Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on January 04, 2023, 01:25:27 pm

Title: 50 watt PT with a 100 Watt OT
Post by: joesatch on January 04, 2023, 01:25:27 pm

Trying to scrounge this build together. I have a 50 watt PT but my OT is for a 100 watt marshall. The 100 watt OT has 4 8 16 ohm taps.  Will those stay in place or do i need to halve something or double something? I'm running 2 x EL34

Title: Re: 50 watt PT with a 100 Watt OT
Post by: jordan86 on January 04, 2023, 01:44:33 pm

Pretty sure with just two EL34, the resistance on the primary side of the OT is double what the OT expects to see. Which means your secondary “doubles” as well. Effectively, those become like 8,16,and 32 ohm outputs.

Title: Re: 50 watt PT with a 100 Watt OT
Post by: joesatch on January 04, 2023, 01:48:34 pm

So the same as pulling 2 power tubes in a 100 watt amp?  You connect an 8 ohm speaker cab to the 16 ohm OT tap?

Title: Re: 50 watt PT with a 100 Watt OT
Post by: 66Strat on January 04, 2023, 01:52:23 pm

Your concern is the reflective load of the output transformer. Two output tubes want twice the load of four output tubes, i.e. connect 8 ohm speaker load 4 ohm tap or 16 ohm load to 8 the ohm tap. To verify this, compare the 50 watt output transformer specs to those of the 100 watt OT.