Re: [Harp-L] Fender Vibrosonic

[Frank Evers <frank@xxxxxxxxx>]

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