Fender 5C1 trouble - solved

[Mylithra]

I built a 5C1 Champ using a pentode (6SJ7) for the pretube. Here is a link to the schemo:

http://www.schematicheaven.com//fenderamps/champ_5c1_schem.pdf


The only mods from the schematic are as follows:
1. The Power tube Cathode bypass resistor and cap changed to 1k 5w and 47uf/50v to better bias the output tube. As it was, the tube ran too hot.
2. The schematic doesnt list it this way but the 6.3v tap has a CT. At first, this was referenced to ground. I could hear this low frequency hum coming out of it so I moved the CT to the Cathode of the power stage. There was a free 28v so my 6.3v CT is now referenced to +28v. The hum did get better after this, but its still humming.

The problems Im having trouble solving.
1. When you turn the Volume up past 3 o'clock. the output tube gets these 2 pinpoint blue glowing beams and introduces a screeching sound. If you just nudge the volume past that same position (say, an RCH) I can hear it go into oscillation. The only thing I could figure was causing this was a lack of grid stopper on the power tube once the Volume pot's impedence got low enough. The only thing I could figure to do with it to prevent the oscillation was to put a 10K reisitor in series with the pot.

2. Im still getting a hum from the amp. It sounds 60Hz to me but I suppose it could be a multiple of it. I had a friend who has a scope check out the B+ for ripple. He said it was clean.  *edit* Spectrum analyser said it was 120Hz*

All my grounds run to the same place (think buss grounding) The amp is wired Point to point and anything bare was heatshrinked. All my voltages seemed to be what I was expecting. The only thing I could think of over all was I was still biasing too hot. In which case, the idea I had for solving it was to put a 1k 5w pot in series with the cathode bypass on the power tube and revert it back to 500R, giving me a sweepable range of 500R to 1.5K, should be hotter than Ill ever need and colder than Ill ever need. My Plate voltage is 382v. My Cathode Current is 30.4ma, which to me still seems a touch high.

*edit* Being that the hum is 120Hz, its got to be something in the B+ causing the hum.

Am I on the right track to solving my oscillation problem? Im at a loss as to the low frequceny hum. Ill put it up on my spectrum analyer later today to find the exact frequency of the hum.

 

[mrphotodude]

here is some stuff to look at:
first off, are is this a metal can 6v6 or metal can 6sj7?
the metal can octal preamps tubes are notorious for being noisey. You might have to try a glass 6sl7 or similar.
The 6v6 metal can cannot handle so much voltage.
You should put in a grid stopper right before the power tube input of say 1k to start with(though this blocks high frequencies)1.5k was stock in later fenders.

i dont know what kind of power transformer you are using,but the original fender small power transformers had 2 leads for the filament.....one to one side of the filament, the other side to ground with the other side of the tube heaters to ground. That caused alot of noise.
Now you say you changed that...did you balance the heaters with a 100ohm from each side of the filaments to ground? If they have no ground reference, that could be a problem.
Another thing to watch for is that some old power trannies had the center tap or one side of the filaments grounded to the transformer can internally.
You should be able to measure a small amount of resistance between the filament taps, see if the resistance between one tap and the transformer can is half of that...that may indicate it has the internal grounded ct.

i would check the heaters as  i suspect that is the main root of your problem, but there may be something else going on as well.

30ma is not much for class A. Tubes get the snot beaten out of them in class a and run very hot! I put a 1k in my champ to get it going around 35-40 which is lax.

here is a thread to read:
http://www.thegearpage.net/board/archive/index.php/t-138465.html

i suspect heater wiring for noise issue(or ground issuess).
are your jacks mounted right to the chassis?

 

[mrphotodude]

also, one thing in PP amps is if the plate leads are reversed, you will get  ahigh squeel right away...
i dont think this is true on SE amps at all though. If you have the red lead to B+ and the other to the tube...should be fine.

 

[emrr]

Your filament CT bias scheme at 6V6 cathode wouldn't in of itself do anything to help with hum, that sort of thing helps with other noise.  You may have only found a slightly better hum orientation, and I have to wonder if the filament CT seeing cathode of the output stage directly isn't possibly creating positive feedback to the 6SJ7 through the filaments.  I don't know for fact if that's possible, but it would seem so. 

A dissenting view on one issue.

I have a ton of metal 6SJ7's in studio gear; never heard a noisy one.  I've tried a ton of glass ones; none have sounded as good as the metal ones.

