Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JZRepair on February 08, 2022, 08:52:36 am
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Ok, I've scoured the internet and I can't find any further information. I've spoken with Doug at magnatoneamps.com and gained about as much info as I can. I'm working on repairing a Magnatone/PANaramic 1200 (96-10041). The power transformer has blown. Under power with only the primary connected, it blows the fuse every time. The problem is, there's no reference to a part number and I haven't been able to find a suitable replacement, because there's no available information on the requirements of the Secondary. All I know from the schematic is that it has a B+ (rectified) at 360V, filaments are 6V3ac, and the rectifier should be at 5Vac.
I've looked into the 213 and the 413 models and still don't have much information. I have learned this was the 6206 Transformer, but even that produces no results.
So, my question is this. Has anyone built one of these units from scratch? Or, has anyone had to replace the power transformer? If so, what did you use for a replacement? If you could provide a part number, or a link, that would be incredible. But, I've reached the point where I'm at a loss at this point and I'd hate to go back to my customer and tell him there's nothing I can do. There has to be a suitable replacement out there, I just haven't been working with tube amps as much as with solid state, so PTs, OTs, tubes, and the like are a bit outside of my comfort zone, but I'm really wanting to learn as much as I can, as quickly as I can.
Unfortunately, this PT was blown when I received it. Not sure what caused it and have no way of referencing voltages to get any idea as to how to find the best replacement. It would even be helpful if someone has one of these amps and has the ability to remove the secondary and take some voltage readings for me.
Thank you in advance for your help.
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Finding a replacement PT should not be too difficult. You may need to drill some new mounting holes. We need more info...
Hi-rez pics of the chassis, inside and outside.
Hi-rez pics of the PT and mounting hole dimensions.
Good schematic. Hopefully your schematic is better than the ones on magnatoneamps.com.
Which rectifier type is your amp, ss diodes or tube?
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In Doug's library, under miscellaneous, Panaramic 1220, might be very close? :dontknow:
https://el34world.com/charts/Schematics/files/_miscellaneous/Panaramic_1220.pdf
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He says he'd got 5VAC at the rectifier, so . . .
While trying to read the schematics -- Doug's library and the Magnatone website have the same blurry photos, one with SS rectifier and one with GZ34 -- got a headache.
The tube complement is like a Fender DR, but the biasing scheme is not the same (as best I can tell from the photos). Nonetheless, wouldn't a Hammond 290BX work here?
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He says he'd got 5VAC at the rectifier, so . . .
No. What he said is "the rectifier should be at 5Vac". And "with only the primary connected, it blows the fuse every time."
The tube complement is like a Fender DR, but the biasing scheme is not the same (as best I can tell from the photos). Nonetheless, wouldn't a Hammond 290BX work here?
The voltages are probably close enough but I think he needs a stand-up style PT, more like the 272BX.
https://www.hammfg.com/files/parts/pdf/272BX.pdf
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> requirements of the Secondary. All I know from the schematic is...
Also that it has two 6L6 (not CG?) in cathode bias at 425VDC, which is 95% of the total load (the little tubes can't suck much).
What do we know with two 6L6? Fender Bassman comes to mind. Although in 5F6a form it is a much more powerful amp it would be conservative. I remember once using Weber's 5F6a PT on a big cathode bias amp and got 415V at low line voltage. That was with solid-state rectifier.
> the 272BX.
300-0-300VAC will make the 425VDC with zero rectifier loss. With 5U4 bottle it will be ~~50V less, so down from original power. We may want 360VAC to keep using the hot bottle?
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Also that it has two 6L6...
Y'all are looking at the wrong schematic. His amp is model 96-10041 and uses 6V6s. Here's the correct schematic...
https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_estey_96_10041_reva.png
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Finding a replacement PT should not be too difficult. You may need to drill some new mounting holes. We need more info...
Hi-rez pics of the chassis, inside and outside.
Ok, here's the links to the pictures:
https://www.jaerllc.com/images/panaramic_1210-clear_schematic.jpg
(https://www.jaerllc.com/images/panaramic_1210-clear_schematic.jpg)
https://www.jaerllc.com/images/panaramic_1210-front.jpg
(https://www.jaerllc.com/images/panaramic_1210-front.jpg)
https://www.jaerllc.com/images/panaramic_1210-inside.jpg
(https://www.jaerllc.com/images/panaramic_1210-inside.jpg)
https://www.jaerllc.com/images/panaramic_1210-underside.jpg
(https://www.jaerllc.com/images/panaramic_1210-underside.jpg)
Hi-rez pics of the PT and mounting hole dimensions.
