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So many 300B schematics...which one?

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I've finally decided to settle the SET Vs. PP amplifier argument for myself once and for all. I want to build a 300B SET. I have already gotten some Transcendar OPTX's, now I'm looking at designs. There is no shortage of them on the internet. I'm not yet evolved to the point where I can see or understand the advantages of a certain design over another. As a matter of fact, it will be a small miracle if I can pull it off at all. No one needs to talk me out of this, or down sell me to something cheaper or less involved. I want an amp that is going to be a VERY good example of the SET philosophy. I am interested in a design that will have enough bass to handle rock, and I have some concerns afetr reading some people's experience with less than adequate bass response. But rock is only about 30% of what I listen to. I have NEVER heard a SET amplifier, and it really doesn't matter if in the end I don't like it, as I'll pass it on.
So, which design? JE Labs, Angela, the Sound Practices WE91A, Walton's design? I have been looking at Thorsten's design (Legacy, see link)with the tube regulated B+ The PS looks a bit daunting to me, but I could probably figure it out after posting a few hundred questions here on DIY. Anyway, I don't need a ton of power as my speakers are at the top of the efficiency chain. But, I want that MAGIC everyone describes, without a bunch of sweet syrup, but with the type of dynamics to handle ROCK when I get the itch...experiences and suggestions welcome here...more schematics, also welcome.
"I KNOW you can hear it, but are you REALLY listening?"
Legacy 3 (Open in New Window)

Edits: 10/03/09

Follow Ups:

12B4a driver - Plinko 05:04:31 10/29/09 (0)
I wanted exactly what you wanted. So, we went with the 12B4 driver and good output transformers (Magnequest FS-030). The 12B4 is a descendent of the 2a3 and has the voltage swing needed to drive a 300B properly (see archives). I'm no expert but in comparing drivers, this 12B4A specs out nicely. Doesn't have a lot of gain, however. The icing on the cake is that it sounds great also.

There are, of course, many other great tubes to use.
RE: So many 300B schematics...which one? - finnman500@hotmail.com 21:54:21 10/16/09 (0)
A slight correction to my post. The RCA input driver is a Red Base 5692.
Also, I notice Derrick (Walton) no longer provides his schematic for the Mark III. This is perhaps due to the degree of difficulty building it which lead to too many emails from inexperienced builders. Just a guess on my part.
RE: So many 300B schematics...which one? - finnman500@hotmail.com 21:37:17 10/16/09 (0)
Google 'Walton Audio' and peruse through his site. You will see schematics for 3 different versions of the 300B mono blocks. I have the Mark I which I dearly love and have yet to hear anything better and I mean anything. I have tried other tube amps and many, many solid-state amplifiers and for the money, you can't beat these amplifiers.
Based on the classic Western Electric design but extremely over-built, the Mark I is easily built but do not skimp on components. They make ALL THE DIFFERENCE. I have upgraded mine with KR300BXLS outputs, RCA red base 12AX7 driver tubes and Mullard GZ-37 coke bottle rectifier tubes. Also, the .22 micro-farad input cap specified (Angela) was changed to V-Cap teflon.
This is about as nice 8 watts can sound; at least to my ears.

PS: The Mark II model is not recommended as it was found to be not an improvement. The Mark III while sonically superior, mostly because of increased headroom (14 watts) is very much more complex and expensive to build and is only for experienced builders. IMO

I hope this post is helpful and presents a viable option. Cheers.
RE: So many 300B schematics...which one? - njjohn 19:21:51 10/05/09 (0)
I thought maybe I ought to go upstairs and listen again before I post but then I may not get to the post. I think you have to be careful where you shine the light or you might just see a part of the animal and think that is the animal.

I would keep it simple and the SET might not win out completely in all areas but you may be able to ascertain some of what it is all about.

What worked for me was a 6sn7/300b with a fi primer 6sn7 circuit, ac on all of the tubes so you need hum pots, and the most interesting opts you can find. David slew Goliath with a simple stone and it does not take the best design to give you the insight you need. The only thing that can beat alluring is simplicity, purity, and substance.

