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47 driving 300B questions

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AudioGeek

Member
Joined 2004
After listening to a 6B4G PP amp for awhile now I think its time i went back to SE. What i am thinking is a JJ 300B based amp using all Edcor iron.

Would a 47 be capable of driving a 300B? Pentode mode. Cap coupled.

Looking for around 10W.

As far as the power supply, any advantage of doing passive vs regulated in a SE class A amp?

Or should i scrap this idea and just do a DHT to DHT with interstage transformers?

Thanks!!!!!
 
Miles Prower

Miles Prower

Member
Joined 2005
dolphin-hsl.com
AudioGeek said:
After listening to a 6B4G PP amp for awhile now I think its time i went back to SE. What i am thinking is a JJ 300B based amp using all Edcor iron.

Would a 47 be capable of driving a 300B? Pentode mode. Cap coupled.
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Dunnow about that, but an FQP1N50 certainly would be up to driving a 300B. All the complaints I've seen concerning disappointing sonic performance all seem to be due to inadequate grid drive. A source follower would solve that particular problem.

Looking for around 10W.

As far as the power supply, any advantage of doing passive vs regulated in a SE class A amp?

Or should i scrap this idea and just do a DHT to DHT with interstage transformers?

Thanks!!!!!
Click to expand...

I definitely think active regulation would be best for any SE design, especially a triode based one. Triodes are sensitive to plate voltage variations, moreso than pents. Also, the PS is very much in the signal path, since you don't have the AC cancellation of Class A, PP operation that nulls out at the OPT center tap. Active regulation would take care of that by making for a very Lo-Z path to AC ground. It would also help the hum problem since SE becomes a good deal more sensitive to common mode noise riding on the DC rail.
 
soulmerchant

soulmerchant

Member
Joined 2006
AudioGeek said:
a triode 47 has not enough gain for 1 stage.

The PP i used a 6SN7 DC coupled to a 6BX7 which has a phase splitter IT.
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47 in Pentode has a gain of 150... I recon that in Triode it has a gain of about 1/3 (or more) of that - easily a gain of 50 lets say. It should work better than most other triodes except 12ax7 or 6sl7.

Of course 12ax7 or 6sl7 are indirectly heated.

PP is totally different. I use 6SN7 for PP 2a3. I have no need for PP 300b
 
soulmerchant

soulmerchant

Member
Joined 2006
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soulmerchant

soulmerchant

Member
Joined 2006
AudioGeek said:
I guess if I wanted a triode I would do something else other than 47 in triode mode. Gain of pentode and triode is dependent on plate load.. So I would adjust to get the gain I require. What my idea is a we91 style amp with a DHP at the first stage. Maybe even a 6A4.
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6A4 looks good except for the incredibly high Rp. With such high Rp's, to avoid a cathode by-pass capacitor you need an impossibly high plate load...

Of course if your tube draws enough current you can think of other solutions such as LED bias. I'm sure you know all this though.
 
M

marcelop

Member
Joined 2012
soulmerchant said:
47 in Pentode has a gain of 150... I recon that in Triode it has a gain of about 1/3 (or more) of that - easily a gain of 50 lets say.
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This is so not true, I´ve actually measured the 47 curves triode strapped and gain is around 7 or so and rp is 3k. A good approximation of triode strapped gain for tetrodes and pentodes is VG2 voltage divided by cutoff voltage, and can be obtained easily from any set of curves.
 
B

Bandersnatch

Member
Joined 2003
AudioGeek said:
Can we get enough gain out of a 47 in pentode mode and still have enough current drive on the grid of the 300B?
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Call up the plate curves, and draw a load line with the required gain. Check its slope to determine the plate load( which will be about what the output impedance will be). If you need lower output Z, or want more gain, apply a CCS and set the load line with a resistor to ground. Take output at the mu output.
cheers,
Douglas
 
soulmerchant

soulmerchant

Member
Joined 2006
marcelop said:
This is so not true, I´ve actually measured the 47 curves triode strapped and gain is around 7 or so and rp is 3k. A good approximation of triode strapped gain for tetrodes and pentodes is VG2 voltage divided by cutoff voltage, and can be obtained easily from any set of curves.
Click to expand...

Thanks! I didn't realize the gain was so low. Too bad it has such high plate resistance as a pentode...
 
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