bias virgin

[mkstylee]

Hi all just picked up a 8pin bias tool that plugs into a multimeter and shows the MA ,my first foray into tuning and i would like some advise please.I read that you should set a jtm45 with KT66's to 40 ma but with the dial flat out i only get 37 max ' should i run it flat out or not?. Also the valves are a matched pair but are about 2-3 ma apart ,does this matter?.Are there any other things i need to take into account that the bias reading will be trying to tell me (what should i be looking for) i was going to get the TAD biasmaster but that only shows the ma reading aswell so the probe into a multimeter is no different really.
Thanks to anyone who can give any advise.

[Dave W]

I am not a technitian, but have biased many tube amps with a bias probe and multimeter.

I have had 2 amps that even with the trim pot all the way maxed would not quite get the ma up to where I wanted it to go. My best guess with this was that the particular set of tubes I had in those amps were just on the lower end and would not fire up to where I wanted them. Does not mean there was anything wrong with them, just where they ran.

As far as the matching being 2-3 ma different, not a big deal. Thats still close enough. Its not unusual for some tube retailers to call that a match and its close enough.

Running at 37 as opposed to a goal of 40.....IMHO I do not think you will notice a difference in tone from 37 to 40. Not enough to pay for a new set of tubes try and get to 40.

In this amp is there one trim pot for both tubes or 2 separate trim pots. You may try switching the positions of the tubes, may or may not affect it.

You mention a 2-3 ma difference in the readings and 37 the max. Is 37 your higher tube or your lower tube. Basically are you getting 37 and 34 or 37 and 40?

See what other advice people have to offer too.

[mkstylee]

Hi dave and thanks for the reply it helps a lot.
The 37 ma was the highest valve the other one reading around 35ish and only one trim pot inside ,Ive pulled the pot back a tad so probably around 35-36 on the high valve now,and it sounds fine .If anyone is interested i think the tung sol KT66 sounds nice, replaced the valve arts with the tung sol and a good improvement i think.Can you find the voltage from a probe and multimeter if so what do you do to get that figure up.
thanks

[Dave W]

My memory tells me I have read somewhere that the Tung Sol KT66 is really their 6L6 in a different bottle. I searched around and could not find anything about that so it may be my memory is skewed. I did find this review......from The Tube Store review....

Tungsol KT66 - Not your average KT66. In fact it’s more like an extremely rugged 6L6GC in a KT88 bottle. Heater current is the same as a 6L6 and the biasing is more like a 6L6. You might want them just for the look but the tone is all there. Big and open with very solid lows and smooth treble response. The mids are well balanced and don’t go to the extremes of sounding scooped or nasal. Like all the Tung Sol branded tubes, the build quality is top notch. The over sized base has the metal band you usually find on a KT88 tube, and the large bottle helps dissipate heat if you like to run your tubes hot or if you have cathode biasing in your amp. If your amp has “bear trap” tube retainers they will have to go because of the size of the tube's base but also the bear trap clamp will ground the metal base to the chassis and cause a malfunction. If your amp requires the tubes to pass through a hole in the chassis for socket insertion you’re also out of luck. Home audio is where I would use these bottles since they are really functional art objects. Bottles like this should be on display. If you have a Marshal head I think you might find them to be an alternative worth exploring. They will give you great tone but since they draw similar current as a 6L6 you will draw 1 amp less heater current in a 50 watt head and 2 amps less in a 100 watt head. Your power transformer will run cooler and you will get closer to the 6550 tone that many Marshall enthusiasts love

maybe if they bias more like a 6L6, its why you cannot get the numbers up to 40 I do not know this for sure though.

You can also read at Eurotubes more about this tube and this possible issues with the metal base and ground problems. Look under the tube descriptions section under KT66 in the last paragraph.

If this amp has the metal style bear trap tube retainers, they may need to-should be removed due to the possibility of shorting out. I know with the Vintage Modern they have casued some shorts.

If they sound good then they sound good. Enjoy the amp.

Also with checking plate voltage, multimeter to 1000v, black to ground-chassis and red to pin 3 on the socket. Go to Eurotubes and look under the How To Biasing Videos, go under the Marshall 800 video and read everthing and then watch the video. It will make sense then.

Remember, its electricity, use extreme caution. Get the info so it makes sense then just becarefull. Its how I learned and I did it, so can you.

[mkstylee]

Thanks again dave ,Yea i read that about the bear traps before ordering and my jtm has the spring retainer type so not a issue really but about the 6L6 biz yea it could be that i did'nt think about that acording to the TAD booklet most 6L6 seem to be 25-40 so it would explain the low reading ,and earth to the chassis and live to pin 3 .got it thanks again.

[LUVMACHINE]

It kind of sounds like a bias probe. I got one I used on my 76 Super Reverb, I set it to 37.2 ma. It sounds good but, because it is a 45 watt tube amp, it is attention getting in nature! I say just be sober when you are doing it and, calmly think all the steps out and, you should be alright! Right now, I would not dare to do it, I had two vodka and orange juice doubles less than two hours ago! Even though I could handle it, for the sake of safety I would not attempt working on a live tube amp right now! Always remember safety first!