The reason to want to listen first was to hear rock again on the 300b. I've been playing jazz on the 6sn7/300b and it's like being in the room at a live performance. The bass is a bass cello, not an electric bass.

I've listened to rock on the 45 and I can say it gives you the cream of the cream by the emphasis of the midrange. The 300b, at its best, is more ambient with less emphasis on the midrange. Both are really quite good.

These are about the best output tubes ever invented.

So its alot about using the finest ingredients like the best sounding output tubes ever invented, the first and most primal circuits discovered like the fi primer circuit, the best iron, and then breathing life into the amp with the ac.
RE: So many 300B schematics...which one? - northern-pi 17:31:46 10/05/09 (0)
I have recently built my 300B SET based on Thorsten's Legacy amp. I wouldn't have built it if it wasn't for Thorsten's encouragement... In my case, my power supply is based on standard 5U4 rectifier into pi filter which drives the Legacy. My heater supply is DC. It took me a while to have the proper operating load voltage for the driver stage, but once I got it the sound is really astounding. I used iron from James Transformer. I compared it against JE Labs with Hammond iron and found it sounded very close, subjectively speaking... I use motor run caps in the power supply section and also incorporated ultrapath in the circuit each time resulted in better music... I listen to it almost every day for the last 10 months, each time I have it turned on for 4-5 hours and it is still going strong... Overall, it has been a great experience.
Audio transformers - Neff 05:18:57 10/04/09 (1)
To achieve the best, no cost cutting can be in the audio transformers period. Visit MagnaQuest and ask more questions about audio transformers.
- 2nd that nt - Caucasian Blackplate 12:25:05 10/04/09 (0)
  nt
RE: So many 300B schematics...which one? - Paul Joppa 23:19:40 10/03/09 (1)
If you want to hear "a VERY good example of the SET philosophy" you will need VERY good transformers. If you also want powerful bass, the transformers will need to have plenty of inductance, especially if you use series feed. (I think most here know I am generally an advocate of parallel feed...) Such transformers will be pretty large and heavy, but neither price nor weight will provide a promise of excellent performance. And sadly, excellent measured performance is not a guarantee of excellent subjective performance.

The next most important thing is the driver stage, in my opinion. I strongly advocate a simple, linear driver with large voltage headroom. Usually that means a single triode with a high plate voltage and a high load impedance (choke/transformer, or current source).

These two things will go a long way towards achieving your goal. I would further suggest you use as few other components and complications as you can.

Finally, especially for series feed, the power supply must be clean and able to handle the signal current fluctuations. That means really good capacitors at the output of the supply.

There are many refinements to these ideas, but too often we get caught up in the refinement and lose sight of the basics. So I would discourage anyone from getting complicated before they have experience with a simple, well thought out design.
- RE: So many 300B schematics...which one? - Audiodyssey 07:38:06 10/04/09 (0)
  This is very well spoken, Paul, and I advice I think I am going to heed. It gives me another idea as well; you and another inmate (at least two of you)mentioned using high quality output transformers, and this was not the first time I have heard this about transformers, particularly with SET amps. I realize the flaw in my thinking; having tried to cut corners with OPTX's, assuming the lofty goal I want to achieve, I'm off to a bad start already. Of course this is a subjective thing, but there are proven transformers, and even a host of TX comparisons on the web. I think I am going to do the build without permanently installing any OPTX's on the chassis. The pair I have could definitely be said to be in the budget range, though I have heard good things about them, I am sure better can be had. In this way, I hope it's at least possible to get the feeling of the SET experience while allowing for possible upgrades in the future.
  
  Of the designs you have seen on the net, do any fit the specifics you mentioned in your post? This all started with the Sound Practices article about the W.E. 91A, but I think maybe the JE Labs SE300Bdx looks interesting and in line with your recommendations also? Ah, decisions, decisions...
  "I KNOW you can hear it, but are you REALLY listening?"
RE: So many 300B schematics...which one? - Donald North 21:49:43 10/03/09 (3)
What about building an Audio Note Kits Interstage monoblock?

