High Voltage 6L6 SE

[TIMBO]

Hi guys, Would running a single 6L6 at approx. 450v too much for a single ended amp.
Using a SS rectifier.
http://www.mojotone.com/transformers/Fender/Power/MT-FENDER-MOJO760EX.pdf
OT
http://www.classictone.net/40-18031.pdf

[John]

Should be good. GE rates their 6L6GC for 500 at the plate, 450 at the screen. So does JJ with theirs.

[HotBluePlates]

Hi guys, Would running a single 6L6 at approx. 450v too much for a single ended amp.
Using a SS rectifier.
http://www.mojotone.com/transformers/Fender/Power/MT-FENDER-MOJO760EX.pdf
OT
http://www.classictone.net/40-18031.pdf

It should work no problem.

25w / 450v = ~56mA
400v / 56mA = ~7.1kΩ (8k oughta be fine)

Might as well cathode bias, and the voltage across the tube will be reduced by the amount of bias voltage dropped across the cathode resistor.  Ideally, you'd like to have a lower screen supply voltage so the bias voltage can be smaller, and therefore the 6L6 will be easier to drive.

[TIMBO]

Thanks HBP, I built this amp some time ago and the PT was not quite a match to the original circuit.
The original was a 320v no CT and a bridge rectifier.
The Mojotone was as close as I could get off the shelf.

[kagliostro]

Ciao TIMBO

can you explain the purpose of the 470K resistor ? Is there as to act as a grid leak resistor for V1a ?

Thanks

Franco

[HotBluePlates]

can you explain the purpose of the 470K resistor ? Is there as to act as a grid leak resistor for V1a ?

It's a grid leak/reference for the output tube, and an a.c. load for V1A.

You've got to tie the output tube grid to a voltage reference for it to have stable bias.  In this case, the voltage reference is 0v ground through the 470kΩ.  Grid current is extremely small, so no d.c. is developed across the 470kΩ and both ends are at 0vdc.

[kagliostro]

Ciao HotBluePlates

I was talking about the 470K resistor that is between V1b and V1a when the TS is excluded using the DPDT switch

I'm not sure 100% but seems you are talking about the 470K resistor that is between V1a and the Power Tube

Franco

[sluckey]

That 470K is the grid leak for V1A.

[HotBluePlates]

I was talking about the 470K resistor that is between V1b and V1a when the TS is excluded using the DPDT switch

I see...

When that section switches to have the 470kΩ to ground, the other section switches in a series 2200pF cap.  So V1a can't use the Volume pot as its grid reference (there's a blocking cap in the way).

[TIMBO]

Hi guys, You can follow the story here..........
http://www.guitargear.net.au/discussion/index.php?topic=47812.0

[kagliostro]

Ciao TIMBO

If I'm not wrong, seems that on the original schematics there isn't the 5K / 8K OT option that is present on your schematic ....

what do you expect from that commutation ?

Ooppss, I think I've understand, is there to swap between 6L6 & EL34 tubes

Franco

[Tone Junkie]

Hi Timbo hope you enjoy your new amp as much as Im enjoying mine. My voltages are a bit lower for the 6l6 but I will definitaly build another one. Went with the 5k output and lower voltages will try a 7k out put and higher voltages next time just to see the difference. The schematic you showed me was helpfull
Bill 

[TIMBO]

Hey Bill,
I like the sound of the Romany Plus,12" speaker and reverb.
I might try something different to the usual one tube reverb (12AX7 and coupling transformer)
Found this
http://el34world.com/Forum/index.php?action=dlattach;topic=17010.0;attach=42975;image
Just happened to have parts.

[TIMBO]

Hi guys, Some progress.

[shooter]

Nice!, did you do a copper etched pcb with turrets?

[TIMBO]

Hey shooter, This was the original project that I made this board for.
http://el34world.com/Forum/index.php?topic=10554.msg96816#msg96816
My method of board construction was limited to the materials I could get.
It seemed at the time a good idea.

[shooter]

It seemed at the time a good idea

works for me, I used to etch  my own boards for IC projects in my other life!

[TIMBO]

Hi guys, need a bit of direction. 
As I expected the voltages are higher than what the original Stinger would be.
This is making it difficult to bias the 6l6.
With the plate load of 8k I have increased the cathode resistor to 660r (560 and 100 resistors in series) and this brought the power output down to about 19w, with the original 560r output was 25w.
I increased the 47r resistor between the first two filter caps to 1k, I'm now down to about 16w.
Do I continue to increase the cathode resistor or increase the 1k resistor ???

