Grew a pair, and fired it up myself - Sluckeys plexi check

[pjd3]

Hey everyone, hope your hangin in their.

I was going to pass my second build here, a 6V6 plexi off to the local amp tech to "take the hit" should I have made a bonehead move in the build but,
I decided to take confidence and hit the switch. (first, I ohmed out things to make sure there were no fatal shorts). I was hoping some of you could just take a look at my findings as I go - being my first time on this process, I don't wish to overlook, "anything"! thanks.

So no tubes installed upon first power application - nothing smoked, no whining, no hissing and the jewel light shown proudly. Thank God.
I made some voltage measurements, at least the ones that you can with no tubes and here's what I found:
--- 7vac on the green heaters - I'm assuming once tubes are in and loading things I'll see the voltage get closer to 6vac
--- See 330-0-330 vac in expected places
--- Bias range at the power tube grids both -31.9 to -58.5. At first, I was a little suspicious at how long it took the bias to settle at a pot position even without any tubes installed but then it struck me that it must be just the time constant of the bias rectification and filter components which I hadn't initially considered.
--- 5vac rectifier filament voltage initially kept showing up as around 1.1 vac. This baffled me and I had to wonder if it had anything to do with the 5 volt filament living with one of the 330vac lines and creating some differential. But, as I massaged the probe connection, 5.6 vac began to intermittantly pop up. Assuming this was either some crud on the wires/lug, or perhaps some of the plastic jacket melting into the solder joint, I thoroughly reflowed the entire contact area with flux and solder, then cleaned will with isopropyl. I did get back the 5.6vac but occasionally it would pop back down to 1.1 vac or so. I just have to call this as an intermittent measurement probing issue on my part. Hell, its only a normal tube lug with wires.
Thank you very much for looking over this. I suppose next I will pop in the 5V4G rectifier tube and look at DC voltages and try real hard to not electrocute my cats or myself.

Thanks again so much!
Best, Phil D.

[martin manning]

So far so good. Might be flux interfering with your measurement of the 5 VAC. Add the rectifier and check the DC voltage. You won't see much drop as you go towards the first preamp tube as there isn't any current being drawn. Are you using a current limiter?

[pjd3]

Oh Shite, no. I'm not using a current limiter. And this is something I meant to put together.

I'm wondering if I should make one before I try the rectifier tube in. Probably, ey.

Thank you for stopping by,

PJD3

[xtian]

Yes, this is the time for the current limiter!

[pjd3]

Uncle Doug just showed me a nice easy plan.

what I get out it is to used a very large watt bulb (250 watt or greater) as the larger power translates to a lower resistance that will have less effect on the current/voltage to the amp.

I bet Home Depot has everything needed for this.

thanks!

Phil

[sluckey]

It's a 15 watt amp! I highly recommend using a 40 to 60 watt bulb to provide some protection. A 250W bulb will only limit current to 2 amps. And that's enough to fry that little plexi.

u. dug and I disagree on his ideas about a current limiter lamp.

[xtian]

+1,000,000. Use a 40w bulb. With no tubes installed (except rectifier) it will light dim orange under normal operation, but bright if there is a fault.

[pjd3]

Thanks xtian. I will set up for that.

I did go forward however with putting in only the 5V4G rectifier tube and powering it up. I'm finding out everyday how green I am with much of this.
I was very, very surprised to find 495 vdc pretty much everywhere. Its not the "everywhere" that surprised me due - since there are no preamp or power tubes in yet, there would be no loading. However, the 495 vdc did surprise me. For erroneous reasons I suppose, I expected a B+ of around 425 vdc or so.

Looks to me that the resultant B+ value is quite contingent upon everything else being there and placing load on the rectifier tube. I hope that the rectifier tube is happy with no load on its output.

I think its time for a 40 watt current limiter.

thanks guys,
Phil D.,

[pjd3]

Just to say, I was also very surprised how well the F & T 32uF cap cans held their charge. Of course, there wasn't really anywhere for the charge to go, but, a minute or two after shut off there was some 450 volts or more just ticking down very slowly. I was very happy though how well my crazy cap discharger worked. Things were down to the single digits withing 5-10 seconds or so. Thats nice.

Best,
Phil

[xtian]

Now that you powered up and measured B+, there’s no more need for the bulb limiter! Cheers!

[pjd3]

You mean you don't trust me to find a creative way of shorting something else out? It ain't over,,,,, yet.

Was it normal to find such a high voltage coming off the output of the 5V4G?
I initially thought that for a 330-0-330 PT, 330 x 1.4 = 462 volts dc would be the highest I would expect to see. But I'm seeing 495 vdc which is 33 volts higher.

Thanks, !
Phil D

[xtian]

Don't worry--your B+ will sag when all tubes are in.

[pjd3]

Again, thanks xtian.

I'm about to solder in the speaker output secondaries/NFB and pop the tubes in, see what we've got.

The power tubes are Sylvania NOS tubes from Mike (KCA). I'm initially going to bias as 70% of a 12 watt tube for starters.
Should I find that I'm being unduly conservative I'll try bumping the bias current up a bit and listen.
first though, fire this up with tubes in and see what plate voltage I have - go from there.

thank you,
Phil

[xtian]

Biasing to 70% as a rule of thumb is very twentieth century. We know better now. Try 50-60% to start. Your tubes will be much happier.

You mention NFB, so I'll mention the possibility that you get POSITIVE feedback (loud howling) when you power up--this means you have to swap the OT primaries. (Most common troubleshooting issue at this point of the build.)

[pjd3]

Good reminder xtian. I tried to remain aware of that in the build, and it seemed to become even more crucial when I installed the PPIMV. I paid extremely close attention to where everything was going, and always tried to answer to myself "why" I was making each and every connection. Boy, I hope my result has the level of resolve as my diligence was to get it right.

I do have a question about the NFB and in particular which tap it should come from. I think I know the answer but, sure would help to hear about it from someone more experienced.

I'm using an OT that was initially spec'd out and used for a pair of 6L6GC's. But as you know, this amp i'm building now has a pair of 6V6GT's - which i understand would be more happy seeing around 8K as its primary impedance load. I did a couple of tests to confirm the winding ratio's of the taps to primary, then did the calculations to arrive at the impedance of the taps. These calculations are based on a pair of 6L6GC's that I assumed around a 4K ohm impedance. All my tests arrived at the same results (2 4 and 16 ohm taps for an 8K primary impedance) so, I'm pretty confident I arrived with the info I needed.
But, question. Will I still use the 8 ohm tap to feed the NFB by which it was designed in Sluckeys schematic, even though I'm using a different speaker tap than in the original amp? (My 8 ohm speaker now needs to be on the "original" 4 ohm tap to properly reflect the best impedance for a pair of 6V6GT's as opposed to the original 6L6GC's that this OT was seeing in the old amp). I'm fairly sure I have heard that it doesn't matter what speaker you use on what tap, only what tap feeds the NFB. And I have changed nothing in Sluckeys Phase inverter section.

Hope I managed to explain things clearly. I'm trying.

Thank you,
Phil