Hi,
about 10 years I bought a VTA kit (tubes4hifi) for the Dynaco ST70. Using the schematic I tried to build up an amp around this board.
Now, I finally had time to 'finish' it - however when engaging the high voltage I only get popping noises from the attached speakers which repeat about very second.
Also the bias voltage which should be adjusted to 0.4V is constantly alternating at around 1.5 to 2 V.
The problem is on both channels
I did not dare to let it run longer than a few seconds to half a minute... In the latter case i go smoke from in side the amp, or an EL34.
Is there a high risk to break anything? (I guess so.) And does anyone have a good idea where I should start looking for mistakes?
Thanks a lot!
about 10 years I bought a VTA kit (tubes4hifi) for the Dynaco ST70. Using the schematic I tried to build up an amp around this board.
Now, I finally had time to 'finish' it - however when engaging the high voltage I only get popping noises from the attached speakers which repeat about very second.
Also the bias voltage which should be adjusted to 0.4V is constantly alternating at around 1.5 to 2 V.
The problem is on both channels
I did not dare to let it run longer than a few seconds to half a minute... In the latter case i go smoke from in side the amp, or an EL34.
Is there a high risk to break anything? (I guess so.) And does anyone have a good idea where I should start looking for mistakes?
Thanks a lot!
What voltages have you checked before trying to “fire-up” your VTA amp? If you measure all the pertinent voltages and report them here, perhaps someone could help figure out the problem.
Do not run it with output tubes plugged in any further! Sounds like you may have a bias supply problem which could easily destroy your output tubes and even transformers.
Do not run it with output tubes plugged in any further! Sounds like you may have a bias supply problem which could easily destroy your output tubes and even transformers.
Remove only the rectifier tube, and check all the voltages in the bias circuitry.
This will be safe to do. Make sure the bias voltage gets to the tube grids.
This will be safe to do. Make sure the bias voltage gets to the tube grids.
Why, oh why you are not using a series filament lamp to limit the current taken from the mains?! The way you are doing it will result in expensive damage that can be avoided. Have some patience and do so research before you do more damage.
I suggest you view some youtube videos by Uncle Doug.
I suggest you view some youtube videos by Uncle Doug.
Thanks for the fast replies.
I should have mentioned that I implemented a solid state power supply, instead of the tube one. Sorry about that.
@Francois: I will definitely check the voltages. It could actually be that they are a little too high. That would have been a stupid mistake by myself, as I remember that I did simulate the power supply back then... Will the voltages be accurate when no load is applied? (I only have a 520 kOhm resistor for discharging the capacitors with the high voltages.)
@rayma: Very good hint. Something I could have come up by myself, but did not..
@edbarx: I was just so tempting to give it a first try. I will look into the topic of filament lamps. Thanks for the hint Uncle Doug, too.
Does anyone think it could be an issue with the negative feedback loop, or do these tend to be stable?
I should have mentioned that I implemented a solid state power supply, instead of the tube one. Sorry about that.
@Francois: I will definitely check the voltages. It could actually be that they are a little too high. That would have been a stupid mistake by myself, as I remember that I did simulate the power supply back then... Will the voltages be accurate when no load is applied? (I only have a 520 kOhm resistor for discharging the capacitors with the high voltages.)
@rayma: Very good hint. Something I could have come up by myself, but did not..
@edbarx: I was just so tempting to give it a first try. I will look into the topic of filament lamps. Thanks for the hint Uncle Doug, too.
Does anyone think it could be an issue with the negative feedback loop, or do these tend to be stable?
Not likely a problem, since the Dyna transformers are always correctly wired, and are color coded.
You can easily disconnect the feedback to verify. A leaking coupling capacitor can cause excessive
output tube plate current as well, but both channels being bad makes this less likely.
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wow.
There is a pretty normal approach to first start ups - you should read about it, understand it and then follow it. It will help you not damage stuff (including yourself) and also give you a logical faultfinding path in the case of problems like this one.
1. Take any speaker you have handy and move it to another room or preferably another building.
2. Remove all the tubes from all of their sockets. Put them next to the speakers.
3. Build a "dim-bulb" test lamp. Do it safe, not with wires hanging out everywhere.
4. Plug your amp into the output of the dim bulb device. With the wall switch off, plug the dim bulb into the wall. Turn the wall switch on. With the amp's power switch on, the bulb should glow but only dim. If it glows normal brightness, you have a problem. Fix it. Repeat.
5. Tube by tube following from the input to the outputs, follow step 4. Measure voltages at each test point and assure yourself they are within range. Fix each fault as you find it.
6. Finally, connect your sh1ttiest speakers and power up to complete your final tests.
There is a pretty normal approach to first start ups - you should read about it, understand it and then follow it. It will help you not damage stuff (including yourself) and also give you a logical faultfinding path in the case of problems like this one.
1. Take any speaker you have handy and move it to another room or preferably another building.
2. Remove all the tubes from all of their sockets. Put them next to the speakers.
3. Build a "dim-bulb" test lamp. Do it safe, not with wires hanging out everywhere.
4. Plug your amp into the output of the dim bulb device. With the wall switch off, plug the dim bulb into the wall. Turn the wall switch on. With the amp's power switch on, the bulb should glow but only dim. If it glows normal brightness, you have a problem. Fix it. Repeat.
5. Tube by tube following from the input to the outputs, follow step 4. Measure voltages at each test point and assure yourself they are within range. Fix each fault as you find it.
6. Finally, connect your sh1ttiest speakers and power up to complete your final tests.
A visual check for dry solder joints or unsoldered components should be your first step followed by checking that all the wiring has been completed correctly especially the tube sockets and earth connections securely grounded. The B+ supply should be disabled by removing a fuse if there has been one fitted or disconnected. With power on and do use a series lamp in the mains supply, check that the filament supply and negative bias ccts are operational. Report back on what you find up to this stage.