampcabinets** · 04-10-2014, 08:50 PM

1) yes I have a scope
2) Everything is new because all I bought from the owner was the chassis and face/rear plates and a rusted out output transformer. I didn't "shotgun" anything, trying to fix what wasn't broken. Everything means everything, including the cap can contents (power supply filters and b+ filters, resistors, wires, as I said you name it)
for future reference
http://66.7.221.96/~ampcabin/showman..._showman_1.JPG
http://66.7.221.96/~ampcabin/showman..._showman_2.JPG
http://66.7.221.96/~ampcabin/showman..._showman_3.JPG
http://66.7.221.96/~ampcabin/showman..._showman_4.JPG
http://66.7.221.96/~ampcabin/showman..._showman_5.JPG
http://66.7.221.96/~ampcabin/showman..._showman_6.JPG
diagram link
http://66.7.221.96/~ampcabin/showman..._with_caps.JPG

tube voltages link
http://66.7.221.96/~ampcabin/showman...n_voltages.JPG

nothing is plugged into the amp and I have switched tubes until I am blue in the face. This is always one of ther first things to check. Although all tubes are new, and tested better than new, I did find one that was leaking, causing dc into the channel 1, making the pot scratchy. Moving it to channel 2 produced the same thing, so I removed it and have now replaced it. And yes, the hum stays, removing one tube at a time, or all at once, until the phase inverter tube is removed, then there is no sound or hum. The PI tube is supposed balanced, but I have substituted it, too, with no change, the problem still exists..

R.G. (funny my first 2 initials backwards)
It is definitely 120 cycle hum, not the deeper 60 cycle hum.

yes, in answer to your question about the PI tube. (see above). The hum stops when I have pulled each preamp tube, one at a time or all together, unti I pull the PI tube, then the hum goes away, as does any sound.
There is nothing plugged into the amps input jacks. I do get the correct hums and noises when I plug a cord into the input jacks and touch the ends. (with a guitar too), except the hum is always there and it is loud.

I would be interested in what you mean by 'snubbing the 6 diode bridge' (half wave?) . Do you want me to put a .01uF cap across each diode?
By the way the diodes are 5408's, including the bias diode, which was a 1n4007. I have rectifier bridges I could use but fender circuits don't use full wave bridges as far as I can see.

looking above, I replaced everything or just built everything from scratch, and also bought new knobs. Only the chassis and face/rear plates were worth saving. The OT was so badly rusted, it was most likely affecting the laminations, as one side was buldged out. All I got was the chasis, plates, the brass plate, and OT. I was able to clean the chassis very well by using an automotive parts cleaner and my trusty mouse orbital sander, with 400G (grain) paper on it. I can still barely see the date stamp, inside by the PT, if I look very hard. It is a '63. The faceplate is in extremely good ahape with only a scratch/mark down by the word 'showman'. The rearplate is perfect, (almost, nothing is perfect if it has been used). Chassis # is 001465

The transformers are from Weber, which may say something.

as for the grounding test(s), I did everything listed on your geofx, and with an alligator clip, to the chassis, and touched every grounding point, with no change(s).

Filiment is ground with 2-100 ohm 1/2 watt resistors to ground.
Also, the transformer CT is grounded to the same spot, but does go to the filter caps (2-40uF/600v caps in series with a 220K 2 watt across each), so I was going to unground them from the main buss and put them on their own buss. Are you saying I should take thie directly to the negative side of the series, and not to chassis ground, or with chassis ground? I am only wanting to be clear on this.
It is nothing to test it either way, since the chassis is still out of the head cabinet. The schematic just shows it grounded, so that is what I did. Fender usually 'does' have the primary ct filters on a different ground than the other filter caps, in the cap can. I have noticed this forever, even on amps where is is on the board (like a 5E11 I built), and I always followed this, until I got on this amp, figuring they did it for a reason. Why Ididn;t do it, remains a mystery, even to me.

