virtual variac switching for 1969 SL build

[young flower]

I recently realized that I have the correct trannies to do the dual voltage switching on my 1969 SL build which I´m going to start soon.
I have the 1203-80-ML PT and C1998 OT.
I´ll have to get a 3pdt standby switch (the red ones).

Single question remaining now is how to wire that up?

Found the night owl 12xxx build instructions and this here: http://home.comcast.net/~jbjdav26/1959/ ... 0grids.JPG" onclick="window.open(this.href);return false;

If I want to have the low voltage option, is there anything more to it than adding the three way standby switch as shown on that layout?

What about the testing section - is there any difference to the standard 100W kit instructions (beside the standby switch being three way now)? The voltage chart in the 12xxx nightowl instructions seems to be the same as in the basic 100w kit instructions (which I´m using). Do I have to do additional/different testing for the low voltage option to work?

I appreciate any advice.

Thanks
YF

[young flower]

Would be great if anybody could give me a hint on how to do that.

Thanks a lot

[N3m0 7h3 Fi5h]

Hi there...

You may get valueable reference in this thread:
http://forum.metroamp.com/viewtopic.php?f=16&t=40814" onclick="window.open(this.href);return false;

At least.. its double bias arrangements, for normal & low bias options inside the Chassis:
http://www.dropbox.com/s/5og36yddsy63bsi/SL68.jpg" onclick="window.open(this.href);return false;

[N3m0 7h3 Fi5h]

If you might go any further...

Its PI filter caps, actually smaller than mains filter caps.. Some people believes it is 32/32 in PI caps.. but Blankenships variplex.. is using 16/ 32 PI caps in that spot... (pls be notice, additional trany for fix heater voltage)..

http://i1196.photobucket.com/albums/aa4 ... hip050.jpg" onclick="window.open(this.href);return false;

[Stratabuse]

Here's a easy to understand diagram that I used when I built my amp. Hope this helps..

[young flower]

Hey - thanks a lot to both of you for posting that much info .
Didn´t have time to have a closer look at the threads yet.
Thanks for the layout!
Stratabuse - Do you know if I just have to add the dual voltage switch to the standard 100W layout (from the kit instructions) and wire it like on the layout you posted? Can I follow the Metro 100W kit instructions normally other than that?
Is the double bias arrangement (second bias poti) necessary? Is it necessary to change the value of the PI filter caps?
What about voltage charts and testing - will there be any difference when using the dual voltage switch?
I imagine I simply use the high voltage setting on the switch and follow the instructions as usual to test all the voltages? But what additional testing do I have to do for the low voltage option - I think I have to determine the right value for the resistor on the switch (22k-33k), right?

YF

[N3m0 7h3 Fi5h]

My mistake... I have to make correction... I think these are screen caps, instead of PI caps..

See bellow reference:

http://i52.photobucket.com/albums/g16/s ... 0213cf.jpg

Check that out, the mains filter caps look smaller than the screens.. Almost looks like it's 32uf for mains and 50 for screens, or maybe the caps are just reversed in the layout?

Also check the OT..

[N3m0 7h3 Fi5h]

Double bias arrangements are.. to make adjustable bias between normal & low voltage switch..

Actually, its depends on your own decision, whether you prefer fixed bias arrangements in low voltage options, as posted above..

It happens.. via trial & error selection of res. value between 22-33kohms...

....or... incase prefer to adjustable bias between normal & low voltage options.. bellow info are for your reference...

That's very interesting info. I prefer Alpha pots to PEC but have never tried the Mallory 150s so I guess I'll give them a go now. Are these the ones you're talking about ? http://www.tube-town.net/ttstore/index. ... 630-V.html" onclick="window.open(this.href);return false;

From the preview, those look to be Mallory 150's. The caps are now manufactured with a white coating as George mentioned earlier.

There are terminal posts/strips mounted along each of the 8-Pin sockets as well. This appears to be Suhr's adaptation for isolated grounding.

Looking at the wires from the PT, it looks to be a dual-tap. He has the twin BIAS trim pots soldered to the turret board. Also the BIAS numbers are marked as 34mA for the high tap and 45mA for the low tap. This is just a guess but it appears that the amp's bias for the higher voltage setting is at 65% of plate dissipation for EL34's. That would make the B+ voltage around 470 dcv. For the lower voltage setting, the bias also being set to 65% of plate dissipation would make the B+ voltage at around 354 dcv. The bias number of 45mA would then make sense.

Friedman mentioned before that the BIAS for Ed's main plexi is from 50 to 52 ma when the variac is set to 90 volts. That would make the bias around 75 percent of plate dissipation for the lower B+ voltage running at 354 volts. With the hotter bias, it all becomes clear that Van Halen would use a sturdy 6ca7 for sound and longevity all these years.

In all cases, all of these amps sound fantastic.

[N3m0 7h3 Fi5h]

My suggestion.. lets start with stock amps or parts available in your hand..

After burning-in phase has been passed.. and then later on.. you familiar with tone of your amp... it is the time you may started lowering filter Caps in some Spots..

Generally.. lowering filter caps will give you less stiff feeling.. but incase too low.. it may give you gosting effect..

Finds the best filter caps combination that suits to your needs.. bellow.. are just your reference in selection filter caps value..

Well going with what Pete Thorn said: "low but not too low" I'm going to say the screens are a total of 32uF. Preamp cap in my amp is 32/32, mains are 50uF, PI is 100uF and now my screens are 32uF. Sounds pretty damn close but I'm still curious as to what value the two Spague Atoms are.

[young flower]

Hey N3m0 7h3 Fi5h - so much information.
Since I´m still a beginner and this is my first amp build, I still need time to digest things a little. But I have a basic idea of what to do now.

Thanks a lot for your effort

[neikeel]

The diagram from Stat abuse is perfect and you can incorporate that from scratch, simply replace the stand by with the red triple width switch. The reason I say that is that the switch is quite wide and the chassis needs a little fettling with a file to make the on/off and standby switches slightly oval to fit hem side by side (better to make each very slightly oval than one very oval in case the dress nuts do not cover them completely). You do not want to have to file the chassis once it is built .

I have put them in a couple of amps and as above you have two options, have one pot and play with the value of the dropping resistor by trial and error until you get in the right range in both modes. This is ok if you have a quad of preburned in tubes. If not I would use a dual bias pot arrangement (replace the fixed resistor with a second bias pot).

You do not need to change any of the rest of the build - just 'paint by numbers' and when you get to wiring the standby switch part give a shout and we can talk you through it

http://metroamp.com/wiki/index.php/Dual ... _switching

[young flower]

Neil - as always: .
I´m asking myself though, if the voltage switching actually offers any advantage to me:
Most people seem to use the low voltage mod to get Van Halen sound. But I myself am not really into Van Halen - and I think I could do without the voltage switching if it´s just about getting VH right. You said in another thread, it´s also a good option to get distortion at lower volumes? I also wonder how it interacts with a low PPIMV setting, like 25% of max. volume on the PPIMV (which I´ll probably use most of the time except on rare occasions)?

Cheers
Karim