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Old 5th September 2005, 10:57 AM   #1
Sjef is offline Sjef  Netherlands
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Default Help in getting more gain out of 300B SE

Can someone help me in redesigning the driverstage of my 300B SE amp to get more gain out of it. I'm using this amp in a three-way active setup with a DEQX PDC2.6 crossover/preamp. The problem is that the 300B amp has about 10 to 15 dB less gain than my other power amps, in fact the PDC isn't capable of driving the amp to it's limits and it clips out to soon at the moment.

So, I would like to redesign the driver stage for about 10 dB more gain. The driver stage is made with one side of a ECC99 mu-staged with a d3A penthode (see schematics). I have one side of the ECC99 per channel left, just doing nothing at the moment.

So the options are either to use this side of the tube for a extra gain stage but that will probably give to much gain and an extra coupling cap is needed (or redesign the stage for dc coupling) or use another tube in the mu-stage here like the 6c45 or something

there's not much space left in the chassis and it's almost impossible to swap the noval feet for octal ones so I'm stuck to noval types.
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Old 5th September 2005, 11:18 AM   #2
SY is offline SY  United States
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12AT7/ECC81, two sections in parallel instead of the ECC99. That will give you about 8 dB more gain.
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Old 5th September 2005, 11:29 AM   #3
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Hmmm, how about something different?

Wire the idle ECC99 sections up as cathode followers. Put the CFs at the amp's I/Ps and use them to drive step up microphone I/P trafos. The stepped up I/P voltage drives the mu follower.

The footprint of the "mike" trafos is small. Courtesy of Steve Eddy, a link to trafo specs follows. You would want to source in the EU, but the link does indicate the sort of stuff that's available.
Microphone I/P Transformers
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Old 5th September 2005, 11:42 AM   #4
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Try choke loaded d3a. Or a CCS loaded d3a.

Triode wire the d3a. Mu=77. Need to drop the voltage b+ for driver stage as well.
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Old 5th September 2005, 01:02 PM   #5
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Sjef, putting your circuit into tube cad your single half of the 5687 at these figures is giving about 11.5db of gain, if you were to use both halves and paralell them, up drop your cathode resistor to 330ohm instead of 750 you should get 16.5db of gain from the 5687. i think i,ve calculated this right tube cad says o.k.

Cheers George
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Old 5th September 2005, 01:14 PM   #6
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Default Re: Help in getting more gain out of 300B SE

Konnichiwa,

Quote:
Originally posted by Sjef
Can someone help me in redesigning the driverstage of my 300B SE amp to get more gain out of it.
Just re-arrange the Mu-Follower so that it makes sense, that is load the triode connected D3a with the ECC99 as Mu-Follower. As a Hint, if you use a J-Fet CCS (2-pin device) with around 10mA current in the cathode of the ECC99 instead of a traditional SRPP cathode resistor you have an excellent Mu-Follower while retaining minimal component count (okay, okay, it adds "sand", but so what, it sounds fine).

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Old 5th September 2005, 04:18 PM   #7
Sjef is offline Sjef  Netherlands
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Hmmm, very interesting answers, thanks a lot. Time for some consideration, hopefully not to confsing for myself, after all I'm a loudspeaker guy not a tube expert.

I have also considered the use of a step-up transformer but was worried about the max- input voltage of these devices before going into saturation. Could they also be driven directly from the preamp (wich has an output impedance of around 200 Ohm)

Paralelling both halves is also what I have had in mind. This would give some usefull enough extra gain to at least prevent the pre-amp/crossover from clipping out occasionally. I think the rest of the mu-stage has to be redesigned as well. To be very honest I don't have the slightest clue about how this mu-stage actually works and cannot calculate it myself but I guess that if I just parallel both halve they both see only halve the current from the current source wich leads to a very different operation point. Am I right about that ?

Triode wiring the d3A seems interesting indeed. I've read a lot of good things about the sound of them in triode connecion (haven't heard it myself) What kind of choke did you use Bass ? Is the output impedance still low enough to drive the 300B with ease ? Can you describe the difference in sound compared to a mu-stage (if you ever compared them that is)
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Old 5th September 2005, 04:29 PM   #8
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Quote:
Can you describe the difference in sound compared to a mu-stage (if you ever compared them that is)
Yes..I used a Lundahl LL1667 and the previous setup was 6n1p with d3a as mu-follower.

I liked the sound better with the choke. Instruments sounded more like the instruments are supposed to sound and more microdetail. The MU-stage delivered a bigger soundstage though. In the end it is a matter of personal taste. And ofcourse it was comparing apples with pears...because my previous driver was a 6n1p and now it is a triode wired d3a

The Lundahl Choke does exactly the same task as the D3A does in the/your current Mu-Stage...i.e. provide a constant current or in other words to provide the load. But it does this with a whole lot less caps and resistors and no need for an extra source of noise...the heater/filament of the d3a (which is AC in your case).

The big drawback of a plate choke is COST!

But if you like you can borrow my Lundahl LL1667's to try it out. I live in Middenmeer. (They are out of my amps now because I'm going to build a DRD amp that Thorsten (Kuei Yang Wang) helped design.

It would be easiest to just add a d3a triode wired and leave the top tube to be d3a..come to think of it.
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Old 5th September 2005, 05:45 PM   #9
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Konnichiwa,

Quote:
Originally posted by Sjef
Paralelling both halves is also what I have had in mind.
Note, I doubt that this will give an appreciable increase in gain as the "Mu" follower already operates the lower valve at nominal "maximum gain".

Quote:
Triode wiring the d3A seems interesting indeed.
You may want to go all the way. Roll my "super mu follower" and the currently existing valves into one....

See attached....

The CR470 X 2 is one option for a simple CCS, others exist and the CR470 does not get much of an endoresement from me, except "it works".

You may also connect the coupling capacitor to the 300B to the D3a anode, but in that case you need either a full cathode bypass capacitor (220uF or so) OR you need to be tricky and add a small "tuning" capacitor in parallel with the cathode untill the frequency response ends up flat at high frequencies, too much capacitance and the top end gets tilted up, too little and it remains depressed.

BTW, while you are working on the Amp's anyway add a 27uF high quality capacitor (Film, PIO, whatever takes your fancy) between +B and the + Pole of the 300B Cathode bypass capacitors.

Sayonara
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Old 5th September 2005, 06:35 PM   #10
hkoetz is offline hkoetz  Netherlands
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Default Help in getting more gain out of a 300B SE

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