troubleshooting problems with DIY champ 5F1

[mike_mccue]

Greetings,

I've begun troubleshooting a small amp I built about 10 years ago. It was loosely based on a Fender 5F1 with a 5U4GB and larger value power filter caps as exceptions to the original design.



The amp played well. I used it hard for many years as a recording amp, but recently it began osciallating at a low frequency. I gave it a general clean up and the amp now seems stable. But it seems like the power transformer has begun to vibrate which I don't recall it having done before. There is a audible hum in the speakers with the volume turned all the way up. Yesterday I let it stand in the on position for a few hours to get it hot and then I played it loud. When I really drove amp hard it would drop out momentarily. I pulled the chassis and did some voltage measurements this morning. This evening I played it a little and now see there is a very small spark near the base of the 6v6GT tube when I really dig into the guitar.



I ordered new power caps and I'll replace them and the bypass cap ASAP.


In addition to the noticable vibration, I've also noticed the power tranny gets very warm... I don't recall that happening before. It's a New Sensor NSC125P1B.


I did both vDC and vAC measurements today:



vDC with tubes at idle:
1) 422
2) 373
3) 327

Someone on another thread suggested that a shorted Cap might draw down the B+. These voltages seem to be pretty well matched.
vDC without tubes:
1) 516
2) 514
3) 512

I was curious about ripple at idle.
vAC with tubes:
1) 3
1) 0
1) 0

Is the 3 vAC at "1" a normal ripple?

I did one other thing out of curiosity. I recently watched a Gerald weber video and he demonstrated a way to discharge the power caps by connecting to pin 1 of the preamp tube and jumpering to ground. The premiss is that there is enough resistance in the circuit that you don't need a resistor in line with your jumper. He recomends 20 secs.

After I "discarged" the caps in that manner I placed a dc voltmeter across the same connections and watched the voltage climb to 8vDc. I reattached the grounding jumper and repeated the test a few times. It seemed to take 20 minutes before the vDC was below 1vDc. Is that normal?

I'm just looking to learn a few things and need experience troubleshooting. Can anyone suggest anything of note to look for after considering some of what I detailed above?

Thanks,
mike

[Enzo]

8 volts is safe enough in my book. At 8 volts, then if you wanted you could do a hard short to ground with a wire from each cap. a few volts in the cap offers no danger in discharge. There is a lot of resistance in that circuit and caps trying to hang on to their charges will fight. There is something called dielectric absorbtion - look it up. That is what causes a cap sitting there on its own to start to recharge even with nothing connected.

I'd say only 3 volts of ripple at the first filter cap is darn clean. Yes it is normal to see ripple at the first filter, and normal to see little or none at the later filter stages.

If you see a spark, something is arcing. That can only get worse, and it offers potential to damage the amp. Probably the socket arcing, in which case it is best to replace it. If it is just the tube itself arcing, then a tube replacement will do, but I'd bet on the socket.

[mike_mccue]

Thanks Enzo,

Power Transformer?
Did you have any thoughts about the power transformer. I've searched up a few posts about the vibration and it seems many times it's not associated with a problem... but i think it's doing it more than in the past.
I also am concerned about it getting hot. Even in the cabinet there is lots of ventilation. I don't notice trannys on my other amps getting so hot.

Tube?:
To elaborate on the 6v6 and the spark. I just inspected the socket and it looks very clean... no carbon, burn marks, or discoloration. The little yellow spark I viewed was inside the glass and I presume happening on the wiring in the bottom stem assembly.


best regards,
mike

[MWJB]

If the obvious things (tube & socket) don't cure your problem, I have a feeling that you are going to have to look at your layout. The chassis is very deep, with a big distance between control panel & circuit board to the tubes.

That wire from the vol pot to V1, 7 should be shortened & shielded, as should the wire from the 220K grid load resistor to 6V6 pin 5. Consider moving the board nearer to the tubes. Also try a grid stopper on the 6V6 pin 5, start with a big value to see if you're on the right track, maybe 10K? If it has the desired effect, reduce value to the smallest that works.

Whilst it is very possible that the arcing is tube/socket related (so eliminate these parts first) the fact that you mention the amp shutting down at high volumes suggests a pararsitic oscillation. At less severe drive this can manifest itself as a "tick" sound, accompanied by a spark in the power tube. In this case, the spark can simply be a symptom of the PO and not the root cause, changing tube & socket will not make a difference if so.

I hope I'm wrong.

Have you tried rewiring the heaters as a twisted pair?

I'd think about seperating your preamp & power amp grounds.

[mike_mccue]

Thank you for the detailed suggestions,

I have received several good suggestions about the amps layout etc with regards to optimizing the performance. At this time, I'd like to continue troubleshooting a problem that has occured after apprx nine years of use as a recording "voice" for a particularly dirty distortion tone. I'm generally happy with the amp and would like to concentrate on the problem first.

