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DC in Pre-amp output

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I have recently built a two stage tube preamp (anode follower + cathode follower)using 12AY7 tubes. Upon its completion, I connected to one of my power amp (Summo 9+), and it fuctioned fine and sounded very good. However, when I connect this preamp to my other power-amp (Classe C200), the protective circuit was tripped. I have observed the practice of waiting for about 1 minute before turning on the power-amp but in this case, it did not help. Using my multimeter, I measured the DC in the output jack and recorded wide flucuation voltage (between 150Mv to about 0.1Mv). Having only the skill to built based on circuit, I can not understand what causes the problem that affact one power amp but not the other. I thought the capacitor before the output will block all DC. Any help will be appreciated.
Certainly, such variation of DC level at the output of the preamplifier denotes a problem with one or more components, like capacitor leakage, resistor instability... Another possibility is insufficient regulation of the power supply.

Try to replace the output capacitor of the preamp with lower value, 0.1 uF for instance... or connect a resistor across the output terminals of the preamp (100 k)

Regards, P.Lacombe

[Edited by P.Lacombe on 09-03-2001 at 04:30 PM]
since you use a simple circuit, it will be simple to fix..
I think it is a bad output cap, try a new one.. and try a lower value grouding resistor after this new cap that is connected to your cathode folower .. like 47KOhm..

what are your component values?


I'm a bit concerned about the fact that the voltage is varying, instead of remaining constant. If switching out the DC blocking cap at the output of the preamp doesn't cure the problem, you might look for an oscillation. I don't know what kind of front end Classe uses these days, but it's possible that it's the difference in the front end of the amps that causes an oscillation, which in turn trips the protection circuit.

Thank you all.

I will try changing caps and report back results. Currently, it is a 0.47uF with a 1M resistor.

Further thoughts, I am wondering the problem might be caused by some other area of the circuit or my error in construction. Here are some more information about the circuit: The first stage is direct coupled to the cathode follower, which grid is sitting at 139V. B+ is approx 210V. The cathode resistor is 22K 5W with bias of 2V. I have noticed the cathode resistor runs very hot. Almost too hot to touch meaning may be the stage is drawing too much current.
I could discard the circuit and try to build another one. But I thought with some help, I can learn more if I stick with it.

Thanks again.
If I'm understanding your description of the circuit, the cathode resistor is running at a dissipation of somewhat less than a watt--you should be okay if you're using a 5W resistor. If it's a ceramic resistor, yes it will run hot to the touch. That's normal. For that matter, even if the tube were treated as a dead short, a 22k 5W resistor could handle the full rail of 210V with a dissipation of about 2W, well within spec. Just make sure the resistor has adequate ventillation, perhaps even to the point of drilling holes in the circuit board around it, assuming that you're using a PCB and not point-to-point wiring.
If you have an oscilloscope, you might try looking at the output of the preamp, both with and without an amp hooked to it.
If you have a feedback loop, you might try disconnecting it (not something solid state people should try!) to see if you've got an oscillation there.
You might also want to try decoupling the rail for each tube by giving each tube its own cap, after a resistor to the main power supply. It's possible that the circuit may be motor boating through the power supply.

thijs, P. Lacombe, and Grey:

I have changed to new caps, changed the resistor value but still have the same problem. the only idea i have not yet tried is decouple the 2nd stage as suggested by Grey. I will try it next week. (need to work to get money to buy parts!!.

May be I should dismentle the whole thing and rewire it again and incorporate a regulator for the HT. Right now, I am using a 6X4 followed by a simple C-R-C filter (total capacity is 240uF). I am thinking about a 6BM8 or using solid state regulator (317t). I will need to work out the component values though.

One thing i am not certain is: am i looking at the right problem? I still have serveral preamps on hand that are built previously based on circuits published in mags and most of them have the similiar occurence in the output. ie flucuation dc. The only exception is the line amp based on Morgan Jones book. This one have a regulated B+ and I am using it with the Classe power amp with no problem. DC in output is measured at 0.1mV and quite steady. By the way, all these preamp work with my Summo.

My question then is: Is the Classe over sensitive in tripping the protective circuit? Should there by any DC in the output jack?
DC in Tube preamp...

Tube preamps does have DC in their output upon power on. Just wait for about a minute of 2 before you operate them. I huv used lots of tube preamps and they huv muting circuits incorporated...

Try putting a delay circuit at the output stage of the preamp and add a muting circuit. This will enable the tubes to stabilize the filament or heater.

Use capacitor with very low DC leakage.

TUBE PREAMP - SOLID STATE AMP combination... has this problem of triggering the protection circuitof your amp. And if your amp has no protection circuitry, It will burn your loudspeaker voice coils.

All tube set up( tube preamp - tube ampcombination), will not encounter problem wid DC bcoz of the output trans4mer blocking the DC voltage and also those siganal capacitors at the predriver n driver stage of the tube amp.

I agree with one of the previous posts,the worrying thing is that the dc is fluctuating from .1mv to 150mv. this points to some instability in the circuit, could be down to the layout. try putting a shorted jack into the input of the preamp, and then take a meter reading on the output after the cap, and see if the small amount of dc voltage is still there.
i have yet to find a perfect capacitor that dosent have any dc leakage. the best capacitor is no capacitor. but with valve amp circuits you dont often have a choice.
also, try testing each stage of the preamp to see if you can find which part the instability originates from. this will only help if the over-all feedback loop, [if it has one] is disconnected.
good luck, v-man.;)
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