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Bypass a tube to reduce power?
Bypass a tube to reduce power?
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Old 18th April 2019, 01:33 PM   #21
petertub is offline petertub  Sweden
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Bypass a tube to reduce power?
I am only referimg to the measurments i have done .

There is no crossover in any power on a sane st70. Hum would be a more urgent issue, but as the poster does not mention hum i guess that the only thing that bothers him is plain level. Adding a resistor on the volume pot input as suggested seems an

elegant method. If the pot is 100k, then a resistor on between source and pot of 200k would make life better ( is should be mounted between source and outer connection on pot NOT the viper)
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Old 18th April 2019, 04:32 PM   #22
RJM1 is offline RJM1  United States
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You could also reduce the value of the 1k feedback resistor to something like 330 ohms, this would
also lower distortion and increase frequency response, The feedback capacitor would not have to be changed as the zero db gain / frequency intercept point would not move up.
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Old 18th April 2019, 04:38 PM   #23
rayma is offline rayma  United States
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Quote:
Originally Posted by johnhogie View Post
I would like to have a wider range of volume control. I have Klipsch La Scalas which have a sensitivity of 105db.
Then you need a preamp with much less gain (0 to 6dB) in the line stage, or a passive line stage with just a volume control. Don't change or attempt to redesign the amplifier at all.

Quote:
Originally Posted by Francois G View Post
Any theoretical or empirical evidence that crossover is not detectable with a ST70 amp and Klipsch Las Scala speakers? I would be interested to know if I heard something else and not crossover distortion.
The output stage operates more and more completely in class A push-pull as the power level is reduced. The ST70 was a favorite of many with Klipsch speakers for many years.
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Old 18th April 2019, 05:16 PM   #24
petertub is offline petertub  Sweden
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Bypass a tube to reduce power?
Quote:
Originally Posted by RJM1 View Post
You could also reduce the value of the 1k feedback resistor to something like 330 ohms, this would
also lower distortion and increase frequency response, The feedback capacitor would not have to be changed as the zero db gain / frequency intercept point would not move up.
This i would advice against. It might cause instability. Feedback in st70
is quite optimal.
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Old 18th April 2019, 05:45 PM   #25
RJM1 is offline RJM1  United States
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It's not like an opamp, it's unity gain stable.
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Old 18th April 2019, 05:55 PM   #26
hpeter is offline hpeter  Slovakia
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Quote:
Originally Posted by jan.didden View Post
Put a resistor in series with the pot, that would shift the usable range downward.

Jan
i would put 5-10k between pot wiper and ground.
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Old 18th April 2019, 07:22 PM   #27
JMFahey is offline JMFahey  Argentina
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* Silly way: attenuate after amplifier or reduce power output by mutilating it.

It will be as uncontrollable as before and distort a ton, in a very ugly way.

* Intelligent way: put a resistor in series with volume pot.

Will NOT distort , be controllable and have huge headroom.


Pick one.
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Old 19th April 2019, 03:01 AM   #28
johnhogie is offline johnhogie
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Default Pragmatic solution for now.

I am interested in experimenting with this amp but need to be realistic about my current knowledge and skills. (Pun not intended.) I do not want to get rid of the speakers so I am going to start looking for a low power (someone suggested an EL84 based SE) amp. Thanks again for everyone's help.
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Old 19th April 2019, 06:32 AM   #29
6A3sUMMER is offline 6A3sUMMER  United States
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RJM1,

Get a Dyna Stereo 70, and change the 1k feedback resistor to 330 Ohms. Do not change the capacitor that was across the 1k, and do not change the capacitor that was from the UL tap to earlier circuitry, and do not change the series RC dominant pole from the pentode plate to ground.
We are still 10dB or more away from unity gain, even though we increased the negative feedback by 9.6 dB.

But, just try different loudspeakers until you find one that causes the amp to go un-stable.

Who has ever tried it?
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Old 19th April 2019, 10:59 AM   #30
pblix is offline pblix  United States
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I see two good options:
1) Keep the amp, sell the speakers. Buy something less sensitive, say under 90dB/W/m. Additional benefit is the WAF will increase
2) Keep the speakers, sell the amp. Same in reverse. Get a flea-power amp.

YMMV
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