Finished My ST-70

knowledge base for the classic Dynaco ST70

Postby kt88pppamp » Sun Nov 30, 2008 3:26 pm

Can tube testers check for filament to cathode leakage?
User avatar
kt88pppamp
KT88
 
Posts: 530
Joined: Tue Sep 26, 2006 12:11 am
Location: Rochester, NY

Postby Ty_Bower » Sun Nov 30, 2008 3:36 pm

The Eico 666 has a big "H-K leak" switch on it.
User avatar
Ty_Bower
KT88
 
Posts: 1494
Joined: Wed Mar 21, 2007 2:50 pm
Location: Newark, DE

Postby EWBrown » Sun Nov 30, 2008 5:25 pm

Even the most basic emission tube testers can perform an inter-elements "shorts" test. The usual indicator is a neon bulb. These should have a "short" button or slide switch, and really egregious shorts will light it up without any operator input. Yellow_Light_Colorz_PDT_11 :o

If there is no tube tester available, just use an ohm meter, at least it will show "cold" shorts or high resistance "leakage" on any tube.

The better testers like the Eico "satan special" 666 and the Hickok testers can perform much more accurate and specific leakage testing.

/ed B
Real Radios Glow in the Dark
User avatar
EWBrown
Insulator & Iron Magnate
 
Posts: 6389
Joined: Wed Mar 19, 2003 6:03 am
Location: Now located in Clay County, NC !

Postby mugsy » Mon Dec 01, 2008 10:11 pm

Are you sure it isn't merely tube microphonics from a vibrating mains transformer? That would change level depending on the tubes tried and how resistant each design is to vibration.

I mentioned this before, but it looks from your close up pic that the mains trannie is mounted directly to the chassis which would likely cause alot of vibration in the chassis, and quite possibly hum due to the valves vibrating. It did with mine and it lessoned considerably mounting the transformer on rubber washers.

- John
mugsy
 
Posts: 59
Joined: Fri Sep 07, 2007 5:36 pm

Postby kt88pppamp » Wed Dec 03, 2008 8:02 pm

The other day I finished mounting my power transformer on rubber washers. To avoid them from being crushed from the torquing of the nuts, I mounted some thin metal washers over top of the rubbers. The rubber washers did not seem to help with the humming so I tried changing around some of my 6SN7s. Sure enough, the loudest hum followed one of the 6SN7's to the other channel. Then I tried one of my EH 6SN7s. That made the hum so loud it could be heard throughout my workshop! It has to be the tubes.
User avatar
kt88pppamp
KT88
 
Posts: 530
Joined: Tue Sep 26, 2006 12:11 am
Location: Rochester, NY

Postby crispycircuit » Sat Dec 06, 2008 11:30 am

I'd try a couple of hum balance controls (1 for each channel). 100 ohm 5 watt pots. Wire them temporarorly with them hanging out the bottom to get at them. This might be the easiest fix..... I'm guessing is the input tube will be most important. So look to see which heater winding is powering the input tube and try a hum balance control on that first. Good luck.........
crispycircuit
 
Posts: 104
Joined: Tue Mar 20, 2007 9:32 pm

Previous

Return to st70

Who is online

Users browsing this forum: No registered users and 19 guests

cron