Tweed Twin: concern about cathodyne PI harshness

[d***@sbcglobal.net]

I want to build a tweed Twin repro and am having a hard time deciding
between the 5E8-A and 5F8-A. Headroom is less of a concern than the
character of the amp when pushed hard. The 5E8-A has a reputation for
smooth crunch and enormous low end, but I'm concerned about the cathodyne
PI. I owned a Victoria 3x10 Bandmaster with the same PI and although it
sounded great either clean or very crunchy, I never liked the harsh way it
sounded on attacks in the in-between zone. Victoria's web page calls the
cathodyne PI "aggressive" and I'd agree. I finally sold the Vicky after
failing to soften the attacks even with radical tube swaps and a few
resistor and cap-value tweaks. It just didn't work with my neck HB and my
preference for a more graceful transition into distortion.

The low-power Twin's reputation is for a very smooth, creamy distortion and
sweet highs, which sounds like what I want as long as it doesn't get too
dark for my neck HB. Still, that cathodyne inverter concerns me. Should I
consider using a split-tail PI instead? From all descriptions, the mojo of
the low-power Twin comes from its unique front end and the hefty output
tranny. No mention is made of the cathodyne PI. Hmmm. I have heard a few
low-power repros but never had the chance to crank them up and listen to the
attack transients. My nagative perception of the cathodyen PI comes solely
from the Vicky Bandmaster and my frustration with being unable to get an amp
that sounded so good clean or crunchy to sound good in between those
extremes.

Any thoughts about this? I'm ready to order parts. I'm also curious about
whether I should spring for the additional expense of a MM output tranny, or
if the one from Mojo is adequate. If in fact the glory of the low-power
Twin lies in its OPT, that's not the place to save mone.

[Fulltone]

I think it's more about the type of components you choose. For example, if you
took any Fender or Marshall design and installed "slower" Paper/oil capacitors
instead of the Orange-drops you get a much more buttery tone. I built a KT66
powered Beano-Bluesbreaker type amp and used some old John E Fast
hermetically-sealed oil caps throughout...I can plug into the bright channel
with the Treble on 8 and it's still smooth and warm. Not the case when I swap
out for any other type caps! Ouch, the raspy treble.
A key mod to any Marshall/Bassman type design (and something I did a long time
ago to all my Marshalls and some of Trower's Marshalls) is to get rid of the
useless .68uf cap on the bright channel #1 12ax7 cathode and sub that for
22mfd/1.5K resistor combination.
Now you have plenty of bass, plenty of gain/touch, and not too much treble.
Michael Fuller / Fulltone Musical Products Inc. / http://www.fulltone.com

[miker]

Post by d***@sbcglobal.net
Victoria's web page calls the
cathodyne PI "aggressive" and I'd agree.

I think the culprit is the "lower half" of the cathodyne... it acts as a
cathode follower, the same thing that gives Marshall preamps some of their
character. Pretty easy to layout your initial design to accomodate either,
no? The long-tail has more parts, perhaps physically designing for that and
doing a little creative rearranging would let you try both.

Or go way nuts and use an interstage transformer. I've done that and it
sounded nice.

[Jerry Shaw]

Post by d***@sbcglobal.net
I want to build a tweed Twin repro and am having a hard time deciding
between the 5E8-A and 5F8-A. Headroom is less of a concern than the
character of the amp when pushed hard. The 5E8-A has a reputation for
smooth crunch and enormous low end, but I'm concerned about the cathodyne
PI. I owned a Victoria 3x10 Bandmaster with the same PI and although it
sounded great either clean or very crunchy, I never liked the harsh way it
sounded on attacks in the in-between zone. Victoria's web page calls the
cathodyne PI "aggressive" and I'd agree. I finally sold the Vicky after
failing to soften the attacks even with radical tube swaps and a few
resistor and cap-value tweaks. It just didn't work with my neck HB and my
preference for a more graceful transition into distortion.
The low-power Twin's reputation is for a very smooth, creamy distortion and
sweet highs, which sounds like what I want as long as it doesn't get too
dark for my neck HB. Still, that cathodyne inverter concerns me. Should I
consider using a split-tail PI instead? From all descriptions, the mojo of
the low-power Twin comes from its unique front end and the hefty output
tranny. No mention is made of the cathodyne PI. Hmmm. I have heard a few
low-power repros but never had the chance to crank them up and listen to the
attack transients. My nagative perception of the cathodyen PI comes solely
from the Vicky Bandmaster and my frustration with being unable to get an amp
that sounded so good clean or crunchy to sound good in between those
extremes.
Any thoughts about this? I'm ready to order parts. I'm also curious about
whether I should spring for the additional expense of a MM output tranny, or
if the one from Mojo is adequate. If in fact the glory of the low-power
Twin lies in its OPT, that's not the place to save mone.

Go to the Weber site and check out the "Paul C. mod", and check your email.

Jerry

[l***@aol.com]

Post by Jerry Shaw
Go to the Weber site and check out the "Paul C. mod", and check your email.

