burned JLH fuse by connecting to Tube preamp

[rchung]

Hi all,

Looking for some help on fixing my JLH (or not repeating the mistake). I recently build a single stage triode preamp and it worked well driving my headphone so I want to check and see how it eould sound with my JLH (with current source). It instantly popped the fuse. Did I do something wrong ? Is that possible that the coupling cap of the pre-amp have a surge when I powered up the JLH (the preamp was powered on first). What is the recommended procedure of checking this out ? My thoughts are to gnd theinput and check out the biasing conditions for all stages with a current limited power supply.

Thanks.

Russell

 

[Graham Maynard]

Hi Russel,

Hard to diagnose at a distance.

If your jlh runs okay on its own then try shorting the input during switch-on.

If power up is then okay, but the fuse blows when the short is removed then suspect the pre-amp.
If not a dc problem, then it could be RF feedbcack which could be filtered at pre-amp output.

Over to you - - -

Cheers .............. Graham.

 

[rchung]

THanks Graham for your pointer. I found out the problem: one leg of the rectifier bridge shorted. I replaced the bridge and powered the preamp with a two 60V dc power supply ganged together to give 120V DC. That works fine with the JLH. I wondered if my ac power supply was not done properly. Is there some pre-caution I should have taken in coupling a tube preamp to JLH type solid state amp ? The only thing that separate the high dc voltage on the plate and teh JLH input is a cap. but I have a 400V 1 uF cap sitting at the very front of the JLH as well so one would think that is okay.

One question for Graham. I found a pre-amp designed by you on the internet. Any update on that design. I am looking for a line level pre-amp since I am feeding it from a CD player. Is there amy recommendation for a erally good sounding pre-amp ? I tried to use my sansui pre-amp but it definitely colored the naturalness of the JLH.

Thank you all for any feedback.


Russell

 

[Graham Maynard]

Hi Russell,

That circuit is a design project that has been labelled as an audio pre-amp. It does not really suit hi-fi usage.

Best look at
http://www.tcaas.btinternet.co.uk
or
http://www.dself.dsl.pipex.com/ampins/ampdept.htm
or
http://www.passdiy.com/amps.htm


Maybe if you put three 1N4148 diodes in series across your amplifier input, plus another three covering the opposite polarity, you could provide yourself with input overvoltage protection. Better to blow diodes than your amp from a charging or discharging tube pre-amp capacitor.

Cheers ............. Graham.