In fact, I have very few glass bottle preamp octals in use.  Typically 6SN7's and 6SL7's.  Lots of metal 6J7's, 6C5's, 6L7's in high gain mic amps and limiters. 

Having said that, definitely swap tubes if you haven't.  A single funky tube can induce hum.  Layout is probably the most critical thing here, as simple as the circuit is. 

 

[Curtis]

If the hum is 120Hz like you say then I doubt it's due to filament hum, which would be at 60Hz.  Could be insufficient filtering on the plate supply or screen supply of the 6V6.  If the B+ is clean it could indicate a ground loop, which would be addressed by altering the grounding scheme (star grounding), or insufficient shielding (is the chassis and PSU 0V connected to mains earth?).

Swapped leads on the output transformer seems unlikely as there's no negative feedback from the speaker, although you could be seeing capacitive coupling that may get better or worse depending on the phasing of the OPT leads.  Can't hurt to try it.

Is you OPT meant for single-ended use?

Layout and lead dress is probably the main cause for oscillation.  There plenty of good resources out there.  Some easy ones to try are ensuring input signals are routed well away from output signals, early stage grid signals run in shielded wire (earthed one end only), wires that cross paths should be done at 90 degree angles.  Try moving wires with the amp switched on using a non-conductive probe (the chopstick test!) and see if the oscillation responds in any way to the wires being jiggled.

 

[AnalogPackrat]

No one has mentioned it, but are your power and output transformers rotated 90deg to each other?  If they aren't and they are in close proximity (hard to avoid on a typical champ chassis) the result is 60Hz coupled into the output.

If you've got a couple of pics of your build that would help us all play "spot" on both of these problems.  A picture is worth at least a few hundred guesses 

A P

 

[Mylithra]

The original champ 5C1 didnt have a power transformer that had a center tap on the 6.3v winding. My power transformer does. (its a Weber transformer made for this application). Originally, I had it grounded. After reading http://paulrubyamps.com/info.html#hum , thats what gave me the idea to move the CT for the heaters to Cathode of the power tube. The idea being referencing the 6.3v winding to something positive. In this case 30v.

As I had said, The spectrum analyzer noted the peak of the noise being at 120Hz. If it was heater noise, it would be 60Hz.

As for grounding, I have everything grounded on a strip of wire around 16ga around 3" long with one end hooked to the chassis so everything is essentially grounded in 1 place. I read on some other boards that having the B+ grounds and the signal grounds at the same spot could introduce this kind of noise into the amp.

I do have some pics and I will post it when Im near them again.

At this point, I think I have 2 separate problems here.
1. 120Hz hum induced from the B+ either through grounding or Ripple. (I didn't have a scope to check it then, I do have one now) I suspect grounding to be the problem here.
2. The screeching when I kick the vol up past 3 o'clock. I still suspect when the Vol pot gets cranked, it gets too low an impedance, the power tube is essentially running without a grid stopper. Ive don't have a ton of experience with this type of problem so troubleshooting is going slowly.

The power transformers are 90Deg from eachother and mounted on the chassis the same way Fender does.

 

[MagnetoSound]

The original champ 5C1 didnt have a power transformer that had a center tap on the 6.3v winding. My power transformer does. (its a Weber transformer made for this application). Originally, I had it grounded. After reading http://paulrubyamps.com/info.html#hum , thats what gave me the idea to move the CT for the heaters to Cathode of the power tube. The idea being referencing the 6.3v winding to something positive. In this case 30v.

Hi Mylithra,

There should be no problem with grounding the CT.

If you want to copy the original more closely, you can always ground one side of the filament supply instead, as in the schematic.

As for grounding, I have everything grounded on a strip of wire around 16ga around 3" long with one end hooked to the chassis so everything is essentially grounded in 1 place. I read on some other boards that having the B+ grounds and the signal grounds at the same spot could introduce this kind of noise into the amp.

If you mean that you have the B+ CT and the filter cap grounds on this bus bar, that could well be your problem. Ground the B+ CT directly to the chassis, away from where the preamp bus is grounded, together with the filter cap negs, and close to the mains ground. These currents need to find their way to ground easily, and away from the audio path.

At this point, I think I have 2 separate problems here.
1. 120Hz hum induced from the B+ either through grounding or Ripple. (I didn't have a scope to check it then, I do have one now) I suspect grounding to be the problem here.
2. The screeching when I kick the vol up past 3 o'clock.

