Re: [Harp-L] Fender Vibrosonic

[Special20 <special20harp@xxxxxxxxx>]

I'm certainly not a tech and I'm trying hard to learn more about tubes and
tube amps.

I am familiar with the concept of using lower gain preamp tubes in V1 or V2
to decrease feedback and increase usable power.  I use a Sovtek 5751 in
place of the 12AX7 in my Champ, for example.  My understanding is that 12AU7
tubes are not suitable replacements for 12AX7... the lowest gain tube that
is suitable is the 12AY7.  Is that not right?

I don't understand how changing the Phase Inverter tube will chage the
gain.  Doesn't the phase inverter simply keep dual push-pull power tubes in
sync?  I'd like to know more about this.

And isn't it the rectifier tube that controls the "sag" in the power tubes
by limiting their voltage?  In amps with two rectifier tubes, such as the
Tweed Bassman, some players actually remove one of the recto tubes to get
really fat sag.

I went the other way on my Champ, using a hotter 5V4 recto tube.  Small
single ended amps don't really get sag anyway, since they are pretty much
cooking at full tilt all the time.  So I heat the power tubes a bit more to
get a little more volume and grind.

Thanks for any guidance you can give me on this.  I'm always ready to learn
more.

-Spec20



On Sat, Mar 8, 2008 at 3:59 AM, David Harris <snakehead_1@xxxxxxxxx> wrote:

> Hi Spec20,
> Putting a lower gain tube in the Inverter helps to reduce feedback. Wen
> lower gain tubes (Like a 12AXT) are used in the Normal channel, the feedback
> will be reduced even more.  Notice I say reduced not eliminated.
>
> A lower gain Inverter allows for more usable range in the gain controls.
> In effect, the amp should be able to get louder if 12AU or 12AY tubes are
> put here.
>
> Putting lower gain tubes in the Inverter also introduces a nice "sag" that
> is great for harp.  Whereas the amp will usually be very clean, the lower
> Inverter tube will help introduce some breakup an distortion.
>
> Dave Harris
>
> *Special20 <special20harp@xxxxxxxxx>* wrote:
>
> Why would you change the Phase Inverter tube?
>
> -Spec20
>
> On Fri, Mar 7, 2008 at 7:43 AM, David Harris wrote:
>
> > Jim,
> > My research show that this is the standard layout of the tube layout for
> > the Vibrosonic:
> >
> >
> >
> http://bb.steelguitarforum.com/viewtopic.php?p=1095549&sid=9b8d27feec6f14b61cf9263c596f73f2
> > (Stock tube layout facing the rear of the amp)
> > 4 6L6GC Power Tubes
> > 12AX7A 1st stage gain
> > 12AX7A 2nd stage gain
> > 12AT7 Reverb Driver
> > 12AX7A Reverb Return
> > 12AX7 Tremolo
> > 12AT7 Phase Inverter
> >
> > Is that close to your current setup?
> > What I would suggest is concertrate on changing the Gain tubes and the
> > Phase Inverter. Don't even use the Reverb and Tremolo for harp. You
> > should, however, make sure that you replace any blown or worn out bulbs.
> > Especially since you mentioned intermittent performance for the tremolo.
> >
> > Here are my suggested tube types:
> >
> > 12AX7 1st stage gain
> > 12AU7 2nd stage gain
> > 12AU7 Phase Inverter
> >
> > or
> >
> > 12AT7 1st stage gain
> > 12AU7 2nd stage gain
> > 12AU7 Phase Inverter
> >
> > or
> >
> > 12AT7 1st stage gain
> > 12AU7 2nd stage gain
> > 12AY7 Phase Inverter
> >
> > Try those and see how they sounds.
> >
> > Dave Harris
> > http://www.myspace.com/superdharris
> >
> >
> > "Priest, Jim" wrote:
> > Anybody have any experience of, or views on, a Fender Vibrosonic Reverb
> > as a harp amp? I just acquired one of about mid-70s vintage. It's in
> > just-about-working order but has been in the wars somewhat and needs
> some
> > TLC. It has a single 15-inch speaker (which looks like a replacement but
> has
> > no discernable id), tubes are 4 x 6L6 power, MESA brand (one of which
> has
> > blown) and 6 assorted pre-amp. It has in-built reverb (working) and
> tremolo
> > (intermittent). And it weighs a ton!
> >
> > Any suggestions on tube config or upgrades/mods that will suit it better
> > for harp?
> >
> > Cheers
> > Jim
> >
> >
> >
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