I'm running my leak stereo 20 single ended. WHAT???

bigwill · 17th May 2008, 12:07 PM

I took one of the EL84s out in each channel of my stereo 20. It goes just as loud and actually sounds much BETTER, especially in the way it clips. WHAT?

WHAT??!

tubewade · 17th May 2008, 10:56 PM

Perhaps you just changed the distortion from symmetrical (odd order) to non- symmetrical (even order). This is largely the difference between single ended and double ended(push-pull) output stages, although probably to an even greater degree in your instance since the output transformer was not designed for this.

Since each valve has his own cathode resistor, no harm will come from running the amp this way. If the cathode resistor were shared by both valves the current though your lone 6BQ5 would be too high and would probably result in an angry red plate.

Wade

bigwill · 18th May 2008, 12:50 PM

It still sounds clean, and the bass is still present in spite of the magnetic bias in the OPT. When I tried it I just did it for a laugh to see what it sounded like but I was surprised that it actually sounded pretty good! I did check the schematic before pulling out one half of the output stage to ensure nothing harmful would happen and as you said, it is safe to run it like this and quite frankly I enjoy the sound!

dnsey · 18th May 2008, 05:38 PM

Be careful - the ST20 output transformer is fairly marginal for the original design, and often fails. Putting a DC offset through it isn't going to help

bigwill · 18th May 2008, 08:09 PM

Could you elaborate? I'm having trouble understanding how it could potentially damage the OPTs. Does having a magnetic bias present a problem? (Other than possibly magnetising the core)

ray_moth · 18th May 2008, 09:45 PM

I'd be surprised if it did harm to the OPT. On the other hand, it will relieve the power transformer of some of its load, which might not be a bad thing.

Merlinb · 19th May 2008, 06:44 PM

Quote:

Originally posted by bigwill
Could you elaborate? I'm having trouble understanding how it could potentially damage the OPTs. Does having a magnetic bias present a problem? (Other than possibly magnetising the core)

That's exactly it- by having a DC current flowing in one half only you permentantly magnetise the core. This means that the transformer is then pushed heavily towards core saturation, and therefore LF distortion and even overheating.
I'm not sure how you degauss a transformer. Maybe you can just run it normally, but at low levels for several hours?

sorenj07 · 19th May 2008, 06:52 PM

You could just buy some relatively inexpensive SE transformers and plunk them in if you plan on making the switch permanent... If you've decided you like the sound, you should do that instead of messing with your PP transformers.

17th May 2008, 12:07 PM	#1
bigwill General Nuisance diyAudio Member Join Date: Dec 2004 Location: UK	I'm running my leak stereo 20 single ended. WHAT??? I took one of the EL84s out in each channel of my stereo 20. It goes just as loud and actually sounds much BETTER, especially in the way it clips. WHAT? WHAT??!

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17th May 2008, 10:56 PM	#2
tubewade diyAudio Member Join Date: Jun 2007	Perhaps you just changed the distortion from symmetrical (odd order) to non- symmetrical (even order). This is largely the difference between single ended and double ended(push-pull) output stages, although probably to an even greater degree in your instance since the output transformer was not designed for this. Since each valve has his own cathode resistor, no harm will come from running the amp this way. If the cathode resistor were shared by both valves the current though your lone 6BQ5 would be too high and would probably result in an angry red plate. Wade

18th May 2008, 12:50 PM	#3
bigwill General Nuisance diyAudio Member Join Date: Dec 2004 Location: UK	It still sounds clean, and the bass is still present in spite of the magnetic bias in the OPT. When I tried it I just did it for a laugh to see what it sounded like but I was surprised that it actually sounded pretty good! I did check the schematic before pulling out one half of the output stage to ensure nothing harmful would happen and as you said, it is safe to run it like this and quite frankly I enjoy the sound!

18th May 2008, 05:38 PM	#4
dnsey diyAudio Member Join Date: Mar 2004 Location: Shropshire, England	Be careful - the ST20 output transformer is fairly marginal for the original design, and often fails. Putting a DC offset through it isn't going to help

18th May 2008, 08:09 PM	#5
bigwill General Nuisance diyAudio Member Join Date: Dec 2004 Location: UK	Could you elaborate? I'm having trouble understanding how it could potentially damage the OPTs. Does having a magnetic bias present a problem? (Other than possibly magnetising the core)

18th May 2008, 09:45 PM	#6
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	I'd be surprised if it did harm to the OPT. On the other hand, it will relieve the power transformer of some of its load, which might not be a bad thing.

19th May 2008, 06:52 PM	#8
sorenj07 diyAudio Member Join Date: May 2006	You could just buy some relatively inexpensive SE transformers and plunk them in if you plan on making the switch permanent... If you've decided you like the sound, you should do that instead of messing with your PP transformers.