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Old 3rd February 2010, 11:08 AM   #1
tubo is offline tubo  Philippines
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Default converting a magnavox pp 6v6 in triode mode

hi i converted my pp magnavox 6v6 in triode mode. the amp sounds fantastic better than it was in tetrode mode.but im getting hum.

heres what i did

1. connected pin 3 to pin 4 of each 6v6 with a 120ohm resistor
2. disconnected pin 4 of each 6v6 from b+
3. disconnected the feedback circuit (it sounds better to me)
4. i removed r11b and r 21b from both of the 6eu7. (both drifted out of spec, will get replaced.
5. removed the 750 ohm balance pot and inserted 2 390ohm resistor to ground.
here is the scheme


thanks

ivan
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Old 3rd February 2010, 02:46 PM   #2
tubo is offline tubo  Philippines
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what will happen if i remove the 47k grid resistor of the 6eu7 which is r11b and r21b?will it change the operating point of the tube?
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Old 3rd February 2010, 02:52 PM   #3
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Quote:
Originally Posted by tubo View Post
what will happen if i remove the 47k grid resistor of the 6eu7 which is r11b and r21b?will it change the operating point of the tube?
No it won't Afaik it's to prevent oscillation.

BTW your schematic is too small to read..
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Old 3rd February 2010, 10:36 PM   #4
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Have you replaced the main filter capacitor C2 (a multi-section 40/40/20uF)?
It could be that the first 40uF (C2A) section is sad, and now you are getting hum into the screens at pin 4 that would have otherwise been masked when on the plates only.
Worth replacing given the age of the chassis. Don't go overboard with the value, the 6CA4 is only rated for 50uF on its immediate output.

Edit: I noticed you replaced the balance pot with a resistor divider. You didn't take the resistors straight to the chassis at some other point than the main grounding point did you?

Last edited by rotaspec; 3rd February 2010 at 10:38 PM. Reason: added question
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Old 4th April 2011, 11:20 PM   #5
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Hi all,

I am glad I ran into this post by tubo. the schema included is of the same unit I have and I identified the comment of the 88-02 series having the balance on the tuner. That was the key to get rid of a previous hum
Anyway, now that I know what I have and a schema I upgraded the electro caps and fixed-resiter wired the balance, and get rid of the tuner part! I have no hum. I would like to introduce a volume pot though. The one on the tuner is 1 meg, which seems too large, and a 50 K pot may be more appropriate, but if I were to use it, I could just put it in between the input signal RCA plug and the input-grid resistor or would I take resistor that out?
I think this is a great little amp for beginners like me, simple, lots of room and though not big in the trafo department, with just one driver per channel to drive horns; more than adequate! Your assistance is appreciated.

Bert
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Old 4th April 2011, 11:40 PM   #6
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It is interesting that on the chassis layout it shows the rectifier as a 5U4 but the schematic shows a 6CA4. You might try a 10uf cap before R4, and then replace R4 with a 5 - 10H choke of the appropriate resistance, voltage, and current rating.
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Old 5th April 2011, 04:04 AM   #7
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I agree the two are not congruent. the amp I have has the 6ca4 rectifier and there's no choke nor balance on the chasis, and as per schema no choke is called for in this model. The only reason I hard wired the balance within the amp is because the balance pot was shot; no use transferring it from tuner to amp chasis. I might add, the caps I used were 47, 47, 22, 22uf for replacement of the electro's. So far so good.

Bert
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Old 5th April 2011, 05:07 AM   #8
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I have what is pretty much the same Magnavox console amp, using a 6CA4 for rectifier. I may replace the tube rectifier with some high quality SS rectifiers. I was also thinking of replacing the 6V6 outputs with 6AU5, and reworking the input stage. I received my amp with no tubes and funky innards, so I consider it a basic score from which to extemporize...
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Old 17th May 2011, 06:24 PM   #9
chd_diy is offline chd_diy  United States
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I have a Magnavox 88-02 series just like this one.
Thank you for posting the schematic.
My power transformer is bad, and I need to know what the secondary voltage is, so I can order a replacement from Edcor.
Although the schematic doesn't specify the secondary voltage, I assume it must be about 275 VAC.
Also, if anyone has advice on how many milliampres the replacement should be rated for, that'd be great.
Thank you.
Chris.
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Old 17th May 2011, 08:42 PM   #10
TheGimp is offline TheGimp  United States
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Try re-connecting the feedback and see if the hum goes away.
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