PT test

[Tommy_D]

Hello everyone

I’ve wired the PT. 290V leads soldered to the rectifier socket but nothing else is wired except heater leads.

I have a 230V LED pilot that I wired in series. Drops about 10V. Mains are at 245V, I get 235V to the transformer.

Connected to mains the LED lights up fine.

I then tried to plug an EZ81 to see if the heaters would work. Unfortunately not. Same with a 12AX7.

Is that normal or does it mean that I’ve either connected something wrong or that the PT doesn’t work?

Thanks for your help!

[JMPGuitars]

Photos please.

Thanks,
Josh

[Tommy_D]

Tried to get a few angles. Not a lot of light unfortunately.

I’ve tested for continuity, no obvious bad solder joints. LED lights up fine

Thanks!

[Tommy_D]

The on/off switch is on the VRM pot and I don’t have a standby switch

[JMPGuitars]

The on/off switch is on the VRM pot and I don’t have a standby switch

Those pictures aren't helpful, I can't see anything with all the stuff in the way. Please cut some of those zip ties and move the orange wires out of the way, and make sure every solder lug is visible on the PT and the tube socket.

Also, please try taking photos without the flash.

Thanks,
Josh

[Tommy_D]

Late in the UK now but will do tomorrow

I take it that it isn’t normal the tubes don’t heat up...

I’ll do that, check connections / continuity again and measure voltages one more time.

They were 0 for heaters at the lugs... 290/290 taps measured 60V but I may have measured incorrectly.

Thanks so much

[crgfrench]

This ground connection appears not to have been soldered at all, or poorly soldered

20200726_214305.jpg

[katopan]

From your description and the photos it looks like you've wired the mains rated LED lamp in series with the power transformer primary. It should be in series with the on/off switch (your switching pot), but then the lamp and PT in parallel with each other. They both need 230-240V across them. In series the lamp will be current limiting the PT.

[Tommy_D]

Now that you mention it makes perfect sense that the LED would have been limited the current...

The unsoldered lead is for the VVR but it isn’t connected yet. Good spot.

Anyway, I fixed the wiring but then there was a really loud vibrating hum. The LED was like flashing very fast too. Plugged an EZ81 and the heaters were heating up.

Reflowed joints which tamed the hum but not completely. Removed the LED and tried to reflow again but no change.

I’ve unsoldered ground connections. By the way this didn’t really change change the hum.

Will rewire the ground connections to a dedicated star ground hole (rather than using one of the PT mounting holes - think Josh wrote that was better). Also put rosin on the solder lugs if it helps.

Weirdly, one of the 290V lugs measures 310V, the other 130V. The second one was 60V before I reflowed it.

Anyway I will carry on and report back.

Is it reasonable to try and eliminate the PT hum completely? For example I notice that the PT of my soldering iron also hum (although less than the amp’s..)

[Tommy_D]

Update. I shorted the switch (located on the VVR pot) and the hum went away completely. Dead quiet.

Perhaps grounding the pot to the star ground would help?

Still get strange voltages. I haven't yet re-soldered all the grounds, that may explain why.


Still digging...

[Tommy_D]

Everything is working now. I get the right voltages (perhaps 5-10% higher than they should be). Tubes light up fine.

Still a mild hum / buzzing sound (it got dead quiet earlier... when I blew a fuse )

Tapping the PT or ground lugs doesn’t seem to make a difference. Neither does isolating the pot, grounding the pot casing to the star ground point or moving wires around. I’ve zip tied cables together so that hum coming from wires cancels itself.

At this point having reflowed all joints a few times and switched the star ground to an independent hole in the chassis I’m not sure what else I can do..

My existing head amp also has a buzz but you have to get your ear pretty close to the grill to hear it.

Hard to say how the buzz will be in the enclosure. It would presumably absorb some of the vibration.

Any advice appreciated! Thanks

[Tommy_D]

I used a cheater plug to lift the ground and it doesn’t make a difference to the mild buzzing noise so that may well be as good as it will get?

[JMPGuitars]

I used a cheater plug to lift the ground and it doesn’t make a difference to the mild buzzing noise so that may well be as good as it will get?

I haven't replied because I was waiting on better photos, but here's all I can tell you right now. Lead dress is essential. I don't like the zip ties, the wires should be twisted and run much neater. BUT an unloaded PT doesn't perform the way a loaded PT does. In other words, twist and clean up the wires, and build the amp before you worry too much about the PT noise.

If you want to know what I mean by cleaner lead dress, go look at Craig's thread: viewtopic.php?p=246806#p246806

Thanks,
Josh

[Tommy_D]

Ok thanks Josh

That’s super neat lead dressing in the 36W!!!

At this point I need to wire the VVR, output jacks, OT, pots and sockets. Then a highlighter check and usual testing steps.

I’ll get some more solid core wire so I can rewire the PT afterwards. Admittedly my PT wiring is sloppy!

Really grateful for all the help!

[JMPGuitars]

Solid core is okay if that's what you're into, but I prefer stranded. Solid core wire can break.

[Tommy_D]

You’re right. Stranded better in the long-term but need pvc jacket. I have teflon which is very difficult to twist.

Only used solid core for heaters. Supposedly better so it doesn’t move / vibrate. At least the heaters wiring is very tidy! (unless you tell me otherwise of course but I had researched that in details)

[geoff 1965]

i've used solid core for the heaters on my amps and it's a lot easier to twist and keep shape than stranded but i used ptfe stranded for the hook up wiring,just make sure you strip the solid core wire! if you use a knife like teflon it grooves the solid core and weakens it.

[crgfrench]

I'm using this everywhere, it twists perfectly with one end in a vise and the other end in a drill. Stays in place really well, almost like solid. It's insulated with teflon. Solders like a dream. https://tubedepot.com/products/20-ga-si ... -ptfe-wire

[Tommy_D]

Thanks for the tips
Looks like great wire, I’ll get a bunch of it

[katopan]

Normally when people talk about a buzzing sound, it's through the speaker. Can I assume here you're talking about the PT itself buzzing?