Looking for PP design when I already have the OPTs

[Wigwam Jones]

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.

 

[danFrank]

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

Hi,
If you use them with 8 ohm speakers, then the primary would be 20K, NOT 5K. If you up the speaker impedance, then you have to up the primary impedance also.

EDIT: You could use these for a PP 1626 amp.

 

[Wigwam Jones]

Hi,
If you use them with 8 ohm speakers, then the primary would be 20K, NOT 5K. If you up the speaker impedance, then you have to up the primary impedance also.

Oops, my bad. So now I guess I'd be trying to find an output tube with a load resistance in that range when in PP.

 

[danFrank]

I added some more info in my post above, probably after you read it.
A pair of 1626 would be good and they are nice sounding. You could do this with your 8 ohm speakers but output power will suffer a little.

 

[Wigwam Jones]

I added some more info in my post above, probably after you read it.
A pair of 1626 would be good and they are nice sounding. You could do this with your 8 ohm speakers but output power will suffer a little.

Thanks!

 

[BinaryMike]

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.

 

[Ketje]

Perhaps this is an idea ? http://www.diyaudio.com/forums/tubes-valves/70532-looking-building-shematic-ecll800.html
Mona

 

[Wigwam Jones]

Perhaps this is an idea ? http://www.diyaudio.com/forums/tubes-valves/70532-looking-building-shematic-ecll800.html
Mona

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?

 

[BinaryMike]

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.

 

[Wigwam Jones]

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.

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.

 

[BinaryMike]

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.

 

[Ketje]

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?

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

 

[rmyauck]

Maybe run them in triode mode for about 6W if you have 4 ohm speakers, and can't hurt to try with 8 ohm ones as your speakers in reality may be lower (like closer to 6 ohms), as some are.

Kingsburys or (Poinz) 's Musical Machine might be a suitable 6V6 amp as it used 10K+ primarys and ran in triode mode.

http://www.diyaudio.com/forums/tubes-valves/160170-new-project-musical-machine.html

http://www.diyaudio.com/forums/tubes-valves/149211-6v6-music-machine-build-help.html

 

[Wigwam Jones]

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.

Got it, thanks!

 

[rmyauck]

More schematics on the Musical Machine:

http://www.diyaudio.com/forums/tubes-valves/283277-poindexters-musical-machine-tube-power-supply-schematic.html

 

[BinaryMike]

One more detail: You need to know primary DC resistance. Measure it, then add DCR/2 * 45mA to the anode power supply voltage target.

 

[Eli Duttman]

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".

 

[djgibson51]

EL91/6AM5

 

[Wigwam Jones]

EL91/6AM5

Nice!

 

[tricomp]

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.