Although I have 2 scopes, I am not an expert with either. But it is easy enough to do what you ask.

tone and volume does not affect the hum, although the presence does act like a tone control, sort of. And yes, I have checked that wiring and all pertinent parts more than twice.

I had started the shotgun approach but stopped when I realized nothing I was doing was making any difference, and I really don't like that approach, anyway.

G-one:

You can look at the tube voltages chart from the link above.

**ampcabinets** · 04-10-2014, 08:56 PM

**ampcabinets** · 04-10-2014, 09:06 PM

it is not letting me make a response, no cookies are showing up from music-electronics-forum.com, either.
It showed it once, but now only shows nothing but your responses.

**ampcabinets** · 04-10-2014, 09:07 PM

**Chuck H** · 04-10-2014, 10:11 PM

You need to answer the questions. We know why we're asking them and can't help without the answers. See this:

http://music-electronics-forum.com/t36224/

**ampcabinets** · 04-10-2014, 10:33 PM

The original build was from a 1963 chassis, front plate, rear plate, brass plate, and old rusted out output transformer, which was chunked, because of lamination separation by rust

and corrosion. No thaving new laminations to put the windings around and not having the expertise to do so, I threw it away, sorry.
The chassis had some rust, so I got rid of the solder from the chasiss, and ran it through an automobile 'bead' parts cleaner, and then used my trusty mouse rotary sander to clean it

until it was perfectly clean. This preserved the nickle or galvanizing, whatever fender used. Al lparts are new, because of this. So this was not a board substitute. Everything was built

from scratch, and all parts, including the filter caps, pots, board caps, resistors, tubes, ceramic tube sockets, wiring, diodes, transformers, down to the knobs, are new.
all boards are on 1/4" stand-offs, which are metal. Not getting a cap can with it, I bought 2 regular cap cans, cut them, soldered them, not welded them, cleaned and sanded it off to

smooth. All you can see is the seam(inside and out), and it holds quite well. If you wanted to beat it with a hammer, you could get it apart, otherwise it will hold forever.
I, also, used spring power tube retainers, because the tubes hang upside down. It doesn't really need them, Fender didn't use them, but I wanted them, anyway.
The transformers are from Weber, which I got a few years back, (before Mr Weber passed away) , for a twin reverb project, and decided to use them here. Both are new. However,

being from weber might say something.
there are also, pictures of the insides, before I removed the inside brass front plate. They should be large enough for you to see detail. If you need another shot or angle, just say so,

I'll post it. It is no problem.
Everything is on a ground buss, all eminating from a single source. I have had extrememly good luck using this grounding pattern. This is the only amp I have had this particular

problem with.
All boards are 1/8 inch fiberglass. If something looks screwed up, it is because I did it, no excuses.

voltages are listed in the link shown, in order to answer your questions. If you want them at 45mv (at pin 8/1 (the cathode) of the power tubes), instead of 8, just ask, I'll post them.
I realize this is a grid biased system, called fixed adjustable.

I have 2 scopes, but am not well versed in using them, but I can, if you wish.
Other than seeing ripples (or such) in the sine or square wave(s), I see no need to use anything other than my trusty fluke meter, or analog simpson meter, both of which are top of the

line and I have used them for many (many) years. They are dusty, but I can dust them off and make measurements, and take pictures, if you like. Both are Hitachi and dual trace.

R.G.

Thanks for the tips, and I'll go back and redo some stuff but i have some questions.
What kind of snubbers do you refer to? .01uF caps across the diodes, (each one)? There are 6.
I think this makes this a half wave bridge? I have full wave bridges, but fender doesn't use these, do they?
The bias board has been totally replaced, and all I had to use was 8uF/450 volt caps (reversed of course), but the voltage shouldn't make a difference. It made no difference in the hum,

either.

Yes, the hum and all sound stops when I remove the PI tube. Removing the preamp tubes one at a time(and replacing them before going to the next) , or all at once makes no difference.

Sound and hum stops with the phase inverter tube. Testing the power tubes, leaving the inside pair or outside pair, also, makes no difference. Of course, pulling the output tubes stops

everything.

