Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on July 18, 2023, 07:31:53 pm
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I installed (2) new PS Capacitors yesterday and fired the amp up. Smoke and a small flame jumped out from the output transformer hole. I was hoping it was the reverb transformer but it wasn't. I noticed now as well i have ground continuity on the positive leads of the new PS caps i installed. I have not removed the OT wires yet to do an ohms test. Edit: The caps i put in are good and not shorted. But the connection points are shorted.
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What is the value and voltage rating of those two black axial caps?
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What is the value and voltage rating of those two black axial caps?
IMO, too low voltage .
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The original caps are 20uf and rated for 600 volts. The caps i used are 22uf 500 volt. Some of the old fender amps i have seen 600 volt caps in but they never produced 500 volts. A gamble. I am removing one of the new caps now i put in and test it to see if it is shorted. Edit: I just removed the caps and they are good. Not shorted. But the cap board connection points are showing (continuity) shorted. The amp actually worked with the dead looking caps in it.
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I bet that if you disconnect the OT primary center tap that short will disappear from the cap connections. You may want to use the Sceptre schematic on my website. Much easier to read.
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I bet that if you disconnect the OT primary center tap that short will disappear from the cap connections. You may want to use the Sceptre schematic on my website. Much easier to read.
Disconnected CT from OT and no change in short. Removed OT wires from sockets. CT & Plate Green /White Tracer 24.0 Ohms. CT & Plate Green Wire 28.2 Ohms.
CT & Screen #1 .083 . CT & Screen #2 .070 Ultra Linear OT. Yes i am using your page of info. Great detail.
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Disconnected CT from OT and no change in short.
Well that means the short on the filter caps is coming for elsewhere.
If you saw smoke and a flame you should be able to smell burnt OT. May have to stick your nose in the hole.
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Disconnected CT from OT and no change in short.
Well that means the short on the filter caps is coming for elsewhere.
If you saw smoke and a flame you should be able to smell burnt OT. May have to stick your nose in the hole.
I am removing the OT now and will remove the bell ends.
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Bell ends removed. Looks like barbecue OT? Still this does not address the cap short issue. I really don't need a catastrophic problem right now. I got one though.
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That OT is expensive. This is the only source I have for the Sunn transformers...
https://store.triodestore.com/a4dyoutr43oh.html
Finding the short should be easy.
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Here's a more affordable Hammond replacement...
https://www.hawkusa.com/search?search=1650na&field_current_stock=All&field_rohs_compliant=All&items_per_page=10
Hawk has the best prices on Hammond stuff.
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The short should be easy to find. I am taking a break for a while. Yes very expensive OT. MM has it for $364)) i believe. $25.00 ship. $400.00. I'll check out your links. Thanks much!
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$161.20 plus ship is present if they are still in stock. Check in out now.
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$161.20 plus ship is present if they are still in stock. Check in out now.
That same web page says they have 22 in stock. Several forum members and I have used Hawk before. Highly recommended.
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$161.20 plus ship is present if they are still in stock. Check in out now.
That same web page says they have 22 in stock. Several forum members and I have used Hawk before. Highly recommended.
Output transformer ordered. Along with Sprauge Atom 20uf 600 volt capacitors. Still have not found the short in the PS rail. Really weird! Disconnected the choke and no change on the first 20uf PS cap. I rechecked the caps i installed and they are good. No short in them. Pulled the rectifier as well.
EDIT: Found short. I removed both caps i installed from the circuit and one of the caps was shorted internally. This amp has a higher voltage than the caps i used could withstand during power up standby on.Thought i could get away with using 500 volt caps. Lesson learned.
