TUBE BIAS, socket to me!

What are the benefits of proper settings (every 6 months) and what are the potential problems if simply ignored, (as I have done for 47 years)?

If the adjusters are simply set in the middle of their rotation, as I always do, and the thing sounds great, should I risk electrocution?

i would say that if the bias is well set at the start (when new quad of kt88’s are installed) then over time as the tubes age the bias will run a bit low for ideal sound but if you can’t hear a difference, then leave it be... the tubes will become under biased as they age and they will run cooler/easier and actually last marginally longer in use - once again, if you cannot sense a sonic penalty then leave well enough alone, just enjoy the music

middle of rotation on bias pots don’t mean much, as there are different sensitivities in the pots and more importantly different bias needs of the tubes set being driven... so best to have a tech read the correct bias voltage for your tubes set and set properly once, then go from there...

as an audio research tube amp user for years i have become kinda OCD on bias settings as there is much lore about failing power tubes and how bias resistors can be damaged in the amps when a power tube fails - so i am conditioned to check bias conditions fairly frequently (once a quarter, say) - but to be honest i have never let the bias run way off under or over the recommended setting, and so i cannot speak to whether the sonic penalty is noticeable... if anything i set the bias 5-10% below the recommended level to get a little more tube life, and i feel there is no sonic penalty i could hear with various arc amps running 6550/kt88's -- vt50, vt60, vt100, ref 75, ref 110 etc etc...

Well now,

I like the KT88’s more than the 6550’s, difference was obvious, as obvious as switching from Triod mode to preferrable ultra-linear mode.

I assumed all the difference was entirely in the tubes.

Seller told me he had it’s bias adjusted just a few months prior.

Now I’m thinking, it is possible seller had it biased for KT88’s, kept them, put factory 6550’s in, shipped to me.

IOW, I just got lucky, and the bias was part of the improvement.

How about I make myself a long wooden stick/screwdriver instead of plastic. Wood safe?

^^^ 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

@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