Extremely low power first amp problem

[m0rg]

Hello!
Firstly appologies, this is a question about a <gasp> hybrid amp not an all tube amp. I'm just starting out learning tube amps so I figured if I vaguely new what I was doing with half the circuit...

Anyhows, I'm trying to build an extremely low power Single ended EL84 amp to get a bit of soft distortion for guitar. I've recovered the output transformer and the tube and tube socket off someones homebrew amp that went wrong, and have based my circuit on the LXH2 extremely low power tube amp http://home3.netcarrier.com/~lxh2/lowpsch.html Except I've had to convert it to single ended without knowing what I'm doing. I kind of borrowed some bits of the Vox AC4 power stage to help me out, and have ended up with an amp that sounds like it getting some nice power tube distortion, but with a horrible fuzz as well.
I've done some tests and found the horrible fuzz is on pin 2 of the el84 and can't trace it any further. The schematic follows, tube heaters are being run of 6VAC transformer, power for B+ and Op-Amps is supplied by a regulated and smoothed +/-12V 50VA transformer.
http://www.bath.ac.uk/~ee1pa/LowPower1.txt (tube pins are put in brackets, and various voltages I thought might be useful are put in)

Sorry about the newbie question, but I'm stumped, and can't find any circuits that are this low power on the net to compare with.

[fragman56]

I think pin 2 for a 6bq5 (el 84) is the grid or input signal pin. So, if that is where the fuzz is coming in it looks like you need to go up to the chips.

I guess you have ruled out power supply problems? Sometimes too much ripple and not enough filtering occurs with single-ended stuff. that is a low B+, too.

I don't know anything about solid state stuff and not much more about tubes but you might try to install a capacitor in series and a grid resistor to ground between the IC and the tube on pin 2. This would cut out any DC getting to the grid from the IC and give the tube a grid leak pathway.

I'm sure someone here can help--they always do!

Rick

[Tube_Dude]

Hi mOrg!

Are you powering the EL84 with 24 volts DC rail voltage??

[m0rg]

Are you powering the EL84 with 24 volts DC rail voltage?? [/B][/QUOTE]

Yup, sure am! Its a pretty low power amp (what I'm looking for is a nice valve distortion that isn't going to offend my neighbours, and can possibly be amplified later...
Are there any problems with having such a low B+ (apart from low volumes..)?

Pete

[m0rg]

Originally posted by fragman56
I think pin 2 for a 6bq5 (el 84) is the grid or input signal pin. So, if that is where the fuzz is coming in it looks like you need to go up to the chips.

Its the input pin yup (I hope anyway ) its getting the noise whether the inputs connected or not

[i]
I guess you have ruled out power supply problems? Sometimes too much ripple and not enough filtering occurs with single-ended stuff. that is a low B+, too.
[/B]

I've sadly not got an oscilloscope here so can't check the power supply hyper accurately, but I listened for noise on it and couldn't hear any, and checked it with a dmm and there was no AC according to that, and there should be headroom in the it (+/-15V transformer, solid state regulators, fairly big caps)
The noise I'm getting doesn't sound like its any relation of 50Hz, its much higher than that more like white noise.

[i]
I don't know anything about solid state stuff and not much more about tubes but you might try to install a capacitor in series and a grid resistor to ground between the IC and the tube on pin 2. This would cut out any DC getting to the grid from the IC and give the tube a grid leak pathway.

I'm sure someone here can help--they always do!

Rick [/B]

I'll try that when I get home-Thanks!!!
Pete

Ps Soz about the last post-I think I've sorted this quoting thing this time (and also using the preview pane.. useful feature if ever there was one)

[Jax]

Try replacing the 100k resistor between the last opamp and the tube grid with a capacitor, some 0.1 to 1 uF.

Connect a 470k resistor between the tube pin 2 and -12V.

You may have to fiddle around with the 150 ohm resistor as well, try lowering it.

You should get some sound even with only 24V over the tube.

[Tube_Dude]

Originally posted by Jax
Try replacing the 100k resistor between the last opamp and the tube grid with a capacitor, some 0.1 to 1 uF.


You should get some sound even with only 24V over the tube.

Hi Jax!

The tube is with 0 volts at the grid ..so it must have more than 100 mA anode current for to get out from this positive grid situation.
A better solution is connect the lower part of the cathode resistor to ground and increase the positive rail...

[Jax]

Read all of my post

The cathode resistor is connected to -12V so that is the point where a grid leak resistor should go to.

[Tube_Dude]

Originally posted by Jax
Read all of my post

The cathode resistor is connected to -12V so that is the point where a grid leak resistor should go to.

I see!!

But connecting to ground the cathode resistor ...it can be direct coupled...

But maybe this is not important for a soft distortion generator..

[m0rg]

Just a little reply to say thanks so much guys, its working fantastic now! I would never have got that without far to much searching on the net and many frustrated nights I guess..

I'm guessing I'll be back soon with more problems as the circuits get more complicated, but I'll be lurking for a while now and trying to learn as much as possible

Thankyou!
Pete