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Old 17th February 2009, 05:17 AM   #1
Ziggy is offline Ziggy  Australia
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Default Low voltage 845SE with 300B driver

Fired up my breadboarded low voltage (450volts) 845SE the other day and decided I would play around with my old 300B spare tubes as drivers.

So, I have a paralelled 6SL7 as input valve(as per the original circuit), cap coupled to 300B as driver, cap coupled to the 845.
It works! Didn't expect it to at this low plate voltage ??

I only have the 300b working at 200volts B+ and operating at 40ma.

The gain increase and dynamics is dramatically better!

No, I can't run the 845 at higher voltages - output trannies won't handle it.

I guess that it's true that the 845 really comes to life with a 3 stage configuration.

Anyone else tried this ?
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Old 17th February 2009, 10:04 AM   #2
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Quote:
I guess that it's true that the 845 really comes to life with a 3 stage configuration.
I knew it was a dirty tube, only giving its best in a 'menage à trois'.
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Old 18th February 2009, 02:24 AM   #3
Ziggy is offline Ziggy  Australia
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I see

Expected a bit more input on this one.
The 845 at low volts can sound magnificent which should allow more tube builders to construct amplifiers with reasonable rather than astronomic voltages. :
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Old 18th February 2009, 03:16 AM   #4
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how low is low?

dave
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Old 18th February 2009, 03:41 AM   #5
Ziggy is offline Ziggy  Australia
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450 plate volts.
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Old 18th February 2009, 04:42 AM   #6
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"How low can you go" is going to depend to some extent on what you are willing to pay for a dwindling supply of 845s unfortunately. Shuguang the maker of many of the more affordable versions of this tube has apparently decided either not to make them any more or to make them on a very sporadic basis. There are so many rumors that it is hard to know.

Read up on strategies for extending their service life, which includes such ploys as not ever removing filament power, but instead running them at about 80% of rated filament voltage with plate voltage removed when not in use. The filaments apparently become progressively more brittle as they are cycled fully on and off. IIRC Guidelines are from old ABC (OZ) guidelines for maximizing filament life in tubes using thoriated tungsten filaments.
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Old 18th February 2009, 05:16 AM   #7
Ziggy is offline Ziggy  Australia
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Hmmmmm.....interesting Kevin. I have these exact valves and run my filaments at around 9.4 volts AC.

the big **** off is that the Chinese or standard Sovtek 300B's has limited availability and/or shot through the roof price wise.

I have two well used chinese 300B's and one and ''a half'' Sovteks (one of the tubes decided to drop off half it's filament !
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