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B+ winding sizing...

knockbill

knockbill

Lunatic Member
What specs in the the tube manual need to be used to figure PT size (not counting heaters)? Looking at the 50W amp schem in the RCA manual,, it calls for a 200mA B+ winding,, however a single 7027A can draw over 200mA, so, obviously, the maximum column isn't the one to use... Other than the B+ tube draw, what else needs to be considered?

Regards,
John
 
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No, that transformer has 200 mA on the 600-0-600 windings, that's not B+.

You still need the 200mA for the 7027, plus a few mA for the phase inverter and preamp tubes. I quickly plugged that power supply into Duncan's Power Supply and with a 225 mA constant current load, the max current on the transformer was 285 mA (edited, I had not hit "simulate" after changing the load) but only for a brief part of the cycle. Looking at the graph of the transformer current, the mean (middle) of the sine wave was a little over 200 mA but there was a lot of the cycle that was either 0 or under 200.
 
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If you need a schematic for a 7027A Amplifier that's Push-Pull I have a schematic from a mid-1950s Newcomb "Pathfinder" Model E-75 75 Watt PA Amplifier that you could take a look at and draw some inspiration from (its a 75 Watt Monoblock Amplifier so its 25 watts higher than you want to work with but it would at least give you an idea of what kinds of voltages you would be looking at for powering the B+ and the type of transformer you would need to power your 7027As.)
 
nashvillebill said:
No, that transformer has 200 mA on the 600-0-600 windings, that's not B+.

You still need the 200mA for the 7027, plus a few mA for the phase inverter and preamp tubes. I quickly plugged that power supply into Duncan's Power Supply and with a 225 mA constant current load, the max current on the transformer was 285 mA (edited, I had not hit "simulate" after changing the load) but only for a brief part of the cycle. Looking at the graph of the transformer current, the mean (middle) of the sine wave was a little over 200 mA but there was a lot of the cycle that was either 0 or under 200.
Click to expand...

Bill,
Thanks for replying,,, Guess I didn't ask the question right...
The amp I'm working on will have 2 x 6L6, 2 x 6SL7, 2 x 0C3,, and 5U4 rectifier, (unless I go SS, with teh gas regulators)... I just used the example in the manual, but, I wanted to figure out how big a tx I'll need to run this thing, with plenty of reserve, if I decide to play with different tubes...
Of course, I'll look for a "donor' tx, so, unless I know what it came out of, I won't know its power capacity, without trying it... I guess bigger is better, at a hamfest!

Sorry for the confusion, I tried using the PS site before, but its beyond me!!!

If the 200mA 600-0-600 winding isn't for B+, which one is? The example has a separate filament tx, but not a separate plate trans... The questions/learning continues!!!

Regards,
John
 
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That winding is what delivers B+ but it's delivering alternating current at a combined voltage of 1200 volts (if you put a voltmeter across the two plate pins on the rectifier tube you get 1200 volts). The rectifier converts this to pulsing DC and then the choke and capacitor act to smooth the pulsations out. The DC voltage that results is around 460 volts. It's actually still fluctuating some.

Another thing worth noting is that the current that flows through B+ is actually fluctuating as well. The current flowing through the transformer is fluctuating even more, it's not a steady current by any means, it looked like a sine wave but with an interval where no current flowed. So the peak current as calculated at the transformer (the top of the sine wave) was higher than the 200 mA rating, but I suspect that 200mA rating is a RMS current rating and not a peak rating.
 
Yep, I get that,,,, 600VAC CT to each end = 1200VAC across the whole coil,,, and I can see it, using the two diodes in SS doubler PS...The output is more than twice the input, so its actually summing the peak voltage thru the trans input, rather than RMS...while lowering the current...
But I digress,,, I wanted to know how to figure the max vA this amp will need, to have a steady power supply...I'm guessing there isn't an easy way to compute it, without learning Duncans calculator...

So,,, IF two 6L6s + two 0C3s use equal or less power than four 6V6s, or 6BQ5s,, I can use something like a PP console PT to power the amp I'm building? I have a separate filament trans for the 6V heaters...

Regards,
John
 
Yes, 6V6 P-P console transformer should be about right, if this is a SE 6L6 amp. Push-pull generally uses a higher voltage and peak than SE, but a lower idle current per tube. In a class A amp, average current is same as idle current. If you want to use metal or G style tubes, you need to limit dissipation to under 19 W - at 300V, that would be 63 mA, total of plate and screen. So total B+ current will be under 200 mA.
 
