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Old 22nd December 2009, 06:05 PM   #1
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Default Low Voltage Tube Preamplifier

I've recently accuired a Cambridge audio A500 which apparently has a faulty power amp stage, I could fix it, but I have no real use for it; however it does have +/-45v PSU and a nice set of RCA connections selector switches and volume controle that I would like to make use of; the pre-amp stage is rather run of the mill so I cant see any advanteage to using it in my system over my NAD 3130s pre.

I've also recently been looking at the Bottlehead Quickie kit, a battery powered preamp; now I know nothing about valve stuff but this bit of kit seems to sugest that if your making a low power amplifier (pre-amplifier) then you don't need the high voltages asociated with Tube power amps; would this be right?
Could I build somthing like the quickie into the CA Chasis using the CA's powersupply and make myself a cheap valve pre?

The quickie seems to consiste of two valves and a handfull of caps and resistors, most of the $99 price tag goes on the fixtures fittings knobs etc so buying the kit would be a waste of money for this project; the Schmatic is not availible on it's own, could anyone sugest a circuit of easy to source components i could use to create a pre at these voltages?
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Old 22nd December 2009, 07:25 PM   #2
Glowbug is offline Glowbug  United States
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John Broskie also offers a 24V version of the Aikido line stage using the 6GM8.

The one downside to these low-power tubes is that they aren't made anymore, so when demand goes up, prices skyrocket. 6GM8s have gone up in price significantly in the past couple years.
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Old 22nd December 2009, 07:58 PM   #3
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At +/-45VDC, you have more than enough juice to keep ECC88/6DJ8/6922/6N23P pretty happy (and there are reports of low distortion even as low as +24VDC). You can search a whole host of preamp designs based on those.
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Old 22nd December 2009, 08:08 PM   #4
Arnulf is offline Arnulf  Europe
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Note that there are plenty of space charge tubes avaliable, which aren't "cool" enough yet to be used as mainstream as ECC86, even though they run with low voltage as well (12V on anode). I won't list any names because it'd be a shame to see the ePay prices of these skyrocket, but some Google searching will lead you to a very nice webpage, listing many commonly avaliable space charge tube types. What you're looking for are either sharp cutoff pentodes or triodes in any combination with other tubes (FM detector diodes, remote cut-off pentodes etc.) in same envelope and just leave those other portions doing nothing. You will want to avoid 12K5 (its price has gone up significantly already) and one of the preamp tubes the name of which escapes me right now but then again you're only interested in preamplification so any triode that looks linear enough will do just fine !

Also remember that vast majority of power will be wasted on heating, not the tube operation. This is even more true with the space charge tubes (10% of power going info amplification and 90% into heating) so make sure you get a decent supply !
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Old 22nd December 2009, 09:50 PM   #5
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Originally Posted by leadbelly View Post
At +/-45VDC, you have more than enough juice to keep ECC88/6DJ8/6922/6N23P pretty happy (and there are reports of low distortion even as low as +24VDC). You can search a whole host of preamp designs based on those.
A whole host indeed! too many infact, and some seem impossibly simple! I'm trying to read up on the tube to make an informed choice but I could do with a solid start, if anyone could sugest a circuit that's tried and tested for my first go I'd apriciate it!

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Originally Posted by Arnulf View Post
I won't list any names because it'd be a shame to see the ePay prices of these skyrocket, but some Google searching will lead you to a very nice webpage, listing many commonly avaliable space charge tube types.
Cheers Arnulf, could you PM me the URL!
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Old 23rd December 2009, 01:19 AM   #6
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Quote:
Originally Posted by JammyBStard View Post
I've recently accuired a Cambridge audio A500 which apparently has a faulty power amp stage, I could fix it, but I have no real use for it; however it does have +/-45v PSU and a nice set of RCA connections selector switches and volume controle that I would like to make use of; the pre-amp stage is rather run of the mill so I cant see any advanteage to using it in my system over my NAD 3130s pre.

