hello, i have built this amp. sound very nice, only expensive parts,only OPT 5K is chinese.(quastionable quality)
i have rollof in high frequencies. in triode mode also in pentode(better situation with HF but still hass rolloff)\
there is my scematic, and measurments. at 20khz -10 db, thats too much..
any advise how to fix this issue ? negative feedback is no-go option as sound is not in my liking anymore.
pictures in this link :
Imgur: The magic of the Internet
i have rollof in high frequencies. in triode mode also in pentode(better situation with HF but still hass rolloff)\
there is my scematic, and measurments. at 20khz -10 db, thats too much..
any advise how to fix this issue ? negative feedback is no-go option as sound is not in my liking anymore.
pictures in this link :
Imgur: The magic of the Internet
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Its posible. Measurment was done with 4 ohm resistor. On 4 ohm tap. I will try with multimeter. But i can hear on my speakers that i realy have issue with HF. Also i tested few transistor amps. They are flat response. +- 0.1 db so problem is in amp. If i remove pentode cathode cap. I get 20khz - 6db. Its better but still not good enough. I believe i have problem with miller effect
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It could be the bandwidth of the transformer itself, you may be able to improve it with some overall negative feedback to the cathode of the driver tube, I have managed to get much more acceptable results from cheap transformers in this way, without some are just plain unacceptable for much other than guitar.
The output grid stopper will not cause this HF loss unless much higher in value. It is swamped by the output impedance of the first stage - that might cause problems with the Miller capacitance of the output stage. Have you estimated the effect of this?
The OPT might be the problem too. Good OPTs are big and expensive.
If you want really flat frequency response from a valve amp then you need negative feedback, which may mean using a better OPT anyway.
The OPT might be the problem too. Good OPTs are big and expensive.
If you want really flat frequency response from a valve amp then you need negative feedback, which may mean using a better OPT anyway.
Is the output also marked? That was the case in my situation, the output was wired with GND going to where the load should go and vice versa.
Bouncing off of this statement, maybe try running the tests again with an 8 ohm load on the 8 ohm tap? Just to rule it out...
Do you have two 4 ohm resistors?
sadly no, only one... in few days i will have super expensive hashimoto with 5K and 7K primarys, i will do test with these OPT, they claim OPT is FR(+-2dB) 25-70,000Hz
so finaly solved problem. it was OPT fault. installed expensive H20 hashimoto, and now in triode mode at 20 khz i have 0.5 db drop )) thats acceptable. sound is lively, more like transistor A class, pianos sounding beutiful. i was pianist long time ago, and i first time listened pianos with real tonality and decays... ps: speakers supravox 215EXC at 2 Amperes.
)) thats acceptable. sound is lively, more like transistor A class, pianos sounding beutiful. i was pianist long time ago, and i first time listened pianos with real tonality and decays... ps: speakers supravox 215EXC at 2 Amperes.
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