HAD's choice of bias on 6L6 in ODS

[jazzyjoepass]

Hi,

Just want to start a discussion on HAD's choice of biasing at lower current than the usual 70% dissipation. It's interesting.

Mike.

[llemtt]

Since CBS acquisition Fender always does it...

more headroom and a cleaner sound

the only thing meant to be overdriven in a ODS gain structure is V2b

cheers
teo

[jazzyjoepass]

I came to that conclusion too. And its starting to make alot of sense to me actually.

I recently built a #102 clone. I was short of the FET transistor and did not install the FET board but still fired it up anyway. The plate voltage on the 6L6 was about 490V and was biased a little bit on the cold side.

I was quite intrigued with the tone I was getting. Lots of headroom and didn't clip unnecessarily. The OD side had so much clarity and command - no fizz but lotsa nice OD clarity - probably less intermodulation?

Overall the amp was not compressed but very punchy. I supposed the power amp section was just mainly used to power amplify and not to contribute to power tube clipping. Because the way it was biased, it was able to swing voltages without clipping early and so there's lots of ample headroom for punchiness and response to big passages.

I'm loving it every tweak I do to my amp. This Dumble circuit is truly a "less is more" amp.

[Structo]

If you forego the FET circuit you should put a 150K resistor at the end of the power supply node to ground.

This will mimic the FET load and bring some voltages down.

[jazzyjoepass]

Yes, I'm aware of that. Just pointing out to the effect of the power amp bias.

[PaisleyTube]

Hi,

Just want to start a discussion on HAD's choice of biasing at lower current than the usual 70% dissipation. It's interesting.

Mike.

1st of all: any measurements from Dumbles with new tubes biased by HAD himself?

70% works well for most EL34 amps
In most Fender style amps with 6L6s I prefer 65% or lower so the low-end stays a bit tighter. Amps with bias-wobble-trem need to be biased a bit colder (let's say 55%) to prevend overheating when wobbling hard.

[Structo]

In my limited experience, with modern power tubes, I find a colder bias, depends on the amp but on my D clones around 60-65% seems to sound the best.

I also have a Carvin Belair, 4x El84 amp, also sounds a little bit colder.

Now I do have a vintage set of two RCA 6l6GC's that I ran in my D'lite and they also sounded better under 70%.

So I don't think there is a golden rule for fixed bias, you just have to try several settings and decide which sounds better.

[llemtt]

The OD side had so much clarity and command - no fizz but lotsa nice OD clarity - probably less intermodulation?

Yes and no, biasing colder actually increases xover distortion a little in power amp which is a cause of IMD. But that's not that bad at all considering such an imperceptible IMD increase is more than compensated by a huge reduction of harmonic distortion.

Applying a small amount of HD to an extremely harmonic rich signal such as the deeply overdriven OD one is what increases IMD and "unmusical harmonics" which are perceived as fizz and/or unpleasant distortion.

cheers
Teo

[ic-racer]

Cold bias can really sound bad. My HRM was not sounding good and I checked the EL34s and found the cathode current around 18ma. It really did not sound good at all like that.

[Aaron]

I used to worry about bias. The way I do it now will probably give an amp tech a heart attack. With the room dark, I turn the bias until the valves red plate and back off till they're just below. Probably shorten life of the valves but sounds great, (to me anyway).

Aaron

[llemtt]

@ic-racer

agree with you 100%! we were talking 6L6s...

have fun
Teo

[talbany]

Cold bias can really sound bad. My HRM was not sounding good and I checked the EL34s and found the cathode current around 18ma. It really did not sound good at all like that.

No wonder 18ma is a really cold (barely on) AB1
Dumble amps!!
34's (Pentodes) in general (too me) sound better biased around 70% (max Plate) at a plate voltage around 470V,,,which would be around 37ma if memory serves..
6L's (Tetrodes) seem to have a larger swing..Between 55-70%..Class AB1 of coarse..(depending on amp and style) and generally sound best run right at 450V..(or slightly lower) IMO!!!
6650's and KT88's sound best run right at 500V and again biased slightly cold..(accept for SSS's using CF driver..I prefer these bisaed HOT 70-or even 75%)Again IMO!!

Tony

[David Root]

Thanx for that info Tony. I must try that on my SSS build, which currently runs at 440V, BUT it's 6L6s (Mesa 415-STRs, a bit tougher than your average 6L6), do you think that would work compared with 6550/KT88s? Somehow I rather doubt it, but I've not tried it, so......?

[talbany]

Thanx for that info Tony. I must try that on my SSS build, which currently runs at 440V, BUT it's 6L6s (Mesa 415-STRs, a bit tougher than your average 6L6), do you think that would work compared with 6550/KT88s? Somehow I rather doubt it, but I've not tried it, so......?

Dave
Sure!..Try biasing the STR's fairly hot around 70-75% in your SSS..IMO since there is virtually no breakup and very little chance of crossover distortion or grid clamp ..Another is that the low driving impedance serves to provide excellent protection from thermal runaway (almost instantaneous bias excursion speed http://www.ampbooks.com/home/amplifier- ... excursion/ due to direct coupling), and permits the use of tubes that would be too gassy for a higher impedance grid circuit. The bias remains more stable (Faster), and the tubes are happier... (I find running them up helps promote sustain as well as increased touch response and note bloom)..
BTW Dumble seems to have the same opinion since he biases his 0121 hot as well..0121 uses a CF Driver similar to Dumbleland!!

quote from Max

AFAIK ODS #0121 technically operates perfectly and without any problems at an idle current setting of 125 mA for a pair of GE6550A as specified by Alexander Dumble on the back of #0121 on the right beside the two bias adjustment pots (see the attached picture). And AFAIK many of those who had the opportunity to use it, including myself, liked the feel and timbre of this amp a lot with this idle current setting. Todd Sharp - the original owner of ODS150W #0120 and #0121 - in "the ToneQuest Report" magazine: "The truth be known, this was a great, great amp, and I gladly return the insurance money if I could have my stolen amp back..."(#0120 was stolen after he sold #0121 - Max)"...best tremolo, reverb & Overdrive ever,..."

http://personalpages.manchester.ac.uk/s ... 20%230121/

Please let us know what you think!!

All The Best!

Tony

[David Root]

Thanx Tony! I will try that 70% of max Pd bias on the STRs. That will drag the idle cathode draw down a bit. I think I'm running about 34 mA/tube on it now which at 440Vp is only 50% of max Pd. If it sounds better I may have to change the PT out for a bit higher voltage.

I have Richard Kuehnel's books including that Power Amp book which goes into bias excursion in mind numbing detail! He sure is a GEB (Giant Earth Brain). I'll have to reread that chapter though, it's been a while.