Currently running 4 x KT120 in my M125's (2 per amp). I recently picked up an extra pair of GZ37 rectifier as spares, figured before the prices go up again. Installed them to make sure they were both OK. But forgot to recheck the bias. So I throw my meter on today and I'm at only .38 to .39V across all 4 bias ports. But I've been listening to the amps for 2 days now and didn't notice any difference? Why is that?
4 posters
KT120 bias voltage
LeGrace- Posts : 390
Join date : 2016-08-07
Location : Ontario, Canada
Bob Latino- Admin
- Posts : 3272
Join date : 2008-11-26
Location : Massachusetts
LeGrace wrote:Currently running 4 x KT120 in my M125's (2 per amp). I recently picked up an extra pair of GZ37 rectifier as spares, figured before the prices go up again. Installed them to make sure they were both OK. But forgot to recheck the bias. So I throw my meter on today and I'm at only .38 to .39V across all 4 bias ports. But I've been listening to the amps for 2 days now and didn't notice any difference? Why is that?
Anytime you change a rectifier or an output tube, you should check the bias. Different rectifiers will give slightly different B+ DC high voltages and will require minor adjustments to each bias pot.
Unless a bias setting is way LOW, you may not notice any difference in the sound of the amp. Output tubes have a fairly wide range over which they will sound good. If the bias setting is way too HIGH, you may start to notice "red plating" on the output tube's plates. Red plating may also be present when or an indication that, an output tube is just plain worn out ...
Check the video below for an example of red plating ...
Bob
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
Generally, unless Zero, and generally, as long as all (tubes) are biased the same, improper bias will be inaudible at normal volumes. What will happen at the extremes is that the tube will run either too 'hot' or too 'cold'. Which will materially affect tube wear, headroom (clipping onset) and overall tone. But in the general rage of "good-to-bad" the major effects are inaudible at normal volumes.
There are lots of detailed articles out there on this. But for a broad-brush, this is what you need to know. Fix it, you should be fine.
There are lots of detailed articles out there on this. But for a broad-brush, this is what you need to know. Fix it, you should be fine.
bluemeanies- Posts : 274
Join date : 2015-02-09
Age : 74
Location : Folsom Pa.
Tubes are designed to operate within a defined set of parameters.
I have read that tubes bias settings fluctuate within a 10% tolerance, but a 66% variance is simply too high.
Never let your ears be the equation when determining whether a bias setting is to high or low no matter how it sounds to you.
Currently I am running 4 KT88's in each m125 with a bias setting fluctuating between .990-.1000
I intend to back down to only using 2 KT 88's for each m125 at a bias of .500.
I have read that tubes bias settings fluctuate within a 10% tolerance, but a 66% variance is simply too high.
Never let your ears be the equation when determining whether a bias setting is to high or low no matter how it sounds to you.
Currently I am running 4 KT88's in each m125 with a bias setting fluctuating between .990-.1000
I intend to back down to only using 2 KT 88's for each m125 at a bias of .500.