Suggestions for a preamp project?

[Jennings]

I’ve got these two amplifier sections from old 50s / 60s reel to reel tape decks that I feel sorry for, and wish to give them new amplifier lives as microphone preamps.


I like the cool aesthetic and the meter, which I’d like to keep for monitoring output. However, I’ve never designed or built mic pres before, so I’m open to ideas and suggestions for the best way to go on this. I was thinking or adding a couple of 1:10 transformer inputs for the mics. Both my chassis are missing the original EL84 output transformers (intended to drive an internal speaker to the tape deck), so I was thinking maybe some kind of 5k to 600 ohm transformer for driving a line out instead?


The schematic shows the whole tape machine, but I only have the amp modules. I will need to also make or source a power supply for HT and filaments.

[glass54]

Hi Jennings,
Some ideas to follow up on:
https://www.jensen-transformers.com/wp-content/uploads/2014/08/as016.pdf
https://www.jensen-transformers.com/wp-content/uploads/2014/08/as020.pdf
https://gyraf.dk/wp-content/uploads/2018/05/DO-IT-YOURSELF-G9-TUBE-MICROPHONE-PREAMPLIFIER.pdf
The Line Driver output is a good idea. Power supply design should be easily achievable with 3 stage R-C filter and a smaller Mains transformer (with say 200 to 240V AC secondary and suitable Heater Supply). It may be of value to have a DC Heater supply for input tubes (Bridge rectifier with minimum 4700uF rated at 25V Electro)
All the best, sounds like a great project.
Regards
Mirek

[kagliostro]

http://www.gyraf.dk/gy_pd/g9/g9pd.htm

http://www.tangible-technology.com/tubes/1566/1566.html

http://www.pmillett.com/micpre.htm

Franco

[Jennings]

Thanks so much both of you for the suggestions and ideas…these materials look great to chew over! I like the idea of perhaps instrument and mic inputs, switchable gain perhaps somewhere round the first stage. Maybe switchable output options too.


As-is these amps use pentodes, which I’m tempted to keep just for fun, so I’ll definitely look to use DC filaments in an effort to keep noise floor down. I quite fancy a valve rec power supply, again just because. But it’s much easier just to take from one of the circuit suggestions you’ve given me for now. I’ll keep the power supply off-board in a little module of its own, but mount the two pres and power supply into one little rack probably.


Are there many/any options for an EL84 based line output stage? Or should I opt for a different output valve.

[kagliostro]

I'm not able to read correctly the tube labels on your schematic, which are the tubes ?

I've also (on the PC but now I'm on the cellphone) schematics that uses 5654/6ak5 tubes, if you are interested I'll give a search for it

Franco

[Jennings]

As stock the amplifier stages use EF86 pentodes, and the output valve is an EL84. I believe the meter is driven by an ECC83. I could alter these though. Happy to take any advise onboard

[kagliostro]

If you want to use the ef86 my council is to use the Mr Merlin approach to this tube (use it at a lower gain than possible) this make the ef86 a quieter tube and less prone to be microphonic

In some preamp circuit schematic you can find around (old ones) you can find that ef86 are used in triode connection as to be quieter

The 5654/6AK5 was specifically used in mic circuit because it is a quiet tube

Franco