Can output transfomers be pick and mixed?

[surfstu]

I'm salvaging an old heathkit S99 amp. I just want to use the output transformers for a DIY power amp project.

Can i Just find another push pull ecl86 design and use the OT's?

I've attached the schematics for the old S99 and one for an AI series 300 amp that i intend to copy.

The final stages of the outputs on both schematics look similar, does this mean I can just swap out the OT's

Any advice much appreciated....

Stu

 

[scott17]

Not familiar with the S99, but looking at the data sheet for the ECL86, I would assume those xfmrs have an 8K - 9K primary. Should work just fine.

I'm sure you noticed that the S99 is connected UL and the AI is pentode connected. Should not make a difference with regard to the transformer impedance.

 

[Tom Bavis]

Since the transformer has screen taps, you can connect it in UL as Heath did. Expect to adjust the value of the compensation capacitor(s) in the feedback network. The AI circuit includes one more stage in the feedback loop, and has a higher level of feedback.

 

[artosalo]

I recommend you to copy the Heathkit schematic. It is more advanced and gives better performance. The main advantages are UL-connection and separate bypassed cathode resistors. Also the cathodyne phase splitter used by Heathkit is more straightforward and trouble free.

 

[surfstu]

Thanks very much for the replies, I have already scrapped the idea of the AI 300 for these transformers.

I have found a schematic based on the original baby huey amp that used the ecl86, this also operated in ultra linear mode. It is not clear so I have requested a copy of the scematic from Gingertubes who produced it....

 

[TheGimp]

I believe the original ECL86 was by Yves:

Push Pull ECL86/6GW8

Although he used straight PP and not U.L. as his religion forbids it.

 

[surfstu]

Yes, thats the original schematic by Yves, Gingertubes did a UL version and it seems to fit perfectly with my S99 transformers that have screen taps

I was under the impression that if UL is possible then it will be better (However, I don't really know what i'm talking about, just remember hearing it somewhere!)

Maybe I should just build the el84 PP version and leave the screen taps off the transformers? I kinda like the idea of recycling the ecl86's but don't rerally want to compromise on sound quality...

 

[TheGimp]

I recommend designing the amp so that you can switch between U.L. and pentode, (Just don't switch while it is powered up.) and decide for yourself.

The ECL86s are fine sound-wise. The difference in power between EL84pp and ECL86pp is not that great, and if you limit it to the transformer design as a constraint (ECL86) it will be the same.

 

[surfstu]

Thankyou Mr Gimp, I will stick with the ecl86's

I just found Gingertubes schematic,

The reason I want to use it is because if I'm honest - I will just be copying the design identically.... painting by numbers....

I want to use the S99 OT's, i'm just not sure if the screen taps are 20% or 40%... will this matter?

cheers Stu

 

[surfstu]

Also my secondaries are 0 - 3ohm - 15ohm, the schematic uses 0 and 4ohm for feedback... will this matter? Do i just use 0 and 15ohm for feedback and connect speakers to 15ohm?

I am insane attempting this schematic? Any offers of a simpler ecl86 for my transformer spec would also be welcome!!

Stuart

 

[rongon]

If those output transformers are any good, you might be able to use the gingertube design with no global negative feedback loop at all (the 100k resistor with the 22pF cap across it), just the local feedback loops across the ECL86's. You'll want to build it that way first, anyway, then add the gNFB loop after the amp is operational.

From what I've heard, you're quite sane to try this circuit. Lots of people have built gingertube's Baby Huey amp and love it to death.

I have all the parts to build one, just been busy/lazy. I need to get to that...

(added) -- What taps does your output transformer have? Just 0 (common), 3 ohms and 15 ohms? That's it?

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[Chris Hornbeck]

An alternative possible with those output transformers is to connect the 3.5 Ohm tap to ground and to use the other taps for cathode feedback to the output valves. This is easiest with separate cathode bias resistors bypassed with large capacitors. And possibly remove the anode follower feedback or move its takeoff to the screen taps.

All good fortune,
Chris

 

[surfstu]

@rongon & Chris - cheers guys, my confidence has just increased - nearly ready to start ordering some parts.

@rongon

The S99 amp was quite rare and was rumoured to have half decent OT's. Yes only common, 3ohm and 15ohm taps. I've attached a diagram of what i think you mean by losing the gnfb at first - is this right?

Stuart

 

[rongon]

Re: gNFB loop -- Yup, you got it.

Most commercial 8 ohm speakers have an impedance curve that dips down to below 4 or 5 ohms at various frequencies. You can usually get good results connecting your 8 ohm speaker to the 4 ohm tap on a tube amp's output transformer secondary. Unless you're using an actual 16 ohm speaker, you could probably use your transformers' 3 ohm secondaries with no problem.

Or, you could experiment with Chris's idea of using cathode feedback, etc. I've never done that with an OPT that didn't have a dedicated cathode feedback winding (like those nice Tango XE series transformers).
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