Bias 6L6s in my Accomplice build?

doveman · Post by **doveman** » Fri Oct 11, 2013 3:06 am

I just think I like 6L6s better than 6V6s. But when I bias them between 30-35 (recommended for this amp and 6L6s) they seem cold and harsh.

I measured the plate voltage off both 6L6 sockets at 420v. It looks like my Tungsten Sol 6L6GC-STR has 30w max plate dissipation. So, if I recall:

30w/420v*50% = 36ma
30w/420v*70% = 50ma

When I bias at 35 (max recommended for 6L6) it feels cold. If I bump it up a little it starts sounding better. I should be safe in the 40s. Am I looking at this right?

I asked David too, but this is just basic math right? I think I'm doing the math correctly. I should be ok with those voltage measurements?

JazzGuitarGimp · Post by **JazzGuitarGimp** » Fri Oct 11, 2013 3:54 am

I'm not familiar with the Accomplice. The first thing that occurs to me is unless the amp is designed to run four 6V6's or two 6L6's, you're going to need to make concessions with the speaker impedance when switching between the two tube types. So make sure you are driving the correct load impedance after changing over to 6L6's. Your bias calculations are correct, but (and, again, understanding that I don't know the circuit in question), if this is a two-tube amp (two 6V6's or two 6L6's), I might be inclined to question whether or not the PT can source the 140mA needed to idle 6L6's at 70%.

trane34 · Post by **trane34** » Fri Oct 11, 2013 3:56 am

I use this: http://www.webervst.com/tubes1/calcbias.htm

Firestorm · Post by **Firestorm** » Fri Oct 11, 2013 4:31 am

If Dave designed the amp to support both tube types, I'm sure the the PT can handle it. But what Jazz says about output impedance matters. At that voltage, 6L6s will want to see less impedance to do their thing. If you want Fender clean, the 50% bias point is best; if you want something else, go ahead and bump it up. But be aware, most BF and SF Fender circuits sound like crap if you bias them to 70%.

M Fowler · Post by **M Fowler** » Fri Oct 11, 2013 5:06 am

The PT is 180mA per his website can run pair of 6v6 22w or 6l6 35w.

I built a home brew of this circuit and I don't care for the raw control.

statorvane · Post by **statorvane** » Fri Oct 11, 2013 8:57 am

At that voltage, 6L6s will want to see less impedance to do their thing.

I have a 1987 clone that I can run with 6V6 or EL34/5881/6L6 etc. The OT primary impedance is 8 ohms with the 6V6, 4 ohms with the others. I would think you'd want to halve your impedance with the 6L6s - may get you closer to your desired tone.

martin manning · Post by **martin manning** » Fri Oct 11, 2013 11:08 am

Hidden in a thread about a Bassman transformer swap in the Dumble section is a discussion about this build and its speaker jack arrangement. The OT has 8 and 4 ohm output taps and Doveman is hip to the 6V6 vs. 6L6 load impedance needs.

doveman · Post by **doveman** » Fri Oct 11, 2013 11:23 am

Thanks guys. David responded just now that I could bump it a little safely. both the PT and OT are designed to cover Vibrolux territory and have dual OT taps for the different tube types. the PT is rated at 180ma.

I think I'm just looking for an adjustment to make the amp a little less harsh/cold. It's not bad but it's not what it should be for both sets of power tubes. A 12AT7 in the PT ( to replace the stock 12AX7) helps. Two sets of 6V6s sounded similar. A couple of different 12AX7 in V1 sounded similar. I think the 6L6s are closer to the sound I want ... so the question I asked.

Any other blackface adjustments for a slightly cool/harsh sound?

ampmike · Post by **ampmike** » Fri Oct 11, 2013 11:45 am

I wander if it just needs a little breakin,The brown Suger is a bit like that too.The second thing ,could you get hold of a NOS set to see if there is any difference.Just a couple thoughts.Have Fun,Mikey

doveman · Post by **doveman** » Fri Oct 11, 2013 11:56 am

ampmike wrote:I wander if it just needs a little breakin,The brown Suger is a bit like that too.The second thing ,could you get hold of a NOS set to see if there is any difference.Just a couple thoughts.Have Fun,Mikey

Yeah ... that's a possibility ... I always have a bit of trouble with a new amp. But I eventually get there. Not worried ... just looking for ideas for this one. But I don't want to trash a bunch of parts playing with it ... just moving along carefully.

wyatt · Post by **wyatt** » Fri Oct 11, 2013 1:22 pm

Edited.

I think you'll easily be safe around 40-43mA, maybe higher. The 35mA recommendation is pretty much a catch-all for most 6L6GC amps. It was what Allen was quoting for the Accomplices (tweed-style chassis) back when their iron really wasn't up to the task of the larger tubes (Allen changed to more robust iron around the same time he changed chassis).

Allens are sort of "neo-blackface," everything people usually rave about on BF Fneders has been enhanced...more gain, Deluxe Reverb preamp voltages for lower preamp headroom, etc. So, many like to mod them closer to Fenders specs to taste...my list of these mods was posted by someone named topbrent over at TGP.

1. 1k/4.7k dropping resistors, Raises preamp voltages,= more clean headroom (Early Old
Flame/Tone Savor amps had this as do Super Reverb and Twin Reverb amps)
2. 56k resistor to ground before entering PI, simulating opto-roach trem losses = more
clean headroom. (Early Old Flame/ToneSavor amps had this resistor)
3. 10pF capacitor in parallel with 3.3M (Adds much needed sparkle to reverb, and is correct
to the AB763 Reverb Circuit)
4. 12at7 in V2 and V4 (Early Old Flame/ToneSavor amps called for this, and also correct to the
AB763 Schematic)
5. Leave the RAW knob on "0" for the clean blackface sound. (even setting the RAW at
'1' will start adding mids/gain, and move farther from the classic Fender sound

So, my old David Allen-wired Accomplice had #3 stock (many early Allens did), and I did #1 and #2 and I loved the outcome. Much nicer, more Super/Twin-like cleans. And a harsh aggressiveness was removed.

I tried the 12AT7's and they just seemed to make my amp anemic, so I went back to 12AX7's throughout.

Roe · Post by **Roe** » Fri Oct 11, 2013 1:38 pm

usually 50-60% heat diss. sounds best with blackface type amps.

martin manning · Post by **martin manning** » Fri Oct 11, 2013 2:43 pm

Roe wrote:usually 50-60% heat diss. sounds best with blackface type amps.

Just to be clear, is this based on true plate current or plate plus screen?

doveman · Post by **doveman** » Fri Oct 11, 2013 2:52 pm

I just had another idea. I have a pair of Tung-Sol 5881 reissues that I ran for a long time at 445v in my Brown Note. They are suppose to be able to take up to 500v according to some but I only have experience with 445v. They should be able to take 420v pretty easy and at 23v plate dissipation, I could run it up to maybe 38ma. I always liked those tubes as they were in between 6V6 and 6L6GC. Might be a good experiment.

martin manning · Post by **martin manning** » Fri Oct 11, 2013 3:19 pm

Have at it! BTW, how are you measuring idle current?

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