stereomour cathode and power resistors--

[howardnair]

-the power resistors which are a 1.2k ohm and 360 ohm
in parallel gives a value of 276 ohm-knowing that i am not going to put a 45 tube in this- can a 300R ohm value be installed-or better yet a 270R-my common sense tells me 10 watts or better --the same question goes for the cathode resistors a 1.2 k should be right on-that being asked -i realize that there are other factors that may or may not be involved-heat dissipation and are we looking for exactly 276R-what makes me pose such a question??--
Delton Horn and Basic Electronics Theory--he assures me in the preface that it will take several readings before  one can begin to understand electronics BUT!! you have to do the math -after skimming the book a time or two- its time to do the math-which opens the door to questions-naturally!! he is so right-that and a second stereomour are causing me to pay much closer attention to the circuit-the math is giving me somewhat of a fit as it has been 38 years since high school-ahhh but i "will endeavor to persevere"

 

[Paul Joppa]

Yes, you can use 270 ohms 5 watts, and 1200 ohms 10 watts, for 2A3 only. If there were room (there isn't), you could even replace the 270 ohm resistors on the PSU board with the old C7-X choke (270 ohms DC, 10 henries). All these parts are what was in the Paramour design; the doubled-up resistors are in the Stereomour so that it is (relatively) easy to switch to the 45 if you want to at some later date.

 

[jrebman]

Paul,

Could I replace the 270 ohm resistors with something of higher value to bring the voltages more in line with the design expectations -- as my line voltage typically runs about 123-125 volts.

I want to be especially careful with my voltages as I have a pair of EML mesh 2a3s that I'll eventually want to use in this amp.

Thanks,

Jim

 

[Paul Joppa]

It would be better to address the line voltage at the source, since the extra voltage will appear at the filament as well. Excess filament voltage is an equally important issue.

That said, you can increase the resistance to reduce the high voltage, with little risk. We do it that way for the 45, after all!

 

[howardnair]

thank you paul!--the choke idea is interesting and possibly doable for me as i have a oversize wood chassis-howie

 

[jrebman]

Paul,

Thanks, and right, I meant to also say that I was going to deal with the filaments separately.  I figured a couple of small value, higher power resistors from the wipers of the hum balance pots to ground would do the trick for the 2a3s, and something similar for the driver.  BTW, I have some nice 50 ohm 10-turn wirewound pots for hum balance so I won't be using the 22 ohm resistors.

I don't know why I forgot this as the filament voltage was the one I was originally most concerned about as EML is pretty clear that one should not go much above or below the 2.5v for maximum lifespan of the tubes.

-- Jim

 

[Grainger49]

Is there a way to put in "power" resistors to shed some of the incoming voltage between the power switch and the transformer input windings? 

 

[2wo]

How about a thermistor?...John

 

[Paul Joppa]

Resistors can work, but I feel it's best to step the voltage down. That way, you add almost no resistance and do not affect the regulation of the power supply - and you do not have the higher transient voltages you would othewise see. You can use a low-voltage transformer as an autoformer to do this. I should write up something about this, maybe another white paper.

 

[howardnair]

maybe a variac???  and another question to follow jim redman using 50 ohm pots--can you elaborate on the 22 ohm resistors and 10 ohm pot and the 50 ohm pot alone-
this from diytube.com
hum pots are only necessary for AC powered filaments, the idea is to reduce (and not totally eliminate) the amount of audible 60 Hz hum, with careful adjustment, generally close t the halfway point. Bottlehead goes ths one step further, they use a 10 ohm WW pot with a 22 ohm, 5W WW resostor connected to each end, this effectively behaves like a 54 ohm pot with a physically "wider", and less "sensitive" adjustment range.

The way the hum pot works is to create a very simple resistor "bridge" circuit, with the filament and the two halves of the hum pot, and by careful adjustment, the AC voltage on the filament can be balanced out in relation to the tube's grid, which will reduce the amount of hum heard on the output