Mounting hole dimensions are 85mm, post to post, squared. It's about 1mm difference from front to back, compared with width, but there is room to move. Front to back, +/-10mm, width +/- 5mm. Part number stamped on the PT is 6206 606133. No other tags or identifiers that I can see.
Good schematic. Hopefully your schematic is better than the ones on magnatoneamps.com.
Sadly, there wasn't a schematic still glued to the cabinet. I had to go off of magnatoneamps.com, but I did find a more clear schematic and have included that in the pictures. There is a slight difference with this amp, where the 100k resistor that connects the 20uF to the .1uF caps in the power supply is missing. instead, it connects directly to the .1uF cap.
Which rectifier type is your amp, ss diodes or tube?
Rectifier is the same with the schematic. 5Y3. I had replaced this, because there was no more silver color on the inside of the tube. No shorts in the tube, but I don't have access to a tube tester. Just with age and not seeing that silver, I knew it would be better to replace. So, I don't know if that blew and that's what caused the transformer to go, or if it went on it's own.
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Wow! You guys are really quick to respond. I wasn't expecting that! Thank you.
I also noticed, it's model 1210, not 1200 like I originally posted. The schematic I posted was the closest thing I could find.
The output tubes are the 6V6 GTs. That small corner of the schematic is the most difficult to read, but it does appear that they have a plate voltage of 350V.
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I still recommend the 272BX but really like the beefier 272DX too. You will have to drill new mounting holes. Hawk Electronics has good prices and fast shipping.
272BX (https://www.hawkusa.com/search-query?search_api_views_fulltext=272bx&field_current_stock=All&field_rohs_compliant=All&items_per_page=10&offset=0) Data Sheet (https://www.hammfg.com/files/parts/pdf/272BX.pdf)
272DX (https://www.hawkusa.com/search-query?search_api_views_fulltext=272dx&field_current_stock=All&field_rohs_compliant=All&items_per_page=10&offset=0) Data Sheet (https://www.hammfg.com/files/parts/pdf/272DX.pdf)
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I would like to be absolutely sure the PT is faulty before ordering one. I suggest you disconnect the PT primary leads. Then plug the amp into the wall and see if the fuse still blows with just the couple switches, fuse, and cap connected to the line.
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6V6? So it is a Fender Deluxe with an extra preamp tube.
(Altho it may be time for a dim-bulb?)
Agree with no-PT shorts test. Would be annoying and expensive to ship iron and still be shorted because of frayed wire or cracked fuseholder.
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I would like to be absolutely sure the PT is faulty before ordering one. I suggest you disconnect the PT primary leads. Then plug the amp into the wall and see if the fuse still blows with just the couple switches, fuse, and cap connected to the line.
Yes, I'm well aware of how expensive the PTs are and shipping for them. So, I had already done this. I first removed all the tubes, so I could eliminate a tube. It still shorted. So, I removed all the secondary wires from their respective connections and still blew the fuse. The only thing I had connected on the primary was the switch and the fuse holder. I tested for continuity across each individual winding, as well as to chassis and against any other winding. It didn't produce any suspicious results, but I've also seen other components test out perfectly with no power, and have a short when power is present.
This amp just sits in one room and never moves. He's never had any problems with it since he's owned it. He got it from family. He said he turned it on and it just started blowing fuses. That's when it ended up in my shop. I did not remove the fuse holder, but I did check it for a short against the chassis, same with the switch. Initially, I thought it was the "anti-hum" switch for polarity. The amp was updated to a 3-prong, but the death cap was never removed, nor the polarity switch disconnected. While the polarity switch was in play, I had a short across that switch, but as soon as I removed it, and double-checked the switch, there was no short. I put the death cap through a capacitance/ESR tester and it tested out great. If I had hair, it would have been pulled out at that point.
That is a good call though on removing the primary to see if one of the others is causing the issue. I'll check that in the morning and get back to you guys.
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I did a little brush on the schematic, might help!
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Ok, since the death cap was previously removed and the short still happened, I wanted to make sure that stayed the same. I removed the primary from the circuit so all that remained was N -> fuse holder, fuse left in. L -> switch and ground to chassis. Red meter probe on opposite end of switch, black probe on opposite end of fuse holder. No result in the off position, flip the switch on, 600mVac. This seems really low to me. So, I moved the black probe to the other end of the fuse holder (to N wire).
With the switch in the off position, it produces 11Vac. Switched to on position, 114Vac.
I feel like the switch should have 0Vac in the off position. The fact that 11Vac coming through makes me think the switch isn't quite disconnecting, but I also feel like under power, I should still read AC voltage from the first reading with the black probe on the opposite end of the fuse holder as N when there is a fuse in play. But none of this still makes me think the PT isn't causing the short.
It is a bit odd to me to only see 114Vac at the switch and fuse, but, I guess I'm only getting 114 from the wall socket (verified).
