Power Transformer As Output Transformer

so TK up in Olytown WA  wants a dual channel amp for keyboards, bass, git fiddle,

always wanted to build a Ampeg B15 Portaflex, so i took some parts up there and built it in his garage, only problem was i had only 1 output trans.

but i did have a hammond pwr trans from an LA2 project,

guess waht, it works great!

ratio is way off, using centertapped sec as pri, and the 6.3 winding as the spk winding.

so if you need a 6l6 output for cheap,  jus sayin,

here is the circuit and here is the chassis, it sounds fantastic,

no 12ax7 stuff, all octals, 6sl7, 6sn7 as splitter to knock down gain for keys, which is also a total mismact, go figure,

ok  easier to us from img tags, attachment browser not too great

CJ


it has been a while since you've been around these parts...we don't diy like that anymore.

it is beautiful  :'(

A homebrew B15?

Awesome!

thanks guys!

orchard supply sheet metal, bent it with vise grips, 5u4 rect for compression,

i like the b15 tone circuit better than fender,

6384 bendix redbank tubes

i think the b15 needs log vol pots instead of lin,

ampeg must have wanted their amps to sound louder


cathode bias, see the 50 watt resistor with the adj  tap?

A friend has a small Garnet amp that uses a 6sn7 as the phase splitter
two 6v6's sounds great , nice productivity cj !

man, last time I tried to build an amp like that, it ended up an oscillating mess...

Pwr tran for output.  Fantastic ! 

cooool, nice job!
whats the resulting output impedance? i guess the 6.3V winding needs to handle the watts? i ask cos i have a little toroid power transformer with 6.3V widing that i wouldn't mind using for a princeton project i think its 230V/150V+6.3V 4amps, would this work with a pair of 6V6 in push pull?

there is not much wire in the chassis, mostly heater wiring, so the flux loop that would catch noise is very small, same principal as api discrete opamp and telefunken v modules, the tighter, the better, tag boards are ok, but the chassis ends up being much bigger,

the main spec to watch is the current rating of the pwr trans sec, which becomes the pri of the output.

freq response does not need to be great for a git amp, most of the noise is at 1 k hz,

if you have 6l6, you need about 80 ma sec current on the high voltage sec,
so if a pwr trans is designed for a pair of 6l6, the current will be adequate to handle the output tube current, get it?

so a 6l6 pwr trans can be a 6l6 output, etc.

thought about using the pri as output sec, but too much dcr and not big enuff wire.

there is no osc, even full up, 1 k feedback resistor helps, as well as solid grounds via terminal lugs being soldered to chassis instead of #6 hardware which can become bad after a while. also, grouping grounds together helps, v1 and v2 star ground , 1 ea per channel,

proper wiring, like keeping b+ lines at a 90 degree angle to grid wires, etc, helps keep the amp stable.

Inspired by CJ's post it got me thinking :

I checked out the ratio of a 2x115:2x55V mains toroid transformer and its 4.18:1. (230/55)

Thats the same as 10K:600 ohms, which would be very useful, so I stuck a signal generator on the primary and a scope on the secondaries and went through the range.

I'd expected it to be good at 50/60 Hz obviously but it only dropped very slightly at 20Hz which is fine for most apps.  Where it surprised me was the top end, it only started to drop at 25KHz.  There was a slight resonance at 200kHz and 1.2MHz just detectable.

So it could be useful for so many tube compressor projects using the junction of the 2 primaries for HT/B+ .  This is a 50VA TX so there is bags of poke.  It cost £17.32 or ~$27.00

Now somebody tell me why it won't work!

best
Dave

Only because Edcor sells them cheaper.....

I've bought Edcor myself and its very nice, but there is duty and vat to pay which pushes the price up a lot.

best
DaveP

Actually i do this a lot with "normal" EI transformers...
take a 220V:12V transformer - (about 0.5A power rating)
take out the secondry winding - count the turns (no more than 100 normally)
calculate the parimary turns from that
and wind to taste...
dcr on the primary is high ~ 250R so not so good for low impedances - but for cap coupled tube output it works great for me.(very good frequency response)
now if you have 2 x 115V primary - even better -like you say- you have a tap for B+ and can use it in push pull designs.

How about this for a U47?

230/36 = almost 6.5:1

If you take the clamp off it would fit, but I dunno what it would sound like.

Too good to be true?  ;D

God idea for low-Z output trafos but:

One thing is how it acts when driven from a low-Z generator - another is how it behaves at significantly higher drive impedances. Here you run into problems with parasitic capacitance ruining high end.

And for small-signals like microphones you'll not want standard iron-core - low-level will be grainy and distorted.

Jakob E.

Yeeeh, thought so. Thanks!

CJ:  Are you still winding transformers?  Haven't tried the Edcor stuff...Do you know if a 417A or D3a (triode connected) single ended trafo is available (app. 2.5 k primary with 600 ohm secondary), with DC in the primary, with linear response 20Hz-20kHz and +24 dbm clean output?

rmburrow said:

CJ:  Are you still winding transformers?

Click to expand...

HA! Is the pope still catholic?

Love that build CJ.

I've been coming to like the Ampeg eq stacks as well - my current build uses something very similar as this B15 eq.

I'm also trying some other Ampeg EQ circuits - they are quite innovative and a nicely different to the common eq stack styles out there.

you can find a lot of SE outputs at Antique Electronics,

https://www.tubesandmore.com/products/transformers_chokes

those Ampeg B15N amps are great, not a lot of power, but a bunch of tone,

using discrete components for the tone controls gives you a better sound than the encapsulated tone parts, or so they say, never did an A/B test,