something rather odd in sound city

[johniss0001]

I have a pa 50 that's been doing my head in recently trying to figure out what is going on with the damn thing the bias is too hot and won't give any more negative voltage it's already at -42vdc i am still on original iron etc. Got the amp on the bench and i am noticing something rather peculiar going on excuse the bad terminology here.
http://www.soundcitysite.com/sc_webpage ... _schem.jpg" onclick="window.open(this.href);return false;

Now looking at that above schematic the .47 the 100k and 10k resistors coming from VR7 and 8 which are my bias pots should go to pin 5 right on el34 bases. I have my amp right infront of me and thing that gets me here is the .47 and the feed from the bias pots go to pin6 on either el34base then the 10k resistor comes from pin 6 to pin 5 although i upped it to 22k resistors. for some reason the amp is producing audio perfectly i am surprised no tech came across this before

Looking over other amp layouts the bias feed goes to pin 5 on el34bases there is no bloody mention of those components going to pin 6 now correct me here if i am wrong pins 6 on each el34base are tied together each with a grid resistor going to pin4 correct?

[toner]

Pin 6 has no internal connection in EL34's so it is just used as a convenient mounting point. Usually the screen grid voltage supply is sent to pin 6 and there's a grid resistor mounted between pin 6 and pin 4. In the schematic, the screen grid resistors (R46 and R47) go directly to pin 4.

It's a little odd to send the bias to pin 6 and then have a resistor between pin 6 and 5 but it will work fine.

If you need more negative voltage, why did you increase the 10k's to 22k? You need to lower the resistance. I would remove the 22k's and send the bias feeds directly from the 100k's to pin 5. If you still need more neg voltage, you'll have to reduce the 100k's.

[johniss0001]

i measured the voltage before and after the 22ks were installed i am still getting the exact samething -42v to be honest i may need to modify the whole bias circuit in the amp. I am getting -42v at the center leg of bias pot and ground and that's on the highest negative setting and -42v at pin 5 so something has to be changed before that. I am looking at doing the hiwatt bias circuit on the mhuss page or i may go with ceriatone's layout which is quite similar to what i have with the 22ks coming off the 100ks with the 10k pot.

Looking over the amp schematic there's a 4k7 resistor there that ties the 2 bias pots I have been advised to up that to a 22k and looking over the ceriatone layout that looks not far off what i am working with except i have 2 bias pots it's a bit of try it and see what gives me the range if it doesn't work i am going to take the iron out put it in a new chasis and do a hiwatt from scratch

[johniss0001]

I have been pondering over this for days and well what i want to do is decrease the current going to the tubes that's why i upped the resistors to 22k well that's my theory because even with those 22ks that isn't making any difference so I need to look back a tad further in the chain there has to be a resistor increase somewhere to LOWER the current going to the power valves.

Here's a scenario going on with my vintage marshalls I put some chinese el34s in them as they are my favourite tube did a bit of calculations and using an amp head bias probe with a voltage there of say 450v at the plates I can bias the amp to a max of 40mV now with the 47k after the bias pot I was only getting 28mV per tube with the pot on the hottest setting so therefore in order to get the tubes hotter I had to decrease the value of the 47k resistor i opted for 33k and that took me to 40mV straigt away.

So my theory is in order to decrease the current in the tube i am going to try a higher value resistor and i have pretty much ended up with ceriatone's hiwatt bias circuitry.