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Old 16th May 2009, 09:48 AM   #1
Ipanema is offline Ipanema  Malaysia
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Default Glass B1. Need advise

Hi, need some advise here because I've never build a tube amp before. I have two 12AU7 sitting there collecting does. So I thought whether it is possible to build a tube buffer ala Firstwatt B1 with bipolar supply so that both input and output can be direct coupled. Thanks
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Old 16th May 2009, 12:53 PM   #2
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The 12AU7 makes an acceptable cathode follower. Eliminating the coupling cap. on the O/P side is tricky and (IMO) to be avoided, but a bipolar PSU is a good idea. Fewer caps. in the signal path simply can't hurt.

Max. performance will be obtained, if the triode is constant current sink (CCS) loaded. The easiest way to achieve that is with the 10M45S integrated circuit.

The bipolar PSU need not be symmetrical. The negative rail need only provide for the CCS' needs and compliance with the I/P drive signal. Upwards of 150 VDC are needed in the positive rail.
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Old 16th May 2009, 10:00 PM   #3
DougL is offline DougL  United States
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Quote:
is possible to build a tube buffer
I would explore Sy's Heritical.
The design is documented here: Heritical Article

It uses a 6DJ8 not the 12AU7, but the concept is the same.
One of the variations discussed eliminates the output cap.
Sy has very well thought out and very lucidly described designs.
The only reason I do not simply copy him is a desire to learn by doing it "my way".

Personally, I would use a cap and avoid the complexity of the bias for my first build.

If you are interested in tube circuits, By Morgan Jones "Valve Amplifiers" Third edition.

Good Luck.

Doug
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Old 17th May 2009, 03:04 AM   #4
Ipanema is offline Ipanema  Malaysia
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Hi,

This is my first try. The loadline is copied from http://www.shine7.com/audio/12au7_pre.htm

I've build a similar topology B1 with 2SK170 (unmatched top and bottom jfet). The output DC offset is <10mV. Do you think that 12AU7 will produce less offset since both valves are contain in the same envelope? I can always trim R1 and R2 to get zero offset. Worst case plug in an output cap, which need not be high voltage type (cheaper).

But do you see any problem with the above schematic before I started building it.

Thanks.
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Old 17th May 2009, 06:57 AM   #5
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Even if you manage to get the offset low keep in mind that tubes age and the two sections may drift apart in time. Is the PS regulated? IMO the risk of blowing something upstream is just not worth it. I will be interested in your appraisal of the sound compared to the B1.
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Old 17th May 2009, 07:44 AM   #6
Ipanema is offline Ipanema  Malaysia
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Good point. I'll just stick on the output cap then. I haven't decide on the power supply yet. Is there any concern on the PSRR of this circuit that it need a regulated supply? Thanks.
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Old 17th May 2009, 08:13 AM   #7
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I mentioned regulation only as a factor in keeping offset stable and independent of mains variations. As far as sound goes i prefer good regulators even in opamp circuits with supposed psrr of just under infinity. Of course regulation removes some of the "tube" character and is not necessarily to everyone's liking.
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Old 18th May 2009, 03:07 AM   #8
Ipanema is offline Ipanema  Malaysia
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Hi, I just check the datasheet. It said that the heater to cathode positive voltage for 12AU7 should be <100V. For the circuit that I posted, the heater positive is higher than cathode of the lower triode by : 12.6+133 = 145.6V. Is this what the datasheet mean?

Thanks.
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Old 18th May 2009, 07:02 AM   #9
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The heater to cathode spec is based on the heater being referenced to ground and the cathode can be more positive by 100v or negative by 200v. For the schematic you show you can get away with referencing the heater to ground, but, referencing to -50v would be a better compromise.
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Old 18th May 2009, 08:26 AM   #10
Ipanema is offline Ipanema  Malaysia
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May I know what is the benefit of referencing heater to -50V? Can I show me a schematic of how to do it? Thanks.
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