Output Transformer question

[Redfishbum]

Greetings!

Earlier this year I thought it would be fun to build a really small el84 tube amp and a 6 inch alnico speaker.  I happened upon the Ricktone 19 Cupid Schematic.  I studied it and began collecting parts.

The schematic calls for an 8 ohm secondary with a 4000 ohm primary impedance.  At the time of my searching for parts, the only one that had listed these stats was the P-T291.  The diagram attached is the only info I have available to me.  I do not currently have a variac to confirm the winding ratios. It seems to suggest that I need to connect Common (C) and 3 on the secondary for 8 ohms but I'm not sure about the primary side.

I also noted after the fact that this is for a push pull and I am only using one output el84.

Any thoughts on how this should be connect?

Thanks in advance!

Bart

[sluckey]

I would not use that transformer for a single ended amp. A Hammond 125CSE would be a good choice for a single EL84.

[Redfishbum]

Thanks Sluckey for the reply.  I just looked at the wiring for the Hammond CSE seemed also confusing.  The amp schematic calls for 4K primary with 8ohm secondary.  I could find a 5K primary but not a 4K.

The output driver I have seems to fit this but not if it isn't gonna work for a single end.  Transformers are black magic.....dang.

Thanks.

[sluckey]

5K is just as good as 4K for this. That Hammond 125se is a really popular OT.

[PRR]

Tubes&More _did_ "tell" you this one is P-P, though very subtle. See attached.

Tubes&More does NOT make it easy to find SE iron. This filter link is supposed to find SE, but in fact it finds a lot of P-P iron which mentions SE alternates.

This sorted filter shows that the true SE iron does not appear until the $40 range. There is some sense to this. For the same Watts, SE iron will be 3X-4X the size/weight of P-P iron.

Impedance 4K or 5K makes no difference. Wider differences are often done. A quick analysis of that Rick-Tone suggests 7K as a "best loading". However this is clearly NOT aimed at "maximum loudness" "stadium blaster". Design is actually constrained by use of two commodity 120:12 transformers back-to-back, with double sag. Ideally this is 330V DC, but actually 300V no-load and under 280V at 35mA load. Power Output is 3 Watts at best, and maybe 2W with 4K-5K loading.

Any "Fender Champ" OT is quite suitable. Mostly not cheap. Any old-old radio *with power transformer* and one 6V6 has a 5K OT, though finding one in working condition could mean buying and shipping a lot of heavy junk. "AC/DC" radios with 35C5 50C5 etc outputs are NOT suitable.

A possible alternative to new-made output transformer is to scour eBay for "Epiphone Valve Jr" OTs. It was one EL84 at much higher voltages. There was a fad to rip-out the stock iron for other iron (assumed better). The rip-outs often appeared on eBay, so many that prices fell. I do not know if this fad is still running, if it may still be a buyer's market.

[Redfishbum]

Thanks for the replies.  Yep, I did see that it was a double ended after the fact; I was so focused on the impedance that I totally missed it.

This is great information.

Bart

[isaac_teller]

https://www.tubesandmore.com/products/transformer-output-8-w-single-ended

[thetragichero]

https://www.tubesandmore.com/products/transformer-output-8-w-single-ended

i used this in my 'big boy' 6l6g vibrochamp 10" build. i don't claim to have golden ears or anything but played this amp at church for the better part of a year. sounds quite nice, i still use it quite a bit for recording

[dude]

i used this in my 'big boy' 6l6g vibrochamp 10" build.

The 8 watt probably saturates a bit at higher volume, in my 6L6 Vibro Champ l used a 10 watt OT, probably still saturates a bit but a little more headroom. The Fender Champ 12 uses a 10w OT, might be the same OT Sluckey recommended as l bought mine from AE.