new caps, but still have a hum in amp...

This on a small tube amp I got. Its a Telvar amp. I replaced all the caps minus the death cap but still have some hum left I cant seem to find...
It plays music nicely from my MP3 thru the mic connector just fine but thru the RCA connector its not nearly loud enough. My guitar hooked to either has very little volume. Could not find a schematic. It was all original so I used cap values like the originals. Any troubleshooting tips on the hum? The chopstick test found nothing..
Rob

You have to start isolating what part of the circuit has hum. Does it stay the same loudness with the volume up or down? Is it coming from the power section, If you take out the tube befor the power tube is it still as loud?

Take the tube numbers, look up the pin outs on the datasheets, start drawing out the circuit with the values. After that measure the voltages, AC and DC. The problem usually does not jump out at you (at least not me) and you have to track it down. Without knowing what you are working with you are just fumbling around in the dark.

Here is a link to a site someone recommended to me.

Amp Debugging (http://www.geofex.com/ampdbug/ampdebug.htm)

Thanks, I had that link for debugging, will move on to some more serious testing. BTW hum stays the same at any volume setting. Will pull tubes and see what happens and do a resistor check.
Rob

This page might help - it looks like your amp.

http://www.radiomuseum.org/r/audar_mas_4.html

Looks like a totoal rat's nest to me. No attempt to isolateand twist the heater wires from the PT. Isolate the heater wires coming from the PT (the hole on the right) and just try pulling them away from the other wires as a first step. Better to rewire them.

You said you had re-capped but did you replace tubes? I also assume by death cap you mean the large electrolytic filter cap, which by the way it's job is to clean up the DC B+ high voltage line.

As the POSTer above mentions, start by tracing the wires and make a wiring diagram. You should be able to make a schematic from the wiring diagram but if you need help I will do it.

Oh, that is a cutie, what is the preamp, 6SL7? They have a reputation of having hum problems that take an odd cure.

Do you know if it is 60hz or 120hz? knowing the difference will tell you a lot about what is not causing it

6string: Thanks! Thats it for sure, good detective work! I cant view the schematic but at least I know what it is and about how old it is.
Cruisin: Its a rats nest but I have seen a lot worse. Death cap off the ac cord is what I meant, not filter caps. Will be putting a 3 wire in soon and loosing the death cap. Only tube I have as a replacement is the rectifier so I will try that too. Will move wires as last resort.
Celeste: When I took it to my local electronics parts store(they have seen every amp in the world) They all thought it was a cutie too. 6sl7 is correct.
Not sure the frequency of the hum.
I did check all the resistors and two resistors are way off spec so I will fix that too.
No time for more tonight, maybe tomorrow I can work on it.
Thanks for all the help on this!
Rob

I've got an old Wards Airline that hums from the transformers. Soon as you turn it on it hums before the tubes warm up.

I do not think the wiring is too bad. The heater wires I would twist but most of the parts and wires cross each other at right angles.

6string: Thanks! Thats it for sure, good detective work! I cant view the schematic but at least I know what it is and about how old it is.


If you register at that site you can download the full schematic and other info that is shown on the page.

Looks like a one time fee of $25 to join that forum...

It is a cutie, didnt mean to offend your baby! Looks like a fun project to tackle.

As mentioned above the freq of noise would be nice to know, if you have access to oscilloscope would be great.

Also agree with one of the POSTs that transformers can become noisy with age (the resin in the windings dries out and allows the wires to vibrate, thus noise).

Here to help if you need it.

Those loose heater wires looping around the sockets are less than ideal. Remove them and replace with twisted pair wires.

You can also add a humdinger - that's a 500 ohm or 200 ohm 5w wirewound pot, each outside lug attached to one heater wire lead, and the wiper connected to the power tube cathode bias resistor.

Doing that should substantially reduce any heater induced hum.

Cruzin, I am thinking you may have something there on the dried up transformers...No O scope here...If they are the problem, I may gut it and make it into a tweed Champ circuit. Will have time enough to look at it this weekend.
THanks

Ok got curious and did some quick testing.
1. Removed all tubes plugged it in. Still buzzes.
2. disconnected the output trans. plugged it in. Still buzzes.
3. changed the two bad resistors. Still buzzes.
4. looks like one heater wire going between the 3 sockets
the other heater wire goes from transformer to ground.
There is a wire from the last tube to ground and the heaters do work.
I did do that artifical center tap thing on another amp and it fixed it.
I do not recall exactly how I did that...
5. If its not a heater issue I figure a buzzing power transformer, if so I will gut it
and make a tweed champ out of it.

Not sure the frequency of the hum.


You should be able to find an instrument tuner on line like this one http://www.world-voices.com/software/nchtone.html then you could match your hum to 60 or 120Hz by ear.

The software is just an example, I have not used it and don't know anything about it other then it was the first hit on a google search that would do what I wanted

Wow, great idea, just tried it and its a 120 cycle hum. Ok, where does that lead me...
again no tubes installed and new filter caps.

120hz hum is usually more indicative of rectified DC voltage (120hz being a 2nd harmonic passing thru the rectifier.), what we call the B+. (ie bad rectifier tube, bad filter cap network, bad ground,etc)

But you dont have tubes plugged in, and you diconnected the OT (which i assume is also the speaker) and still get noise??!! Is the noise coming out of the speaker? If so can be electromagnetic noise coupleing thru air onto speaker. If not , disconnect speaker, if still noise then I suspect it is mechanical vibration noise from your PT and thru its mount. It can be resonating the cahassis that it is connected to. does it dampen if you press on the chassis? can you feel vibration?

120hz says heaters are not the problem. With no tubes, is the OT connected to the power supply? I am assuming the hum you are chasing is coming from the speaker?

I thought he said ot was disconnected? Need clarification.

No tubes in it, OT not connected, sound is NOT coming from the speaker.
I suspect a bad power transformer. Just did some more checking...Found
the problem. It is the transformer. Its a huge paperwound one and when I
press hard on one side of it I can make it stop humming.
Its putting out 200 volts on each side to the 5y3gt.

Easy to find replacement. Try triode electronics inc.

Thanks for all the help on this one! Since this has no collector value, rather than fix it up the way it was, I will gut it and make it into a 5C1 tweed amp. I have just about everything needed minus the proper transformers. I had the Silvertone/Valco version of this a while back and it sounded great.

Do you have access to an oscilloscope? That little piece of equipment can tell you loads about where and what your problems are.


Thanks for all the help on this one! Since this has no collector value, rather than fix it up the way it was, I will gut it and make it into a 5C1 tweed amp. I have just about everything needed minus the proper transformers. I had the Silvertone/Valco version of this a while back and it sounded great.