As mentioned in my introduction, I am working on finding a short in the captioned DIY amp. I have checked and rechecked wiring, connections and solder joints. I have checked continuity of virtually every wire and every tube socket pin. I have rolled rectifiers and power tubes and preamp too.
I fire it up through a variac plugged into our 120vAC main and then a light bulb current limiter plugged into the variac ....no smoke, no odor of burnt anything, no arcing in the dark. The thing starts life as it should and about 30 seconds after power on, the current limiter goes full on bright, so I shut it down and keep looking.
I know that my variac and current limiter are functioning as I tested a good amp on it. Like I said above, I have tested all resistors and caps for values within tolerance, no broken wires no bad solder joints...I'm at a loss! Oh, I even checked the resistance on both sides of both transformers before installation and after.
What am I missing?
Schematic attached.
I fire it up through a variac plugged into our 120vAC main and then a light bulb current limiter plugged into the variac ....no smoke, no odor of burnt anything, no arcing in the dark. The thing starts life as it should and about 30 seconds after power on, the current limiter goes full on bright, so I shut it down and keep looking.
I know that my variac and current limiter are functioning as I tested a good amp on it. Like I said above, I have tested all resistors and caps for values within tolerance, no broken wires no bad solder joints...I'm at a loss! Oh, I even checked the resistance on both sides of both transformers before installation and after.
What am I missing?
Schematic attached.
Before unhooking any caps, remove the octal tubes and power up. The bulb tester should not glow. Then replace only the 5AR4 / GZ34 and try again. If the bulb glows you likely have a bad filter cap (the 33uF electrolytics) or a bad OPT. If the bulb does not glow, but does glow with the 6V6 tubes installed, you may have a bad tube or a shorted 1 uF cap. Look carefully at the 6V6 tube sockets for any signs of an arc around pin 3, especially if they are black plastic.
Thirty seconds sounds like a thermal issue of some sort. If Tubelab's suggestions don't produce the issue, work back with each preamp tube in turn. I doubt any of the preamp tubes have a thermal fault, but you'll need to verify that. If the tubes look good, then I'd insert an ammeter into the heater circuit and watch for the spike there. Perhaps the heater balance resistors or the pilot light have issues.
A short or other severe overload on the B+ will not show up until the rectifier tube is hot. The preamp tubes are all downstream of the main B+ being fed through the 4.7K and 12K resistors. These resistors would not pass enough current to make a bulb tester glow without noticeable damage. A short on the heater circuit should blow the line fuse.
I did not realize that this is a DIY amp. If so, the first question is, "Did it ever work."
Agreed, a fault in the heater circuit would blow the fuse, if current weren't limited by the light bulb. Even a 100 watt bulb will limit to less than one amp, though: far below the 2A Slo-Blo fuse that the schematic says is in there. I also agree that the 4.7K and 12K resistors should limit current to the preamps, provided they are of the correct values. Given this is a DIY, all bets are off. (And I mean no disrespect to the builder, weird stuff happens all the time in one-off builds.) Absent issues with the amp sections, the heater circuit is my best guess for where else to look.
Hi, Good thought, I use it regularly with no issues but I checked the current limiter bulb just to rule out the possibility and it is an "incandescant 100W 120V GE". No, not a stereo stereo's are over my skillset at this point. The 0.1uf caps are new Spragues.
Thanks for the suggestions.
Hi!
Yep I had checked initial power-up with no tubes installed, as it is my SOP for first time fire up. I did it again just now and no glowing current limiter, I rechecked the 6.3vAC heater wiring and got 3.4vAC at all heater pins. All's well until I install the rectifier tube.
I had already rolled 3 rectifier tubes to be sure I have a good one and in desperation I even pulled one from a working amp to be sure I have a good one. But, to be sure I reinstalled the 5AR4 and powered it on. This time I timed it with my watch (no estimates) ...no current limiter glow at all until 24 seconds after power on at which point it went suddenly and fully bright. Again, I had already checked my current limiter function with a working amp...so I was certain it works. I use a 100W incandescant GE bulb.
The filter caps are new Vishay's of the correct values and they are installed with the correct polarity, I can't get good readings on them installed, so I guess I will have the pull them.
The OPT is a new Mercury Magnetics FTDO-59 I'm suspecting it even though I checked it for continuity and resistance against the spec sheet attached.
Agreed, I figured it was a thermal issue given there's no issue until the rectifier is hot. But a blown resistor usually looks blown and if it does go bad it is generally open right? All downstream voltage dropping resistors values are checking out fine and no signs of burning or shorting either.
Nope the amp has not worked since built. The problem was discovered in my initial fire up sequence. Yes, I built it and I take no offense, I am here to learn. I had a new Sprague go bad in a Tweed Bassman that I built last year, so I am familiar with the possibility of new bad components.
And, I agree it must be the first filter cap or the OPT.
My build and my initial fire up.
The sequence I use starts with no tubes and all pots to zero or 1 then I bring it up slowly on the variac, the current limiter is plugged into the variac and the amp plugged into the current limiter. I check for any arcing, smoke or odors and then proceed to check the heater voltages that's my first step.
Second step is to shut down and install the rectifier (only) and fire it up again. This is as far as I got so far.
If I wasn't experiencing an issue I would shut it down, plug in a speaker and the power tubes and fire it up again.
If no issues, I shut it down and install the preamp tubes let it get warmed up and check voltages and power tube bias.
If all's well, I plug in a guitar and start fiddling with the volume and tone knobs.
If all's well at this point I am a happy camper.
The higher the wattage of the bulb in the limiter, the less protection. You want to use the I=P/E formula (max-current equals wattage divided by voltage) to see the maximum current allowed by the limiter.
no it wouldn't matter...that post was impulsive...i mistakenly pictured a scenario where there was no bias to the output tubes! (the older i get the more Homer moments i have. Doh!)
Update: I have had time finally to revisit this thing. Since consensus here seemed to confirm my suspicions about the Output transformer I swapped OT's from a working 5E3 and there's no problems with the OT. Good news because they are expensive, bad news because now I have to start unsoldering and testing the filter caps.
Thanks for the help from all so far. If you think of something I missed please let me know.
Happy New Year.
Thanks for the help from all so far. If you think of something I missed please let me know.
Happy New Year.
What would happen if you forgo the bulb tester, just use your Variac to bring up the voltage slowly, while monitoring the B+ with a DMM? By 90 VAC it should run. The worst is you take out the 2A fuse?
Even using the dim bulb, by slowly increasing the Variac output, you could at least find the B+ value when the bulb takes off. Might be a piece of diagnostic information that could point out what's going on.
Even using the dim bulb, by slowly increasing the Variac output, you could at least find the B+ value when the bulb takes off. Might be a piece of diagnostic information that could point out what's going on.