Bias Drift Question

[SFW]

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!

 

[harddriver]

Sounds like the Ruby STR are failing and are going into runaway bias and drawing excessive current. It might be just one or two of them going bad. If you can determine that two out of your quad have a stable bias/current draw you could run those two in either the two outside or two inside power tube sockets to get you by at 50 watts until your new quad comes in. Just remember to half your impedance value if you do this.

If you are running a 16 ohm cab you need to set your amp to 8 ohms if you run the amp on two power tubes as suggested above.

 

[FourT6and2]

Bias will drift for a while until amp is fully heated up and stable. 10 minutes isn't long enough. That said, it's possible drift can be from other sources.

 

[Fret-Shredder]

Can I ask what the plate voltage is? Just curious how you arrived at 32mA. Thanks.

 

[SFW]

I settled on 32mA because that's what Scott recommended. I've had the Rubys in the amp set in that range for four years now, and it has sounded great. Just recently started to notice that the lows were no longer defined and that the highs and mids had gotten very harsh.

To be honest, I didn't measure the plate voltage, as I trusted Scott to know where his amps sound good. (Not to be a smartass..lol!)

 

[lespaul6]

I settled on 32mA because that's what Scott recommended. I've had the Rubys in the amp set in that range for four years now, and it has sounded great. Just recently started to notice that the lows were no longer defined and that the highs and mids had gotten very harsh.

To be honest, I didn't measure the plate voltage, as I trusted Scott to know where his amps sound good. (Not to be a smartass..lol!)

32ma is perfect for longer life with that amp... it probably has 480ish PV..... scott knows

 

[Monomyth]

Put a brand new quality battery in your meter and try again, 10mins should be fine

 

[311boogieman]

Yeah - that's about right on the Plate Voltage.

What Hammered said sounds about right too.

I'd stick the probe back on there and see if it levels out.

Sadly, my SED =C= had a short life so I popped my old Mullards back in there and rebiased but couldn't get them above 26 mA so I'm off to get another pair.

fred-shredder - have you seen my Splawn biasing thread on the Splawn boards? There is a long (and short) explanation on how to bias and how to determine the range you want to be in. Pretty common though that anything between 28 mA - 38 mA is going to be fine. I actually say - don't bias to the numbers bias to the tone you want.

Put a brand new quality battery in your meter and try again, 10mins should be fine

This too. I actually was using my new meter and getting some wonky readings. Check batteries (with my old meter) and they were fine. Then I checked the fuse in the meter. Solid white

And yeah, I usually can see things leveling out after 5 min or so. I'm not an EE nor an amp tech though so carry on guys

 

[napalmdeath]

I just biased my Competition to 35mA with KT77's. The E34L's sounded best around 33, so to each their own.

PV in mine was about 429, so even 35mA is closer to 60%.. I tend to find 60-65% in most amps are about right to my ears. I found, (in my Splawn, and to my ears, anyway), 32mA sounded a bit cold and less lively. With the KT77's at 35mA really gives it more attitude.

FWIW, when I put both the E34L's, and then the KT77's in, It took a good 45 minutes or better of adjusting/playing until they stabilized. Of course, I use a Pro One, which allows you to play while dialing it in. Once I settled on 35, it's rock-solid.

 

[Fret-Shredder]

Hey 311splawndude. Indeed I have seen your tutorial. I bias using the standard formula of getting the plate voltage , dividing the tube wattage and plate voltage to get the max bias range then going with 70% of that max range. I just feel its important to put your volt meter on there and see where the plate voltage is. Cant go wrong if you do those things. I trust Scott too but it's good to go through the steps and be proficient at it. You can always tweak it a smidge hotter but the tubes will wear out faster. To each his own. Take care and be safe.

 

[napalmdeath]

Hey 311splawndude. Indeed I have seen your tutorial. I bias using the standard formula of getting the plate voltage , dividing the tube wattage and plate voltage to get the max bias range then going with 70% of that max range. I just feel its important to put your volt meter on there and see where the plate voltage is. Cant go wrong if you do those things. I trust Scott too but it's good to go through the steps and be proficient at it. You can always tweak it a smidge hotter but the tubes will wear out faster. To each his own. Take care and be safe.

70% is closer to 40mA in my Splawn. I went as high as 38mA, and it didn't sound as pleasing.... It "sounded" too hot.. Loosens up, and loses some of its tightness/focus.

I take Scott's suggested 32mA and bump it a smidge. Keeps things tight, but adds just that touch more of warmth, and a wee bit more of attitude. I know we're talking 2-3mA, (in my case), but I certainly hear it.

