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4S Universal Tube Preamp Project (12A*7)
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Author:  Renaissance Man [ 11 Sep 2018, 09:10 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I seem to have clouded the question with too much information. In my reading I have seen that some builders of tube / SS integrated amplifiers add a zener voltage clamp or turn on delay relay between the tube preamp and SS power amp specifically to deal with a potential voltage spike when the tubes start conducting. I don't understand the mechanism of how this voltage spike would occur when the tubes first start conducting when the cathode warms up and the B+ is already present, and don't have the equipment to measure it so I am not sure if I need to worry about this in my 4S / F5 build.

Author:  ILoveHiFi [ 11 Sep 2018, 14:16 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I've designed many valve pre amps, including hybrid, pure valve op amp and classic valve schematics.

The only time you need to be worried about voltage spikes is when you are building a hybrid valve op amp, this means something that uses transistors and valves and inside a op amp.

Classic valve schematics like 4s universal and virtually all valve only schematics you can find online do not have voltage spikes during power on.
I've designed two valve only op amp schematics and they also do not have power on votlage spikes.

If something does give you power on spikes then its likely to be from a hybrid or transitor design, normally cause the output is capacitor couppled this means you will only see the ac votlage spikes.
DC dosen't pass through capacitors.

Only time where dc may pass through a capacitor is when you have one side of the capacitor not ground refrenced by a resistance of 1M ohm or smaller

Author:  Renaissance Man [ 11 Sep 2018, 23:35 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Ok. Thanks to all. My concerns have been relieved and I have a better understanding of what is going on.

After I finish reassembling the amp in it's new case, I will remeasure voltages during startup because the numbers I remember seem a little off. The preamp output capacitor is ground referenced by a 50 kOhm resistor. The volume potentiometer is on the input side of the tubes.

I am quite happy with the sound of the preamp (in combination with the F5) and appreciate the simplicity of the circuit. In order to lower the output resistance of the preamp I ran two triode sections in parallel for each channel (two tubes total) to better match the 50 kOhm input resistance of the F5.

Author:  mwhouston [ 12 Sep 2018, 01:19 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I assume you are aware that HT is always higher until the tube reaches operating temp 20 seconds or so after turn on. The tube being non-conductive when cold there is no load on the PS so the HT floats high briefly.

Author:  Jakub [ 28 Sep 2018, 07:33 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi Is this preamp suitable for amplifier control with input impedance of 18 kOhm and 106dB? Thank you very much for your answer.

Author:  Ja Patt [ 02 Oct 2018, 15:26 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Having difficulties picturing connections regarding the chassis/lift grounding.

Question: How is the signal ground switch wired to chassis and "lift", and how are the signal wires connected to ground so that they can be "lifted" or chassis ground? perhaps a wiring diagram anyone...

Author:  Ja Patt [ 29 Oct 2018, 14:47 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

preamp complete! playing with Tubelab SSE KT88 and Pioneer PLX1000. Sounds great.

Author:  Suncalc [ 29 Oct 2018, 17:43 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Nice setup! Looks great!

Author:  mwhouston [ 29 Oct 2018, 17:49 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I'm being very picky but it is my pet hate; not into raw Al when a spray can can make a huge visual difference. Im sure it all sounds good.

Author:  Ja Patt [ 29 Oct 2018, 18:16 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Matt, thanks for sharing your design.

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