Interesting Bias Discovery

[Dave_C]

During rehearsal last night, I was getting frustrated with these sudden changes in tone the Glaswerks SOD II was making. After we finished, I decide to measure bias. (Easy enough to do with the handy bias taps on the back.) Sure enough, it was reading 58mA per side, even though I had just set it on Sat to 62mA. I don't like the tone of this amp when bias drifts much below 60-61mA (30-30.5 per 6L6). So, I knocked it back up to 62.

Tonight, I was jammin' away on the Glaswerks and same thing happens again. Every so often, I'm noticing these sudden changes in tone...not huge, but noticeable. Pull out the meter and now the bias is up to 66 mA. So, I start watching the bias voltmeter and my Furman AR15 meter and, sure enough, with all the power surges due to everybody's AC and other cooling cycling away, the Furman is jumping between the 120 and 125 taps which were producing about 118 and 122 VAC at the output, respectively, well within spec for the product, but enough to jump my bias from 62 to 66 mA and change the tone! I played with a couple of different bias points and the 62-66 range (31-33mA/tube) resulted in the least noticeable tone shift and the sweetest overall tone from top to bottom, so I just left it there.

But, wow, this was just another confirmation that AC regulation is super important to consistent tone!

[GT100]

Originally Posted by Dave_C

During rehearsal last night, I was getting frustrated with these sudden changes in tone the Glaswerks SOD II was making. After we finished, I decide to measure bias. (Easy enough to do with the handy bias taps on the back.) Sure enough, it was reading 58mA per side, even though I had just set it on Sat to 62mA. I don't like the tone of this amp when bias drifts much below 60-61mA (30-30.5 per 6L6). So, I knocked it back up to 62.

Tonight, I was jammin' away on the Glaswerks and same thing happens again. Every so often, I'm noticing these sudden changes in tone...not huge, but noticeable. Pull out the meter and now the bias is up to 66 mA. So, I start watching the bias voltmeter and my Furman AR15 meter and, sure enough, with all the power surges due to everybody's AC and other cooling cycling away, the Furman is jumping between the 120 and 125 taps which were producing about 118 and 122 VAC at the output, respectively, well within spec for the product, but enough to jump my bias from 62 to 66 mA and change the tone! I played with a couple of different bias points and the 62-66 range (31-33mA/tube) resulted in the least noticeable tone shift and the sweetest overall tone from top to bottom, so I just left it there.

But, wow, this was just another confirmation that AC regulation is super important to consistent tone!

Yeah, I've heard of this before with those type of voltage regulators (transformer taps).
That's why I would stear away from that design.

Lloyd

[scottl]

So what design would you recommend? No regulator is far worse.....

Originally Posted by GT100

Yeah, I've heard of this before with those type of voltage regulators (transformer taps).
That's why I would stear away from that design.

Lloyd

[Dave_C]

Originally Posted by GT100

Yeah, I've heard of this before with those type of voltage regulators (transformer taps).
That's why I would stear away from that design.

Lloyd

What design is better? I'm not aware of any other way to regulate AC but to use a multi-tap transformer. Only way to make it better would be to wind closer taps so whenever there's jitter, the voltage swing is less. But, you'd also have to provide more taps so you could cover the same input range and that increases cost. Still, are there any other regulators out there that regulate tighter than the AR15 Series II? I haven't seen any.

[Dave_C]

Originally Posted by scottl

So what design would you recommend? No regulator is far worse.....

Yeah, that's for sure! Instead of a 4-V step, you get a nice smooth 10-15 volt curve with spikes, dips and noise thrown in for good measure!

[michael.e]

Whoa, this amp is THAT suceptable to voltage swings?

[Dave_C]

Originally Posted by michael.e

Whoa, this amp is THAT suceptable to voltage swings?

They all are, but if the amp is producing mush to begin with, you won't pick it up as easily. It also depends what bias point the amp is centered at. The swing from 62 to 58 mA was more noticeable than that from 62 to 66 mA.

In any case, one volt at the input is about 4 volts at B+. So, a 4V swing on the input is 16V at B+. It's like having your plate voltage swing between 450V and 466V. You don't think that effects operating points and tone?

