Peavey Classic 30 - Speaker Short

[Amp Kat]

That doesn't sound right because if the bias voltage was missing it would red plate and likely blow the HT fuse or maybe it did. But still check that bias voltage good as Jazz pointed out.

[bkahuna]

Like Amp Kat said, if the bias voltage is missing, those tubes would be getting extremely hot and glowing red.

I'd be inspecting those little wire jumpers that connect the pcbs together. (What a great idea that was, Hartley).

[Shawnobi]

Ok, seems I've numbered the socket pinouts the wrong way. When I originally measured them after the build pins 2 & 8 both gave -16. So it's actually pin 8 that has 0v not pin2 (which explains a lot!)

Another thing. ON the input jack it seems that by inserting a jack it connects the screen of the wire to the signal wire creating a short. I don't understand why this is. However that looks to be correct from the drawing? Can somebody explain this?

Thankyou.

[Shawnobi]

Ok, seems I've numbered the socket pinouts the wrong way. When I originally measured them after the build pins 2 & 8 both gave -16. So it's actually pin 8 that has 0v not pin2 (which explains a lot!)

Another thing. ON the input jack it seems that by inserting a jack it connects the screen of the wire to the signal wire creating a short. I don't understand why this is. However that looks to be correct from the drawing? Can somebody explain this?

Thankyou.

Update.
I've pulled the output transformer. When I check for resisitance between the wires on the secondary winding they all show a dead short with the meter. Could the Output tranny be dead then I take it?

[Ronsonic]

Update.
I've pulled the output transformer. When I check for resisitance between the wires on the secondary winding they all show a dead short with the meter. Could the Output tranny be dead then I take it?

It could be dead - BUT - The DC resistance of an OPT is generally very low, like ranging from .4 to 1.5 ohms depending on the tap and the meter and phase of moon. So be careful of the measurement. If you are sure of your meter in low-ohms mode or have other methods of checking, that confirm (Inductance measurement is good, so is running a voltage off a variac and observing / measuring the results) then yep it's bad. If that is the usual tranny wrapped in yellow tape look for signs of heat and stress.

My old Dick Smith ESR meter, as a coincidental virtue, tends to see the inductance of common transformer secondaries as a value close to their nominal impedance rating.

[Amp Kat]

You want to check the primary not the secondary. You should measure across the 2 winding wires which are usually brown and blue and from the red wire to each should be half of that. so if for instance the blue and brown read 150 ohms the red to brown and red to blue should read about 75 ohms. If it does, it's probably good. I'd be looking at jumper wires with a magnifier at this point. You may also try taking the send out to another amps return or imput and see if you get sound. If you do it's in the output section from the return to outs and you have at least cut the work load in half.

[tubeswell]

The other (figity) thing about the C30 is the multi-strand grey 'ribbon' jumper wire (the one with 6 or 8? wire strands in it) that connects the 'middle' board to the extn speaker board, is actually part of the circuit that routes the OT secondary to main onboard speaker via the extn speaker socket (which has switches in it). If that ribbon wire is broke, then the signal won't be getting from the OT to the main speaker. I had this happen to me once - ended up taking the infernal thing apart and putting it back together 13 times before I found the culprit.

[Shawnobi]

You should measure across the 2 winding wires which are usually brown and blue and from the red wire to each should be half of that. so if for instance the blue and brown read 150 ohms the red to brown and red to blue should read about 75 ohms.

I have done this. I'm getting 100 Ohms & 75 Ohms. I also injected a 10VP2P sine wave into the primary and seem to get a clean stepped down wave on the secondary.

Regarding the jumper wires and the ribbon. I'll take a look. I still have the nagging point that this thing was powered with no speaker attached and the wires possibly shorted on the speaker cage though. But I shouldn't rule anything out I guess!

[tubeswell]

Regarding the jumper wires and the ribbon. I'll take a look. I still have the nagging point that this thing was powered with no speaker attached and the wires possibly shorted on the speaker cage though. But I shouldn't rule anything out I guess!

Look at the schematic and you'll see how its connected and on what wires on the ribbon you need to be examining. But check them all for continuity anyhow.

[Amp Kat]

If you are getting 75 ohms from CT to each primary leg doesn't sound good as it should be about 50 or 60 at the very most. Try the 10vpp from secondary to primary and see if you get a big voltage on the other end or use RG's troubleshooting OT guide .
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#OTlive

[Shawnobi]

Does anybody know the stepdown ratio of the C30 output transformer?

[Enzo]

NO idea, but it won't be wrong. The number of turns will not change once it leaves the factory. If you intend to test it, the test is for open or shorted, not turns ratio.

Did we ever find out of B+ was present on BOTH screen and plate pins of the power tubes? Dead amp, first thing to check is ALL the power supplies. And just because B+ is present at the rectifier doesn't mean it is getting to the tubes.

Your shorted speaker leads likely harmed nothing. These transformers are not all that frail.

The primary is wound simply, so the wires are differnt lengths inside, so DC resistance will be about 100 ohms from the red wire to the blue, and about 75 ohms from red to brown. That is normal.

Your transformer is fine - it is the last thing on the list of likely problems.

MY guess? Your screen supply resistor opened. Easy enough to see if I guess wrong or not.

Look at the pair of fuses on the board. From them look along the edge of the board, past the large capacitors until you see two 5 watt ceramic resistors next to one another. The one away from the edge ought to be 400 ohms (or possibly 390 ohms in some amps). On one end of it you should find +332v, and on the other +328. Now your voltages are not likely to be exactly that, but you ought to find 300 and some volts on BOTH ends of it. One end serves the power tube plates and the other end the screens.

If those voltages are not present, there will be no sound.

[Shawnobi]

Thankyou Enzo I shall look into those resistors. On checking the power tubes there seems to be a lack of -16V on pin 2. The scheme seems to say that pin 2 is not connected. But I'm sure I measured 16V there after the build.

[bkahuna]

On checking the power tubes there seems to be a lack of -16V on pin 2. The scheme seems to say that pin 2 is not connected. But I'm sure I measured 16V there after the build.

"There's no bias....there IS bias....there's NO bias"
This is what you seem to be saying. I think you need to find out for sure which pin you're probing, and go from there. Pin 2 on the schematic referenced above is most definitely hooked up. It has to be. It's the grid, where the bias and signal enter the output tubes. If the bias is really gone, and you left the amp on more than a minute or two, your tubes would be glowing red hot.

[Shawnobi]

Hi Enzo,
It's a 390 Ohm ceramic for the record. I'm seeing 360V on one side and 370v the other. Interestingly the speaker gives a crackle when the test probe touches either side if that gives any hint...