Hi All,

For the past three days, I've been having this problem with my stock
Dynaco ST-70 in which sudden loud pops and crackling noises would
appear after 5 to 15 minutes of listening. When I switch-on the unit,
it'll work fine for awhile, then all of the sudden, I'd hear
"bang...pop...bzzt...bzzz...pop...". After the first bang, there is
also a 120 Hz hum in the left speaker and a 60 Hz hum in the right
channel. When switched off, the fizzing sound fades, and ends with a
soft "pop" (sounds like as if though someone gently tapped a finger on
the speaker). I use a quad of 6V6GT's with 5U4G rectifier.
Swapped-in a proper ST-70 tube set (EL-34's and 5AR4) and that didn't
help at all. Back when I was rebuilding the set, the original Black
Cat capacitors have been replaced with "Orange Drops" and electrolytic
caps in the bias supple were also replaced. The original multisection
capacitor cans tests okay on my Eico 950B R/C tester/meter. Also
checked the preamp, which is okay. Has anyone experience a similar
problem. I would appreciate suggestions. Thanks in advance.

C.W.

Casino wrote:
Hi All,

For the past three days, I've been having this problem with my stock
Dynaco ST-70 in which sudden loud pops and crackling noises would
appear after 5 to 15 minutes of listening. When I switch-on the unit,
it'll work fine for awhile, then all of the sudden, I'd hear
"bang...pop...bzzt...bzzz...pop...". After the first bang, there is
also a 120 Hz hum in the left speaker and a 60 Hz hum in the right
channel. When switched off, the fizzing sound fades, and ends with a
soft "pop" (sounds like as if though someone gently tapped a finger on
the speaker). I use a quad of 6V6GT's with 5U4G rectifier.
Swapped-in a proper ST-70 tube set (EL-34's and 5AR4) and that didn't
help at all. Back when I was rebuilding the set, the original Black
Cat capacitors have been replaced with "Orange Drops" and electrolytic
caps in the bias supple were also replaced. The original multisection
capacitor cans tests okay on my Eico 950B R/C tester/meter. Also
checked the preamp, which is okay. Has anyone experience a similar
problem. I would appreciate suggestions. Thanks in advance.

C.W.

I've seen (heard) that effect on an amp that had bad coupling caps
between the phase inverter and output tubes.

Does your unit do it in both channels simultaneously, or just one
channel? That bit of info might help to narrow it down.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

The unit had been working for less than a year. I collected original
parts from here and there and reassembled the Dynakit using original
instructions. Other than coupling caps, bias supply caps and bias
supply rectifier, everything is original. Even the internal hook-up
wires are old. The capacitor can is also old but tests okay on my
Eico 950B. Is it possible to have an electrolytic capacitor that
tests good on a tester and yet be problematic?

C.W.

(TubeGarden) wrote in message ...
Hi RATs!

How long did the unit work flawlessly before this new problem appeared?

Sounds like cap problem. Or bad connection on ground, somewhere.

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Yup.. it is completely possible for caps and cap testers to test good and
the caps be BAD. I have taken 'trashed' PSU caps, tested and measured them
will cap testers and found them to test OK but when on a real-live circuit
they actually draw about 10-20 milliamps (not a good thing) and get warm
(they also spit and crackle.)

The best way to test these FP metal Can caps is as shown in the following
procedure.

Excerpt from the 'Angela Instruments' Web page. Believe me I have used this
and sorted through bins of NOS PSU caps are beyond use. It has saved me many
times (and the related Power Transformers also):


"The process consists of applying rated voltage through a resistance (about
30,000 ohms, five watt) for five minutes plus one minute for each month of
storage. As the capacitor reforms, the voltage across the resistor will
drop measured with a DC Voltmeter across the resistor. If that voltage will
not drop below 10% of applied voltage after one hour, the capacitor is
probably beyond help"

DC Volts (less than 5% of rated applied DC voltage )
| |
(+)------------- 33K----------------
|
Capacitor section tested
|
(-)-----------------------------------

In my experience with PSU HV Caps even 10% of applied voltage is suspect
across a 33K resistor. I like to see the current go practically to zero
milliamps after let's say after 10 minutes evidenced by a very small voltage
drop across the series resistor (maybe lets say 5 volts out of 450 Volts or
1%.)

