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Old 4th November 2013, 09:01 PM   #1
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Default The ULTIMATE 300B

The ULTIMATE 300B

Well, I know that the name is somewhat pretentious... No importance.

I have chosen this name just because it is the last version of my serie of prototypes that I experimented with VT25/10Y, 50, 2A3 and ultimately 300B. It was on Feb. 6th. 2008... But the final model is now on construction... Since a few days...

This amplifier is built in 2 monaural identical "mono blocks" that I wanted to built as compact as possible : 275mm x 175mm x 65mm for the chassis, with a vintage electronic instrument look.

It has a bi-power switching : LO 4Wrms / HI 8Wrms nominal power.

Here are the performances measured on the 2008 prototype :

THD for 1KHz Sinus @ 8ohm resistive load, no NFB :
HI 10W = 5.32% (at the threshold of clipping)
HI 8W = 3.75% (at the nominal power)
HI 4W = 1.69%
HI 1W = 0.7%
LO 5W = 4.96% (at the threshold of clipping)
LO 4W = 3.50% (at the nominal power)
LO 1W = 0.85%

Bandwith @-3dB/8Wrms = 8Hz-25khz

The 2008 prototype, still alive of course :
Click the image to open in full size.
Click the image to open in full size.

Tracing the chassis :
Click the image to open in full size.

Starting the drilling :
Click the image to open in full size.

Drilling completed :
Click the image to open in full size.
Click the image to open in full size.

Wait and see...

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Old 5th November 2013, 09:01 AM   #2
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Looks like a nice project! Good luck!
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Old 5th November 2013, 11:57 AM   #3
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Thanks Vincent77,

I will post pictures of the ongoing construction au fur et mesure... I expect finishing them before the end of 2013 (since 2008 !). I am waiting for the custom engraved plates now.

Wait and See...

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Old 5th November 2013, 06:23 PM   #4
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I need some custom engraved plates, me too... can you recommend a company?
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Old 5th November 2013, 10:57 PM   #5
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Quote:
I need some custom engraved plates, me too... can you recommend a company?
Unfortunately, no...

I used to go in a multi-service shop where the boss had a laser-engraving machine which was compatibe with .jpeg fichiers. His machine was not enough powerful for engraving metal but gave excellent results on plexiglas and dilophane plastic materials.

The shop closed its doors last year for lack of business , but the boss accepts to still engrave me custom plates as a "part-time" job .

I think that in Belgium you would certainly be able to find some specialists in that field, if you are willing to pay the price for a custom job : I will have to pay 100E for the 6 different custom-engraved plates of my ULTIMATE 300B. I will display them here when I will have it.

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Old 22nd February 2014, 10:04 PM   #6
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Work is in progress...

Click the image to open in full size.

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Click the image to open in full size.

A+!
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Old 23rd February 2014, 03:13 PM   #7
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More pictures taken outdoors :

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

A+!
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Old 23rd February 2014, 03:35 PM   #8
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Hi,

Well done.

Can you tell us more about the driver stage and how it all sounds?

Ciao,
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Old 23rd February 2014, 04:20 PM   #9
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Quote:
Hi,

Well done.

Can you tell us more about the driver stage and how it all sounds?

Ciao,
Yes, thanks ! I can only speak about the results of the protoype, since the 2 definitive amps are not wired yet...

1 - this amp has only one driver stage, so a total of 2 stages including the 300B power stage. The driver is a // 12AX7 SRPP with a voltage gain of 64, RC coupled to the 300B. Power output is 8W rms/8 ohms @ 0.776V rms input.

2 - The performances are much better than the 6SN7 direct coupling 2 preamp stage configuration found everywhere (that I also tested on my prototype). More power, more headroom and linearity : the 300B is suitably driven, giving 8.0W rms @ 3.75% THD with no NFB, and the harmonic gradation is regular, H2 predominant.

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Old 23rd February 2014, 04:52 PM   #10
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Hi,

Quote:
The driver is a // 12AX7 SRPP with a voltage gain of 64, RC coupled to the 300B. Power output is 8W rms/8 ohms @ 0.776V rms input.
That's what I figured seeing the pair of 12AX7 on the chassis.
Personally I'm not a fan of using //'ed triodes as it tends to create a somewhat "diffuse" sound. This is (and it's just a guess) probably due to small electrical differences within the twin triodes.

Either way, in order to obtain similar gain and IMHO markedly better drive characteristics using a pair of EC91 may be a viable option?

OTOH: see your signature.

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