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Old 15th July 2012, 09:59 AM   #1
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Default Sowter 8650 in SE

Hi,

i got some of this transformers which are normally used in preamp PP output stages:
Sowter Type 8650
I would like to use them in SE, with triode wired 1620/6J7. It looks like rp for this tubes is ~7,5k in my circuit, i'm not sure what to expect from such transformer in SE.

Cheers, Miha
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Old 15th July 2012, 10:43 AM   #2
jane is offline jane  Norway
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The datasheet says 10Hz to 70kHz (+/-0.5dB) driven from a 8k source. On the other hand the frequency plot for the 8650 shows -2dB at 70kHz driven by a 2k source
Anyway, in single ended mode no DC is permitted.
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Old 15th July 2012, 11:38 AM   #3
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well you can always parafeed the amp... either choke load it or resistor load it. For calculations there are several excel based sheets such as the one listed here:

parafeed plate choke calculation

The transformer can then be un-gapped however component value and choice is critical to a parafeed design.
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Old 15th July 2012, 02:38 PM   #4
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Thanks for confirmations. I would like to keep it transformer coupled with DC into it, like originally built by RCA. I looked into options Alex mentioned, but i don't want redesign here, i need to get as close as possible with modern transformers.
I have several big 50%Ni ungapped output transformers, those could be tested for the start. I know my lows will get worse at a certain point when Al value gets lower, but i don't have much current after all. Some people seem to get away with up to 4mA with this cores, this is what i have more or less. Does anyone of you have experiences with this? If it won't work well i can still buy gapped transformer, although this would cost me and i already have a few ungapped...
I just want to properly build something that works similar to original and see how it behaves. If i recall correctly, original output transformers were attenuated by ~1-2dB at 18kHz, 600 into 10kohm. I will try to find graphs. Not too good when compared to modern transformers, i will probably have different response with new transformers.

Miha
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