5F2a, built and at the starting gate

[dig22]

I built a 5F2a using Weber transformers -- PT & OT.
I checked the continuity from the AC plug up to the switch -- all good.

I've gone through the board point by point a couple times looking for shorts and bad grounds and corrected the couple I found.

Using Paul Ruby's "firing up your amp for the first time", I found continuity in the both low voltage lines. If I understand Paul correctly, that's not right. Is it?
If so, what might I check?
-James

[tubeswell]

Unplug the rectifier and the (other) tubes. With your R meter, you should have DC continuity between each of the ends of each respective winding, but not between the windings.

Then check that you haven't accidentally dead-shorted the B+ directly to ground (which you should also check with your R meter) - I'm not talking about the centre tap of the HT winding, I'm talking about the wire off pin 8 of the rectifier - it shouldn't be making direct contact with the chassis.

Then plug the amp in (without any rectifier or other tubes installed) and with your VAC meter carefully check your VAC on each of the PT secondaries. You should have high VAC on the HT winding and ballpark heater VAC on the 6.3V winding and on the 5V winding (for if you have a tube rectifier). When I check voltages I use meter leads with insulated 'gator clips, and I hook them up before I plug the amp in and switch it on etc, so I don't have to probe inside a live amp.

Then if the thing looks good (switch the amp off and) plug in the rectifier, and then with your VDC meter check the B+ at the +ve end of the reservoir cap/pin 8. You should have high VDC and no smokin' or buzzin'. No need to keep the amp on for any long period of time at this point (the B+ will keep climbing up quite a bit higher than the 'usual' operational B+ without the other tubes installed anyhow)

If that checks out okay, then plug the rest of the tubes in and check the VDC elsewhere on the plates, screen, and cathodes of those other tubes. Watch it for a while on the bench with the back off at this point and keep and eye out for (unwanted) smoke (but don't burn your house down). And I mean watch it - don't walk out of the room and leave it on while you go and do whatever. Keep your eye closely on it for a good 5 mins

Hopefully you won't have any major hum or other wanted noise issues, but don't be surprised it you do - its all part of the DIY process.

BTW, if you get a high-pitched squeal when you turn the amp on with all the tubes installed, it probably is the NFB hooked up wrong, giving you (unwanted) positive feedback. Reversing the OT primary leads around will fix that.

[WholeToneMusic]

Your in the trenches of troubleshooting now, don't look back.

[dig22]

Thanks Tubeswell. So here's what happened:
Continuity on the pt secondaries checked out ok.
So with no tubes in, and using a 1/4 amp fast blo fuse, I flipped the switch for a second. The fuse didn't blow. But no hint of pilot light. I switched the fuse to a 1 amp and still no pilot light. I checked all my pilot bulbs on another amp and they were fine.
So here are the readings from the 5y3 socket.

(I should add that on another thread, someone sugested the use the red & white striped HV line instead of solid red on the Weber PT. So I tried that and reads are from those lines.)

Pin 2 0 Vac
Pin 8 5.4 Vac
Pin 4 320 Vac
Pin 6 320 Vac

6 V filaments 3.3 each line

What says ye? What might I try next? With no pilot light glowing, I imagine something is wrong. I think I'm stuck at the starting gate.

[tubeswell]

Hi James

The HT winding VAC (between Pins 6 and 8 of the rectifier) looks OK.

For the 5V filament winding for the rectifier you should have ~5VAC between pins 2 and 8, but Pin 2 should not be reading 0V. I take it that the rectifier is unplugged?) Check that you haven't got Pin 2 shorted to ground

Also you absence of lamplight makes me suspicious about your heater wiring. You say you have 3.3VAC on each side of the heater winding, so that is 6.6 VAC across the winding? Yes? (Can you confirm that by measuring it?)

Make sure you have got the correct opposing ends of the heater winding 'correctly' wired, thus:

For the heater, the 'ideal' layout is to take one end of the heater winding and run it to one side of the pilot lamp, and thence to pin 7 of the 6V6 socket and thence to pin 9 of the 12AX7 socket. The other side of the heater winding goes to the other side of the lamp and thence to pin 2 of the 6V6 and thence to pins 4&5 (tied together) of the 12AX7 socket. If your heater winding has a Centre Tap, that either goes to ground at the same location as your reservoir filter cap ground and High Voltage centre tap, or you can connect it up to Pin 8 of the 6V6 socket (which will elevate the heaters to the same 'ground return reference' as the 6V6 cathode (i.e.: about 19-21V or so - to help reduce hum from this potential source).

