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PostPosted: 28 Jan 2012, 23:16 
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Mark, I'm really confused. With a forward voltage of 400v across the tube and a 56v filament implies about 300mA (i.e. the plate would be melting). Looking at your voltages and the 300B plate curves, I can't find the operating point. Is there a possibility that the 300B bias resistors are 560Ω instead of 750Ω? I think that would explain some of it, but I'm really just guessing at this point.

Here is the original 300B plate design for reference.
Attachment:
Power Stage Design 3.jpg
Do you see my confusion?

If there is 400v across the tube and a 750Ω bias resistor, then the bias voltage should be up around 80v, not 56v. In fact, I cannot imagine any combination that could yield a 56v filament along with a 400v forward voltage. The only other thing I can think is that some part of the 300B filament supply got grounded somehow. Other than that, I'm out of ideas.

Keep us informed.


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PostPosted: 28 Jan 2012, 23:36 
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Matt: you may be onto something when you said part of the filament cct. may be grounded. Qian's friend replaced all filament regulators and may have the regs common running to common ground?? Please explain how this would affect the amp?

But 401V plate to filament at 74mA is only about 30W. I think these tube should handle at least this without a red face??

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Last edited by mwhouston on 28 Jan 2012, 23:40, edited 1 time in total.

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PostPosted: 28 Jan 2012, 23:37 
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Mark, You might also want to measure the voltage across the 4.7Ω HT droping resistor to verify total current draw.

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PostPosted: 28 Jan 2012, 23:43 
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mwhouston wrote:
Please explain how this would affect the amp?
It depends on the filament supplies. But if the negative side of the bridge was grounded then there could be some unaccounted for currents in the regulator circuitry. This could cause all kinds of havoc with the o/p stage operating point. Even some HF oscillation in the 100s of kHz range. A schematic of the new filament supplies would help. I kept thinking that the originals were still in there.

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PostPosted: 06 Mar 2012, 22:29 
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Don,a local DIY valve amp friend, is building this amp. He is using different components but is following the same schematic. One of Don's amps has recently been put on the gallery and another (6C33C) is about to be posted.

It will be interesting to hear the difference in the two 300Bs. As an aside Don has completed a 2A3 amp that only has a buzzy distortion for an output. I suggested he check all resistors and voltage levels. He has done this and claims it is no different. Don is bringing the amp around for me to look at. I think it is a missing earth from one of the valves, a very wrong plate or bias resistor or missing HT from driver tube. I'll let you know what I find. Whatever it is is common to both channels.

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PostPosted: 07 Mar 2012, 01:39 
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Ok still off thread but for a single entry it seems silly to start a whole new thread. Should this post evolve into something bigger I'm happy to have the postings moved to a new thread.

The issue with Don's 2A3 is he had the coupling cap (0.33uf Russian military cap) connected to the plate of the of the 2A3 and not the grid. The amp played well when this error was fixed. BUT tapping on one of the driver tubes produces noisy crackling through the amp. At a guess I'd say Don has a dry joint (they light easier) at the socket of the tube. I'll point this out to Don and he can worry about it.

I'm keeping the amp for a few days to have a listen. Skip this if you hate subjective banter.

All the tubes are new with ZERO hours on them so any remarks here could change in the weeks ahead. The amp sounds excellent with one of the fullest rounded basses i have ever heard. Apart from a small hum when you are close to the speaker there is no buzz. I'm very impressed by the sound of this amp.Others of Dons have been a little average apart from a 6C33C which he is rebuilding. I think I would have to put this amp down as one to build. It sounds very sweet and natural. I just want to keep listening to it. Pianos are to live for.

The amp has James OPTs and Brimar drivers. The 2A3s are Chinese. Don lives on a pension so I'm guessing the amp was not expensive to build. The power tranny and Choke are locally wound Dynes.
Attachment:
Don2A3.jpg


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PostPosted: 07 Apr 2012, 02:27 
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I have comment here before that when I throw the standby switch on the Silver Dragon amp sparks go off inside the Golden Dragon 5U4 rec. tube. Matt has suggested to put a resistor prior to the first cap on the PS. Once when I turned on the HT fireballs rolled down the inside of the rec tube. I don't want to destroy this tube but at this rate I will. I have changed the Golden Dragon 5U4 for an old (and large) Australia made Miniwatt. The plates appear bigger and the tube looks more rugged.

There is but a slight blue flash now when I through the HT. Sometimes there is nothing. I don't want to destroy this tube either but better than the new Golden Dragon. The amp still sounds excellent though the new 807 comes close.

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PostPosted: 07 Apr 2012, 20:43 
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Mark; I hope you'll forgive me. :blush: It seems I goofed.

If you look at this post here, I said to put the switch before the first 47µf capacitor. This is causing a hot switching transient of the rectifier that is causing arc over. Luckily, the fix is really simple. the switch needs to be moved from it's current position to the other side of the first capacitor. Like in this schematic.
Attachment:
Modified PS Schematic.png
This should fix the problem and eliminate the rectifier torture currently taking place. If not, then the switch could be moved to the other side of the 100µf capacitor, but I don't think that's really required.

Have you tried starting up the amp with the standby switch in the on position?


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PostPosted: 07 Apr 2012, 21:27 
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Not sure I have ever tried the amp with the standby on. But the fix you have posted above is a an easy one and I'll try it soon. It maybe a few weeks before I get around to the change but It is one I will definitely try. I want to use the Golden Dragon tube because: 1, I think it is a good quality rec. tube and 2, it has a dragon on it and goes with the whole "Dragon"amp theme. Also it is coke bottle shape the same as the 50 Year Treasure 6SN7.

Thanks Suncalc for the updated suggestion. I was thinking the 47uf poly was too big to be the first cap after the rec. tube.

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PostPosted: 20 May 2012, 02:30 
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Don, my old amp building buddy, who turns out nearly a tube amp a week, has knocked up this Shuguang based 300B (300B-98) Currently the 6SN7 is Shuguang also but Don has lots of premium and rare 6SN7 to drop in. The rec. tube - Golden Dragon. The iron is locally wound Dyne. Don likes his horizontal power trannies. High end passives are used throughout and MKP coupling caps. Matts cct. has been used but Don used AC on all heaters.

On the 6SN7 heaters he just has two 100ohm resistors to the negative rail to balance the heater AC. On the 300Bs he has two 22ohm resistors (one off each side of the filament) joined then connected to the 750 cathode resistor. The 750 ohm cathode R is bypassed by a high-end cap. Hum is very controlled. Only by pressing up to the speaker do you hear a hum. From a meter - nothing. The amp plays very loud without clipping. Its early days but the top and bottom end are crippled and mids sound veiled. I have the amp for the next two days but it may need 100 hours. It still smells of burning paint. Don makes all my bases now. I have a spare to build a KT88/EL34 amp on.

A very compact build with heavy ABS top plate (no bottom plate). The bloody thing weighs a ton. I herniate every time I lift it. mind you just the amp section of my 807 is heavier and it has no power tranni - Edcor iron.
Attachment:
Dons300B.jpg


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