Re: [Harp-L] sound

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

Hi Stephen,

If the filter caps have never been changed, then it is a good idea to get these 
replaced, along with the cathode bypass cap at pin 8 of the power tubes (use a 
50-100v rating here).

But first step will be to determine whether your hum is related to caps 
specifically, or not, as filter caps can go bad long before the amp hums 
objectionably. To do this unplug the amp from the wall & pull out the 12AX7 
tubes...replug the amp into the wall & power up (you can't damage the amp by 
doing this), does it still hum? If no, the hum is related to the preamp 
circuit/tubes. Power down & replace the 12AX7 nearest the power tubes, power up, 
any hum? If yes, try a known good 12AX7. If no, repeat process with the 12AX7 
farthest from the power tubes.

If the amp hums with no preamp tubes fitted, then the hum relates to the power 
tubes, power transformer, or heaters. When trying new/different 6V6s be sure to 
replace both tubes at the same time, do not try these one at a time. If it's not 
the power tubes themselves, then also check that R37 & R38 (both 68ohms) are not 
charred or burnt - these are the virtual centre tap for the heater wiring, if 
one of these has blown open the amp will hum...but if this was the case, it 
should hum irrespective of the volume control setting.



________________________________
From: Stephen <boogieblue@xxxxxxx>
To: harp-l@xxxxxxxxxx
Sent: Sunday, 12 December, 2010 22:47:34
Subject: [Harp-L] sound

Harp brothers, I just picked up a silvertone 1472 one 12 great 

Condition. Now ,but it has a slight hum when I turn the 

Volume pass 2.5. have been suggested to check or change 

The caps any other thoughts?