Valve preamplifier, modifications in original schematic

[MidnightArrakis]

Hi Ian,

Yes, the first schematic is from Tape Op, but I can´t see the power supply since I don´t have a subscription to the magazine, but I designed my own, check the attached file, and considerer the grounding that I forgot in the drawing and a 10H choke.

Nelson: I can only assume that the voltage source shown in your schematic is an AC voltage that is then being rectified by D1 and D2, correct? In my updated schematic I have shown that as a standardized "Voltage Source" schematic symbol. However, I am confused by your nomenclature of (0 - 225 - 60). I do not understand whatever that means. SORRY!!! Maybe "225 Volts referenced to GROUND with a current draw of 60mA"? I just don't know. Maybe I should???

I have shown your D1, D2 and D3 diodes as Type 1N4007's since they are standard, cheap and readily available 1,000V @ 1A diodes that are more than adequate for this application. Personally, I like using "Full-Wave Bridge Rectifiers" in a 4-Pin DIP package that are available in 1A to 3A versions. However, another quick check on your 10H choke on Digi-Key shows 9 models ranging in price from $10.00 to $202.00. I believe you will need to provide some additional information about this choke's specifications before one particular model type can be selected. Is it possible to come up with some other way to offer the needed filtering action here? That's a lot of money just for one part!!! And, you will also need to point out which resistors are -- quarter-watt -- and which ones are -- half-watt -- or something else.

I have also updated the basic audio circuitry schematic with the proper capacitance units (converted to -- uF -- since that is used more often). In addition, your schematic shows various electrolytic capacitors, but you do not show what voltage-rating they need to be. I did a quick check on the 220uF capacitor and the only readily available type that Digi-Key had in-stock is a 220uF @ 400VDC capacitor by either NIGHICON or VISHAY. Since your B+ voltage is shown as being 225VDC and the closest value capacitor on Digi-Key is a 250V unit, I barely know enough to know that that is too close to the B+ voltage to use and also be reliable. So, a 400V capacitor will need to do. It would also be generally helpful to know both the -- type (ceramic, film, etc.) -- and -- voltage -- specifications for the other capacitors shown with your schematic. Otherwise, "assumptions" need to be made and everyone knows the story about "assumptions", right???

 

[ruffrecords]

Nelson: I can only assume that the voltage source shown in your schematic is an AC voltage that is then being rectified by D1 and D2, correct? In my updated schematic I have shown that as a standardized "Voltage Source" schematic symbol. However, I am confused by your nomenclature of (0 - 225 - 60). I do not understand whatever that means. SORRY!!! Maybe "225 Volts referenced to GROUND with a current draw of 60mA"? I just don't know. Maybe I should???

This is because it is a schematic from a simulation of a possible PSU rather than a 'real' schematic. The AC sources are 225V volts at 60Hz.

Cheers

Ian

 

[ruffrecords]

Hi Ian,

Yes, the first schematic is from Tape Op, but I can´t see the power supply since I don´t have a subscription to the magazine, but I designed my own, check the attached file, and considerer the grounding that I forgot in the drawing and a 10H choke.

Do you rememeber which issue of Tape Op it was in? I am sure I have seen the complete schematic somewhere. I have a free subscription to Tape Op so maybe I can find it.

Cheers

Ian

 

[Nelson Paschoal]

Do you rememeber which issue of Tape Op it was in? I am sure I have seen the complete schematic somewhere. I have a free subscription to Tape Op so maybe I can find it.

Cheers

Ian

It´s the issue #30, that would be awesome if you have!!!

 

[rock soderstrom]

You will find the schematics here:

https://groupdiy.com/threads/hamptone-pre.52448/
AFAIK the TapeOp Basic subscription is for free.

 

[MidnightArrakis]

This is because it is a schematic from a simulation of a possible PSU rather than a 'real' schematic. The AC sources are 225V volts at 60Hz.

Cheers

Ian

THANKS!!! for the enlightening information about the simulation symbology!!! I am not involved with any form or type of simulation, so that aspect is something I never would have considered imagining. THANKS!!!

While reviewing the original schematic that was referenced by Soderstrom, I noticed that both the circuitry AND the XLR connectors are all tied together using the same GROUND symbol. Does this mean that the original circuit design suffers from the same error like the one you pointed out in my version of the schematic? If so, I am then guessing that this manner of error must be rather common within the audio industry?

And, in a related manner.....the original power supply schematic shows a power transformer having a 225VAC @ 50mA secondary. Would it be possible to use a power transformer with a 100VAC @ 100mA secondary, > IF < a voltage-doubler circuit was used instead? I looked up the plate currents for the 12AT7 and 12AU7 tubes and they average 10mA to 11mA each, then " - X2 - " for a 2-Channel preamp. Would that work?

/

 

[Nelson Paschoal]

THANKS!!! for the enlightening information about the simulation symbology!!! I am not involved with any form or type of simulation, so that aspect is something I never would have considered imagining. THANKS!!!

While reviewing the original schematic that was referenced by Soderstrom, I noticed that both the circuitry AND the XLR connectors are all tied together using the same GROUND symbol. Does this mean that the original circuit design suffers from the same error like the one you pointed out in my version of the schematic? If so, I am then guessing that this manner of error must be rather common within the audio industry?

And, in a related manner.....the original power supply schematic shows a power transformer having a 225VAC @ 50mA secondary. Would it be possible to use a power transformer with a 100VAC @ 100mA secondary, > IF < a voltage-doubler circuit was used instead? I looked up the plate currents for the 12AT7 and 12AU7 tubes and they average 10mA to 11mA each, then " - X2 - " for a 2-Channel preamp. Would that work?

/

That´s a good question about the voltage multiplier, I wanna know about it, I´m planning the PS for the phantom power and maybe I go in that way, with a 20+20v xformer, I heard that noise is higher with this technique.

 

[ruffrecords]

You might be able to read the original here:

Build a tube Mic Pre: Build Your Own Two Channel Vacuum Tube Mic Preamp!

Cheers

Ian

 

[moamps]

https://groupdiy.com/threads/scott-hamptons-2002-tape-op-tube-pre-anyone-built-it.61049/
post 9

 

[Nelson Paschoal]

Hi friends, I took some photos, I didn´t remember but about the input and output the pin 1 is soldered in that pad which connect the case os the plug in the chassis. In saturday next I will do the changes Ian told before.
Please take it easy with me, it´s my very first attempt with a tube pre.

 

[ruffrecords]

No problem Nelson. We are here to help. It is good to see another point to point tube project.

Cheers

Ian