TUBE BIAS, socket to me!

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

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....

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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...

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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