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But none of this still makes me think the PT isn't causing the short.
DId you read the PT wires while disconnected, to see what impedance it have or if it is shorted or anything else?
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DId you read the PT wires while disconnected, to see what impedance it have or if it is shorted or anything else?
About 4.5 Ohms across the primary. Double-checked for any shorts to ground or to any of the secondary, still nothing.
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So let me ask this...if that rectifier diode would have blown, could that have caused the PT to short out? Looking over the datasheets for the two transformers provided, the current is awefully small on both of these for the primary. The fuse is a 3A fuse. It's probably my limited knowledge of transformers, but at 224mA-230mA of the max on the primary side winding, it suggests that the transformer would blow before the fuse would.
Again, maybe I'm misunderstanding something here, but it's not making sense in my head. I guess the better question would be, which would be the better transformer to go with?
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Don't lose focus. If the fuse held when you disconnected the primary leads then you should order a new PT. I'm favoring the bigger one, but the voltages will be higher, probably around 400VDC. If you want to keep the B+ around 360VDC then use the smaller one.
Meanwhile, read about the current limiter light bulb (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf), aka, dim bulb tester. Build one and use it when you first test the amp with the new PT to prevent any meltdowns. You need about a 60 watt bulb for this amp.
Yes, a shorted rectifier tube or shorted filter cap, or shorted filament wiring can damage a PT. And they can also blow the fuse, but it usually doesn't blow immediately when you turn the power on. In fact, it may not blow at all, allowing the PT to melt down, which will then blow the fuse. So, ckeck for shorted filter caps and rectifier tube. If your fuse is blowing immediately the problem is usually the PT or wiring on the primary side.
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Don't lose focus. If the fuse held when you disconnected the primary leads then you should order a new PT. I'm favoring the bigger one, but the voltages will be higher, probably around 400VDC. If you want to keep the B+ around 360VDC then use the smaller one.
Meanwhile, read about the current limiter light bulb (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf), aka, dim bulb tester. Build one and use it when you first test the amp with the new PT to prevent any meltdowns. You need about a 60 watt bulb for this amp.
Yes, a shorted rectifier tube or shorted filter cap, or shorted filament wiring can damage a PT. And they can also blow the fuse, but it usually doesn't blow immediately when you turn the power on. In fact, it may not blow at all, allowing the PT to melt down, which will then blow the fuse. So, ckeck for shorted filter caps and rectifier tube. If your fuse is blowing immediately the problem is usually the PT or wiring on the primary side.
Thanks for the advice.
As for the caps, yes, I've pulled the vast majority of the caps and tested them with my capacitance/ESR tester, including the multi-cap can electrolytic. I have seen where this tester doesn't do the greatest job, but being able to monitor ESR makes a lot of difference. Usually caps are one of the first things I check, because as we've all seen, a shorted cap can wreak all sorts of havoc. All the caps appear to be original, but in great shape. The Can is the only one that appears it may have been replaced at one time. Also, the rectifier was the first thing I suggested be replaced, since that silver coating within the tube was nowhere to be found, just little flecks of white "dust."
As a final test, I just purchased a 60W filament bulb, and connected it to the switch, the fuse, and chassis ground. I then rigged up an electrical socket for a clamp light so I could put a load on the switch and fuse. Everything worked fine, so I've eliminated the switch and the fuse. The benefit is, I can use this as a current limiter in the future, which I will do after installing the new PT.
Thank you, BTW, for connecting me with Hawk Electronics. WOW! Those are much better prices than what I was previously finding and will definitely be using them in the future, as well as for replacing this PT.
So, ideally, I'd like to keep the circuit as close to original as possible. I'll leave it up to my customer, but, what are the benefits of the higher voltage? I know cathode biased tubes like the higher voltages, but I don't know why. Could you explain? Or link me to some material that will paint a picture?
Also, because I'm a solid state guy, I don't know much about tubes or tube amps. I understand them enough, but I want a better understanding. I do this as a business and I love electronics. I am super passionate about electronics and repairing them. I want to soak in as much info as I can cram in my tiny little head. Are there any books, websites, or other materials anyone can recommend for a deeper understanding of tubes and tube amps?
Again, I can't tell you guys how much I appreciate the quick responses and help with this issue. You all have been amazing and I can't thank you enough!
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Plenty of reading material here...
https://el34world.com/Forum/index.php?topic=3273.0
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recommend for a deeper understanding of tubes and tube amps?
http://www.valvewizard.co.uk/index.html
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Ok, new transformer came in last night. I just noticed that the heater winding doesn't have the center tap the original had. This shouldn't cause any problems, should it? I mean, the whole reason they did this was to help eliminate noise with the 2-wire mains, combined with the death cap to tie either line or neutral to the chassis. With the ground wire tied to the chassis, the vast majority of noise should have been eliminated, correct?
