• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

what do you tube gurus think about this?

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Mohan Varkey

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Joined 2001
Topology seems to be OK. At the expense of incurring your wrath, I have to say that it is not my cup of tea.

I guess, the purpose of this preamp is for use in a band and when the band is not playing you would like to play CD or radio through your stage amp. Am I correct?

Why do you need to raise the cathode voltage on the input tube? I am referring to the 47K resistor. If you remove this resistor, then you may not need the 10nF input capacitor. I do not prefer cathode followers in general unless you want to run long cables from the preamp.

Mohan
 
G

GRollins

Member
Joined 2001
Micke,
I agree with both of the previous posts:
--Drop the cathode follower at the input. There are a number of reasons to use a follower, but for the life of me I can't think of one to justify the use of one as an input buffer. One at the output is fine. (Incidentally, I'd consider raising the 100k at the output to 1M--it's there to drain DC off the back end of the coupling cap at turn-on, which is fine, but it will also be in parallel with the Zin of the amp/crossover and constitutes an unnecessary load.)
--Omit the cap for the cathode. The gain will drop when you do so, but you'll not only avoid an electrolytic, you'll pick up a bit of local degeneration (aka feedback) which will reduce distortion. These days, you don't really need a lot of gain in a preamp.

Grey
 
Thatch_Ear

Thatch_Ear

Member
Joined 2001
Take that back....for stereo you need 1 ECC83 (1/2 per side) and 2 ECC82s (1 per side). The 7247 is 1/2 each. For a nice SE EL84 stereo amp design check out the Eico AF-4. I am building this with 1 12AX7 and 1 12AU7 instead of 2 7247s, leaving the tone controls out and adding a choke and larger caps in the power supply. Note that the signal goes through the 12AU7 side if the tube first. The copy I have of this is too large to post here.
 
H

hifi

Member
Joined 2001
I finaly made a board and build that darn thing...


Sorry for the poor quality of the pictures but ill post better ones when i get the casing done..

Btw the pcb:s size is eurocard (100x160mm)
 

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