If your heater winding doesn't have a CT and you are using a 100R resistor from EACH side of the winding (i.e.; that's 2 x 100R resistors altogether), then run the other end(s) of both 100R resistors to the same ground return reference point that you would've otherwise run the Heater winding CT to as described above.

Either way, keep the pairs of heater circuit wires - that you end up with between the Pt and the lamp, and between the lamp and the 6V6 and between the 6V6 and the 12AX7 - twisted together as much as possible, and keep these clear of other wires (particularly signal wires). This helps eliminate another potential source of hum because the EM field containing the AC wobble surrounding each wire is cancelled out by the equal and opposing EN field containing the opposite AC wobble surrounding the other wire.

When you have got the 5V filament winding sorted and confirm the heater is correctly wired then report back.

[dig22]

It is 6.6 across heater windings at pilot lamp

I corrected the heater wire connections between 6V6 and 12ax7. Still no pilot light. Rectifier pin 2 & 8 are not grounded, however, with the CT, the heater wires are grounded. I think that may be correct but I'm not 100% sure. I'm rushing off to work so can't check anymore till tonight (I'm in Los angeles). Thanks for your help, 'TW'.

[tubeswell]

Re: the lamp, try a different bulb.

Unsolder the 5V winding from Pin 2 of the rectifier socket and carefully keeping it floating (i.e.; not coming into contact with anything but your VACmeter), measure the VAC between pin 2 and pin 8 - should be about 5VAC - if so, then check (with your Rmeter) that pin 2 on your rectifier socket isn't accidentally shorted to the chassis before you re-connect the lead to pin 2. (we are trying to find shorts here)

[dig22]

Tested all bulbs on another amp. All working.

Hi. Well....first off, when reporting voltages, I reversed info.
On the rectifier, pin 2 had 5.4v and it was PIN 8 that was 0. That 0v was consistent when soldered in, unhooked from the pin and with the wire to B+ both hooked and unhooked.
Voltage across the 5 V read 5.4. Then 5.4 on pin 2 alone , and 0 on pin 8.
And that's the good news. Now for the bad:
I unhooked all on the rectifier socket. I sprayed a bit of flux remover on the pins and went at the socket with toothbrush to scrub clean. Then I rehooked all and voila! no voltage anywhere. I'm baffled, certainly, and my knowledge is limited, but I'm thinking the PT is a dead soldier. I checked the continuity from AC line in through switch and all is well. Fuses and bulbs are good.
I have a salvaged PT and a long weekend. Maybe I'll swap it out unless anybody has another suggestion? Sadly, James

[JHow]

Did you go back through the checks in tubeswell's first post? He is explaining how to diagnose dead xformer.

Transformers are expensive. Before buying another one, I would go back to the beginning and re-trace my steps from plug, cord, fuse, switch, transformer primary and back out to the line. Then move to un-hooked, un-plugged secondaries and do tubeswells checks, then move on to hooking of the rectifier socket and so on. If it makes life simpler while you sort things out, use solid state diodes to get your circuit running and then add your rectifier back into the mix.

I know it sounds like a pain and its ground you already covered, but I think chances are good you have some simple error in there that you haven't hit on yet. Transformers can die, but they are usually fairly robust.

[Bruce / Mission Amps]

My rambling thoughts on this... ha ha

A long night for me and I'm a bit dull this morning...

Assuming the PT is not bad.... (a dangerous thing with some cheap transformer winders) ...
1st thing I think you need to do is strip all the PT secondary wires off and start over.
I wouldn't waste time with checking this and that.
If there is not +5vac across the bare leads when disconnected, you have a bad PT.

But if you won't or don't want to take the wires off...

As mentioned... you should only have +5vac across lug 2 and 8... don't measure these lugs with respect to ground... in this amp, they should never be connected to ground
There is no +5vac center tap and there should be NO reference to ground across the +5vac wires unless there are bleeder resistors in the B+ rail somewhere, which this amp does not have.

