What am I missing?

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morcey2
Posts: 132
Joined: Mon May 12, 2008 5:31 pm

What am I missing?

Post by morcey2 »

Just finished an Express and I've got a weird problem going on.

It's really buzzy and distorted, but not in a good way. Think square wave. I say think square wave because I can't actually look and see whats going on. My o-scope died a couple of weeks ago. :x

When I was playing it, I had the multimeter hooked up to the bias ports on the back (1-ohm resistors) and one of the EL34's reacts how it should. Hit a chord and the current jumps from 45mA to 150-ish mA. I plugged into the other one and when you hit a chord, it jumps, but in the wrong direction. It drops to 15-20mA. I swapped tubes and it stayed at the same socket. I swapped OT primaries and grid wires, to keep the NFB from becoming PFB, and it followed the grid lead. Long story short, I narrowed it down to the grounded-grid side of the PI. I tried a different PI tube. I resoldered the socket. I replaced the socket. I replaced the grid-to-ground cap. I replaced any wires anywhere near that socket, and the problem won't go away.

I'm ready to shoot the dang thing, but someone else paid for all the parts.

I'd post pictures, but I think the PI tube in my digital camera went out also. The pictures come out looking like a really bad Picasso.

Any ideas?

Matt

EDIT: Not sure why I put this in Technical Discussion and not Trainwreck. Probably the same reason I can't seem to hook up a PI.
Andy Le Blanc
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Re: What am I missing?

Post by Andy Le Blanc »

so ..... your confident its not the power side of the push-pull circuit......
what about the rest of the inverter circuit?...... or the preceding driver....
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morcey2
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Re: What am I missing?

Post by morcey2 »

Andy Le Blanc wrote:so ..... your confident its not the power side of the push-pull circuit......
what about the rest of the inverter circuit?...... or the preceding driver....
I am. I disconnected the NFB to keep from having squealing while I and I've swapped the everything around and the only thing it follows is the non-inverting side of the PI tube.

Here are the voltages I get at idle for the PI and power tubes

V3 -- 12AX7

1: 194VDC
3: 35VDC
6: 188VDC
8: 35VDC
Top of tail resistor: 33VDC


V4 -- EL34

1&8: ~40mVDC
3: 392VDC
4: 375VDC
5: -33VDC
6: 384VDC

V5 -- EL34

1&8: ~43mVDC
3: 394VDC
4: 374VDC
5: -33VDC
6: 383VDC


The only deviations I've made from this schem: https://tubeamparchive.com/files/twreck ... ic_421.pdf are 2.2k screen resistors and 10uF cathode bypass caps.

Matt
Last edited by morcey2 on Sun Jun 22, 2008 6:06 pm, edited 1 time in total.
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jjman
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Re: What am I missing?

Post by jjman »

I'm glad you mentioned the 2.2k screen resistors since the voltage drop on the screen resistors looked too high for 1k's. I'm trying to wrap my head around the dc going down in the el34 when driven by the offending side of the PI. I guess it's sending a signal that has more negative swing than positive swing so maybe that side of the PI is saturating from somehow being biased too hot? How do pins 2 and 7 look on the PI?

I bet not having the scope for this one is a major drag. :twisted:
If it says "Vintage" on it, -it isn't.
Andy Le Blanc
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Re: What am I missing?

Post by Andy Le Blanc »

is that a typo.... or are you really connected to pin 9
pin 9 is a heater
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Andy Le Blanc
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Re: What am I missing?

Post by Andy Le Blanc »

with the voltages youve given for the inverter stage......
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morcey2
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Re: What am I missing?

Post by morcey2 »

Andy Le Blanc wrote:is that a typo.... or are you really connected to pin 9
pin 9 is a heater
yep. typo. fixed.
morcey2
Posts: 132
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Re: What am I missing?

