Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smilch on December 17, 2017, 10:11:45 am
-
Hello Forum . Im sure this topic has come up before. When Wiring up an amp with a three way sector switch . EX: Marshall style with 4, 8 , 16 Ohm taps, the feedback loop is most likely connected to the tap that is planned to use . Is there a diagram showing how to wire the selector up such that if you change the selector from 8 to 16 the feedback wire is in the circuit .What if you take you head to your friends house and they have a 16 ohm cabinet and you wired the tap to the 8 ohm tap? . i can't imagine that your going to open up the amp and change the feedback wire on the 16 Ohm lug .
Food For thought .
-
read this, might help;
http://el34world.com/Forum/index.php?topic=9696.msg88380#msg88380
-
It is typical to use the same tap for feedback regardless of which output is used. In principal, the 8 ohm tap (for example) should have the same signal voltage regardless of which one is connected to the speaker-- assuming the speaker is matched to whichever tap you use, and the amp is therefore operating normally.
If you pulled feedback from different taps you'd need to make other adjustments to the feedback network to compensate for the difference in voltage-- the same power from the 16 ohm tap into a 16 ohm speaker involves 2x greater voltage than the 4 ohm tap operating into a 4 ohm speaker.
If you did want to switch it and you use a rotary switch, it should be as simple as connecting the feedback wire to the "output" of the rotary switch, or the actual output to the speaker. I'm not sure I'd recommend this for the above reasons, however.
-
2 approaches:
1. Feedback is sourced from the highest ohm secondary tap - which produces the highest NFB source voltage, regardless of speaker load used.
2. Use a rotary SW to match the OT secondary taps to the speaker load, and also source NFB from that secondary tap > SW output lug. The theory for #2 is that NFB voltage will correlate to the speaker voltage.
-
The 4/8/16 taps are 99.9% identical except for voltage. Pick one, any one, it will NFB the amp on all taps.