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Old 13th September 2015, 07:29 PM   #1
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Default Time for another dumb question

I've been playing around with a lower-voltage B+ tube amplifier using parts I have around the house. I realize that most of what I'm doing is sub-optimal, but it's all in the nature of an experiment. Please understand that this is a scrapheap, and it's going to stay one. Just playing around with parts I have - not wanting to optimize or buy a bunch of stuff (other than OPTs, which I will need to buy).

The PT is a salvaged power transformer from a huge solid-state receiver. It offers 80VCT, 40VCT, 20VCT, and 4VCT secondaries. It's freaking huge, too. At least 10 pounds. I believe it was made by Tamura, it has that funny logo on it. So, I don't think I'm going to overtax it. Running for nearly 8 hours yesterday, it never even got warm to the touch under load.

Tubes are two 50C6G (a 50-volt heater variant of the 6Y6G) and one 6SL6GT.

I used a solid-state rectifier/double circuit to get my B+ from 80VCT (which developed about 58 volts DC) to just over 110 volts DC.

The 40VCT outputs are used to heat the 50C6G tubes - a bit cold, but they seem to be OK with it.

I am using 1/2 of the 20VCT secondary to heat the 6SL6GT tube, with appropriate dropping resistors, I went from 10 volts to 6.3 on the money.

I even made use of the 4VCT output to power a 6.3v pilot light.

I am using this schematic, more or less:

Circuit - 6Y6 SE ??

Mild modifications - I did use a set of grid leak resistors in front of the 6SL6GT. My power section is quite a bit different, of course.

My B voltage is right where I want it on the 6SL6GT, but I'm about 50 volts lower than the B+ given in the circuit described in the link. Doesn't seem to matter much - I am getting good output, and I really don't want to get too wrapped up in 'fixing' the B+ when the goal was to use these specific parts, which is working out pretty well.

However, I am so happy with the outcome so far that I am thinking I want to put this into a chassis, and that means picking out some output transformers.

The various schematics I have seen out on the web seem to all want 3.5K SE output transformers into 8 ohms, but the data sheets all seem to show 2K as the optimal primary impedance for 135 volts B+ (and I'm a bit lower than that).

So, doing my best to figure out the correct value for OPT primary impedance and comparing that to what's available out there, I'm thinking that what I want is about 1.7K for my primary impedance.

As it turns out, Edcor has a dandy set of 15-watt SE 1.7K OPTs for just about $20 each, not including shipping and waiting time.

So my dumb question of the day - am I figuring this right? For my lower-than-135 volt B+ on the output tubes, my optimal value for lower distortion ratings is going to be something less than 2K, am I right?

Well, that's it. If I am way off base, please let me know. Thank you!

And again, my goal here is to figure out the best OPT impedance value for this scrapheap I have put together - not to make the sow's ear into a silk purse. I know I could hit the recommended B+ values with a different transformer, but I wanted to use this one.
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Old 13th September 2015, 11:31 PM   #2
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Before chock you can connect 47 uF to ground to decrease ripples.
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Old 13th September 2015, 11:39 PM   #3
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Quote:
Originally Posted by azazello View Post
Before chock you can connect 47 uF to ground to decrease ripples.
Not to worry, my power supply is not the same as the linked schematic. My DC is nice and clean. I'm running 3 47uf caps, with 100 ohm between the first two and a choke between the second two. Since I have a voltage doubler arrangement, there are also two 100uf caps as part of that apparatus, before the smoothing caps, resistor, and choke.

I'm really more interested in choosing the right output transformers.
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Old 14th September 2015, 12:05 AM   #4
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Quote:
Originally Posted by Wigwam Jones View Post
I used a solid-state rectifier/double circuit to get my B+ from 80VCT (which developed about 58 volts DC) to just over 110 volts DC.
Actually, you have it configured as a 4 diode bridge for 110 volts. A 2 diode doubler circuit will get you to around 220 volts B+ if that is a better voltage for you. All you need is 2 diodes and 2 capacitors to get 220 volts.

Look at figure 4 on how to wire it up:

https://en.wikipedia.org/wiki/Voltage_doubler
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Old 14th September 2015, 12:10 AM   #5
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Quote:
Originally Posted by danFrank View Post
Actually, you have it configured as a 4 diode bridge for 110 volts. A 2 diode doubler circuit will get you to around 220 volts B+ if that is a better voltage for you. All you need is 2 diodes and 2 capacitors to get 220 volts.

Look at figure 4 on how to wire it up:

https://en.wikipedia.org/wiki/Voltage_doubler
Thanks, I will look into that. At the moment, though, I'm thinking about output transformers.

Thanks again for the link. The design I used was this one:

Enhanced Voltage Doubler Power Supplies

I used the "Classic Full wave CT transformer circuit, " minus the optional bit.
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Old 14th September 2015, 03:03 PM   #6
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The 50C6 should work nicely on 220VDC, using 110V for the screen (half voltage point on the doubler). Quick way to estimate load Z is V/I, V is plate voltage I is idle current. With 220V, would use 3.5K output transformer, which would be more useful in the future... SE 6L6, for instance. Power should be around 3 times what you can get at 110V.

You can use the 10V winding in series with the 40V to get 50V for the output tubes - and still use if for the 6SL7.
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Old 14th September 2015, 03:30 PM   #7
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Quote:
Originally Posted by Tom Bavis View Post
The 50C6 should work nicely on 220VDC, using 110V for the screen (half voltage point on the doubler). Quick way to estimate load Z is V/I, V is plate voltage I is idle current. With 220V, would use 3.5K output transformer, which would be more useful in the future... SE 6L6, for instance. Power should be around 3 times what you can get at 110V.
I always appreciate your wisdom and advice, Tom. Thanks! I don't think I am going to get 220VDC on the plates, though. I realize how advantageous it would be, but I just don't want to do it. I have the B+ I am happy with, I just want to figure out what my output transformer impedance should be. With that forumula you gave, I guess I can figure it out on my own.

Quote:
You can use the 10V winding in series with the 40V to get 50V for the output tubes - and still use if for the 6SL7.
That is a very interesting thought! I am confused how I could get there, however. Both the 40 and the 20 are center-tapped. I am currently using the 40 VCT for the heaters, with the center tap grounded. The 20 VCT I am taking one end and the center tap to make 10 volts for the 6SL7 heater (with dropping resistors, of course).

I'm trying to picture it - I can connect one end of the VCT to one end of the 20VCT, for sure, but that's going to give me two weird center taps, won't it? Neither one will represent 'center' of anything. I guess if I tied them together like that, and took one end of the 40VCT and the center tap of the 20 VCT, I'd have 50 volts, but no center tap anymore (I'd have to ignore the old 40 VCT center tap). But how then would I derive the voltage for the 6SL7? All I would have left would be the unused end of the 20 VCT - would I take the center tap, even though it is also being used by the (previous 40VCT). Sorry having trouble wrapping my mind around it.

Thanks!
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