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In future if you run into a similar issue, you can use series connected caps. Two series connected caps results in double the voltage rating on the cap and half the value, however it is important to use resistors in parallel with each cap so the caps will share the load if doing this. For Sunn amps I usually do this for the first two cap stages. With today's wall voltages, the DC voltage inside the amps is higher than what is on the schematics, so you have to uprate the caps also. Sprague Atoms are completely overpriced and I wouldn't use them under any circumstances myself. They also put a modern cap inside the cap case in some examples. Much cheaper and better to use quality modern caps, and radial caps are going to be cheaper and better quality these days in most cases since there are so many made for other industries. You may have to drill a hole or two in order to mount terminal strips to attach the parts to, but that is the best way to go about it.
An example would be in the Sunn first stage after the power transformer, they call for a 30uF cap at 525 volts. You could get two Panasonic radial caps, 68uF at 350V for $2.99 each from Mouser. That results in essentially one 34uF cap at 700V when connected in series. Use a couple resistors in parallel with each cap (1 watt 220k metal oxide is a good choice), buy some terminal strips, and you are still WAY under the cost of a Sprague Atom cap at 600V, and you end up with a 700V cap that will be safe for any voltage that amp will see. Do something similar for the second stage, but use the 47uF 350V caps which results in a 23.5uF 700V cap for that stage when connected in series. Don't forget the resistors in parallel with each cap. The third stage is usually ok with a single 500V cap, though you can use series connected caps there also for better voltage protection.
For Sunn bass amps like the 200S or 2000S I have found that going up in value and using a solid state rectifier instead of the GZ34 results in a tighter bass response that many players like, if any are still using vintage Sunn amps that is. Most bass players these days seem to have switched to super light Class D amps and a 12 inch cabinet or something else similarly light, though you wouldn't catch me doing that. I still like vintage Sunn 200S and 2000S and Ampeg SVT's for bass, and if forced to use solid state I have an old Trace Elliot that is too heavy but gives good sounds. For guitar, I don't use Sunn personally. :)
Greg
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It's funny how you mentioned series caps. That's what i was thinking of doing but didn't want to lower the mf value. This amp is using a solid state rectifier plug. The bottom line is i should have just waited and ordered the 20uf 600 volt caps. There is 550 volts at the standby. Dah! All after the fact. Thanks for the info!
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Steve what are the Blue & Brown wires of the primary? Got a little dementia comin on lately. Plates?
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Blue and Brown are plates. Blu/Yel and Brn/Yel are screens. If the amp howls or screams after replacing the OT you will need to swap the primary leads in pairs, ie, swap plate and screen leads to the other tubes. Take your time and don't screw this up. Could be expensive. Don't cut the leads until you are sure the primary leads and NFB is correct. See pic...
Study the secondary connections and don't screw those up either. Notice the FB is connected to the 16Ω tap.
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Blue and Brown are plates. Blu/Yel and Brn/Yel are screens. If the amp howls or screams after replacing the OT you will need to swap the primary leads in pairs, ie, swap plate and screen leads to the other tubes. Take your time and don't screw this up. Could be expensive. Don't cut the leads until you are sure the primary leads and NFB is correct. See pic...
Study the secondary connections and don't screw those up either. Notice the FB is connected to the 16Ω tap.
Yes have been going over the diagram and am making sure i got it right. Thanks for all your help. I am sure it's one great sounding amp. Like the other Sunn i worked on a few months ago built into the cabinet instead of just the head. Killer amp!I don't have the OT yet. Should be a week or less. I always take plenty of detailed pictures before and after of my work.
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Got the OT this afternoon. Wired up the speaker wires,CT and feedback. Going to try and make the Plates & Screen connections to the tubes right tomorrow on the first shot. Don't have the PS Caps yet so no rush.
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Plates & Screens tacked until i get the caps and am sure they are not in positive feedback.
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Don't you like the way those old Sunns are laid out?
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Don't you like the way those old Sunns are laid out?
I love the way there laid out. I love the HiWatts to but these amps are super easy to work on.
