Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: neskor on April 26, 2025, 03:25:06 pm

Title: EL34 pin 1 connected via resistor to pin 3???
Post by: neskor on April 26, 2025, 03:25:06 pm

Hi.
I just received stereo amp with parallel SE EL34 on output with cathode bias.
It has a blown Power transformer.
I can see that pin 1 on El34 is connected to pin 3 with 1K resistor. Never seen that before.
Is that normal practice?
Usually pin 1 is connected to pin 8.
Also pin 4 is also connected to pin 3 with 150R resistor, triode mode?
Thank you in advance!

Title: Re: EL34 pin 1 connected via resistor to pin 3???
Post by: Merlin on April 27, 2025, 05:23:34 am

Yeah it's in triode mode, with both the screen and suppressor grids tied to anode. The resistors are a just-in-case measure to ensure the grids don't pass more current than they can handle, as well as discouraging oscillation.
It's rare for a triode amp to blow an output transformer, I wonder what happened.