Original sex mono new power tranny

[najo49]

Paul, I have re-wired the amps and I am waiting on a set of 8.1 MQ transformer I am getting for a fellow membe here. I have a problem. The 1k 5w is overheating and drawing to much current. any Ideas? Jann O.

[Paul Joppa]

Jann, until now we have been talking about using the PT-7. If you will use the PGP8.1, you must use different circuits and components because of the greater voltage.

From your earlier descriptions, it appeared you had the paraSEX circuit. But that circuit (per the manual) removes the 1000 ohm 5 watt resistor at Step 2. So I do not know what circuit you have now. There are two useful options with the PGP8.1, but so far in this thread I have not described either of them. So I also do not know what circuit you want to have. If I am to make any useful comments, I need to know these things!

For the PGP8 circuits, you can use its high voltage, with a 5K series feed output transformer, or you can reduce that voltage with the original 1000 ohm resistor followed by a filter capacitor to ground. But I would use a resistor of at least 10 watts rating, mounted so it gets some cooling airflow; it will dissipate a lot of power and will get hot! The original 5-watt rating does not have the safety margin that I use today.

[najo49]

Paul,thanks for you reply. Sorry about the confusion. I have a 2.5 or 3.5 James opt.And I am using the original version not the parasex which I did have.,I bought a new MQ 8.1 power trasformer. So I need the 1k resistor to drop the voltage. and I have now a 12W1k resistor on the way. The amp sounds great w/o any feedback! In the sex article you sent me a while back it talks about the High voltage ,and low voltage option. Which version is the high voltage to be used with? The parafeed I suspect.

[Paul Joppa]

Thanks, Jann - you are right, you want the low voltage version. There should be a filter cap after the 1000 ohm resistor, from that point to the paralleled output triode cathodes. This creates a variant of the "Ultrapath" circuit which has some of the virtues of parallel feed. I'd suggest at least 27uF and at least 450v; 500v or greater would be best. This cap is in the output signal current loop, so make it a good one!

Change the cathode resistor to 330 ohms 5 watts (original was 270 ohm 1 watt, if I recall correctly), and replace its bypass capacitor with a 25v or greater rating. A good value would be 10 times the filter cap mentioned above, so 270uF or greater.

I recommend grid stopper resistors on each of the power triode grids; the value is not critical (we use 220ohm) but good quality carbon composition is the traditional gold standard.

Start with the 3.5K taps; you can also try the 2.5K but let the transformer break in for 50-100 hours of music before making critical judgements.

The high voltage design requires a 5K output transformer primary impedance. The only suitable options which have a chance of fitting on the chassis are all series feed.

[najo49]

Thanks paul, I was pretty much on track,but having your comments always is a learning experience. questions arise the grid stopper resistor installed on the paralleled grids on each end? and just to understand more clearly the cap after the 1k is connected to the cathode directly. Jann

[Paul Joppa]

One stopper to each grid, mounted as close as possible to the pin. That leaves the other end flying since the original did not have a dedicated terminal strip lug for the grid, but just wrap the wire and solder together. Yes, negative end (if polarized) to the cathode, positive end to the resistor. THe hot end  than supplies the output transformer and the driver.

[najo49]

Got it. And thank you soo much. I now have two working amps w/o any feedback and they sound great!  I will let you know how they sound as the OPTs get some break in. I think the James tranny is a good thing. Thanks again Paul. Jann o.

[najo49]

Paul Et al, check out the finished Pics of the amps ,you helped me with. The post is in graingers corner. Thanks to all. JANN OLSEN