Optimizing Bias on 5F6A

[waldner]

I just measured the bias using the Compu-Bias with dual probes.  Wondering what is ideal for these EH6L6 tubes I got from Doug and if everything else looks good or not?  Some 6L6 tubes are rated for a max of 30 watts and others much less.  Not sure what these EH tubes are capable of.

The Compu-Bias readout is showing the following:

Vp= Plate Voltage (voltage between cathode and plate)
IK= Cathode Current -
D= Tube Dissipation in Watts (total as measured at cathode)

My results are:
Vp= 475
IK= 26
D= 12 watts

Any thoughts?

[eleventeen]

475 volts sure seems on the high side for plates. What voltage do you measure on your G1's? I usually like to see about -50 or -52 but I've seen plenty of real Fenders cruising along at -45 (would be hotter) but I have little experience with those particular 6L6 tubes. Is the 12 watt diss supposed to be for both tube or for one tube? I guess it would be for one tube with the separate socket-bases on your tester. I'm thinking you're running those tubes too cold, meaning, the bias volts might be -62 or -65. When I finish a build and am turning the thing on for the first time, I like to set the bias way too cold, somewhere up near -65. I have not measured 6L6 current under those conditions, frankly, because I know I have them choked off on purpose with the bias set so cold.

I am also thinking that if your ecaps are rated 450, you are past that. If so, don't run the amp this cold for long. Proper bias should pull that very high B+ down, but I am thinking it should pull it down by maybe 25 volts and then you are still on the high side. Almost like you have a SS rectifier in there, but I see you do not.


Ultimately, you may want to reconfigure your power supply to be choke input (afficianados will argue this will affect your sound vis a vis sag) to knock it down 20-40 volts.

[waldner]

This build is the straight Hoffman 5f6a right down to all the parts and tubes coming from Doug.  I thought the 475 on the plate voltage seemed kind of high, but wasn't sure.  I also don't even know what G1 is, so not sure what to measure.  I turned the pot up quite a bit once I got this amp up and running and maybe I'll try to turn it up further.  It sounds like the tubes can put out more than 12 watts.

My electrolytic caps are the Illinois rated at 500v, so I think I'm ok on that front.

Any other thoughts are appreciated.
Thanks

[PRR]

ALL modern guitar-market "6L6" should be good for 30 Watts (the 6L6GC rating).

Hardly any (only very-old) guitar amps were designed around the 19W-21W versions of 6L6.

5F6A was designed for 5881 which was an improved 6L6. The 6L6GC's ratings are higher even than 5881.

26mA seems a little low, and 475V seems high. The classical condition is near 430V and 33mA. Dialing-up to 33mA will drop your B+ a mere 6V or so. Your B+ is about 9% "high". If the PT is designed for 115V wall-voltage and you have 125V walls, that's about right.

Bring it up to 36mA (in proportion to your higher B+). That's 17 Watts per plate, which ought to be plenty safe in any modern "6L6". (And in-the-zone for that bias-meter's "6L6GC" suggestion.)

[waldner]

Just the kind of info I was hoping for PRR.  Is it ok then to leave the plate voltage where it is and just bring up the cathode current to around 36ma?  It sounds like you are saying that bringing the cathode current up will bring down the plate voltage.   

Also, how do I bring the cathode current up?  Is it a change of cathode resistor value?

Thanks,

[waldner]

Update.  I just turned the bias pot up some more and got the cathode current to around 36 and that brought the plate voltage down to about 466 and i'm showing about 17 or 18 watts.  So I've learned that plate voltage and cathode current have an inversely proportional relationship. 

Is it probably good to leave it here or should I be doing something to get the plate voltage further down?

Thanks,

[PRR]

How does it sound?

[waldner]

Sounds good and less ice-picky than where it was before.  Thanks