Yes it is SE,, I have 6L6GC tubes, which seem to be good for better than the 330V I'll run them at... Also planning to use 2 x 0C3s for screen regulators, I believe they require 5mA each, so that would add 10mA, and figure 15-20, adding a buffer...

Regards,
John
 
What you probably want to do is download PSUDII - power supply simulator, it's VERY accurate.

Link:
http://www.duncanamps.com/psud2/

ask questions to get it going it's not very difficult once you get someone to get you started.

Here's what 400-0-400 200mA transformer gets you (~500v with whatever load you need - this one is set for 100mA). You trick is getting a transformer that can handle your load. Basically 7027A is between a 6L6GC & KT88 @ 35W dissipation. I don't mess too much with Push Pull but suggest IF You want all that power and voltage to look to Rowe Juke Box transformers (the older ones that ran tubes). They will have the juice for what you need and more to spare.

At the end of the day with my amp running I had 479v at the plates with tubes in place. Here's the PSUDII for the amp/transformer/PSU I'm talking... this would certainly be enough for those tubes you like. I'm running some Russian 6L6 tubes with that PSU sounds great.

attachment.php
 
Hi Bob,,,
I have it DLed,,, never was able to run it, let alone understand the graphs/results... I'll try it again, and see if it makes any more sense...

I built one channel of a simple 6L6 SE amp, and am finding the PTs I have aren't quite up to the task, let alone two channels,,,

I think a console PP tx will work for me here... biggest problem is knowing the capacity of a used PT...

Regards,
John
 
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knockbill said:
Hi Bob,,,
I have it DLed,,, never was able to run it, let alone understand the graphs/results... I'll try it again, and see if it makes any more sense...

I built one channel of a simple 6L6 SE amp, and am finding the PTs I have aren't quite up to the task, let alone two channels,,,

I think a console PP tx will work for me here... biggest problem is knowing the capacity of a used PT...

Regards,
John
Click to expand...

6L6 are quite robust tubes and they work across a wide range of voltages.

You can use PT from 300-0-300 up to 450-0-450 secondary voltage just be sure the current is over 100mA. Of course the higher the voltage/mA ratings the more $$, but you should be able to find an old stock or even NOS PT that will work - check your heater current for the 6L6 on the data sheets so you have enough current for your heaters and you're good to go.

For a 6L6 I'd be looking for say 350-0-350 with 120mA, 6.3V center tapped heater winding with at least 3amps, and a 5V rectifier no CT with 3 amps there.

A Hammond 373BX or 373X I think those are more than enough. Those are in the 1 hundy price range, if you want to find something cheaper used say Ebay is your friend there look for Triad or Sencor brands they are both good ole USA made will last forever. I have a standard search for a particular Sencor - the prices can fluctuate all over the place. Typically I wont spend more that $50 for a good PT. I like to spend less of course, so always on the lookout for a old 6V6 console pull, usually it's push pull and will have enough juice for 2 6L6 SE.

Yeah getting PSUDII running so you can understand is a very good tool for modeling power supplies, it's not hard just no directions so pretty much word of mouth to learn. Ask ?? most folks here that build tube amps will use this tool and help you get it going.

Cheers,
Bob
 
bobrown14 said:
6L6 are quite robust tubes and they work across a wide range of voltages.

You can use PT from 300-0-300 up to 450-0-450 secondary voltage just be sure the current is over 100mA. Of course the higher the voltage/mA ratings the more $$, but you should be able to find an old stock or even NOS PT that will work - check your heater current for the 6L6 on the data sheets so you have enough current for your heaters and you're good to go.

For a 6L6 I'd be looking for say 350-0-350 with 120mA, 6.3V center tapped heater winding with at least 3amps, and a 5V rectifier no CT with 3 amps there.

A Hammond 373BX or 373X I think those are more than enough. Those are in the 1 hundy price range, if you want to find something cheaper used say Ebay is your friend there look for Triad or Sencor brands they are both good ole USA made will last forever. I have a standard search for a particular Sencor - the prices can fluctuate all over the place. Typically I wont spend more that $50 for a good PT. I like to spend less of course, so always on the lookout for a old 6V6 console pull, usually it's push pull and will have enough juice for 2 6L6 SE.

Yeah getting PSUDII running so you can understand is a very good tool for modeling power supplies, it's not hard just no directions so pretty much word of mouth to learn. Ask ?? most folks here that build tube amps will use this tool and help you get it going.

Cheers,
Bob
Click to expand...

