Quickie problems: hum and microphonics

[snugbottlehead]

I built my Quickie last year and it has been running with a Dynaco St-70 and Heresy II's.  This was my first kit, but I took my time, checked the instructions twice and was careful with the iron. 

I have two problems with the Quickie that I hope to resolve on this Forum. First, when I use the volume control, there is quite a bit of hum.  I don't even have to touch the volume knob - even coming close to it with my fingers will result in hum.

Second, microphonic high-pitch "whistle" occurs at almost all levels.  I have two sets of tubes and the problem persists.

I have a digital meter to help with testing.

The Dynaco amp was new to me when I began using the Quickie.  I've never run the amp through a different preamp, so I cannot say that amp is not part of the problem.  I know someone who runs Dynaco in my area (Buffalo, NY), and could run the amp through another preamp to test.

I really enjoyed building the kit, but it would be much more satisfying to resolve the issues before moving on to something else.

Thanks to all who respond.

Paul

[earwaxxer]

hey Paul - how's the weather in Buffalo? - you find some good hum advice on:
http://www.bottlehead.com/smf/index.php/topic,3354.0.html

also there is a post on hum that I cant find, I started it, and I think its call 'hum me a few bars' - I solved my hum prob by grounding the quickie to the DAC.

tons of microphonics post that you can peruse.

Ringing is a situation of 'damping' - there I got that right.
happy new year! - Eric

[corndog71]

The hum when touching the volume knob I think is normal.  That's just a product of the very closely mounted parts and sensitivity of the 3s4 tubes.  If they're ringing constantly then it's time to get some more tubes.  You could try damping them but personally I would toss them and find better tubes.  If you want to be anal about it reheat every solder joint to make sure there are no cold solder joints.  Remember never to blow on a solder joint and allow it to cool on its own.  3s4 tubes in my experience are a mixed bag.  But many of them sound really good.  While the stock quickie is a terrific deal, it's also a product of numerous compromises to keep the price down.  The circuit is amazingly great and benefits considerably from a different layout.

[Paul Birkeland]

The hum when touching the volume knob I think is normal.  

No, this is not normal.  The OP needs to connect the ground of the Quickie to the ground of his source.

[corndog71]

The hum when touching the volume knob I think is normal.  

No, this is not normal.  The OP needs to connect the ground of the Quickie to the ground of his source.

I didn't see that in the manual.

[Paul Joppa]

Older ST-70s have a couple issues that can play into this:

1) They had 2-wire power cords, so the chassis is not grounded to safety ground. This allows some ground current among other components, depending on how exactly they are grounded.

2) They were quite sensitive. Add to that your sensitive speakers, and a very tiny amount of hum or microphonics will get amplified to a loud output from the speakers.

We don't have a really good ST-70 thread in the archive that I know of, but there are a few owners on this board - hopefully they will post. Meanwhile, there are many other forums more devoted to those classic amps. For item 2) check my article on Signals and Noise, linked off the Community page.

It is also possible that you are acting as an antenna, bringing in radio-frequency hash from the environment, and the non-conducting chassis of the Quickie is allowing that to get to the circuit. Light dimmers and florescent are often culprits.

[Paul Birkeland]

I didn't see that in the manual.

In cases where the ground side of a sources RCA jacks are floating, this will be necessary.