View Single Post
Old September 28th 19, 05:34 AM posted to
Phil Allison[_4_]
external usenet poster
Posts: 469
Default Dynaco ST-35, A First

Peter Weaky Total ****** wrote:

> After a bit of research, and after learning that this driver consisted of half-a-12AX and half-a-12AU7, and not having the time to search through my tube stash, where I am sure I have several, I decided to try a pair of clear-top RCA 12AU7s from the couple of sleeves I snagged at Kutztown last Saturday ($4 each, NIB). I was warned from the research that the gain would be lower, but that there were no specific hazards in using this option.
> I dropped them in this morning, they are singing sweetly as I type. And, yes, I need to increase the volume from the preamp some to get the same volume, but not excessively much. From about 9:00 to about 10:15. Still lots to go.


** FFS, the ****** has just thrown away the entire NF loop from his amp.

The "12AX7" half of the 12DW7 input tube is THE main forward gain stage - about 100 times - now it is less than 20.

So the NFB loop, being unable to function, has collapsed allowing the overall voltage gain the amp to drop as well. Add that to the wrong bias conditions and you have an amp working in "limp mode".

Good enough for old Weaky though.

Too lazy to even go through his own tube stash.

...... Phil