Stepped att for 10$.

[MASantos]

DACT Type 21 Stepped Attenuator Potentiometer 50K 300B - eBay (item 120473999989 end time May-24-10 11:05:02 PDT)

Check these out, they look rather nice, any reason why I should try one out? The Rk27 on my millett is suffering from a much annoying channel inbalance! One of these would fit right in!

[luvdunhill]

you might very well be trading one set of problems for another...

[n_maher]

With as much gain as the Millett has I'd never put a stepper in there. I suspect that Marc may have been hinting at this...

[MASantos]

With a 50k Rk27 I usually have the volume pot between 11-15 o'clock position for regular listening levels, using AKG 701's and a alien dac, so I guess that the gain is ok.

I ordered one, for 15$ I can give it a try, I'll report back!

[MASantos]

Well I ordered one anyway, for 15$ I can give it a try! I'll report back once it arrives!

[luvdunhill]

With a 50k Rk27 I usually have the volume pot between 11-15 o'clock position for regular listening levels, using AKG 701's and a alien dac, so I guess that the gain is ok

that all depends what clock position is zero, right?

Assuming 7:00 is zero... I'm surprised you're have tracking issues in the middle range of a rk27. I assumed it was at the start of the travel, and as Nate inferred a stepper will be a whole lot worse here in terms of step size.

[cetoole]

I am also quite surprised you are having channel imbalance with the RK27 turned up that high. Sounds like something wrong to me, either with the RK27 or elsewhere.

[MASantos]

When I turn the volume all the way up, and reduce on the digital volume of winamp, the imbalance is less perceptible, but still there.

When I move the volume knob with no music playing, I hear a "scratching sound" on the right channel (right channel is the one that sounds lower than the left one), I suspect this has something to do with the pot?

Or could it be the tubes? I have these in the amp for a long time, and the amp was unused for about 6 months.

[deepak]

All of the RK27s I've tried only had the imbalance right after the 0 position and none after that.

[cetoole]

Could be dirty, could be DC. DC could cause a change in tube bias, messing other stuff up. You might want to measure the pot, see how the resistances look when it is turned all the way up.

[MASantos]

Ok, I measured the resistances on both channels with the volume in different positions, and they have a slight variation of about 0,15k to 0,30k between them across the range, the left channel always having lower resistance. Could this small difference cause this?

I'm replacing the tubes and checking all the bias points again, will report back!

[n_maher]

Resistance of the individual channels is hardly a good measure of matching since that resistance is simply used as a voltage divider so the two wipers could be wildly different and still well matched. A better way to check and see if it's in the pot is to feed the amp a 1kHz wave and measure v-in and v-out at the pot. You can use your laptop as a signal generator for this purpose and get much better and easily interpreted results. And lest anyone think I was smart enough to come up with this on my own, Justin is the one who suggested it to me when I was fighting a similar problem on a different amp.

[MASantos]

So I changed the tubes for a pair of RCA 12FK6.

The scratching sound is gone, as well as lots of background noise, and the volume is perfectly centered!

Lesson learned: check tubes and bias before ordering new parts!

I remember reading somewhere about the differences in the millett hibrid tubes, can't find it though, does anyone have a link?

Thanks for helping guys!

[n_maher]

I almost always run 12KF6s.

[MASantos]

So the attenuator arrived, and it looks quite nice for the price. The resistors are well matched, with 0.5 to 0.02 differences across the 21 steps. I was quite impressed by such a good matching. I haven't used it yet as I don't have an amp needing this for the moment.

[cetoole]

The resistors are well matched, with 0.5 to 0.02 differences across the 21 steps.

What is that, dB, ohm, %?

[TomB]

So I changed the tubes for a pair of RCA 12FK6.

The scratching sound is gone, as well as lots of background noise, and the volume is perfectly centered!

Lesson learned: check tubes and bias before ordering new parts!

I remember reading somewhere about the differences in the millett hibrid tubes, can't find it though, does anyone have a link?

Thanks for helping guys!

Any of the MAX/MiniMAX websites will have a page on the "Millett Hybrid" tube types. Beezar also has short descriptions of each of the three tube types, their advantages and disadvantages.

As you discovered, scratchiness and imbalance on a Millett Hybrid type amp is not the fault of the RK27. Imbalance can be caused by lack of proper bias adjustment or improperly matched tubes. Bad bias adjustment will result in an off-center focus, but improperly matched tubes will usually result in one channel not sounding as lively as the other, even though the sound may be centered.

As for scratchiness, maybe Pete Millett, Dsavitsk, or Colin can explain it, but there's a scenario where high-gain tubes and or speed of the caps in certain parts of the circuit will result in scratchiness. It's almost as if the tubes are unable to keep up with the gain changes when turning the volume and scratchiness results. I've had MAXes with BlackGates that had scratchiness on any volume change, regardless of tube brand, but then almost never experienced it with ES's. Almost always, this is with high-gain 12AE6's when it happens. It's a nuisance, but does not affect the sound when the volume position is stable.

[luvdunhill]

You should compare the total series and total shunt value across each position and then convert into dB.

[MASantos]

What is that, dB, ohm, %?

This was using ohm, with a fluke 179 self ranging multimeter. Kohms, or ohms, the difference between the right and left channel values was always below 0,5 and this more in the kohm range!