Very low bias and hum on Dynakit ST-70

lagwil wrote:Say the happiness that lasts forever. Just when I was so pleased with my set up, the amp gave up on me...

Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

Any help on troubleshooting is very much appreciated!

Bob Latino wrote:

lagwil wrote:Say the happiness that lasts forever. Just when I was so pleased with my set up, the amp gave up on me...

Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

Any help on troubleshooting is very much appreciated!

If this is an original Dynakit from the 1960's ..

1. Try replacing the quad cap. On older Dynaco amps, these caps start to leak. When the do leak, the amp won't bias properly.

2. If you still have the selenium rectifier for the bias system, replace it with a diode.

3. Replace the two 50 uF caps on the 7 lug terminal strip.

Bob

Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

Bob

lagwil wrote:Say the happiness that lasts forever. Just when I was so pleased with my set up, the amp gave up on me...

Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

Any help on troubleshooting is very much appreciated!

Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

Bob

lagwil wrote:

Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

Bob

It measures 61.8 Ohm. I also measured the 15.6 ohms resistor and they measures 15.7 wich I guess is good enough?

MechEngVic wrote:

lagwil wrote:

Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

Bob

It measures 61.8 Ohm. I also measured the 15.6 ohms resistor and they measures 15.7 wich I guess is good enough?

Start following that low bias voltage backwards towards its source, you might have a bad component/connection back in the chain. I don't see how the buzz is explained by this but it does need to be resolved. I know it's probably the first thing you checked but you haven't mentioned them: Have you tried swapping around your tubes? A bad tube can cause buzz and skew voltages.

lagwil wrote:Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

Any help on troubleshooting is very much appreciated!

Bob Latino wrote:1. Take out the tubes and measure the AC voltage to chassis ground from pin 4 and then pin 6 of the rectifier tube socket. See if you get about 350 to 375 AC at both measuring points. If you don't then the transformer is bad.

2. Also measure the AC voltage to chassis ground where the RED/BLACK wire connects to the diode that replaced the selenium rectifier. You should get 50 to 60 volts AC to chassis ground. If you don't then the transformer is bad.

Bob

lagwil wrote:

Bob Latino wrote:1. Take out the tubes and measure the AC voltage to chassis ground from pin 4 and then pin 6 of the rectifier tube socket. See if you get about 350 to 375 AC at both measuring points. If you don't then the transformer is bad.

2. Also measure the AC voltage to chassis ground where the RED/BLACK wire connects to the diode that replaced the selenium rectifier. You should get 50 to 60 volts AC to chassis ground. If you don't then the transformer is bad.

Bob

Hi Bob and many thanks for all great help!

I have now made more measurements on my Dynakit. I live in Sweden and the socket should give 220–240 V 50 Hz. I measured 235V. The amp is stamped 220V.

On pin 4 and 6 on the Rectifier tube I get 386V
Before the diod I get 57V

/Wilhelm

peterh wrote:

lagwil wrote:

Bob Latino wrote:1. Take out the tubes and measure the AC voltage to chassis ground from pin 4 and then pin 6 of the rectifier tube socket. See if you get about 350 to 375 AC at both measuring points. If you don't then the transformer is bad.

2. Also measure the AC voltage to chassis ground where the RED/BLACK wire connects to the diode that replaced the selenium rectifier. You should get 50 to 60 volts AC to chassis ground. If you don't then the transformer is bad.

Bob

Hi Bob and many thanks for all great help!

I have now made more measurements on my Dynakit. I live in Sweden and the socket should give 220–240 V 50 Hz. I measured 235V. The amp is stamped 220V.

On pin 4 and 6 on the Rectifier tube I get 386V
Before the diod I get 57V

/Wilhelm

Are you by any chance close to gothenburg ? Asking since i can fix it...

PeterCapo wrote:Some mechanical humming from power transformers is not unusual even under normal circumstances and results from flowing current.  So, electricity and mechanical humming are definitely associated.  Sounds like he described the mechanical hum as varying with the setting of his bias pots.  The varying mechanical hum sounds like a symptom of another problem, probably whatever is causing his biasing difficulty.

If the amplifier was working fine for some time and then this problem suddenly came up, it means some part or parts, including possibly tubes, in the amp are failing.  Questions:

- were there any changes to the amp shortly preceding the onset of the current symptoms?
- is a 3A fuse in the fuse holder, and not more than 3A?

If the problem is in one channel only, I'd try swapping the tubes from one channel to the other and see if the problem follows the tubes.

I'd also still refer back to Post n°13.

lagwil wrote:Nope but I do have a 5U4GB and exactly the same resultat with that rectifier. Alao swaped all EL-34 Tubes. Same...

mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

lagwil wrote:

mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

PA521 is what I got.

/Wilhelm

peterh wrote:

lagwil wrote:

mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

PA521 is what I got.

/Wilhelm

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