I've also recently been looking at the Bottlehead Quickie kit, a battery powered preamp; now I know nothing about valve stuff but this bit of kit seems to sugest that if your making a low power amplifier (pre-amplifier) then you don't need the high voltages asociated with Tube power amps; would this be right?
Could I build somthing like the quickie into the CA Chasis using the CA's powersupply and make myself a cheap valve pre?

The quickie seems to consiste of two valves and a handfull of caps and resistors, most of the $99 price tag goes on the fixtures fittings knobs etc so buying the kit would be a waste of money for this project; the Schmatic is not availible on it's own, could anyone sugest a circuit of easy to source components i could use to create a pre at these voltages?

JBS,

Some of your assumptions are wrong, but you should be able to make a decent line stage out of the defunct "solid scrape" unit.

The 45-0-45 you give will rise, when asked to provide only a few mA. of current, instead of the considerably larger draw the OEM planned on. To get a feel for what will actually occur, disconnect the PSU from the "dead" circuitry and measure the unloaded voltage.

Gain structure requirements will govern what you do. What drive level does the power amp you plan to use require? If, as is frequently the case, less than 2 VRMS are needed, a near unity gain setup will serve you well. Remember, a "standard" CDP produces 2 VRMS peak.

PSU's are naturally differential. We make them single ended or bipolar by where we place the ground connection. If you move the ground connection to the negative end, you get a highly satisfactory B+ supply for use with a 6922. A single 6922 set up as cathode followers (1/channel) fills the "unity" gain role well.

Let's resolve the gain structure issue before moving forward.
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Old 23rd December 2009, 08:24 AM   #7
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Originally Posted by Eli Duttman View Post
JBS,

Some of your assumptions are wrong, but you should be able to make a decent line stage out of the defunct "solid scrape" unit.

The 45-0-45 you give will rise, when asked to provide only a few mA. of current, instead of the considerably larger draw the OEM planned on. To get a feel for what will actually occur, disconnect the PSU from the "dead" circuitry and measure the unloaded voltage.
I'm afraid I havent phisically got the thing yet, but I have the circuit diagram. PSU is +/-45v unregulated, it comes with LM3x7regulators, set by default to 30v, but this could be easily adjusted to +/-37v or replaced with higher voltage regs to get +/-43v. Or most possibly higher when as you say the voltage will rise unloaded.[/QUOTE]

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Originally Posted by Eli Duttman View Post
Gain structure requirements will govern what you do. What drive level does the power amp you plan to use require? If, as is frequently the case, less than 2 VRMS are needed, a near unity gain setup will serve you well. Remember, a "standard" CDP produces 2 VRMS peak.
I'd say your right; The service manual for the Power stage of the NAD 3130 states:
Input Impedance = 22Kohm
Input sensitivity for TW/40W out = .15/.84 V (perhaps you could clarify for me exactly what that means, TW??)
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Old 23rd December 2009, 08:28 AM   #8
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You should manage 125V from your transformer. If you can find some simple chokes then you can keep almost all of this. I regularly use florescent light ballasts as chokes in preamp duties. Very cheap and readily available from lighting suppliers.
A version of the Aikido using ECC88's would be my first choice. 60V on each plate is just adequate. Forget the ECC86, because the 24V Aikido has turned prices stupid.

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Old 23rd December 2009, 10:13 AM   #9
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Originally Posted by Shoog View Post
You should manage 125V from your transformer. If you can find some simple chokes then you can keep almost all of this. I regularly use florescent light ballasts as chokes in preamp duties. Very cheap and readily available from lighting suppliers.
A version of the Aikido using ECC88's would be my first choice. 60V on each plate is just adequate. Forget the ECC86, because the 24V Aikido has turned prices stupid.

Shoog
I like the sound of this, there seems to be alot of easy to follow info here: http://www.tubecad.com/Nine-Pin_Aikido_PCB.pdf and the the ECC88's seem easy to get hold of; I just need to see how much voltage I can get out of the PSU. some more info on the light ballasts choke would be good.
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Old 23rd December 2009, 12:28 PM   #10
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These are what you are looking for;
http://upload.wikimedia.org/wikipedi...80/LAjaHID.jpg

Every single strip light will have one, and so can easily be got as replacements from lighting suppliers. About $10.00 a throw.

Shoog
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