OT: 18watt rectifier.. tube vs. diode bridge "sag"

[nightingale]

got bitten by the DIY amp  insect~
i've collected lots of parts for the minimalist 18watter... the diode rectifier looks alot easier to build/layout...
my question is: will i be losing out on alot of "sag" that the tube rectifier will produce..? some say that the diode rectifier is alot more "responsive" in stage situations.. i have been on the fence for a few days thinking this over..
any outside opinion is welcome here..

here's the schems:

diode bridge: http://18watt.com/Storage/18minwatt.gif

tube rectifier: http://18watt.com/Storage/18w%20lite%20III.pdf

[The Tone God]

Some people like a quick responding amp like hard rock and metal players. Others like slow response like blues players. What you want depends on what you like or want.

The sag occurs because of the internal resistance inside the rectifier tube. Si rectifiers do not have this high resistance. Sag is not the only thing introduce with tube rectifiers. There is a substantial voltage drop that occurs when using a tube rectifier. This act in a way like power supply attenuation proving a warmer and easier to distort sound. Si does not as high of a voltage drop.

Just a few thoughts.

Andrew

[nightingale]

thanks andrew~
i think the tube is the way to go for me... the only problem is that i bought the hammond 261M6 for the PT, and the hammond 125E for the OT... (the transformers listed in the SI rectifier version)...

will this be a problem bor the tube recto version? i am hoping i don't have to hock them to get the hayboer set..?

i am not very experienced with transformer data sheets, or building amps... yet!

[The Tone God]

Just working off the top of my head and not looking at the specs myself I don't think there will be much of a problem using a tube rectifier. As long as you have the enough voltage to overcome the drop in the tube rectifier, which I belive the AX84 does, then it should be cool.

Do you have a 5v winding for the tube rectifier's heater ?

As a side note: good luck and please be very careful.

Andrew

[Aharon]

You don't need a 5V winding.The 18W uses an EZ81 rectifier that has 6.3V heaters but you could use any rectifier that provides enough mA for the job.The best would be to go to the 18W site and ask there.There's schems and layouts too.
Aharon

[csj]

Hey Ryan,
I've built several of those...excellent choice for an amp - I know you'll love it.

IMHO the amp stock is too spongy for my tastes.
The power supply seems underfiltered and when coupled with the voltage drop across the ez81 the sound is too soft for me.
Great tone in some applications, like Andrew says...blues mainly.
Set up like that it's a great "plug and play" amp with a Les Paul and quality pickups.


I like a more stiff supply with as little sag as possible.
This allows me to use different pedals in front without losing each pedal's personality. But then you run into the "well, it's not a Bluesbreaker then" criticism.

Recommendation:
Keep the stock filter supply.
Cut it out for tube rectification but set it up with the capability to bridge the tube socket with SS diodes.
See which one works for you.

Good luck on the build.

Clay

[The Tone God]

Thanks for the correction Aharon.

Has anyone done a SS/Tube switch for the rectifier with the AX84 ?

Andrew

[puretube]

some tube-amp designers put a hi-watt resistor in series with the ss-rectifier to get some sag...

[R.G.]

Tube amps are one of my other afflictions. I have a collection of ideas surrounding the concept of the Immortal Amplifer - a tube amp where you eliminate the failure points other than tubes.

It's good practice to put two 1Kv silicon diodes in series with the tube rectifier, one per anode, in any tube amp. If the tube rectifier shorts, the diodes take over the rectification, and nothing dies. The B+ goes up a bit and you get less sag, but otherwise, it works until you can fix it.

You can fake a tube rectifier by putting a resistance in series with each of two solid state diodes, or one resistor after the junction of two Si diodes.

You can switch from SS to "faked tube" by switch-shorting the resistor(s).

You can make a SS diode pair with or without resistor-faking and a tube rectifier, then use an SPDT switch to select which rectifier you use.

As to which to use - your ears will know.

Weber Speakers makes a solid state tube emulator that's considered superior by many amp users and plugs into the same socket as a tube rectifier.

[Aharon]

Not much to add to that.The Copper Cap is what RG is refering to and you can get them with sag emulation  or without sag for a number of the most popular tube rectifiers and the price difference is minimal.
The advantage is that if used properly they will outlast the amp and everything in it and they allow the PT to run cooler since the 1 to 3 amperes used by a tube recto are not employed.
I have a Copper CAp in every amp I've made except my Spitfire.

[Aharon]

Thanks for the correction Aharon.

Has anyone done a SS/Tube switch for the rectifier with the AX84 ?

Andrew

I know,I also think tube recto=5V.
You can have a SW but you have to keep in mind that the voltage will go up with SS and it's not adviseable to switch on the fly.
Aharon

[Paul Marossy]

If your amp is Class AB, you can add a "sag resistor" in the B+ supply from the transformer - about 100 to 200 ohm/10 watt. Also, you can reduce the size of the filter caps a little bit to get things sounding more spongy, but if you go too low, you can get some hum.

This won't work on a Class A amplifier, though, as I recently found out.

[Aharon]

To me there is 3 major ways you can approach this.
One is "the perfect clone" approach in which you get all the same parts as the object of your desire and proceed......costly but hey you got your clone.

Two is the "Uberamp" or let's get the "bestest" of the best out there and build an amp.Very costly and not necessarily the best way.

The third way is to wait and gather your parts when oportunity calls.Takes longer to build,may not be a XeroX copy of your dream amp but IT IS THE CHEAPEST WAY and guaranteed to be an original.

I just picked up an output transformer yesterday for a dollar which will go towards my next build.

Take your pick.
Aharon

[Hal]

honestly, i dont think there's too much of a differance.  I've played around with mesa boogies that have switchable rectafiers, and the differance is negligable at best.  Often its even unnoticable.  For a first amp, I think you should just go with the easier, solid state recti.

[nightingale]

very helpful posts~!
i apprecdiate it alot guys.. i am just waiting to collect all of the parts before i do any drilling.. i plan to make the hole for the tube recto, but just go with the diode pair until curiousity gets the best of me..

csj~

But then you run into the "well, it's not a Bluesbreaker then" criticism.

that is why i didn't post at the 18watt forum.. those cats seem pretty die hard hayboer transformers...
thanks again guys,