You can remove one coupling capacitor to only one of the output tubes (same in each channel).
Then that output tube will not amplify the signal, but its DC grid bias voltage will still be present.
The DC bias current must still flow as usual in both tubes of each channel, to prevent saturating
the output transformer. You can't just remove an output tube.
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This amplifier is a push-pull amplifier that can not be changed easily in the way you wanted. Even if you wire it for triode output you will consume about the same amount of power. Let’s first what is the motivation behind your goal to reduce power in the ST-70? Lower electrical consumption? Less heat generation? Speakers super efficient? Save on expense of replacement tubes, etc
If you really want to conserve electrical power get a small Solid State amp. If you must have a more “economical” or lower powered tube amp I suggest you trade your St70 and buy or build a EL84 type amplifier. The “El Cheapo” amplifier designed and described by Eli Duttman, a regular on diyaudio.com, uses 12AQ5 output tubes. They are like EL84 in a smaller bottle and NOS is still very affordable.
Or if you have very efficient speakers that will give you the sound volume you want with a 2-4 watt input, you could get or build a single-ended EL84 type amp.
Wanted more volume control.
I appreciate everyone's quick responses. I would like to have a wider range of volume control. I have Klipsch La Scalas which have a sensitivity of 105db. The change isn't worth adding distortion or damaging the amp. I saw a solid state rectifier and thought maybe there was an equivalent for a power tube.
I appreciate everyone's quick responses. I would like to have a wider range of volume control. I have Klipsch La Scalas which have a sensitivity of 105db. The change isn't worth adding distortion or damaging the amp. I saw a solid state rectifier and thought maybe there was an equivalent for a power tube.
You cannot run it with only 1 EL34 / channel as the transformers will
saturate due to absense of magnetic gap ( needed for SE amps )
If problems are to high level from preamp, a simple resistive divider is what you need. For $$ you can buy readymade "dampers" on the RCA inlets. For less $ you solder some resistors. Or get a better preamp.
If this is a push-pull amplifier (and I think it is), removing one tube of the output pair breaks the amp.
The best route to lower dissipation is to lower the supply voltage and/or adjusting the standing current.
Edit: reading again I realize he doesn't want less power. He wants less gain. Duh.
Jan
The best route to lower dissipation is to lower the supply voltage and/or adjusting the standing current.
Edit: reading again I realize he doesn't want less power. He wants less gain. Duh.
Jan
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Yup. Quite often people cannot formulate what they want.
Lots of ways to reduce gain, but perhaps the easiest is a fixed attenuator at the input. And it is pretty much the only one that will leave damping factor and distortion unchanged.
Put a resistor in series with the pot, that would shift the usable range downward.
Jan
With such sensitive speakers I understand your concern now. A friend of mine had a pair of very efficient, large Klipsch speakers (don’t remember the model) and he drove it with an ST70 clone - to my ears it was not a pleasant-sounding match.
I believe the crossover distortion region in (relatively high-powered) push-pull amplifiers become more noticeable if the volume is turned down to a fraction of a watt output power, which is sufficient to drive the La Scalas. You may want to consider single-ended amplifiers to drive them, if you intend to keep them. Even low-powered ones should be satisfactory on your La Scalas, but I’m not sure how low you could go. Anyone with experience in the topic?
I do not think crossover is detectable in this amp. But a voltage divider
on input is what is needed here.
If the speakers will be kept another amp with lower power might be
an alternative.
I would agree with you that in a “normal” setup with 80-90 dB speaker it may not be detectable, but at very low power such as is used with 105dB speakers at normal listening levels, the crossover region becomes relatively more prominent. I believe it is detectable under these conditions because I heard what we thought was crossover distortion on the setup as describes above in my post.
For some discussion and actual graphs showing crossover non-linearity see: What is Crossover Distortion?
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.
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