Power Up; Time to push some electrons!

[Firestorm]

Re the heaters -- with tubes out, they'll be essentially at ground if you're using a center tap; should be 100 ohms if you're using the two 100 ohm resistors.

Now, with tubes out, you've got 400ish volts up and down the power supply, right? Probably no shorts then, since that would create a very noticeable current draw.

Also unlikely that there's a problem in V1 or V2 (probably not V3 either) or you'd see a big a drop occurring across the associated 9K1. So the problem is creating an 80ma draw through the 25W 1K. A short would draw more than that. But that IS close to what you'd see if all the current from both output tubes was going through the 1K!

Did you accidentally connect the OT center tap to the wrong side of the 1K? You should have the rectfier output, the main filter, the two sides of the standby switch and the OT center tap all on one side of the 1K -- the screens on the other side.

If that ain't it, we'll think some more. (I still think you may have your OT primaries reversed, but we'll see.

[gearhead]

I spent hours checking for shorts, especially around the power sockets and B+2 connections. Did check around the IK resistor to make sure I got the connections right. Got around to swapping the OT primaries only just before I went to bed.

Kablam, no screech, no 80 VDC drop across the 1k 25w resistor. B+s mostly in the zones; plates okish. Didn't do a thorough comparison, which I will tonight.

Said what the heh, and plugged in a guitar. Very low volume even when cranked all the way, but something! Don't know if tubes or components were damaged, or there is some other miswiring, but am on the path again.

Thanks FS!

Are there any typical low volume candidates?

[UR12]

Now that you got the OT primary leads sorted out and nothing is smoking or on fire, it would be very helpfull if you measured the voltages on the tubes and post them in a voltage chart so we can help maybe identify where your problem is.

[Firestorm]

+1 on Dana's request for a voltage chart. Include the AC voltage on the heaters -- you mentioned that were were seeing near continuity to ground there, but didn't say whether you using a filament center tap or the dual 100 ohm resistors. If the heaters aren't heating somewhere the tube won't work.
Next, make sure the feedback resistor is actually 100K and not 100 ohms or some other variant.
Then, check all your grounds, especially on the preamp cathode resistors.
I was pretty sure you had your OT primaries reversed 'cause I've heard THAT sound before. But I'm still a little perplexed about the 80VDC drop across the 1K. I don't think the PI can pull that much current even at full signal.
Let's see them voltages. Pics wouldn't hurt either.

[gearhead]

Will do all the voltage measurements; I should have swapped the OT primaries first!

Will check the resistors to make sure they're right. Did it before, but fresh eyes can make a -big- difference.

I'm using Mooses PT/OT set, so it has a center tap on the heater. Tied it to ground at the grounding tab nearest the input/fuse. I did measure the voltage between the two taps (using the tube socket connections), and it was 6.3V. I thought it was an AC measurement, but sometimes . . .

Have noticed that the preamp/power tubes take quite a while to get hot, but was just thinking it was my heightened sense of anticipation. LOL.


BTW, if you've never heard that screeching, it is very difficult to describe the impact. It is FULL volume, and sounds like a higher pitch version of fully locked brakes with tires squelling. Literally sounds like a death cry. Awful, awful sound.

[UR12]

BTW, if you've never heard that screeching, it is very difficult to describe the impact. It is FULL volume, and sounds like a higher pitch version of fully locked brakes with tires squelling. Literally sounds like a death cry. Awful, awful sound.

Yes but now that you have heard it, you will know at a moments notice just what is causing that sound from now on We can talk about this stuff forever and it just isn't the same as hearing it for yourself for the first time.

[Structo]

If the heaters are run right off the PT 6.3v taps, they should be 6.3 vAC.

[Allynmey]

Gearhead, write out a voltage chart. This will tell us where the problem lies. You can also check to make sure the tail of the PI is ground (very common with this build to forget). If there is a ground issue with the PI, you will see weird voltages and attenuated volumes.