Also, thank you for pointing me towards the links. I haven't had a chance to look at them yet, but they're on my list and I'm hoping this weekend I can set aside some time to spend digging deeper.
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Ok, new transformer came in last night. I just noticed that the heater winding doesn't have the center tap the original had. This shouldn't cause any problems, should it?
Yes, it will cause 60Hz hum in the amp. There's an easy fix. Just build an artificial center tap. Easy to do. Connect a 1/2W 100Ω resistor from pin 2 to pin 8 on a 6V6 socket. Connect another 1/2W 100Ω resistor from pin 7 to pin 8 on the same 6V6 socket. That's all.
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> the whole reason they did this was to help eliminate noise with the 2-wire mains, combined with the death cap...
No. Hum comes in layers like an onion. Hum from ungrounded chassis is different than hum from ungrounded heater circuit.
Two resistors to ground usually works. Sluckey just gave you an extra layer, connecting to a DC source (tube cathode).
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Thanks guys. I don't know why I don't ever think of a bridging the resistors together to make a ground. For some reason it never crosses my mind.
I also notice that the red/red-yellow/red (300/0/300) should be connected to the heater pins of the 5Y3 (pins 2/8). But, I am not supposed to have a center tap. This seems like a stupid question, but not asking it would be even dumber. So, I can just clip and cap, that off, correct? Or would I tie it to ground? It's been a long time since I've done anything with a transformer. So forgive my stupidity.
And then there's supposed to be a center tap on the Yellow wires, which there is not, so I am good with tying together two more 100R 1/2W resistors together. Can I ask why the 100R and not something larger? Wouldn't a 10k be better to limit the amount of wasted current/heat?
There's also the extra (gray) wire for the 115V supply. I should also be able to cut and cap this, correct?
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Thanks guys. I don't know why I don't ever think of a bridging the resistors together to make a ground. For some reason it never crosses my mind.
I also notice that the red/red-yellow/red (300/0/300) should be connected to the heater pins of the 5Y3 (pins 2/8). But, I am not supposed to have a center tap. This seems like a stupid question, but not asking it would be even dumber. So, I can just clip and cap, that off, correct? Or would I tie it to ground? It's been a long time since I've done anything with a transformer. So forgive my stupidity.
And then there's supposed to be a center tap on the Yellow wires, which there is not, so I am good with tying together two more 100R 1/2W resistors together. Can I ask why the 100R and not something larger? Wouldn't a 10k be better to limit the amount of wasted current/heat?
There's also the extra (gray) wire for the 115V supply. I should also be able to cut and cap this, correct?
Yeah, I'm kind of an idiot and being nervous about this repair. So I just looked at the old PT and yes, there was a center tap on the 600Vac winding. That was tied to ground. Also, the schematic shows a center tap for the Yellow wires, but, the original PT didn't have it either.
So, the only question I have is the choice of resistor values for the ground. Why those 100R rather than a 10k, or even a 100k? Just seems that the lower current would be more desirable.
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Whoa! You're about to blow up your new PT.
Tape the gray wire off. Don't cut it.
Connect one red wire to pin 4 of the 5Y3. Connect the other red wire to pin 6 of the 5Y3. Connect the red/yellow wire to chassis ground.
Connect one yellow wire to pin 2 of the 5Y3. Connect the other yellow wire to pin 8 of the 5Y3. (The B+ wire will also be connected to pin 8.)
You should already have figured out the two green wires.
READ, UNDERSTAND, AND FOLLOW THESE INSTRUCTIONS. Exactly as stated. Don't read between the lines because there is nothing there. Don't do anything else.
The schematic does not show a center tap for the yellow wires.
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Whoa! You're about to blow up your new PT.
Tape the gray wire off. Don't cut it.
Connect one red wire to pin 4 of the 5Y3. Connect the other red wire to pin 6 of the 5Y3. Connect the red/yellow wire to chassis ground.
Connect one yellow wire to pin 2 of the 5Y3. Connect the other yellow wire to pin 8 of the 5Y3. (The B+ wire will also be connected to pin 8.)
You should already have figured out the two green wires.
READ, UNDERSTAND, AND FOLLOW THESE INSTRUCTIONS. Exactly as stated. Don't read between the lines because there is nothing there. Don't do anything else.
The schematic does not show a center tap for the yellow wires.
Ahh, thank you. I misunderstood, because of the 360V coming off the 8/2 pins.
So the 600 actually goes to the plates. Glad I'm a little apprehensive today. Thank you for the direction and correcting me!
