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Bias Drift Question

SFW

Member
Messages
1,393
So I was going to retube my 2005 Splawn QuickRod this afternoon. It's been running the same set of Ruby STRs in it for the last four years. I had noticed that the amp was starting to sound harsh. Popped in a set of GT Mullard reissues from a few years ago, and they were so far out of range I couldn't get them below 42mA. So I turned the amp off and after cooling down, I re-installed the Rubys- until I can pick up a new set. Went to just the bias back to 32mA, and noticed that after a few minutes, the bias began to climb. It got to 36mA and I readjusted it back to 32mA. Once again, the bias began to climb. I let it get to 38mA before I shut the amp off.

All readings were with the amp idle. No signal going through. I let the amp warm up for 10 minutes before I took any readings. I've never seen an amp do this. Wanted to see if anyone has had a similar experience. I definitely think I will be ordering new tubes this evening. Any input from you tube amp gurus would be greatly appreciated. Thanks!
 

sickboy79

Member
Messages
12,900
Bias can sometimes fluctuate for a few minutes as the amp warms up. How long did you let it idle and watch it? That said, seems like 6mA over a few minutes is a lot pretty fast. Probably a good call to shut down.

If you retube, set the bias and let it idle for 15-20 minutes. The initial bias will sometimes drift a bit during this time.
 

DonaldDemon

Member
Messages
8,646
I've seen that happen on my Splawn and other amps as well. I always end up returning those tubes because it usually means there is a leak or some other issue. If it drifts that much in a short time and continues to climb then that tube is no good imo. QC on tubes just isn't what it used to be and they aren't as robust as they once were.
 

SFW

Member
Messages
1,393
I had let the amp sit on standby for about 20 minutes for the first set of tubes. They stayed constant, but were at 42mA and I couldn't get the bias to adjust down any further. They were just too hot for the amp. I know that I can change the resistor to assist with getting in in the correct range, but I have heard that the Groove Tube EL34M tubes don't last long these days. My set was purchased two years ago, but I'm not sure I want to replace components in the amp for a set of tubes that may not last.

The Rubys that I put back in I let sit on standby for about 10 minutes before I pulled a reading. At the rate they were drifting, I didn't want to take any chances. I'm guessing that's why my amp was not sounding good anymore. The bottom end had gotten flabby and the highs and mods were extremely harsh and grainy.
 

support

Member
Messages
700
There's a pretty obvious clue in your description of events.

"I had noticed that the amp was starting to sound harsh"

Probably because the Ruby power tubes were already causing issues. Then you changed the tubes and *then* tested the bias (based on my understanding of your event timeline) only to find you couldn't get a decent bias setting with those *used* tubes. The implication is that you didn't test the Ruby's before replacing them. If that's true, and you're assuming your other used tubes should be good, that may be your problem. Your assumptions are getting in the way of reality. Both sets are likely bad. So before complicating things further, get a known good set of power tubes and see what happens.
 

HotBluePlates

Member
Messages
10,002
... bias back to 32mA, and noticed that after a few minutes, the bias began to climb. It got to 36mA and I readjusted it back to 32mA. Once again, the bias began to climb. I let it get to 38mA before I shut the amp off.

... I've never seen an amp do this. ...
I've seen it plenty, especially in fixed-bias amps. Tube idle current drifts a bit over time. It is alluded to in different places in RDH4, with the overall recommendation to allow a tube to burn-in for 24-48 hours to stabilize.

You will see a lot less drift in cathode-bias applications, as the cathode resistor tends to being self-correcting.
 

SFW

Member
Messages
1,393
There's a pretty obvious clue in your description of events.

"I had noticed that the amp was starting to sound harsh"

Probably because the Ruby power tubes were already causing issues. Then you changed the tubes and *then* tested the bias (based on my understanding of your event timeline) only to find you couldn't get a decent bias setting with those *used* tubes. The implication is that you didn't test the Ruby's before replacing them. If that's true, and you're assuming your other used tubes should be good, that may be your problem. Your assumptions are getting in the way of reality. Both sets are likely bad. So before complicating things further, get a known good set of power tubes and see what happens.
Perhaps I was not clear in my original post. The Groove Tubes EL34M quad that I installed when I felt it was time for new tubes where purchased two years ago, but never used. They have been setting on a shelf in my office. So basically, they are “new” tubes.

When I installed them and gave them a while to warm up, they were running hotter than I wanted a set of tubes to run. (42mA) I was unable to sweep the bias pot to a mA measurement that I was comfortable with. So I shut the amp off and pulled them. Yes, I am aware that I can change the bias resistor to make these tubes set in the correct bias range. I do not wish to do this.

When I reinstalled the used set of Ruby EL34BSTR tubes, it was with the intention of ordering some more new tubes. Upon seeing the rapid bias swing, I shut the amp off before these tubes could cause damage to the amp.
 

HotBluePlates

Member
Messages
10,002
... Upon seeing the rapid bias swing, I shut the amp off before these tubes could cause damage to the amp.
"Rapid swing" from 32-38mA is "nothing."

I've had a pair of 6L6s that on power up, the bias climbed to within ~2 seconds to 100mA (on 1 tube) and was still climbing when I shut off power to the amp.

Your situation may not be ideal, but it's not unusual either. Most are completely unaware of what current their tubes are drawing.
 

support

Member
Messages
700
BSTRs tend to run hotter on average than other typical 34s.

EDIT: In a weak/aged tube, transconductance goes down due to reduced cathode emission. As a tube heats up overall (not just the cathode), emission increases slightly. In a weak tube, this total system heating effect on cathode emission is increased noticeably. This could be called "bias drift" though that's a pretty generic term. Which is to say, there isn't anything abnormal going on here that a good new tube probably wouldn't fix. In a 30-50 year old amp, component leakage might be a logical place to look, but highly unlikely in this case.

Are the GT rebranded Russian Mullard RI or what GT calls their Mullard copy? If the latter, I’d recommend getting a set of the current Mullard branded offering. They’re reliable, sound good and transconductance is where it should be, so should give you no problems when biasing.
 
Last edited:

Red House

Gold Supporting Member
Messages
1,999
Maybe the root of your problem is a bad/leaky coupling cap to the grids of the power tubes.
 




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