adjusting gain?

[ThSpeakerDude88]

ok now that I have built some basic circuits with tubes...

how do I tweak the gain on a preamp tube?


currently I'm using the triode in a 14gt8, but the cuircit I have has hardly enough gain for a guitar. How do I change this?

below is my current circuit

forgot to mention , that un-named resistor on the plate is 100k

 

[Tom Bavis]

You're getting close to the maximum gain of the tube with 100K and a bypassed cathode resistor. For more gain, you'll need another stage.

 

[ThSpeakerDude88]

so this tube doesn't have enough gain for a guitar? I know it has a gain of 70, a 12ax7 is around 100, and a 12ay7 is about 40. I've heard of replacing ax7's with ay7's for less gain, so shouldn't I have enough to at least be able to run my guitar into it? Currently it has almost no gain... its about the same as running my cd player or guitar directly into the output tube.

 

[Jeb-D.]

Replace the plate resistor with a constant current source. You will get far more gain then passive loading it will.

 

[Tom Bavis]

The circuit as drawn should have a gain of 40-50 - what is it driving?

 

[ThSpeakerDude88]

a 50c5 right now..

how do I apply constant current sourcing to tubes?


I tried a 12au6 today instead of the 14gt8, it has enough gain to start to push the output tube, but still not enough drive. Only when I use an overdrive pedal , or booster which only boosts the volume, can I get it to break up nicley. Otherwise its not loud enough to distract a loud conversation.. much. It also stays mostly clean which I'm not quite sure about... I've never heard the real silvertone so I don't know how its supposed to sound but shouldn't it have enough gain to push the output tube to break up? I'm using the basic parts of the silvertone 1430 circuit ( not line operated of cource, with proper traqnsformer, and I have a seperate power supply for the filiments. )

 

[Jeb-D.]

You know those tubes you listed are nothing alike, eventhough they are all miniatures right? But all of those tubes are intended for a much higher supply voltage. Do you know how much peak voltage your guitar puts out? What kind of output tube are you trying to drive? If you let me know I may be able to help you with tube selection and your stage design. But off hand I think a 6DJ8 with a solid state active load or perhaps a choke load, would make the most of that 120V supply.

 

[ThSpeakerDude88]

I know their not, I'm just experimenting

I'm using it to drive a 50c5. I don't know what voltage my pickup puts out, I wonder if thery would show it on their website? I'm only using a single humbucker , its a dimarzio elemental.. In an acoustic guitar, but hey its all I have for now. Still sounds great , especially for blues.


I really don't have many tubes to work with.. just the 14gt8 , 3 12au6's and a 50c5. I really just want a 1 watt small practice amp. I'm eventually going to get a 12ax7 , but am unsure of what to use for the output stage. I would like to build a champ, but the price of the trannies is too much for right now. I was thinking along the lines of maybe a 12c5 or 6aq5 ( I think) and a 12ax7, solid state rectifier to keep it simple.

 

[Jeb-D.]

is this the schematic your going off of?

http://webpages.charter.net/dawill/tmoranwms/Elec_50C5SET.html

What are you using for the output transformer, and what kind of speaker are you using?

With that small of output power you should be using a very efficent driver around the neighborhood of 95dB/1W.

 

[Jeb-D.]

Also, after looking at curves. The 14gt8 driver biased as in the drawing should be capable of prducing enough output voltage to drive the 50c5 to it's potential as it is biased. Even with the the weakest of pickups being used as a signal source.

 

[cogsncogs]

so this tube doesn't have enough gain for a guitar?

As you may know guitar amps have a pre-amp stage as you are dealing with a low-level signal. You will need one more gain stage. One single (section) tube, no matter how configured, whether pentode or triode will have sufficent gain for your purpose.
Using both sections of a 12AX7 with a gain of 40x each section = 1600 which should be more than enough for some 'overdrive'.

But all of those tubes are intended for a much higher supply voltage.

Hmmm, 'cept for the 50C5, Max ->150V plate, 130V screen. They were used in AC operated table radios.

Cheers
Wayne

 

[Miles Prower]

a 50c5 right now..

how do I apply constant current sourcing to tubes?


I tried a 12au6 today instead of the 14gt8, it has enough gain to start to push the output tube, but still not enough drive. Only when I use an overdrive pedal , or booster which only boosts the volume, can I get it to break up nicley. Otherwise its not loud enough to distract a loud conversation.. much. It also stays mostly clean which I'm not quite sure about... I've never heard the real silvertone so I don't know how its supposed to sound but shouldn't it have enough gain to push the output tube to break up? I'm using the basic parts of the silvertone 1430 circuit ( not line operated of cource, with proper traqnsformer, and I have a seperate power supply for the filiments. )

Go Here for a preliminary design of a 50C5 amp I did. Pay attention to the first stage, which uses CCS loading over passive, resistance loading. In this case, as with yours, the main problem is getting enough current to drive the 50C5's Miller, input, and stray capacitance. Passive resistance loading with Rp= 100K, and a Vpp= 120Vdc isn't going to get the job done by any means. A CCS will allow for more driver plate current, and enough current to meet the "Rule of Five" for the 50C5 grids.

 

[cogsncogs]

Miles is correct, I was thinking gain only. 120V for the driver stage won't cut it using a resistor as the plate load. A CCS plate load (or a much higher B+) will solve that problem. But still it would be good idea to have a single stage pre with possibly a volume control between the the pre and the driver, i.e. the other half of the 12AX7.

Wayne