In all likelihood, in a circuit this simple both these problems are going to be caused by the same fault. As well as the grounding scheme, check the third filter cap (the one after the 25k). Bad filtering here will cause feedback from the screen grid to the input grid of the 6V6.

HTH,

Dan

 

[peterc]

WRT the screeching, I had a similar issue in a Champ cct (not this one).

Try a cap across the 250k Anode resistor, 47pF or so. Then fiddle with the value as this can reduce the HF response of the amp.

Bigger cap will give more HF reduction.

You could also try raising the value of the last filter cap to about 47uF. Dont do this to the earlier ones, the rectifier tube sometimes does not like a higher value here. Also try parallel poly caps of 100nF or so across the 8uF caps to tame the HF.

Peter

 

[Mylithra]

Hi all,
Sorry it took so long for me to get back. Been waiting on parts. They got here today. First, the pics.
The first one was the first run on this. I tried doing it true point to point with phenolic strips to hold the Power supply. Think really old school.

I thought the layout was a little too conducive to being noisy so I totally redid it on a turret board I constructed.

The grounding scheme I got off another builder's forum post. The ground is a 10ga with the pre and power sections grounded seperetely.

The power section ground is the 1st and 2nd filter cap, the power tube cathode resistors/caps, the HV sec. CT and the ground off the power cord and is terminated at the Power Transformer's mounting bolt.

The pre section ground is everything going to ground off the pre tube, the 3rd fitler cap, and the volume pot all terminated at the ground of the imput jack.

The output is grounded to the chassis. The heater CT is grounded to a different Power Transformer bolt than the power section ground.

The only difference from the schematic I have now is the 25K in the power section is 5w. (I couldnt find the value at a 1w-4w locally) The cathode bypass has been redone with a 470R 5W resistor in series with a 1K 5w pot with the cathode bypass cap (changed to a 47uf 50v to accmodate the increased voltage from having a large bypass resistor) in parallel with the resistor/pot) With this mod, I was able to set my bias for the tube I use. Ive set my cathode current to be 28.5ma (though I could run it as hot as 33ma).

I figured out what was giving me the screeching noise. I had a 22k in the power section where a 25K should have been. Once I got the volume pot low enough, I was feeding too much voltage to the input grid on the power tube. With the correct power resistor in place, I no longer get that noise and I get a much better overdrive out of the amp. (power plate voltage and all)

The bad part. I still have the 120Hz hum problem. The grounding scheme was meant to take care of that but alas it hasnt. Im open to new ideas if you have them.
EDIT: as a test, I pulled the pre tube and ran the amp. The hum was still there even without the pretube inserted.

 

[mrphotodude]

is your input jack mounted right to the chassis?
i have found inputs need to be isolated from the chassis to get lower noise. It looks like you have a ground wire from that jack, so if it is not isolated, that is one ground loop right there.
i usually isolate my output jack as well.

is that 472 going from the power to ground? 
if so that is what they call the death cap! they used to use those back in the day to ground one side of the line to the chassis because they had no 3 prong cords in use.
if that is the case, get rid of it!!! you should have a grounded cord on it.

i would pull the input jack out as well and leave it floating safely not touching anything and turn the amp on and see if the hum goes down at all.
i could be way off...but i would isolate it anyway.
its hard to tell, but if you have the ground lug on your pot soldered right to the pot...it has to go through the chassis to reach ground as well.
run a wire from the lug to a ground point on your preamp section on the board instead.

i would seriously just pull out the input jack and tape it or something and see if the hum goes down right away(unless it is isolated from the chassis already).
if that helps...do the output jack and pot as well.

 

[Mylithra]

The amp is wired with a 3 prong plug. The cap was on the schematic so I put it there for completeness sake but you are correct. It will simply need to come off.

With the Pre Tube pulled, I was still getting the hum. That effectively eliminates it as the possible problem. I ended up adding another RC filter in front of the first filter and that improved the hum considerably. At this point, Im going to put it up for a little while and come back to it in a week or so.

 

[mrphotodude]

yeah..if you need a break from it,by all means!
but seriously... make sure the input jack is isolated from the chassis and put a ground wire from the jack to the grounding on the preamp.

 

[Mylithra]

My band practiced today so I got to use this little amp today. Not for the whole practice session but for a couple of songs..
Connected to my 412, it was plenty loud. I could still hear the 120hz hum but it was barely audible (no one else in the band noticed it till they got their head right up in front of the cab) . The amp itself was very clean as far as other noises (RF, EMI, etc). Start getting the volume past 5 and it starts breaking up very nicely. At full volume, it has a TON of crunch and is very smooth. Im very happy with how this has come out.

