Interesting Stiletto bias observation

I just picked up a Stiletto Ace combo and was checking the bias with my Weber Bias Rite meter. In doing this I found that the amp bias changed when switching the channels. It was biased colder when on the clean channel than the dirty. With the stock Mesa tubes I was getting 24ma per tube on the clean channel and 34ma on the dirty. Anyone else experience this? I wonder if they are switching the bias resistor via relay when the channels change?

fjs1962 said:

I just picked up a Stiletto Ace combo and was checking the bias with my Weber Bias Rite meter. In doing this I found that the amp bias changed when switching the channels. It was biased colder when on the clean channel than the dirty. With the stock Mesa tubes I was getting 24ma per tube on the clean channel and 34ma on the dirty. Anyone else experience this? I wonder if they are switching the bias resistor via relay when the channels change?

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You don't have one of your channels on tube rectifier and the other on silicon diodes, do you? This will alter the plate voltage as well as the current draw. The Bold/Spongy switch will also do this.

JOEY B. said:

You don't have one of your channels on tube rectifier and the other on silicon diodes, do you? This will alter the plate voltage as well as the current draw. The Bold/Spongy switch will also do this.

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Good question. I don't remember where I had the recto set, but power was on full. I'll have to do some further investigation when I get another chance. 9-10 ma seems like a pretty big difference for the recto to make.

OK, I went back and did more thorough testing. The results are very interesting. First let me say the Mesa STR447 power tubes are a very close match, and in all cases weren't more than 0.5 ma apart! Here's what I measured, in plate voltage/current:

Full power, SS rectifier:
channel 1 492/45
channel 2 495/40

Spongy power, SS rectifier:
channel 1 398/32
channel 2 400/25

Full power, tube rectifier:
channel 1 458/32
channel 2 465/23

Spongy power, tube rectifier:
channel 1 368/17
channel 2 370/13

I think what I was running when I first tested was full power, tube recto on both channels and I had the ma for the channels backward in my first post. For all tests I had the amp idling with no input signal and the volume controls all the way down. The amp sounds great on all settings, so I guess this stuff doesn't matter too much, but I thought it was interesting that it varies so much.

rabies said:

so according to this, your stiletto runs hot @ ~500V and cold @ 400V and cold @ 375V....

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Yeah, that's what I got from it too. I guess with all the different power possibilities it's kind of hard to come up with one bias setting that works all the time.

The interesting thing is there are some differences in the voltages when switching channels even with the power setting staying the same...

interesting. i know the maverick runs the power tubes at a higher voltage for the clean channel compared to the distorted channel. i don't know about the bias change though.

fjs1962 said:

The amp sounds great on all settings, so I guess this stuff doesn't matter too much, but I thought it was interesting that it varies so much.

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As long as there's no crossover distortion and nothing is redplating, the actual bias doesn't make any difference other than how you like the sound.

Good stuff! I wonder what my Recto is running @ with 6L6's.

Oct 31. Figured it was a good day to revive a post from the dead.
I just got an adjustable bias modkit from granger today. Haven't even pulled the chassis yet and don't know when I'll be able to but I was just snooping around online in advance to see what the plate voltage on a Stiletto Ace is and WOAH!!! That's quite the swing in modes and rectos right there. HOLY!
I know that by ear is the best way to finetune your bias once you're in the hood but where are y'all biasing your Mesas that have the selectable voltage and rectifiers etc? Are you setting it in full bias diode mode? Yeah obviously are unless you want to melt your tubes quickly if/when you do kick it over to fullpower/diode modes.
It'll be cool to see/hear the differences.
Oh...so if you bias at say 70% dissipation (I usually go just a little under that) at full power with diode rectification, when you engage the "variac" and the plate voltage drops does the dissipation also "compensate" so to speak and maintain the ~70%? Meaning, when you switch to spongy and let's say ALSO tube recto is it still "properly biased"?

There was something else I was meaning to ask.
Oh...probably should start a new thread but...the stupid preamp tube jackets/sleeves and their springs...GEEZUS!
I put a sweet old Mullard in V1 and a Dutch Philips in V2 a few days ago and didn't try it out. Maybe I did post this last night??
Don't forget. It's a Stiletto and I'm an old Marshall guy so I want it to run "proper" glass at "proper" bias.
I get to rehearsal last night and the amp is silent :-(
I was using a new speaker cable I had made/repaired so thought maybe it was to blame.\To make a long story short...the tube covers cracked both tubes!! Well I did I suppose...not the amp. Dammit! Stupid amp owes me around $150...well $100 for SURE!
GOD!

EDIT: Sorry...now I remember what it really was.
Where have any of you who have done it, did you mount the wee pcb...or in the case of just using your own resistor and trimpot, where did you mount them?
Thanks