My First Tube Preamp

[macdelf]

My heater voltage checks out fine. I think the output cap theory is correct. I will first go to a regulated B+ supply, and if I still have a problem I will reduce the capacitance.

 

[Einric]

I used the Obbligato Gold 1uf for my output capacitor and it works very well.

 

[cjkpkg]

1uF is the max recommended for coupling cap. Somewhere George(Tubelab) wrote a terrific post on why that is. IIRC it had to do with the corner frequency, saturation of the cap itself and time it takes to "recover" if I am describing it correctly. Also I think he went into why guitar amps tend to be 0.22uF or less and why hi-fi amps tend to be between 0.22 and 1.0uF.

I will try and dig it up...

JACKPOT!!!

 

[macdelf]

Thank You for all the help. I think the post is in:

http://www.diyaudio.com/forums/tubelab/177194-tubelab-se-coupling-capacitors.html

I will now reduce my output cap to 1uF. I expect all will be well.

 

[macdelf]

All is well 1uF reduced the fluctuations to less than 0.2 mv under the high load impedance of my volt meter. I have also learned a new word "bias drift". Tubes are different but cool.

 

[cjkpkg]

Indeed, after starting this hobby about 5 years ago I am still a complete noob but know enough to be dangerous!!!

Someone here has a cool signature..."Real amps glow in the dark"...

 

[Einric]

Alright, I'm going to change my input tubes from 6CG7's to 6DJ8's, I need a little more gain.
The manual shows the R10 value but that looks to be in the output section.
If I look at the input section the only value that looks to change would be the R3 & R4 but the voltage suggested for R3 & R4 is at 150V.
I am running @ 290V w/ 7.3 mA to my 6CG7's, @ 300V the chart shows 8mA to the 6DJ8 but that is set by R10 which is in the output section.

Will my higher voltage (290 vs 150) @ the R3&4 change the values needed for R3&4?
Should I just plug them in and hope for the best?

There doesn't seem to be much listed for changes in the input tubes.
The short of it is that I want to run a 6DJ8 @ 290V on the input of my glassware Aikido 9-Pin All in One.

Thanks

 

[Einric]

SO.....for 6CG7 R10 @ 300V it shows about 83kR & 6DJ8 @ 300V it shows 87kR.
Since I'm mixing tubes should I just shoot for half?.....Say about 84.5kR?
Or does R10 only have something to do with the output tube (which is staying 6CG7) and I should leave it alone?

 

[cjkpkg]

I would leave it and see if it behaves. more important to get the cathode resistors right.

 

[cjkpkg]

also you may want to consider 6n1p for the higher voltage... you must pay attention to the plate to cathode voltage very carefully for 6dj8 especially at higher voltages.

 

[Einric]

OK well.... I have already removed my 870R R3&R4 resistors and destroyed 2 of them in the process.
I will swap them out for my 470's that I have pulled or should I go 1k instead?

BTW my loaded B+ is 284V.

 

[Einric]

The manual shows Cathode R value for 6DJ8 between 100-330R but it doesn't say which R it is.
Pin 8 & 3 are cathode correct? That would mean R3&4 are the cathode resistors correct?
Should I go for somewhere around 200R then for R3&4?

 

[Einric]

I have a jumper in R22 just FYI.

 

[cjkpkg]

I would stick with 330 and measure.

 

[Einric]

I have 340's, I will use them.
Measure what exactly?

 

[cjkpkg]

The current by measuring the voltage across the cathode resistors. I=V/R

R=340
V=?
I= let's assume 10 ma

So you want about 3.4v dropped across the cathode resistor.

 

[cjkpkg]

check out this post for an explanation of heater to cathode voltage.

But with 300v b+ and heaters at b+/4 or 75v above ground you will probably be OK.

You might want to consider trying to drop b+ to 250 or so.

 

[Einric]

I found some 330's, and I measure 2.74V +- a few mills across all r3's &4's

 

[Einric]

So that looks like 8.4mA since I matched my 330's @ about 326.

 

[cjkpkg]

OK so you are getting around 8...that's good. Now measure the voltage difference from heater negative and pin 3 and pin 8.