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Old 29th March 2008, 08:30 PM   #1
eexdm2 is offline eexdm2  United Kingdom
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Default Tube Preamp Hybrid new builder, lots of question!

Hey guys, not really a tube pro but am confident with solid state.

I currently have a 100W power amp module and would like to build a tube pre-amp. Ive searched the forum but carnt seem to find anything (maybe i didnt search well enough!), i have a couple of questions.

1) Can i run the tube pre-amp from the power amps supply (plus minus 40V)

2) Does anyone have an reference designs for this sort of application

sorry if these sorts of things have been asked before and cheers for your time


Damon
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Old 29th March 2008, 08:50 PM   #2
Gordy is offline Gordy  United Kingdom
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Over the last few months I've seen dozens and dozens of different pre-amp designs all over the net, so you just have to search a little harder and wider.

Most vacuum tubes work at high voltages, however I believe that some tubes were designed specifically for low voltage operation, such as portable gear, car radios, etc..

It is likely that a low voltage tube, without the high voltage range to swing through, will loose the inherent advantage of a triode (i.e. linearity). However if you still wish to pursue it then try the low voltage Aikido design of Mr. Broskie...
see
www.tubecad.com
http://www.tubecad.com/2008/01/blog0132.htm
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Old 29th March 2008, 11:47 PM   #3
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Default Re: Tube Preamp Hybrid new builder, lots of question!

Quote:
Originally posted by eexdm2
Hey guys, not really a tube pro but am confident with solid state.

I currently have a 100W power amp module and would like to build a tube pre-amp. Ive searched the forum but carnt seem to find anything (maybe i didnt search well enough!), i have a couple of questions.

1) Can i run the tube pre-amp from the power amps supply (plus minus 40V)

2) Does anyone have an reference designs for this sort of application

sorry if these sorts of things have been asked before and cheers for your time


Damon

Damon,

What sort of drive voltage does your SS power module need to reach full power O/P? If the 2 VRMS of a "standard" CDP can drive the SS stuff into clipping, a tubed buffered level control powered by your existing bipolar PSU is possible.

Before going any further, please state clearly what your drive voltage requirements are. Also, please provide the I/P impedance of your SS power module.
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Old 31st March 2008, 02:06 PM   #4
eexdm2 is offline eexdm2  United Kingdom
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Thanks for the responses,

Ok the I/P of the amplifier is 50K, the amplifier goes into a clipping with an input of around 1Vpp,

The module I am trying to use and have setup is the 100W MOSFET Maplin kit from a few years ago.

Damon
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Old 31st March 2008, 03:25 PM   #5
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Quote:
Originally posted by eexdm2


The module I am trying to use and have setup is the 100W MOSFET Maplin kit from a few years ago.

Damon

I believe the photo is the amp you are using and I reworked it to improve the HF thd.. The original 2 Mos;4 transistor 1 diode design came out in 1978 by Hitachi and somehow kept itself to power hi-fi systems when top notch FM quality was around. It was reliable but care was required with bias transistors to avoid the output offest dc drifting higher than 100mV.
Another circuit proved over time.


richj
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Old 1st April 2008, 01:24 AM   #6
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Damon,

It seems you're in luck. 1 V. peak to peak is approx. 354 mV. RMS. The last thing you need is gain. AAMOF, to avoid a "hair trigger", you will have to pad the volume controls with a fixed resistance.

Tubes (valves) are set up more or less the way a "N" channel JFET is set up. You will use the 6GM8/ECC86 twin triode, with both sections (1/channel) set up as cathode (voltage) followers. Examine the 6GM8 data sheet. Please notice that the type was intended for service in low voltage automotive applications. That combined with its reasonably high transconductance (gm) makes it a good choice to work with your existing bipolar PSU and as a volume pot. buffer.

You will use adjustable 3 terminal regulators to bring the positive rail down to 30 V. and the negative rail down to 35 V. The use of a bipolar PSU allows the elimination of caps., in the I/P circuitry.

Constant current sink (CCS) loading will be employed. CCS loading ensures linearity and makes control of the operating point easy.

The buffers are coupled to the load via 680 nF. capacitors. I'd look for Soviet surplus paper in oil (PIO) parts for that role. If you can't find 680 nF. or a larger value in surplus PIO, Mundorf 0.68 muF. M-Cap Supreme Silver/Oil are quite suitable, but costly.

Are you still with me?
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