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Old 6th January 2006, 07:11 PM   #1
sgerus is offline sgerus  United States
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Default Ideas for input/driver stage of 300B amp

I have just completed a JE Labs / Walton 300B mono-block and it sounds great…
Now I’m looking for ideas on the input/driver section for the 2nd amp so I can do
a side by side listing tests….

What I now is a 6sn7 input/driver (cascade?), running at 350V, -76V 50ma into a 5K output trans. (it’s actually 2.5K using the 4R output)

My speakers are 98db/w/m horn type. Turns out the 300B was overkill
(Someone on this message board warned my…. But)

I open to running at less power. Keeping HUM as low possible is important.

Would like to stay with a 6 volt octal tube, but that’s not a requirement.

Has anyone compared a 6sl7 SSRP to the 6sn7 cascade?

How about a 6sl7 cathode follower?
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Old 6th January 2006, 08:46 PM   #2
JimW is offline JimW  United States
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Default 6sl7 srpp

First set I built was a 2a3 using 6sl7 in srpp configuration. The driver stage showed a pretty significant rolloff in the region above 10khz (miller cap!). I was running the 'l7 with 2.2k cathode resistors, which only allowed the tube to run at about .75 ma. I think you will need to run it much hotter to have enough drive to overcome the miller. I have used a 6sn7 in srpp mode to drive 2a3 and 300b, running at about 6ma - works very well, but you will need some gain from a line stage. Since the 'l7 and 'n7 have same pinouts, you can set up for the 'l7, and drop in the 'n7 for comparison without any mods. Gordon Rankin donated a schematic using a 12at7 in srpp as a driver for a 2a3 (see baby ongaku) and it works well, has enough gain and current to drive either 2a3 or 300b. Another option, if you are open to experiment, is to drive the 300b with a el84 (see T.L. Legacy) I am presently using a variation of this design, sounds very good to my ears, but the power supply needs to be well filtered.
Jim
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Old 6th January 2006, 10:10 PM   #3
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Well Im in same situation as Sgerus .But im just starting .
I have allready JÉLabs amp and im starting to build second amp
based on w32b Krone tube .Idont know what kind of driver Im using but I start with 6sn7 ,so that i have space in chassis for additional Interstage /Choke .Secondly i use two driver tubes per channel so that only half is used. Maybe one alternative is this

http://www.simpletube.com/databank/S...L7_6BL7_3K.gif

I Have bad experience using Cathode follower in preamp project but maybe Im wrong??? Isuppose somewhere was circuit for parallel connected 6sl7 srpp?

Papparazzi
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Old 6th January 2006, 10:32 PM   #4
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Quote:
First set I built was a 2a3 using 6sl7 in srpp configuration. The driver stage showed a pretty significant rolloff in the region above 10khz (miller cap!). I was running the 'l7 with 2.2k cathode resistors, which only allowed the tube to run at about .75 ma. I think you will need to run it much hotter to have enough drive to overcome the miller.
Seeing that the Pd of the 6SL7 is just 1000mW, there is a limit as to just how "hot" that can run. Since this is a power stage, I'd fugedaboud the 6SL7 altogether. Use a 6SN7 (Pd= 3.5W).
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Old 6th January 2006, 11:55 PM   #5
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Check into using a 6BM8 wired as a mu follower. There are a number of projects on the 'NET.

On the SRPP front, choke loading a 5965 might give you the gain you need.

Check out Bottlehead. Their hole shrinker allows 9 pin mini tubes to go where Octal tubes have been.
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Old 7th January 2006, 01:46 AM   #6
arnoldc is offline arnoldc  Philippines
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I have built the SRPP 2A3 with the 2k2 resistors on the SRPP. I have not made any measurements, but I am not about to complain just yet. Had this for over two years already.

Tony in this forum suggested replacing the 2k2 Rk with 1k for more current and a colleague of mine said "like a different amp" after the simple mod.

