I spec'd out and bench tested these mono block amps using a (tested good) NOS 5Y3GT get a perfect 320v then when I built it i put in a brand new 5Y3GT and I'm over voltage by 40v. I want to sell the amps so I can move on to a new project but don't want to put it out there knowing some sol will throw in a new tube and the thing will red-plate on them.
What would you guys do ?
Amp specs 275-0-275 into 5Y3GT into 15uf cap - choke - cap choke config 320v ~140 ma The amp is a PP 6V6 parallel using a 4 6V6's and a 12AT7 with a CCS.
Any help would be great.
What would you guys do ?
Amp specs 275-0-275 into 5Y3GT into 15uf cap - choke - cap choke config 320v ~140 ma The amp is a PP 6V6 parallel using a 4 6V6's and a 12AT7 with a CCS.
Any help would be great.
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Mark02131 I suspect your transformer is really a 550v Ct one and not a 550-0-550 as noted in your thread. If it is a 550v ct trans., I would expect the cap input supply to produce something on the order of 380v dc at the first input cap if there were no load. Loaded at 140ma, I would guess it might be something like 330v or so. With the privious thoughts in mind, I would be suspicious that the current load on the power supply is different in either the bench test scenario or the final configuration. Confirm the load is as you noted (140ma). Hope this helps in sorting out what is going on. Mickeystan
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Opps yes the trans is 275-0-275 (550v) The load is 4 6V6's and a 12AT7. I would think the load is 70 mA for a pair of 6V6's so doubling that would give me ~140 mA. Loaded I get 360v with a new 5Y3GT and 320V with a vintage one. Currently I'm running vintage 5Y3GT's I need to get the voltage correct using a New 5Y3GT tube. There's a few options just not sure what you guys would do in this case. I'm even into pulling some voltage off to run the CCS and get rid of the 100v trans for that But I'd like to see what you guys think
Are you sure that your modern 5Y3GT are actually 5Y3s? I've had this problem in the past and upon closer inspection it was pretty obvious that they were.... 5AR4s.. These have much less forward drop than a real 5Y3 and an indirectly heated cathode which was seen mainly in the 5Y3WGT, and rarely in garden variety 5Y3 back in the day.
NOS 5Y3GT are still relatively common on eBay and elsewhere so recommending NOS is not that big an issue.
You should measure the output stage current rather than assume and make sure it is in the range you expect. Easy enough with cathode bias.
I'd recommend subbing another NOS 5Y3GT for the one you tested with and make sure you get the same result, voltage sounds a bit low to me in that first scenario.
NOS 5Y3GT are still relatively common on eBay and elsewhere so recommending NOS is not that big an issue.
You should measure the output stage current rather than assume and make sure it is in the range you expect. Easy enough with cathode bias.
I'd recommend subbing another NOS 5Y3GT for the one you tested with and make sure you get the same result, voltage sounds a bit low to me in that first scenario.
there are a lot of tricks to losing voltage on a tube amp, so i do not consider this as a big issue, the bigger issue imho is when you do not have enough....
I thought the same as Kevin when first read the OP's problem. One way to see if that's the case, 5Y3s have very little warm up time and 5AR4s are much longer, how long does it take for the B+ to come up?
Craig
Craig
One easy way to remove the excess voltage would be to remove the 1st cap, and parallel it with the second.
The rectified AC would have to cross the choke before reaching both capacitors.
This method would reduce the maximal DC voltage, and improve load regulation.
It's worth a trial, as you'd only need to displace a single lead. 😉
The rectified AC would have to cross the choke before reaching both capacitors.
This method would reduce the maximal DC voltage, and improve load regulation.
It's worth a trial, as you'd only need to displace a single lead. 😉
Thanks Guys, I checked I am using a pair of Magnavox NOS 5Y3GT's at least that is what there labeled. Both mono blocks get the same voltage with both tubes. I will check the actual current tonight. Turning it into a choke loaded supply may be a good idea. It's already a 2 choke supply with 15uf - 200ma choke - 100uf - 200ma choke - 200uf - (resister - 100uf for 12AT7)
I just checked and the new tubes are also 5Y3GT's I suspect that the current draw is high enough that it's loading down the old tubes and the newer tubes can handle the higher current so less voltage drop.
It is very easy to check the current through the 5Y3. Measure the DCR of your first choke and then power up the supply. Measure the voltage drop across the choke and use Ohm's law to determine the current.
Ok took the voltage drop across the first choke. I'm getting a 3v drop with both tubes so 29 ohms DCR v3 drop should put me at 103 mA. It's so strange that the tubes would be that different.
with 120v In
NOS 5Y3GT 307v first cap - 304v after choke
NEW 5Y3GT 350v first cap - 347v after choke
with 120v In
NOS 5Y3GT 307v first cap - 304v after choke
NEW 5Y3GT 350v first cap - 347v after choke
They test good (well into the green) on my heath kit tube tester. Another thing I just found out my Edcor (XPWR007-120) Trans (120v primary 275-0-275) puts out 600v at 120v and 550v at 110V. I didn't use that trans when designed the amps I used a trans that put out the correct voltage.
So again I ask, what would you guys do, just sell it as-is or fix it so it works with a new tube and if so what would you do, (resisters before the rectifier, bleed off voltage with a big resister, use some of it for the CCS) there's so many ways to waste voltage, I'd love the easiest way. I'd thinking of trying to change it over to a choke loaded to see out that affects it.
So again I ask, what would you guys do, just sell it as-is or fix it so it works with a new tube and if so what would you do, (resisters before the rectifier, bleed off voltage with a big resister, use some of it for the CCS) there's so many ways to waste voltage, I'd love the easiest way. I'd thinking of trying to change it over to a choke loaded to see out that affects it.
Choke input will probably be the best way, and there's only one lead to reconnect.
If you're disappointed by the results, it'll be really easy to go backwards!
If you're disappointed by the results, it'll be really easy to go backwards!
Thanks I changed it over and now the voltage is within range. (275-310) depending on bias. I still have to figure out where I need to set it to. I have -22v and I had it set around 1v I'm thinking lower now maybe around .3v to give me around 300v I'll have to listen to it and see where it sounds best.
thanks everyone for all the great advice.
You sure the "new" tubes aren't relabeled 5AR4's? I don't know of any factories making new 5Y3s. Post up a pic of the "new" 5Y3's. Show us close-ups of the internal structure.
As for the testing, 5Y3s are rated for 440mA PEAK current per plate. Unless your Heath tester can actually supply and measure that peak rating the test won't be reliable.
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