Whenever you see output tubes on a cheap circuit board, and especially a board that's not highly perforated, you know you've got an amp destined for progeria.
As for the overall design, cough, cough, it's not exactly what I'd do...
As for the overall design, cough, cough, it's not exactly what I'd do...
Holes are "free"
Can fix holes, both in pcb & chassi...
Next tip please!
Arne K
1000 x thanks to mississippi for making the great drawing...
Can fix holes, both in pcb & chassi...
Next tip please!
Arne K
1000 x thanks to mississippi for making the great drawing...
I always like seperate transformers/rectifiers for low and high voltage.
You can keep the transformer you have now for high and just add another for the heater. Not real exspensive.
You can keep the transformer you have now for high and just add another for the heater. Not real exspensive.
I found this: http://www.arrakis.es/~igapop/russianotes.htm#6N1P /6N2P How to use them
-and it show a very similar circuit...
How can this be improved?, (sorry, no space for an extra tube).
Arne K
-and it show a very similar circuit...
How can this be improved?, (sorry, no space for an extra tube).
Arne K
I guess you have to first define what you're after. A test bed for learning and experimentation? A long-term amp to use unmolested for listening? Different answers to those questions.
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
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
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.
Run bandwidth at different powers while observing waveforms for distortion. You're most likely to see problems at the low end if the iron is light, high frequency problems if the winding is poor, both if you're dealing with a really bad specimen.
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
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
Don't be afraid to use that output stage for power testing.
If you really only get 25kHz and peaking from the transformer at small signals, that's an indication that trying to modify this amp is like polishing a turd.
If you really only get 25kHz and peaking from the transformer at small signals, that's an indication that trying to modify this amp is like polishing a turd.
I really guessed the trafos were the cheapest part...
There are space for replacement...
So, what should I look for / where? Price-range? (no, not the gold-plated, silver-wired-ones).
(offers welcome)
Arne K
There are space for replacement...
So, what should I look for / where? Price-range? (no, not the gold-plated, silver-wired-ones).
(offers welcome)
Arne K
I know the brand-names...and a few more, but, what properties should I look for? Ohm/ratio/power/?...
BTW, are the stock trafos usable for anything? heater-supply?
Arne K
BTW, are the stock trafos usable for anything? heater-supply?
Arne K
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.
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.
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?
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?
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.😉
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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