Modding pwr amp

[rzenc]

Hi Fellas,

It's my first time with an amp that has such wierd - at least too me  - power tubes cathode arrengment. Will it hurt if I install 1 ohm @ 1W cathode bias sensing resistors in order to adjust current thru tubes?? The amp do not have them and O.T. leads are directly soldered at sockets, so using O.T. shunt method is unlike due to the mentioned fact.

Thanks in Advance.
Best Regards

Rzenc

[FYL]

It's my first time with an amp that has such wierd - at least too me  - power tubes cathode arrengment.

It's a bog standard fixed bias stage with grounded cathodes.

Now have a look at the added parts: a fuse with a LED + limiting resistor in //. The fuse shows a negligible impedance, the R+LED combo is much more resistive, nearly all current flow thru the fuse, so cathodes are still grounded. If the fuse blows, current will only flow thru the R+LED leg, the LED will light and you'll know that it's time to check the amp and change the fuse...

Will it hurt if I install 1 ohm @ 1W cathode bias sensing resistors in order to adjust current thru tubes??

Absolutely no problemo.

[rzenc]

If the fuse blows, current will only flow thru the R+LED leg, the LED will light and you'll know that it's time to check the amp and change the fuse...

Isn't it strange to make use of a 0.8 A fast blow fuse? I believe this amp would never reach 0.8 of cathode current... it's a 50W - pair of 6L6. My 6550 beast drags around 0.29A's @ full blast... how come a pair of 6L6 will eat 0.8? Short between elements??? The only shorts I recall were between plate and fil. aka. pins 2 and 3 and it smoked the offending power tube socket  but I know I will live to see other bizarre stuff happening.

Thanks for your answer FYL.

Best Regards,
Rzenc

 

[FYL]

A PP of 6L6GC in AB mode will typically draw 200 mA max and maybe 250 mA fully cranked. A fast 800 mA fuse will only blow if something goes very wrong, typically an internal short.

Here's a pic from one of my studio amps (a VTL monoblock) after a 807 shorted a while ago. The 5W 10R sense R and the three 2W 50R anode Rs have overheated before the main fuse blew. The MercMag OT and PT are still fine.