DIY Audio Projects Forum

300B SET Design Project
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Author:  Suncalc [ 01 Jul 2011, 10:09 ]
Post subject:  Re: 300B SET Design Project

mwhouston wrote:
I replaced them with 100uf 400V. Yes over-kill but all I had. I don't think the 100uf will hurt anything.
This is correct. All that changing the 50µf to 100µf will do is lower the low frequency half power point on the output stage to 4.0Hz (-1.6dBv@20Hz). The output transformer is still the limiting factor on the low end rolloff.

Glad to hear that the Dragon has risen again. :twisted:

mwhouston wrote:
To protect these very expensive and limited edition tubes, fuses in the plate cct. may have been a good idea. Most say you can "hear" a fuse.
I believe that this complaint is usually due to the mechanical contact issues with the way most people mount fuses. In this situation I would generally use a pigtail fuse like this one, with some heat shrink tubing over it, wired directly into the B+ leads at the output transformers (one for each channel). This requires a soldering iron to fix but they should only blow when something else fails so it shouldn't matter too much. These fuses should be "silent" in the amp.

The key is finding the right value. The current rating is simple, it's the I^2*t value that is difficult. This is a measure of how fast the fuse will open given a specific fault current. It needs to be long enough to handle the initial transformer core magnetization inrush current, but small enough to open circuit quickly in the event of a real circuit fault. It's a balancing act. Let me know if you'd like to go this route and I'll see what I can find. I'll warn you now, it will probably take a little experimentation to find the right value.

Keep us informed.

Author:  mwhouston [ 01 Jul 2011, 19:28 ]
Post subject:  Re: 300B SET Design Project

Today I return the amp after about 6 to 8 hours of play. It has performed perfectly. It doesn't quite perform as well with the EH's and at this point I don't want to put my TJs in. Once at Qian's I'll inpect his TJs and meter them for S/Cs. If Ok I'll drop them back in if not Qian can use the EHs until a new pair of TJs arrive. The EHs have a dryer sound.The TJ have a big fat sound.

I don't think the original fault was with the tube because they worked for 4 weeks without any clicks or pops or fuse spitting. Qian has been moving the amp around and taking it to friends house to compare with their 300Bs though. I have him remove the TJs when he does. So there is a chance they did receive mechanical disturbance or damage.

If I know Qian he will want the TJ back in. I can get them for about USD$650. I have been offered a deal of USD$550 each if I take two pairs.

Author:  mwhouston [ 02 Jul 2011, 02:52 ]
Post subject:  Re: 300B SET Design Project

I now have absolute proof that God is into DIY audio. Qian's 300Bs are pertect. God was watching that day. I metered the tubes out looking for O/C filament and S/C between other elements. All good. I then examined the big compound carbon plates for diss-colouring or marks which may have shown the tubes had been stressed. But they looked perfect.

I placed them back into Qian's amp and light them up. A few minutes later they were playing the beautiful music I knew they could. Amp sounds great and everything back to normal. It appears the limiting resistors in the PS popped prior to any damage to the 300Bs.

While I had Qians amp I increased the snubbing around the filement rectifiers. Though the hum from the amp was only low I thought it worth while to see if I could reduce it more. I put 0.01uf polys between all legs of the rec. bridges. Qian tried serveral times to find a hum and could not. A big win here. The amp is dead quiet even with you ear pressed againts the driver. Also while the PS had the bottom off I tidied up the wiring by zip tying groups of wires together. It may have also helped with hum issues. Looks much neater though.

So a win-win result. He's a happy Qian. And the Silver Dragon is in full flight.

Author:  mwhouston [ 25 Jul 2011, 06:57 ]
Post subject:  Re: 300B SET Design Project

Qain, for whom I built the Silver Dragon, has been using the Electro Harmonix Gold 300Bs lately. Claims in some areas he feels they sound better than the TJ Full Musics 300B/SEs.

Author:  mwhouston [ 22 Sep 2011, 21:05 ]
Post subject:  Re: 300B SET Design Project

I'm off to an Opera meeting tonight hosted by a member of the Melbourne Audio Club, Qian. Qian has the Silver Dragon I built for him. I have been informed he has swapped the very expensive TJ Full Music 300BSE carbon plates for Shuguang 300Bs. Supposedly they had a warmer sound than the TJs so Qian swapped them.

I'm going to be hard to convince ANYTHING could ever sound better than the TJ. I just fixed an amp for a guy who used the Shuguang 300B (cheapest ones) and Shuguang rec tube. I got a chance to listen to the amp, it had a big sound with a strong bass but from what I can remember of the Silver Dragon, I'm not sure the Shuguangs was as good. I'm told there are a number of different models of the Shuguang 300B tubes. Qian's could be the ceramic based expensive ones.

Tonight will tell.

Author:  mwhouston [ 23 Sep 2011, 23:11 ]
Post subject:  Re: 300B SET Design Project

I listened to Qian's Silver Dragon with the Shuguang 50 year Treasure black glass 300Bs. We only listened to one Opera which spanned two CDs. The sound was big and impressive with what appeared to be unlimited dynamic head room. We were listening at a reasonable volume level when this suprano would let go with both lungs bursting. I though I would wear the driver cones as ear muffs. It was really loud already from a loud sound level. The Silver Dragon showed absolutely no sign of strain or clipping - very impressed. This was more about Matts design work than the Shuguangs characteristic.