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Check everything two three times, make sure things are in the right place, in doubt do ask questions.
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Connect one yellow wire to pin 2 of the 5Y3. Connect the other yellow wire to pin 8 of the 5Y3. (The B+ wire will also be connected to pin 8.)
If you read the previous reply, I deleted it. I misspoke. I see thank you.
Does it matter if it's pin 8 or pin 2 that's connected to B+?
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So, pin 8 and pin 2 should be shorted together?
Nop
Connect one yellow wire to pin 2 of the 5Y3. Connect the other yellow wire to pin 8 of the 5Y3. (The B+ wire will also be connected to pin 8.)
One side of the 5v wire to the pin 2 and the other to pin 8. from pin 8 a wire will feed the high voltage or B+.
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Does it matter if it's pin 8 or pin 2 that's connected to B+?
A 5Y3 tube doesn't care if you connect the B+ wire to pin 8 or pin 2. I see from your photos that the B+ wire is connected to pin 2. That's fine.
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Does it matter if it's pin 8 or pin 2 that's connected to B+?
A 5Y3 tube doesn't care if you connect the B+ wire to pin 8 or pin 2. I see from your photos that the B+ wire is connected to pin 2. That's fine.
Thanks. I didn't think it would, but I'm not making assumptions at this point. I'd rather be that annoying guy who asks far too many questions and be sure, rather than screw something up and be that much further behind.
I can't stress how much I appreciate all the help this forum has provided already. Tubes confuse me. Give me an op amp or a transistor any day of the week! But, that's what I love about electronics, I'm constantly learning and I appreciate everyone who takes the time out to give me a hand in grasping things I've never fussed over before, or fussed very little over.
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Does it matter if it's pin 8 or pin 2 that's connected to B+?
A 5Y3 tube doesn't care if you connect the B+ wire to pin 8 or pin 2. I see from your photos that the B+ wire is connected to pin 2. That's fine.
Thanks. I didn't think it would, but I'm not making assumptions at this point.
On some rectifier tubes it does matter. Indirectly heated rectifier tubes, 5AR4/GZ34, 5V4, you have to take the B+ from pin 8, on directly heated rec tubes, 5Y3, 5U4, it doesn't matter.
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Ok, Red wires to pins 4/6, yellows to 2/8, green to pilot socket, 2x 100R 1/2W resistors off the pilot socket to create a center tap to pin 7 of 5Y3 (pin 7 connects to 20uF cap and 250R resistor from the cathodes of the 6v6. That 250R should be the cathode bias resistor(?), the other end is connected to ground. Gray(115V tap) wire taped and bundled neatly out of the way. Black and White wires to their respective terminals on the power switch and the fuse holder. I don't think I'm missing anything else.
I need to rewire my outlet with a lamp for a current limiter, then I should be ready to power it on unless I'm missing something. Here's a couple pictures to verify. A second or third set of eyes would be appreciated.
https://www.jaerllc.com/images/pan/pt1.jpg (https://www.jaerllc.com/images/pan/pt1.jpg)
https://www.jaerllc.com/images/pan/pt2.jpg (https://www.jaerllc.com/images/pan/pt2.jpg)
https://www.jaerllc.com/images/pan/pt3.jpg (https://www.jaerllc.com/images/pan/pt3.jpg)
https://www.jaerllc.com/images/pan/pt4.jpg (https://www.jaerllc.com/images/pan/pt4.jpg)
I did not need to drill new holes. Instead, I shimmed out the transformer from the chassis to allow for the wires to use the same hole and wrapped them in heat shrink to add an extra layer of protection. The holes actually lined up by turning the PT 90 degrees.
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That'll work.
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That'll work.
Thank you, Sluckey. Just got done wiring the current limiter. Gonna test that out now, then I'll move on to the amp. I don't know why I get so nervous with tube amps. Give me a solid state and I'll run circles around it, but tubes confuse the crap out of me and it's as if I allow that to screw with me. Building confidence more and more, but transformers are something I've rarely had to deal with. Tubes? Man, they make sense to some extent, but it's still a little magic in a bottle.
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Tubes? Man, they make sense to some extent, but it's still a little magic in a bottle.
Don't dwell on it too much. Learn what the purpose of each pin is and what voltages should be present. The magic inside the bottle is no different than the magic inside an op amp. When the magic stops, replace it.
That's a bit simplistic and you can get a lot deeper than that, but how often do you look at the schematic for the internals of an op amp? There are thousands of people working on tube equipment that have no deeper understanding than what I just said.
Not trying to steer you away from learning more about tubes. More knowledge means a better understanding and better chance of becoming a better technician. But you don't have to know it all today, especially to replace a transformer. Take your time. I've invested about 55 years in electronics at this point. You'll be surprised at how much you can pick up just by hanging out in places like this forum. I've seen quite a few people greatly improve their electronics skills since I've been here.