I'll find a PT at a hamfest or such,,, this is just a hobby for me, not into buying newest etc...I'm using a separate trans for the 6V heaters, just to not add to the PT load, and cause I have one! Even leaning towards using a SS doubler PS, but still will need a healthy enough tx for that also...I have a NOS Merit 40W isolation trans, that I used in the SS version of the schem PS, but its not big enough...

So, what ever shows up first, is what I'll use... The schem I'm using is only running the 6L6s at 330V,,, so a 6V6PP console tx may work, as was said previously...
Yep,, I have to get the PSUD running, and understand it,,,at least then, I'll know what parts I need...

Regards,
John
 
knockbill said:
I'll find a PT at a hamfest or such,,, this is just a hobby for me, not into buying newest etc...
The schem I'm using is only running the 6L6s at 330V,,,
Click to expand...

330V at the plate on a 6L6 is fine you can lower the cathode resistor value to get more juice (mA). The 6L6 tube is VERY flexible why it's probably the most used tube in audio, and it sounds good thru a wide range of settings.

You can run it lower voltage with higher current, higher voltage with high current, or high voltage lower current... I doubt you'd be able to hear any discernible differences. I've tried them all possible ways, probably like the lower to mid voltage (340ish) high current sounding one the best tho with 3DG4 moto rectifier. Really gets the 6L6 tubes to glow a nice blue color. :thmbsp: So ya a setup that was running 6V6 push pull you can re-purpose to run 6L6 single ended - that was my first tube amp build still sounds awesome.

Remember class A all the way with single ended, bias these tubes to 18w dissipation and kick back and enjoy. Try a 450 - 470ohm cathode resistor bypassed with say 800-1000uF 75v cap there, bypass the bypass cap with poly cap .... good to go.

With decent OPTs you'll have audiofull sound. :smoke:

Cheers,
Bob
 
Thanks for all the info,,, once I get a big enough PT, I'll try all the changes you mentioned,,, basically the only way I learn is by doing, and then sorting out the results... I'm using a pair of 0C3s for the screens, and can see, even with less B+ this thing is going to sound pretty good....
Built a 5U4 PS, and am trying a SS doubler,,, never tried 3DG4....

Thanks again,
Regards,
John
 
Reason I mentioned the 3DG4 is I know you've been messing with some Motorola amps and many of them came with that rectifier. Its a VERY good one. Hold a 5AR4 in one hand and the 3DG4 in the other. The 3DG4 is MICH beefier and A LOT cheaper just need different (lower) heater voltage and of course more current. These are used in Moto TVs back in the day, I think they were trying to get everyone to use them.

Cheers,
Bob
 
I built a Maggy 175 (more or less) on a Moto chassis, only one I've messed with,,, It uses a 5U4,, but I've read about guys liking 3DG4s... don't have any, tho...

I just found two matching 100W train transformers (120 to 16VAC),, and connected the secondaries to make an Iso trans,,, I get 117VAC out, but V drops to 110V with the amp connected...I was surprised, as they seem to be pretty beefy, twice the size as the one I was using, but the V drop is greater...
Back to the drawing board...

Thanks for the info,
Regards,
John
 
bobrown14 said:
Remember class A all the way with single ended, bias these tubes to 18w dissipation and kick back and enjoy. Try a 450 - 470ohm cathode resistor bypassed with say 800-1000uF 75v cap there, bypass the bypass cap with poly cap .... good to go.
Cheers,
Bob
Click to expand...


Quick update,,,Main build thread is here...http://www.audiokarma.org/forums/showthread.php?t=598368&highlight=heathkit+A9

I've been listening (CDP, rebuilt Advents), and tweaking this amp for a couple weeks with the test OPTs I have on hand... Right now, I have about 330V, with a 200 ohm cathode resistor, and 500uF cap, bypassed with .47 film cap... It sounds very good, excellent highs and mids, decent bass, hindered by the marginal OPTs, I'm sure...Didn't try raising the B+ yet, as I'm not sure how much the OPTs can handle...

From the Weber bias calculator...
Voltage measured across cathode resistor---11.20
Actual resistance of cathode resistor--- 200
DC voltage measured between plate and cathode. ---330VDC
Plate dissipation per tube (Watts) --- 18
Approximate plate current in each tube --- 53mA

What do you think of the numbers?

Think the next step is a decent pair of output transformers, then dial it in,,,
Thanks again to everyone that contributed, I learned a lot building it, so far, and it looks better than I expected recycling that old chassis!

Regards,
John
 
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