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Putting in some new resistors and cathode bypass cap. Still waiting on the power supply caps to arrive. My solder roll is going bad. Never seen this before. Wont flow nicely at all. I have a new roll of 60/40 rosin core lead roll out for delivery today. I thought my solder gun was going bad but i have 5-6 other guns and they all wont make the solder i have flow either. It just lumps up and is lumpy. I have flux pencils as well and the solder still wont flow nicely. What the hell! Edit: Got new of 60/40 1mm 1LB roll of solder delivered today. It flows really good and shinny.
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It's funny how you mentioned series caps. That's what i was thinking of doing but didn't want to lower the mf value. This amp is using a solid state rectifier plug. The bottom line is i should have just waited and ordered the 20uf 600 volt caps. There is 550 volts at the standby. Dah! All after the fact. Thanks for the info!
By using different values for your caps you don't lower the mf value as I noted in my post above. A 34uF cap (two 68uF in series) instead of a 30uF cap is within the 20% tolerance that electrolytics usually are, and newer radial especially, and modern axial caps will hold closer to their value when compared to vintage Sprague Atoms. Not sure if you have seen the pic of the new Sprague Atom Sluckey cut open years ago? A small cap inside the large case, and you pay a 5x premium for that.
Greg
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Yes i remember the Sprague cap that Sluckey cut open. I need (3) 50uf /150 volt caps and it looks like i have to pay the premium for the Sprague Atoms. Can't find any 50uf/150 volt caps. It's getting harder to find other brands and ones that are not on back order.
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AES has a generic 47µF @ 160V (https://www.tubesandmore.com/products/capacitor-160v-axial-lead-electrolytic) for only $1.40. They also have that 30/20/20/20 @ 525V cap can (https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-30202020-f). This image shows the correct way to install that cap can. I suggest you take a look because all the Sunn schematics are wrong. They only show one 20µF @ 600V but there is clearly one on each side of the choke...
http://sluckeyamps.com/sunn/sceptre_filter.jpg
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Yes there (2) 20uf 600vdc caps on each side of the choke. NOW FOR THE CRAZY NEWS!
I am still waiting on those (2) 20uf 600vdc Capacitors. They were returned for some unknown reason. I generally always get my parts at my address on file when purchased. Anyway there on the way again. Something is screwy! I found the cap can a month ago at Antique Electronics.
https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-30202020-f
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Yes i remember the Sprague cap that Sluckey cut open. I need (3) 50uf /150 volt caps and it looks like i have to pay the premium for the Sprague Atoms. Can't find any 50uf/150 volt caps. It's getting harder to find other brands and ones that are not on back order.
https://www.mouser.com/ProductDetail/CDE-Illinois-Capacitor/476TTA160M?qs=3tP%252BN51vMXc7yVnwLsohBA%3D%3D
--Pete
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If they are for the bias ckt. I'd use these:
https://www.digikey.com/en/products/detail/vishay-beyschlag-draloric-bc-components/MAL204211479E3/5632494
--Pete
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AES has a generic 47µF @ 160V (https://www.tubesandmore.com/products/capacitor-160v-axial-lead-electrolytic) for only $1.40. They also have that 30/20/20/20 @ 525V cap can (https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-30202020-f). This image shows the correct way to install that cap can. I suggest you take a look because all the Sunn schematics are wrong. They only show one 20µF @ 600V but there is clearly one on each side of the choke...
http://sluckeyamps.com/sunn/sceptre_filter.jpg (http://sluckeyamps.com/sunn/sceptre_filter.jpg)
Most Sunns with today's line voltages go over that 525V that cap is rated for. They are generally around 550-580V depending on the vintage and that is operating voltages, not to mention what they might spike to with inrush current. I've used series connected caps with great results and less money spent myself. Usually the first two stages on the 60W and 120W Sunns need the series connected caps to get to a sufficient voltage rating.
Greg
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I love compiling all the information i can get on most every amplifier ever made. It's Golden Info and more valuable than the amp itself to me. I know i can just wire caps in series but haven't had to do this before. I still haven't gotten those caps.