On brand new caps, they will charge real fast, and the voltmeter will drop
to millivolts and if on auto-range mode the display will eventually start to
dance around to indicate that the cap has fully charged and that it has
approached an Open Circuit
condition for DC!

If you EICO Cap tester applies the full rated DC voltage to each section in
your ST-70 cap then I hope that you are not using 7199's SOVTEK that have a
real bad reputation for not working properly and humming their life away.

If your 7199 are NOS then I would check to see if they still have life
left. This tube is world famous for being more finicky than Morris The Cat!
Also make sure that your rectifier and power tubes are also known good.

Regards,

Rich

"Casino" wrote in message
om...
The unit had been working for less than a year. I collected original
parts from here and there and reassembled the Dynakit using original
instructions. Other than coupling caps, bias supply caps and bias
supply rectifier, everything is original. Even the internal hook-up
wires are old. The capacitor can is also old but tests okay on my
Eico 950B. Is it possible to have an electrolytic capacitor that
tests good on a tester and yet be problematic?

C.W.

(TubeGarden) wrote in message
...
Hi RATs!

How long did the unit work flawlessly before this new problem appeared?

Sounds like cap problem. Or bad connection on ground, somewhere.

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Casino wrote:
Fred Nachbaur wrote in message ...

Casino wrote:

Hi All,

For the past three days, I've been having this problem with my stock
Dynaco ST-70 in which sudden loud pops and crackling noises would
appear after 5 to 15 minutes of listening. When I switch-on the unit,
it'll work fine for awhile, then all of the sudden, I'd hear
"bang...pop...bzzt...bzzz...pop...". After the first bang, there is
also a 120 Hz hum in the left speaker and a 60 Hz hum in the right
channel. When switched off, the fizzing sound fades, and ends with a
soft "pop" (sounds like as if though someone gently tapped a finger on
the speaker). I use a quad of 6V6GT's with 5U4G rectifier.
Swapped-in a proper ST-70 tube set (EL-34's and 5AR4) and that didn't
help at all. Back when I was rebuilding the set, the original Black
Cat capacitors have been replaced with "Orange Drops" and electrolytic
caps in the bias supple were also replaced. The original multisection
capacitor cans tests okay on my Eico 950B R/C tester/meter. Also
checked the preamp, which is okay. Has anyone experience a similar
problem. I would appreciate suggestions. Thanks in advance.

C.W.

I've seen (heard) that effect on an amp that had bad coupling caps
between the phase inverter and output tubes.

Does your unit do it in both channels simultaneously, or just one
channel? That bit of info might help to narrow it down.

Cheers,
Fred


Hi Fred,

There is popping in both channels, though it is louder in the one with
the noisier 7199 driver tube (confirmed by swapping). With no tubes
on the driver board, I can run the ST-70 without pops and bangs.
Then, since I don't have spare 7199's, I made an adapter to run 6JW8's
(a TV tube, apparently better sounding and less noisier than 7199s)
and/or 6GH8A's (NOS) as drivers and problem came back with tubes on
the board. So it seems that although the problem is not due to faulty
tubes, noisier ones do tend to magnify the bangs and pops. The driver
board should be fine, since coupling caps have been replaced. Could
the filter can capacitor be dying?


Yes. Especially since we've determined that it has to be something
common to both channels. ;-)

You can often narrow down electrolytic power supply cap problems by
temporarily clipping a known-good capacitor across each section in turn.
Does the problem (hum, noise, crackling) decrease? If so, there's very
possibly your cuprit. If not, look elsewhere.

Cheers,
Fred

(btw - 6JW8's are terrific tubes! Definitely one of the "sleepers of the
year.")
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+