If you do have some reference to ground on the 5vac leads, you have a short somewhere and it could be just a simple internal misswiring of the
PT (not your misswiring, one from the factory in China if it is one theirs).

By the way, the secondary center taps are only the +6.3vac winding and the HI-V winding.
red/yellow = High-V CT
green/yellow = low-V 6.3vac CT

I think you said with the two yellow wires off the socket, there is +5vac across the two wires... if that is true, then with the wires soldered back onto the socket, you might have a short on the socket somehow if one lug goes to zero while the other stays at +5vac.
I can only think of one or two other way for that to happen and one is if the CT of the HI-V winding is shorted to the 5vac leads.
Have you checked with a DC ohm meter to see if there is any continuity between any other wires with respect to either of the yellow ones?

If you said, with the yellow leads on the socket and the wire from lug 8 to the eyelet board is disconnected ...and there is still 0v on one, then it must be the socket itself.
Another common problem with novice builders is too much heat when soldering. The soft plastic on these leads melts, flows down the bare wire into the lug and becomes an insulator again at the lug.


6.3vac:
Are you using one of the awful three lug Chinese plastic pilot lamp assemblies Weber used to include in their kits?
If so, you should look again at the lugs as they are not what you'd normally see in a pilot lamp assembly and you might have it wired wrong.

[tubeswell]

Are you using one of the awful three lug Chinese plastic pilot lamp assemblies Weber used to include in their kits?

Hi Bruce

Whaddaya mean "used to"?

[JHow]

Assuming the PT is not bad.... (a dangerous thing with some cheap transformer winders) ... (not your misswiring, one from the factory in China if it is one theirs)...

Bruce: I am not in the manufacture or repair business just a hobby-builder, so I recognize my range of experience is quite limited. In terms of PTs and OTs I am mostly involved with Hammonds... but in one case, a Weber PT and OT for a champ-style build (seems to work OK). My impression has always been that transformers generally don't cause a lot of issues, and that new-out-of- the-box usually work.

Your post (and others that I have seen recently on the board) seem to suggest a less-consistent experience. In particular, you mentioned Chinese transformers as being poor. Can you add some perspective on how often you see failures? Were you burned by Chinese stuff previously, and now just avoid it altogether?

Don't get me wrong, I'm no fan of chinese technology - my 1940s Southbend lathe is way better than the 1990s chinese one, for example - but I read a lot of posts where the poster suspects the transformer and it turns out something else. But then again, I don't see the volume that you do, hence my question.

[dig22]

The pilot light? She a glowin'

I swapped out the pilot light holder and bingo. For the record, the bad part was not from Weber.
And with great embarrassment, I report that a press of the reset button on my power strip fixed the 0v readings of precious post.

I still unhooked all on the rectifier socket to take readings:

660 vac across the hi v
6.7 across the 6 v
5.5 across the 5 v (hooked to pins 2 & 8 and unhooked from pins)

Though I understood Bruce's post to mean don't read from rectifier pins to ground, I still I took readings from 5 V pins as we'd gone over that earlier.

with pin 8 connected to B+ wire to first capacitor: 0v on pin 8.
Without that connected wire: 2.6 vac. on pin 8 / 1.9v on pin 2

So if all sounds right to you, I'll mush onward and report back my squeals and hums

Lastly, any advice or comments on Weber PT hi v options? There are two pairs:
1. solid red specs say 660V
2. red & white stripe specs say 600V
I've used the red & white striped because another thread on 5f2a mentioned it was a better choice with the Weber Pt. But the read is still 660v across. Should I reconsider the choice or march on forward?

[MWJB]

I prefer the higher VAC secondary (600VAC has little headroom, even for this kind of amp), 330-0-330 is still not particularly high for this kind of amp, your 600VAC reads high because there is no current draw (tubes installed). Either set of secondaries will "work" fine. "Best" is simply the one you prefer.

[JHow]

Just for the record, what was the failure on the pilot light? Was it shorting to ground?


Re: the PT, I have a 5f2a with weber w022772 transformer. I used the 600VAC red/white leads. I get about 307VAC and about 370 VDC at the top of the power rail. I think you will get a high B+ if you use the 660V leads.

What is your B+?