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Old 25th March 2008, 04:56 AM   #21
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Default Finally!

It's up and running.
My first mistake made last week was that I set the output tube bias at 2.5 mA (thinking the filament current is 2.5) instead of 50 to 60 for my 2A3 tubes. That's what blew the fuse when I turned up the volume a bit. All this hassle just from not reading throughly...

I still have one issue though. It's feeding my spare speakers which is around 86 db efficient but there is hum. Could it be from all these wires around it? Right now it looks just like Tubelab's "Checkout" process picture (no chassis, everything sitting on a piece of board).
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Old 25th March 2008, 06:49 AM   #22
wicked1 is offline wicked1  United States
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I just did the checkout tonight, and have no hum. Mine is also spread out all over the place. I don't have any wires crossing, tho.
Are you using a choke?
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Old 25th March 2008, 02:08 PM   #23
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I have one but not hooked up yet. Thatís just an upgrade though, isnít it? The hum from mine is very audible as is.
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Old 25th March 2008, 07:52 PM   #24
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As a note, here are the measurements during last test run.
B+ = 376 vdc, B- = 232 vdc, input tube plate to ground = 178 vdc, 180 vdc, output tube bias current = 50.1 mA, 50.8 mA

Is it normal for B+ and B- resistors to get hot? Mine got hot to the point of deforming the plastic tips of test probe hooks.
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Old 25th March 2008, 11:27 PM   #25
wicked1 is offline wicked1  United States
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Hi, I'm using the hammond 272jx transformer.

B+ = 320v, input tube plate =176v output tube current 70ma

the resistors on mine are hot, but I can keep my finger on them for almost 10 seconds before I can't take the heat. I've seen that used as a general rule for electronics heat... not sure what it means, really.

Power transformer remains cool, the choke tx is pretty warm. Everything else other than the tubes stays cool



the choke is an easy upgrade. Remove R4 (double check this on the tubelab site!) and wire the choke in its place. (as long as you got the choke he recommends, which is the same resistance as r4)


I love mine! The highs might be a little muted, or maybe in my last amp the highs were exaggerated. I've got to do a little more testing w/ other amps to figure it out. This one definitely has more bass than my other amp! almost don't need the sub anymore!!
Other amp is a 6v6 PP
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Old 26th March 2008, 02:41 AM   #26
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Default Choke filter in.

The hum has been reduced once the choke filter was put in but still audible from 1 ft away (86 db speaker). I'm wondering if something else is at fault.
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Old 26th March 2008, 02:44 AM   #27
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Quote:
Originally posted by wicked1
Hi, I'm using the hammond 272jx transformer.

B+ = 320v, input tube plate =176v output tube current 70ma
What kind of output tubes are you using? Tubelab suggested in his website 50 to 60 mA for 2A3 tubes.

Here's the latest measurements after the choke connection.
B+ = 350, B- = 228, input tube plate = 178, 2A3 bias current = 56 mA.
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Old 26th March 2008, 03:07 AM   #28
wicked1 is offline wicked1  United States
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OH That makes a difference. . 300b

I have no hum on mine at all. I'm actually quite surprised!

do the standards, like making sure the power is rotated 90 degrees from the output and choke tx's. Twist all of your interconnecting wires. My power wires aren't twisted or braided(but I don't think it would hurt), nor is the choke(I dont know for a fact, but twisting the choke wires together seems like a bad idea to me). The wires going to my external capacitor are twisted, and the wires going to the output tx's are twisted. The wires leading from my rca jacks to the amp are twisted. (I'll use shielded wire when I put it in its case) Try to set it up so that nothing is crossing over any of the power wires.

modifications I've made.... the 100uf power capacitor is a motor run cap. I added some 1uf poly/foil caps across the 4700uf caps on the input tubes. (not sure if it makes any difference or not, but had them on hand, so used them)

I think that's it... I did put all the tall components on the bottom side of the pcb, but that wouldn't make a difference w/ hum.

Can you tell if the hum sounds like standard 60hz power noise? I'd guess the noise is coming from either the power, or getting introduced before the input tubes, and then getting amplified.

That's about all the advice I can offer, as Ive mentioned, this is my first amp, too. I know it's a good design. If nothing Ive mentioned helps, start checking any parts you've used that aren't new, or aren't what Tubelab recommended.

I've got a few synth projects on my bench that have been finished for months, but I haven't powered them on for the first time yet. Troubleshooting electronics projects is frustrating! I use a bunch of old opamps and transistors, etc from surplus stores in my synth, and so besides mistakes Ive made, I often have to hunt down bad components. Troubleshooting always takes longer than building the thing in the first place.
So, much sympathy, and good luck!
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Old 26th March 2008, 02:57 PM   #29
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Default Hum

I connected it to my high efficiency main speakers. Yikes, loud 60Hz hum! I must have have mis connected the transformers or something else may have been damaged. Here's how it's wired.

PT: Hammond 272JX
Section 1 (600V CT @ 250mA DC) red-red to T1-4 & 5, red-yellow to T1-1
Section 2 (5V CT @ 4A) yellow-yellow to T1-2 & 3, black-yellow not connected
Section 3 (6.3V CT @ 8A) green-green to T1-7 & 8, yellow-green to T1-6
Primary (115V 60Hz 236VA) black-black to power supply.

OPT: Hammond 1627SEA
Secondary 8 Ohm yellow & black to speaker terminal, green & white not connected.
Primary 2500 Ohm - blue to T3-2 (left) & T2-2 (right), red to T3-1 (left) & T2-1 (right), yellow-blue not connected.

Any help will be appreciated. Thanks.
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Old 26th March 2008, 05:42 PM   #30
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Here are a few things to try.

If the amp is still in the breadboard state see if moving or rotating the transformers makes any difference. Sometimes the 60 Hz magnetic field from the power transformer can get into the output transformers and cause hum. If the hum is audible as soon as the amp is switched on (before the tubes warm up) this is likely the case.

Try disconnecting the input wiring from the PC board. Run the amp with no input to see if you still get hum. If the hum goes away the problem is with the input wiring.

Make sure the metal case of the volume pot (if used) is grounded.

Does the hum level change with the volume pot setting?
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