Fire up

[basschops1528]

Well it's time to finally fire it up and the current bulb shines bright and the speaker hums on just more than quietly. I followed Paul Ruby's guide and everything seems OK. Bias pot set to middle and all tubes in. There's a short somewhere and I suspect a power tube. Though maybe a preamp tube. I remember in HS when I fired it up and one of the Power tubes arcd internally but alas I never marked it. Can I remove tubes one at a time (preamp and power amp) to possibly find the culprit?

[PRR]

Take all the tubes out.

But it is true that preamp tubes can't cause a line-limit-lamp to glow (they have lots of resistance in circuit); the POWer tubes (including rectifier) are what can cause trouble and can be conveniently yanked for test.

For thought: I would work the other way. Wire *just* the switch and PT primary, all secondaries taped/insulated. Lamp-test. Run the heater wires, lamp-test. Insert tubes, test. Now you know the 117V and the 6V systems are non-shorted (and probably heaters lit). Then rectifier-- the DC output will look low, but it should be hundreds of V. Then first filter cap, and now you should have a high (no-load) DC voltage. If a bias supply, build and verify that it makes -negative- DC of several dozen Volts. Go on with OT, output tubes, and their bias. You should have no limit-lamp-glow, ballpark voltage and current around the output tubes.

See, if _I_ try to build the whole thing *before* any live testing, I'll screw-up somewhere and can't find it without back-tracking the build. Whereas when built in stages which can be tested as I go along, I'll stop when I find fire/smoke/glow and re-examine what I just did.

[basschops1528]

Sorry I should have been specific. Let me list the order of where I'm at:

1. With HT unwired, heaters tested good and all tubes glowed with no limit bulb glow.
Checked HT secondaries at 180-0-180.  No tubes in.
2. Wired in HT and still no bulb glow. B+  483V. Screens 483V  V1,2,3 @ 445V. No tubes in.
Checked bias at all power tube sockets. Between -28V and -46.2V.
3. Wired in OT primaries and secondaries. Put in all tubes including unknown suspect Power tube.
4. Engage power switch, heaters glow, no limit bulb glow.
5. Engage HT switch, the above happens, limit bulb glows, hum in speaker, no arcing at all.

[PRR]

So take the power tubes out.

[basschops1528]

One power tube was shorted between Pins 3 and 2/7.   The heater coil is jusssssssst touching the metal spacer at the top. AHA!!
I tested all the other PTs for shorts and they're fine, though one tube has the heater very close to the spacer, it's not shorted. I will be keeping an eye on this.
BTW, they're EL34's from Electro Harmonix. You get what you pay for. I'm checking the preamp tubes as well.

[basschops1528]

Success! It works! Sounds decent on $25 subwoofers so far After playing for about a half hour, the power tubes were hot but not red plating, preamp tubes were fairly warm, PT was warm on the cover but laminations were less warm than it, OT dead cold.

The only thing I still need to do is to set the bias properly. I had it set at -37.4 mV  for now. I have a feeling that the 1 ohm cathode resistors are giving me a false reading because they are actually 1.5 ohms x 3 and 1.6 ohms for the set of four. Would that mean that even if I want a bias current of 35 mA, I would need to at least set it for 52.5 mV for 1.5 ohms and 56 mV for 1.6 ohms?   52.5 / 1.5= 35 mA  or 56 / 1.6= 35 mA

[Jack_Hester]

The only thing I still need to do is to set the bias properly. I had it set at -37.4 mV  for now. I have a feeling that the 1 ohm cathode resistors are giving me a false reading because they are actually 1.5 ohms x 3 and 1.6 ohms for the set of four. Would that mean that even if I want a bias current of 35 mA, I would need to at least set it for 52.5 mV for 1.5 ohms and 56 mV for 1.6 ohms?   52.5 / 1.5= 35 mA  or 56 / 1.6= 35 mA

What is your plate voltage for each power tube?  To get the calculated values that you give, you would have 500vDC on the plates, assuming a Bias setting of 70%. 

Jack

[sluckey]

Would that mean that even if I want a bias current of 35 mA, I would need to at least set it for 52.5 mV for 1.5 ohms and 56 mV for 1.6 ohms?   52.5 / 1.5= 35 mA  or 56 / 1.6= 35 mA

That is correct. However, many meters have trouble accurately measuring low resistance values. Does your meter read zero ohms when you short the probes together? Do you have a relative button on your meter? If so, use it.

I would just buy 1Ω 1W 1% resistors and trust that they are within tolerance.

[basschops1528]

I have two meters:

Meter 1: On 200 ohm setting, probes shorted measures 0.4 ohms (after the digits bounce around), continuity setting measures 0.001 ohms,

Meter 2: On 200 ohm setting, probes shorted measures 0.6-0.8, continuity setting measures 0.00 ohms, and the sound pitch slowly increases (dying battery? I used it to test continuity in a transformer primary once and the pitch climbed until the battery light came on, same way ever since).

I checked my parts order and the resistors are :
R-262-1 2 Watt  Metal Oxide Power Resistor Value: 1 (4) @ $0.15 = $0.60
https://tubedepot.com/products/2-watt-metal-oxide-power-resistor

[PRR]

> the resistors are : https://tubedepot.com/products/2-watt-metal-oxide-power-resistor

"5% tolerance"

These resistors are more accurate than any bench-meter can be around 1 Ohm. Take them at face value.

35mV-40mV should be a perfectly playable bias.

[basschops1528]

When I went to check the bias last night, the meter went from 0 up to -40 and was still going when I shut the Power off. I couldn't bring the volume pot past 10-15 degrees or so without it being too damn loud! It was clipping too, sounded gooooooodddd.    But its at -35 now and I can get the vol up a little more then before but still.... This thing has a lotta horsepower. Pictures and video to follow in some time