Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on October 04, 2010, 09:20:55 pm

Title: GZ37 max capacitance?
Post by: firemedic on October 04, 2010, 09:20:55 pm

I just ordered a NOS Mullard GZ37, the most I've spent on a tube to date. But maybe I should have done some more research because the max capacitance it can see is 20mFd, according to some, and the datasheet reports a max of 4mFd! My first filter cap is 32mFd, have I made a mistake? And does anybody else really notice a big difference from the GZ34?

Title: Re: GZ37 max capacitance?
Post by: Geezer on October 04, 2010, 09:43:37 pm

See the note on this page>>>

http://www.andycowley.com/valves/rect/GZ37/GZ37-02.html

Quote

typical value for capacitor input,very conservative, it is known to work well up to 60 μF

Title: Re: GZ37 max capacitance?
Post by: PRR on October 04, 2010, 10:45:23 pm

> the datasheet reports a max of 4mFd!

That's NOT a Max rating; that is a "typical".

The max rating is 750mA peak per plate. If you are doing something odd, assume a tube rectifier's peak current is maybe 3 or 4 times the DC current.

The math for that is hard. However you can run a pair of 6L6GC or EL34s very hard with 100uFd main cap, as long as the PT secondary resistance is at least 25 ohms per side.

Title: Re: GZ37 max capacitance?
Post by: FYL on October 05, 2010, 05:30:38 am

Quote

I just ordered a NOS Mullard GZ37, the most I've spent on a tube to date. But maybe I should have done some more research because the max capacitance it can see is 20mFd, according to some, and the datasheet reports a max of 4mFd! My first filter cap is 32mFd, have I made a mistake?

Up to 60 µF for C1 is OK, depending on the type of PT used and it's source resistance.

Quote

And does anybody else really notice a big difference from the GZ34?

The GZ37 draws close to 3 A vs. 2 A for the GZ34, the PT core will behave differently. It's plate resistance is higher, leading to more voltage drop and slightly more sag in class AB amps.

Title: Re: GZ37 max capacitance?
Post by: firemedic on October 05, 2010, 10:19:10 am

Thanx for the replies, a little reassurance from those in the know never hurts. I would have tried it anyway. My rectifier filaments are supplied by a separate 5v3A transformer so my PT should not be affected by that, at least.

Title: Re: GZ37 max capacitance?
Post by: firemedic on October 09, 2010, 03:15:18 pm

It just came in the mail, worth every penny. It barely fits in my head. I'll need to stock up on a few of these before all the old Mullards disappear. It's hard to describe the effect- the amp sounds more open & expressive, softer. Buttery. It sounds clean all the way up to 10, too, where it broke up around 7-8 before. Probably just a loss of gain but very nice.