[GT100]

Originally Posted by Dave_C

What design is better? I'm not aware of any other way to regulate AC but to use a multi-tap transformer. Only way to make it better would be to wind closer taps so whenever there's jitter, the voltage swing is less. But, you'd also have to provide more taps so you could cover the same input range and that increases cost. Still, are there any other regulators out there that regulate tighter than the AR15 Series II? I haven't seen any.

I'm no expert, but, can't you get designs that use solid state control instead?

Lloyd

[rhythmrocker]

Wow - another question would be:
are solid state power amps subject to those power fluctuations?

Note: I read somewhere that Brad Paisley? used a variac(s) to keep the voltages to his amps constant AND had a tech monitoring that variac(s) so that any wall voltage fluctuations would be compensated for by that monitoring tech. We need less issues here, not more . . .

OK then . . .
So let’s follow up on this.

Note that I am not looking at the practicality of any solution. I am only looking at the possible solutions. Practicality can be addressed after the research is done (e.g. if you gig anywhere on a regular basis that has consistently bad voltage fluctuations, then have someone bring the extra and probably heavy gear or have it wired in or whatever).

1) Use the Amp Maniac. Plug it into an extension and set it somewhere where it can be read during the show so that manual adjustments can be made. http://shop.vintagesoundworkbench.com/product.sc?productId=5&categoryId=1 Here’s a pic:

2) Use one of these, a voltage regulator that (If I read it correctly) holds output voltages to a +/- 1.0% or less with input variations as great as +/- 10 to 20% (see spec sheet: http://www.solahd.com/products/powerconditioning/pdfs/operchar.pdf Here’s the website: http://www.solahd.com/products/powerconditioning/cvs.htm Here’s a pic:
Don’t know which model though or if this thing needs to be hardwired to the outlet (me no read electrical so good).

Any other ideas?

[teemuk]

I'm no expert, but, can't you get designs that use solid state control instead?

Yes you can, but building a switching-type AC regulator that outputs clean sine wave is a lot harder task than controlling few thyristor switches that select alternative taps from a series transformer.



No need for generating suitable modulation signal to drive the switching devices. No need to DC rectify and filter the incoming mains to generate a proper DC power supply for the switching devices. No need to low pass filter the output signal of the switching power supply. etc.

Wow - another question would be:
are solid state power amps subject to those power fluctuations?

Yes they are, unless they use regulated power supplies.

[TD_Madden]

http://www.newegg.com/Product/Produc...a-6c554c7386aa

[RobH]

The only way to maintain a constant voltage & frequency is to use an online type UPS.

It is a dual conversion type UPS that will supply a constant voltage via a true sinewave inverter.
I have installed them on critical control systems where the power was too dirty...

They are expensive, but that is the only way you are going to get a constant AC voltage 100% of the time.

[Jef Bardsley]

Originally Posted by TD_Madden

http://www.newegg.com/Product/Produc...a-6c554c7386aa

That basically has three steps, with bigger jumps than the Furman.

[harryjmic]

I guess I'm impressed you care that much but I also question why? I'm careful about not doing stupid stuff, but at the same time I realize life happens and I make the best of what I am given. Seems like a lot of hoops to just enjoy what you do.

My amps are cathode biased and the only time it sounds off to me is when the amp has been in the car all day and it's mid January.

For outdoor gigs which could be problematic I bring a voltage regulator otherwise I just try to play better and not get caught up in the minutia.

[rhythmrocker]

Originally Posted by harryjmic

I guess I'm impressed you care that much but I also question why? I'm careful about not doing stupid stuff, but at the same time I realize life happens and I make the best of what I am given. Seems like a lot of hoops to just enjoy what you do.

My amps are cathode biased and the only time it sounds off to me is when the amp has been in the car all day and it's mid January.

For outdoor gigs which could be problematic I bring a voltage regulator otherwise I just try to play better and not get caught up in the minutia.

Your right Harry but at least my idea here in offering "solutions" is to take that journey to find out what is avaialble. Without so doing, you don't find out for yourself that "it may not really be worth it" (e.g. too heavy, hardwiring necessary, too expensive, etc.).
Thanks all.