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Old 19th August 2008, 02:28 PM   #1
bbuz is offline bbuz  United States
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Join Date: Jun 2008
Default EL Cheapo Quad amplifier???

Ok, I have not even built the El Cheapo yet.... but I have a large pile of nice 12aq5's and got to wondering.... what would need to be changed... or added ( aside from more tube sockets) if one wanted to have 4 output tubes per channel ( instead of two) in order to increase the output wattage?

12aq5's are cheap... but would the reward in beef be tangible... would it be worth it? or would there be overwhelming negatives?... could it even be done? Would UL be out of the question in this configuration?

More for my curiosity really but would love to see a proposed schematic reflecting the changes: larger transformers( AC and output)?...... need more tubes to split up the load- could it even be the same class anymore( BTW, would love to know more about the theory of the El cheapo design)......... would power supply caps need to be larger... etc etc.

Thanks!
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Old 22nd August 2008, 06:24 AM   #2
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Join Date: Oct 2004
Location: Adelaide South Oz
I noted that no-one had replied to you - so here is my view for what it is worth.

First I am a big fan of paralleling output tubes. Not everyone agrees but I find that the doubled gm of paralleled output tubes makes the amp sound:
"less restrained"
"more muscular"
"effortless"

I believe that Eli's front end will drive parallel output tubes without any real problem.

Mods required:
Just duplicate everything from the phase splitter tube anodes onward - that is coupling caps, Rg1 to 0V, separate cathode bias resistor and bypass cap for the 2nd pair of tubes etc.
The output tranny Raa will need to be about half that used in the standard El-Cheapo and of-course double the power rating. A Hammond 1615 would probably do or a 1645 would be fine.
The power supply would need to be beefed up to provide double the current.

Send Eli an email - he is a approachable sort of fellow.

Cheers,
Ian
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Old 22nd August 2008, 02:27 PM   #3
bbuz is offline bbuz  United States
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Join Date: Jun 2008
Default EL Cheapo Quad amplifier???

Ian,

Thank you for the reply : )

The info you gave is very helpful- and exciting to hear as the degree of difficulty does not go up exponentially and the benefits seem worth while.

I will email Eli and hopefully he has further thoughts and insight.


Thanks again!

Bill
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Old 22nd August 2008, 11:23 PM   #4
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Join Date: Apr 2004
Location: Monroe Township, NJ
Current hogging is an important issue, when you parallel tubes. Cull the 12AQ5s into TIGHTLY matched pairs. A pair of tubes will be used in each "leg" of the O/P stage. In order to balance the O/P stage idle currents, combination bias will be employed. Each parallel tube set will share a 150 Ohm/470 μF. cathode bias network. A bias trim pot., in combination with a negative rail, will set the drop across the 100 Ohm resistor at 9 V. That's the 30 mA. "idle" current/tube we want. Because a substantial amount of the total bias voltage comes from the RC network, retaining 470 KOhm grid leak resistors (which is important for keeping splitter/driver gain up) does not risk instability.

Use an individual 1 KOhm g2 resistor on each 'AQ5.
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