Valve amplifier OP load vs frequency. - UK Vintage Radio Repair and Restoration Discussion Forum

Diabolical Artificer · 9th Oct 2020, 6:10 pm

If we have a valve amplifier with an output transformer with 4, 8, & 16 ohm secondary taps, does connecting an incorrect load, EG using a 4r on the 8r tap effect frequency response? If so why? And by how much?

Andy.

kalee20 · 9th Oct 2020, 6:36 pm

Could do, depending on what's stopping the frequency response extending from DC to light in the first place.

The output transformer is the biggest culprit. Use a larger-resistance load than designed for, and the finite primary inductance will start to clobber the bass response at a somewhat higher frequency.

On the other hand, the leakage inductance, caused by less-than-100% coupling between primary and secondary, won't make its effects so noticeable until frequency is higher.

So you worsen your bass, but potentially get more treble.

The reverse happens if you use a load of lower resistance than designed-for.

Of course, using non-optimum load, distortion will start to rise at a lower volume level. But, I'm assuming that the volume control is kept below this point, in all of the above.

Negative feedback acts to iron-out this anyway. And, it could be that response is deliberately limited in earlier stages, in which case you probably won't notice.

GrimJosef · 9th Oct 2020, 7:03 pm

Valve amps may have significant output impedance. There will be an interplay between this and the strongly frequency-dependent impedance of the speaker, which will affect the overall frequency response of the amp/speaker system. Changing the tapping on the OP transformer secondary will change the output impedance. But saying anything quantitative about this will require a knowledge both of the amp's output impedance and the speaker's impedance. I suspect there's no simple answer to your question Andy.

Cheers,

GJ

Diabolical Artificer · 10th Oct 2020, 8:52 am

Thanks both. I was thinking in general terms after I re-called a conversation I had with a now long gone forum member years ago and recently saw a thread where changing the sec tap on an OPT was posited to get a better Ra-a.

Andy.

9th Oct 2020, 6:10 pm	#1
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,670	Valve amplifier OP load vs frequency. If we have a valve amplifier with an output transformer with 4, 8, & 16 ohm secondary taps, does connecting an incorrect load, EG using a 4r on the 8r tap effect frequency response? If so why? And by how much? Andy. __________________ Curiosity hasn't killed this cat...so far.

9th Oct 2020, 6:36 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Valve amplifier OP load vs frequency. Could do, depending on what's stopping the frequency response extending from DC to light in the first place. The output transformer is the biggest culprit. Use a larger-resistance load than designed for, and the finite primary inductance will start to clobber the bass response at a somewhat higher frequency. On the other hand, the leakage inductance, caused by less-than-100% coupling between primary and secondary, won't make its effects so noticeable until frequency is higher. So you worsen your bass, but potentially get more treble. The reverse happens if you use a load of lower resistance than designed-for. Of course, using non-optimum load, distortion will start to rise at a lower volume level. But, I'm assuming that the volume control is kept below this point, in all of the above. Negative feedback acts to iron-out this anyway. And, it could be that response is deliberately limited in earlier stages, in which case you probably won't notice.

9th Oct 2020, 7:03 pm	#3
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Valve amplifier OP load vs frequency. Valve amps may have significant output impedance. There will be an interplay between this and the strongly frequency-dependent impedance of the speaker, which will affect the overall frequency response of the amp/speaker system. Changing the tapping on the OP transformer secondary will change the output impedance. But saying anything quantitative about this will require a knowledge both of the amp's output impedance and the speaker's impedance. I suspect there's no simple answer to your question Andy. Cheers, GJ __________________ http://www.ampregen.com

10th Oct 2020, 8:52 am	#4
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,670	Re: Valve amplifier OP load vs frequency. Thanks both. I was thinking in general terms after I re-called a conversation I had with a now long gone forum member years ago and recently saw a thread where changing the sec tap on an OPT was posited to get a better Ra-a. Andy. __________________ Curiosity hasn't killed this cat...so far.