Metropoulos Forum

B+ is 515 I'm going SS rectification, what do you think will the 500volt caps hold ? It will be a JTM50 with a 784-128

jonamojo wrote:B+ is 515 I'm going SS rectification, what do you think will the 500volt caps hold ? It will be a JTM50 with a 784-128

They probably will, but their life expectancy will also probably be less than half they would at 450V.

approximately how much would a 200 ohm 10 watt sag resistor lower my B+??? thanks in advance for your replys

OK they make 5 versions of that particular model numbered G1-G5. All of them have different rectified B+ values.

The one you have is spec'ed at 515 with SS rectification. Way too hot for the application. Would've worked with tube rectification since it would've given you a 450V B+, but way too high with SS rectification. If you had the plate and screen filter caps set up like a 100 watter does with the two filter caps in series for the plate and screen filters with bleeder resistors across each cap in series it would work, but would require drilling extra holes and the JTM-45 layout wouldn't be the same. The best thing to do in this case would be to exchange it for the G2, which is spec'ed to give a solid state rectified B+ of about 420 volts.

Here are the links to the data sheets of each one so you can take a look. IMHO, if that's a brand new tranny, the best thing to do would be to exchange it rather than to modify the circuit from its original scheme. I feel you would be less than impressed via band-aiding it with a sag resistor, so a tranny exchange would be the best route to go IMHO.

G1 - Approx SS Rectification B+ = 475V (bit too high...probably could drop its voltage some with a zener diode to ground on the center tap though if the G2 is not enough voltage for your liking)
http://metroamp.com/wiki/images/0/01/To ... 0JT-G1.pdf" onclick="window.open(this.href);return false;

G2 - Approx SS Rectification B+ = 420V (perfect)
http://metroamp.com/wiki/images/b/bc/To ... 0JT-G2.pdf" onclick="window.open(this.href);return false;

G3 - Approx SS Rectification B+ = 515V (this would work well with tube rectification at a 450 volt B+ but won't work for SS rectification in your application)
http://metroamp.com/wiki/images/4/4f/To ... 0JT-G3.pdf" onclick="window.open(this.href);return false;

The last two are a bit on the low side.

G4 - Approx SS Rectification B+ = 380V
http://metroamp.com/wiki/images/5/58/Ax ... 0JT-G4.pdf" onclick="window.open(this.href);return false;

G5 - Approx SS Rectification B+ = 320V
http://metroamp.com/wiki/images/c/ca/Ax ... 0JT-G5.pdf" onclick="window.open(this.href);return false;

thanks for all the info unfortunately the transformer is used so I cant return it , guess I'll have to go with tube rec

jonamojo wrote:thanks for all the info unfortunately the transformer is used so I cant return it , guess I'll have to go with tube rec

By "used" do you mean "installed, wires cut and soldered"? Or do you mean "was used in another amp at one time"? If it's a brand new transformer but the wires have just been cut and soldered in, Mercury has a "tonal satisfaction guarantee" that will allow you to exchange that tranny for a different one.

And since you're down in SoCal, Merc isn't far from you in Chatsworth...it's worth giving them a call about.

ok thanks I'll give them a call

If you aren't successfully at MM, then ask me for the zener trick to lower B+

Larry

novosibir wrote:If you aren't successfully at MM, then ask me for the zener trick to lower B+

Larry

I was gonna post that up but thought that having to knock 50-100 volts was a bit much to use a zener diode for. So this raises a question...

What's the highest amount of voltage you've knocked down with a zener diode? And how much zener current does the zener have to be rated for when used in this fashion (I know it's one of the chassis mount ones with the anode case)?

I use a 125v 25w stud mount Zener to drop B+ in a little 6V6 amp I built.
It had a whopping 465v on the secondary.
I installed the Zener on the center tap to ground.
Cathode to case Zener.

Works great!

Structo wrote:I use a 125v 25w stud mount Zener to drop B+ in a little 6V6 amp I built.
It had a whopping 465v on the secondary.
I installed the Zener on the center tap to ground.
Cathode to case Zener.

Yes, a stud mount zener w/ reversed polarity is required,
and you must bypass the zener w/ a filter cap, plus facing to ground.

Larry

Yeah the zener trick is cool and all...great for testing to try out an exact certain plate voltage. However, my thought was that if he could get MM to exchange the thing why not just get the right tranny for the application.

Another PT is the best way to go, because zener tricks are crutches and I've already had dead zeners in the past, though well dimensioned (50W chassis mount types).

Larry