DIY Audio Projects Forum

300B SET Design Project
Page 33 of 54

Author:  Suncalc [ 11 Dec 2011, 13:30 ]
Post subject:  Re: 300B SET Design Project

Well Mark, this one has me stumped. A number of thing are disturbing me here.

    1. The output power is definitely low. But I don't understand your numbers. 15.3vp-p is 5.4vrms (15.3÷2×0.7071=5.4vrms) which across an 8.2Ω load should translate to 3.6Wrms (5.4^2/8.2=3.6). This is REALLY low. Time to check the bias points.
    2. Upper rolloff design point is 76kHz, 47kHz is too low.
    3. Channel separation seem good.
    4. The 5.6Ω across the 8Ω tap lowers the reflected load impedance to around 2.5kΩ. The band pass opened up to design points (which is good) and the power went up slightly as expected.
    5. A 5.6Ω load across the 16Ω tap lowers the load to ≈1.2kΩ. When I put this load on the original load plot, I calculate about 10.5W, so this matches. But your second order harmonic distortion should have increased SUBSTANTIALLY. I don't understand this at all.

As to running your speakers on the 16Ω tap, I would recommend against it. With the reflected load the upper swing is significantly above the plate dissipation limit and played loud for an extended period, you may melt your plates.

Without having the amp in front of me I really can't offer any more insights that this. Something seems wrong with the amp's performance with the matched load on the output. Some investigation may be in order to see what other changes may have been made to the circuit.

Author:  mwhouston [ 11 Dec 2011, 16:28 ]
Post subject:  Re: 300B SET Design Project

Firstly I did some simple mods to the amp section and it looked like nothing had changed there. Funny part was there was a few screws missing from the bottom plate and some only half screwed in. I think the guy who modded the heater cct. was taking a look. In the PS section only the heater ccts. were altered. I checked the voltages and they appeared OK.

I took the amp to a friends place who has some $26K Osborn Grand Monuments. These 6' 200lb lead lined speakers have two 12" Focal woofers but present an 8ohm load. The amp drove them well but not as well as the 28W 845 amp in residence. We tried the Monuments on the 16ohm tap. It broadened the sound stage and that was about it.

I will leave my 6ohm Eclipse on the 8ohm tap. My speakers are much easier to drive than the Monuments and we both agreed the amp sounds a lot better at my place. I think the Monuments are a capacitive load and it kills low power tube amps. My 3W 6AS7 crumbled under the load. The 6AS7 drove the Osborn Epitomes extremely well but they were 94db efficient.

I will attempt to check bias points. If I do I'll let you know. As I have stated in earlier posts the amp is a fabulous all-rounder and continues to surprise and delight me each time a play something different on it. This amp is here to stay for a long while.

Author:  rock4016 [ 12 Dec 2011, 10:02 ]
Post subject:  Re: 300B SET Design Project

mwhouston wrote:
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.

FYI with any Directly Heated tube, all your signal passes through the heaters...

Author:  Suncalc [ 12 Dec 2011, 10:13 ]
Post subject:  Re: 300B SET Design Project

rock4016 wrote:
FYI with any Directly Heated tube, all your signal passes through the heaters...

Understood. See this post...

Author:  rock4016 [ 12 Dec 2011, 11:57 ]
Post subject:  Re: 300B SET Design Project

Also the hammond E I transformer you specified in the original heater design will reject mains noise better than any torroid. So unless the new simpler design has better filtering than your more complicated design, I would presume that the amp does not sound as good as it did when first delivered to Quan.

Author:  mwhouston [ 12 Dec 2011, 15:59 ]
Post subject:  Re: 300B SET Design Project

It is hard to know whether the amp sounds as good, better or worse than before the mods. It has been 6 months since I heard the original amp. Regardless I don't think the mods have degraded the sound. I guess there is more than one way to skin a cat. The amps is absolutely dead quiet. Even with CD player and preamp on you cannot hear anything from the speakers at all. My speakers are 91db efficient and they will reveal any system noise. There is NONE. I'm guessing then the mods have not degraded the amp at all.

Author:  mwhouston [ 12 Dec 2011, 22:13 ]
Post subject:  Re: 300B SET Design Project

When I got back the Silver Dragon it had different 300Bs. I replaced them with a pair of TJs I had purchased some months before. But now I have a pair of matched Shaguang 300B-Z black glass and ceramic base (US$496 a matrched pair). Matt maybe way in the furute we can build another 300B, not as high-end as this one with say a 6N1P driver or 12AX7. It is ashame to see such nice tubes go to waist. A three stage integrated and even mono-blocks would be a nice touch. I suppose we have the power end we just need the driver stage??

With a 12AX7 preamp/driver stage I could use one of the triodes for my single tube universal preamp and the other triode to drive the 300B. Any thoughts?

Author:  Suncalc [ 14 Dec 2011, 23:33 ]
Post subject:  Re: 300B SET Design Project

mwhouston wrote:
With a 12AX7 preamp/driver stage I could use one of the triodes for my single tube universal preamp and the other triode to drive the 300B. Any thoughts?

The real challenge in this design is getting the clean 142vp-p swing necessary to drive the 300B while being limited by the 220kΩ max grid leak resistance. This drives you to use a driver stage that's lightly loaded. Thus to get the voltage swing you're driven to lower µ tubes. Actually, the 6SN7GTB really is ideal in this situation. Personally what I would recommend for a mono block is sticking with the 6SN7 using one triode in a slightly modified design and the other configured as the 6SN7 version of the 4S preamp.

Here is the load line design I would suggest for the driver stage.
6SN7 driver.jpg
Of course the preamp is shown here. (

Author:  mwhouston [ 15 Dec 2011, 07:24 ]
Post subject:  Re: 300B SET Design Project

Matt, I think you got out of this too lightly: The 6SN7 single stage preamp you designed back whoop-whoop (Ozzi for "some time back when") coupled to the original Silver Dragon. I can do that.

After Paris I got the sense that an integrated amp has something over a preamp and power amp combo. No extra loses and signal down grading from more RCAs and inter-connects. I know, it has its limits.

Not so quick Matt: Drag out the slide rule and "knock-up" a couple of PSs to suit the mono-block scenario. Maybe simpler DC heater ccts. Also scale back the OPTs??? Or is that too much for any engineer?

I guess what I'm after, and others may wish to build, is an easier and "cut-back" 300B. Maybe a Tin Dragon or an Iron Dragon. The Silver Dragon was intense and expensive to build (yes my fault, Silver foil Audio Note caps etc). I would like to see the Shaguang 300B-Z in there own special amp. They are too nicer tube to just have laying around, regal looking

Author:  Suncalc [ 15 Dec 2011, 19:44 ]
Post subject:  Re: 300B SET Design Project

Well here it is. It's an "all Edcor" design.
300B Monoblock Amp.jpg
300B Monoblock PS.jpg

The Preamp/Driver is slightly redesigned to take advantage of the mono-block concept. The PS is universal 120v/240v and quiet. The 300B filament supply is the same. Too good to throw away.

mwhouston wrote:
I think you got out of this too lightly:
I think that one of these days I'll tell you just how much :$: I make so that you'll better appreciate the value of the Engineering services provided. ;)

Page 33 of 54 All times are UTC - 6 hours [ DST ]
Powered by phpBB® Forum Software © phpBB Group