ebay 6J1 valve pre-amp - circuit diagram/gain
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02GF74
diyAudio Member

Join Date: Jul 2010
ebay 6J1 valve pre-amp - circuit diagram/gain

There is a plethora of 6J1 valve pre-amps on ebay from china/hong kong.

Photo of a typical example is below.

I would like somebody to confirm the circuit diagram is the correct one. I've buzzed 3 resistors (not the ones in the voltage multiplier) and they match so I am reasonably sure it is correct.

But I would like to know how to convert the circuit to unity gain (i.e. to behave as a buffer). The potentiometer is to be removed and resistors to control the gain changed - but so that there is not clipping and frequency response is unchanged - hopefully that is possible.

My guess gain is set by (R13 / R14) = 470 k / 200 or by comparing to another circuit I have, is it (R16 / R150 = 100 k / 4.7 k ?
Attached Images
 6j1 x2 amp.jpg (115.8 KB, 401 views) 6j1 x2 amp1 - circuit.jpg (97.2 KB, 400 views)

 3rd December 2017, 09:33 AM #2 jazbo8 diyAudio Moderator     Join Date: Jan 2011 Location: In Transient Just move C10/C12 to pins 2,7 (the cathode).
02GF74
diyAudio Member

Join Date: Jul 2010
Valve pre amp, what is the gain?

What is the gain of the valve Valve pre amp circuit?

I suspect it is set by 2 or 3 resistors.
Attached Images
 DSC_0294.jpg (905.2 KB, 428 views)

 6th December 2017, 06:56 PM #4 aardvarkash10   diyAudio Member     Join Date: Mar 2007 Location: Auckland, NZ hard to know since the tube that is drawn (a triode) doesn't match the data sheet for its designation - the 6J1 is a pentode... __________________ "It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
 6th December 2017, 07:14 PM #5 Eli Duttman   diyAudio Member   Join Date: Apr 2004 Location: Monroe Township, NJ As no NFB loop is present, the gain (α) of a triode is controlled by the amplification factor (μ), the plate resistance (RP), and the net load. A complication is the unbypassed cathode bias resistor. The absence of a bypass cap. introduces degeneration, AKA local current NFB, which raises the effective RP. The 6J1 data sheet is (at least to me) confusing. The type is a pentode, but the plate curves shown are for triode wiring. Somebody with Hanji skills might offer a better insight. The schematic provided shows, in an unclear manner, the tube being triode wired. Pin 6 is the screen grid (g2), but it's shown as a plate connection. __________________ Eli D.
 6th December 2017, 08:02 PM #6 Sorento   diyAudio Member     Join Date: Aug 2017 gain of unloaded grounded cathode triode is: A=(mu*Rp)/((rp+Rp+(mu+1)*Rk) where mu and rp are properties of the tube, Rp is the external plate R, Rk the cathode R. 6j1 is a pentode which is triode connected in this case; Chinese datasheet does not give numbers for trioded mu and rp but curves for triode connection; from these a rough estimate gives mu of 30 and rp of 5kohm. So gain is approx. 24, unloaded. Gain is lower when loaded. Now there is already a load of 100kohm which has to be calculated as parallel to Rp. This reduces gain to 22. If we assume that whatever is driven by this pre has input R of again 100kohm, we end up with something like 20.
 6th December 2017, 09:13 PM #7 Alllensoncanon   diyAudio Member   Join Date: Mar 2017 Location: NorCal, East Bay If the schematic came from one of those low cost tube line amp from ebay and Amazon. They operates at 24V DC plate much lower than typical 6J1 plate. The gain is about 5 with inverted output. Non-clip Input range is about 0-2.5Vp-p.
 7th December 2017, 05:02 AM #8 auriga2001in   diyAudio Member   Join Date: Nov 2013 Location: Bangalore This preamp is explained here: Tube preamp kit FE-6J1-2.0B | Note: R16/R21 must be cnnected to ground.
 7th December 2017, 10:18 AM #9 DF96   diyAudio Member   Join Date: May 2007 The 5654 pentode was designed for good VHF/UHF gain: low grid input conductance and hence low grid noise were the aim. It is not particularly linear, so I don't understand why it is so popular for audio. Anyway, to turn this circuit into a buffer you need to rejig it as a cathode follower.
 7th December 2017, 11:35 PM #10 Eli Duttman   diyAudio Member   Join Date: Apr 2004 Location: Monroe Township, NJ I would add that a properly functioning buffer is transparent, most definitely not an effects machine. High transconductance (gm) leads to low O/P impedance in a cathode follower. Combine high gm with reasonably high plate current (IB) and you have something that can drive some pretty miserable loads. The 5687/6H30Π (6n30p)/ECC99 group of "super triodes" fill the need well. __________________ Eli D.

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