I have made every substitution I can, including pulling all the preamp, pi tubes and putting different ones in. It makes no difference.

next.....
And, are you telling me the red/yellow wire from the PT (CT) goes only to the 2 first stage filter caps ,negative connection only (whether I am using the series or parallel system) and

not to ground?
CURRENTLY I am using series, 2- 40uF caps, with 220K/2watt resistors across each, but I can return thet to 2- 40uF caps in parallel if you think that is better. Fender shows it just

to the chassis ground, which is what I followed.

I do have them on the same ground buss as everything else, but I think you are telling me to put them on their own buss, to the ct and it's own ground.
I normally separate them out to another ground leg (eminating from the central ground point), this is the first I haven't done that on. Fender's normally have that done.

cost is not the object. Getting the amp to fire up and work as silently as it can and as properly as it can, is. I rebuilt everything using genuine fender parts, including new old stock

handles, chrome corners, JBL K-120 15 inch speaker, etc. Just one of those metal logos cost 60 bucks. Lucky I had one in stock. The cabinets are solid #1 pine, dove tailed, not

finger jointed, glued clamped and finished and covered by me. The tolex is as close as you can get to the real deal. WAY better than what mojo sells as rough blonde tolex. Cabinets all

painted brown before tolexing like fender did on these amps. I didn't have a tone ring, but it did't make a difference to my ear, when I tried one. Baffle is also, solid wood, not plywood,

or particle board. Solid wood resonates ,the others don't. I got a 2x10 cabinet from a friend who bough it from some "high falootin' " cabinet a maker on fleabay, and the dern thing

had pressed paper for a baffle, so I had to rebuild it. The wheat grill was, also, on sideways, so I had to replace that.

anyway...
On the diagram, I put the cap can contents on it ,as fender never shows this and it is easier to understand this way.
My schematic shows the exact same. Pictures of the cap can contents is also provided.
You might notice the brassplate, but that has been removed. I had kept it in the chassis for posterity, but thought it might be part of the problem and removed it.

There were only 5 ground points from the board to the ground buss. All have been checked, and the ground buss on the back of the pots and the grounds from the pots to the bubss

have, also, been checked, as well as the grounds on the output jacks. There is a leg of the ground buss to the 1 ohm 1 watt resistors, also, and they have been soldered to the chassis,

as well, for strength. There is no circle (loop) to the ground buss. Both sides (front and back) eminiate from one point on the power transformer, which is connected to the ground leg

of the ac power cable. The filiment ground is 2-100 ohm 1/4 watt resistors to the same ground point on the tranny. The green/yellow (filiment ct) is not connected, although it does

exist.

I hope this helps.
I know it is going to be something simple, but finding it is the problem.

http://66.7.221.96/~ampcabin/showman..._showman_1.JPG
http://66.7.221.96/~ampcabin/showman..._showman_2.JPG
http://66.7.221.96/~ampcabin/showman..._showman_3.JPG
http://66.7.221.96/~ampcabin/showman..._showman_4.JPG
http://66.7.221.96/~ampcabin/showman..._showman_5.JPG
http://66.7.221.96/~ampcabin/showman..._showman_6.JPG

diagram link
http://66.7.221.96/~ampcabin/showman..._with_caps.JPG

tube voltages link
http://66.7.221.96/~ampcabin/showman...n_voltages.JPG

chuck H.
No one was trying not to answer any questions.