Allynmey

[Bob-I]

BTW, if you've never heard that screeching, it is very difficult to describe the impact. It is FULL volume, and sounds like a higher pitch version of fully locked brakes with tires squelling. Literally sounds like a death cry. Awful, awful sound.

I've heard that , and I've taken another stratigy to power up a new amp. I disconnect the NFB from the speaker jack. Once the amp works I TAP the lead to the speaker with my teeth clenched. No screach, I'll hold it on a few seconds more, no screach, power it down and solder in place.

[gearhead]

Thanks all,

Here it is:

Power In: 121 VAC
PT Secondary: 605 VAC

Heaters: 6.3 VAC Tap to Tap, 2.9 VAC Tap to Ground

B+1 406 VDC
B+2 390
B+3 311
B+4 294
B+5 278

V1a plate 175 VDC
V1a cath 1.43

v1b plate 199
vab cath 1.84

v2a plate 267
v2a cath 4.1

v3a plate 191
v3a grid 11.4

v3b plate 197
v3b grid 12.9

V3 common cath 36

V4 plate 401
V4gs 382
V4 grid in -27

V5 plate 401
V5gs 383
V5 grid in -27

The V2a plate and cathode voltages look kinda wacky. I actually had a hard time getting a good read on the plate; it sometimes was as low as 245V. Could possibly be my cheap ablative meter, but it settled on the others pretty quick. Had a real hard time getting a voltage across that plate resistor; it was all over the map. The B+4 was very stable tho.

Feedback much appreciated. Thanks

[Allynmey]

V@ Cathode looks high. The plate is high also. Do you have any good pics to look at?

[slajeune]

V@ Cathode looks high. The plate is high also. Do you have any good pics to look at?

Allynmey is absolutely right here. Check the cathode and plate resistors. Make sure that the cathode is 10k and the plate is 100k not the other way around...

Cheers,
Stephane.

[jjman]

Could also be a switcharoo in the PS between B3 and B4 ?

But I'm guessing that slajeune is correct on the 100k vs 10k switcharoo.

[gearhead]

Pic below.

Whoa, this baby looks much uglier in pics. The heaters look like I used thick jumper cables, LOL.

All potential places that looked like there could be a -possiblitliy- of unwanted contact I've checked and rechecked, but you never know, so ask away.

Thanks!

[Firestorm]

Allynmey is absolutely right here. Check the cathode and plate resistors. Make sure that the cathode is 10k and the plate is 100k not the other way around...

Well, that would certainly give you very low volume, but I think the voltage at V2k would be QUITE a bit higher if that were the case (like 150V).
HOW low was the volume when you tried playing through it. Just barely audible? Or like a volume control set on 1 or 2?
I think we can conclude that the PI works (or at least one of its inputs, based on the "feedback explosion.")
So do recheck the resistor values (feedback resistor too) and if nothing shows up, let's do the stage isolation thing.
First, with the amp off and unplugged, put a guitar cable or a patch cord in the input and make sure you have continuity from the tip to the V1a grid. Also make sure you DO NOT have continuity from the tip to ground (should be 1 meg). If you used shielded cable, it's possible that in soldering, the shield got hot enough to melt through the insulation and short out your signal. Also possible (I hate to admit) to accidentally solder your lead to the shorting lug of the jack instead of the tip lug.
If none of that, double-check your connections to the tone stack, then DISCONNECT V2 from the PI and temporarily connect the wiper of the volume pot directly to the .01 input cap on the PI (short jumper cable with tiny alligator clips works, or just tack it on with a little solder). Be sure you're not connecting to anything with DC on it!
Turn the amp on, plug in a guitar, turn up the volume and see if you get anything. It won't be terribly loud at this point. If that works, move your jumper from the volume pot to the grid of V2 and see if V1b is working. If you get nothing out of V1a, the problem is either there (recheck connections) or in the power amp (starting with the PI itself).
Keep us posted. Pics would help.