Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 09, 2023, 08:09:03 am

Title: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: kagliostro on March 09, 2023, 08:09:03 am

I'm curious to know if in the past there were amps with Power Tubes that were Cathode Biased with a Positive Voltage given with a Power Supply instead of a Cathode Resistor

and also I would like to know what will happen with that configuration

Thanks

Franco

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: Latole on March 09, 2023, 08:40:09 am

The resistor acts as a brake to limit the current in the tube.

 Maybe a current regulator could do the same thing, I don't know but it would cost 100 times more

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: kagliostro on March 09, 2023, 09:03:00 am

Ciao Latole

Thanks

 :think1: :think1: :think1:


I don't think a current regulator will give the desired effect

I'm thinking (wrong or correct it is  :w2: :dontknow: :w2: ) more to a cathode resistor to perform a Mixed Bias


But also I think ..... this is the reason because I'm asking for

Franco

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: astronomicum on March 09, 2023, 10:19:28 am

I am not aware of any amp that does what you are asking but an interesting thought experiment  :think1: Of course you could not apply a voltage from a power supply to a cathode without a cathode resistor between the supply and ground because you would short the power supply, but applied between the cathode resistor and cathode to “trim” the bias, good question. If it would work, you could choose a resistor value which would bias the tube hot and trim it with positive voltage to cool it down. Conversely, you could choose a cathode resistor that would bias the tube cold and apply negative voltage to warm it up, which would make better sense in the event you lost the bias supply voltage. My concerns would be what happens with a power supply hooked directly to a cathode if there is a failure of the cathode resistor or if the tube fails  :w2:

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: kagliostro on March 09, 2023, 11:55:18 am

Ciao astronomicum

Quote

Of course you could not apply a voltage from a power supply to a cathode without a cathode resistor between the supply and ground because you would short the power supply, but applied between the cathode resistor and cathode to “trim” the bias, good question

I think that without a resistor you must connect the Positive voltage of the dupply to the Cathode, disconnecting it from ground, the Power Supply will have the negative connected to ground by himself

Quote

My concerns would be what happens with a power supply hooked directly to a cathode if there is a failure of the cathode resistor or if the tube fails

To me if the PS didn't supply any voltage, as the Grid Leak at the junction are grounded, is the same as to disconnect the Cathode in a Cathode Biased Amp

Franco

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: sluckey on March 09, 2023, 12:26:41 pm

I think the fact that none of the major amp manufacturers have produced an amp such as this sends a strong unspoken message.

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: PRR on March 09, 2023, 01:17:17 pm

You can put a battery in the cathode circuit to bias. This battery will not go flat: the tube will charge it up.

So why make a power supply for a job of negative power?

A resistor, a Zener, or a current-limiter, will do the job cheaper.

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: kagliostro on March 09, 2023, 01:34:33 pm

I was thinking to a mixed bias and to define better the matter I readed about Bias at Aiken Amps

when I was reading this curiosity come to me

That is

Franco

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: sluckey on March 09, 2023, 02:31:03 pm

Not what you were originally asking, but there are some amps that have mixed fixed and cathode bias. They put a negative voltage on the grid and they put a resistor on the cathode. Fender had several such arrangements in the later amps.

This Super Reverb AB568 (https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab568_schem.pdf) is an example.

And this Magnatone M10-A (https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_m10a.pdf) is another.

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: kagliostro on March 09, 2023, 03:18:18 pm

Ciao Steve, Thanks

At DIYItalia we are wondering about to realize a Mixed Bias amp, so, I was trying to inform me about

I didn't find an answer to the question I had (there is a reason to choice a certain percentage of Fixed Bias in a Mixed Bias ?)

But I refresched my memory about other aspects reading at Aiken Amps

EDIT: Interesting the Fender, seems that Fender did it as to perform a sort of "balance" between the tubes and also Biased  a tube in the oscillator, unusual olso the cap connected between cathodes of power tubes, that remember me a power circuit developed here on the forum, if I'm correct it was Geezer

Franco

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: Latole on March 17, 2023, 05:12:05 am

Differents ways.   https://www.guitarscanada.com/threads/asking-for-info-on-this-se-cathode-bias-design.300921/

Title: Re: Power Tubes Cathode Fixed Bias with positive voltage ????
Post by: kagliostro on March 17, 2023, 07:43:13 am

Ciao Latole

Thanks

I've seen that configuration that uses CCS on FiFi circuits

thank you for reminding me

Franco