Uneven readings on pin 3 of power tubes (fixed)

[SolidStateCircuitReject]

Hi Kids.

Checking bias on a 40 watt amp (2 x 5881), v5 plates at 440VDC. v4 on the other hand reads 420VDC and squeals emitted from the speaker every time I touch pin 3 of the socket reading 420V with my tester lead.

I tried changing tubes, re-hit solder joints, etc. Voltage at center tap was 450 VDC if I recall correctly.

What are the odds that it is a tube socket failure - the tube socket is a few months old.

Thanks!
SSCR

[PRR]

> touch pin 3 with my tester

That's the biggest signal in the amp.

If it sneaks back to low-level stages, it gets amplified around and around to total squeal-out.

Your test lead is a "better transmitter antenna".

"Usually" it won't squeal if all Volume knobs are at zero (so not much gain to power tube plates).

[SolidStateCircuitReject]

thank you.

I have heard it before, but never so consistently... and it is a coincidence then that side of the tube reads 20VDC less than the other side? I wasn't like that previously, I had stable readings with in 1 or 2 VDC, and that was more likely fluctuations in the wall AC. At least I suspect as much.

AND I renamed the topic for clarity.  Thanks!!!!

[pompeiisneaks]

Basically means one tube is conducting more than the other, causing a larger voltage drop.  It may be that one tube is aging worse than the other, or it may have always been that off.  If the tubes are old, it wouldn't hurt to try new balanced tubes and see if they seem more 'normal'  You could also check related tube resistors to see if they've drifted out of spec too I'd wager...

~Phil

[Tony Bones]

I tried changing tubes,

Not sure exactly what that means, but as Phil said, your problem is that one tube is conducting roughly 3x as much as the other. That's more than just a poorly matched pair.

The easiest thing to try is to swap the tubes. If the problem moves with the tube then you have your answer.

If it stays with the socket then you have more work to do. Measure the voltages at pins 3, 4, 5, and 8 of both tubes.

[PRR]

> it is a coincidence then that side of the tube reads 20VDC less

When squealing, the amp de-biases itself.

And you are checking Plate Voltage, NOT B+.

Does the amp work OK when not poking the plate with a wire?

[SolidStateCircuitReject]

The amp works fine.

Last time I biased, which was maybe four or five days ago everything was fine, as in within 1 or 2 mA.

The problem stays with the socket. I swapped tubes, V4 to V5 - V5 to V4. Same socket gave the same result.

All other pins read as they should on both sockets. Screens (pin 4) were at 438, bias at -50 (pin 5), heaters at 3.3 each ( pin 2 and 7)... Pin 8 and 1 is grounded.

I guess I can lean on a bad socket at this moment.

By the way, I do use balanced sets of tubes.

[SolidStateCircuitReject]

Since my last post I have done the following...

Replaced the v4 tube socket.
Swapped tubes (GT 5881 matched)
Changed tubes (Tung-Sol 6L6GC-STR matched)
Rewired/re-soldered the filter cap board (including cutting fresh ends on OP Tx CT and primaries).

I drew a layout schematic of the power and output sections and circled test points in thick black marker (attached to post!).

The readings are as follows...
1. 140VAC
2. 140VAC
3. 165VAC
4. 165VAC
5. 459VDC
6. 458VDC
7. 405VDC
8. 330VDC
9. 414VDC
10. 445VDC
11. 458VDC
12. 458VDC
13. -59VDC
14. -59VDC

all readings were rounded to the nearest whole number.

VOLTAGE DROP between OP Tx center tap and OP Tx primaries with amp on;
OPTx to v4, Pin 3 = -21.43VDC
OPTx to v5, Pin 3 = -3.81VDC

v5 is running normal, -3.81VDC voltage drop equates to 44mA current using the "Plate/Cathode current" method of biasing.

After changing the socket to a brad new one, changing tubes etc, Pin 3 at v4 still screeching when i measure it to ground. I am at a loss what to try next.

