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In Reply to: RE: VT 100 Mk II driver tubes posted by dgaapc7 on December 29, 2015 at 17:50:59
Yes, I know that the driver tubes present a real PITA to replace and bias. I read Mr. Collins terrific how-to post on this issue several years ago when I rolled them the first time. I have no intention of tackling this myself, but will let a good tech do it and pay for the time it takes. I just want suggestions as to better selections that are within my discretions for the driver sections as to price, sound and availability.
LowIQ
Follow Ups:
For their ruggedness I use EH 6922 tubes
6550 tubes are SED winged C 6550 power tubes.As for the tech you use to bias the 6922 input and driver tubes I suggest you have him show you the finish product plate voltage measurements of each dual section of each 6922 tube before he puts the side panels back on the amp.
If you are going to be the one buying the 6922 tubes I suggest you buy a few more matched tubes than you will actually need for the tech.
That's what I ended up doing the last time I retubed the 6922 tubes in the VT50 amp. I first installed the vendors "closely matched" pairs of tubes and found they were not so "closely matched". So instead of playing the shuffle the tubes around game for hours on end again I tried something new.
Not knowing how it would turn out I tested the new matched pair tubes in a Hickok 6000a Mutual Conductance tube tester. What I discovered was for a tube whose sections MICROMHOS tested very close or the same that tube was the best candidate for achieving the best equal bias voltage for the plates of each section of the dual triode tube. This is a must for the ARC VT series tube amps. (Note, not so for the VT100 mrk 3 amp)
Example, the ARC VT50 amp. See numbers 4) and 5) for setting the input and driver tubes plate voltage on the PDF Link provided below.
It tells you to set the voltage on pins 1 and pin 6, on the input tube to +160Vdc and the driver tube to +170Vdc.( pins 1 & 6 are the plates of the 6922 dual triode tube).You can't set the two plate voltages of each tube individually. RV1 trim pot is for the input tube and RV3 is for the driver tube.
Next read note #7)
And there is the rub. Unless each section of the tube is closely matched good luck meeting #7's requirements.
I am not going to name the Vender I bought the last matched several pairs of EH 6922 tubes from. His close matching of 6922 tubes used as driver tubes in other audio circuits may work just fine, but not in an ARC VT50 amp.
Example:
Here are the tube vendor's test readings of some of the his tubes markings on each tube box.Section 1)14700, section 2)14000
No way this tube could be biased in the VT50 as a driver tube let alone an input tube. And yes I tried.
Another one.
section 1)14800, section 2)14300
For the driver it still would not meet the ARC specs.another one.
section 1) 14000, section 2)14600
same problem as above.another one.
section 1)14200, section 2) 14200.
BINGO! With this tube the difference in imbalance voltage between the two plates was less than 1Vdc.This is the reason, I believe, why ARC charges so much for matched 6922 input and driver tubes used in the VT series mrk 1 and mrk 2 amps. As well as the VT50.
As always YMMV.
Edits: 01/01/16
Unless the vendor matched the tubes for plate/cathode current the Gm readings are relatively meaningless in the bias scheme. The current can differ greatly between tubes of the same (or nearly the same) Gm. It was likely as much coincidence as anything else that the one pair worked and the others didn't.
Plate voltage in this circuit is a function of current through the plate load. More current = lower voltage at pin 1 or pin 6 (these are the plate pins of the two triode sections) due to the higher voltage drop across the plate load. So while the Gm match is important, the CURRENT match is critical!!
The tubes for these amps need to be very low noise, and have matched transconductance AND current to successfully bias up in the VT100. Matched transconductance alone will not get it done!!
Thanks for the information. These are the main characteristics that led to the mixture of driver tubes I mentioned. I'm wondering if I just shouldn't leave well enough alone and stick with what I've got, since it sounds satisfactory. I've got three Amperex Bugle Boy (Holland/white label) in the D 70 MkII. Would they make a good start on a new set of similar stuff if I decided to become less frugal than is my normal inclination?
LowIQ
Thanks for the detailed warnings regarding tube matching. Very informative. I figured I would have to get more than 8 if I wanted to try new, matched, drivers across the board. I am running a mish-mash of Sovtek, Philips, and others, chosen for no other reason than their ease of matching and biasing. I wanted to sell the amp a few months ago, but after swapping it in and out with a D 70 MkII, I decided I liked the VT 100's sound better, despite its being possibly the most user unfriendly amp ever made, (surely in the top 10 all-time.) I'd dump the damn thing if I didn't like its sound so much.
It might be a lot to ask, but given the cost of tubes these days, is there any description you might give me regarding the effect on the sound quality the EH tubes have (warm, neutral, cool, strong/weak bass, etc.) Much appreciated.
LowIQ
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