Enjoy the smell of old tube gear. AND BE SAFE!
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Hah, very well said. And yeah, it takes time. Problem is, I worked as a test tech for a while and didn't use a lot of skills, so I've forgotten some things and just need to refresh a little.
So, I made my current limiter, but shouldn't the bulb come on? The bulb is good. The fixture is good. Everything is wired properly (hot to bulb fixture, which returns as white (painted it black in the outlet box just so I have reference if I ever forget how I wired it)) I get voltage coming out of the outlet when the bulb is present and no voltage when the bulb is removed, but the bulb doesn't come on. I'm using a 60W flood lamp. It's not supposed to be an LED bulb, but that is about the only thing that would make sense.
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Put a jumper between the line and neutral slots on the outlet. This simulates a dead short on the amplifier and the bulb should come on full bright. This assumes you wired the limiter correctly. :icon_biggrin:
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Put a jumper between the line and neutral slots on the outlet. This simulates a dead short on the amplifier and the bulb should come on full bright. This assumes you wired the limiter correctly. :icon_biggrin:
Hah, what you're saying is maybe I should have read a little bit more and not ASS-U-ME. :laugh:
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Put a jumper between the line and neutral slots on the outlet. This simulates a dead short on the amplifier and the bulb should come on full bright. This assumes you wired the limiter correctly. :icon_biggrin:
Should my light be on at all with the amp plugged in? It is glowing, but not full on. I've got my bias probe on one of the output tubes and I'm not getting any current readings. I'm gonna double check all the connections to make sure nothing has moved and touching something it shouldn't.
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This may be of some help !
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No fuse blowing ? I would check all the cirtcuit 1st before turning it on, read everything you can with your meters.
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It is glowing, but not full on
That's good. Now you can put the limiter under the bench because it's done it's job. Time to plug the amp directly into the wall. Set your voltmeter to measure DCV and connect red probe to pin 2 of the 5Y3. Black probe connects to chassis. Turn the power switch on but keep your hand on the switch and be ready to quickly turn it off if things go badly. It will probably take a few seconds for the 5Y3 to warm up and put out some B+.
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No fuse blowing ? I would check all the cirtcuit 1st before turning it on, read everything you can with your meters.
Hahah, yeah, thanks for sharing that video uki. That was a great explanation and I think I'm going to end up building one like that, minus the variac (can't afford that right now).
So, yeah, Fuse was good. Unit stays on, pilot light illuminates and stays illuminated. I was forgetting that the bulb would cut down the amount of current, so the tubes might not glow that much. I think I'm gonna call it for today. And start in on this again in the morning. I'll start with leaving it in the current limiter and reading voltages everywhere. I guess, I'm curious, what voltages I should see with a 60W bulb in play. I'm also curious, in the video, he used 2 100W bulbs. Obviously, that's going to limit the current much more, acting like a big resistor, but I've also heard of people suggesting 200W bulbs. I guess, how do you know what bulb should be used?
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It is glowing, but not full on
That's good. Now you can put the limiter under the bench because it's done it's job. Time to plug the amp directly into the wall. Set your voltmeter to measure DCV and connect red probe to pin 2 of the 5Y3. Black probe connects to chassis. Turn the power switch on but keep your hand on the switch and be ready to quickly turn it off if things go badly. It will probably take a few seconds for the 5Y3 to warm up and put out some B+.
I will do this in the morning. I think I'm going to call it a day. I have to be out to dinner in 30 mins, and need to wrap things up here. I just set the probes up, so I'm ready to start right away tomorrow.
One thing I noticed with this is, is there's no standby. I'm assuming the same rules apply, you don't want to hook anything up to the inputs until you give the amp/tubes a chance to warm up? This is for normal operation and not right now. I have no intention of hooking a signal up to this just yet until everything is checked out without a signal. I'm just curious, because I don't think I've worked on a tube amp without a standby switch yet.
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The bulb wattage depends on the device wattage.
Use a light bulb limiter when you turn it on after installing the new caps. For a Twin, 100w bulb is fine. Don't go larger, the higher the bulbs wattage, the less protection to the amp.
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...Should my light be on at all with the amp plugged in? It is glowing, but not full on....
Consider two extremes. A 60W amp on a 3W lamp, and a 60W amp on a 1,000W lamp.
60W amp on a 3W lamp, the lamp will get most of the voltage, maybe 95%, and the lamp will be at least 9/10 full brightness.
60W amp on a 1,000W lamp, the amp will get most of the voltage, maybe 95%, and the lamp will be at 5% full heat, which is NOT bright at all. (A trace of heat if you put your lips on it.)
