Motown Direct Amplifier-inspired Preamp?

[DaveP]

I think we discussed earlier that when paralleled the gm doubles, rp halves and mu remains unchanged.
Cheers
Ian

That is what I have done Ian, but the exercise shows how far reality differs from the published tube specs and how important it is to calculate the figures in the actual circuit.
best
DaveP

 

[ruffrecords]

I agree. Published specs seem to be designed to show the maximum gm achievable without melting the tube and bear no relation to the values actually achieved under normal circuit conditions.

Cheers

Ian

 

[DaveP]

>> Shouldn't your schematic also be showing a 1/4-inch instrument input jack as well as a switch toggling between it and the XLR? Just wondering.....

>> Do you have any suggested recommendations as to what both T1 and T2 "could/might" be? With my knowing who makes them and what their Part Numbers are, I could layout a series of hole cutouts on the top of the chassis which would then be used to accommodate any one of them.

>> Is there an accompanying power-supply schematic for this circuit somewhere?

>> Are all of the resistors 1/4-Watt (other than R12)? Or, do any of the resistors need to be 1/2-Watt?

I can draw a jack circuit and switch if it will help.

For T1 I would use a mumetal case 600:50k from RS https://uk.rs-online.com/web/p/audio-transformers/2106352?gb=s, but there must be others that are similar. For T2 I would use a 2.5W edcor 15k:600 but people may turn their noses up at that! This thread has discussed the transformers in some detail. Look Midnight, trying to get all the mounting holes predicted may not be wise, because that chassis is small and you may have to move the transformers around to avoid hum pick up. Look how they mounted that choke for instance.

I can draw a suitable power supply but I can't predict the performance of your chosen power transformer, adjustments will probably be necessary.

I only use 0.75W metal film resistors for ruggedness and lowest noise.
best
DaveP

 

[DaveP]

For Midnight:-

best
DaveP

 

[DaveP]

Power Supply:

The power transformer secondary is assumed to be 250V and depending on the regulation, should give around 350V after the bridge.
best
DaveP

 

[MidnightArrakis]

I can draw a jack circuit and switch if it will help.

For T1 I would use a mumetal case 600:50k from RS https://uk.rs-online.com/web/p/audio-transformers/2106352?gb=s, but there must be others that are similar. For T2 I would use a 2.5W edcor 15k:600 but people may turn their noses up at that! This thread has discussed the transformers in some detail. Look Midnight, trying to get all the mounting holes predicted may not be wise, because that chassis is small and you may have to move the transformers around to avoid hum pick up. Look how they mounted that choke for instance.

I can draw a suitable power supply but I can't predict the performance of your chosen power transformer, adjustments will probably be necessary.

I only use 0.75W metal film resistors for ruggedness and lowest noise.
best
DaveP

>> DaveP: -- THANKS SO MUCH!!! The information you have provided in your response here is all good information to know just in a general sense. But, perhaps you are mistaken or perhaps what I have been doing and saying within my comments hasn't been very apparent or clear. Let me explain.....

The efforts I have put forth and have shown within my comments in this thread -- HAVEN'T BEEN FOR ME!!! -- My creating a chassis design and requesting a more complete schematic, etc. -- ARE FOR THE MEMBERS OF THIS FORUM!!! NOT ME!!! -- In other words, the members of this forum who would like to build this project and who also may not be so mechanically-inclined to design and layout a chassis (as one "certain member" in this thread has personally admitted that they are "mechanically all thumbs"), could simply use my eventual mechanical detail fabrication drawing and/or perhaps the 3D CAD-file I will generate, to have their own chassis fabricated in such a manner that it will actually work out well for them. That's all.....

[you may have to move the transformers around to avoid hum pick up] -- Then, how about possibly using a steel chassis and/or also including a steel shield over the transformers? Would an aluminum chassis with a steel shield placed over the two transformers work well enough? Doing something like that is easy enough for me to do and to also include with the chassis design.

[I can't predict the performance of your chosen power transformer] -- Since I am not a "Circuit Designer" type of person in order to know what happens electronically but, isn't there some kind of "voltage-range tolerance", where a few volts either side of some ideal voltage still allows the circuit to operate well enough? Meaning, if the 300V in someone's build actually ends up being only 296V, that the circuit will sound just awful? I find something like that rather hard to believe. But, what do I know?

[I only use 0.75W metal film resistors for ruggedness and lowest noise] -- There are far more resistor values available in a 1-Watt 1% Metal-Film package than there are for a 3/4-Watt package.

/

 

[DaveP]

[you may have to move the transformers around to avoid hum pick up] -- Then, how about possibly using a steel chassis and/or also including a steel shield over the transformers? Would an aluminum chassis with a steel shield placed over the two transformers work well enough? Doing something like that is easy enough for me to do and to also include with the chassis design.

[I can't predict the performance of your chosen power transformer] -- Since I am not a "Circuit Designer" type of person in order to know what happens electronically but, isn't there some kind of "voltage-range tolerance", where a few volts either side of some ideal voltage still allows the circuit to operate well enough? Meaning, if the 300V in someone's build actually ends up being only 296V, that the circuit will sound just awful? I find something like that rather hard to believe. But, what do I know?

[I only use 0.75W metal film resistors for ruggedness and lowest noise] -- There are far more resistor values available in a 1-Watt 1% Metal-Film package than there are for a 3/4-Watt package.

An aluminium chassis is usually best because it does not conduct a magnetic field, steel shields can help.
4 or 5 V either way will not matter but 10 or 15V will shift the working point.
1W metal film are even better.

Thanks for explaining
best
DaveP