Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on December 27, 2019, 03:41:30 pm

Title: Exceeding heater winding filament capacity
Post by: Diverted on December 27, 2019, 03:41:30 pm

I built an AB763 for a friend. It uses a Heyboer/Mojo 762 power transformer which is rated at 4a on the 6.3v winding.
Currently the heater winding is seeing 3.75a draw (2x6l6gc, six 12A-types and a #47 pilot light)
He would like to add three more #47s which if added in would equal a 4.2a current demand - over Heyboer spec.

My question is how conservatively rated are Heyboers?
The lights are for a backlit nameplate on the baffle. I suppose I could use just two for a total draw of 4.05a. Just don’t want to fry it out.

Thanks!

Title: Re: Exceeding heater winding filament capacity
Post by: sluckey on December 27, 2019, 04:30:06 pm

I would not hesitate to add the extral lamps. BTW, they do make LED replacements for #47s. Very low current...

https://www.ebay.com/itm/10-Pack-White-Pinball-6-3v-LED-Round-Replacement-Bulbs-44-47-Bayonet-Base-BA9S/233153741387?hash=item36490bc24b:g:BiMAAOSwGMFcfCLZ

Title: Re: Exceeding heater winding filament capacity
Post by: Diverted on December 27, 2019, 04:50:37 pm

Thanks Sluckey, those look pretty slick. How’s the brightness compared to standard 47s?

Title: Re: Exceeding heater winding filament capacity
Post by: sluckey on December 27, 2019, 06:41:01 pm

I've never compared the brightness but it was good enough for me. I use them in the Fender style red lens holder. They are available in several different colors.

Title: Re: Exceeding heater winding filament capacity
Post by: davidwpack on December 28, 2019, 04:17:48 pm

Ones I got were frosted. They are very bright, possibly brighter but slightly alter the color of the jewels. IE, Amber jewels appear slightly yellowish. I probably wouldn't have noticed if I wasn't the one that exchanged the bulb.

Title: Re: Exceeding heater winding filament capacity
Post by: Greg on December 28, 2019, 11:40:47 pm

They still make indandescent 130v 0,03A version bayonet-style and the old neon type 60V + serie resistor fitting the same 47 type socket. We usually see a 6.3v, 150ma bulb in those type of socket, so you need 120 v. spec clearly written somewhere.

Title: Re: Exceeding heater winding filament capacity
Post by: Greg on December 28, 2019, 11:52:39 pm

If you only exceed by 5 % there is no problem, especially if you don't overcharge the other windings. I would not worry by that small amount.

Title: Re: Exceeding heater winding filament capacity
Post by: Colas LeGrippa on January 08, 2020, 06:36:00 pm

I built at least 3 amplifiers with Heyboer trannies and I had to put diodes in series wirth the heaters circuit in order to lower their voltage.
So add the tubes you need and even if the voltage drops to 6.2, it will be just fine


Colas