I appreciate that some of your suggestions may address the problem. I'd like to consider them slowly.

I had a BIG problem with the low freq oscilation, but I can't get that to happen anymore.

The audio drop outs are only occuring when I purposely overdrive the amp and the sound seems stable until it simply "turns down" momentarily. I don't notice a change in character.

The power trans vibration and hum seems to be a new problem. The self noise is very quiet with the exception of the specific 60Hz hum.

I have not noticed any "tick" sounds

best regards,
mike

[mike_mccue]

I am feeling a bit foolsih and wouldn't be surprised if some of you think I'm a whack job :-)

I have several Fender amps. This morning I turned on a 1970 Champ, a 1975 Vibro Champ and a late 70's Deluxe Reverb ad let them all warm up at idle.

I was surprised to learn that after a lifetime of playing guitar I had never observed that all my power transformers get hot.

I just never stuck my hand on one after using it for while. :-).

The vibration from the amp at hand is what finally caused me to pay more attention to the trannys.

best regards,
mike

[MWJB]

I wouldn't feel too bad, often amps that are otherwise outwardly identical can run at noticably different temperatures. Heat in itself may not suggest a problem ("hot" is a relative term and human hands aren't the best tools for guaging temperature)...smoke on the other hand, always does.

How bad is the vibration? Just a gentle tingle? Pull the tubes and power up so that you can get at the PT secondaries, with a chopstick (a dry, wooden one), or some other NON-CONDUCTIVE implement, try moving the secondaries around and see what effect that has on the vibration.

[mike_mccue]

Thanks,

I'd like to try what you are suggesting by moving the PT secondaries.

Are you suggesting I pull the rectifier tube as well as the audio tubes?



On another note I'm still waiting on the caps to arrive but I did try a different 6V6 tube. I haven't seen any sparking in this one. The socket looked clean. Are there any board components that I should specifically check because the the short in the tube might have damaged them?

best regards,
mike

[MWJB]

Well, you don't really have to pull the tubes, I was suggesting it because of the physical layout. If you can securely rig up the chassis upside down, with the tubes, that would be better for eliminating any unwanted noise along with the vibration.

If you're getting good voltages at the 6V6 pins then you're probably OK. Screen grid resistors can suffer.

[mike_mccue]

Thanks MWJB,
I checked my voltages with the replacement tube:

1) 428vDC
2) 376vDC
3) 328vDC

so I'm feeling lucky.

The vibration is just a tingle. The hum in the speakers is not very loud. But I'd like to learn a bit more.

I have homemade jigs for many of my amps. So I tried poking around with the wiring while the amp was sitting at idle with the vol turned all the way up.

I have 3 questions about what I observed

1) I didn't find any notable difference when moving any of the wires. I used a ball point pen body/shell to move the wires. I wasn't sure if I was imagining it but when I held the pen firmly against a wire it seemed the vibration I felt with my other hand on the work bench seemed to drop in amplitude. Is that something that could happen, or just my imagination?

2) The hum and vibration goes away completely when the Volume knob is turned down. Does that imply I can minimize the vibration by optimizing the preamp portion of the circuit?

3) I also noticed that when driving the amp with a distorion pedal and engaging in acoustic feedback drone that the tranny seemed to be motionless, the vibration would return when I reigned the feedback back by stopping strings etc. Any thoughts on this?

I'm going to review your suggestions above and maybe do some shielding after I get the new caps installed.

best regards,
mike

[MWJB]

1) Hmmm, you've got me there!

2) OK that suggests that something earlier in the circuit/wiring layout is the cause. So, yes it's possible.

3) See "1)"

[mike_mccue]

thanks again I'll post after I do the cap job and try some of your suggestions.

best
mike

[MWJB]

Just a thought, may not be relevant to your specific PT/symptoms, but some mfrs specs for 125P1B PTs quote a 2A 5V secondary for the rectifier, thus will only take 5Y3, 5V4, GZ34. Do you know for sure that you have the 3A winding required fora 5U4?

[mike_mccue]

WOW,
That's a pretty good thought. I don't think I ever considered that but a quick look at my RCA manual made it pretty obvious :-).

I do have a 5Y3 at hand.

I went looking for specs on the New Sensor PT and I can't find any. Do you have any links etc?

I'm going to try the 5Y3

best regards,
mike

edit: I don't know if I was ever aware that the 125P1B was a legacy fender part number.
I found this useful link:

http://www.unclespot.com/FenderXFMRs.html

and I wrote a email to New Sensor.

the 5Y3 works fine.. but I think It was louder with the 5U4.