"SHAKA TUBE" - almost working ??

[markr04]

Marty,

Yeah, it's a nasty sound. I hope to fix it with you guys' help though. It's not the recording itself - I made sure of that. I get that sound out my amp also.

[Paul Marossy]

Paul: I don't know how to fix the odd voltages at the tube, or what to compare them to. Will you elaborate?

Well, I can't remember exactly what I had voltage-wise in my Shaka Tube, but you should have something like 12 to 14V on Pins 1 & 6 (V+) and around -12 to -14V on Pins 3 & 8 (V-). Bias should measure somewhere around -12 to -14V or so. These numbers are all dependent on how you set the bias, of course.

Something is definitely awry...  :wink:

EDIT: You are absolutely sure that you have the 470uF caps connected up properly, right? If you look at the schematic, two of them have the positive lead connected to ground. Just a thought.

[markr04]

Paul, I just compared to your pdf and I'm absolutely sure I have them oriented correctly.

[Paul Marossy]

Paul, I just compared to your pdf and I'm absolutely sure I have them oriented correctly.

Hmm... that's perplexing.

[aron]

Pin 1 and pin 6 should have DC voltages on them - higher than tenths of a volt. Not only that, they should be positive voltages, not negative(as in in 6). There is a miswiring somewhere in the pin 1 and 6 connections.

Build an audio probe and verify that the signal is good and clean after the op amp.  (pin 6)

Debug pin 1 and 6 on the tube.

[markr04]

Paul,

I messed with the trimpot, so my voltages are about -.8v different from my voltage posting earlier. +V is now 12.5v.

On the audio board:
I have 12.5v at V+, IC pin 7, and all the way up to the two 50K resistors. On the other side of those resistors, the voltage goes negative as shown in my voltage post before. About -9.4 now.

This kills me. I don't understand. Why does it go negative there? :roll:  :x

[Paul Marossy]

That sounds better, but if the voltages on the tube are still the same, you have more work to do.

[markr04]

I can't measure at the socket pins because I used heatshrink. I'm measuring at the wires coming off the board toward the tube. I'm under the impression that the measurements would be the same (at our level).

So... it goes negative at the 50K resistors (pin 1 and pin 6), but it should be positive (if I understand you correctly). Why is that? How do those resistors turn +12.5V into -9.4V? And it seems I should remove and jumper them if I want +12.5V, right?

[aron]

No, the resistors don't turn positive into negative. The problem is before the resistors. At one end of the each 50K resistor should be the full V+ (positive DC voltage).

[markr04]

Aron,

There is +12.5v (V+) at one end of each of the resistors (and at pin 7 of the opamp). It's on the other side of those resistors that there is -9.4v.

[aron]

Pull out the tube and measure again and tell me the voltages.

[markr04]

12ax7
1: 14.9
2: -9.4
3: -12.8
6: 14.9
7: -9.4
8: -12.8

When I put the tube back in, I get -3.7V at pin 6 and .3 at pin 1.

[Paul Marossy]

Well, your voltages on Pins 1,3,6 & 8 look about right with the tube pulled out...  :?

[markr04]

I futzed with the trimpot some more and found more gain. Again, turning it CCW to just before oscillation.

bias: -9.5
V+: 12.6
V-: -10.4

With the tube:
1: 3.7
2: -10.1
3: -10.1
6: -0.3
7: -9.9
8: -10

without tube:
1: 14.9
2: -11.6
3: -12.8
6: 14.9
7: -11.6
8: -12.8

Any ideas?

[aron]

Do you have another tube to try out?

I guess you can try one more thing. Try changing the 10K bias pot to 50K and see if it helps.

I don't know why it would be oscillating.

[markr04]

I just noticed and guess I should mention I raised  :oops:

the .01uF input cap to .022uF
the .01uF cap between tube pin 6 and tone to .1uF
the .01uF between tone and ground to 1uF(NP)
(these last two are on the bottom right of Paul's layout)

I was looking for some low end. Would those caps cause this problem?

[markr04]

To tell the truth, I don't understand biasing. I modded my Peavey 5150 to have an external bias pot because I was temporarily caught up in the "run power tubes to just before meltdown" thinking. While I could perform the "surgery", I still don't understand the "hows and whys" of it all. I turned that pot back to give the factory -50V setting and can't tell a difference myself. But I digress:

I turned the trimpot even further. Now this thing has gain and strong output! I hear some hum, which was silenced when I had it biased the other way, but maybe a bit of noise is expected? The oscillation I spoke of might've been feedback. I can turn my guitar down, twist the trimpot all the way CCW and it doesn't squeal.

now:

Bias: -10.7
V+: 13.6
V-: -11.4

12ax7:
1: 5.2
2: -11.2
3: -11.1
6: 2.6
7: -10.9
8: -11.1

How are those numbers?

[markr04]

A new recording:

http://www.siteofmark.com/shaka_maybe_fixed.mp3

[puretube]

[SOT]anybody ever heard/read about the "circlotron" with the 2 individual floating power supplies?
:wink:

[Peter Snowberg]

Cecil Hall.

No DC in da OT.

Bridge circuits can be wonderful things.