How much mis-match can an OT take?

bereanbill · 8th May 2009, 09:49 PM

Ok...I have a SE amp, 1 x EL84, 1 x 12AX7, diode rectifier...it has an OT designed for 16 ohms. Am I safe running it into an 8 ohm speaker?

kevinkr · 8th May 2009, 10:25 PM

It's probably safe, it will just sound really BAD..

ray_moth · 9th May 2009, 01:26 AM

I agree with Kevin, if the EL84 is running as a pentode. If it's in either ultra-linear or triode connection, then the load impedance would be less critical and you might find it doesn't sound too bad; however, you will lose some power.

Johan Potgieter · 9th May 2009, 01:29 AM

Er ... well ...

Respecting Kevin, but my take is that it will more likely limit the maximum output (which is already low) than generate severe distortion. But it depends for what value of primary impedance the 16 ohms is valid. Hopefully that is on the high side.

Johan Potgieter · 9th May 2009, 01:34 AM

To follow:

I notice you ask about the mismatch of the OPT. Just a small point, perhaps unnecessary: It is not really that the mismatch concerns the OPT, but the tube itself. Within limits the transformer simply 'supplies' an impedance ratio.

kevinkr · 10th May 2009, 12:22 AM

Nothing speaks to this like direct experience and I have tried it with both triode and pentode connected SE EL84 amplifiers, and I believe the reduced output power is the least of your problems. Distortion IMHO was intolerable (to me) - however I am noted for liking relatively undistorted sound, tube or otherwise..

Note: I haven't done this experiment on more than a few console amplifiers and a few DIY types so your experience could be different.

Think about driving a 1.5K load SE with a 6BQ5 and you might see an issue.

Johan Potgieter · 10th May 2009, 01:40 PM

OK Kevin, accepted then. I have little practical experience of SE in this manner.

8th May 2009, 09:49 PM	#1
bereanbill diyAudio Member Join Date: Jan 2009	How much mis-match can an OT take? Ok...I have a SE amp, 1 x EL84, 1 x 12AX7, diode rectifier...it has an OT designed for 16 ohms. Am I safe running it into an 8 ohm speaker?

8th May 2009, 10:25 PM	#2
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	It's probably safe, it will just sound really BAD.. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

10th May 2009, 12:22 AM	#6
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	Nothing speaks to this like direct experience and I have tried it with both triode and pentode connected SE EL84 amplifiers, and I believe the reduced output power is the least of your problems. Distortion IMHO was intolerable (to me) - however I am noted for liking relatively undistorted sound, tube or otherwise.. Note: I haven't done this experiment on more than a few console amplifiers and a few DIY types so your experience could be different. Think about driving a 1.5K load SE with a 6BQ5 and you might see an issue. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

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9th May 2009, 01:26 AM	#3
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	I agree with Kevin, if the EL84 is running as a pentode. If it's in either ultra-linear or triode connection, then the load impedance would be less critical and you might find it doesn't sound too bad; however, you will lose some power.

9th May 2009, 01:29 AM	#4
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	Er ... well ... Respecting Kevin, but my take is that it will more likely limit the maximum output (which is already low) than generate severe distortion. But it depends for what value of primary impedance the 16 ohms is valid. Hopefully that is on the high side.

9th May 2009, 01:34 AM	#5
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	To follow: I notice you ask about the mismatch of the OPT. Just a small point, perhaps unnecessary: It is not really that the mismatch concerns the OPT, but the tube itself. Within limits the transformer simply 'supplies' an impedance ratio.

10th May 2009, 01:40 PM	#7
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	OK Kevin, accepted then. I have little practical experience of SE in this manner.

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