If that does not make sense, yet, sleep on it.
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Consider two extremes. A 60W amp on a 3W lamp, and a 60W amp on a 1,000W lamp.
60W amp on a 3W lamp, the lamp will get most of the voltage, maybe 95%, and the lamp will be at least 9/10 full brightness.
60W amp on a 1,000W lamp, the amp will get most of the voltage, maybe 95%, and the lamp will be at 5% full heat, which is NOT bright at all. (A trace of heat if you put your lips on it.)
If that does not make sense, yet, sleep on it.
Thank you for that explanation. I'm trying to look at it as a resistive element. You use a current limiting resister to dial down the amount of current running to a specific leg of a circuit. The higher the resistance (wattage), the less current can flow to that leg. The lower the resistance, the more current. You're essentially choking off what the rest of the circuit can have delivered to it. This is why I questioned the use of the 100R resistors to create the center tap for the 6V3 heater windings. It's only 6V3, but 100R allows a lot more current to flow than a 10k. From a design standpoint, that's a lot of wasted energy. Granted, this is running from mains, but it's not efficient and will produce more heat than the 10k.
Essentially, it's the same concept with the light bulb limiter in that the light bulb is limiting what the amp (or whatever is in series) can receive. It's the concept of power that throws me off, rather than the current. With a resistor, you're limiting it with that resistance level and the resistance is given. With a bulb, I have to calculate it out and that's where my mind goes blank. It's not that I can't do it, it's just that math isn't second nature in my brain. I'll get it though and I do appreciate the picture you painted. That helped put it in perspective, as did uki's explanation that it's based off the power of the amp, as well. I just have to think of both of them as resistive elements and not two separate entities that share nothing common, like my brain wants to perceive it.
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This is why I questioned the use of the 100R resistors to create the center tap for the 6V3 heater windings. It's only 6V3, but 100R allows a lot more current to flow than a 10k. From a design standpoint, that's a lot of wasted energy. Granted, this is running from mains, but it's not efficient and will produce more heat than the 10k.
Don't even try to relate the artificial center tap to the current limiter bulb. They are two completely different things with two completely different purposes.
The sole purpose of a center tap on the filament winding is to be able to provide a ground reference to the filament circuit. There is no current flowing into this ground reference. But this ground reference will reduce the amount of undesirable 60Hz hum being coupled into the amplifier circuit. In the absence of a real center tap you can use two resistors to accomplish the same ground reference. Ideally, you would want this ground reference to have zero resistance, but that's impossible because doing so would short the filament winding. So, you must use some value of resistance. It has been found that 100Ω resistors provide an adequate ground reference without adding too much extra current to the filament winding. (Two 100Ω resistors across a 6.3VAC winding only draw 31.5mA from the winding. Barely noticeable.) Using 10K resistors would defeat the whole purpose of the artificial center tap because the ground resistance would be 10K and that will allow a lot (probably all) of the 60Hz hum to get into the amplifier circuit.
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Have you look into tube datasheets ? Lots of info on it !!
https://frank.pocnet.net/index.html
6V6 on this page
https://frank.pocnet.net/sheets66.html
Look at the current need for the tubes heaters.
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This is why I questioned the use of the 100R resistors to create the center tap for the 6V3 heater windings. It's only 6V3, but 100R allows a lot more current to flow than a 10k. From a design standpoint, that's a lot of wasted energy. Granted, this is running from mains, but it's not efficient and will produce more heat than the 10k.
Don't even try to relate the artificial center tap to the current limiter bulb. They are two completely different things with two completely different purposes.
The sole purpose of a center tap on the filament winding is to be able to provide a ground reference to the filament circuit. There is no current flowing into this ground reference. But this ground reference will reduce the amount of undesirable 60Hz hum being coupled into the amplifier circuit. In the absence of a real center tap you can use two resistors to accomplish the same ground reference. Ideally, you would want this ground reference to have zero resistance, but that's impossible because doing so would short the filament winding. So, you must use some value of resistance. It has been found that 100Ω resistors provide an adequate ground reference without adding too much extra current to the filament winding. (Two 100Ω resistors across a 6.3VAC winding only draw 31.5mA from the winding. Barely noticeable.) Using 10K resistors would defeat the whole purpose of the artificial center tap because the ground resistance would be 10K and that will allow a lot (probably all) of the 60Hz hum to get into the amplifier circuit.
So in essence, the resistors are creating a filter with the natural inductance and capacitance of the windings for the 6V3 portion of the transformer?
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Have you look into tube datasheets ? Lots of info on it !!
https://frank.pocnet.net/index.html
6V6 on this page
https://frank.pocnet.net/sheets66.html
Look at the current need for the tubes heaters.