I also built the 6SN7-300B of JE Labs but I prefer the 76-6SN7-300 better. It stayed with me for a couple of months but I just can't get the hang of the 300B character. I found both of these to have too much gain when used with a Bottlehead Foreplay.

I also built the Gordon Rankin Bugle 45 with I believe SRPP drivers too, and I think it sounds great.

More recently, I have been using 5842/417 drivers in my amp. Thorsten gave me a tutorial on using this tube on both 45 and 50 amp. These tutorials are buried in the Audioasylum Tube DIY and here at DIYAudio.

My 45 amp uses plate choke on the 5842 and grid choke going to the 45. One person who borrowed it has this to say - "The midrange of your 45 was spooky, haunting and surreal. It was like having wispy fingers of sound touch your skin. The amp is a real spell caster when listened to with DB99 speakers playing vocals and small ensembles. Presentation was dark and rich but paradoxically light and ethereal like mist or slowly rising steam."

I have no idea what it translates to, but I like the sound anyway.

Having said that, I bothered Thorsten again for using the 5842 as drivers for the 50 tube, which he gave me. So, if you like to play around with 5842, they could well match the 300B.

Now, 300B experts here maybe can tell me why, why is it that when I ran the 300B at Vp=350/Ip=60 it sounds a lot livelier and faster the the usual 450V/80mA recommendation?
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Old 7th January 2006, 10:17 AM   #7
JimW is offline JimW  United States
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Default 300b sound

Actually, I run my 300b amps at 325/70ma, and I think they sound better than when ran at higher plate voltage - perhaps it is a better point for the tube, or the OT. I adjusted voltages by varying the input cap, maybe the smaller input cap contributes. With a low parts count and only two stages, almost anything you change is audible. These amps do sound very good.
Jim
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Old 7th January 2006, 09:54 PM   #8
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Default Re: 6sl7 srpp

Konnichiwa,

Quote:
Originally posted by JimW
Another option, if you are open to experiment, is to drive the 300b with a el84 (see T.L. Legacy) I am presently using a variation of this design, sounds very good to my ears, but the power supply needs to be well filtered.
The Amp schematic I provided MUST be used with the powersupply published with it. If you do not do that change the circuit by adding 1K/100uF filtering to theAnode supply of the EL84 and take the capacitor between screengrid and +B away.

Also change the output stage (if you use self biasing) to use a 47/50uF cathode bypass capacitor and a 12uF MKP Capacitor between cathode & +B.

That will do quite nicely using standard LC filtered supplies...

As for the 300B, I like it best at 350V/60mA and with a 5K load instead of 2K5, good for around 6W.

Sayonara
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Old 9th January 2006, 11:15 AM   #9
JimW is offline JimW  United States
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Default legacy ps

Thorsten, thanks for your comments. I have mine implemented with 2.2k and a 47u to decouple the driver from the ps (this was actually based on your suggestion at an earlier date - you did suggest 100u, but I had 47u, so I tried them and they work well). I also used a clclc filter for the main b+. Hum measures right at 1mv on one amp, .8-.9 mv on the other, and my dmm has a residual of .2-.3 mv, so my real hum is on the order of .6-.8 mv at the output. I agree in your asessment of running the 300b in the low 20w range of anode dissipation. I have no hard data, but they sound good to me. I have a pp 2a3 that I am running at lower voltages also, for the same reason. My next project is to implement the WE connection for the output. I did this on the pp 2a3 with very good results - I may be tight on real estate in the 300b chassis! I am amazed at how well the WE guys understood getting the most out of their tubes. The article on Lynn Olsen's site "the rosetta stone for triodes" is a must read!
Jim
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Old 9th January 2006, 04:48 PM   #10
kevinkr is offline kevinkr  United States
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I totally agree on your assessment of the "Rosetta Stone for Triodes" it is a worthwhile read for anyone not familiar with the early work at Western Electric.

kevin
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