Re: [Harp-L] Fender Vibrosonic



Here is a link to a nifty little Flash application at The Tube Store
that shows which 12A?7 preamp tubes are suitable substitutes for each
other: (click on the little tube icons)
http://thetubestore.com/gainfactor.html

As you can see, the Tube Store says 12AU7 and 12AV7 tubes are not good
substitutes for the usual 12AX7.  Is that not right?

Here is a link to a handy list of rectifier tubes, from higest to
lowest voltages, and least to most sag:
http://www.kcanostubes.com/content/newsletter_details.asp?ArticleID=3

1) Solid State Rectifier (NOT Weber Copper caps)
2) 5AR4/GZ34
3) GZ37       (3 amp filament current)
4) 5V4G/A    (Possibly lower if plate current of the power tubes is high enough)
5) 5U4/G/A/B  (3 amp filament current)
6) 5R4G/A/B
7) 5Y3G/B

Mike at KCA NOS Tubes is pretty much an expert on this stuff.  Also,
Gerald Weber in his book about Vintage Tube Amps said all Champs sound
better with a 5V4 rectifier.  I emailed him and asked if that applied
to harp as well, and he said it was a matter of taste.  I tried it and
I think it sounds great.

Everything I know about tubes (and it ain't much) I learned by trial
and error.  My 1970 Fender Champ has a Sovtek 5751 in the preamp
socket, NOS RCA 6V6 power tube, and a JAN Sylvania NOS 5V4 coke bottle
rectifier.  I have a pot in the negative feedback circuit so I can
adjust the grit, and it has a 10 inch Weber Vintage alnico speaker
(10A125-O).  The tone is remarkable.

-Spec20



On Sun, Mar 9, 2008 at 1:03 AM, Frank Evers <frank@xxxxxxxxx> wrote:
> Am Samstag, 8. März 2008 schrieb Special20:
> > 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?
>
> No, by far the lowest gain tube is the 12AU7, then 12AY7, 12AT7, 5751,
> 12AX7. The 12AU7 _can_ be suitable replacements, however they have
> quite high current compared to a 12AX7 and the circuit must be
> prepared to stand that high current
>
> > 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.
>
> There are several options how a phase inverter can be built. One often
> used design uses onla one system of a double triode, some use both,
> and some amplify the signal more, other designs less. You cannot
> generally say the phase inverter stage adds gain or not.
>
> > 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.
>
> In an A/B class amplifier the power tubes don't have much current when
> the amp doesn't have to work, however if you make them work hard
> there's qute a lot of it.
> Rectifier tubes wont  generally have an effect on the voltage and
> changing them would be a very bad idea then, because the power tubes
> wouldn't run in their specs.
> However a rectifier tube only delivers a limited amount of current. If
> you use a less powerful rectifierer or remove one of them when the
> amp is designed to hold two of them, the  power tubes will get
> underpowered if you make them work hard, which generates an
> compression effect often described as 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.
>
> The Champ is a class A amp and the compression effect will work much
> less compared to a A/B amp.
> Heating the power tubes will of course reduce their lifetime ;)
>
> --
> Gruß,Frank
>
>
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