6418 valve booster/drive/preamp PSU help!

[MissionBrown]

Hi folks,

I bought a kit off ebay consisting of 1 tube 3 resistors and 2 caps powered by a pair of 9v's and a single AAA.
Kit works just fine on batteries but I want to run it all off an 18v PSU.

I thought doing it via a voltage divider might work but experimenting on The breadboard produces zip voltage when connecting the filament.

The stock circuit(Designed by P. Woodland, have file but can't find a link) comprises of

Input is .1uf Cap, 1m resistor to ground and in on pin four of the valve.
Filament is 1 x AAA between pins 3 & 5 with an 18 ohm resistor in series. -V of 18v is connected at junction of pin 5 and the AAA cell(this is also circuit ground).
Triode mode Output is pin 1 & 2 joined at the junction of a .1uf cap to output and a 100k resistor to +V
Pentode mode is as above but connect pin 2 to +V

The voltage divider I used consisted of an 18k and 1.2k resistor, was getting about 17v over the 18k and 1.15v over the 1.2k
Like I said, the 1.2k when connected to the circuit in place of the AAA battery measures nearly 0v and looks to me like it might be shorting somewhere.
Could it be the filament resistance being too low?
Thinking about it now suggests to me that this might be the case, should I connect the filament in series with the voltage/resistance path?
Pretty tired right now but I think I connected it in parallel with the 1.2k resistor.
http://tubedata.itchurch.org/sheets/127/6/6418.pdf

Thanks to anyone who could help!

[MikeH]

I don't suppose you have a link to a schematic, or you could sketch one out yourself?  I'm having a hard time visualizing...

[Renegadrian]

I uploaded it some time ago...Here it is
http://www.aronnelson.com/gallery/main.php/v/Renegadrian/6418+Tube+Preamp_001.pdf.html

How's that circuit btw?! What can you expect from it?! boost, overdrive, what!?

[MissionBrown]

Yup that's the circuit, couldn't find in in all my googling yesterday.

I measured the filament Resistance and it's 50 ohms, I think I answered my problem in my original post.
The filament needs to be in series between the 1.2k resistor(pin 3) and the 18k resistor(pin 5).
Connecting that resistor as a battery was the wrong way to go about it.

Will experiment more when I get home from work.

As far as sound goes, in triode mode a single valve makes a nice clean booster with a lot of output.
The diagram & explanation uploaded is the same one that came with my kit.
The bit about pentode giving lots of distortion is kind of a lie. It will distort but not with just the one valve(the gain and frequency brightness changes).
I found cascading two triodes into a pentode works best for creating a bit a gritty overdrive.
Not a lot of sustain but definitely a lot of bark and growl.

I put in a switch to shift pin 2 either side of the 100k resistor to change pentode & triode mode easily.
I also put a switch on the ground end of the 1m resistor to switch between the negative side of the 1.5v battery and true ground.
Doesn't do much in triode mode but changes the way the circuit clips in pentode mode.

And drawing on my experience building single transistor boosters I put a 1m resistor between pins 1 and 4 to get some negative feedback going.
Any higher and there's no change in sound but at 1m it gives that little bit of extra distortion. Lower values and shorting didn't produce an oscillation(I hoped it would).

I found the sweet spot for the circuit is at 18v, 9v was poor headroom across the board and 30v created an ugly compression.

At the end of the triode stages I used a 250k pot to control the gain to the next stage(Configured like and LPB1), but on the last valve in the chain I used a 100k pot.
When wide open it will cause buffered buffered pedals to clip but not in a nice musical way, more like crappy digital peaks on a computer.
Turn it down a bit and it plays nice with way more volume than your guitar could provide on it's own.

This is beneficial when driving the front end of an epiphone valve junior as it caused the 12ax7 to saturate and will give satisfying distortion even as the lowest volume.

I have plans to make a buffered fx loop with blend control from a pair of these suckers, a reverb recovery circuit for my accutronics tanks and also use as a clean boost to cause some of my low HFE soviet era Ge transistors to clip(which they do readily when configured in a fuzz face type circuit).

[maarten]

The control grid (connected to the input) should be negative with respect to the cathode; that is why you see two batteries employed in Renegadrian's schematic. Your voltage divider probably will not work this way. Let me know if your experiments don't work out, next week I'll probably have more time to dig out the notes I've somewhere on my experiments, I remember I tried to find a solution for this second supply source, but at the moment I do not recollect what I did. I do remember though that I planned to do some more experimenting with this tube as it sounded very promising to me, but I haven't had the time yet...

