50W Lead/Bass Plexi

[bbaug14]

So I'm building a 50W Plexi with a lead/bass switch and I'm having an issue. With tubes in, I get this loud squeal/him and when I turn the mains off, I get a loud pop that decays with a delay until power is drained. The only thing I can think of is I have the OT primaries backward, but I'm not sure if that were the case that I'd be experiencing these symptoms. Below is a link to my original build thread with good photos and more detail. Any help is appreciated.

http://forum.metroamp.com/viewtopic.php?f=7&t=44089

[Bob S]

Could be a defective switch drawing an arc.
Is there a grounded center tap on your power transformer?
If so does the schematic call for one with a full wave bridge recto?

[bbaug14]

Could be a defective switch drawing an arc.
Is there a grounded center tap on your power transformer?
If so does the schematic call for one with a full wave bridge recto?

I'm using a Marstran PT and I am grounding the center tap. I'm also using the standard recto scheme found in the Metro instructions. I've seen others use the same set up with success, so from a schematic standpoint, it's correct.

The switch is an interesting idea. How would this affect anything but turning the amp off though?

[bbaug14]

Could be a defective switch drawing an arc.
Is there a grounded center tap on your power transformer?
If so does the schematic call for one with a full wave bridge recto?

I'm using a Marstran PT and I am grounding the center tap. I'm also using the standard recto scheme found in the Metro instructions. I've seen others use the same set up with success, so from a schematic standpoint, it's correct.

The switch is an interesting idea. How would this affect anything but turning the amp off though?

Okay, I did a bias check with no tubes and I was getting a positive voltage on Pin 5. That's no good as that means the OT primaries are swapped and I'm supplying positive feedback instead of negative. Instead of switching the primaries (which would be a pain as they were cut to length), I swapped the 5.6k grid resistor lines instead. It "should" have the same net affect in a push/pull set up such as this.

I'm not sure this is my problem, but it's at least one thing that was wrong. I should point out also, I'm only seeing 2.8VAC to ground at the heaters. I'm seeing 6.1VAC across the heaters though, and I've read this isn't all that uncommon, so I don't think it's an issue.

Anyone who has any suggestions, please feel free to let me know.


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[img1152]http://i58.tinypic.com/11tlogl.jpg[/img]
[img1152]http://i60.tinypic.com/33ek2g0.jpg[/img]

[Jana]

bbaug14 wrote: "Okay, I did a bias check with no tubes and I was getting a positive voltage on Pin 5. That's no good as that means the OT primaries are swapped and I'm supplying positive feedback instead of negative."

This makes no sense at all. How can you check the bias with the tubes removed?
How does having a positive voltage at pin 5 indicate reversed primaries and positive feedback?

Tubes or not, you should have about -30 to -40 volts on pin 5 (assuming EL34).

[bbaug14]

bbaug14 wrote: "Okay, I did a bias check with no tubes and I was getting a positive voltage on Pin 5. That's no good as that means the OT primaries are swapped and I'm supplying positive feedback instead of negative."

This makes no sense at all. How can you check the bias with the tubes removed?
How does having a positive voltage at pin 5 indicate reversed primaries and positive feedback?

Tubes or not, you should have about -30 to -40 volts on pin 5 (assuming EL34).

Exactly. Prior to swapping the 5.6k resistors on pin 5 I was getting a positive 30 to 40 volts. This is wrong because the primaries are swapped. After the swap the voltage was -30 to -40 volts as it should be. I've read this has happened to others with MM OT because its not clear what tube to route the primaries to.

[ToneMerc]

Kind of convoluted here; so just so I understand,the ONLY issue that you have is when turn the mains off and you encounter the squealing issue? Otherwise voltages are fine, all controls and functions are normal?

TM

[bbaug14]

Kind of convoluted here; so just so I understand,the ONLY issue that you have is when turn the mains off and you encounter the squealing issue? Otherwise voltages are fine, all controls and functions are normal?

TM

Sorry about that. No. The squealling that decays when I power off is one issue. The other is when powered on I moved a control and got a loud squeal. I assume this was due to the primaries being backward. I'm going to do a deep dive into voltages tomorrow. Right now I get 2.8VAC to ground on the heaters and 6.1VAC across them. About 450 at the AC HV with stand by off. -40V at pin 5 of the power tubes after the swap.

[Bob S]

Sounds like you're confusing negative feedback with bias voltage.
Swapping the pin 5 control grid wires might well have fixed your squealing problem if it was positive feedback.
The pop at turn off might be a different problem. I had that with a switch not breaking contact cleanly. The magnetic field in a transformer doesn't like to collapse when you turn it off. Arcs in the switch - pop.

[bbaug14]

Sounds like you're confusing negative feedback with bias voltage.
Swapping the pin 5 control grid wires might well have fixed your squealing problem if it was positive feedback.
The pop at turn off might be a different problem. I had that with a switch not breaking contact cleanly. The magnetic field in a transformer doesn't like to collapse when you turn it off. Arcs in the switch - pop.

Thanks for the suggestion. I've had it happen where it was just a single pop, but never like this where it echo's as power drains. Its weird.

[martin manning]

-40V at pin 5 of the power tubes after the swap.

Just to be clear, swapping the grid or primary leads is not going to change the bias voltage from positive to negative. Swapping your meter leads will do that though ;^)

[bbaug14]

-40V at pin 5 of the power tubes after the swap.

Just to be clear, swapping the grid or primary leads is not going to change the bias voltage from positive to negative. Swapping your meter leads will do that though ;^)

That's very odd. I literally was checking pin 5 to ground (no tubes) and it was positive. I swapped the lines going to each tubes pin 5 and checked it again and it was negative and I'm certain I didn't switch the probes as I didn't move the grounded negative probe.

[sluckey]

That's very odd. I literally was checking pin 5 to ground (no tubes) and it was positive. I swapped the lines going to each tubes pin 5 and checked it again and it was negative and I'm certain I didn't switch the probes as I didn't move the grounded negative probe.

That's impossible. The only way to get +40vdc on pin 5 of the output tubes (unless you pipe it in from an external source) would be to unsolder the three components in the attached pic, flip them around, and resolder.

If you really had +40v on pin 5 you would have been chasing a totally different set of symptoms.

[bbaug14]

Well, if I was wrong then so be it. I will say this also, I tested today after swapping the 5.6k connections to the tubes and all my issues are gone. All voltages are correct the amp is functioning properly. Very happy.

[M Fowler]

Well, if I was wrong then so be it. I will say this also, I tested today after swapping the 5.6k connections to the tubes and all my issues are gone. All voltages are correct the amp is functioning properly. Very happy.

Your build looks very good to me, glad it is functioning correctly. Something happened for sure whether wrong DMM leads showing + voltage.
Maybe you can just read voltages again now that it is working just to make sure all is well and enjoy your amp.

Mark