Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lectroid on December 24, 2025, 03:17:34 pm

Title: PT safety margin?
Post by: Lectroid on December 24, 2025, 03:17:34 pm

I need to replace the power transformer on a Bell&Howell derived amp. (Two 6V6s, maybe 15W?).  I have an unused ClassicTone that looks like a good fit.

For the filament supply, it puts out 6.3V at 1.65A while total filament current requirements for the tubes is 1.5 A.  So there's an excess 10% (0.15 A) unused current available. Good enough.

Or is it?  Should I be adding in a safety margin when speccing a power transformer?  What percent of the transformer's current output should be a safety margin.  Is 10% excess enough?

Title: Re: PT safety margin?
Post by: shooter on December 24, 2025, 03:25:39 pm

should be good to go

Title: Re: PT safety margin?
Post by: tubeswell on December 24, 2025, 06:49:21 pm

150mA for the lamp?

Title: Re: PT safety margin?
Post by: tubenit on December 25, 2025, 06:09:08 am

quick reference info

Title: Re: PT safety margin?
Post by: Lectroid on December 25, 2025, 10:24:50 am

Thanks guys.  What I am hearing is that the amp will probably not catch fire or explode.  Good enough.

So is that a general rule?   If the proposed new xfrmr has enough B+ and rectifier amperage to meet the necessary totals calculated from the datasheet values, then the amp should function normally, and no further safety margin is required.  Got it. 

The datasheet is below, ClassicTone's answer to the 5E3 market.

@shooter, thanks for the nudge. I'm doing it.

@tubeswell, good catch on the lamp--I had totally forgotten about it.  150mA--wow.

Lately I've been using the LED versions of those #47 bulbs.  They use 16 mA and they're almost too bright behind a Fender jewel.

Thanks again all.