Maarten

[MissionBrown]

The control grid (connected to the input) should be negative with respect to the cathode; that is why you see two batteries employed in Renegadrian's schematic. Your voltage divider probably will not work this way. Let me know if your experiments don't work out, next week I'll probably have more time to dig out the notes I've somewhere on my experiments, I remember I tried to find a solution for this second supply source, but at the moment I do not recollect what I did. I do remember though that I planned to do some more experimenting with this tube as it sounded very promising to me, but I haven't had the time yet...

Maarten

Yup, attempt 2 on the voltage divider didn't do it.
9v A negative > 1.2k > Filament > 18k > 9v B positive.
I did get the right voltage over the filament, but the voltage at pin 1 was 18v when normally I was measuring around 8v with the 3 battery config.

Anyone think using a voltage regulator might do the trick?
I'm not really familiar with working on valves so this is all a bit new to me.
On paper the look like they're easily substituted for transistors, but it doesn't appear to be the case 

I don't mind using batteries, but they sure do take up a bunch of room in the pedal case.

As you said these tubes do have promise to them, even as simple buffers.
I'm one of those folk that prefer a decent buffer to true bypass and these valves have a great clean tone, though my preference is for 0.2uf caps on the in and out.

[MissionBrown]

Hmm... I'm at an impasse.
My breadboard of the circuit (powered by batteries) worked just fine with two triode preamps feeding a 3rd which is swtichable between triode/pentode configs.
Now that it's soldered onto protoboard, I've encountered a problem with the last preamp when it's running as a pentode.
What happens is that when a chord is struck then released there is a swell of background noise which quickly fades.

I think the issue is with the way I'm powereing it, but that didn't matter on the breadboard.
With exception of the in/out's of the preamps they're configured in parralell, with the filament resistance bing about 26 ohms (One by itself is around 50 ohms).

The configuration is identical to the way I'd had it on the breadboard, so I think maybe it could be my batteries being a bit run down (They're on th charger at the moment).
Or could it be a bad cap? Maybe the caps I used are too large? The caps I used were .22uf as I'd run out of .1uf

Output pentode/Triode measures 8v at pin 1
Middle triode measures 9v at pin 1
Input triode measures 14v at pin 1

This has me stumped at the moment.

[PaulBass]

I'm building this same kit and still can't get a signal.   I am newbie to tube preamps and can't figure out where each pinout goes. I was using this schematic that shows the red dot at Pin 1. I connected pins 1 & 2 together and connected that to the 100K. the tricky part is pins 3, 4, & 5 and I can't figure it out from the Raytheon 6418 datasheet. I'm trying to find a resource on the web on how to read a tube schematic. can somebody please sketch out a simple diagram of the schematic that shows where each pin goes? Then I can compare the sketch to the schematic and know how to build future tube preamps. then I'll share the results and knowledge to other newbies. TIA! 

[PaulBass]

Here is the text that came with the schematic above:

"This Preamplifier is based on a Military grade Raytheon 6418 Pentode valve. Power is supplied by one 9 volt battery and one 1.5 Volt battery. The wires on the Valve are, 1 anode, 2 Screen Grid, 3 Filament, 4 Input Grid, 5 Filament. Pin 1 has a Red dot for indication. Pin 5 on the valve is the common ground connection. All input and output shields from cables connect to this point. To turn the preamp off just disconnect the 1.5 Volt battery. The 9 Volt battery has no current when the 1.5 Volt supply is disconnected. Battery life should be greater than 100 hours. The circuit below shows the Preamplifier connected as a Triode circuit. This is used for Microphone, guitar, and low level audio enhancement.
To use the Preamp as a Pentode circuit, that is to get lots of guitar overdrive, then disconnect the wire between pin 1 and pin 2 on the Valve and connect pin 2 to the 9 Volt positive lead. Then connect the output capacitor to Pin 1. If you want more gain you can add more 9 volt batteries in series, up to 5 to get 45 volts. You can experiment with this circuit but never connect pin 3 and 5 to any more voltage as it will burn out the filament."

[PaulBass]

I got it to work and it sounds GREAT!  lots of great clean boost at 27 volts

[PRR]

> Could it be the filament resistance being too low?

What IS that filament resistance?

1.25V. 10mA, which is 0.01A.

1.25V/0.01A= 125 ohms.

> I measured the filament Resistance and it's 50 ohms

Cold. Filament resistance is 3 to 10 times higher when red to white hot. Hmmmm,,, you have "only" 2.5 times change cold-hot, this filament runs cooler than most, which makes sense for a hearing-aid tube.

> voltage divider I used consisted of an 18k and 1.2k resistor

A voltage divider will only give the calculated ratio when there is NO load on it. In practical cases, when the load is much-much higher than the resistors. For 18K+1.2K, I would load with over 100K, or maybe 10K and expect sag, or 1K and heavy sag. But 50 ohm load on an 18K+1.2K divider will sag it "to nothing" (0.05V).