[kagliostro]

Give a try to increase G2 resistor

if you decrease the G2 voltage also the power stay lower, see how TubeTown use his "VVR" (look also to the attached .pdf file)

https://www.tube-town.net/ttstore/Kits/Power-Supply/Kit-TT-VoCoM-Variable-Voltage-Regulator::5651.html

https://www.tube-town.net/cms/?DIY/Amp-Tools/VoCo_Variable_Voltage_Control

or

tame B+ vith a VVR (no pot, use it fixed using a trimmer)

Franco

[John]

Hi guys, need a bit of direction. 
As I expected the voltages are higher than what the original Stinger would be.
This is making it difficult to bias the 6l6.
With the plate load of 8k I have increased the cathode resistor to 660r (560 and 100 resistors in series) and this brought the power output down to about 19w, with the original 560r output was 25w.
I increased the 47r resistor between the first two filter caps to 1k, I'm now down to about 16w.
Do I continue to increase the cathode resistor or increase the 1k resistor ???

I would increase that 1K to something like 2.2K, see where that gets your B+, and then go from there. And like K says, you can increase the screen resistor as well. With a cathode bias SE though, your 6L6 can run at pretty much 100% at idle. I assume you're talking about plate dissipation with your 25W, 19W etc? That's very safe as far as I know.

[shooter]

This is making it difficult to bias the 6l6.

SE builds for me have always been a compromise between voltage, current, and resistance.  My KT88 settled in at 16W clean, 23ish full roar, 42W plate diss, no matter how I shuffled the B+ and cathode R.  try the suggestions you got, be happy with the sound and not so much the power, well unless you're melting tubes for the sound

[HotBluePlates]

... I have increased the cathode resistor to 660r (560 and 100 resistors in series) and this brought the power ... down to about 19w, with the original 560r ... was 25w. ...

Are you going to be using metal 6L6's or 6L6G's (not GB)?

If no, then you should be able to idle at 25w dissipation without issue.

Also, you can increase the power supply dropping resistance to knock down screen volts, but you may not want to make the series screen resistors larger.  Screen current at idle for the 6L6 is pretty small, so it would take high-valued resistors to really lower screen volts.  Meanwhile, you'd be adding power compression when you crank up the amp.

You could just tack in 100Ω or so between the rectifier and first filter, or raise the 47Ω resistor between caps to 100-200Ω.

[kagliostro]

Ciao TIMBO

a bit of espionage

Which is B+ at node A with that 330V AC transformer ?

and .....

Which are the measures of your board ? 

Thanks

Franco

[PRR]

> output was 25w.

Measured how?

You certainly did not have 25W of clean sine power output with one 6L6.

For 8K load, take cathode resistor in the 600-800 Ohm range. Then fiddle B+ dropper to, as Shooter says, find "a compromise between voltage, current, and resistance" and hit your safe plate dissipation.

When done, voltage *across* the tube, divided by plate current (~= cathode current on 6L6) should work out very near 8K, the load. +/-20% is all the same, +/-40% is not wrong.

[TIMBO]

Thanks PRR, I thinking that the 6L6GC I had, measured 25w may have been a bit dicky.

Put in another RCA 6L6GC and these are the results.......

Top left, circuit as per original schematic
Top right, cathode resistor increased
Bottom left dropping resistor between first two filter caps increased.
I used the weber bias calculator to calculate the output wattage.
http://www.tedweber.com/webervst/tubes1/calcbias.htm

[frus]

There's no output power in that calculator, as far as I can see...

[HotBluePlates]

Thanks PRR, I thinking that the 6L6GC I had, measured 25w may have been a bit dicky.

PRR was only getting at the fact you were calculating idle dissipation not audio power output.

This is also why I asked about which version of the 6L6 you would be using.  Assuming a suitable load, idling at/near 100% for the later types lets you use a smaller cathode resistor and (more importantly) a smaller bias voltage which implies the output tube is easier to drive to full output.

[TIMBO]

When done, voltage *across* the tube, divided by plate current (~= cathode current on 6L6) should work out very near 8K, the load. +/-20% is all the same, +/-40% is not wrong.
Thanks guys, with PRR's calculation all the variations fall within the 8K +/-20%