• 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.

6V6 line preamp

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#4,001
Lingwendil said:
Parts came in today! Sneak peek at the populated boards- going 6AQ5 for the octal/seven board, and 6p43p-e on the noval board. The wima output caps I ordered are huge compared to what I usually use, it's been a while since I used such a large film cap! Thankfully everything I ordered fit the boards perfectly.

In the next few days or so I hope to have a power supply together so I can test them, and go from there. I think that will be a new thread, with better pictures and more discussion.
Click to expand...

Wow thats so clean looking, nice! The FKP are a PP foil cap so larger than the metalized film version. That tall electro is a stage decoupler maybe?
 
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#4,005
I'm not dead I swear!

Lots of things going on- new supervisor position at work, long unpredictable hours,and a totaled family car has limited my free time. I will hope to get back to this soon. I need to put together a quick PSU and I will be able to do smoke testing and basic sound.
 
#4,012
I’ve been happily running my 6V6 preamp for several months now. Works great!
But there’s something that I wonder about. I am using (2) 10k 5W kiwame resistors in parallel as the original spec, but they get bloody hot! How much air flow do I need under the chassis to make sure things don’t cook? The chassis is on 20mm standoffs and much of it is open perf. Is that sufficient?

I turn it off after use, but I always think what happens if I forget to turn it off for a few days. Thoughts? Am I just being paranoid?

(Btw I’ve only ever touched the resistors which I know are at HV, once powered off, tested with a multimeter, also standing on one leg, on rubber, with left hand in back pocket)😎
 
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#4,014
I’m not near the amp now, but I’ve measured in the past and everything is at spec and normal voltage-wise. It’s been running for over a year, I don’t think anything is wrong. I’m just always a bit surprised at how hot they get, and wondering what kind of air flow is needed from under the chassis. They’re also spaced about 25mm away from the pcb, and nothing is brown from heat so I’m suspecting all is ok. But let’s say I forget to turn it off at night and it runs for a few days or a week. Are there designed to run continuously like that with no issue?

Might just be some diy ocd kicking in lol
 
#4,016
Thy can get hot, they normally burn 2-2.25W together in the standard circuit. How much hot depends on ambient temperature around their location also. It adds to their surface temperature rise for dissipated power. The "Kiwame" characteristics are: KOA SPR
 
#4,017
Thanks for the data sheet, I should have checked. Looks like they’re designed to get hot so looks like nothing to worry about. Chart shows 50 C surface temp at 25% per rating, which is appx where we are +ambient temp gets us to 70-80C. Yup, feels about right! Amazing materials in the lowly resistor.
 
#4,018
Do I need a grid leak resistor on the gain or CF version? Didn't see it in the original schematic.

I have a 100K pot, then a switch that goes either to the CF or gain version. Seems like when one is operating, and the other isn't then one grid is dangling perhaps.
Have another switch that goes from the gain version back to the input of the CF, in case I want to run one into the other. Not sure if that matters here.

Also I notice a tad bit of noise when the pot as 50% (on the gain version, not CF), and I've read on this forum the reasons why. Would a grid leak resistor help? And also, is there that same mid-point slight noise issue with a stepped attenuator?

Can't tell you how much I've learned from this little project!
 
#4,019
Since you switch, better not let one grid dangle.

Pot at 50% is at its max series insertion impedance (1/4 its nominal value). Combine the tube's input capacitance and you get least bandwidth at that point.

RMS noise in a system increases with bandwidth, just press the 20MHz button on your scope and see its line getting thinner vs full bandwidth as an example.

So no, it doesn't seem to compute creating noise due to volume mid position alone in this case.
 
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#4,020
What seems more plausible a scenario is the input circuit needs some kind of investigation about interference since at the pot's max impedance point should be more susceptible.

Or simply compare noise behavior with a smaller value pot.
 
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