 

[Fret-Shredder]

Hey 311splawndude. Indeed I have seen your tutorial. I bias using the standard formula of getting the plate voltage , dividing the tube wattage and plate voltage to get the max bias range then going with 70% of that max range. I just feel its important to put your volt meter on there and see where the plate voltage is. Cant go wrong if you do those things. I trust Scott too but it's good to go through the steps and be proficient at it. You can always tweak it a smidge hotter but the tubes will wear out faster. To each his own. Take care and be safe.

70% is closer to 40mA in my Splawn. I want as high as 38mA, and it didn't sound as pleasing.... It "sounded" too hot.. Loosens up, and loses some of its tightness/focus.

Right on. I understand all amps are different. The way I explained is a good starting point. You can always adjust to taste. It's just good to know those basic readings and the parameters you have to work in. Cheers man.

 

[napalmdeath]

Hey 311splawndude. Indeed I have seen your tutorial. I bias using the standard formula of getting the plate voltage , dividing the tube wattage and plate voltage to get the max bias range then going with 70% of that max range. I just feel its important to put your volt meter on there and see where the plate voltage is. Cant go wrong if you do those things. I trust Scott too but it's good to go through the steps and be proficient at it. You can always tweak it a smidge hotter but the tubes will wear out faster. To each his own. Take care and be safe.

70% is closer to 40mA in my Splawn. I want as high as 38mA, and it didn't sound as pleasing.... It "sounded" too hot.. Loosens up, and loses some of its tightness/focus.

Right on. I understand all amps are different. The way I explained is a good starting point. You can always adjust to taste. It's just good to know those basic readings and the parameters you have to work in. Cheers man.

Absolutely. My point was, 70% in my Competition sounded kinda crappy.

But I follow the same rule of thumb as you. Having a Pro one that reads PV and mA is a great tool - especially when you can play while it's plugged in.

 

[Fret-Shredder]

Hey 311splawndude. Indeed I have seen your tutorial. I bias using the standard formula of getting the plate voltage , dividing the tube wattage and plate voltage to get the max bias range then going with 70% of that max range. I just feel its important to put your volt meter on there and see where the plate voltage is. Cant go wrong if you do those things. I trust Scott too but it's good to go through the steps and be proficient at it. You can always tweak it a smidge hotter but the tubes will wear out faster. To each his own. Take care and be safe.

70% is closer to 40mA in my Splawn. I want as high as 38mA, and it didn't sound as pleasing.... It "sounded" too hot.. Loosens up, and loses some of its tightness/focus.

Right on. I understand all amps are different. The way I explained is a good starting point. You can always adjust to taste. It's just good to know those basic readings and the parameters you have to work in. Cheers man.

Absolutely. My point was, 70% in my Competition sounded kinda crappy.

I understand.

 

[311boogieman]

I just biased my Competition to 35mA with KT77's. The E34L's sounded best around 33, so to each their own.

PV in mine was about 429, so even 35mA is closer to 60%.. I tend to find 60-65% in most amps are about right to my ears. I found, (in my Splawn, and to my ears, anyway), 32mA sounded a bit cold and less lively. With the KT77's at 35mA really gives it more attitude.

FWIW, when I put both the E34L's, and then the KT77's in, It took a good 45 minutes or better of adjusting/playing until they stabilized. Of course, I use a Pro One, which allows you to play while dialing it in. Once I settled on 35, it's rock-solid.

Yeah - I'm going to be on the lower end of the bias range too. Man, this reminds me that I really need to try some KT77s in her and also get one of these Pro One probes - then I can bias more to ear more quickly.

Hey 311splawndude. Indeed I have seen your tutorial. I bias using the standard formula of getting the plate voltage , dividing the tube wattage and plate voltage to get the max bias range then going with 70% of that max range. I just feel its important to put your volt meter on there and see where the plate voltage is. Cant go wrong if you do those things. I trust Scott too but it's good to go through the steps and be proficient at it. You can always tweak it a smidge hotter but the tubes will wear out faster. To each his own. Take care and be safe.

Oh yeah. For sure. Only takes a second to check the plate voltage. 70% is way too hot on my Splawn too. For me shit starts to get warbly and almost mushy. For a minute I thought maybe I wrote 70% in my tutorial so I had to go look. If I have it there somewhere else though please call me out on it so I can correct.

He're what I wrote though since I'm right there

https://splawnfans.proboards.com/thread ... g-tutorial