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Recommended 300B SE Amplifier Designs

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tcqanh

Member
Joined 2011
kevinkr said:
The D3A would be a great choice and since you have it on hand.. Choke load or IT couple it to a fixed bias or cathode biased 300B.. I'll post something over the next few days - I've used D3A with Lundahl 1635 IT to drive copper plate GM70 with very good results, something similar should work quite nicely for the 300B.
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Hi Garbo,

D3a is very good tube. an other choice is maybe E810F, 5842/ 417A or 6C45P.

If you like sweet, warm and silked sound in the Jazz, Country, Instruments.... you can have regard to 6SN7 in which 1 triode is directly coupled to another. sound quality depends much on quality of your parts. in my ampli, I use totally AB resistors and many film capacitors.

here is my schema. sound is really attractive.
 

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T

tcqanh

Member
Joined 2011
SemperFi said:
You are right some audio signal will also go that way. Any alternating voltage, audio signal or noise signal will follow any path to a steady (non-alternating) node, either gnd or b+. In this case 1/10 of the audio signal will go to b+ since the cap to gnd is 10 times as large as the one to b+.
Click to expand...

The problem here is that audio signal will be bypassed through that C not linearity/ not equally. So freq response is not flat ?
 
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SemperFi

Member
Joined 2002
tcqanh said:
The problem here is that audio signal will be bypassed through that C not linearity/ not equally. So freq response is not flat ?
Click to expand...

No it doesnt matter if bypassing goes to gnd or b+, but any steady state node gnd or b+, needs to be real 'steady'. The effect is like having a 10uF in parallel with 100uF as far as the audio signal is concerned. The point is to bypass the cathode so it is the tube's reference point for the grid.

In a SE design you have no benefit of the ripple cancelation you have in a PP design. Since SE runs class-A with high current, it is usual to have relatively high ripple on b+ and hum on the output, espescially with low rp tubes such as triodes. So a trick is to inject some ripple out of phase into the cathode. It really does reduce hum but I find I need to experiment a bit with the final values, (parallel small caps over the large untill I get it right).

Thanks Kev for correcting me, I always seem to round off that '+1' factor since I usually end up fine-tuning the values after build anyways.
 
S

SemperFi

Member
Joined 2002
tcqanh said:
Hi Garbo,

D3a is very good tube. an other choice is maybe E810F, 5842/ 417A or 6C45P.

If you like sweet, warm and silked sound in the Jazz, Country, Instruments.... you can have regard to 6SN7 in which 1 triode is directly coupled to another. sound quality depends much on quality of your parts. in my ampli, I use totally AB resistors and many film capacitors.

here is my schema. sound is really attractive.
Click to expand...

Nice design tcqanh.

I wonder though if AC coupling the two 6SN7s is just as good, maybe better for that last 6SN7 because it gets more voltage to swing around. it is no more AC coupled than your schema because it doesn't matter if the cap is from one stage to the next, or if the cap is bypassing a large cathode resistor, that cap is just as much 'in the signal path'. It would perhaps be an interesting experiment.
 
soulmerchant

soulmerchant

Member
Joined 2006
SemperFi said:
It shunts some power supply ripple to the cathode to cancel the ripple on the output. The value of the cap from cathode to b+ should be [(C cathode to gnd)/mu]. I've found some fiddling with the final value necessary for complete ripple cancelation.

You'll find lots of this at tubecad.com...
Click to expand...

This technique works exceptionally well.

Read this: Lowering SE Amplifier Noise (page 2)

I use motor run capacitors in the upper C1 position and tune the cathode by-pass capactor with small value films if necessary.

Ian
 
soulmerchant

soulmerchant

Member
Joined 2006
PhilippeS said:
Best sounding in pentode mode (on my plan) : 12GN7, 12HG7, EL802, EL183E
Click to expand...

I have done similar, but use DC heaters and get rid of the hum-buck pot. Then connect the cathode-by-pass at the heaters.

Also, ever try a 6sn7 cathode follower (broskie style) after the D3a?

The D3a has lowish ~2k2 output impedance, but a cathode follower should be even better...
 
G

gaborbela

Member
Joined 2005
Hello

I'm still in the process collecting the parts for my 300B.
I have several question
Can I use other 10H choke because the LL1638 a bit pricey.
Like Hammond 159P??
Also I have a pair new Transcendental 3.5K OP transformer. Andrea use 5.6K in these circuit.
Can I use my transformer with out a major modification?
From these circuit also missing one capacitor value.
Kevin if you have at your hand your 300B (circuit) driven by D3a tube it would be great to share with us.
Please! Thank you

Greetings Gabor
 

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ErikdeBest

Member
Joined 2003
tcqanh said:
I bought these ITs from a friend. He don't know their origin. I suspect they from Hammond cause of the same style, but not sure. These use Fe 32mmx50mm and their size is equal an OPT Tango U808. 126B is much smaller.
Click to expand...

Thanks, I was just curious as the 126B seems a very nice interstage and is cheap. Indeed the U808 has almost double the volume as the 126B, so it is a sort of 'misterious' IT then :)

BTW, very nice looking amplifier!
 
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