Yes, looked at them very briefly, but never paid any attention to the current on the heaters. 450mA isn't too bad. So about 1A for the outputs. I'm going to bookmark these pages. I usually just Google them every time, but this is much handier! Thank you.
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That's good. Now you can put the limiter under the bench because it's done it's job. Time to plug the amp directly into the wall. Set your voltmeter to measure DCV and connect red probe to pin 2 of the 5Y3. Black probe connects to chassis. Turn the power switch on but keep your hand on the switch and be ready to quickly turn it off if things go badly. It will probably take a few seconds for the 5Y3 to warm up and put out some B+.
Yep, all good. I read about 340V, so it's a bit low, but my outlet is only putting out 115, so I'm not surprised. I need to figure out what's going on with this outlet, because the other one is on a different circuit and I get 123V. Might be the surge protector I have on it, but not sure. I feel comfortable with the 115V on the mains for right now, though I do want to plug it into the other outlet to verify it's still going to be ok, and for my own knowledge of seeing what I would normally get.
My bias current is only about 10mA with no signal, and that seems low in my head, but I haven't done the biasing yet, so we shall see.
**EDIT**
Ok, plugged it into the outlet with the 123V. Tubes are all glowing, no red plating. Output plate current is about .03A. B+ is ~365V. Everything appears to be running smoothly. Haven't prodded around with the probe, since I had it clipped on to pin 2 and always makes me nervous disconnecting it on a live circuit, because I don't want to be jittery and short something. Will check around different points to verify all the other voltages are good.
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Look at the current need for the tubes heaters.
One thing that I find confusing with the datasheet is on page 2.
https://frank.pocnet.net/sheets/093/6/6V6GT.pdf (https://frank.pocnet.net/sheets/093/6/6V6GT.pdf)
There's 3 columns, but nothing is labeled. Also, I've been told that the 6V6 are usually pushed with regards to the plate voltage of 400+V. Yet, the datasheet shows the three values of 180, 250, and 315. Looking at page 1, I can see that a class A amp is max rated for 315, but is this a class A? I've always been confused with the classes, even when in school. Oddly enough, when I studied electrical engineering, I had no desire to work on audio and because of that, I didn't pay as much attention as I should have. It wasn't until I worked with hearing loops that I got interested in audio, but those are constant current and most of what I worked on was class D, but it still never made sense.
**EDIT**
This is simply a guess, but on page 2, is the left column minimum ratings, middle is max, and right is ideal/nominal?
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So in essence, the resistors are creating a filter with the natural inductance and capacitance of the windings for the 6V3 portion of the transformer?
Everything you need to know about heaters...
http://www.valvewizard.co.uk/heater.html
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> minimum ratings, middle is max, and right is ideal/nominal?
There's no "minimum". Except in the commercial sense: you don't buy a 2-ton truck to haul a six of beer.
Irving's Intercoms wants a box to speak. Power but not a lot of it. 2 Watts? There are smaller tubes, but frequently the 6V6 has been the most available and often within pennies of smaller tubes. Hey, here's an old ad! 6V6GT is in the 45c bin, along with 6F6 6K6 (similars), 6AU6 (maybe 1/3W), 35/50*5 (110V radio 1W outputs). Yes, TAB was a bottom-barrel supplier, 6V6 "should" cost $0.95-$1.10 in those days. But Irving doesn't expect a lot of tube-abuse. Or a lot of sales. And if he has no sales at all, he can always turn and dump 6V6 at not too much of a loss.
The 250V condition is classic in good parlor radios. 4+ Watts is fine. All the numbers are there.
Fender wants the MOST he can get for his buck. The 5.5 Watt numbers are there, and if that eats tubes GE/RCA won't blame Leo too much, will replace his early failures.
Any ONE tube audio stage MUST run class A, tube conducts for the ENTIRE audio cycle. If it ever cuts off, that's gross distortion.
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Irving's Intercoms wants a box to speak. Power but not a lot of it. .... Fender wants the MOST he can get for his buck....
Here's another market survey. Olson sold many amplifiers, here are two.
That Olson LoudMouth sure looks like a Fender Champ!! 8" PM speaker. Oh, it has Tone, the first Champs didn't. 6V6GT power. Claims 6 watts, which is probably the 5.5W condition on the datasheet, rounded-up.
The intercom head for $9.99.... they can't be using one-buck 6V6 in that. In fact by the tubes they did use we know this is a transformerless "hot chassis" amplifier; don't put your lips on it! (Note "plastic cabinet".) However at a higher class of product (Bogen?) a designer might pick 6V6 (a solid choice) and run it at the 2 watt conditions for long life and good filtering.