Help: Biasing 5881's in a Deluxe Reverb

[Robert Trent Burkey]

Just purchased a 70ish Deluxe Rev with original tubes... I want
to put a set of 5881's in it & wanted to know if anyone had a
recommended bias current for this situation (Using Weber's
Current method).

Thanks....

Bob Burkey
rtb...@tamsun.tamu.edu

[Tremolux]

I've done this exact thing, except mine is a 64 Blackface (pre CBS). It
sounds great, incidently. More punchy and more headroom than with the
6V6s.

Anyway, I set my bias for 30 ma per tube, and I'm using the Russian
5881WXT tubes. The current method (transformer shunt) method is what I
also use.

If you have a 70s silverface Deluxe, you can make it really loud with the
5881s. You first must replace all the electrolytic filter caps. I
believe the originals are all 16 uf at 450 volts. I replace them with 20
or 22 uf at 500 volts. Then you can substitute a Russian 5AR4 rectifier
in place of the stock 5U4 (it just plugs right in). Replace the bias
filter cap as well. I use a 100 uf at 100 volt cap here. The rectifier
sub will give you more B+ on the 5881 plates, which translates to more
power output. Don't do this unless you change the filter caps, or they
might blow. My buddy, who I gig with also has a 64 Deluxe. At the gigs,
we set them virtually side-by-side. Mine sounds louder, with more bass
(without farting-out), and is cleaner. He even says it sounds real
"chunky". If you really want maximum loudness, instead of the 5AR4, use a
plug-in solid state diode module. Your Deluxe will sound like a little
Twin!

A word of caution. The stock speaker in the Deluxe is rather wimpy. If
you do as I described, it might blow. You can replace it, but by all
means, keep the original in case you ever want to sell the amp. If you
wan to try a replacement, I recommend the Celestion Vintage 30. A very
un-British sounding speaker. Warm, smooth and very efficient. I have one
in mine. Better than an old Jensen! Trust me on this, since I have
direct experience. Go for it!!!!

Regards.

[Robert Fries]

Weber recommends 35 ma, start there.

I recently installed 5881's in my '70 Deluxe Reverb, along with SS rectifier,
and beefier filter caps. I also upped the voltages to the preamp and
phase inverter, which involves changing two resistors. I'm waiting for
a new Eminence 12" speaker - this amp sounded thin with it's original
12" Oxford, it sounds great plugged into the speaker in my '65 Deluxe
Reverb Reissue.

Mine's set at 35 ma. now - I'll probably fiddle with it more after
I get the new speaker.

So far, I like it - but I haven't had a chance to take it out and really
crank it, yet. And there is some possibility, (small, I hope), one of the
transformers will start smoking...

Robert

[OmarNepo]

Bob,

I work on amps that have 5881 and like to run them from 35 ma up to 50 ma.
I have tried to put 5881 in a 6v6 amp and was dissappointed. I love the
sponginess that 6v6 tubes give. Give it a try you might like it!

Later,
Omar Nepomuceno

[Tremolux]

Careful making blanket statements like that regarding biasing, or someone
will blow up their amp. The LAST thing you would want to do is put 5881s
in a Deluxe and bias them for 50 ma per tube. No way, Jose!!!

The Deluxe power supply and output transformer were not designed for
anything near that. I run mine (I have Russian 5881s in my 64 Deluxe
Reverb) at 30 ma per tube, and this results in the output transformer
primary winding dropping over 5 volts between center tap and plate due to
IR losses. Any more would be too much. The 5881s in my Deluxe give it a
real strong and clear clean tone with mondo low end, and a sweet high end
as well.

Bear in mind that the Deluxe's magnetics were not designed for 5881s, so I
have measured the output at 25 Watts at the onset of clipping. I'm
running a solid-state rectifier in it, and I have replaced all the
electrolytic filter caps (originally 16 uf @ 450V) with 22 uf @ 500V
units, in order to stiffen up the supply a bit for the 5881s.

This is not to sell short the 6v6s. These tubes sound great, if you can
get NOS American tubes. The current production Russian and Chinese 6V6s
suck big time. The Groove Tubes 6V6HD is NOT a real 6V6, it's really a
wimpy Russian 6L6, so forget it. I put in the 5881s specifically to get
the thing to go as loud as possible before breaking up, and it worked. In
the Deluxe, the 5881s are dead reliable.

The 50 ma bias figure sounds more like Calss A operation to me. Class A
amps usually run at lower plate voltages so as not to over dissipate the
tubes since they run at high idle currents. Don't try to get an amp
designed for Class AB1 to run Class A, because the magnetics probably
won't take it.

In amps designed for 6L6/5881s from the beginning, such as Super Reverbs,
etc., the right AB1 bias value is around 35 ma. The 50 ma value will
result in the power transformer running really hot (maybe too hot),
probably hum due to inadequate filtering for this current level, short
tube life (they would really be cooking), very early break-up, and
probably a somewhat mushy sound.

Sorry, Omar, but I disagree with you on this one, without your putting in
any qualifiers.

Regards.