Troubleshooting a 5f2a build

[BadDave]

Howdy Everyone,

I'm trying to put together a 5f2a using the Mojotone schematic, but substituting a full bridge rectifier for the 5Y3, I've attached the modified schematic here.

Now that it's all put together I'm having some problems that I just can't track down. When plugged in to the amp I get harsh breakup very early, and suddenly, at relatively low volumes.  There is also squealing at low volume, and feedback at high volume.  I also have my plate voltages on the second half of my 12AX7 changing a huge amount with the position of the volume knob from 133 V at low volume to 252 at high volume. There's also a big change in the voltage labelled XB on the schematic, from 220 V to 287 V. I've attached a table of the specific voltages and volumes to try to make it clear.

Please excuse the mess of wires in the attached photos, as I understand that lead dressing is a likely cause of noise, but I'm trying to get this problem sorted before I clean that up.

I've run down all of the standard listed troubleshooting methods like chopsticking and highlighting the circuit without any luck, and I have a bunch more voltage readings, if that would be helpful but I'm not sure what would be relevant.

Thank you guys so much for your help - Dave

[BadDave]

Voltages

[BadDave]

Schematic

[BadDave]

Board

[Esquirefreak]

Have you tried reversing the OT primary leads?

/Max

[BadDave]

Yeah, I've run it in both directions with similar results.  I yanked this out of a working amp and I believe I've now got it in the same orientation it was

Thanks!

[sluckey]

Surely that PT is not connected as shown in the schematic. There is 640VAC between the red secondary leads. That would produce about 900VDC unloaded when used with a bridge rectifier! Tell us how you really have it wired.

Also, disconnect the NFB wire from the speaker jack. Any better.

[BadDave]

Ah, good catch, I should have removed part number from the schematic, I'm not actually using that transformer. I salvaged one that puts out about 240V AC.  I've got one I ordered from Primary Windings (250V 100mA 6.3V 2A CT HT Valve transformer) that I'll switch in before I get it all buttoned up, but I would be surprised if that's the issue, given that I measure the rectifier output at around 350V DC.

I'll check on the NFB when I get home and see how it reacts

Thanks!

[sluckey]

Can you post another hi-rez pic that shows the entire chassis, not just the board?

[BadDave]

Sorry, I realize I broke the cardinal rule by not posting these originally, the bends on this chassis make it hard to get everything clearly, so let me know if there's more detail that would help

[Esquirefreak]

Routing heater wires very close to the input jack is usually not ideal.

/Max

[WimWalther]

Routing heater wires very close to the input jack is usually not ideal.

/Max

Good eye.

[Esquirefreak]

Also, the PCB for the output jacks (I assume?) looks like it's been cooked. Either with a lot of current or a soldering iron.

Where does the long, white wire go inside the chassis?

/Max

[BadDave]

The long white wire is one of the OT primaries that is too from switching the leads.

I think maybe what you're seeing on the output jack PCB is the copper showing through the green mask, but I can get a better picture of it when I get home if you're interested

Thanks,
Ryan

[Esquirefreak]

OT primary leads close to the input jack is also usually not ideal. You may have some cross talk and oscillation, due to lead dress. I suggest routing OT primary and filament leads away from the sensitive preamp stuff. Look at known good working designs and copy.

/Max

[BadDave]

I'll get to work with better routing and shielding, but should the anode voltage on my second 12AX7 really be swinging from 133 V at low volume, to 122 V in the middle, and 252 V at high volume? Maybe I totally misunderstand the interaction between the grid and the anode, but it seems off to me.

Thanks again for all your help!

[BadDave]

Here's a picture of that output PCB

[sluckey]

There is also squealing at low volume, and feedback at high volume.  I also have my plate voltages on the second half of my 12AX7 changing a huge amount with the position of the volume knob from 133 V at low volume to 252 at high volume. There's also a big change in the voltage labelled XB on the schematic, from 220 V to 287 V.

None of this is normal. The changing voltages are probably related the squealing. The squealing is probably related to your poor layout and wiring. Possibly related to NFB too. Did you ever disconnect the NFB wire from the speaker jack as I asked earlier? Is the black wire on the OT secondary connected to chassis ground?

[BadDave]

Yeah, I just disconnected the NFB, and it made the swing on the anode about half what it was throughout the run of the knob but at max volume, but it's still at about 250 at high volume.
How much change would you expect out of the anode? When I model it in spice I don't really see any, is that what I should see?

[Esquirefreak]

Which anode?

/Max

[BadDave]

Sorry, it's the anode of the second stage of the 12ax7, so pin 6 in my layout

[sluckey]

Measure resistance from V1 pin 7 to ground. Measure resistance from V2 pin 5 to ground. What have you?

[BadDave]

From V1 to ground I get whatever the volume pot is set to, from 0 to 1m

V2 pin 5 to ground measures 210k

[sluckey]

How much change would you expect out of the anode? When I model it in spice I don't really see any, is that what I should see?

With nothing plugged into the input jack there should be no change of plate voltage as you turn the volume knob. Spice cannot explain or fix this problem.

It's looking more and more like a layout/wiring issue. Leave the NFB wire disconnected until this is resolved.