To get rid of the last of the noise, Im going to try either adding another RC with a Choke in place of the resistor or just replace the Whats now the first RC resistor with a choke. I might do that first just to see if that kills the noise. Mercury Magnetics makes a mini choke for this application. I will most likely use that. I know Im being rather nit picky now but I know I can get this thing to dead quiet. As it is now.. it wispers.  ;D

PS.. I did pull the Death cap out of the thing.. I think it actually helped with the hum a little

Edit: Here are some pics.. now that she's dressed

 

[peterc]

Something to try, which I do AC30s that I service:

Take out all he valves, plug in the speaker & switch on. Listen closely to the speaker for any hum. On AC30s, the hum is transmitted from the transformers thru the chassis to the speakers.

My solution to this is to mount the transformers on some kind of rubber or plastic washers to raise the transformers off the chassis slightly.

Gets em dead quiet.

 

[Mylithra]

I tried pulling all the tubes and turned the amp on. There was nothing at all.

I had to take the amp apart today to fit in the handle for the cab. I had just gotten my O-scope and signal gen so I decided to once and for all track down the noise.

First thing I did.. No signal gen, amp connected to a dummy load. Turned the amp on, let it warm up. I started tracing back through the signal path till I saw the noise show up on the scope. It didnt show up on the scope till the speaker jack. So then I started tracing the PS. The pre section PS was dead flat. The Screen Grid tap had ripple in it, so did plate, only more so.

So there is the definitive answer to the cause of the hum.  The only ways I know of to get rid of ripple is more filtering but adding more capacitance changes the character of the amp. In this particular case, the changes have been positive but I know there will be cases where that will not be the case. How do you get rid of the ripple and thus the noise without changing the character of the amp?
(I get the feeling the ultimate answer is you dont but I thought I was throw this out there)

 

[ampman1961]

I tried pulling all the tubes and turned the amp on. There was nothing at all.

I had to take the amp apart today to fit in the handle for the cab. I had just gotten my O-scope and signal gen so I decided to once and for all track down the noise.

First thing I did.. No signal gen, amp connected to a dummy load. Turned the amp on, let it warm up. I started tracing back through the signal path till I saw the noise show up on the scope. It didnt show up on the scope till the speaker jack. So then I started tracing the PS. The pre section PS was dead flat. The Screen Grid tap had ripple in it, so did plate, only more so.

So there is the definitive answer to the cause of the hum.  The only ways I know of to get rid of ripple is more filtering but adding more capacitance changes the character of the amp. In this particular case, the changes have been positive but I know there will be cases where that will not be the case. How do you get rid of the ripple and thus the noise without changing the character of the amp?
(I get the feeling the ultimate answer is you dont but I thought I was throw this out there)

Lots of excellent points on this thread, though I didn't read all of them. Here's my 2 cents worth.
First of all, make sure your B+ and screen supply filter ground is referenced to the main system ground IE: fil CT and B+CT to chassis. Then, the preamp B+ filter ground should be routed to the input jack ground along with the 1.5K cathode resistor and bypass cap ground leads. A ground buss is not needed for the entire amp. What you are trying to do here is keep them separate at opposite ends of the chassis. As to the volume control, simply solder the grounded tab or CCW terminal to it's metal cover. Keeping the leads as short as possible and dressed close to the chassis floor is paramount here. Lead dress is very important, even in little single ended amps like these. The grid input lead should be short as possible and should be at right angles to any filament lead. In fact, a single ended filament layout like the originals, if done right can operate very satisfactorily. You may also opt to lift the filament CT altogether and try two 100 ohm MF resistors from each filament leg to ground. Gibson and some Valco/Supro's ran the filament CT through the power tube cathode resistor with decent results.
Again, keep it simple and clean, double check all solder joints and wire snippets and make damn sure the the grounds are firmly bonded to the chassis.
I have restored and built 100's of these little amps using the prescribed approach and it always works out perfect. I hope that helps.
Billy Yates

 

[analag]

No cathode resistor on the 6SJ7?

 

[emrr]

nope, grid leak. 

 

[MartyMart]

As Mrphotodude has said, use isolated Jack sockets ( plastic body and plastic washers where screwed to chassis )
I think that will help a lot, did that on my little SE5a from ampmaker.com

MM.