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Vacuum Tube OTL power amp!!
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Old 9th July 2011, 11:30 AM   #131
Alastair E is offline Alastair E  Wales
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Ah--OK here we go....

No 470K in the FB loop. Its joined directly, so that the 18K on bottom end of phase-splitter sits on the O/P line....

C2 is 0.22 or 220N I'm using and works well....
If you wanted to pull up the LF pole, you could reduce its value to 0.1 or even 0.047....
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Old 9th July 2011, 04:17 PM   #132
M Gregg is offline M Gregg  England
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Ok thank's,

Just short out the 470K..OK

Have you used any special type of fuse on the OP (Quick blow?)

Started building the chassis..<<<< always takes a while!

Looking at components I'll use the IRF710

Should be interesting

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M. Gregg
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Old 9th July 2011, 04:28 PM   #133
Alastair E is offline Alastair E  Wales
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I'm using a standard, Anti-Surge fuse inline with the speaker, its possible with a QB type a powerful transient could pop it....

Chassis work is always a time consuming process!
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Old 19th July 2011, 04:25 PM   #134
M Gregg is offline M Gregg  England
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Just some progress

For interest, this was just to try the components for fit.
Some more work required..and ideas to try.



Regards
M. Gregg
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Old 19th July 2011, 05:27 PM   #135
Alastair E is offline Alastair E  Wales
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Hey--Thats coming along really nicely!

Wont be long now till you have it done!
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Old 24th July 2011, 03:29 PM   #136
woody is offline woody
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How much current are the 6c33c's drawing. I was wondering if I could use 100 ohm
cathode resistors and reduce the voltage to 120 + and - for a 10W amp?
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Old 24th July 2011, 07:35 PM   #137
Alastair E is offline Alastair E  Wales
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Yup--Dont see why not....

Ive run the same scheme at 100V, and 100 ohm, get around 160-180 through
the tubes, depending on whether Ive got 6C33, 6C41C or single heater 6C33C rigged up....
At 100V we get around 5-8 W depending on what tube/dual/single heater, into 8ohm

At 120V and 100ohm, you'll prob be puling about 200-220mA through each 6C33C tube (Both heaters) with around 100V across the actual tube itself....

Just make sure that you burn the tube heaters for a good couple of hours first-(Pref a day or so), then you can plug 'em into the amp, with dummy load across speaker-term --10 ohm WW will do--and run the amp for around 3 hours before trying to match up the valves, and shooting some sound through.

--This above, applies obviously only to New, Unused 'green' valves that have never been used....
-If you have some with some time under their belts, just fire 'em up and check the DC Offset....

Matching the valves for min DC Offset--

There's NO need to buy 'matched' valves--Which are difficult to find in 6C33 and always drift a little anyway.

You can measure the DC Offset across the 10 ohm WW and switch round the 6C33c until you get below 100mV offset.--(Best Ive ever achieved, was 15mV with a couple of pretty old 6C33, running single-cathode operation.)

--Normally this is pretty easy, you're sure to find a combo of tubes that give you below 100mV for both channels, if you have say, 6 tubes, just chop/change 'em round and test the offset....

You can quite happily pull the valves while the amp is still on and switch them round for the offset matching process, but I wouldnt recommend doing this if its attached to a speaker--Although thats the way I actually do it....

--Take Care, Oven-Glove time, they get HOT!

Even though 6C33C take 20 mins to stabilise, the 'offset test' can be checked as soon as the valves have fully hot cathodes, They seem to 'track' reasonably well over time in hours and time as in weeks, it varies very little, as they seem to age together.
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Old 25th July 2011, 06:57 AM   #138
M Gregg is offline M Gregg  England
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Alastair,

Couple of questions:

How many watts are you dissipating in R9/R10?
What voltage do you have across them?


The reason I ask is Mills resistors seem to be 12 Watt and you are using 17W. (I did not want to parallel if not required).

I know you have used standard WW. I just like Mills Resistors.

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M. Gregg
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Old 25th July 2011, 10:49 AM   #139
Alastair E is offline Alastair E  Wales
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Well, you're looking at around 200mA and 20V at 120V supply, so be in the order of 4W....

Your P-diss will be somewhat higher, being a higher value 150 Ohm and 150V supply, but will prob be in the order of 6-8W...

A 12W resistor will be fine, Not sure of the Mills construction--I'm going to look them up now....

I used 17W as those are what I had around, They get hot--but not excessively so...
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Old 26th August 2011, 03:06 PM   #140
M Gregg is offline M Gregg  England
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More slow progress.. work is getting in the way..LOL

I can start the internal build now the chassis is strong enough.

I will add more vent holes as I build, so it's still pulling the air from the front and around the top via the tubes and cathode resistors.

Inputs mounted! Lots to do.

Regards
M. Gregg
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