Donald North
- http://www.audionotekits.com/monob01.html (Open in New Window)
- RE: So many 300B schematics...which one? - Audiodyssey 23:10:10 10/03/09 (1)
  Though these look very appealing, the price for admission exceeds my budget. I have access to everything I need to build outside of tubes & OPTX's. Plus to be honest, building this is definitely part of the allure for me. Thanks, Don.
  "I KNOW you can hear it, but are you REALLY listening?"
  - RE: So many 300B schematics...which one? - Donald North 13:54:10 10/04/09 (0)
    They also offer the Kit 1 which is less money and sounds great - I own one.
    
    One benefit of building a kit is resale value: If there comes the time and you want to sell your amp, known quality kits command higher resale values than scratch built creations.
    
    Donald North
- RE: So many 300B schematics...which one? - Caucasian Blackplate 21:58:13 10/03/09 (0)
  Why is all the iron aligned?
RE: So many 300B schematics...which one? - paba 20:11:35 10/03/09 (16)
Just so your list of schematics is complete before you make your final choice...

the T-Rex (John Broskie and Dick Olsher and Steve Bench)

I have some modified Welbourne DRD monos but if I had the need I would go T-rex.

cheers
paba
- T-Rex 300B (Open in New Window)
- RE: So many 300B schematics...which one? - Audiodyssey 23:12:38 10/03/09 (15)
  Thanks for this, I hadn't seen it. I kind of like the idea of the 5687 in front. Do you know of anyone that could give me any feedback on performance?
  I would have to dispense with the use of Plitron TX's. That is one hell of a regulated supply. How often do you suppose those tubes in the PS would need replacing?
  "I KNOW you can hear it, but are you REALLY listening?"
  
  Edits: 10/03/09
  - RE: So many 300B schematics...which one? - paba 09:59:54 10/05/09 (14)
    I think the power supply is so often neglected in 300B set that I have seen. I would say that the order of impact on the sound is:
    
    1) topology (2 stages or 3 stages or interstage transformer etc.)
    2) quality of the iron
    3) power supply
    
    choices like AC or DC heaters and how clean your B+ is will play big because you are probably using high eff speakers (95dB and more if you can find some) with your 7-8 watts and hence you will hear the power supply effects.
    
    cheers
    paba
    - Forget the 300B with efficient speakers - drlowmu 10:27:20 10/07/09 (13)
      Hi paba,
      
      Intersting post. I would say it differently, from my experiences:
      
      1) Topology, two stage vs: three, AND topology, 300B vs 2A3.
      
      (On efficient speakers, a 2A3 is the superior tube choice, and a good two stage amp performs better than ANY good three stage. KISS rules.)
      
      2) Power supply
      
      (The chokes and inductor(s) have to be 20 Ohms DCR or less, or you are wasting your time, money, and badly deluding yourself). The SE amp's wiring and lay out needs to be optimized also!!
      
      3) Output Transformer quality
      
      (Yes, it is certainly a factor, but it is "totally trumped" by doing 1) and 2) above. With 1) AND 2) done as best as possible, and a wire-modded Transcendar will very easily outperform ANY of the highest quality outputs you, or any one else, may choose.
      
      The output transformer is the LEAST important of the three, on the basis of overall audio performance, when the amp is built correctly. Very few SE amps are !!
      
      Jeff Medwin
      - RE: Forget the 300B with efficient speakers - paba 11:24:16 10/08/09 (12)
        
        In Reply to: RE: Forget the 300B with efficient speakers posted by drlowmu on October 07, 2009 at 10:27:20
        
        Hi Jeff,
        
        not sure I get all of your post. So I will state what I understood.
        I place Transformer last on my list as you so we agree there.
        
        By two stage and three stage I mean to stay two different tubes before the 300b (to me means 3 three stages) and 1 tube before the 300b means 2 stages. (Even if the one tube is double triode which some might argue that it is two tubes in one envelope). Interstage transformer is obvious. I believe in KISS but blind KISS is not always the best. We can make something simple but not too simple. My 300b amps are two stage by my definition but I've never heard the highest end 300b implementations.
        
