Re: [Harp-L] Re: Vibrosonic

[Frank Evers <frank@xxxxxxxxx>]

Am Montag, 10. März 2008 schrieb Jim Rossen:

> I don't think it is necessary to prepare an amp that uses 12ax7
> tubes to use 12au7 tubes.

The 12AU7 has significantly higher current than a 12AX7, Especially 
the working resistor (is that the term used in english?) may heat up 
beyond it's specs if doesn't stand the extra wattage.
It might or might not work fine, but i have worked two amps so far 
heating up a 1/4W resistor with nearly 1/2W when using a 12AU7! You 
see they were running out of specs and became pretty hot. You can 
take the risk of course, but i'd recommend to make sure changing 
tubes not not a risky thing.

> Changing a rectifier in a class AB amp WILL change the operating
> voltage as well as current.

Yes, you're right, but the voltage drop will be only slightly as long 
as you don't make the amp work. The change in tone won't come from 
these few volts. However IF you make the amp work hard B+ will drop 
significantly more than it would with the original tube setup. This 
is the effect we were talking about. It adds whole a lot of 
compression without feeding back. I'm sure we can find agreement here

> Reducing the operating voltage may be a GOOD thing for harmonica
> use and is a common practice.

Yes, for sure. However if you tweak the A/B amp in this way you at 
least have to rebias and probably take some more action requiring 
professional work. 
This thread was about tube changes, not tweaking the amps guts!

-- 
Gruß,Frank