help designing 6L6G S.E. amplifier

[pilovis]

I am designing a stereo tube S.E. amplifier.

This amp uses one 6FQ7 (driver) and two 6L6G (triode connection).

I am looking for help, comments, suggestions.

Thank's for your help.

Maurizio




Early prototype:

[Eli Duttman]

6L6s need a fair amount of drive voltage. I don't think a single 'FQ7 section is going to get the job done, unless a line stage with gain is employed.

Study of the 6L6 data sheet might be beneficial.

A nice thing about the 6L6 is that it is safe to use in a positive g1 current configuration. Class "A2" operation will let you squeeze more power out of triode mode. Check out Tubelab's PowerDrive

[TheGimp]

Given the drive requirement of 20V peak in class A triode operation I think the 6FQ7 is doable but on the edge.

Drive = 20V peak

Given a mu of 20, and fully bypassed cathode resistor one can expect a gain of around Av = (mu * Ra) / (Ra + ra) with Ra=27K and ra = 7000 (guestimation from data sheet) gives Av= 15.88

So it should be acceptable as long as his input device can provide 1Vrms drive or better.

The low plate resistance of the 6FQ7 should allow for some entry into A2 operation given that his drive input is sufficient to achieve A2.

I'd look at the 100K grid resistor and see if it can be increased which will allow for a smaller coupling cap value. (cathode bias up to 500K)
http://scottbecker.net/tube/sheets/127/6/6L6GC.pdf


I recommend dropping the first 400uF cap to a lower value like 100uF and increasing the second cap value to 330uF or better. This will increase your conduction angel of the rectifier bridge and improve efficiency. It will also give a better filter for the 120Hz noise you need to suppress in a SE design.

Use low noise diodes and bypass with either 10nF caps or 10nF caps with 4.7r series resistors. Do not use 1N4007 without bypass snubbers and or ferrite beads.

I presume he is requesting help with the TBD resistor values. If you want 250V on the plate at 40mA (wrong cathode resistor, but data sheet bogie) you need around 625 ohms for the resistor between the first and second filter caps. This assumes you only get 1.25* 220VAC as the rms voltage at the first cap.Pd = 25*.04=1W so use a 3W or so resistor.

250-180V = 70V drop across the dropping resistor to the input tube anode resistor. Given you want 50% voltage drop in the anode resistor this gives 90V, dividing by the anode resistor value gives 3.33ma bias current (haven't checked the anode resistor value vs bias).

70V at 3.3mA is 21K.

Pd = 2.3W so use a 5W resistor.

[chrish]

This data sheet may be helpful http://www.retrovox.com.au/STC807.pdf The 807 and 6L6 are from the same family of tubes, so the data will be valid.

[TheGimp]

Using a 470K grid resistor you can drop to a .056uF coupling cap and still achieve a 6Hz -f3. 0.022uF and 470K would be 31.6Hz.

[pilovis]

Thank you for your suggestions!

[cerrem]

Originally Posted by TheGimp

Using a 470K grid resistor you can drop to a .056uF coupling cap and still achieve a 6Hz -f3. 0.022uF and 470K would be 31.6Hz.

Did you take into account source impedance... the 27k plate resistance ??

Chris

[pilovis]

Based on your suggestions, this is the modified schematic:



Note: I increased the plate voltage of 6FQ7 and I removed the input coupling caps.

Any other improvement, suggestion?

Thank's.

Maurizio

[TheGimp]

Originally Posted by cerrem

Did you take into account source impedance... the 27k plate resistance ??

Chris

No, I did not, but if you want to do that combine with the anode impedance for full analysis and you get:

f = 1/(2*pi*(470K + (27K||ra))*0.022uF)
27K||ra ~= 27K||7700 = 5.99K
f=1/(2*pi*(470K + 5.99)*0.022uF)
f=15.198Hz

I made a mistake in the original calculation, it should have been 15.39Hz, not 31.6Hz.

15.39Hz vs 15.198Hz is noise compared to the tolerances of the components.

The difference due to the anode impedance is negligible.

[dirkwright]

Originally Posted by Eli Duttman

6L6s need a fair amount of drive voltage. I don't think a single 'FQ7 section is going to get the job done, unless a line stage with gain is employed.

Study of the 6L6 data sheet might be beneficial.

A nice thing about the 6L6 is that it is safe to use in a positive g1 current configuration. Class "A2" operation will let you squeeze more power out of triode mode. Check out Tubelab's PowerDrive

Replace the 27Kohm load resistors with constant current sources and the triode will have more gain.