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JPeitzman 6th May 2008 04:20 AM

300B Summer Build Question
 
I am deciding on a design and gathering parts for my summer 300B amplifier build. This is a project I have wanted to build for quite some time now. I have looked at a few different designs and I guess I have some questions about the different layouts. This is the main schematic I have been planing on using.

http://homepage.mac.com/parisatnight...s/300B-AMP.jpg

I haven't decided yet if I will use 6J5's or a single 6SN7 since I have a lot of NOS 6SN7's. I am wondering though how this design would compare to something like the Angela Model 91. It uses a 6J7 for the input/driver but I think I would switch it to a C3g or something else, 310A maybe if I can get my hands some. I hear the pentode is a better driver for the 300B but I have never heard one. Would this design (the Model 91 clone) benefit from a choke load. Or would there be another design that would be as good or better than either of the two previous ones?

For reference I am planing on using Electra-Print output transformers (3k) with EH 300B Gold Grids. For the PSU I will be using RCA NOS 5U4G's with Angela or Electra-Print power transformers. I don't know yet if I will us Hammond chokes or Electra-Print chokes. I am planing on using a C-L-C layout with ASC Oil capacitors. I may switch this to a L-C-L-C layout. They will be built as mono-blocks one wooden frames with aluminum or copper tops. These will be driven with a 12B4A line stage or JFET Bride of Zen line stage. In the future these may be joined by a 26 or 10Y choke-loaded line stage.

I am open to any suggestion and feedback, thank you in advance!

Cheers!

James

dsavitsk 6th May 2008 04:31 AM

If you search through this forum a bit, you will find lots of opinions on how the 6SN7 is a poor driver choice for the 300b. Moreover, the 12b4 would be a great driver. Why not switch these around. In commercial products, certain amounts of gain, or certain input sensitivities are necessary, but in DIY, you can do what you like. So, make a 6SN7 preamp with lots of gain, then a 12b4->300b (or 6J5->12b4->300B) power amp with a low input sensitivity.

Also, even if you don't want a full DRD, setting up that last cap as an ultrapath cap would be a good idea.

Miles Prower 6th May 2008 04:36 AM

^^^^

The design looks pretty good. It could stand to have a grid driver, though. DHTs like the 300B are rather difficult to drive, due to the large Cin, and that the grid starts pulling current even before Vgk actually goes positive. A 6J5 with a plate current of 4.0mA seems iffy there. I would include a power MOSFET source follower grid driver for the 300B, as the disappointing sonics of this type seem to be related to inadequate grid drive. That would also allow direct coupling between the MOSFET and the 300B to remove "blocking" as a cause of sonic degradation. It also makes for easily adjusted fixed bias instead of cathode bias.

I'd also make provision for adding some gNFB as well.

If you decide to go with the 6J5s, remember to ground the metal shells (connected to pin #1).

JPeitzman 6th May 2008 06:59 AM

Thanks for the replies. I have been doing some searching both on and off the forum and have come up with a few more ideas. Out of interest though, why is the 76 preferred (by some) at the input over the 6J5/6SN7? I was looking at the Electra-Print DRD and am doing some reading on it now. If I went this route would it still be a good idea to have a FET source follower? Also can this be done with a tube? Just as a basic cathode follower?

Regarding the ultra-path cap in the Electra-Print and other schematics, do I still have to worry about excessive hum? I have read a few places when I was looking at building a 6J5 Ultra-Path line stage (also Electra-Print) that hum can be a major problem if not properly designed.

I was looking too at the JE Labs 300B DX design, which is where I saw the 76 input, and had a few questions. Instead of can I use both halves of the 6SN7 and change the operating point to better drive the tube, or still is a power follower a better idea? I do however like the PSU on this amp better than the one that was designed to go with the schematic that I first posted. I was looking it though and noticed that it uses a 47uF cap at the input, to me this seems large for a 5U4. I was also wondering, is there a purpose to the 50k resistor to ground other than a bleed off resistor? I am just wondering why this has to be a 50watt resistor.

I am interested too if anyone has tried the Electra-Print / Welborn Labs DRD designs or anything similar. I am especially wondering about the new designs on Electra's site. They are using large electrolytic caps instead of oil caps (in the output).

Would DRD be a better option than choke loading the driver / changing operating points / FET power follower / something else?

Thanks and Cheers

James

coresta 6th May 2008 07:08 AM

Nothing new under the summer's sun ? IMHO , the WE91 IS the best : high gain, one coupling cap ... and a single 6J7 instead of the scarce WE310A ;)

Miles Prower 6th May 2008 07:35 AM

Quote:

Originally posted by JPeitzman
I was looking at the Electra-Print DRD and am doing some reading on it now. If I went this route would it still be a good idea to have a FET source follower? Also can this be done with a tube? Just as a basic cathode follower?
The DRD is choke coupled, and uses a pretty stiff driver VT. If you go this route (or use xfmr coupling) then a MOSFET driver wouldn't be necessary. It's probably a good design, though I don't like all that iron in the signal path. Also, forget about any gNFB if you do this.

Quote:

Regarding the ultra-path cap in the Electra-Print and other schematics, do I still have to worry about excessive hum? I have read a few places when I was looking at building a 6J5 Ultra-Path line stage (also Electra-Print) that hum can be a major problem if not properly designed.
I'd say it's not a problem. This includes a power supply design.