The best I have ever heard this amp was in my main listening area, with the Osborne Eclipse three-way towers, Capless BoZ/SE preamp and Oppo BDP-83. Of course at this stage the TJ Full Musics were installed. I can't say the sound was "bad" without the TJs I just feel it would have been better. Anyhow Qian is happy and for what the amp cost him, that is all that matters.

But it has made me hungrier for my own slice of the 300B pie. Patients is not one of my virtues.
Shuguang 300Bss.jpg

Author:  mwhouston [ 23 Nov 2011, 16:57 ]
Post subject:  Re: 300B SET Design Project

I have trhe Silver Dragon back. Qian the guy I built it for listened to my new 6EM7 - "Paris" amp and liked it more. Paris is only 2W but the mid-range is excellent. See the thread on this forum. When I got the Silver Dragon home and back in my system it didn't seem to have the richness I can remember it for. Then I realised Qian had swapped out the TJ Full Music 300BSE carbon plates for some Shuguang, top of the range, black glass, ceramic socket 300Bs. I replaced these with a new pair of TJs and wow what a difference. I never realy liked the Shuguangs at Qian place with his gear and I told him so.

Boy am I happy man that big fat rich fully-blown 300B sound is back. The Shuguangs run lean, too dry for me. The TJs will every air molecule with music. Parts for Paris-> about $400, parts for Silver Dragon -> $3.5K. I guess I got the better end of the deal. Of course not according to Qian (haha). Actually the Siver Audio note caps in the Silver Dragon cost $400, about as much as all the parts for Paris.

Author:  mwhouston [ 30 Nov 2011, 18:41 ]
Post subject:  Re: 300B SET Design Project

Having the Dragon back and the TJ tubes back in has given me a chance to re-eval. the amp a little more. I have been playing some really big orchestral pieces through it (Mahler - don't come much bigger). The Dragon can easily play a whole orcgestra, going full tilt, and let the solo trumpet, flute or violin shine through with full weight to each instrument. I guess this where a good amp and a great amp part ways. In a good amp each individual instrument would just get buried in the mix, though the mix would be very well reproduced, with the solos just rising above the raw. With a great amp, like the Dragon, each musician his own space and attention and allows the soloists to rise-up and be highlighted.

Where the Pinot KiSharn (6AS7 SET) and Paris (6EM7 SET) really show thier metal is small chamber groups, sextets, octets etc. Or two piece like piano and violin, or voice with simple backing. In fact when listening to these combos I would rather have Paris. But the Dragon is an amp for all seasons and for all reasons not just a few.

Author:  mwhouston [ 04 Dec 2011, 17:01 ]
Post subject:  Re: 300B SET Design Project

Matt: You have stated on past threads you don't like tube rings because they hides the heater glow. Don't ever build anything with TJs. You see no heater glow in fact you would swear the tubes are not on. Even when you look down into the middle of the plates when it is totally dark there is barely a soft red glow. The treasure 6SN7 is not much better. Compared to my image of Paris this is a dead looking amp. Thank God it sounds so bloody good!

Author:  mwhouston [ 10 Dec 2011, 22:27 ]
Post subject:  Re: 300B SET Design Project

Having the Silver Dragon back meant I had a chance to run some tests with some surprises. Firstly while the Dragon was with Qian he had a friend remove the filament transformer and replace it with two small toroids, remove the elaborate regulator s Matt had design and replace them with simple versions. Worst than this, to mount the toroids my fancy domed chromed nuts were replaced with plain hex head nuts.

I could get all a*na**l about the reg down grade but in the end it is just providing DC to the heaters and possibly has no effect on the overall sound so to stay. The hex nuts is not obvious so sit back Houston and listen to how well this amp plays. The Golden Dragon 5U4 was replaced by something else, I had a second so swapped it back. Now for the test results.

These tests with 8.2 ohm resistor on 8ohm tap:

2.86Vpp in 4.66Vpp out - gain is very low. I knew it was this low but this is extremely low. My BoZSE has stacks of gain so all good.
-3db points 10hz -> 47khz (see later notes for a shock)
Max power: 9.13Vpp in and 15.31Vpp out -> 5.3WRMS (agree)? and lower than I expected (see later notes)
Both channels perfectly balanced and noise with channel shorted -> 2.0mv (lowest I have seen)
Xtalk 4.88Vpp one channel -> 2.8mVpp in other. Excellent separation (agree)?

NOW with 5.6ohm resistor on 8ohm tap (my speakers are 6ohm so this is closer to what I should hear).
-3db <9hz -> 80khz (wow what an improvement)
Max power 5.5W no gain here but with 5.6ohm across 16ohm OPT tap a whopping 11WATTS!! with no loss in any other test. This means ( I think) a lower OPT secondary impedance could have been used (agree)?

Only the SQR wave at 1K was any good. at both 10Khz square wave was poor. The FFT results showed very little distortion at all. The best amp I have ever tested when it cam to FFT results. Almost no added distortion to the original input signal in this respect. I have to repeat what I just said, cleanest FFT results I have ever seen. Does this explain why this amp sounds so good??

The 64million dollar question is should I run my 6ohm speakers on the 16ohm OPT tap? I realy don't need the extra volume. What may I lose?

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