el34 8ma bias issue

[Pochie45566]

Hey, so i have this 2204. I put two el34's into it but one tube is getting a voltage drop of 1.2v from center tap to plate (which is good) and the other is pulling 300mv. So I switched the tubes out for ones I know are good, and now each tube is giving me a voltage drop of about 300mv. With a plate voltage of around 460, i need about a 35ma bias. So my resistance from center tap to plate is 38 ohms, voltage drop is 300ma, thats giving me about 8ma of current on those tubes and well needless to say it sounds like shit aha. Ive checked voltages on all the pins and they check out ok per the voltage chart. Where do I go from here?

[sluckey]

You can start by telling us exactly what your voltage measurements are for all the tube pins.

[Pochie45566]

You can start by telling us exactly what your voltage measurements are for all the tube pins.

v4:
1:0
2: heater
3: 457
4: 461
5:-39
6: no connection
7: heater
8: 0

v5:
1: 0
2: heater
3: 460
4: 462
5: -39
6: no connection
7: heater
8: 0

[sluckey]

Those voltages look fairly good to me. How does the amp sound?

[Pochie45566]

Those voltages look fairly good to me. How does the amp sound?

muddy as hell. dark, yet shrill. distorts easy.

[sluckey]

Put a 1Ω resistor between cathode and ground of each tube. Measure the voltage on the cathode of each tube. What have you?

[Pochie45566]

Put a 1Ω resistor between cathode and ground of each tube. Measure the voltage on the cathode of each tube. What have you?

Didn’t I already measure 0 volts on each cathode? I’ve never biased this way, sorry. Store is closed right now, any way to do this with like 47 ohm without running tons in parallel 

[sluckey]

When you put the 1Ω resistors on the cathode the voltage will no longer be zero. If you divide that voltage by the 1Ω resistance you will know what the current is. I proposed this method because you were having difficulty with your method.

I noticed that this is the same amp you have been struggling with for some time now. Most people probably have not made that connection. Please keep all your discussion about this amp in a single thread so we will have some history and maybe avoid repeatedly going over the same ole ground.

[Pochie45566]

When you put the 1Ω resistors on the cathode the voltage will no longer be zero. If you divide that voltage by the 1Ω resistance you will know what the current is. I proposed this method because you were having difficulty with your method.

I noticed that this is the same amp you have been struggling with for some time now. Most people probably have not made that connection. Please keep all your discussion about this amp in a single thread so we will have some history and maybe avoid repeatedly going over the same ole ground.

2, but okay yeah. I didn’t want to have misleading titles for I continually find something new wrong once something is solved but I’ll keep it to this one from now on. I’ll
Pick up a 1ohm on Monday and update then. I appreciate all I can learn from you guys! Thanks :)

[PRR]

> I’ll Pick up a 1ohm on Monday

Two.

Or just use the 47 Ohm resistors (but tell us you did so). The 1.5V-2V drop at a happy current will not screw-up the amp.

[Pochie45566]

> I’ll Pick up a 1ohm on Monday

Two.

Or just use the 47 Ohm resistors (but tell us you did so). The 1.5V-2V drop at a happy current will not screw-up the amp.

Using the 47 ohm resistor I got 1.7 volts on the cathode and that equated to about 36 ma on the cathode.

[pdf64]

...Using the 47 ohm resistor I got 1.7 volts on the cathode and that equated to about 36 ma on the cathode.

Is that with 47 ohms on each cathode return, and both cathodes' voltage is 1.7V?

[Pochie45566]

...Using the 47 ohm resistor I got 1.7 volts on the cathode and that equated to about 36 ma on the cathode.

Is that with 47 ohms on each cathode return, and both cathodes' voltage is 1.7V?

Yes sir

[Pochie45566]

that has the screen grid current included though right? so I am still biased pretty cold.

[shooter]

my math, assuming 25w max plate shows ~~~60% if you subtract a couple watts for G2

[Pochie45566]

60% just about right. . However that’s with the bias pot giving me the least negative voltage possible. So up the resistor before the negative voltage pot or raise the one after it going to ground?

[Pochie45566]

so where should I go now? it sounds like shit when distorted and all pin voltages seem to be fine. Im lost frankly. things to check anyone?

[PRR]

Why do you think it is the power tubes? There are many reasons an amp can sound like crap. Preamp?

[Pochie45566]

Why do you think it is the power tubes? There are many reasons an amp can sound like crap. Preamp?

I’ve replaced all cathode bypass caps, checked resistors in the preamp, checked all voltages and it seemed to be okay to me but if you have something I could look at in the preamp
I’m more than open. I thought it was the power amp just due to my previous issue with bias. However using the other method suggested, it appears to be fine.

[pdf64]

I don't trust any measurements made when probing a plate of an operational power amp, as doing so can easily induce oscillation, especially so with high gain pentodes. Always remove the phase splitter tube.

Is the amp a real Marshall 2204 or a copy?
Has it ever sounded good.
Have you gone through it with a scope and sig gen, verifying stability and some degree of linear operating range at all control permutations?

[Pochie45566]

I don't trust any measurements made when probing a plate of an operational power amp, as doing so can easily induce oscillation, especially so with high gain pentodes. Always remove the phase splitter tube.

Is the amp a real Marshall 2204 or a copy?
Has it ever sounded good.
Have you gone through it with a scope and sig gen, verifying stability and some degree of linear operating range at all control permutations?

copy, it has sounded good. yes. and no, I do not own a scope.

[Pochie45566]

well, any ideas?

[pdf64]

Voltage survey (all controls at min)?
Detailed internal chassis photos?
Is the issue that the amp sound bad at ANY amp/instrument control setting? Or can it sound good with some settings but bad with others?
It may be helpful to record a clip demonstrating the issue on a smartphone etc, upload eg to YouTube, and provide a link.

[VMS]

one addition to pdf64's good questions, when did it start to sound bad, was there a tube failure?

[Pochie45566]

Here’s a video clip, I think the popping is a bad ground connection.

It sounds decent clean but pretty damn muddy when turned up.

Also, it blew a fuse a while back maybe 10 years ago. Sat in a closet until I found it. Put a new fuse in and it seemed to have lots of issues. All the pots were wide open, bunch of stuff.

This popping just appeared last night when I was digging around in it so we know the popping isn’t the cause of the issue. It had this problem before of sounding like crap.

Here’s a thing, when I hit the second plate (pin 6) of v1 I get a weird noise but the solder connection looks good