Mini-toggle hum changes with tube type

[sthebluesman]

This is a follow-up from my earlier post asking about extra hum in the up and down positions of the mini-toggle. One observation I made this weekend was when I put in a 6072 (12AY7) tube in V1, the hum reverses. With that tube, I have more hum in the center position and less hum in the upper and lower positions. I thought that was interesting and might be a clue to you guys to understand what I am experiencing. I still have not gotten my cabinet yet. I am thinking that once I get everything in the cabinet, the extra noise will not be noticeable in the boosted positions. Right now, I have both speakers sitting on a table so any extraneous noise is in my face.

[Robin]

Assuming that it’s wired correctly, changing the bias to V1 (that’s what the mini “boost” switch does) can increase (or decrease) noise, probably more like hiss than hum though. Which makes sense, just as raising or lowering gain can increase of decrease noise (or hiss). Changing from a 12AX7 to a lower output (less gain) tube like a 12Ay7 can decrease hum/hiss. Both of my 2Strokes are really quiet (with 12AX7s), but there is a detectable increase in “noise” when the highest gain option is selected with the “boost” switch. It is possible that when you get the chassis buttoned-up in a cabinet with a shielded cover over the exposed side you will have a reduction in noise.

[sthebluesman]

You are right. It is more of a hiss. I’m sure everything is OK. The amp sounds good in the boost positions. I just noticed the extra hiss and was checking to be sure I had done everything correctly.

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