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**frus** · 10-30-2014, 07:27 AM

so the difference between the "hottest" and "coldest" tube at idle is 10%, and under maximum drive it's 14%
not THAT much of a difference

**nevetslab** · 10-30-2014, 04:46 PM

After moving the tubes around to even out the differences, and re-adjusting the bias voltage to get there, it's not that big of a difference. Where it started, with the coldest tube running 14.3mA and the hottest at 28.5mA, that was nearly 99%.

In the distribution after balancing the two other used tubes, it was interesting to see the Svetlana tube running around 9% above average (of the four Sovtek's) under drive. Might be the Svetlana hadn't run as many hours as the Sovtek's. One of those and the Groove Tube behaved much the same.

I thought I saw a comment of the Ampeg SVT-CL/AV's Window Comparator for the Bias LED's GRN range to be around 8mA. It's more like 14.7mA, computing the two voltages of the window comparator's voltage reference string (147mV on the Low side (NO LED LIT), 294mV on the High Side (RED LED LIT). That's 100% change, so it is tolerant of a wide difference of current in the two triads of tubes in the output stage. With 14.7mA being at the low threshold, 22.1mA is the center of the range. I'm usually setting them up in the 22-24mA area, and move tubes around to get as good of a balance as I can within the sextet. Maybe it's not worth all the trouble, but it seems like it would be better to get the load sharing as even as possible....thinking in the long run, it may extend the overall life of the sextet. I've never tracked them that far, but usually have the idle current values recorded in my service notes for later reference.

**TimmyP1955** · 10-30-2014, 09:52 PM

On one hand, I'm surprised that the comparator window is that broad, given the number of good pulls I've had that would not work with the five good ones in an amp.

On the other hand, I believe that so far, if our Orange tube tester says that a tube matches the ones in the amp, all will be well. And sometimes it can be a number off and still work. (IMHO, the Orange's algorithms do not place enough weight on bias current, as the bias current of "matching" tubes can be as much as 25% off, and tubes that are a "number out of match" can have identical bias currents.)

**nevetslab** · 10-31-2014, 12:13 AM

Next SVT CL I get open, I'll have a look at the comparator window again. I had both written down measured window voltages in 2010, and had also computed the string based on the schematic values. Doesn't say there haven't been revisions to that circuit. The amp I just serviced had two tubes installed that were near the edge of the window, and it still liit up GRN for both upper and lower sets.

I wasn't aware of the Orange VT1000 Tube Tester....just looked to see what it was. Nice and simple, takes care of a lot of functions. I'll add that to my database on tube testing. Never did get around to turning an old Fender The Twin (in our boneyard) into a test set for matching tubes and such. A job for laterman.

**TimmyP1955** · 10-31-2014, 08:20 PM

The Orange is a useful tool, but not perfect (per my previous comments). Here's some results (Ignore the JP number) (first number is Orange rating, second is bias current e.g. 8/35) (Hope this isn't too jumbled up):

Amp Model JP# Orange Rating/Current Draw (or Orange Rating/Did it Work in SVT when Subbed for #2 ?)
Twin 3 8/36.5 8/31.3 8/30.7 8/37.8 All 8s, not very well matched
Twin 15 9/18 9/20 10/22 9/18 I did not leave the bias so low of course
Twin 15 10/38 10/28.2 10/32 10/35.7 All 10s, not very well matched
Twin 42 10/36 10/37 10/37.3 10/38 Good match
Twin 10/38.6 10/34.9 10/34.7 10/36.3 Pretty good match
Twin 9/31.5 9/33 10/39.6 9/35.4 A bit of a spread on the 9s

59 Bassman ? 11/40.2 9/22 9 and 11 almost 20ma apart
59 Bassman ? 9/36.1 9/34.9 Good match
JCM800 12/33 12/26.8 12/23.2 12/26.5 All 12s, not very well matched
Plexi 1 10/32 10/29.7 11/33.3 11/33.3 11s matched, a 10 was good with the 11s, a 10 was only a fair match to the other 10
Plexi 2 13/24 14/31 13/31 14/35 14s were a fair match, a 14 and a 13 matched, the other 13 was not even close to any of them

**nevetslab** · 11-03-2014, 05:43 PM

After I looked at the manual for the Orange VT1000, I didn't see where you're getting your current values. The Matching number is, of course, one of the LED's that lights up. Orange doesn't seem to want to tell you any specifics, though. Is there an LED or LCD numeric display on this box? I couldn't tell from the photos. I'd guess the Matching number takes into account the transconductance of the tube along with the plate current, maybe also the mu. No idea what their internal plate and screen voltages are.

At first, overlooking the 'matching number', I would have been inclined to re-sort some of your tubes, then saw similar currents, but associated with different matching numbers, after which you're looking at that usual bell curve in sorting thru tubes on hand to batch them into sets. Still, a handy device for pre-sorting and matching tubes.

When I don't have cathode (or plate) resistors in circuit, I'll use a Compu-Bias Bias Probe meter on the lower plate voltage ratings, and a fixture to use the probes on my 4-Ch HP 3467A Logging DMM on the higher voltages, it having scaling networks in the fixture, as the Compu-Bias can't run on 600V+ supplies.

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