Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ChubeRules on May 04, 2015, 11:55:55 am

Title: Strange Cathode Voltage Reading
Post by: ChubeRules on May 04, 2015, 11:55:55 am

I'm currently modifying a 5E3 to a 5E3P Proluxe.  The mods include upgrading to 6L6s and a fixed adjustable bias from a cathode bias.  Both PT and OT have been upgraded as well as filter caps to accommodate the new tubes.

With all the new parts installed the amp fires up without any fireworks and produces 5E3-like sounds when a guitar is played through it.  But, at a much reduced power output.

At this point I started checking voltages and aside from a high B+ (450 vdc on the plates) all voltage readings appear to be in line with the high B+ reading, save one. 

I have 1 ohm resistors installed for biasing and I'm reading +12 vdc across them !!!  So I connected my bias probe and found the same reading (in milliamps, of course).  Was I really reading 1.2 amps?

My cathode to plate voltage reading also shows the same +12 vdc discrepancy.
Has anyone ever seen such a thing before?

I've swapped in two known good 6L6s with the same results.

Plate... 449 vdc
Grid 2...424 vdc
Grid 1...-32 vdc
Cathode... 12 vdc

all measurements to chassis ground.

Title: Re: Strange Cathode Voltage Reading
Post by: John on May 04, 2015, 01:05:23 pm

If it's cathode biased, then you have your 1 ohm resistor in series with the 250 ohm, correct?  If so, then 12 volts is at least in the ball park.

Title: Re: Strange Cathode Voltage Reading
Post by: AZJimC on May 04, 2015, 01:26:05 pm

Are you sure the resistors are 1 ohm, not 10 ohm?

Title: Re: Strange Cathode Voltage Reading
Post by: sluckey on May 04, 2015, 02:20:14 pm

Quote from: ChubeRules on May 04, 2015, 11:55:55 am

I have 1 ohm resistors installed for biasing and I'm reading +12 vdc across them !!!

12v across 1Ω would be 12 AMPS! That's not realistic. Take a second look and try describing the problem again. A picture would be helpful.

Title: Re: Strange Cathode Voltage Reading
Post by: ChubeRules on May 04, 2015, 03:14:45 pm

Bullseye!  I had 470 ohm resistors where 1 ohm resistors were supposed to be.  Can't rely on my eyes anymore... 


What mystifies me now is that for the last couple of years, when the amp was in an otherwise stock configuration as a 5E3 with a stock cathode bias, it's been working and sounding great!  In fact, I was running a 440 ohm 5w bias resistor (instead of the stock 250 ohm).  For a grand total of 910 ohms !!!


Anyway, thanks for the leg up, until next time ...