Nitro Bias and Tubes/Power (50w/100w) Question

Bash_Man

Bash_Man

Well-known member
so after figuring out bios in 50watts mode and 100watts mode, etc.. I have a couple some questions:
1 - When biasing the amp in 100 watts mode (full power), I can bias the tubes to approx 34mA (Plate voltage is 490). however when I switch the amp to half power mode (50watts), it seems to lower the bias rating on the 2 outer tubes. The 2 middle ones dont measure anything with the Multimeter. so in this case, do I bias it to the "recommended" bias (34mA) at the mode i am staying with full time? or do I bias it at that at 100watts mode and let it do the "math" it self when i switch it to 50watts mode?

2 - When switching between 50 watts and 100watts, do the 2 middle power tubes still get used? or do they just turn ON, heat up and have no involvement in the tone/sound, etc.. ? Reason I ask this is.. when it comes time to swap tubes out and I use the amp in 50watts mode, can I simply swap the tubes around (put the 2 middle power tubes to the outside and vice versa) and pretty much have 2 "fresh" tubes in the middle?

Thanks
 
Hey Bash_Man - I'll take a stab at this but parts of what you are saying see off (not you...the amp).

Unless the Nitro is different, it should be the middle pair that are active not the outer pair. If you are primarily in 100w mode, then bias the amp for that mode and then just use your half power switch as needed. When in half power mode, the other pair is disconnected from the circuit but they will still light up as the heaters are active. They wont' affect tone/sound. No worries. If you plan to run at 50 watts for longer term then pull the outer pair of tubes completely out, cut your ohms in half, and rebias. That second pair will actually stay 'fresher' outside of the amp.

Not sure if that answers your questions. I made a biasing tutorial if you have not seen it. I personally would call Scott on the outer pair being active in half power mode. Not really the end of the world or anything - I doubt it makes much difference. It may be a KT88 thing. What kind of tubes are you running and have you dropped ohms to cab at all in half power?

PS: I'm only aware of the Quick Rod variant and EL34s.
 
Last edited:
311splawndude said:
Hey Bash_Man - I'll take a stab at this but parts of what you are saying see off (not you...the amp).

Unless the Nitro is different, it should be the middle pair that are active not the outer pair. If you are primarily in 100w mode, then bias the amp for that mode and then just use your half power switch as needed. When in half power mode, the other pair is disconnected from the circuit but they will still light up as the heaters are active. They wont' affect tone/sound. No worries. If you plan to run at 50 watts for longer term then pull the other pair of tubes completely out, cut your ohms in half, and rebias. That second pair will actually stay 'fresher' outside of the amp.

Not sure if that answers your questions. I made a biasing tutorial if you have not seen it. I personally would call Scott on the outer pair being active in half power mode. Not really the end of the world or anything - I doubt it makes much difference. It may be a KT88 thing. What kind of tubes are you running and have you dropped ohms to cab at all in half power?

PS: I'm only aware of the Quick Rod variant and EL34s.
Click to expand...
Thanks 311splawndude. I'll have to double check but I was certain it was the outside pairs that were active. I had a local amp tech add external bias points and bias adjustment dial so I don't need my old weber bias probe or even take the amp out of it chassis. I'll send Scott an email to confirm the tubes used in 50 watts mode.

So long as I can bias the amp in 100watts mode and use the 50watts mode without an issue, then that's great. It'll help save on buying tubes. I put JJ EL34II matched quad in there.
 
OK. If you had a tech wire up test points I suppose maybe there is more going on. I think even any standard Marshall amp the inner pair work together and the outer pair work together and people pull the outer tubes when going half power. Doesn't mean it is wrong on your amp. Maybe ask that tech if he is still around. I'd be curious. I'm not sure there is any advantage one way or the other.

And yeah, I went to 50w more or less permanently years ago because I couldn't tell a difference and wanted to save on tubes. Scott told me to pull the outer pair, drop ohms and rebias. May the force be with you. :yes:
 
311splawndude said:
OK. If you had a tech wire up test points I suppose maybe there is more going on. I think even any standard Marshall amp the inner pair work together and the outer pair work together and people pull the outer tubes when going half power. Doesn't mean it is wrong on your amp. Maybe ask that tech if he is still around. I'd be curious. I'm not sure there is any advantage one way or the other.

And yeah, I went to 50w more or less permanently years ago because I couldn't tell a difference and wanted to save on tubes. Scott told me to pull the outer pair, drop ohms and rebias. May the force be with you. :yes:
Click to expand...
Thanks buddy. Appreciate the responses. Let's see what Scott has to say. I can also test with my Weber bias probe and switch it to 50watts. Technically I should only get ready from the middle tubes if it's in 50 watts mode. No?

Reason I use 50watts mode is it's plenty loud in that mode.. and I actually like the overdrive sound in that mode. But that's just me.
 
@Bash_Man

I would put the bias probe on to double check.
The bias test points he put in should be completely passive.
I'd love to see pics of how he did it and the variable resistor (PM me)
With what I know, yes, you should technically only be getting bias readings off the middle pair when in half power.
Again, not saying it is wrong if yours is the other way around. The only thing I can think of is that maybe Scott is using the outer pair on Nitros and Pro Mods because of the KT88s? To keep them cooler?

