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Looking for PP design when I already have the OPTs

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Wigwam Jones

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Joined 2011
#1
I happen to have four unused but very old PP output transformers. They are very small, look like old All-American Five OPTs in terms of size.

They are from SNC Manufacturing, Glenview, IL. According to what I've been able to Google, SNC is still around, moved from Glenview to Wisconsin just after WWII, so these are indeed quite old, I guess.

Model number is P316. Labeled 6W output 10M CT to VC.

I was able to find a list of old transformers online and the person who made the list states these are 10K PP to 4 Ohm.

So here's my question. Given that these are unused, I'd like to use them in a PP tube project. Naturally it would have to be low output given the size of the transformers (6 watts seems a bit optimistic given their size).

So I am casting about for a topology that would match these output transformers.

Given that these will probably end up matched to a set of 8 ohm speakers, should I rate them at 5K instead of 10K?

Open to suggestions, and thanks for your time.
 
#6
You need 316Vrms across the 20K OPT primary for 5W out. That implies 224Vp across each 5K half of the primary, requiring 45mA peak current. B+ will have to be around 300V depending on how far the power tube can pull its anode down at 45mA. A pair of 1626s could work in class AB2 if you're willing to drive the grids up to +12V. Grid bias will be around -48V to keep anode dissipation down to 5W at idle, so you need a total of 60Vpp drive at each grid. Judging by the curves on the datasheet, linearity doesn't look stellar.
 
#8
#9
6AQ5 (5AQ5, 12AQ5) looks like a good bet. As a pentode, with anode and G2 both supplied from 250V, it can pull the anode below 25V at 50mA without driving the grid positive. And it stays in class A all the way. I would drive 'em with a classic pentode input stage and a DC-coupled split-load triode phase inverter made with a 9-pin triode/pentode; 6GH8, 6AN8, 6BL8, etc. The pentode input stage has the gain to accomodate global NFB and it isolates your preamp or input level control from excessive capacitive loading.
 
#10
BinaryMike said:
6AQ5 (5AQ5, 12AQ5) looks like a good bet. As a pentode, with anode and G2 both supplied from 250V, it can pull the anode below 25V at 50mA without driving the grid positive. And it stays in class A all the way. I would drive 'em with a classic pentode input stage and a DC-coupled split-load triode phase inverter made with a 9-pin triode/pentode; 6GH8, 6AN8, 6BL8, etc. The pentode input stage has the gain to accomodate global NFB and it isolates your preamp or input level control from excessive capacitive loading.
Click to expand...

OK, I follow most of that. Curious on the PP 6AQ5 though. Datasheet shows 10K load for PP, and yes, these tiny OPTs are 10K into 4 ohms, but I thought if I use more traditional 8 ohm speakers, then they reflect a 20K load to the plates? Am I mistaken in my understanding?

EDIT: And I also see 10 watts output for PP. These OPTs are physically tiny. Even though they say 6W, I tend to doubt that. I realize PP OPTs are physically smaller than SE, but I suspect I would be pushing way too much horsepower on these. Again, unless my understanding is way off.
 
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#11
Not mistaken. Your 20K load will yield 5W instead of 10W output, which was my putative design target, and it has the advantage of staying inside the class A operating envelope. You could use cathode bias if the anode supply is 263V.
 
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#12
Wigwam Jones said:
Neat! What are your thoughts on compactrons?

Mini-Tube Audio Amplifier - Part 1

Do you think this might work? The author says he ended up going with a 20K PP output transformer; if I am going to use these 10K:4 ohm opts with 8 ohm speakers, do you think this might do the trick?
Click to expand...
Not a very stable solution to get the opposite phase.You have to adjust the trimpot on the brink of oscillation to get the amplitude right.If the voltage divider on the OT is half way there is no output to the grid from the potm.Adjusting one direction givers NFB , the other PFB !
With the lower triode grid on gnd level and the 150ohm replaced with a (low drop) current source it could be possible.
Mona
 
#13
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#17
The 6V6 family is large. In addition to the 'AQ5 already mentioned, include Loctal 7C5/14C5 and Noval 12AB5/6CM6. in the list.

BTW, the "weak link" in the 'AQ5 is the screen grid. Protect the screen grid from excessive power dissipation and quite respectable plate to cathode potential differences can be safely employed. Refer to "El Cheapo" a reasonably close relation of Poindexter's "Musical Machine".
 

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#20
The transformers are small; you'll probably want to go with a compact design.
Triodes sounds arguably 'better' so why not kill a couple of flies in one swat ?
Take a look at the 6N7, which is the same 6A6, 53, ECC31, 5694 and to some extent 2C34 and RK34 besides the Russian 6H7.
Inexpensive Push-Pull in one bottle !
A couple of those for Stereo run in class AB1 should perform well from a supply voltage of around 275~300V.
Some recommends to not use fixed bias with 6N7 but I would because of the larger plate-swing available.
P-to-P loading not ideal with 20KOhm but it will still sound quite good, NFB applied, imho.
 
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