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HELP: High impedance load for SET amplifier

Posted on August 6, 2020 at 01:17:03

Posts: 2
Joined: August 6, 2020
Hello there to all SET audiophile fellows,

I would appreciate any hint/help concerning the following use case:

I own a commercial SET amplifier (Coincident Frankenstein Mk3); it has 8 and 16 Ohm speaker taps. I would like to experiment with a field-coil speaker/chassis that exhibits an impedance range of 24-30 Ohm.

Generally speaking, I know that with SET amplifiers "higher" is better than "lower" when it comes to speaker impedance...but there might be also more subtle aspects.

So, my question:
Could I encounter issues concernign power transmission, tube life span or dammage risk for the output transformer ?

Thanks in advance!
Best regards,


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RE: HELP: High impedance load for SET amplifier, posted on August 6, 2020 at 06:57:28

Posts: 2327
Location: The South
Joined: December 24, 2012
You will not harm the amp. The 16 ohm tap will be a fine match for the field coil. If you were able to do a frequency sweep to measure the impedance of the speaker it is highly likely that some frequencies would have less than 16 ohms impedance. Speakers are not resistors and the impedance is usually an average number.


RE: HELP: High impedance load for SET amplifier, posted on August 7, 2020 at 00:38:49

Posts: 2
Joined: August 6, 2020
Many Thanks!
I will try to do an impedance measurement. Given the outcome would really be a narrow range from 24 to 30 Ohm, do you think that the amplifier would deliver less power via the 16Ohm output...meaning compared with an idealized load (16Ohm). The manufacturers web pages suggest that it would deliver even „more" power...not sure if right.


It will make less power but, posted on August 7, 2020 at 11:44:47

Posts: 3931
Location: Minnesota
Joined: April 24, 2002
its also likely that the tube will make less distortion since it will be seeing a higher impedance as well.


RE: It will make less power but, posted on August 8, 2020 at 16:15:12

Posts: 11774
Joined: May 14, 2002
everywhere except for the LF extremes...there there will likely be a shortage of inductance to drive such LF content into the load.

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.


Not really, posted on August 9, 2020 at 08:02:17
Paul Joppa
Industry Professional

Posts: 7007
Location: Seattle, WA
Joined: April 23, 2001
Think of the load line oval at low frequency - the peak current demand depends primarily on the inductive reactance and is relatively insensitive to the resistive part.


RE: HELP: High impedance load for SET amplifier, posted on August 10, 2020 at 04:39:52

Posts: 1427
Location: Ontario
Joined: March 20, 2006
At 30 ohms, and the 16 ohm tap, you will have a little more than half the power available as a 16 ohm load at the 16 ohm tap.
Any transformer mismatch either higher or lower will result in less power.

Dan Santoni


RE: HELP: High impedance load for SET amplifier, posted on August 11, 2020 at 13:02:53

Posts: 194
Joined: August 21, 2018
At the moment you drive a higher impedance the loadline that your powertube will see will be more flat. So less distortion (for a triode), less power output, but higher output voltages from the tubes.

If the tube is a triode probably the higher output voltages won't distroy your transformer, only penthoses are very risky.

If you are not sure about the risks or want to prevent any danger, put an extra resistor parallel at your speaker, about the same value as your speaker is. The transformer/output tube will be happy again.


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