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Old 21st October 2005, 01:31 PM   #11
SY is offline SY  United States
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You might have a problem with the output tubes' cathode resistor and/or bypass cap. First, check to see that the cap is OK and not leaking. Second, double-check that your cathode resistance is what you had planned. Maybe one or the other is out of spec?

You might also have a massive oscillation. Have you looked at the output with a scope?
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Old 21st October 2005, 01:55 PM   #12
Danko is offline Danko  Hungary
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The resistors are OK. That's 146ohm. I changed the capacitor, but no changes

But, after all, this is only a learning project, like James wrote. It works fine, and there are a lot of things to tweak, measure, learn on this amp :-)
For example a CCS in the LTP's "tail" :-)
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Old 21st October 2005, 04:59 PM   #13
kevinkr is offline kevinkr  United States
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21V across the 150 cathode resistor implies a combined plate current of about 140mA which is way beyond what you should be using. These tubes can dissipate about 12W max and you are asking them to dissipate over 20W each, no wonder they glow. Something on the order of 60 - 70mA total cathode current would be about right - Try changing that 150 ohm resistor to about 250 - 300 ohms and see how things go.

I would configure for triode connection as it will sound cleaner, pentode connection should give you about 10 - 15W, but this is 3 - 5dB and it won't sound that much louder. To get it to sound twice as loud acoustically would require 10 times the power.

I have a somewhat similar amplifier with 7189 and about 400V plate voltage which produces a room filling 7 1/2W per channel


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Old 21st October 2005, 05:12 PM   #14
SY is offline SY  United States
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I'd want to know why the tubes are running so much current with that cathode resistance. Wiring error? Leaky coupling caps?
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Old 21st October 2005, 05:46 PM   #15
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There is another permutation that can be used......try common resistor in o/p stage (cathodes strapped together)
Use 150R common res; keep cathode decoupling cap. (300V B+) G2 strapped to anode.
THD will be around 1% depending on o/p stage matching. With triodes neither the ouput power nor thd will depend on the load resistance between 8-10Kohms.
OMO.....EL34's will give better power i.e 14W in triode p-p with B+400V.

richj
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Old 21st October 2005, 06:50 PM   #16
Giaime is offline Giaime  Italy
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I think that 150ohm is right for a COMMON resistor, not just one for tube.
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Old 21st October 2005, 07:06 PM   #17
SY is offline SY  United States
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Quote:
Originally posted by Giaime
I think that 150ohm is right for a COMMON resistor, not just one for tube.
That's what he's using, 150 ohms as a common resistor. So... either there's something broken (e.g., coupling cap, wiring mistake) or his output tubes are wildly out of spec.
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Old 21st October 2005, 07:08 PM   #18
Giaime is offline Giaime  Italy
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Oh excuse me, my mistake

I've overlooked the schematic
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Old 21st October 2005, 07:15 PM   #19
Danko is offline Danko  Hungary
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Hi!


I don't really know, what's wrong in this amp (on the breadboard), but it doesn't matter so much.
BTW, i'm made thoose pictures with two used tubes, but after that I changed them to new 6P14P tubes, and they were also glowing.

I works, it sounds, so I can continue experimenting on the amplifier.


I couldn't increase the output power, but I learned some things, and this is the porpuse of this amp!
Thank you all your help!
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Old 21st October 2005, 07:54 PM   #20
cerrem is offline cerrem  United States
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There are a number things to do ...
Most paramount thing with Hi-Fi pentodes is to REGULATE the screen voltage...this is VREY critical for linearity....

I would not use anything bigger than 1K for the screen grid resistors.... ALso regarding the HUM, make sure the screens are well decoupled to ground... You can elect to use a LC filter for the screens... Figure a 10H inductor and a 100uF cap to feed the both screens should be fine.....
Or use a voltage reg for the screens...but that would make the circuit very busy...

Chris
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