1959 Supro Coronado 1690T on the bench

[TubeGeek]

Thought this is worthy of sharing pics...


Serial T8364


It is one of the sweetest blues tones I have EVER heard.  I absolutely love this one, too bad it is not mine.


It came to me with the speakers wired in series for a 16ohm load when it should be a 2 ohm load for the OT. It sounded very weak and under powered. The speakers in this are a CTS and Oxford 10", both 8 ohm. I wired them in parallel for 4 ohms and the amp sounds 1000% better.  I know it is not the optimal 2 ohm load but it is closer.  I'll mention to the owner he may want to install new 4 ohm speakers in parallel at some point.


I did a simple electrolytic cap job and that's about it.  It doesn't need much more than this in my opinion.  The owner is really going to love the tone improvement.

[PRR]

2-Ohm only??

Personally, I'd look around for a top-quality 40W OT with 2-4-8-16 secondary taps (any primary). You lose 1dB from autotransformer loss, but heavy mis-match is 5dB or 10dB loss.

[eleventeen]

Long ago and far away circa 1974 I got a Supro something or other amp w/2 6L6 and 1-15" for free as part of cleaning out somebody's storage space. I don't even remember what happened to it---unfortunately I regarded it as a piece of junk. Probably because it didn't have reverb.

[uki]

Very interesting it have 2 chassis, I bet it is very quiet?!

[birt]

i was looking for a schematic for this amp and found this:


http://www.dirtygirlamplification.com/Amp_site/Schematics_files/1690T.pdf

[TubeGeek]

Very interesting it have 2 chassis, I bet it is very quiet?!

Yeah it is quiet when idling.

[TubeGeek]

2-Ohm only??

Personally, I'd look around for a top-quality 40W OT with 2-4-8-16 secondary taps (any primary). You lose 1dB from autotransformer loss, but heavy mis-match is 5dB or 10dB loss.

Yeah only 2ohm output on the OT.  I advised the owner of his options.  He is leaning toward replacing the speakers down the road to Weber 10F150 (3.2ohm).  We feel he should try and use the original OT as long as possible.

[TubeGeek]

i was looking for a schematic for this amp and found this:


http://www.dirtygirlamplification.com/Amp_site/Schematics_files/1690T.pdf

Yes I found that in the schematic library here and it saved a bunch of guess work!  It's a nicely drawn schematic.  It certainly came in handy.

[PRR]

> use the original OT as long as possible.

Of course.

I am suggesting a second transformer *just* for converting the 2 Ohm output to 4 and 8 ohm loads.

Ideally we want a single-winding "autotransformer" for lowest loss. However plate-speaker transformers are SO much more common, and the loss from ignoring the hi-Z primary isn't worth the cost of getting a custom winding.

EDIT: E-V changed their site, less info, more cookies. Link removed.

[TubeGeek]

Where does the primary of the new transformer connect?  to the secondary of the first OT? (2ohm)

[HotBluePlates]

Where does the primary of the new transformer connect?

To nothing; insulate and stow those ends.

PRR is suggesting you attach the 2Ω secondary of the existing OT to the 2Ω & common terminals of the added OT's secondary. You get your 4Ω or 8Ω output between the 4Ω & common, or 8Ω & common.

That is, the newly-added OT is just an autotransformer (step up/step down via taps on a single winding).

[TubeGeek]

Where does the primary of the new transformer connect?

To nothing; insulate and stow those ends.

PRR is suggesting you attach the 2Ω secondary of the existing OT to the 2Ω & common terminals of the added OT's secondary. You get your 4Ω or 8Ω output between the 4Ω & common, or 8Ω & common.

That is, the newly-added OT is just an autotransformer (step up/step down via taps on a single winding).

I see. Thanks for the clarification.  I learned something new. Thanks.

[octal]

This is the only source I've found for the purpose-built speaker autoformers:


http://www.tedweber.com/wzc-50


Or you could just buy an output transformer from Doug. :-)

[lovetone]

Sorry to join this thread so late.

Just wondering if you measured the primary of the original transformer?

I was looking to buy one with a 2 ohm secondary for a project but thought this might be 5K like other Supro transformers.

[TubeGeek]

I did not measure at the time, or I can’t remember.  It was quite a while ago and many amps ago.

[DummyLoad]

> use the original OT as long as possible.

Of course.

I am suggesting a second transformer *just* for converting the 2 Ohm output to 4 and 8 ohm loads.

Ideally we want a single-winding "autotransformer" for lowest loss. However plate-speaker transformers are SO much more common, and the loss from ignoring the hi-Z primary isn't worth the cost of getting a custom winding.

link is broken.

--pete

[PRR]

> link is broken.

As is the TedWeber link in a later post. I have removed mine in disgust at E-V's cookie pile-up.