        Power supply. Here is what I was trying to say... design choices like AC or DC heaters and the quality of those supplies feeding the tubes before the 300b will have a big impact on the sound of that amp especially if using high eff speakers.
        
        thanks
        Paba
        
        RE: Forget the 300B with efficient speakers - drlowmu 14:24:58 10/08/09 (0)
        
        In Reply to: RE: Forget the 300B with efficient speakers posted by paba on October 08, 2009 at 11:24:16
        
        Hi paba,
        Using both sections of a dual triode, for different functions, ie: input function and another section as a driver function - the 300B amp has a three stage topology. We are talking about the SE amp's number of stages doing differing functions, not number of tubes employed.
        Using a single stage, functioning as a input AND driver, powering a 300B, is an amp with a two stage topology. Even if the first stage is a dual triode, sections connected in parallel, performing the same functions, it is a two stage amp !!
        If two and three stage topologies are done (each to the best possible execution), the two stage will outperform the three stage, and that is why I build them. The three stage WILL be less transparent, due to the extra (third) stage.
        So, proper KISS rules, and when each topology is optimized, less is more !!
        Jeff Medwin
        
        Edits: 10/08/09
        
        How about EQUAL importance? - Ray Moth 12:17:28 10/08/09 (10)
        
        In Reply to: RE: Forget the 300B with efficient speakers posted by paba on October 08, 2009 at 11:24:16
        
        Get any one of those three factors wrong (topology, PS or OPT) and no amount of improvement in the other two will save you.
        
        No, Not Really !! - drlowmu 14:27:57 10/08/09 (9)
        
        In Reply to: RE: How about EQUAL importance? posted by Ray Moth on October 08, 2009 at 12:17:28
        
        Ray,
        
        In practice at the highest possible build and design level, not at all, the SE OP XFR is truly last of the three, but it is nice to have a good one, of course.
        
        Jeff
        
        RE: No, Not Really !! - Donald North 17:04:15 10/08/09 (8)
        
        In Reply to: RE: No, Not Really !! posted by drlowmu on October 08, 2009 at 14:27:57
        
        Jeff:
        
        The question of 2 or 3 stages is dependent upon if there is an active preamp upstream or not. In my experience, you need 3 gain stages, be they all in the amplifier or 1 in the preamp and 2 in the power amp.
        
        No doubt, the choice of output tube is significant. So is topology: cap or transformer or direct coupling between stages.
        
        As for output transformer quality/construction, I can assure you: You will heard a marked difference between a standard copper wound, M6 EI core and a silver wound, nickel double C cored transformer. :) I would not put it at the bottom of the list. To me it is equally important.
        
        Donald North
        
        RE: No, Not Really !! - drlowmu 19:23:13 10/08/09 (7)
        
        In Reply to: RE: No, Not Really !! posted by Donald North on October 08, 2009 at 17:04:15
        
        Hi Donald,
        
        I totally believe you about C Cores, and I have NOT had build experience with them, hard to source in the USA.
        
        You state:
        
        "The question of 2 or 3 stages is dependent upon if there is an active preamp upstream or not. In my experience, you need 3 gain stages, be they all in the amplifier or 1 in the preamp and 2 in the power amp."
        
        If you go to the Serious Stereo web pages, you will see a two stage DC 2A3 amp and audio system that requires NO active preamplifier stages in front of it, but rather, a passive attenuator.
        
        I have had lots of listening experience to this system, at four RMAF shows, and I can tell you the gain works out fine and it is NOT dependent upon an active preamp upstream at all.
        
        Hence, my comments on loss of transparency with a three stage amp. With a three stage, you can pick out a delicious sounding input stage, and or a beefy low Z driver stage, but they will not together quite do what a well designed two stage will do, transparency-wise. This is my experience.
        
