• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

845B problem

I swapped stock 845 tubes in my Ming-da 845C to Valve Art 845B. After I turn off the amps I saw redish glow on the plates. It may mean that the bias to high. But every source states that 845B can be used as a direct replacement for standard 845. What is the electrical difference between standard 845 and 845B?
What should I do if my amps are cathode biased?

Thanks for ansvers

Choky has it right.
With Cathode Bias its easy to measure Vak (Voltage anode to cathode) and calculate Ia (Anode Current) from the voltage drop across the cathode resistor. From there multiply the values together to get Pa (Anode dissipation) for the Tube. If its more than 75W then increase the cathode resistor. 845s are meant to have 100W Anode Dissipation BUT a lot of the new 845 Tubes will only cope with 75W.
CAREFULL - use a multimeter with decent probes and lead insulation, Vak could be anything between 700 and 1200 volts.


Paid Member
Actually the 845 is an audio amplifier/modulator tube, and afaik is not generally used in RF output stages. These tubes probably should not be operated with glowing plates unlike the 805. No 845 amplifier I have worked on has ever run them that hard.. 60 - 70ma at 1KV with fixed bias is reasonable..

See: http://www.mif.pg.gda.pl/homepages/frank/sheets/111/8/845.pdf

Note that at least the Amperex version of this tube is rated at only 75W dissipation.

It is also possible that your amplifier employs fixed bias in which case a minor adjustment to the bias divider string is all that may be required (if no pot.) Otherwise you have cathode bias and you will need to adjust the value to reduce the current and hence the dissipation to a safe value.

Kevin K.

Edited to fix typo.