Smoking OT

[lmolter]

If you live in the States, you could send it to me. I'll look at it no charge and send it back. Just a thought.

Larry

[foxlemieux]

I appreciate the offer, and it might come to that. I'll see what other local options I can exhaust first.

[loudthud]

In the first picture it looks like one of the red wires going to the 5Y3 socket has a yellow stripe. That can't be right, that wire should go to ground, usually one of the transformer bolts.

[foxlemieux]

Thanks for taking a look at it, loudthud. The is the sort of mistake I'm sure I made somewhere but have yet to find. That wire is going to ground along with earth ground on a PT lug. The angle at which I took the picture makes it look like it's connected to the 5Y3.

[foxlemieux]

A quick update: I'm back working my second job which is far less time consuming than my other job. I can finally put a bit of time into trying to sort this problem out.

In the mail tomorrow I'm expecting a new OT (the third for those playing along at home) and new a 5y3 and 6V6. I don't believe the tubes are the problem, but it's safe to say I'm not an authority on the matter. I figured as long as I'm already paying shipping I might as well since I can't buy those tubes locally.

We'll see how it goes.

[erndawg]

Before you install the new OT, why don't you temporarily substitute a power resistor in place of the OT. A suitable value would be 5k at 10 watts.

Disconnect the OT primary, and install the resistor in its place. Also, disconnect the 22k resistor used in the negative feedback loop. Just lift the leg that gets connected to the 12ax7's cathode.

This way, you can measure voltages without worry about burning up the new OT. Report back with your measured voltages.

[theunrulychef]

Maybe you could put a fuse on the OT before blowing out another one? Someone smarter & more experienced than I would have to chime in & say whether this should work & give you a fuse rating suggestion.

[foxlemieux]

As per the good advice I ordered a 10W 5k from Mouser. Thanks erndawg for the resistor value. Thanks to theunrulychef and actually Gibsonlover as well who suggested something similar a long time ago. I feel less likely that I'll burn out another OT with a power resistor in circuit rather than a fuse on the OT. Color me paranoid.

Just so I'm 100% clear I just directly connect the 10W 5k from pin 8 on the 5Y3 to pin 3 on the 6V6? Or should I jumper from the filter cap eyelet? Does it matter?

For my further education if the 22k NFB resistor is still grounded by the output jack does it still need to be disconnected from the circuit because the OT isn't wired in? I'm still studying up on negative feedback theory and function, so the signal path is still a bit nebulous to me.

[erndawg]

What you need to do is disconnect the OT primary leads from the 16 uF/10k/pin 8 node, and pin 3 of the 6V6.

Then connect the 5k/10W power resistor between the eyelet board 16 uF/10k/pin 8 node and pin 3 on the 6V6 socket.

[foxlemieux]

Gotcha, erndawg. That was plan A. A bit more soldering but seemed the best way to go about it. The part comes in tomorrow. A long weekend of work ahead of me, but I'll try to get some voltages posted by mid week.

[robotman]

I didn't see anyone suggesting that each lead from the OT be checked for resistance to the case of the OT. On the output side or speaker side of the OT one of those leads could be to ground normally. On the power tube side neither the blue or red should have any ground ref. at all if measured out of the circuit, ie un-hooked.

[foxlemieux]

Current config as suggested: NFB resistor leg lifted. 10W 5k resistor from 16 uF/10k/pin 8 node to pin 3 on the 6V6. Voltages in the configuration:

5Y3 pin 8: 412V

6V6 pin 3: 205V (low from the 10W 5k resistor?)
6V6 pin 4: 353V
6V6 pin 8: 21V

12AX7 pin 1: 190V
12AX7 pin 3: 1.62V
12AX7 pin 6: 190V
12AX7 pin 8: 1.60V

Those numbers seem in line with what I've read elsewhere except for that 6V6 pin 3 voltage. Any thoughts?

[erndawg]

The voltages look good; a 200 volt drop across the 5k/10W is to be expected. The 6V6 is drawing roughly 40 mA, so the numbers are right.

A Champ OT can have a DC resistance, depending on the OT, of somewhere between 200 - 700 ohms. Its impedance, depending on the OT, can range between 5k and 8k.

The 5k/10W resistor limits the current sufficiently to protect burning out your PT, in case you had an ultrasonic parasitic oscillation or an unexpected circuit path.

For the record, give us the resistance readings of the failed OTs. Measure from red to blue, red to black, blue to black, and yellow to black. I assume your black lead is still connected to the transformer's mounting bracket.

I looked back at your photos, and was wondering about photo 2. The resistor below the screw head, to the left of the 22k (red, red, orange). That is supposed to be 10k (brown, black, orange) but it looks like brown, black, brown (100 ohm). Not that it would burn out the OT...

Report back on the OT readings and the resistor value.

[erndawg]

Also for the record, are you still using the original 6V6 that you have used all along?

[foxlemieux]

Erndawg, when I ordered the 10W 5k resistor I also ordered a new 2W 10k resistor for that position. I didn't like how I seated the original resistor and how it sat over that first position eyelet, especially with all the soldering and desoldering I've been doing at that point. I offset the new resistor by making an 'L' with the leads so that it sits slightly lower on the board leaving room to work around it.

Long story short, the old resistor was brown, black, orange and a quick resistance check verified the rating. The exposure in that 2nd shot shot is what makes the band look brown instead of orange. Kudos for the sharp eye though.

As very much an aside - when I bought the new resistor I got a metal oxide resistor as the original in the kit was also a metal oxide resistor. Would a metal film resistor have done the same job in the same way or would there be a substantial difference in the quality of the power through the different type of resistor?

To answer your second question, no, when I bought the new OT I figured as long as I would be paying shipping I'd go ahead and get a new 5Y3 and 6V6. Those voltage measurements are with the new tubes installed; however, the numbers are very similar to the readings I was getting with the old tubes. I'm still a bit wary of trying the new OT in the circuit just from changing the tubes out.

Wait...actually your second question was about the OT resistance. Unfortunately (and perhaps foolishly) the old OT's found their way to the garbage. The first I actually took apart to get a look at the insides. The second very quickly found the circular file with a bit of force. I'll chalk that up to frustration. Where the red lead tucked into the winding was charred black and chrispy, so I assumed it was well beyond repair or future usefulness.