Re: [Harp-L] Fender Vibrosonic

[Frank Evers <frank@xxxxxxxxx>]

Hi

Am Sonntag, 9. März 2008 schrieb Special20:
> 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?

They take care of their customers ;)
The 12AU7 is a _possible_ substitude for a 12AX7, but as described 
somewhere else in this thread it's not always a safe option and i 
wouldn't recommend it without advice of a tech.

Besides having different gain rates there are significant differences 
between the various 12A?7 tubes. Unfortunately this is not much 
communicated. The 12AX7 and 12AT7 are in fact tubes with very 
different characteristics, even though they are safe subtitutes. Also 
the gain factors communicated all the time can be misleading. These 
preamp tubes have two systems and both of them have the gain factor 
described in their specs. Even more the 12AT7 is more efficient then 
the 12AX7, so in fact their _real_ gain factor is only slightly 
different - much less then the 100-60 suggests.

> 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

Hmm, you're playing a Champ. In my original post i was referring to a 
bigger beast, because you mentioned removing one rectifierer tube 
where an amp holds two. Changing/removing tubes does in fact have 
different effects depending if you do it on a small (ClassA) or a big 
(Class A/B) amp.
But lets start with a few basics.

I hope my english will be good enough for this, there are a lot of 
technical terms i know only in german.

You got to understand what  high or low voltage actually means in this 
scope. All rectifierer tubes give more or less the same voltage to 
the filter caps - the voltage that comes from the transformer. Whats 
different about them and what makes some more powerful than others 
ist the maximum current that they can have.

The rectifierer tube's output is a rectified sinus wave and it's 
output voltage turns from zero to (sqrt2 times the eff. output 
voltage of the transformer) and back 120 times a second.

This output loads the filter caps BUT it loads only when the tube 
output has higher voltage than the filter caps, so they are loaded 
with short pushes of high current. The rectifierer tube must be able 
to stand this high current.
The filter caps are on the other hand drained continiously by the 
power tubes (mainly, the preamp tubes don't drain much in 
comparison). So all the power thats continiously drained must be 
loaded from  the rectifier in short time strong pushes.
the Voltage will rise until there's a balance of load and drain. This 
is the operation voltage B+.

What happens if you now use an undersized rectifier tube depends on 
the type of your amp.
Your Class A Champ's power tube  is set up in a way, that it'll drain 
more or less al the time the same  medium current from the filter 
caps, wether you make it work or not (wether you play or not). 
However it drains some power and the undersized recifier will not be 
able to stand this compared to the original tube. The voltage on the 
filter caps drops, which means on the other hand, that the rectifier 
tube has more time in each cycle to reload the caps. At some voltage 
below the original one there will be a balance of loading and 
draining again.
This way an undersized tube makes your class A amp run  at lower B+, 
which is fine. There'll be not much difference in B+ between playing 
hard and not playing at all.

It's different wirh a class A/B amp though. If you don't play and the 
amp doesn't have to work the power tubes will only drain low current, 
so B+ will only drop slightly with an undersized rectifier tube, less 
than with an class A amp. this is important, since the A/B setup 
requires that the power tubes run in their specs for this mode.
However if you make the power tubes work hard by playing your amp, 
they will drain much current from the filter caps, and similar to the 
effect described above B+ wll drop significantly. This means, the 
more the power tubes have to work, the lower is the amplification by 
your amp. This effect is called compression and you want tat for 
harp!

Now, all this is still somewhat simplyfied, but it should help to 
understand what effects changing rectifier tubes has.
 

> Also, Gerald Weber in his book about Vintage Tube Amps said all
> Champs sound better with a 5V4 rectifier.  

Hey, i don't know that book, can you give me the title? Gerald Weber 
was the one who originally made me understand what the main 
differences between guitar amps and harp amps are.


Gruß,Frank