ST35 rev. E w/ AI Series 300 Transformers?

for the DIY ST35, the Dynakit and every other PP EL84

ST35 rev. E w/ AI Series 300 Transformers?

Postby gonzo » Sat Feb 07, 2015 11:11 pm

I have already ordered your ST35 pcb.
I would love to hear your thoughts on using the AI S300 output transfromers with either 47 or 100R from a - g2. The standing current could have to be reduced by raising Rk ..... Then comes the playing by ear. They are 9K('ish-not too sure?!):8R. The ST35 calls for 8K:8R. My seEL34 sounds better on the 8R taps than the 4R ones, even with KEF 103/3 (4R). The output available would only be (max)10W, due to transfo, but would it work?

Next question is the mains tranfso from the AI s300. Can I run the EL84 from this? Ia maybe down to 30mA? (your board looks nicely adjustable!).

re Mains transformer: I can find no ratings for the transformer, but it did feed 4xECL86 and 3xECC8*, so should just about manage the heaters. As for the HT, I have little hope, showing only +285VDC at CT of O/P transformer in most schematics. Maybe keep the o/p's and find a new mains?
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Re: ST35 rev. E w/ AI Series 300 Transformers?

Postby Shannon Parks » Mon Feb 09, 2015 7:24 am

Do you have a schematic for the Audio Innovations Series 300?

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Re: ST35 rev. E w/ AI Series 300 Transformers?

Postby johnf » Wed Feb 11, 2015 1:46 pm

Here are a couple.I saw a rumour on another forum that the transformers were used in the Edison 12. There is a thread about that kit amp on this forum.

ai300mk2.gif
ai300mk2.gif (14.93 KiB) Viewed 5707 times
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300mk12.jpg
300mk12.jpg (193.37 KiB) Viewed 5707 times
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Re: ST35 rev. E w/ AI Series 300 Transformers?

Postby Shannon Parks » Fri Feb 13, 2015 10:10 am

gonzo wrote:Next question is the mains tranfso from the AI s300. Can I run the EL84 from this? Ia maybe down to 30mA? (your board looks nicely adjustable!).


No center tap and too low of a B+. I'd save this for another project and just use the PA774 or Edcor.

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Re: ST35 rev. E w/ AI Series 300 Transformers?

Postby gonzo » Fri Feb 13, 2015 11:08 am

The t/x's are from Series I (, or just original ,?, yes, the pcb had a QC label dated December 1987. Well before mk.II) 300 Series.

My question is: Can I not run a p-p EL84 output stage at around 35mA per valve, with 100R from HT to g2 connection on pcb (, which has another 100R to the terminal). The HT current will be down, even with my planned 7025/ECC82 mods. I do not hope for 17.5Wrms. The output transformers (Also 9K'ish:8R, no 4R) can only do 10Wrms max. Given the HT voltage on the mains t/x HT, I can only reasonably run this as the Mullard 5-10, with higher value cathode resistors to reduce the current.
The mains t/x will probably get replaced when funds allow, but am I right in thinking that the output transformer are around 4K5-0-4K5:8R? I will try to send photos, but I don't know your site too well.
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Re: ST35 rev. E w/ AI Series 300 Transformers?

Postby gonzo » Fri Feb 13, 2015 11:36 am

Some pics:-
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IMAG0068.jpg
From AI Series 300 MkI.
IMAG0068.jpg (911.97 KiB) Viewed 5674 times
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Re: ST35 rev. E w/ AI Series 300 Transformers?

Postby Shannon Parks » Thu Feb 19, 2015 5:29 pm

gonzo wrote:My question is: Can I not run a p-p EL84 output stage at around 35mA per valve, with 100R from HT to g2 connection on pcb (, which has another 100R to the terminal).


You can definitely reuse the OPTs using a pentode connection with 100 ohm resistors. But with all these differences, I would suggest re-tuning the overall feedback bypass cap (with scope and square wave) to be assured of stability.

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