^^^ My ARC REF-75 is biased from the front of the amp through little holes. ARC supplies a long plastic rod with a screwdriver tip for the purpose of biasing. Works just fine.
Frank
Frank
TUBE BIAS, socket to me!
BIAS: (I'm starting from zero understanding)
I have never measured/adjusted bias in the 3 tube amps, 3 tube receivers, and 2 tube preamps I have acquired over 47 years. I just switched my current Cayin from 6550's to KT88's. Adjust bias? Adjusters inside, scary electrocution warnings. I could pay someone else to do it, i.e. Steve at VAS 1 hr away in NJ, soooo,
What really counts? (personally I don't care about either heat or life, but would like to understand)
Heat?
Life?
Output stays Matched when adjusted?
Acoustic Performance?
_________?
Over the years, fronts off, bottoms off, I hose em down with contact cleaner/lubricant, compressed air, all controls and switchers, any adjusters, swish full spin back and forth. Kill any spiders, look for, replace the rare burnt resistor.
Then leave any adjusters (whatever they are) in the middle position, button it back up.
Two tube testers, my big hickock always agrees with small portable one, test strength, shorts, matched strength old and newly purchased. Large collection of NOS, used. Often used test essentially same strength as new ones.
When they go, it's usually a short.
I have never measured/adjusted bias in the 3 tube amps, 3 tube receivers, and 2 tube preamps I have acquired over 47 years. I just switched my current Cayin from 6550's to KT88's. Adjust bias? Adjusters inside, scary electrocution warnings. I could pay someone else to do it, i.e. Steve at VAS 1 hr away in NJ, soooo,
What really counts? (personally I don't care about either heat or life, but would like to understand)
Heat?
Life?
Output stays Matched when adjusted?
Acoustic Performance?
_________?
Over the years, fronts off, bottoms off, I hose em down with contact cleaner/lubricant, compressed air, all controls and switchers, any adjusters, swish full spin back and forth. Kill any spiders, look for, replace the rare burnt resistor.
Then leave any adjusters (whatever they are) in the middle position, button it back up.
Two tube testers, my big hickock always agrees with small portable one, test strength, shorts, matched strength old and newly purchased. Large collection of NOS, used. Often used test essentially same strength as new ones.
When they go, it's usually a short.
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- 91 posts total
@elliottbnewcombjr yes a wooden or plastic stick with a screwdriver blade at the end of it would insulate you from any spurious shocks for sure... that is why audio research supplies the skinny long plastic screw driver for bias adjustment -- here is one you can just buy quick from amazon https://www.amazon.com/Potentiometer-Tools-Hardware-ADJUST-TOOL/dp/B06Y6FTGNT/ref=pd_bxgy_img_2/139-... you probably know this but a proper bias setting is specific to a particular set of power tubes (it is not to the amp, irrespective of tubes)... change power tubes in the same amp, you need to re-bias... i have had different tube sets (even of the same make/type of tube - say a sovtek 6550c quad) need quite different bias settings to achieve the correct biasing level question -- you use a wooden or plastic handled screwdriver as per above, cool - but how are you going to measure the correct bias level for each kt88 pair? i did not see instructions for your a88 amp how/where to measure it... @oregonpapa dunno if your ref75 is your first arc tube amp -- they made power tube biasing much easier on that amp (long overdue)... on earlier models you needed to open top cover, use multimeter with hook prongs clip onto both sides of a bias resistor and read 65 ma at proper bias level... reaching into the running amp with hot tubes with the clips or with the biasing screwdriver took some nerve and fine motor skills lol here is the process for the vt100 -- point is, you need a way of reading the correct bias setting!! https://www.youtube.com/watch?v=GnBuI0sLhcg |
I’ll say this and let it go.. It’s better to stay out of trouble than get out of trouble... Bias is part of maintenance. Very simple. There are three types of bias adj. 1. The way the factory says to BAIS and WHEN.. 2. The way others tell you to or not. OR 3. NOT at ALL... The correct answer is what the factory says... Plane and simple. Yes it changes the sound.. especially if you bais low, it will sound muddy. If you bias HOT the amp will distort sooner (tube break up), and burn valves quicker... Bias is very important... Bias hot and turn the volume up...LOL.. go ahead... Bias low LISTEN.. muddy as all heck. Especially 60-1500hz.. muddy. YUK... jjss49, we are on the same page, 4 times a year is good for me.. YUP.. ALWAYS with a valve change..... ALWAYS.... <<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>> Billwojo... I just got my New Maxi. It does 4 large valves at a time. The Mini has been on back order for over 6 months.. They should have started shipping the 15th. There were 16 or 18 before me.. I love um.. A LOT faster a lot more accurate, than the older ones.. My 533 and 39 are OLD... They WORK very well, the 533 for small valves, and 539 for small or large valves. I use them to compare... I use a couple of known test valves. Pretty good information... <<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>> Last but most important.. OP I’m glad your happy... What else counts.. NOW if your looking for a little nugget..... :-) It may change nothing... One way to find out.. Regards |
https://www.pinoydvd.com/index.php?topic=139769.0 OP check this thread out pretty good, pics in all... Regards |
As tubes age, they lose transconductance. As they lose transconductance if the grid bias voltage is fixed (not varying with respect to cathode voltage), you gradually run out of oomph (for want of a better term). The amplifier may begin to sound a bit limp. But this happens so slowly that the user is often only dimly aware that the sonics are not quite as wonderful as they once were, with fresh tubes. And eventually, one buys new power tubes. In this way, I once got 10 years out of a set of power tubes in a Futterman OTL. However, as the owner of an amplifier with adjustable bias and a built-in method for measuring and correcting bias current when it inevitably goes off, by adjusting grid voltage, I have to say it is a revelation to hear the amplifier restored to normal bias current after a periodic bias adjustment. By "periodic" I mean once a year or so. If you can adjust bias safely and know what you are doing it for, I highly recommend monitoring it and keeping it up to snuff. Also, owners of amps with some sort of autobias circuit need not be concerned. I have a Hickok 533. I use it only to test for transconductance of small signal triodes. I do not use it to evaluate power tubes because it does not have the capacity to subject such tubes to plate voltage and plate current parameters where such tubes need to operate. |
- 91 posts total