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New OT = Howler Monkey?

8/12/2004 9:23 PM
Drew
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New OT = Howler Monkey?
I've been prototyping a PP EL84 amp that sounds very good. the only problem is, it eats OT's. I've been through 2 inexpensive/generic Deluxe OT's (8k:8). The circuit is fairly tame - 304 plate, 287 screens, 150 ohm shared bias resistor, so I think the transformers are not up to snuff. I've also recently heard of several other 18 watt EL84 designs eating OT's (RI AC15's for instance).  
 
So, in the search for better iron, I tried a Bandmaster/Super replacement (4k:4) but using an 8 ohm speaker for, I assume, an 8k primary impedance.  
The problem is, on power up with the new transformer, the previously well behaved, quiet amp howls like crazy. The amp has no NFB, so, I don't think the problem is PFB. Just to be safe, I reversed plate leads, but the howl is still there.  
 
I am starting to think that the bigger core has the LF rolloff so low that I am getting oscillation or, maybe there is some flyback problem. Any ideas?

 
Replies:
Carl Gigun Just curious, what does the amp do ... -- 8/13/2004 11:25 AM
Drew BTW, I should have mentioned that t... -- 8/14/2004 6:37 PM