Yeah, in 50w mode, you will have less head room so the power tubes are compressing and doing their thing sooner. I still can't hear a difference though :)
 
311splawndude said:
@Bash_Man

I would put the bias probe on to double check.
The bias test points he put in should be completely passive.
I'd love to see pics of how he did it and the variable resistor (PM me)
With what I know, yes, you should technically only be getting bias readings off the middle pair when in half power.
Again, not saying it is wrong if yours is the other way around. The only thing I can think of is that maybe Scott is using the outer pair on Nitros and Pro Mods because of the KT88s? To keep them cooler?

Yeah, in 50w mode, you will have less head room so the power tubes are compressing and doing their thing sooner. I still can't hear a difference though :)
Click to expand...
I'll try to test it out this week at some point if possible and get back to you with some results and hopefully pics as well.
 
Bash_Man said:
I'll try to test it out this week at some point if possible and get back to you with some results and hopefully pics as well.
Click to expand...
Sounds good. Again, I'm not an expert in this area just trying to help you think through it.

You could always post your question on the main forum. This question is not really unique to Splawn and there are some very smart people here and some that build amps very similar to Splawns. Shea of Monomyth or MichaelR/T for example.
 
311splawndude said:
Sounds good. Again, I'm not an expert in this area just trying to help you think through it.

You could always post your question on the main forum. This question is not really unique to Splawn and there are some very smart people here and some that build amps very similar to Splawns. Shea of Monomyth or MichaelR/T for example.
Click to expand...
Hey I appreciate you responding and offering your input. Thank you :2thumbsup:
 
So i went back and rechecked things.. and I had made a mistake and you guys were correct. When switching to the 50watts mode, the outside tubes dont provide a reading, only the ones in the middle. This is even on the bias test points the amp tech installed for me.

So the question I have is this.. in 100watts mode, I biased the tubes to around 34.5 and they seem to hover around .5 to .7 but stay within the 34mA range. If I switch it to 50watts mode and I do a check on the bias, it shows the tubes at 39mA range for the 2 middle tubes. if I switch back to 100watts, it shows 34mA range.

do the different modes affect the tubes bias like that?
 
I believe 50 watts goes into B+ mode and it will run hotter, my Supersport is in B+ full time and Scott said 40ma is optimal, so I went to 38ma just to be safe (although I don't know what 2ma will do but it makes me feel better :) )
 
kmanick said:
I believe 50 watts goes into B+ mode and it will run hotter, my Supersport is in B+ full time and Scott said 40ma is optimal, so I went to 38ma just to be safe (although I don't know what 2ma will do but it makes me feel better :) )
Click to expand...
is that the case for older "1st gen" Nitros? mine is back from 2007 or so.
 
Bash_Man said:
So i went back and rechecked things.. and I had made a mistake and you guys were correct. When switching to the 50watts mode, the outside tubes dont provide a reading, only the ones in the middle. This is even on the bias test points the amp tech installed for me.

So the question I have is this.. in 100watts mode, I biased the tubes to around 34.5 and they seem to hover around .5 to .7 but stay within the 34mA range. If I switch it to 50watts mode and I do a check on the bias, it shows the tubes at 39mA range for the 2 middle tubes. if I switch back to 100watts, it shows 34mA range.

do the different modes affect the tubes bias like that?
Click to expand...
yes
 
Bash_Man said:
Thank you. so would the recommendation be if I am using 50watts mode, is to just bias it in 50watts mode?
Click to expand...
Scott told me that if I plan to play mostly in 50w mode (which I do) them pull the outer tubes, drop the ohms in half, and rebias.

But the huge deciding factor IMO is how does it sound and which do play in more often (50 or 100)? If the answer is you desire 50 watt mode for more power tube compression, lower headroom and to save on tubes then dial it all in that way. I mean, you can always go back to 100w. I do periodically but usually when I have the amp out of the chassis when rebiasing etc.

I think you are asking is it better to bias in 50w mode and then switch back and forth or bias in 100w mode and then switch back and forth? Because I think it is safer to bias on the cooler side then biasing your amp at 50w would be better. If that's the question. But really do what sounds good to your ears. There is probably a happy medium.
 
311splawndude said:
Scott told me that if I plan to play mostly in 50w mode (which I do) them pull the outer tubes, drop the ohms in half, and rebias.

But the huge deciding factor IMO is how does it sound and which do play in more often (50 or 100)? If the answer is you desire 50 watt mode for more power tube compression, lower headroom and to save on tubes then dial it all in that way. I mean, you can always go back to 100w. I do periodically but usually when I have the amp out of the chassis when rebiasing etc.

I think you are asking is it better to bias in 50w mode and then switch back and forth or bias in 100w mode and then switch back and forth? Because I think it is safer to bias on the cooler side then biasing your amp at 50w would be better. If that's the question. But really do what sounds good to your ears. There is probably a happy medium.
Click to expand...
I always play My Nitro in 50watts mode. Its plenty loud, even though I have the master volume (on rear) at 3 o'clock and my main volume for channel 2/3 at just before 9 o'clock.

Would leaving the 2 outer tubes in the amp while in 50 watts wear out those tubes at all? I know they still glow and produce heat, which I am would imagine wears them out over time, but perhaps not as much as the ones getting used in the middle for 50watts mode.
 
Bash_Man said:
I always play My Nitro in 50watts mode. Its plenty loud, even though I have the master volume (on rear) at 3 o'clock and my main volume for channel 2/3 at just before 9 o'clock.

Would leaving the 2 outer tubes in the amp while in 50 watts wear out those tubes at all? I know they still glow and produce heat, which I am would imagine wears them out over time, but perhaps not as much as the ones getting used in the middle for 50watts mode.
Click to expand...
Correct.
 
You must log in or register to reply here.
Back
Top