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Chinese EL34 amp to modify

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mississippi

mississippi

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amplifiers-with-valves.nl
Here is the schematic diagram of this amplifier.
 

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Cobra2

Cobra2

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Since it barely can make a pigs ear, I see no hope as a silk purse...
It will mostly be a test-bed,(training/learning), but first; it has to be usable.
As it it(was); it has no bass or punch...and had a glary top-end.
And not even close to a varm midrange.

Arne K
 
SY

SY

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SYclotron.com
I'd spend some time measuring the output transformers. If they've got the bandwidth and power handling to make a decent amp, then it could be worth stripping everything else out and starting with that as a base. If they don't, just reconnect them, sell the amp, and use the money to buy some decent iron.
 
Cobra2

Cobra2

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My "lab" is not exactly set up for high-power testing, I could only get 10V out of my generator. (~250mV over load)

With a 8 ohm load, a sinus looke fine from 10+Hz to 23kHz, started dropping, increased to a resonance-peak at 25kHz, and fell thereafter quite fast...
With square, it looked good from 10-5kHz, then it showed a small ringing, squares got curvey upward...still "square" past 15kHz then got fairly out of any shape...

Does this tell anything about the trafo? (other than sh**?).

Arne K
 
SY

SY

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Start from the beginning. What performance parameters are you looking for? Power, especially.

The power transformer might be OK; it would be helpful to measure the output voltages no-load and under load, then run it loaded for a few hours to measure thermal rise. A resistor load is good enough for this sort of test.
 
Cobra2

Cobra2

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It is rated 2 x 35W (in China-watt) no idea about distortion-level.
30-40W should be reasonable to get?
I ran it hard/LOUD for an hour, into speakers(8 ohm/90dB), powertrafo did not get hot...maybe 5 degree rise from idle, or less.
So I guess it can stay...

Arne K

looking at specks, typical pp 2 x EL34 trafos have Raa from 750-5k, lower for higher power? How does this influence other parameters? Need higher Volt/Amp?
 
SY

SY

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I'd look for something 4k3-6k6 plate to plate, preferably with ultralinear taps (whether or not you end up going that route, it's best to be flexible). Appropriate power rating. In that power and impedance range, you'll want to consider EL34, 6L6-oids, and maybe paralleled EL84. There are other choices, but these are tried, true, and come with decades of documentation.

The choice of 9k p-p for a pentode EL34 amp is not something I understand. But this designer is working under a set of goals and constraints that I don't and can't know, so maybe there's some logic to it. Maybe not.;)
 
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