[tubenit]

You could also check related tube resistors to see if they've drifted out of spec too I'd wager...

I would check every resistor to see if something has changed.

With respect, Tubenit

[jjasilli]

1.  Specific voltage points seem suspicious:


* 1 & 2 (wall voltage 140VAC???
* 11 & 12 (screens) much higher than plates.


2. What is measured static DC resistance of OT primary: end-to-end; and CT to ea end?

[Tony Bones]

> it is a coincidence then that side of the tube reads 20VDC less

When squealing, the amp de-biases itself.

PRR, please help me understand what you think is going on.

[shooter]

OPTx to v4, Pin 3 = -21.43VDC
OPTx to v5, Pin 3 = -3.81VDC

swap the OT plate wires and see if it moves with the OT or stays with the socket, make sure you have NO signal and volume down, maybe disconnect NFB or she'll squeal being outta phase

[SolidStateCircuitReject]

You could also check related tube resistors to see if they've drifted out of spec too I'd wager...

I would check every resistor to see if something has changed.

With respect, Tubenit

All check okay within specs.

1.  Specific voltage points seem suspicious:

* 1 & 2 (wall voltage 140VAC???
* 11 & 12 (screens) much higher than plates.

2. What is measured static DC resistance of OT primary: end-to-end; and CT to ea end?

The ac voltage would be around 230 to 240 VAC if I measured it from #3 to #4, but these were measured to ground. I am in Belgium, supposed to be 220VAC, but well over 230VAC in reality.

As for #11 and #12, I don't know what the hell happened there... just took some fresh readings... starting at #5;

5. 458 VDC
6. 457 VDC
7. 402 VDC
8. 328 VDC
9. 428 VDC
10. 458 VDC
11. 456 VDC
12. 456 VDC
13. -56 VDC
14. -56 VDC

sorry for the weird confusion, perhaps I mis wrote it, but I was extreeeeeemly careful when writing them down this time.

I didn't include more bias details originally because some people will throw some red flags with the CT to primary DCR readings, but this transformer - by design - is uneven, but here you go...
OP Tx CT to v4 Pin 3: 110Ω
OP Tx CT to v5 Pin 3: 85Ω

These are within spec if you look at the data sheet for Hammond 1750JA, which is a direct replacement for the OP Tx of the Fender Hot Rod Deluxe (the amp from which I harvested the trannies :-) )

so having a voltage drop from the same points above (taken earlier today, not part of this latest readings):
OP Tx CT to v4 Pin 3: -21.4 VDC
OP Tx CT to v5 Pin 3: -3.81 VDC

V5: voltage drop (3.81) divided by resistance (85Ω) = plate current (45mA)
V4: voltage drop (21.43) divided by resistance (110Ω) = plate current (195mA!! I'm sure this exceeds the maximum allowable plate current, not red plating at any time though)

Knowing this, the amp still sounds awesome.

OPTx to v4, Pin 3 = -21.43VDC
OPTx to v5, Pin 3 = -3.81VDC

swap the OT plate wires and see if it moves with the OT or stays with the socket, make sure you have NO signal and volume down, maybe disconnect NFB or she'll squeal being outta phase

Will do as soon as I get back to it in a short time.

THANKS!

[SolidStateCircuitReject]

1.  Specific voltage points seem suspicious:

* 1 & 2 (wall voltage 140VAC???
* 11 & 12 (screens) much higher than plates.

2. What is measured static DC resistance of OT primary: end-to-end; and CT to ea end?

CT to each end of the primary = 200Ω

[PRR]

Remove driver tube and read that plate again.

I don't think you have a real problem *until* you connect a long probe wire to that output plate, radiate big signal back into early stages to induce squeal.

[SolidStateCircuitReject]

Remove driver tube and read that plate again.

