P782S substitute

Hi everybody

And thanks for this forum.

Do you people think that Hammond 378X could be a substitute for the P782S?

What confuses me is the 50V tap on the 378X.

Thx for your answers.

Éric.

The easy answer is probably. I went to the Dynakitparts.com site and pulled up their P782S specs. Lets compare with the Hammond 378X. From the Hammond schematic info both 120 VAC windings can be in series for 240VAC. Then the rating is 230ma vs 200 mA so no problem there. The 5 volt winding is the same and the 6.3 volt winding has a slightly higher rating of 6 amps. Both are fine. The 378X has a 400-0-400 VAC rating vs the 410-0-410 rating for the P782S. That will give you a slightly lower B+ but only by 2.5%. That is not enough to change the power output or distortion ratings etc. It may actually help with the next issue.

The 378X has a 50 VAC winding which is lower then the 55 VAC of the P782S. This is the winding used in the bias circuit. Using 55VAC the selenium rectifier with light load will yield about 70 VDC at its output. The stock schematic will show a 1K, 10K adjustment pot, and a 23K ohm resistor all in series to ground. Each drops a portion of the 70 VDC. The important calculation is the mid point of the adjustment pot will be somewhere around -58VDC that goes towards the output tube grids on the circuit board. The 378X has a 50 VAC winding which should translate to about -63.6 VDC at the rectifier. Same resistors to ground. It seems like you can rotate the pot to find that -58 VDC point. It will be more to one end than the other but still have some more adjustment capability. However these are 60 old amps and the resistors were not exactly per the schematic even when new. Have you checked them? Did you replace any of them? I have seen a number of the adjustment pots off by several thousand ohms. At least measure them and think about what all this means. I do think you will still will be OK but you should spend some effort to check the resistor values. You should be able to fully check out the voltage limits of the bias adjustments with out causing problems with the output tubes. Pull the GZ34 tube out so you have no high voltage DC. Power up and measure all the bias resistor voltages to make sure you can get the -58 VDC reading at the tap point to the circuit board. The lower B+ voltage will make it so you may only need -55 or so. Another helpful item is to replace the selenium rectifier with a silicon diode (1N4004). This will help get another volt or two more from the source.

My two cents. Others may not agree. Hopefully they will give you another response or two.

Hello WLT,

Tanks for this fine answer. I had the same thoughts.

If you check the P782S on triode electronics site, you’ll see it is 800 V CT, and 60V bias tap... which is quite coherent with your calculations.

And if we check all the Mk3 schematics on the web, we can notice that in the resistor’s string ( 1K, 1K pot, 18K), some say the last res is 18k, others 22k... So as you say, i could recalculate this third res...

My project : I bought these Mk3 for a décent price, seeing that they needed to be fully rebuilt : melted resistors, paleolithic caps, and... lousy wiring. In fact : electrical rat trap.

And I learned from my other amps buildings that : old caps go to junk, melted resistors signifie a real electrical problem...

All the new resistors have arrived yesterday. Kiwame 2w, and they measure the right values within 1pcent.

For the KT88 bias resistor, I took the option of a 10Ohm, since the 11.2 are widely over priced. Will just set the bias to 1.4V.

Had new pots too and they measure right... by 10pcent in excess

What lead me to Hammond trx was that I never had any problems when building other amps with them. And their availability in France.

Building an amp is at least quite easy. Refurbishing an amp is not the same job...

The Hammond site lists a 50 VAC rating and yet you find a 60 VAC rating elsewhere. Only one way to find out. However the 60 VAC rating should work as well. Just a rotation of the pot in the opposite direction. I have early MK III manuals and the schematic does show 18K ohm for the last resistor. I also have later manuals with 22K ohm shown. It is about the only change in the 20 year production of the amps that I have found. Both work and there is little to no info on why this change was made. The adjustment is a little sensitive and it may have been an attempt to make it less sensitive. We may never know.

Kiwames are a great choice. I use them as well. You can see my MK III restorations on this site in the photo section here https://dynacotubeaudio.forumotion.com/t3641-kiwame-rebuild-of-my-mk-iiis Post your photos once you are done.

Setting the bias for 1.4 VDC with a 10 ohm resistor yields 70 mA per tube at idle. If you search here and elsewhere on the internet you will find the recommendation is to lower the current to about 50 mA each tube. I have adjusted mine using those recommendations and find the amps sound great and do run cooler. It should also translate to much longer tube life. So setting closer to 1.0VDC is recommended. Try it and if it is not good enough you can always raise it some.

1.0 V bias? Whyr not? My scope won’t explode, and I’ll let my ears decide.

Sas your pics: fine job I haven’t begun to dismantle my rat traps yet, I wait for te power trx to arrive...March...

My chassis are not clean. Still wondering if I just de-rust and polish., or have them painted. I know, painting will make them loose money value. But I don’t intend to re sell them.

I will post my pictures on the Dynaco Addicts group on FB. Under the name Eric Li.

I just finished restoring a pair of MK IIIs that were rusty. The nickel plating on these amps may get pitted but is heavy and for the most part is worth saving. If you try use the usual cleaning methods to the point where something has to be done with the rust. I then used aluminum foil as a sandpaper. It did a surprisingly good job of taking the rust down to the metal substrate. It did not hurt the plating around the bad spots. A picture of the bottom (side) and front shows the bad spots but it is not bad. I have no pictures to show of them before but the change is pretty dramatic.