Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on May 23, 2018, 03:14:30 pm

Title: 12AU7 sub for 12BH7 ?
Post by: mresistor on May 23, 2018, 03:14:30 pm

anyone know if it's ok to sub in a 12AU7 for a 12BH7.   Marshall 5C.


thx

Title: Re: 12AU7 sub fir 12BH7 ?
Post by: 92Volts on May 23, 2018, 03:43:35 pm

It looks like it acts as the power tube in this amp. The 12AU7 is also used like this for very low power outputs. The 12AU7 has lower transconductance so it won't drive a low load as well and biases differently. It also has lower plate dissipation. I have no idea how close this design is to the 12BH7's limits and whether this might be a problem.

I'd guess it would work, better than a 12AX7 for example. But it's not ideal and I'd worry it wouldn't last long.

Title: Re: 12AU7 sub fir 12BH7 ?
Post by: mresistor on May 23, 2018, 05:37:10 pm

thanks 92v   I noticed it has less current demand on the filaments too.

Title: Re: 12AU7 sub for 12BH7 ?
Post by: PRR on May 23, 2018, 07:35:28 pm

The BH is an extra-big AU. Usually if a BH is specified, the AU will not do the job.