68 12,000 with 2" high impedance OT

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vh junkie
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Re: 68 12,000 with 2" high impedance OT

Post by vh junkie » Thu Dec 13, 2012 4:34 pm

neikeel wrote:
vh junkie wrote: Another thing to consider here is the UF4007 vs 1N4007 for the diode. Try both and see if you can hear the difference Dave F. said the UF was acceptable here, but not for the mains.
Do you have a reference for that quote - I would like to read. I have UFs in two amps, one of which sounds great and would not want to change the other not so that I am going to fiddle with when I get chance!
Not sure he really explains why...

http://forum.metroamp.com/viewtopic.php ... alpin+FRED
http://forum.metroamp.com/viewtopic.php ... 2&p=141940
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Re: 68 12,000 with 2" high impedance OT

Post by mightymike » Tue Dec 18, 2012 12:30 am

emmjaydubya wrote:
Strat78 wrote:Your not using the 1ohm resistors to ground to measure bias so we need to go over how to bias the old fasion way (better but more dangerous; separates the men from the boys! :lol: ).
Here is a good start to begin to understand this method thanks to Robert it's pretty easy but remember you are waving around your other meter probe with a live 500v.
http://forum.metroamp.com/viewtopic.php?f=28&t=23950&" onclick="window.open(this.href);return false;
Thanks, I have that thread saved. I was inspired by your posts on the matter and I thought I'd try it without the 1 ohm resistors. More dangerous and more time consuming but I'm not afraid of a little extra math, I have a calculator. :lol:
Strat78 wrote:Oh, start with 47k resistor, that should put you in range. Not sure I would trust one of those little 1/4w looking resistors, you mite have to cave and use a piher in that possition. :(
Funny! Piher it is. I'll start with the 47k and go from there. First I need a Variac for forming and then a set of 6CA7s. NOS Sylvanias would be awesome but after pricing 'em, that's not gonna happen. What tube source do you guys like for the EH? I would like to find a source that matches tubes in-house, rather than trusting the factory numbers. Ideas?


Really enjoying this build thread like a good meal. Great theme on parts.. Here's another bias method Chris taught me years ago. It's called the crossover notch method: Basicly you run a 1khz signal into your amp, hook an occiliscope at the speaker Jack to see the sine wave. Then increase volume of amp until the peaks start to lose their roundness ( called clipping)and back off to the high point without clipping. At this point: Looking at your waveform where it crosses the 0 line, you will see a little spike in the wave. This is the crossover notch. Now adjust the bias pot till the notch disappears or is minimized the most.



I love biasing this way. I still go back and check my milliamps, and it's always very close to the 70% Max dis method.
I picked up an occiliscope on.EBay for $20, and used a free sig generator program to create the 1khz sine. You could even convert it to an mp3 for future use.

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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Tue Dec 18, 2012 6:16 am

What sig gen program did you use, and how did you connect into the amp. I have a scope but no connectors yet. Not sure what I need or where to get them.

Now that would be a great thread topic.
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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Tue Dec 18, 2012 3:33 pm

vanhalen5150 wrote:What sig gen program did you use, and how did you connect into the amp. I have a scope but no connectors yet. Not sure what I need or where to get them.

Now that would be a great thread topic.
I have been thinking about a scope but have no idea what are the basic needs for amp and/or echoplex or other repairs/usage.
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Wed Dec 19, 2012 7:32 am

I got one from an army surplus for $45. Guy said it worked, if not to bring it back. Hopefully it does.

Is this the program?

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=17724" onclick="window.open(this.href);return false;
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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Wed Dec 19, 2012 11:58 am

rgorke wrote:
vanhalen5150 wrote:What sig gen program did you use, and how did you connect into the amp. I have a scope but no connectors yet. Not sure what I need or where to get them.

Now that would be a great thread topic.
I have been thinking about a scope but have no idea what are the basic needs for amp and/or echoplex or other repairs/usage.
I've been watching a lot of youtube vids, but we need someone with the meat and potatoes crash course. Here is some basics for you. Basically your sending in a Sine wave on the from, and seeing whats comming out at the other end. There is a 10 part series on youtube fro diagnosing hum and noise. The guy put me to sleep on vid 4.

http://www.tone-lizard.com/Oscilloscopes.htm" onclick="window.open(this.href);return false;
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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Wed Dec 19, 2012 4:39 pm

vanhalen5150 wrote:
rgorke wrote:
vanhalen5150 wrote:What sig gen program did you use, and how did you connect into the amp. I have a scope but no connectors yet. Not sure what I need or where to get them.