Quote:

I was looking too at the JE Labs 300B DX design, which is where I saw the 76 input, and had a few questions. Instead of can I use both halves of the 6SN7 and change the operating point to better drive the tube, or still is a power follower a better idea?
With this design, you're back to the inadequate drive problem.

Quote:

I do however like the PSU on this amp better than the one that was designed to go with the schematic that I first posted. I was looking it though and noticed that it uses a 47uF cap at the input, to me this seems large for a 5U4.
It is. I did a project using the 5U4GB for the positive rail. The input capacitance I used was 34uF (two 68uF in series) to get some 900mA of Isurge. 47uF exceeded the max Isurge rating of 1.0A / plate. I forget by how much, but the 5U4GBs'll last longer if you stay within spec.

Quote:

I was also wondering, is there a purpose to the 50k resistor to ground other than a bleed off resistor? I am just wondering why this has to be a 50watt resistor.
He's loading down the PS, probably for improved voltage regulation. Be less wasteful of power to make a choke input filter.

dsavitsk 6th May 2008 07:52 AM

Quote:

Originally posted by JPeitzman
Regarding the ultra-path cap in the Electra-Print and other schematics, do I still have to worry about excessive hum? I have read a few places when I was looking at building a 6J5 Ultra-Path line stage (also Electra-Print) that hum can be a major problem if not properly designed.
Basically you multiply the B+ ripple times the mu (reduced by the OPT winding ratio) to get the hum. With the line stage, mu is 20 and it gets amplified again in the power amp, so it can be an issue. On the output stage, since the 300B's mu is low, it isn't a problem.

Quote:

Originally posted by JPeitzman
I am interested too if anyone has tried the Electra-Print / Welborn Labs DRD designs or anything similar. I am especially wondering about the new designs on Electra's site. They are using large electrolytic caps instead of oil caps (in the output).
Jack seems to not think this makes much difference. I think it does, and these oil caps are cheap anyway. Keep in mind that the idea here is that rather than the cathode bypass cap and the final PS cap both being in the signal path, the design is supposed to remove one of them from the signal path. So, an electrolytic that might be in the PS anyway is not going to be terrible, but is probably bettered by the film cap.

Fwiw, the DRD design is really Jack's. Welborne used it, but Jack did the original. I have DRD amps and like them a lot, but I have not compared them to much else so I can't say they are better. Well, I can say they are better than my old Sumo, or a Gainclone :)


Quote:

Originally posted by Miles Prower
The DRD is choke coupled, and uses a pretty stiff driver VT. If you go this route (or use xfmr coupling) then a MOSFET driver wouldn't be necessary. It's probably a good design, though I don't like all that iron in the signal path. Also, forget about any gNFB if you do this.
For my money, more iron and less gNFB is a good thing. There isn't a right answer here, just different ways of doing things and different preferences.

Search around for user Kofi Annan. He build some DRDs (d3a->300B iirc) and there is information about issues he ran across.

andyjevans 6th May 2008 09:29 AM

Have you considered that the improvement in sound you get by using a DHT like the 300b can be continued right through the amp by using more DHTs as input and drivers?

Keep the circuit simple - pretty much as you have it, but use something like:

Input: 201A/01A or 26 or 10Y

Driver: 46 or 10Y Other choices but less gain, 31 and 71A

If you want the maximum gain and probably the best sound, I suggest 201A into 10Y. If you can afford 10y into 10y that's even better.

In any case, if you use 4 pin UX sockets for input and driver you can play about with some of these DHTs. If you want to experiment, some bench power supplies are great to drive the DC filaments at various voltages. The 46 is a 5 pin UX socket. You can get 4 pin UX sockets that fit in an octal hole (and if you're lucky 5 pin ones), or you can get the big oval ceramic UX sockets, which they also do in octal. If you use those, you could start with 76 (UX5) and play about with DHTs later - at least the sockets would be compatible.

I've built with all the above, and I'd say that the improvement in transparency and detail and treble finesse was well worth it. Andy

kstagger 6th May 2008 12:05 PM

A number of years ago I built a 300B amp and it used a pentode SV83 driver. Thorsten Loesch has a design - the Legacy - which has morphed over the years, starting with the SV83 and eventually using the C3m.
http://www.diyaudio.com/forums/showthread/t-52347.html

I liked the amplifier quite a bit, but didn't have the power to drive the speakers I had at the time.

DaveMcLain 6th May 2008 01:47 PM

I think you'll be quite pleased with the sound of the EH Gold 300B output tubes. I have them in my 300B amp and I like the sound a great deal. I've been using them for about the last 7 months since I built the amp and I've used them a lot with no problems at all.

I'd love to get some other 300B's to try but they are quite expensive, maybe I'll leave well enough alone.

I have an old set of Altec Iconic speakers and the amp drives them easily. The speakers I have are 16 ohms and that's no problem with the One Electron output transformers which have a tap for 16 ohms. The setup is not really super loud or anything but it sounds amazingly big. My listening room is approx 26X40ft yet the sound fills the room and the whole house easily my friends are amazed at what 16 watts peak can do!


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