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Question 4 Lovekraft


ROBERTLATHAM1
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lovekraft

i dont know if you remember but you helped me fix my harmony 535.

it is working. i replaced the power tubes with 2 mesa boogie 6L6GC's and had to replace the resistor going to ground from the tubes#8 pin. i replaced a 100ohm 7watt resistor with 2- 50ohm 10watt resistors in series. this worked and the amp is working, but it has had a very large drop in "power" or volume! it does not get as loud as it did before the tube and resistor repair. could you shed some light on whats happing hear. the resistence is the same, just higher current capacity with the use of 2- 10watt resisitors in stead of 1- 7watt 100ohm. would the circuit actualy go to "hot" and cause the tubes to saturate early and produce more distortion in less volume because of the high current capacity?

:D

thanx

robert latham

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Not a clue, I'm afraid! A pair of 50 ohms in series is electrically identical to a single 100 ohm. The only thing that comes to mind is that the original carbon comp resistor might have "self-adjusted" well below its rated resistance, which would have allowed more plate current to flow, but that's just a wild guess! Did you ever manage to grab a schematic for this beast? Is there a 100 ohm resistor on the other 6L6's cathode?

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no havent found scheamatic. so if i lost say one of the resistors in series would this give me back my power or would this burn my tube?

if anyone know where to get a schematic for a harmony 535 i would love to know where

my brother is a electronics tech and he said that a cap might have went bad also but the only caps i can see are on a different pin than the resistors not knowing tubes i wonder if this is a possability?

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  • 2 weeks later...

OK, Robert, if I'm reading your schematic right, we've found your problem - according to the schematic (again, if I'm reading it correctly), the shared cathode resistor is 200 ohms, or about twice what you've got in there. IIRC, that should be running those tubes at about, what, 110+mA? Way too hot, at least for my somewhat conservative tastes (I'm kinda surprised nothing melted) - replace that pair with a 200 ohm, 10watt (or a 220 ohm if that's easier to find - probably no audible difference) ASAP! Hopefully that'll solve most of your troubles (although those M/B labelled Chinese tubes wouldn't be my first choice - but that's just me :D ).

The rest of you guys please chime in here if I'm missing something - my view of the forest is occasionally obscured by all those trees, efyuhnoewudimeen! :rolleyes

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thanks i misprinted the resisitor values in my post i do have 2- 100ohm resistors in place. and yes i agree about the china tubes would really prefer the sylvainia 6L6GC much more! dont know where to get them though? i am wondering if it might have also poped the 30/450 capacitor on the 370V leg of the tube circuit? i have been told that this would make a dramatic loss in volume level as well.

thanx again for your help :D

just a ignorant electrician trying to save money here :D

robert latham

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thanks i misprinted the resisitor values in my post i do have 2- 100ohm resistors in place. and yes i agree about the china tubes would really prefer the sylvainia 6L6GC much more! dont know where to get them though? i am wondering if it might have also poped the 30/450 capacitor on the 370V leg of the tube circuit? i have been told that this would make a dramatic loss in volume level as well.

thanx again for your help :D

just a ignorant electrician trying to save money here :D

robert latham

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Yeah, if that cap's leaking, it'd be a problem, no doubt - pull one end loose, and check it.

well i checked all voltages and resistances and everything looks fine but the voltages are a little high (15 volts average over scheamatic voltage) i really dont know how to check the caps though my meter only has a palce to plug the cap into it so other than pulling them of the board i dont know . they are in a metal can with 3 seprate in each can. so...........

but i think you r right with the cheap chine/mesa tube thing!

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