The 'Mule Project'- learning about valve/tube amps on a B25.

[pentode]

Hi Steve,

Thanks for your input!

I popped a 12AX7 into channel two and it became quite 'gainy' and noisy, rather like channel one.

I tried using a pair of 12AU7's and whilst there was less volume it sounded a lot cleaner which suited the bass better.

There was still a speaker ripping distortion near full volume. Now is this just a trait of the B25 or is there an issue with this example?

I thought I'd take measurements at the valves and see if there was anything obviously amiss.

There was.

The two preamp valve voltages were wildly out. Why? Because I still had the 12AU7's in there...!!

Changed them over to the proper 12AX7's and things looked much better in the preamp section. However things seem to be out on the 7199 PI valve. The B+ is a bit high but even so....

Here's the schematic -

http://www.kbapps.com/audio/schematics/t...es/b25.gif


The voltages on mine -


B+ - 523

Nodes

C - 456
E - 353


V1 - 6L6GC

1 - 523
4 - 522.3
5 - -61.3


V2 - 6L6GC

1 - 523
4 - 522.3
5 - -61.4


V3 - 7199

1 - 326.5
2 - 200.4
3 - 81.6
6 - 2.6
7 - 0
8 - 134.8
9 - 111


V4 - 12AX7

1 - 239.7
2 - 0
3 - 2.3
6 - 251.7
7 - 0
8 - 2.4


V5 - 12AX7

1 - 233.8
2 - 0
3 - 2.4
6 - 245
7 - 0
8 - 2.4


So, any thoughts?

Many thanks
Grahame