Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on January 07, 2012, 04:30:27 pm

Title: In general terms what change having Higher or Lower B+ on Preamp Tubes ???
Post by: kagliostro on January 07, 2012, 04:30:27 pm

In general terms what change having Higher or Lower B+ on Preamp Tubes ?

What is to expect from a lower B+ or a Higher B+ ?

Many preamps are supplied in the order of 290v

other with less B+ (200v or less), someone with very High B+ (see Matchess Clubman 35 with 410v on 6SH7 tube)

Thanks

Kagliostro

Title: Re: In general terms what change having Higher or Lower B+ on Preamp Tubes ???
Post by: bigsbybender on January 07, 2012, 05:26:26 pm

Put most simply.

Lower Voltage = Dirty, more distortion (not always good distortion if voltage is too low). Less Treble, muddy or loose lows. (usually less gain too).
More Voltage = Cleaner, more treble response, tight bass response.


These may vary depending on the overall design of the amp.

j.

Title: Re: In general terms what change having Higher or Lower B+ on Preamp Tubes ???
Post by: jjasilli on January 09, 2012, 12:26:20 pm

Ditto.  But note: Difference in tone & distortion gets bigger towards, and below, 100VDC on the plate of a 12ax7.  Standard values, per the tube charts and reference material:  plate voltage should = 1/2 B+ supply voltage; the latter standard being 300VDC.  This will maximize the clean headroom of a 12ax7 stage.  This is another way of saying that it maximizes the clean, amplified, +/- voltage swing of a sine wave, provided that the tube is biased "correctly" with respect to voltage. 

Voltage works in conjunction with bias.  Deviations in voltage, and/or bias (cathode resistor value), will produce harmonic distortion, i.e., bent waveforms; and can also be designed to clip the top or the bottom (asymmetric clipping), or both (symmetric clipping) of the waveform.

What you are really asking about is "Load Lines".  There's a lot of great info on this Forum.

Title: Re: In general terms what change having Higher or Lower B+ on Preamp Tubes ???
Post by: kagliostro on January 10, 2012, 06:18:52 am

Thanks to both friends

Kagliostro

Title: Re: In general terms what change having Higher or Lower B+ on Preamp Tubes ???
Post by: HotBluePlates on January 10, 2012, 02:13:20 pm

Quote from: bigsbybender on January 07, 2012, 05:26:26 pm

Lower Voltage = Dirty, more distortion (not always good distortion if voltage is too low). Less Treble, muddy or loose lows. (usually less gain too).
More Voltage = Cleaner, more treble response, tight bass response.

Said another way:
If you need a big output signal, or need to handle a big input signal, you need a higher supply voltage. Low supply voltage + proper biasing = smaller input/output signal before distortion.

Funny enough, with a smaller supply voltage, the actual gain of most tubes is reduced. We hear the effect as "more distortion" because gain does not equal distortion.

Title: Re: In general terms what change having Higher or Lower B+ on Preamp Tubes ???
Post by: kagliostro on January 11, 2012, 09:07:13 am

Thanks HotBluePlates

that is an interesting way to explain me that

Kagliostro