        Jeff Medwin
        
        RE: No, Not Really !! - Donald North 20:54:23 10/08/09 (6)
        
        In Reply to: RE: No, Not Really !! posted by drlowmu on October 08, 2009 at 19:23:13
        
        Hi Jeff,
        
        Yes, I've read through that site before. Do you know is the gain input-output of that amplifier? I estimate around 22dB. In my experience using passive preamps and true 95dB efficient speakers, 22dB is not enough. I've found 30dB or slightly higher provides the best micro dynamics and life to the musical presentation and listening experience. I like to use transformer volume controls like the Silver Rock. From my measurements, they're best performing at their lower settings (-40 to - 60dB), requiring 30dB+ gain amplifiers.
        
        Donald North
        
        RE: No, Not Really !! - tube wrangler 00:08:07 10/09/09 (5)
        
        In Reply to: RE: No, Not Really !! posted by Donald North on October 08, 2009 at 20:54:23
        
        I had that trans-pot and I sold it. It lost energy like a Yugo with a fuel system leak.
        
        I then tried every passive attenuator out there-- regardless of price, and I finally got Tribute's trans-pot, and built my own box and wiring around it. Jay Fisher has this one and he loves it. It is very dynamic, has great dynamic shadings, and gets all levels and layers of the midrange out there very well.
        
        I wanted more bandwidth for my own systems, and set about building a Ladder type out of Shallco 45 position switches, and then I built a L-Pad unit out of military-style wirewound pots with the same strict attention paid to low-loss wiring. I love the thing, and also the LADDER.
        
        You can forget about losses and needing more gain with these the way I build them. You have gain and dynamics to burn because the losses are minimized before the amp sees the signal.
        
        What's left of it after it goes thru the attenuator is-- in this case-- MOST of it!
        
        ---Dennis---
        
        RE: No, Not Really !! - Donald North 08:18:27 10/09/09 (4)
        
        In Reply to: RE: No, Not Really !! posted by tube wrangler on October 09, 2009 at 00:08:07
        
        It lost energy like a Yugo with a fuel system leak?! That's not what I and my friends have heard. Of course with TVCs, they're particular about system synergy and specifics like output impedance driving them, cap or direct coupled, input impedance, etc.
        
        Donald North
        
        RE: No, Not Really !! - tube wrangler 17:21:46 10/09/09 (3)
        
        In Reply to: RE: No, Not Really !! posted by Donald North on October 09, 2009 at 08:18:27
        
        Two different worlds here.
        If you've got an active preamp driving it and you have excess gain from an extra stage in your amp, then you'll not notice the attenuator losses.
        What I've found is that taking that 3rd stage out of an amplifier and replacing it with a single one of high gain, doesn't yield quite the total gain of most 3 or 4 stage amps, but that is a moot point because those amps can't compare in overall fidelity to music and what some refer to as "jump factor"-- rhythm, pace and accurate event timing.
        The simple two-stage amp simply walks all over amps with more active stages.
        If you then merely add the 3rd stage back-- say, into an active preamp, then you're back to the same topology-- too many active stages.
        So, if you're going to run the simple 2-stage amp and use a passive attenuator for a volume control, you will definately notice losses which occur in that attenuator.
        I've noticed that, and since I'm not willing to go back to too many amp or preamp stages, I have built an attenuator that really rocks-- has almost unmeasurable signal losses or signal changes in character.
        This approach has produced a system that excels in dynamics, correct rhythm, pace and event timing, and fine introspection into music's many attitudes, player's intentions and intents during a musical performance.
        Today, the difference is just too great. I could never go back.
        ---Dennis---
        
        Edits: 10/09/09
        
        RE: No, Not Really !! - Ray Moth 21:39:10 10/12/09 (2)
        
        In Reply to: RE: No, Not Really !! posted by tube wrangler on October 09, 2009 at 17:21:46
        
        Dennis,
        It seems to me that there could be scope for trying a suitable pentode to drive the 2A3 in your amp, as per the original L-W. (I know that was actually a tetrode but you know what I mean.)
        You might hopefully get the improved performance that others have discovered from such an arrangement. Maybe you could offer such a topology as an alternative to those who prefer the sound?
        