Post by morcey2 »

jjman wrote: I bet not having the scope for this one is a major drag. :twisted:
My kingdom for a scope! (Of course, my kingdom is deep in debt so that's not much motivation)
jjman wrote: I'm glad you mentioned the 2.2k screen resistors since the voltage drop on the screen resistors looked too high for 1k's. I'm trying to wrap my head around the dc going down in the el34 when driven by the offending side of the PI. I guess it's sending a signal that has more negative swing than positive swing so maybe that side of the PI is saturating from somehow being biased too hot? How do pins 2 and 7 look on the PI?
They both measure at about 12V, but that's with a DMM that I'm sure is loading them down. (5M grid impedance || 1M DMM impedance causes problems.) But at least I know they're connected. I've got a VTVM that I usually use to measure that, but I've misplaced all my leads to it. That's actually what I'm looking for right now because I think I can get some of this solved with that as much as with a scope. Like watching the needle sweep for AC voltage on the grids without loading them down.

As for the DC going down, the plate and screen voltages are sagging with the power supply when the other side starts pulling more current. It sags about 50 or 60V. I think the screen supply actually sags more and that's what really kills the current on that side.

Matt.
morcey2
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Re: What am I missing?

Post by morcey2 »

Found the VTVM. There is a signal going to the 'dropping-current' tube, but it's about 1/4 what's going to the normal EL34.

Correction on the last post. The screen sag is 50-60V. The B+ sag is about 15V. I tried 6V6's just for fun and it does the same thing. I think I'm going to pretend it doesn't exists for the rest of the day and come back to it tomorrow. That'll give me time to work on something else. Not amp related. OK, maybe I'll finish the turretboard for the 18watt. That's not totally amp related. Nevermind. I'm addicted. But aren't we all? :?

Matt
morcey2
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Re: What am I missing?

Post by morcey2 »

Stole a camera. here's pictures.
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morcey2
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Re: What am I missing?

Post by morcey2 »

One more picture.
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DonMoose
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Re: What am I missing?

Post by DonMoose »

How confident are you that C14 isn't a tad leaky?
Or that R19 really is 1k5 and not 15R0 or something?

I'm trying to imagine a failure mode where the 'push' is a relatively stronger and/or the 'pull' is relatively weaker. Hey, is R17 really 220k and not, say 2M2?

/ Obviously I mean measured values, not marked values.

Hope this helps!
morcey2
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Joined: Mon May 12, 2008 5:31 pm

Re: What am I missing?

Post by morcey2 »

DonMoose wrote:How confident are you that C14 isn't a tad leaky?
Or that R19 really is 1k5 and not 15R0 or something?

I'm trying to imagine a failure mode where the 'push' is a relatively stronger and/or the 'pull' is relatively weaker. Hey, is R17 really 220k and not, say 2M2?

/ Obviously I mean measured values, not marked values.

Hope this helps!
I measured all the resistors as they went in and I've replaced both of the coupling caps. The behavior follows the non-inverting side of the PI, wherever that's connected. When I swapped PI plate leads, the behavior moves to the other power tube. It also moves when I swap PI grid leads. That's why the leads in the pictures are crossed now.

I'm probably going to have to find someone w/ a scope, or buy a new one before I can figure this out.

If anyone out there has a setup that's easy to measure plate (or in this case, plate and screen) current under load and tell me what they see, I would really appreciate it. That way I know that I'm not just chasing ghosts.

Thanks,

Matt

Matt
Andy Le Blanc
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Re: What am I missing?

Post by Andy Le Blanc »

are the plate leads from board to the socket pins ment to be twisted.....
ive had an amp where that was an issue......
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jjman
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Re: What am I missing?

Post by jjman »

Andy Le Blanc wrote:are the plate leads from board to the socket pins ment to be twisted.....
ive had an amp where that was an issue......
Out of phase signal bucking jumping from one to the other on the twisted pair. And I'm watching bucking broncos on TV at the moment. :shock:

Do we have a bingo?
Last edited by jjman on Mon Jun 23, 2008 3:27 am, edited 1 time in total.
If it says "Vintage" on it, -it isn't.
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