“I’ve got one hand in my pocket, and the other is biasing.”
- Alanis Morrisette

I managed to switch my tubes with this video and set the bias myself. Thanks for being very thorough and giving safety warnings, which I think a lot of videos out here lack. It was actually very easy to do. My amp sounds like new again 👌🏾😄

Apart from most of this being incredibly 'Average Joe' standard, I did cry a little when he switched the power and standby switch immediately one after another, then stated to let it warm up. Let it warm up with the the power on and the standby off.

Thanks for the knowledge, this makes it much clearer for me.  I always thought that if I didn't set the bias pot to the "absolutely correct" spot (that could only be derived at through an engineering degree) I would blow the amp up entirely.  Thusly, I would until recently only buy solid state amps because I didn't want to get someone else to bias my tubes.  Now I am learning to do it myself, and this vid provides a nice piece of the puzzle.

That bias point is total cathode current.. A little screen current (5%) is included in that number..
So a number like 70mV at the test point is ~ 66mA plate dissipation for two tubes or 33mA for each tube. The B+ voltage with a cold bias (60mV) is 431V. A little OT loss and you have 429V plate. Multiply Volt with mA and you get the plate dissipation at idle. This is 14.16W or 47.2% of max plate dissipation for a 30W 6L6GC tube.. That is too cold,, you want to be above 50% to avoid crossover distortion. Between 50% and 60% is a good number for the hot rod.. Going higher and you will loose a lot of headroom and spank in the bass..

Lets run the bias to 80mV at the test point..
deduct 5% as screen current = 76mA of total plate dissipation.. Multiply with you plate voltage which should be a little lower as you are drawing more current at idle.. So lets say it has dropped to 424V (80mV) from the previous 429V (60mV).
Now you get an idle plate dissipation of 16.11W which is 54% of max dissipation.. That should be a good bias...

With the Hot rod deville you want to have a lower mV number at the bias test point as the voltages are higher in that amp..
With 60mV at the test point and correct wall voltages and a pair of 30W 6L6GC tubes you should get around ~484V on the plates..
Doing the same math you get that 60mV is 57mA plate dissipation (I assume 5% screen curent). Plate dissipation is (0,0285A x 484V) = 13.8W dissipation (or if screen current is even lower) around 14W which is 46.6% of max dissipation.. Again a little too cold.. Bring it up to 55% if you have a strong 30W 6L6GC tube..

I'm 76 years old and 'been messing with analog and digital electrical projects since the 1950s, when my grandfather showed me (more or less) how a simple radio worked on the inside.  Got into rock music in the '60s, playing sax and keys along with a number of guitar players in many bands.  All their amps used tubes, which were still a mystery to me, as to how they worked.  I got pretty good as an amateur, with solid state transistor projects including mic preamps.  And I've known that there WAS such a thing as "biasing your tube amp" but I'VE NEVER KNOWN THE THEORY BEHIND IT much less HOW TO DO IT until watching YOUR VIDEO!  Thank you very, very much for your well presented lesson and vivid demonstration.  So I did get into playing guitar along the way, and now look forward to replacing my guitar amp tubes the proper and safe way!

You are leaning on the metal chassis with your left hand, adjusting the bias pot with a uninsulated  screwdriver with your right. A recipe for 'thoughts and prayers'.

Great video. Fender sets this amp at 60mV. Raising it to 70mV gives a better sounding gain. Higher bias shortens valve life.

EXCELLENT!!! So glad you did this tutorial. I didn’t want to take my HRD to a tech for such a simple maintenance task. You shed light on the mystery.

On an interview the tec guy for Angus ACDC said he runs the valves hot.bias high a bit on the edge...which explains Angus,s eq  only on 3 bass treble & mids...great info
Thanks...you have saved me a few quid...and amp down time.

Kennis, want to say thank you for sharing your knowledge on this.  I just bought a FHD and it was so flat and dead sounding.  Bought a set of matched JJ's and with your help got them biased to were I am so happy now!  What a difference :)

This video is a little biased.

Great explanation, thanks. I followed your suggestion to set it at 71 and the result was so good for me.

Good stuff Kennis. I'm acquiring a USA HRD that most likely needs to have a total tube overhaul, and I'm just glad biasing can be done relative quickly and easily. Having no prior experience because all my other tube amps either have fixed bias (Mesa Boogie ROV25) or are self-biasing (THD Bivalve 30), so this video is super useful!

Ahh man THANK YOU!! Have been wanting to switch from SS to tube but this whole bias thing kept me from doing so. Finally got myself a   HRD iii and could not believe this is all there was to it!! Thank goodness I have the potentiometer!! Thank you!!!!

Thank you for the education on biasing the power tubes. I have a Hot Rod Deluxe and I'm replacing all the garbage GT's with JJ's (both preamp and power) but never knew anything how to check the bias. Looks simple enough, thanks for the video and safety tip!

really appreciate your effort to make these guides. Very well explained!

Thanks for the tutorial!!!  Didn't know it was something I could do by myself...Amp sounds GREAT now!!!

Great video for someone who knew nothing about biasing tubes! Thanks!

I love your videos! Very informative, clear, and concise. Thank You!!