300b at 3.35V after bridge replacement

[tom1356]

I replaced the bridge rectifier in one of my 300b amps with some fast soft recovery controlled avalanche rectifier diodes I had on hand. The voltage has dropped to 3.35V from the normal 5.0V. It seems to work very nicely but with reduced gain. Any ideas as to fixing this? Does it hurt to run the 300b's at 3.5V or less?
During warm up, after a few mins. it did get a little louder, but not the way it used to be.
The tubes are TJ mesh plate golds.
Thanks in advance
Tom

 

[fdegrove]

Hi,

That's really odd.

You could try to use a bigger filter cap but this voltage drop shouldn't have happened.

Cheers,

 

[tom1356]

Frank
Yes it's strange. It's just over 3.5V now and rising very slowly. Sounding very sweet though.
I'm tempted to do the other just to see what happens but if I get the same drop then what?

 

[tom1356]

The cap across the 5.0 volt (suposedly) wires is 10,000uf.

 

[fdegrove]

Hi,

Sounding very sweet though

I'm not surprised, you're starving the 300B which actually linearises the bottom end of the curves.

I'm tempted to do the other just to see what happens but if I get the same drop then what?

If you have a DVM, set it to lowest resistance and measure the series resistance of the diodes before you do the other channel.

Maybe you received the wrong kind of diodes?

If anything, I'd expected a slight rise in voltage, not a drop.

Cheers,

 

[fdegrove]

Hi,

The cap across the 5.0 volt (suposedly) wires is 10,000uf.

Should be more than ample....strange, really strange.

Cheers,

 

[dhaen]

Hi,

The Fast-Soft Recovery Diodes that I have come across have high forward voltage drop. Schottky diodes are more suitable for low voltage applications.

Cheers,

 

[tom1356]

"If you have a DVM, set it to lowest resistance and measure the series resistance of the diodes before you do the other channel."

Could you clarify at which two points do I take the measurments?

Maybe as you say these diodes are not suitable for this application.
Is there any long term problem with using the 300b at 3.5V?

 

[fdegrove]

Hi,

The Fast-Soft Recovery Diodes that I have come across have high forward voltage drop.

But of course, I forgot about that entirely...in HV sullies this doesn't really matter but at 5V it sure would.

Good of you to remind us of that.

Maybe as you say these diodes are not suitable for this application.

See D'Haen's reply...you're losing too much voltage.

Is there any long term problem with using the 300b at 3.5V?

Not a problem as such but it won't exactly be a 300B you're listening to...could be better, could be worse.

I'm not going to take side on this one....

Cheers,

 

[tom1356]

Well that explains it. Thank you both for the help.
I think I'll listen to it this way till I can get the shottky's.
It's warm and thick like a quilt on a winter morning.

 

[fdegrove]

Hi,

It's warm and thick like a quilt on a winter morning.

Yeah...I expected that...

The Schottkys will bring it back to a more lively listen....let us know what you experience.

Cheers,

 

[Saurav]

Now that gives me a few ideas I could try out

 

[tom1356]

I havn't shut this thing off since I did the mod. It keeps getting louder and louder and better and better. And it's in stages. Meaning during a song it will just get louder all at once.
Any ideas why?
I guess the diodes are braking in and passing more current.
I'm off to do the other one.

 

[tom1356]

Oh, it's still warm but not thick anymore.

 

[dhaen]

Oh, it's still warm but not thick anymore.

Hi Tom,

Yes, that's typical of an underrun heater. Just a few degrees more, and a big change.
Frank is right about the linearity, but other parameters associated with the circuit will suffer when the current passed is lower than the design value.

Cheers,

 

[fdegrove]

Hi,

Meaning during a song it will just get louder all at once

That's not considered as normal break-in observation.

Cheers,

 

[dhaen]

Hi,



That's not considered as normal break-in observation.

Cheers,

Indeed not, it's the valves sputtering into life, as the internal temperature rises.

Really, they'll work best at the rated heater voltage. Give 'em a chance...

 

[fdegrove]

Hi,

Indeed not, it's the valves sputtering into life, as the internal temperature rises.

No, don't think so.

Really, they'll work best at the rated heater voltage. Give 'em a chance...

No, not necessarilly either...have a Stella on me and a headache....stupid brewers...

Cheers,