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HP8903B 7th November 2010 07:00 PM

Which tube does my amp use? 845 or 211

astouffer 7th November 2010 07:16 PM

Who makes the amp? If the amp uses fixed bias you could measure the voltage at the grid contact.

HP8903B 7th November 2010 07:26 PM

2 Attachment(s)
I don't know who makes the amp. I attached some pictures.

Burnedfingers 7th November 2010 07:38 PM

I think that you need to send this amplifier to me:D:D:D

HP8903B 7th November 2010 08:02 PM

To measure the grid voltage, do I put the RED probe on the Pin 1 (Grid) and the BLACK probe on the Pin 2 (Filament)? Do I do this with the tube installed? If so, which tube should I installed? I do notice that the 211 and 845 have very different DC grid voltage, 211 is -61V and 845 is -145V at 1000V plate voltage.

What do I do if it has adjustable bias?


Palustris 7th November 2010 08:16 PM

You better know EXACTLY what you are doing to poke around in an amp with a B+ of 1kV!

HP8903B 7th November 2010 08:19 PM

Thank you for the EXCELLENT advice.

That is why I am here seeking for specific instructions.

Frank Berry 7th November 2010 08:54 PM

To measure the bias voltage, the black probe should be connected to pin 1 (grid). The red probe should be connected to chassis ground.
Do this without the tubes installed.
What are the other tubes in the amplifier?

astouffer 7th November 2010 09:01 PM

Put the black probe on the chassis ground and measure the grid voltage without the tubes. Don't try measuring the plate voltage. Most digital meters top out at 1KV and unloaded the supply will read much higher. Judging by the pics the amp looks homemade but well built. If the bias has enough range then you could run either tube depending on the transformer impedance.

HP8903B 7th November 2010 09:24 PM

Thank you very much for the information.

So does that mean the two factors that determine which tube to use are the grid DC Voltage and output transformer impedance?

I am pretty sure the output transformer impedance is 7k. If it is 7k, is it for 845 or 211?

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