Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big chief on August 30, 2020, 09:24:45 am
-
Try to find out if this component is a cap or resistor
Micamold square with 6 dots top 3 black green black bottom has 1 dot in middle silver. Shows ohm 90 shows capacitance 1.7 is there another test that will tell me thanks for any help
Have 3 in the same area
-
A capacitor. .05 I think, but not sure.
You should be able to surf up the code.
-
i called them Chicket caps
should be esay to trace out and see if their in a cap "place' or resistor place
-
I think you are describing a "domino" cap, most likely with a picofarad value. You should disconnect one lead to check accurately unless you are 100% sure no other components are parallel.
A pic is worth a thousand words.
-
Domino cap colour codes
https://technifest.com/technical-documents/resistor-and-capacitor-color-codes-complete-with-mica-capacitor-codes/ (https://technifest.com/technical-documents/resistor-and-capacitor-color-codes-complete-with-mica-capacitor-codes/)
-
photo
-
photo
Yes that’s a domino cap. Sorry picture is too low-res/blurry to take a guess at the colors
-
Black = military. Then green-black-brown = 500pf, or 500µµF, or 500 mickey-mike as some of us elders say.
-
Thank you very much
-
Hey Chief. I'm a fan of old Valcos, curious as to what model you have. And it looks like a lot of shielded cable in there?
-
It's unlikely that the cap is bad.
-
It is National 1215 Ch. 510 as near as I can tell serial number indicates 1949
Found at music store in St Pete beach Fl. I had one back in the 50’s I’ve been around Country music for 70 years love these old amps. Have a few old Gibson’s and Gretsch
Thanks for asking
-
This amp has a hum that I’m trying to locate checking caps and resistors replaced all the tubes having speaker re coned now amp help would be great
Thanks
-
hum usually comes in 2 flavors 60hz or 120hz, knowing which helps
60 usually filaments, tubes or sensitive signal near filament
120 usually sensitive parts/signals near PS wires or big signals
-
The first thing to check should be filter caps. I see the original cans have been clipped out. Are there any replacements? If so use a test cap and gator clip leads to bridge each filter cap, one at a time.
-
This is your schematic, eh? Lot of tubes in that amp! Per my earlier comment I see in the schematic several places where shielded cable was spec'd. If your filter caps check out, you may want to make sure that none of those shielded cables have gone astray. As wonderful as those old Valcos are, they are hard to work on - they crammed a lot of components on those terminal strips. It looks like a lot of caps have been replaced in the past. More often than not I find hum to be associated with a weak or broken ground connection.
-
The filter caps have been replaced and as soon as my speaker is re coned and back to me
I can continue my search as this has a field speaker
Thanks so much for all your help
-
I had a 1951 National model 1215... I found it a very challenging amp to work on. The component count per square inch is high with this one! :)
If I recall correctly, Terry Dobbs (Valco amps guru) described this as one of the earliest amps with active tone controls. (see the schematic bmccowan posted).