Also open to opinion. I haven't found any personal advantage yet to
any of the popularly-circulating mods to the split-phase PI. Some
others claim to increase "drive" for "more power tube clipping", but
this is impossible since the driving signal will always clip at the PA
tube grids at the same voltage no matter what you do the the PI. All
those mods do is give you the same PA signal clipping point at a lower
volume setting, which just means a dirtier amp with less, not more,
headroom as claimed.

This other mod (submitted) causes the split-phase PI to operate more
symmetrically by increasing the DC biasing of the stage. I fail to
see or (so far) hear how this can widen the clean-to-dirty zone, and
it may narrow it somewhat. With the split-phase PI stock, it clips
asymetrically at the PA tube grids (and resultant output signal as
well), meaning that only half the signal enters clipping first. With
symmetrical operation similar to a long-tailed pair that is
well-balanced, both halves of the signal clip at the same playing
level/settings. This will typically give an amp that stays clean a
little longer, then starts to enter full clipping. The result is also
harmonically different & those nuances of the assymetrical arrangement
are gone. As always, a balanced scope trace & a tonally balanced
signal are 2 different things.

This is something easy to try on an already-built amp as a reversible
experiment that may or may not be a keeper, but I wouldn't go
designing a new build for a specific playing goal by counting on a PI
mod idea I'd never played through to carry the day & make a whole
project pass/fail.

My sense is that most of these mods are chiefly attempts to make a
player/axe/amp combination that doesn't get along well work somewhat
passably. A much better way to self-build, if one doesn't have a lot
of proven design experience, is to first find an existing amp that
fills the bill or is very close with only a few other features missing
or different, and build that with those changed incorporated,
confident of how it will sound from the outset. And in just as may
cases, the real sonic problem is axe or player-related. These, too,
are better discovered by finding out if there is anything alreay
existing that sounds right before creating something else.

[l***@aol.com]

Post by d***@sbcglobal.net
I want to build a tweed Twin repro and am having a hard time deciding
between the 5E8-A and 5F8-A. Headroom is less of a concern than the
character of the amp when pushed hard. The 5E8-A has a reputation for
smooth crunch and enormous low end, but I'm concerned about the cathodyne
PI.

A very subjective matter. Many of us - I am one - get exactly what
you say you are looking for with a split-phase (cathodyne) PI amp
driving 6L6's. What you are experiencing may or may not also be
related to your HB p/u's characteristics, or your technique. FWIW I
spend most of my time in the "zone" you describe, formerly with a very
early stock Tele neck p/u & presently with a 7.5k custom-wound
minibucker. It is so important to me that if it wouldn't do it & with
plently of touch-"width" of the "zone" from quite clean all the way to
quite dirty without touching any controls, I'd throw the amp away, so
I know what you mean. But I have experienced this very problem to the
point of distraction on many Fender-type amps with the usual array of
stock f/s buckers (neck), and for some reason I have never fully
figured out, only a SR (the 4x10") did the deed with buckers no matter
what the others had inside. I am talking real-life gigging at high
settings - YMMV.

I've also got this great result for decades from these amps with no
ckt tweaking of any kind, and only in recent times did I get into
de-farting them, which also is not related to the threshold width you
describe.

Post by d***@sbcglobal.net
Should I
consider using a split-tail PI instead?

You must mean long-tailed, but if you believe that PI config is a
problem for you, you can easily play a number of amps with both & find
out first. FWIW my '64 BFVR, also a 2x6L6 amp w/long-tailed PI & orig
orange JBL's, was enough to make you quit playing guitar with any
ordinary neck 'bucker such as an early 335. But put a Tele through it
& it was jaw-dropping dynamically & tonally in every way.

Post by d***@sbcglobal.net
My nagative perception of the cathodyen PI comes solely
from the Vicky Bandmaster and my frustration with being unable to get an amp
that sounded so good clean or crunchy to sound good in between those
extremes.

Yup, when one has this problem, it is just like playing through a
typical SS amp - onset of high distortion comes on fast with little
lattitude in-between. But I suspect you are misplacing its blame.

Post by d***@sbcglobal.net
Any thoughts about this? I'm ready to order parts. I'm also curious about
whether I should spring for the additional expense of a MM output tranny, or
if the one from Mojo is adequate. If in fact the glory of the low-power
Twin lies in its OPT, that's not the place to save mone.

It cannot have anything to do with the iron used as long as all are of
similar weight, ratio & DCR, which they are. I personally see no
point in trying to glorify a primitive & simple old ckt with sexy &
pricey iron, but in any case it won't have any bearing on what you
describe. BTW, I never found the NPT OPT to be "massive", either,
though it isn't wimpy. But the bigbox was harsher from those pushed
5881's running at or past their ratings, and for the NP 6L6 job, I'd
forget about its 2 toob rectumfriers & build it with diodes, just as
Fender would've if they'd been available.

You may, or may not, only be trying to play the wrong axe through the
wrong amp for it.(?) I don't think the PI matters for it, but for YOU
it may.