rectifier-bias

[andrew]

Hello, I have a 6v6 push pull, cathode biased amp. It was designed for a SS rectifier and a bias control with voltage on 6v6s at 383v. If I use a SS plug in and a 1k variable resistor pot in series with a 500ohm 10w cathode I can get the 383v ( pin 4). With a tube rectifier I can only get 366v. Since I play at low volume is there any advantage to tube rectifing it? Thanks!

[Ears]

Hello, I have a 6v6 push pull, cathode biased amp. It was designed for a SS rectifier and a bias control with voltage on 6v6s at 383v. If I use a SS plug in and a 1k variable resistor pot in series with a 500ohm 10w cathode I can get the 383v ( pin 4). With a tube rectifier I can only get 366v. Since I play at low volume is there any advantage to tube rectifing it? Thanks!

That's a big ask Andrew. Volumes could be and have already been written on this. In short - At low volume, no. Tube rectification introduces more impedance into the power supply cct. In comparison to SS, rect tube rectification impedance has its greatest affect in high load situations when it allows the supply voltage to drop - "sag". In most cases this affects the pre amp as well. When supply drops the active amplification devices will approach their limits earlier, resulting in lower headroom, compression of peaks and more attack sensitive overdrive. SS power supplies are cleaner (more headroom) and subjectively more transparent. Depends on what you like. I'm quite sure others have more to say or can supply you with good links.

Go to Duncans amp pages for a great little download to mess around with PSU design. It'll show you the effect of diode type, rectifier config, cap values etc on output waveforms and values.

[dartanion]

In addition to sag and the compression effect, the lower plate voltage will lower the output a bit too. The lower output will hardly be noticeable, yet it has the advantage of browning the sound.

[andrew]

My 20w amp calls for a GZ34 but I tried a Sovtek 5Y3 tonight and I love the fuller sound, however, the tube sure gets red. Is there a way to emulate this 5y3 effect with the GZ34 or is the 5Y3 safe to use? Maybe a different brand? Would a Weber Z-Dump do it? Thanks for any help!

[andrew]

Sorry, I meant a Weber Z-Dropper zener diode.

[statorvane]

is the 5Y3 safe to use?

Don't know why it is red, but 5Y3's typically draw 3 amps, whereas GZ34 /5U4s draw 2 amps. Is your transformer's 5VAC supply rated for 3 amps?

[andrew]

The 270FX is rated for 3A. To clarify the 5Y3 ( Sovtek ) gets a deep dark red while the Gz34 looks " safer". Thanks!

[Ears]

From Duncan's short Data:
5Y3 is rated to supply 125mA max DC current, 350-0-350 Vrms input

GZ34 160mA, 550-0-550V

From GEC data:
6V6 in AB1 pp at 285V on plates draws 74mA idle 105mA peak.

A guitar amp biased hot and with 380v on plates may well pull considerably more juice so its probably no wonder your tube is red.

Don't know about the diode but incorporating resistance into the filter can induce more sag.

[andrew]

Thanks for the info. I'll try a 10w resistor going to the first filter cap.thanks again.

[andrew]

I tried a 250ohm but it didn't drop enough. Has anyone tried this idea?http://web.archive.org/web/200502122108 ... ayout2.GIF

[Ears]

I tried a 250ohm but it didn't drop enough. Has anyone tried this idea?http://web.archive.org/web/200502122108 ... ayout2.GIF

What's the Mosfet do?

[andrew]

He used the mosfet to bring down the voltage about 50 volts. I was also wondering what wattage the two resistors needed to be. This would be a $2 project as opposed to Weber's $20 Z-Dump voltage dropper.We will see. Thanks!