The filament wants 1.25V at 10mA. You have 18V. You want a resistor to waste (18V-1.25V) or 16.75V at 10mA. 1,625 ohms. 1.6K will work. 1.5K will lean to 11mA, which is probably acceptable.

BUT there is another problem. The heater MUST run a certain way to get proper grid-cathode(filament) bias. Usually a small-tube filament pins are marked + and -. And the Tung-Sol datasheet seems to show both filament leads as "+", a typo. I found a Raytheon 6419 sheet showing what I assume is the conventional polarity.

> shows where each pin goes?

You have pin numbers. Why do you need to understand....

Oh, because you are modding the power AND the plan in this thread is missing any ground.

Do this:

Expect -roughly- those voltages. Close is fine if it works. If it don't work and one of those voltages is way-whack, you got a Clue.

[PaulBass]

I'm a total noob to tube preamp assemblies. this is my first one. I was following somebody else's method on another forum that was wrong. he had the ground pin wired incorrectly. now I got it and it sounds great!

> shows where each pin goes?

You have pin numbers. Why do you need to understand....

Oh, because you are modding the power AND the plan in this thread is missing any ground.

Do this:

Expect -roughly- those voltages. Close is fine if it works. If it don't work and one of those voltages is way-whack, you got a Clue.

[chris2002rocklin]

Hey gang, what is the status here?

I have a story about this kit, and I think this is a case of "the emperor's new suit". Nobody wants to say they can't get it to work. Well, you and I are the exceptions.

The schematic is not correct. If you look at his narrative that accompanies the schematic, it does not match the image.

I asked for a correct schematic and he admitted it was bogus but then kept saying he hadn't got around to fixing it.

I think what happened was the guy who sold them last year did not know what he had. He took it on faith that it was legit, but he could not support the project.

So maybe this new guy (it is a new account with ebay, a different username). Maybe this guy is legit, but the schematic is the same IIRC.

Let me know if you guys need anything from me, I found an excellent kit that costs close to $30 but has 2 pentode tube stages and a mosfet stage. I plan to also build a higher voltage version than this kit, which runs on a single 9V. I'd like to run 18V to the plates (or more).

[MissionBrown]

Not sure what you mean. Mine works. The claim on the schem is bs from a single valve perspective, but I got it going sweet on my breadboard. Need to revisit my last board as I think I made a mistake when transposing it to a permanent circuit.

[PaulBass]

this kit can run on 45 volts. as you add 9 volt batteries the gain increases by 6 dbs. I have it running at 27 volts for 18 dbs of clean gain. the sound is awesome. that 2 pentode + mosfet sounds interesting but I can't read a schematic to build it

Hey gang, what is the status here?

I have a story about this kit, and I think this is a case of "the emperor's new suit". Nobody wants to say they can't get it to work. Well, you and I are the exceptions.

The schematic is not correct. If you look at his narrative that accompanies the schematic, it does not match the image.

I asked for a correct schematic and he admitted it was bogus but then kept saying he hadn't got around to fixing it.

I think what happened was the guy who sold them last year did not know what he had. He took it on faith that it was legit, but he could not support the project.

So maybe this new guy (it is a new account with ebay, a different username). Maybe this guy is legit, but the schematic is the same IIRC.

Let me know if you guys need anything from me, I found an excellent kit that costs close to $30 but has 2 pentode tube stages and a mosfet stage. I plan to also build a higher voltage version than this kit, which runs on a single 9V. I'd like to run 18V to the plates (or more).

[MissionBrown]

Not sure what you mean. Mine works. The claim on the schem is bs from a single valve perspective, but I got it going sweet on my breadboard. Need to revisit my last board as I think I made a mistake when transposing it to a permanent circuit.

I suppose I should expand on the BS bit.
What I meant to say was that the pentode mode doesn't distort with one valve on 9, 18 or 27v that I tried.

On 18v it gave my valve jr a nice boost on the front end causing it to clip the 12ax7 as extremely low volume.

[PaulBass]

can a pot be substituted for one of the resistors to act as a gain control? If not, can a pot be added to the circuit to control the gain?

[PaulBass]

can a pot be substituted for one of the resistors to act as a gain control? If not, can a pot be added to the circuit to control the gain?

forgot to follow up.. added a 100K pot at the output. sounds great! 

[dorrisant]

Do you have a final schematic?

[PaulBass]

Do you have a final schematic?

it's exactly like the schematic above (not the hand drawn one) with these changes: supply voltage - 27 volts, output cap - .22uF metal film, 100K volume pot AFTER the output cap (1 - input, 2 - output jack, 3 - ground)