• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

OTL designed by Tim Mellow with 4 6C33C?

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C

cnpope

Member
Joined 2009
goddlediddles said:
I'm using 1.5 Amp quick-blow fuses in the speaker O/Ps. A bit low maybe but then again, I'm pushing only a few Watts at the moment and they haven't blown yet. I'm not concerned with the ultra purist notion of non-linearity of fuses in the O/P chain; to me the idea of no O/P transformers in a tube power amp overrides most other considerations. Anyway My Tim Mellow OTL is now singing happily day in, day out.
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I agree on all points. I too have quick-blow fuses in series with the speaker outputs. The only time I've ever blown them is when running into dummy loads, testing the maximum power before clipping. (Which, by the way, really does reach the expected 25 W into 8 ohms.)

Chris
 
V

victor dascalu

Disabled Account
Joined 2015
cnpope said:
I wonder sometimes about whether output capacitors would really be that beneficial in the event of a catastrophic tube failure. If the output voltage on the upper tube's cathode suddenly went to one or other of the B+ or B- rail voltages because of some failure condition, I could imagine the surge current through the speaker would be pretty massive even if there were a capacitor in series with the speaker coil. I could imagine the coil would get fried either way, with or without the series capacitor. Fuses hopefully take care of the situation anyway after a short time, but the initial surge before they can act is maybe the bigger risk.


Anyway, I just hope for the best with mine, and it's been OK so far, which is about 5 years with using the amplifier almost daily.

As to whether an output capacitor would affect the sound, since the power supply capacitors are in the audio path anyway, I wouldn't think there should be much difference to the sound either way, as long as the output capacitor is large enough.

Chris
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Old Romans sais "no risk, no win"
But I stay with ot coupled otl (otl/esl)
 
TonyTecson

TonyTecson

Member
Joined 2003
goddlediddles said:
I'm using 1.5 Amp quick-blow fuses in the speaker O/Ps. A bit low maybe but then again, I'm pushing only a few Watts at the moment and they haven't blown yet. I'm not concerned with the ultra purist notion of non-linearity of fuses in the O/P chain; to me the idea of no O/P transformers in a tube power amp overrides most other considerations. Anyway My Tim Mellow OTL is now singing happily day in, day out.
Click to expand...

do you use air conditioning? and what is your ambient temperature.....
 
K

Koonw

Disabled Account
Joined 2013
One issue with force air cooling fan that blew air through the top from beneath is that if a big capacitor is busted, the stream of hot electrolytic chemical can also sprout to a few feet high that could be dangerous to a person looking down near the amp.

The chassis temperature raised is mainly due to radiation from tube that heat up chassis, just like solid state device cooling fins are expected esp. for small chassis. If some cooling fins are mounted on tubes, they will also cut the radiation therefore reduce the chassis temperature. Look here for some more interesting cooling arrangement
 
goddlediddles

goddlediddles

Member
Joined 2010
atmasphere said:
If you don't have fuses for the individual output tubes you are living dangerously.

They should be rated no more than 1.5amps each. If you have them installed there will be no need for a speaker protection fuse (unless you are running a lower powered speaker like a PHY driver) and no need for an output coupling cap.
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Do you suggest slow-blow or quick-blow?
 
B

Black Jack

Member
Joined 2015
Hello
My version of Tim Mellow OTL.
Some mods
- Balanced inputs
- ECC808 input tubes (Valvo, Hamburg, 60`s)
- Microcontroller based HT power delay (~ 15 min.)

An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.





An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.



More pictures
https://postimg.org/gallery/15zbrq7o6/
 
Last edited:
B

Black Jack

Member
Joined 2015
atmasphere said:
Before using any 6C33 they should be pre-conditioned for at least 3 days and nights by running the heaters with the plates, cathodes and grids tied to one side of the heater circuit. This will minimize arcing and double the life of the tubes.
Click to expand...

Hello,
I`t clear to me why tube should be conditioned with heater voltage applied.
But why need connect plate, cathode and grid with heater?

Could you explain, please?
 
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