        Edits: 10/12/09
        
        RE: No, Not Really !! - drlowmu 12:47:20 10/15/09 (1)
        
        In Reply to: RE: No, Not Really !! posted by Ray Moth on October 12, 2009 at 21:39:10
        
        Hi Ray,
        
        Having just spent three full days listening to Dennis' amps at a show, (as I have done these last four years at RMAF) any change as you suggest would likely be a degrade.
        
        They are superbly balanced, superb sounding, totally honest amps, just as they exist !! It is probably the best sounding low powered amp ever available as a manufactured product. At least, this is my own opinion, from all my own listening experiences in various systems.
        
        Jeff Medwin
        
        RE: No, Not Really !! - Ray Moth 08:05:34 10/16/09 (0)
        
        In Reply to: RE: No, Not Really !! posted by drlowmu on October 15, 2009 at 12:47:20
        
        People's tastes differ and there are those who say they really like pentode drivers. I'm not suggesting it necessarily as an improvement, just an alternative that might be interesting to try.
Kimmel mu stage - 4season 17:28:15 10/03/09 (1)
Sorry, I can't find schematics posted online, but maybe Welborne Labs can sell you the assembly manual for the Laurel IIX monoblocks. It's been 9 years since I built myself a set of those, and they're still my primary amps and have outlasted the DRD monoblocks which came after. In fact, I think my next attempt at a major upgrade won't be another SET, but perhaps an output transformerless amp.
- Here you go ... - danlaudionut 02:17:50 10/04/09 (0)
  Googled and found it for you -
  
  DanL
  - http://www.enjoythemusic.com/magazine/sound_practices/6/welb (Open in New Window)
RE: So many 300B schematics...which one? - danlaudionut 16:49:53 10/03/09 (23)
Audiodyssey

I have been toying with the idea
of a 6EM7 driving a 300B like the
Coincident Frankenstein 300B which
seems to be getting great praise.
Here is my version of the amp.

DanL
- Enjoythemusic.com Review (Open in New Window)
- RE: So many 300B schematics...which one? - Audiodyssey 23:17:24 10/03/09 (2)
  Dan, nice try...gettin a newbie to build your prototypes :)
  I appreciate the schematic, and it's worth a second look. Hey, what do you know about the EML's? What's a pair of those cost? Have you used them? I was looking at the KR Ballons...
  "I KNOW you can hear it, but are you REALLY listening?"
  - RE: So many 300B schematics...which one? - danlaudionut 00:15:19 10/04/09 (1)
    Audiodyssey
    
    My building days are over - sorry.
    All that is left is dreaming up designs.
    I left here because I could not build anymore.
    I eventually decided to return because
    dreaming was better than nothing.
    
    It is not really new but a modification of a known
    and highly praised Gary Kaufman design - seen above.
    Google it and you will have many people
    testifying they love the sound of the 6EM7.
    Some prefer it over the DHTs they have.
    I increased current in the first stage
    and decreased it in the second stage
    then used an interstage to the 300B
    for more power and control.
    
    EML are more reliable than KR so I've heard.
    EML 300B-XLS $590/pair
    Never could afford a pair.
    
    DanL
    - EML Tubes Dealer (Open in New Window)
    - RE: So many 300B schematics...which one? - Paul Joppa 11:11:12 10/04/09 (0)
      And Ron Welborne's design for a 6EM7 driver (which doubles as an amp) was published in VALVE in 1998 (v5n2). It was cap coupled, but my own direct-coupled driver design using the 6DN7 was in VALVE in 1997 (v4n9).
- I have been thinking along similar lines. - RC Daniel 19:48:55 10/03/09 (2)
  Despite the recent popularity of the EM7 series of TV dissimilar triode, I will be using something with a lower impedance 1st section (more like a 6SN7 than 6SL7) so I can L load it. All inductive loading on my beast - it should be nice light to carry to show and tells ;^) Its gain structure should also be a little more useful than the EM7s, which have lots-o-gain in their 1st sections (tho I guess it would not be by some folks reckoning).
  
  Thanks for sharing the schema Dan.
  
