Radio to guitaramp conversion hum problem

[Krille]

Hi all,

Can't remember if I've ever posted anything on this forum Well, anyway: I got this small Philips tube radio some years ago and now I've converted it to an guitar amp. It's just to tubes EABC80 for input and EL86 for the output stage. The problem is I'm getting tons of hum. I've stripped out everything that had to do with radio reception, so the diods of the EABC80 is not connected. Should these be terminated in some way?

Any pointers are much appreciated

[Miles Prower]

Your problem is probably due to way inadequate DC ripple filtering. These cheap radios seldom paid much attention to ripple suppression, relying on the tiny OPTs and their horrible low frequency response to eliminate hum. If you have one of those diode/triode tubes, connect the diode plates to the cathode to eliminate the diode action (that way, the diode plates become internal shields).

[DF96]

We need rather more information. "Tons of hum" does not tell us much.

How does the hum relate to volume position, whether an input is connected, is it 50Hz or 100Hz? I assume this radio has a proper isolating mains transformer? Do the main reservoir and smoothing electrolytics (probably in the same can) look OK, no bulges or leaks? Did you reform them, or was the radio in regular use before you converted it? Have you used screened cable for any new audio wiring?

[Krille]

Originally Posted by Miles Prower

Your problem is probably due to way inadequate DC ripple filtering. These cheap radios seldom paid much attention to ripple suppression, relying on the tiny OPTs and their horrible low frequency response to eliminate hum. If you have one of those diode/triode tubes, connect the diode plates to the cathode to eliminate the diode action (that way, the diode plates become internal shields).

You were right, the ripple filtering was the main problem. I looked through my heaps and found two pretty big ones, 2200µ 200V so I put these after the rectifier with a resistor of 150 ohms inbetween. I also grounded the diod plates, but that didn't make any difference actually. The OPT is tiny, farting when playing the low E string There was a 22nF series cap on the input, I removed that. Also, there's no volume or tone control or anything, it's real simple. I'm mostly a drummer, so I'm gonna use this for some guitarpractising and maybe rehearsing at home along with electronic drums, so it's not very loud. Thanks for the help!

[DF96]

2200uF is rather too much for a typical rectifier, even with 150 ohms in series. It may kill the hum but it will probably slowly kill the rectifier too.

[Krille]

Originally Posted by DF96

2200uF is rather too much for a typical rectifier, even with 150 ohms in series. It may kill the hum but it will probably slowly kill the rectifier too.

It's a solid state rectifier, so it'll be ok.

[DF96]

You mean a modern silicon one, not an ancient selenium rectifier?