DIY Audio Projects Forum

I was recently asked to do the electrical design for a 300B SET stereo amp. I thought that as part of the activity I would walk through the various design stages on the forum as I went. This will provide the opportunity for some input from the community at large (engineering and design should be a collaborative process) and it should serve as a nice demonstration for how to approach this type of project for anyone new to tube amp design.

Lastly it should also end up producing a nice design which people can use directly or as a point of departure for their own designs.

So, without further eloquence, lets be off...



Thread Links

• summary post, complete with schematics for the SE 300B stereo amp

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

First things first... Requirements

Given that I was asked to do this and am not designing for myself, it is appropriate to take a look at the initial requirements for the project. These mostly come from the individual who requested the design, but some are self imposed.

1: An 8W/channel class amplifier.
2: Low distortion for classical music listening
3: Class A operation
4: Two stage design
5: Zero negative feedback
6: 6SN7 driver tube (A nice one is already in hand)
7: Preamp drive (i.e. the amp can be driven by a preamp)
8: Edcor output transformers
9: DC heaters on the 300Bs

This may look like a pretty simple list of requirements but they do place some restrictions on the design. As I go through the design process I'll point out when I am looking at particular requirements and how they are affecting my design decisions.

Even when I design for myself I like to start with a list of requirements. This simple step tends to help focus the design and prevent "drifting off" as the design matures. Without it, I tend to get distracted and want to try too many different things. If I have a list of requirements (and keep referring to it) I can evaluate each change I want to make and see if I'm drifting. And if one of my "good ideas" doesn't fit into the requirement list, I'll either reevaluate the requirements and see if something needs to change or I just jot it down for use on some other project or prototype.

The requirements simply help keep me focused on the end goal.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Thank goodness there is an active preamp, the 6SN7 doesn't have enough gain to drive a 300B. Are you allowed to use CCS as a plate load?

If you CCS load the 6SN7, with a gain of about 20 or so, you'd need over 3 volts input to get the 300B to full power.

I presume single ended, but it didn't say.

I think it would be fun to do a 2 stage PP 300B amp...

I think use 300B only in SE for good and nice sound! PP is not for Hi-End, IMO, with its metal sounding in result switching 2 shoulders of output tubes. /For PP better use KT88/. ODDbLock that I like is between SE and PP with SE sounding and power like 50% of regular PP!
In the past I told Sampleacurate who wants build 2 stages with 300B and 6SN7, that 6SN7 is not for
this....ampl. factor is very, very small!! If your CD player gives 0,6 v output, this multyplay to 30 in SRPP
will = 18 v, that is very, very small to drive 300B with bias -40-50 v.
You can use parallel 12AX7, but it is not good tube for Hi-END sounding. It is only for guitar ampl.
You can use only penthod E180F, E280F, EF861, russian 6E5P, 6P15P.....or some RCA low noises
metal penthode.

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/

Awesome. I'll be watching this thread with keen interest.
Cheers

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]

Same here.
Suncalc - Really enjoy reading the posts with the smooth writing style and precisely focused goals.
Excellent.

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Kind Regards,
Dinesh

Some of my diy audio projects:

• Elliot Sound Products - P3A 100W Amplifier
• TMM 2.5 Way Loudspeaker with Scanspeak 18W8546
• 2-Way MTM Loudspeaker with Scanspeak 21W8554 and D2904-7100

If it were really true that SE is per se better than PP then using two triodes in parallel SE would produce a better result than two in PP. Yet if the two PP tubes were run in class A the amp would be equivalent for AC signals to the parallel SE amp, yet have the profound advantage of zero DC in the output transformer. The myth that SE is inherently better than PP is simply that, a myth, and such a remarkably easily disproved myth it's staggering that it persists. Getting a PP amp right might be harder, because it requires the output valves to be driven in opposite phases.

Just because it's harder to get right doesn't make it fair to compare great SE with mediocre PP - which is what I suspect is happening. The likes of Quad, Leak, etc. only ever produced PP amplifiers. In the UK SE was used for black and white TV sets, portable record players and kitchen radios.

Most of brands that produce speakers, produce for all public non expencive low sensitivity comercial speakers.
For driving they, another brands for tube amplifiers produce power PP "comercial" ampl. for all public.
I think IMO QUAD is good for all people, but not for High top of "Ice cream".
Please, don't mix both thinks....SE with high quality /Audionote..../ is not for all public that listen Disco and Bum, Bum....
This is only for High Quality sounding with expensive fullrange drivers without filters, that press the sounds....
/I made SE and PP 6P6 - or 6V6 and different of sounding with the same 96 db sensitivity speakers and the same player and PP loosed!/. Sorry, but this is my IMO and IMO my audiophil-friends with SE amplifiers 2A3, 300B, 6S33S, 6S4S, KT88, 6V6.

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/

Wow this thread has stuck a fire in everyone’s underwear. Godda love it!

Firstly: I requested Suncalc to design the 300B for me. Yes it is an SE design. Now how did this come about? Funny you ask: I am currently the Melbourne Audio Club Opera convenor after two years as DIY convenor. Yes Opera.. ... OK stop laughing I happen to like it.

Seven members of the club put in a very long day at my house with a Handle Opera on 4 vinyl LPs played through my 6AS7 SET amp. At the end of the long day a member asked me to make him one of my SETs as he was so impressed how good the sound was. We tried my amp at his place on his system but he felt he wanted more power. Qian plays clasical music loud! Introducing the 300B project and I asked Suncalc (Matt) to once more do the design for me. I do intend to build Matt's 807 design but as this is a paying job it must come first.

Originally, Qian (said Chan), the customer, wanted silver everything. But with one quote of $US1500 per silver wound OPT I convinced Qian to go for cheaper OPTS. We will be using silver interstage caps.

Qian, being Chinese wanted me to use Chinese tubes. Qian is now the very proud owner of a matched pair of TJ Full Music 300B/SE premium carbon plates and Shuguang Treasure 6SN7 (X4). I have read many reviews about the 300B/SE and on all occasions the tube has outperformed the likes of W.E 300Bs.

Secondly let's add some dry timber that fire burning below: I believe SE design may deliver a better “sound” then other designs. I have read many articles where the writers have written the same. For some time now I have only built SE tube amp and I like the detail and texture they can deliver. This is not the thread to debate PPs over SE. The SET thread may be better.

Currently I wait for Matt to finish the design and the arrival of some very expensive tubes. Once the design has been finalised I will order the OPTS. I like Edcor top-end OPTs but Gi o has suggested Electra-print and others. I will investigate.

http://www.grantfidelity.com/site/catalog/89/tj_full_music_

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Projects:"Rosso" - E88CC SRPP preamp | "Pass F5 trubo” - dual mono high-end amp | “MWH” - 1920s UX-171A preamp | ”Image” - Low Profile 100W Class D amp |