I don't think you have a real problem *until* you connect a long probe wire to that output plate, radiate big signal back into early stages to induce squeal.

now we're getting somewhere...

pulling the PI, v4 was 449, v5 was 450. But what could this mean? 

FWIW, I replaced the 12AT7 PI with a balanced 12ax7 and it shows all the same symptoms.

[SolidStateCircuitReject]

OPTx to v4, Pin 3 = -21.43VDC
OPTx to v5, Pin 3 = -3.81VDC

swap the OT plate wires and see if it moves with the OT or stays with the socket, make sure you have NO signal and volume down, maybe disconnect NFB or she'll squeal being outta phase

forgot to add, i did this and the symptom stayed with the socket, it did NOT switch with the primaries ... giving weight - at least in my version of "common sense" - to what PPR is saying... introducing a long lead into this area of the circuit is injecting signal back in an earlier stage inducing squeal.

To further this theory, I switched bypass off, right away attached the leads and as the amp was "fading in" i could see that the voltages were falling on v4 as the squeal was rising in volume.

[shooter]

now you know it ain't the OT

I don't have a schematic so;

I switched bypass off

bypass cap on cathode?

how "good" is your meter?  I have got bizarre problems like you have with a scope, but my fluke doesn't seem to cause "non-issues"

from what PRR suggested you're feeding back from the pre section, wondering if that could be induced into the NFB

[HotBluePlates]

how "good" is your meter?  I have got bizarre problems like you have with a scope, but my fluke doesn't seem to cause "non-issues"

from what PRR suggested you're feeding back from the pre section, wondering if that could be induced into the NFB

It appears we know probing with the meter induces the oscillation.  I'd ask, "Where are you grounding your meter's black lead for these measurements?"

I've measured output tube plate voltage in amps and did not induce feedback.  I clipped the black lead to the PT mounting bolt where the center-tap and filter cps are grounded.  The rest of that lead tends to stay over to the PT-end of the chassis, away from the preamp.  The red lead gets clipped to the output tube plate.  The rest of that red lead tends to run right to the chassis-edge near the output tube sockets.  Meter & leads stayed away from the preamp, as much as reasonable.

So I wondered if the chosen ground-point for the black lead connection was somewhere over in the preamp, and contributed to the issue...

[PRR]

It probably does not matter where the black lead goes. The meter is a near-infinite impedance.

The red *wire* (any wire) added to the power section plate adds a couple feet of "transmitting antenna" to the largest signal in the amp. It may be stable with the shorter OT lead, not with another yard of added antenna.

Location of the lead may be critical. A lead from plate that runs away from preamps and outside the chassis ASAP may be stable where a lead laying near preamp invites oscillation.

"Real" bench tools, vintach VTVMs with original probes, had a 1Meg resistor in the probe for DC measurements, and a shielded lead from probe. The 1Meg and cable C greatly reduces high-frequency radiation. But these probes failed a lot due to fine shielded cable and too-compact switching.

[SolidStateCircuitReject]

First, let me say that I have had this amp on the bench for a few weeks, playing it, changing parts one at a time and taking bias a dozen times and not had this issue... UNTIL I made a big change in the preamp... originally I was running half of v1 (v1b, v2a, v2b, v3 (the PI)). So I added a superlead type channel to utilize V1a. THIS is when the issue started. Coincidence?Starting to looking like it is not.

I have the amp oriented with the controls facing me, so the red lead does reach across part of the bias area to probe v4 and v5. The black lead is clipped to the chassis near the power/standby switches. Though I have also clipped it at the chassis nearest the input.

Good meter? Probably could have spent the $100 in a better place... it is a Ryobi Tek 4 - and I am not crazy about it. Before this one, which I have been using for about 8 years, I went through a string of $40 Radio Shack ones  - having moved to Belgium 6 years ago I have stuck with the Ryobi - perhaps it is time for a change.