Now that would be a great thread topic.
I have been thinking about a scope but have no idea what are the basic needs for amp and/or echoplex or other repairs/usage.
I've been watching a lot of youtube vids, but we need someone with the meat and potatoes crash course. Here is some basics for you. Basically your sending in a Sine wave on the from, and seeing whats comming out at the other end. There is a 10 part series on youtube fro diagnosing hum and noise. The guy put me to sleep on vid 4.

http://www.tone-lizard.com/Oscilloscopes.htm" onclick="window.open(this.href);return false;
This one seemed to give the basics.

http://hipofutura.com/images/diag_scope.pdf" onclick="window.open(this.href);return false;
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

vh junkie
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Re: 68 12,000 with 2" high impedance OT

Post by vh junkie » Wed Dec 19, 2012 6:16 pm

Well, lets talk about buying a scope first: A lot of them (150mhz, etal) are designed for use with higher speed digital computer circuit analysis. We don't need that! 20mhz is plenty for audio!

Probes: At some point you will want to stick this thing on your 500v line to observe sag. If the probe can't handle this, you will hear a pop and that will be it. Get a probe that can handle 600v. To make this more confusing, most probes have a 1x/10x switch to adjust their sensitivity. The 1x is unity gain for voltage measurements. The 10x is not ten times more, it is ten times less voltage getting thru to the scope. While this means that you are less likely to blow up the probe, it means that you need to compensate in your thinking when setting the the voltage range on the scope. I think I got my probes from apogeekits.com... I think the scope came from weber.
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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Wed Dec 19, 2012 7:30 pm

Mine is a used Hitachi 100MHz. Found some 600v 1x/10x probes on eBay.
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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Sat Dec 22, 2012 11:23 am

Might be hijacking a thread but I got a Conar 255 off Craigslist for $30. Not sure if it is the right kind. Has a probe though...

http://gretschpages.com/forum/other-equ ... 816/page1/" onclick="window.open(this.href);return false;

Maybe good for Echoplex??? We should start a thread on how to use these things...
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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Tue Jan 15, 2013 11:33 pm

Starting to get back to finishing this thing and it's about time! After the busy (and expensive!) holidays, I finally scraped up enough money for a set of EH 6CA7s and I scored a 7.5 amp Ohmite variac off ebay for $45 which I thought was pretty good considering some of the ABSOLUTELY RIDICULOUS prices I have been finding for used american made variacs. Some of these dudes on the bay have some pretty huge balls asking what they do for the JUNK they put on there. Anyway, got this one:

Image

It's smooth and tests fine and only starts to drift off the dial by a few volts towards the top. Here's the wiring as it stands with no ground, just line and neutral:

Image

It obviously needs a grounded socket and 3-prong cord, so I set to work. Have a few square pop-in Leviton sockets that I like to keep on hand for stuff, gonna work perfectly in this new hole:

Image

Put in the Leviton socket and replaced the cord with a heavy-duty 14-guage 3-prong cord and put in a brass nut and bolt for ground and rewired:

Image

Then put a little white paint in the dial slot so I can see where I am and presto! Grounded Ohmite variac ready for service!

Image

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Re: 68 12,000 with 2" high impedance OT

Post by Strat78 » Wed Jan 16, 2013 10:39 am

Great job on that! I did this to one of mine and have one more that needs a three pronger as well. You got to love these nice old Ohmite transformers. It's odd how people on eBay are trying to get over $200 for just the naked rheostat, when you can grab the very next listing like yours, for under $80. :what:

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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Wed Jan 16, 2013 1:38 pm

Yeah it's crazy! I've been looking for quite a while and I was lucky to find this. I haven't seen a whole lot of Ohmites lately, alot of General Radios but the prices have been crazy! Nothing under $60 or $80 in months and thats for ones that are untested or have cut cords, etc. And the test equipment companies that sell a junky scratched up one for $200 or $300? :stars: I hope they choke on it! Ha.

So I think I'm gonna form the caps this weekend, any pointers? I've read though Larry's thread a bunch of times and seems I need to use the variac to make sure I don't exceed the voltage limit of the Eries in the mains (350V) and the Hunts in the pre (same). Seemed like the jist of the thread is that you want to apply as much current to the caps as you can up to B+, but not overload, to form them properly. What would a safe input voltage be to start at? 100V? 110?
Last edited by emmjaydubya on Thu Jan 17, 2013 7:36 pm, edited 1 time in total.

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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Thu Jan 17, 2013 6:49 am

Where did you get the plug for that? I need one of those for mine. Good buy for that price.
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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Thu Jan 17, 2013 7:30 pm

I searched for them online a while back when I was building a powersupply box for my pedalboard. Actually, I just looked at the package and looks like I got them from http://www.allelectronics.com.
cat# ACS-31
15 amp grounded outlet

Here's the link:
http://www.allelectronics.com/make-a-st ... LET/1.html

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