Using mains transformer as output transformer

[Shoog]

I use at least 150VA units for my output transformers.

Shoog

[Elias]

Hello,

Originally posted by jerluwoo
Have you tried simply grounding the cathode through the secondary so that you have the same amount of dc through primary and secondary? Curious if this would have a cancelling effect on saturation or not.

Connecting windings like that is one way to include cathode feedback. But for DC current compensation it does not work because for proper magnetic cancellation the current in the secondary winding should be approksimately the same as primary current times winding ratio, which condition is not fullfilled in this.

Doing this way can reduce the distortion though and thus is interesting.

- Elias

[Elias]

Hello,

Originally posted by john_ellis

I don't think transformers like to be treated the way you supposed. If you are going to the trouble of adding a compensating DC current, why not operate two valves in Class A push-pull?

Well, cannot say it didn't come to my mind doing PP. But as far as I understand PP operation, it cancels even order distortion and what is left is nasty odd order. Exactly the opposite I'd like to achieve! I want to find out how an amplifier with dominating even order distortion sounds like. That's why my primary interest is SE at the moment.

- Elias

[Elias]

Hello,

Originally posted by Shoog
... However I think the whole idea of SE toroidals is a big mistake and wouldn't recommend it at all. Having said that, DC compensation would seem to be the way to go. If I did that I would put a CCSink in the cathode to make it rock solid stable over time. Basically choke loads are expensive and introduce all of the nasties which toroidals avoid - so no advantage and no cost saving.

I have three amps using toroidals in PP amps and this is where they shine the most. Response down to 10Hz and up to 60kHz. DC balance is essential and can be achieved with CCsinks in the cathodes, or with "Garter Bias". I like to get my CCS to within 1mA, bass suffers badly after that. LM317's make reasonable candidates for CCS. One thing to watch is because of their extreme bandwidth they do tend to ring and may need dummy loads. ...

I agree, DC compensation is essential to makes toroids to work.

How do you implement CCsink in the cathode, do you mean in SE as well, or in PP? In PP I can understand cathode current balance being important but does CCS bring any benefit in SE?

Originally posted by Shoog
My personal experience of using toroidals is with
- 6080's running at 100V 100ma, very low output impedance.
- ECL82 with plate to plate feedback which reduces output impedance down to CF levels.
- 807 with plate to plate feedback as well.

...

However I believe Steve Bench made his Matrix amps with toroidals as outputs and he measured very high inductances and very good results. I think experience is stacking up to show that they work well and work well down to very low frequencies - better than many SE transformers.

Great to hear you have a lot of experience with toroids. And yes I think also toroids are a great potential!

- Elias

[Elias]

Hello,

Originally posted by Merlinb

That depends mainly on the primary inductance. If you want any bass at all you should be looking for the highest primary inductance you can get.
Cheap toroids may have less than 1 Henry, which is useless. You might get lucky and find one with around 10 Henrys, which would be "useable" for lo-fi applications using pentodes (such as a guitar amp or radio), but if you want to get anything that remotely looks like hifi you''ll have to use it with a low impedance valve, like a nice beefy triode. Couple of triode-strapped ECL86s maybe.

Ok. I didn't measure inductance yet. If it's too low EL34 does not seem to care about it, since signal goes much less than 20Hz and it's quite clean. However, can be that this toroid has enough high primary inductance for proper impedance transformation also at low freqs. Measurements will show eventually.

- Elias

[Elias]

Hello,

Originally posted by 316a

Well done ! Are you going to share the design ? I'm surprised at your upper rolloff . I got -0.5dB down at 20khz using a 6B4G into a 230:9+9 Talema 30VA unit .

Thanks Currently I have only the output stage on the table. I need to make a driver as well. At this point I'm investigating different options to use a toroid at the output. One day this is will turn out to be a whole amplifier I'm sure

Originally posted by Shoog
I use at least 150VA units for my output transformers.

Interesting, two people using totally different size of transformers. Regarding the high freq roll of, which size toroid has wider bandwidth in the top, big or small VA ratings? Any rule of thumb in this?

- Elias

[boiss]

I'm about to start back in on these Toroids late next month with my home wound 350VA tranny as parafeed. Class A2 SE with GU48, ex computer 10V heater psu, MOT as choke. Solid state transformer coupled drive with positive Vg. HT still undecided but preferably much less than 850V.
Keep up the good work.
Mike

[Shoog]

Interesting, two people using totally different size of transformers. Regarding the high freq roll of, which size toroid has wider bandwidth in the top, big or small VA ratings? Any rule of thumb in this?

The bigger transformer will suffer most - but my experience is that it just doesn't matter. In theory the smaller VA will have higher primary inductance. I prefer the greater immunity to saturation offered by the bigger transformer.

How do you implement CCsink in the cathode, do you mean in SE as well, or in PP? In PP I can understand cathode current balance being important but does CCS bring any benefit in SE?

I did put a CCS in the cathode of an SE amp but it didn't make any significant difference to the sound. What I was suggesting was putting a simple CCS in the cathode when using DC compensation. My thinking is that it will lock the arrangement into stable current through the valve even when the valve ages. This will make the transformer immune to saturation under most conditions.

I'm about to start back in on these Toroids late next month with my home wound 350VA tranny as parafeed. Class A2 SE with GU48, ex computer 10V heater psu, MOT as choke. Solid state transformer coupled drive with positive Vg. HT still undecided but preferably much less than 850V.
Keep up the good work.
Mike

I tried a MOT transformer on a RH807 SE design. It worked, but when comparing it to other amps it seemed to show phase shift issues. I think this is because though the inductance of the MOT is enough to make it work, it isn't enough to dominate the arrangement. Most parafeed choke loads seem to have inductances well over 50Henries, the MOT can probably only manage something around ten. I had it set up before with a pentode CCS and this seemed to work better.

Shoog

[boiss]

Shoog,
I'll try it anyway, If it is poor I'll cut the welds on the MOT and rewind the primary with the same wire as the secondary which should bring it into the right area re "L".
Thanks,
Mike.

[boiss]

By the way, excuse my ignorance but what is CCS (Constant Current Source?) This appears to be referred to continuosly in posts. Pls direct me to any source material.
Because my Toroid was made for a PP amp and it also has seperate UL windings, any of these could be used for balancing the Ia in an SE setup. Also any extra secondary winding could be added for this purpose. How should signal voltage on this winding be treated seeing as it also presents a load to the tube?
Regs
Mike.