  Cheers
  Raymond
  
  "As long as we have any intention to be right... we should be wary. So long as words have the slightest ego attachment, they are dishonest."
  Charlotte Joko Beck
  - RE: I have been thinking along similar lines. - danlaudionut 20:23:43 10/03/09 (1)
    Raymond
    
    I love plate chokes too.
    In my design it is linear with
    a Zout of 17K and gain of 30.
    Draw a loadline for yourself
    and you will be surprised.
    That is why I have a 30K load
    and it is operating at 2.5mA.
    Most designs use a larger plate load and
    lower current but if you want no phase
    deviation @ 20KHz you need a lower impedance
    drive for the second half of the 6EM7.
    Input sensitivity is about right
    at .55V for max output.
    It all worked out quite nicely.
    
    DanL
    - Cool. I look forward to seeing your progress! nt. - RC Daniel 21:30:49 10/03/09 (0)
      .
      "As long as we have any intention to be right... we should be wary. So long as words have the slightest ego attachment, they are dishonest."
      Charlotte Joko Beck
- RE: So many 300B schematics...which one? - mach1 19:06:55 10/03/09 (11)
  Dan,
  
  Should be a kickass amp. Is there any particular reason for having a 30k plate resistor on V1 (Ri =40k) when you have 130k potentially available? Should the plate resistor be 100k and the dropping resistor 30k ?
  - Good Question - danlaudionut 20:11:09 10/03/09 (10)
    Mach1
    
    The reason for the 30K plate load is:
    1) Reduce the gain of the stage to 30
    (Still has over 150 gain to the 300B)
    2) Reduce the output impedance so that
    the second half is driven to 200KHz F3.
    (200KHz means 20KHz has no phase shift)
    
    DanL
    - RE: Good Question - Donald North 21:45:29 10/03/09 (2)
      Hi Dan,
      
      With a 30K load impedance, aren't you going to have high distortion from the first stage?
      
      You might be better served going with the 6DN7 which has a first stage Rp of 9K and mu of 22.5. You should still be able to have high bandwidth driving the second stage and maintain a similar gain into the 300B.
      
      Donald North
      - RE: Good Question - danlaudionut 23:48:38 10/03/09 (1)
        
        In Reply to: RE: Good Question posted by Donald North on October 03, 2009 at 21:45:29
        
        Donald
        
        > > With a 30K load impedance, aren't you going
        > > to have high distortion from the first stage?
        
        Not according to the loadline.
        1.5V bias @ 2.5mA and a 30K loadline
        is Very Linear from 0-3V bias.
        
        I looked at the 6DN7 but the second stage
        just isn't as linear linear as the 6EM7.
        Also you had alot more gain than needed.
        A 2K to drive the 300B into A2 peaks
        doesn't quite hack it IMO.
        
        DanL
        
        RE: Good Question - Paul Joppa 11:00:34 10/04/09 (0)
        
        In Reply to: RE: Good Question posted by danlaudionut on October 03, 2009 at 23:48:38
        
        More important, the first stage can produce a peak of +/-50v or so but you need only 15v to drive the second stage to the onset of output stage grid current. That headroom reduces the first stage distortion substantially from its value at peak output!
    - RE: Good Question - Caucasian Blackplate 20:26:39 10/03/09 (6)
      Why not use a slightly different tube? Something like the 6EW7 wouldn't completely kill you in gain.
      
      I do like the design idea. A low Rp VA driving DHT's is a very viable option. With the interstage transformer, you suddenly also have the ability to dicker with A2 if you really wanted to... (That interstage transformer looks quite good, have you tried any? The 3K looks like a nice way to do 2A3-> 845)
      - RE: Good Question - danlaudionut 20:39:42 10/03/09 (5)
        
        In Reply to: RE: Good Question posted by Caucasian Blackplate on October 03, 2009 at 20:26:39
        
        CB
        
        The 6EW7 has half the gain that
        you want in a 3 stage amp.
        A 3 stage amp should not need a preamp.
        So I increased the current and loaded
        the 6EM7 down with a 30K plate load so
        that the gain and output impedance
        are right for the design.
        