I plan on buying a better grade of tools, oscilloscope etc - but doesn't help me ATM not having them. But I have dumped a HUGE chunk in building 4 amps, trying to find two circuits that I love since I play classic rock AND hardcore/metal 

now you know it ain't the OT

I don't have a schematic so;

I switched bypass off

bypass cap on cathode?

No - bypass switch on the amp - I never know quite how to express this... bypass off? Bypass on? When I say Bypass off, I mean the bypass switch creates an open circuit. Meaning power is free to flow to the rectifiers.

how "good" is your meter?  I have got bizarre problems like you have with a scope, but my fluke doesn't seem to cause "non-issues"

from what PRR suggested you're feeding back from the pre section, wondering if that could be induced into the NFB

It appears we know probing with the meter induces the oscillation.  I'd ask, "Where are you grounding your meter's black lead for these measurements?"

I've measured output tube plate voltage in amps and did not induce feedback.  I clipped the black lead to the PT mounting bolt where the center-tap and filter cps are grounded.  The rest of that lead tends to stay over to the PT-end of the chassis, away from the preamp.  The red lead gets clipped to the output tube plate.  The rest of that red lead tends to run right to the chassis-edge near the output tube sockets.  Meter & leads stayed away from the preamp, as much as reasonable.

So I wondered if the chosen ground-point for the black lead connection was somewhere over in the preamp, and contributed to the issue...

As I added first above, I am clipping black at the chassis and I have tried it a few places with no change. I too have several amps that do not go into oscillation like this - including THIS very amp, until I added a second channel.

It probably does not matter where the black lead goes. The meter is a near-infinite impedance.

The red *wire* (any wire) added to the power section plate adds a couple feet of "transmitting antenna" to the largest signal in the amp. It may be stable with the shorter OT lead, not with another yard of added antenna.

Location of the lead may be critical. A lead from plate that runs away from preamps and outside the chassis ASAP may be stable where a lead laying near preamp invites oscillation.

"Real" bench tools, vintach VTVMs with original probes, had a 1Meg resistor in the probe for DC measurements, and a shielded lead from probe. The 1Meg and cable C greatly reduces high-frequency radiation. But these probes failed a lot due to fine shielded cable and too-compact switching.

I will try reading from the other side of the amp (speaker jacks facing me) and the combination of the chassis beings rather small jtm45 sized mixed with jamming circuits in, though rather neatly for a prototype, does not help.

[SolidStateCircuitReject]

Hi Folks.

Thank you all for the replies.

I removed the "lead" channel circuit i added in v1a that was previously unused and it stopped squealing on bias.  The circuitry in theory should have been leaner and a bit more distorted (using .68µF/1.5k), but in practice it wasn't so different that I continue to struggle with not being able to bias it... probably because the cathode bypass caps further down the circuit are adding back in fatness (they being 47µF and 1µF later in the circuit).

So, on to the next thing... it biased up within 5mA - I was hoping for tighter results but I am not overly disappointed about it since the ORTx is so unbalanced for whatever reason Fender saw to do that.

[PRR]

> meter?... it is a Ryobi Tek 4

?? That seems to be a power screwdriver? (Actually a battery used in several tools.)

Yes, putting More Gain in the line makes it more likely that output sneak-back to input will cause howl (in this care screech or bat-calls?).

[SolidStateCircuitReject]

> meter?... it is a Ryobi Tek 4

?? That seems to be a power screwdriver? (Actually a battery used in several tools.)

Ha - it does seem that way. Same battery between several products I guess. More specifically http://www.toolstop.co.uk/ryobi-rp4020-tek4-4-volt-digital-multimeter-with-battery-and-charger-p13384

[pompeiisneaks]

I've seen a lot of those around but they don't sell them anymore and I ehar they were pretty good... Bummer.  I ended up with a Klein Tools one myself because I didn't bring my Fluke 117 along on a trip to see family (why would I?) but then my in laws wanted me to try and fix a power supply so I needed one :P....