I typed in a lengthy answer and the site asked for my name and password, again, and it lost the entire post, which took some time to type in. This is why I asked if I was not accepting

the right cookies or not.
I block a LOT of unwanted and un-neccessary cookies from all kinds of sites. I don't mind them from the originating site, but no one else needs to be setting cookies on my

computer, because they want to track my habits, so I block a whole lot of them, and though I might be blocking the ones I needed from this site.
If you will give me a minute, I'll sure enough answer all of your questions, as best I can. However, if you are expecting an immediate answer or are accusing me of skirting your

questions....well.... beg to differ. I am not gong to sit here all day. I have work to do elsewhere.
If you will look at the length of this answer, it will show you that is surpases the time limits for typing any answer, on this forum, so I had to go to note pad, type in the response, and

then come back and cut and paste it back into the response grid and then try and post it.
It is unfair that the site lost my orignal post, just because it logged ME out. It keeps doing that. Especially when using the "quick reply" field or button.
I am doing the best I can, therefore I may not be as good as you or anyone else on this forum. But I am trying.

If you wish me to stop, that's fine, but if you want me to do what you ask, ask someone else.
I know why you ask the questions;
"SO YOU GET ANSWERS!!!"

But them please slow your ass down and give us time to get you the answers. I can;t type and jack with the am,plifier ,.post pictures, come back here and beg like a dog, (like you want) all at once. Give me a blamed minute or 2.

**The Dude** · 04-11-2014, 12:06 AM

ampcabinets: "I have 2 scopes, but am not well versed in using them, but I can, if you wish. Other than seeing ripples (or such) in the sine or square wave(s), I see no need to use anything other than my trusty fluke meter, or analog simpson meter......"

Well, there are lots of reasons. The scope is your best tool for figuring this out. That is why I asked if you had one. Knowing for certain what the frequency of the hum is would narrow down the problem substantially. Also, the first thing I would do would be to scope all of the B+ lines and see what amount of ripple exists on them. That's a start.

**ampcabinets** · 04-11-2014, 12:54 AM

definitely not a 60 cycle hum.
I believe it is most likely 120cycle hum.

**Chuck H** · 04-11-2014, 01:21 AM

Don't be a poor sport. I bet you SUCK when you lose at Monopoly

It was just a bit of ribbing. I should have followed with a wink emoticon. I did it because your post following the questions didn't answer any questions but did provide photo's that don't. Which is admittedly similar to the linked post. You did mention that you had written a lengthy post. My bad. Relax and take a deep breath. You have plenty of time to learn to hate me. I didn't know I was dealing with a tender man.

I'm sorry you had trouble with the forum. It happens. In fact I'll wager I've lost many more posts here than you due to the forum logging off or logging me off. I'm still here because it's the best place to be.

I'm inclined to agree with The Dude. Find out the frequency for certain. I'm not saying you don't have 120Hz or that you can't be trusted to tell the difference. There was a thread recently where someone had an unusual hum frequency because of partial rectification. That might be good to know. My dog in this race goes to a ground loop. Because you've altered and then recreated something like the original ground scheme there may be a redundancy. However, the grounding advice given is sound. Whether Fender got away with something or not doesn't matter. Sometimes happy accidents are in the particulars. It's always best to separate signal grounds from power grounds. That includes the power supply capacitors. One reason is that the capacitors conditioning voltages for the power tubes primary function is filtering while the most important function for the preamp power supply caps is decoupling AC.

Look for redundant grounds immediately prior to and within the PI circuits.

**ampcabinets** · 04-11-2014, 06:35 PM

1) I never play games, not poker, not monopoly, not anything, not even pc games.
2) cookie jar is still empty on pc, and after email from admin, I am not blocking anything, either.
3) you would naturally have more posts than me, 1) because you have been here longer than I have, 2) you answer more posts than I do, because of your knowledge3) I am an old "stukid boy", according to my wife of 35 years, so, what do I know?

No redundancy found.

All grounds eminate from a single source point, from one bolt of the power transformer.

You can look at the pics, if you wish.

60 cycles is a low hum, this is a higher frequency, much like input hum.

I am still confused about

- incorrect wiring of the PT center tap; the PT CT has to go to the minus terminal of the first filter cap, nowhere else, period

It shows in the schematic to be grounded. Should I ground it to another point on the PT and then to negative of the first stage filter, or just to the negative of the filter.

Don't think I don't appreciate help. I always appreciate people taking the time to respond and help with my mistakes.

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