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New project 300B
New project 300B
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Old 13th April 2018, 05:56 PM   #11
xcicc is offline xcicc  Italy
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Exactly there is no mistake in the design, this project reminds me that I had copied it from an old book (which obviously can not be found). I do not see why I have to look for exotic products like 6H30 just because they go "fashion" ....
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Old 13th April 2018, 06:02 PM   #12
waltube is offline waltube  Italy
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6H30 is not fashion and exotic, it is very fine. One of the best as linearity and is not so expensive. 6H6 is a good alternative ( same family).

I think you copied wrong

Walter
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Old 13th April 2018, 06:33 PM   #13
xcicc is offline xcicc  Italy
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2 x 6H30Pi Electro Harmonix Gold balance select & matched pair - 6H30 Röhre Tube | eBay
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Old 13th April 2018, 06:40 PM   #14
waltube is offline waltube  Italy
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Look this:
6H30PI Sovtek :: Sovtek :: Tubes :: Banzai Music GmbH same tube, with 10% more you can ask for matching.

or
www.thetubestore.com - Russian 6N6p / 6H6pi Audio Tubes

Walter
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Old 13th April 2018, 06:42 PM   #15
xcicc is offline xcicc  Italy
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Thanks, good to know.....
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Old 13th April 2018, 07:00 PM   #16
Redbeard is offline Redbeard
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Hi xcicc,

Wow, this is my first post to this forum.

Ok, whilst your circuit diagram is incorrect - g1 and g2 are transposed on the pentode half of the ECL82, it's nice to see -ve feedback being used in a SE triode amp.

As a first-poster, i must be careful what i say but my favourite way of driving a 300B (or any output triode) is a 5687 wired in SRPP "series boot-strap", as this gives you the voltage gain (µ of the triode) and the drive current from the cathode of the upper of the two triodes.

Xx.
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Old 13th April 2018, 07:03 PM   #17
hpeter is offline hpeter  Slovakia
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rg on 300b is too low. (selfbias) it will load driver too much (high thd), and decrease gain.
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Old 13th April 2018, 07:06 PM   #18
waltube is offline waltube  Italy
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with a srpp of 6h30 or 5687 ( less linear but strong) you have a low Z, around 300 ohms so the risk is limited also in this case.


Walter
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Old 13th April 2018, 08:04 PM   #19
Adolf Corkscrew is offline Adolf Corkscrew  Finland
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I don't see anything wrong in using an ECL82 as a driver for the 300B. I might, though, opt for using the pentode half to drive the output tube directly. But that poses additional considerations to the design. As long as it's kept in class A1 operation, there should be not much trouble with the schematic posted, given that the driver tubes are wired correctly and not as they're shown in the schematic. It won't probably extract every last inch of what you can get out of a 300B, but I'd expect it to work just fine.

Some considerations, though. The +B voltage is quite low as already mentioned, especially with a tube rectifier. Secondly, I'm not quite sure that a 5U4 would enjoy that much input capacitance. 100µF might just be eating away it's useful life faster than necessary.

Third thing to consider is the feedback loop. This should be applied only when you've measured the thing open loop, and can assess it's response both in the amplitude and phase domain, lest you wish oscillations to occur. Many conventional transformers drop their response quite gracefully, but some - especially toroidals, I've found - have some quite nasty high frequency resonances, which can spell big trouble. If you want to do things right, you'll need measuring capabilities at least up to 1 MHz.

Now, if I was designing a 300B amp, I'd probably go with a single pentode driver stage with high quiescent current and a low plate load resistor. That'd give enough gain from a single stage, and a low enough driving impedance for the output tube. Or a high gain triode and a buffer. Either option would give the option of tailoring the distortion behaviours of the driver and output stage to match and partially cancel out before global feedback was applied.
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Old 13th April 2018, 08:34 PM   #20
6A3sUMMER is offline 6A3sUMMER  United States
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The essential effect of the 300B fixed negative bias circuit is 250VAC driving a half wave rectifier, driving a 10k/10k 2:1 divider. That is (1.414 x 250V) / 2 = -176Volts.

The schematic should either use fixed Bias (but adjustable fixed), or Self Bias.
The 820 Ohm and bypass cap is self bias.
The fixed bias (-176) could be used instead, but only if it is revised, and the self bias circuit is removed.
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