DIY Audio Projects Forum

300B SET Design Project
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Author:  mwhouston [ 22 Jan 2013, 21:33 ]
Post subject:  Re: 300B SET Design Project

I thought I may buy another pair of TJ Full Music 300BSE Carbon plates and started looking on the Mable Audio site. Couldn't find the tubes so gave up. The next day I was looking through some boxes of tubes (one being the Shuguang 300B-Z black glass) and notices another box with TJ Full Music 300BSEs. Glad I found them and didn't order yet another pair. Though these tubes RRP for $1200 I did manage to purchases two matched pairs for $650 each pair.

Somehow I had forgotten I had purchased another pair realising that I will probably always going to use the Silver Dragon as my main amp. I'll build others but the Dragon remains my main axe, period. I must build an amp around the Shuguangs. Nice looking tubes and to me they sound excellent.

Author:  mwhouston [ 31 Jan 2013, 06:54 ]
Post subject:  Re: 300B SET Design Project

As in a previous post I checked some of the critical voltages of the Dragon. To calculate (better) the idle current I needed to know the exact resistance of the cathode resistors. I measured 71V across the cathode. To my shock (and horror) one side was 740 ohms the other 703???

I had to use two resistors in series to get close to Matt's 750 ohms. One of the resistors was 470 and the 270 for 740 ohms total. It appears one of the resistors marked 470 was actually 429ohms. Yes really! Not hard to fix but it should have never happened. I know Bruce checks all his parts with a meter. This example is a good reason to follow that practice.

Author:  mwhouston [ 04 Mar 2014, 13:18 ]
Post subject:  Re: 300B SET Design Project

I had been listening to my new 2-ways (VoXDeuX65) with a new 6EM7 amp and for short time a tube and chip amp. I swapped back in The Silver Dragon after it having been out of my system for months. As a wrote under the cheap two way speaker thread there was a huge difference in how my whole system sounded. Much more controlled and fuller bass and greater weight in the mid range with a more mellow extended highs.

The new 2-ways with their bigger 6.5" driver can really deliver a very complete musical picture and add this to the finesse, power and control of the 300B with it's huge OPTs you get a great combination. I didn't expect such a big improvement. I have sold a lot of amps and stuff over the years the the Dragons here to stay. Once more, thanks Matt.

Author:  jpchissitt [ 14 Mar 2014, 04:52 ]
Post subject:  Re: 300B SET Design Project

Hello All

I was considering this project as my next amp build. I was also considering this:

I've got a couple of questions:

I assume the JE Labs/Walton design could be built as a stereo unit with both channels running off one power supply? Would it still be worth duplicating the power supply circuitry after the rectifier, for each channel?

They also have this to say about the element supply:

"300B Heater Supply - AC for the heater is preferable from a sonic point of view. But there are also other reasons why AC is preferred to DC.

Directly heated tubes with dull emitters, such as the 300B, which have a barium oxide-coated filament, when subjected to DC can produce an uneven coating of metallic barium - whereas AC heaters will produce an even coating. This uneven coating can lessen emissivity and shorten the life of the tube."

This thread seems to say that DC is better. Who is right here, or is this one of those PP v SE, sand v tube type arguments which will never be resolved?


Author:  mwhouston [ 14 Mar 2014, 19:05 ]
Post subject:  Re: 300B SET Design Project

I followed the designer's (Suncalc) original design where DC was used on the heaters. I would think Suncalc would have highlighted any issues by using DC. All SE amps are extremely susceptible to an AC. By separating PS and amp I was able to ensure only DC entered the amp section and a chance of AC getting into the amp was very low.

My 300B, "The Silver Drgaon", is absolutely dead quiet. Even on $20,000, 94db efficient 6' tall, 400lb speakers no noise can be heard. I'd go DC on the heaters if I was you.

Author:  jpchissitt [ 13 Jun 2014, 06:10 ]
Post subject:  Re: 300B SET Design Project

I've collected just about all the parts now for my 300b build. One quick question on the 300b heater supply if I may....

Earlier in the thread, it was said that limiting channel crosstalk would require seperate transformers for the heater supplies, or at least seperate windings. Using IC regulators, this was not quite as vital, and seperate windings are used in the design. If using IC regulators, can I use one winding for both channels? (Poor planning/ordering here - I've only got one transformer with a single winding for the heater supplies).


Author:  mwhouston [ 29 Aug 2014, 20:42 ]
Post subject:  Re: 300B SET Design Project

Flashover: ever since this amp was completed I have had flashover in the rec tube. Very violent flashover but luckily as yet not destroyed a rec tube. An 5U4 appears to hold up the best. Matt has suggested small resistors in the primary legs from the power tranni. Tried this didn't help a lot. I reduced the first cap from 47uf to 22uf, didn't help much. Matts suggestion was to place the standby switch after the fist cap. I have it before. This has not been tried yet.

I had tried having the standby switch on when the amp first comes on, it wasn't pretty with lightning going off in the rec tube. But since the smaller first cap and small surge resistors the standby switch has failed locking on permanently. Guess what? No flashover. I am yet to try the original Golden Dragon 5U4 back in the amp. I removed it because it was one of the worst offenders for flashover. Time to try it back in.

Author:  mwhouston [ 29 Aug 2014, 21:46 ]
Post subject:  Re: 300B SET Design Project

While typing the above my left channel has failed. The 300Bs are expensive so I fused them cathode leg. I fuse may have blown.

Author:  mwhouston [ 31 Aug 2014, 06:30 ]
Post subject:  Re: 300B SET Design Project

mwhouston wrote:
While typing the above my left channel has failed. The 300Bs are expensive so I fused them cathode leg. I fuse may have blown.

Though I fiddled with the left inter-connect 5 times it wasn't until I fiddled with it for the 6th time that I realised I had a faulty inter-connect. Not fixed but a fiddle or two brings back the left channel.

Author:  mwhouston [ 31 Aug 2014, 12:30 ]
Post subject:  Re: 300B SET Design Project

In trying to stop the rec. tube flash-over problems I have reduced the first cap to 22uf and added some 33ohm resistors in the secondary of the power transformer. These efforts helped a little bit have not fixed the problem. I moved the standby switch to after the first cap. Still no huge improvement.

What I am thinking now is to place a 1K 5W wire wound resistor across the standby switch. When the power goes on and the standby is open the HT will very slowly start to rise being limited by the 1K resistor. But after a few minutes when the caps have been slowly charged up throw the standby and thus short cct. the 1K resistors. Most of the caps should be charged so there should be no big in-rush. Any comments?

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