fuse blowing now and then.. ideas?

[guitarpsych]

I have had my Dynakit Stereo 70 for a year or two now and it is great. I have one problem. Twice, the fuse has blown. Here's what happens... I will hear a hum, very faint at first (both times I've mistaken it for part of the song), then it builds louder and louder and then the fuse blows.

A friend suggested when I put a new fuse in to try to turn on the amp without the rectifier tube in to see what happens. The amp powered up fine without the rectifier tube. I then put the rectifier tube back and it worked fine. But since this has happened twice, I think there must be something I should fix on the amp. Any ideas what the problem might be?

other minor symptoms:

1. Occassional popping/crackling, usually during the first few minutes of powering up.

2. Sound is very thin if I try to play my ipod through it... though I suppose that is due to the ipod, not the amp.

My experience level: I have done some tube guitar amp repair and built a few tube amp kits, but I'm relatively new to this.

Thanks for any suggestions.

[Shannon Parks]

Is this a stock ST70 with stock driver?

[guitarpsych]

Yes, I believe so. I didn't buy it new so cannot be 100%, but my guess is this has not been altered and is stock.

[WA4SWJ]

Hi,

I'd like to offer some guesses :

1. Your bias pot(s) may be flaky.
2. Your power tubes may be loose in their sockets causing loose connections to the control grids and you are losing bias causing the tubes to run away (probably most likely of these guesses).
3. Your selenium bias rectifier might have a problem.
4. You could have leaky coupling capacitors to the power tube grids from the phase inverter.
5. Power tube oscillation - fairly long shot - put a 100 ohm, 2 watt resistor at the tube socket in series with the screen lead and reconnect the lead. Do this for each power tube.

It would be interesting to hear what you find. My first check would be the tube sockets. Make sure the EL-34s are tight in the sockets. It might be worth replacing those old sockets.

Good luck and regards,

Ed Long

[erichayes]

Hi All,

I think Ed's instincts are right on--when the bias starts crapping out (for whatever reason) the output tubes start drawing more current, which taxes the power supply, which causes excessive ripple voltage, which causes 120~ hum. Ultimately, enough current is drawn to pop the fuse.

After spending considerably more than half my life bailing kitbuilders out, I have formed the following postulate: Dr. Murphy licensed all of his laws to the kit manufacturers (yes, including Heathkit) so that they could justify having service departments. Dynaco minimized PC board errors by supplying pre-stuffed boards (except on the tuners, but don't get me started on that), but that still left all of the interface and chassis wiring vulnerable to Murphy.

Bottom line: check every tube socket, terminal strip, filter capacitor and eyelet connection on the PCB to make sure they were soldered properly. I can't count how many times I've found a "good mechanical connection" as the instructions state, only to have it overlooked when soldering. Damned things would work for years and then oxidation or metal fatigue would finally catch up, causing a failure or, worse, an intermittent problem.

Good luck, and keep us posted.

[s2kov]

try to replace all filter caps in the bias and psu! :)