        DanL
        
        RE: Good Question - Caucasian Blackplate 20:43:43 10/03/09 (4)
        
        In Reply to: RE: Good Question posted by danlaudionut on October 03, 2009 at 20:39:42
        
        You are quite correct, I did not notice the current in the 1st stage (when I see EM7, I generally assume 1ma). Have you built this, or are you still mulling it around a bit?
        
        RE: Good Question - danlaudionut 23:54:01 10/03/09 (3)
        
        In Reply to: RE: Good Question posted by Caucasian Blackplate on October 03, 2009 at 20:43:43
        
        CB
        
        I am stuck with dreaming stuff up as
        I am too broke to afford just the 300Bs.
        I don't see me building anything
        in the near future either.
        That is why I left before but I figured
        my dreaming may give someone else ideas.
        And dreaming was more fun than giving up.
        
        DanL
        
        RE: Good Question - Caucasian Blackplate 12:34:22 10/04/09 (2)
        
        In Reply to: RE: Good Question posted by danlaudionut on October 03, 2009 at 23:54:01
        
        300B's are damn expensive! I run them now, but I figure when I chew through the two sets that I have, I will switch to a different tube.
        
        Since you are in the "dreaming up" mode, maybe you can help me out. I have an ultimate set of iron sitting in my garage (not my iron) and can't settle on just what to do with it.
        
        1 pair - TFA-2004 Ni/Cobalt junior (5 watt, 3k)
        1 pair - MQ plate chokes (60ma IIRC)
        1 pair - MQ filament CMC's
        
        I am thinking at 3k/5w that a 2A3 makes a lot more sense than a 300B. I was thinking AT7 (6C4?) -> 12B4 -> 2A3 (CCS on first stage, choke or resistor on the 12B4). I thought that perhaps it made sense to cap couple the AT7 and 12B4 while providing a generous negative rail for the 12B4 to allow for easy direct coupling to the 2A3, but I'm interested in any ideas....
        
        RE: Good Question - danlaudionut 16:37:49 10/04/09 (0)
        
        In Reply to: RE: Good Question posted by Caucasian Blackplate on October 04, 2009 at 12:34:22
        
        CB
        
        I have a 6GK5 DC coupled to a 2A3
        which needs a 500V PS to complete it.
        A 0A2/MQ EXO-010/6GK5/1.5V NiCad to a
        Sovtek 2A3/3.2KR Cathode/3uFd PF cap
        with your iron and a 6.8uFd bypass cap
        Email me for the schematic
        
        DanL
        
        RE: Good Question - danlaudionut 15:19:36 10/04/09 (0)
        
        In Reply to: RE: Good Question posted by Caucasian Blackplate on October 04, 2009 at 12:34:22
        
        CB
        
        Another tube I am curious about - 6GK5.
        A 7 pin little wonder
        Mu of 78 and Rp of 5400
        Should be plenty for the 2A3
        Interstage it to a Sovtek 2A3
        
        DanL
- RE: So many 300B schematics...which one? - JANDG 17:38:01 10/03/09 (4)
  Can I build this without changes useing 5k outputs..?
  Joe
  Wisdom for today.........
  A GREAT tube amp or speakers will allways be GREAT & bieng relevant today is irrelevant.
  - RE: So many 300B schematics...which one? - danlaudionut 20:06:00 10/03/09 (3)
    Joe
    
    It has a 5K OPT.
    Post me and I will send you a full
    sized schematic with PS and all.
    
    DanL
    - RE: So many 300B schematics...which one? - nicoch 14:47:38 11/17/09 (2)
      HI DanL ,I use this amp, suggestion for improve a bit?
      thanks manu
      
      Edits: 11/17/09
      - RE: So many 300B schematics...which one? - nicoch 14:49:11 11/17/09 (1)
        
        In Reply to: RE: So many 300B schematics...which one? posted by nicoch on November 17, 2009 at 14:47:38
        
        .
        
        RE: So many 300B schematics...which one? - nicoch 14:51:13 11/17/09 (0)
        
        In Reply to: RE: So many 300B schematics...which one? posted by nicoch on November 17, 2009 at 14:49:11
        
        I have 230v on section 2 6em7
        
        Edits: 11/17/09 11/17/09

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