Supro 1690T

[EFK]

Dear lord, does ANYONE have a schematic for this thing? This is the so-called Coronado II, a 2X10 amp with (2) 6L6GB cathode biased and a 5V4 rectifier, (3) 12ax7 in the preamp. Preamp and power amp are two different chassis sections. It is all real point/point and tag strips and thus a rats nest. Mine works pretty well, although it has a faint 60 cycle hum when dimed and the trem is making some thumping noises when turned on. The power amp was left alone, and I can map that out, but the preamp is a real mess and on at least two different occasions, quite some time ago judging by the dust, many of the caps were replaced. The problem is, some of the values I'm seeing seem atypical for a 1957 amp (going by the Rola speaker dates). .01 coupling caps off the preamp plates seems a little odd for an amp this old considering most of the other caps in the amp are .05 and some .02. Furthermore, I found what seems like a mistake, as someone had two different value PI output couplers in there, one a .05 and one a .01. That couldn't be right, I don't think. So, does anyone have a schematic? I have searched the internet high and low. Possibly there may be a Gretsch or National amp of the same layout w/ a different name, but I have not found anything yet other than that it might possibly be the same as a Gretsch 6161T, another amp for which I can find no schematic!!!!!

Anyone?

[EFK]

Also some speculation that this *might* be the same as a Supro 1615T (which had a 15" speaker) - anyone have a schematic for that????

[EFK]

OK I don't think it's the same at the Gretsch amp mentioned above, but I do seem to have some confirmation that the 1690T and the 1615T are the same other than the speaker difference and slightly modified power amp chassis on the 1615 to accommodate the huge 15" speaker bell.

Since I'm apparently talking to myself right now , I'm going to keep posting details here as I uncover them so at least it will pop up in the search engines if anyone else is looking for info.

As I mentioned, it is a cathode bias 6L6GB amp, 5V4 rec and uses (3) 12ax7 in the preamp. Most of the original signal caps are still present and are ceramic disc, and while some of the coupling caps have been replaced with brown Mallory PVC I'm pretty positive that originally the preamp was all ceramic.

The speakers in mine are 1957 dated Rola 10" smooth cones, flat felt dust caps, all original, and they are wired in parallel for a reading of 1.6 ohms total!!! Why so low? Well I'll tell you. (stop it, stop it, we'll have no singing here....): while the power transformer is a big beefy job, the output transformer (also original, speaker mounted) is this little tiny thing that looks like it ought to be in a 6V6 amp - in fact, that's exactly what it seems to be. Supro must have gotten a deal on a boatload of 6V6 transformers, because the impedance ratio is about 5300 to 1! This means for a 6L6 amp, which this is, even a 1 ohm speaker load is already beginning to creep up a little higher than is considered "ideal" for 6L6. Closer to a 2 ohm load, as stock, means the impedance being reflected back to the tubes is something like 9K-10K. Wow.

(As a side note, if this REALLY is the so-called Zeppelin 1 amp, and Page popped a 12" speaker in there w/o swapping the OT, what do you think the reflected impedance must have been if it was even a minimal 4 ohm speaker?!!! That would be around 22K!!!! What would this do?)

I'll get more into voltages as I move along. I was going to make a replacement 12" baffle as I want to try a 12" in here, but given this OT situation I'm not so sure now...

[custom_amp_30]

I spent two days searching the web trying to find this schematic, No Luck. I reviewed The Tube Amp Book, and three other manuals that I had laying around. This is the best that I could do for now. More websites.

Supro Early 60's Supro1690T Combo Amp Super Sound Zep ¡ÚJ-Guitar.com ¾¦Éʾܺ١Û
Jimmy Page Supro Amp - Gearslutz.com
http://www.suprousa.com/
http://www.schematicsunlimited.com/?z=supro
http://www.valcopages.com/Schematics.html

[EFK]

Hey, thanks for trying! I've seen some of those, all dead ends. The amp on the Japanese site is later than mine and Supro made a few changes apparently - cab is a bit different, the preamp section looks the same but the power amp section is organized slightly differently (probably to accommodate mounting the OT on the chassis, not the speaker). Looks like the same scrawny little OT though. I wonder how they managed to get two Celestions to work, ohm-wise? Or maybe just didn't care? Too much focus on whether this was the model Jimmy Page used, frankly I doubt it because shoving a 'common' 8 or 16 ohm 12" speaker in there would be reflecting back 40K to 80K!!!!! Wouldn;t that self destruct? And who would think to change the OT just because you change the speaker?

The more I think about this OT, the more I think this probably was designed as a 6V6 amp (I get 392V on the plates plugged right into the wall [@123 VAC, no bucking transformer which I usually use with these old jobs] and being cathode bias, the actual working plate voltage is less) and someone at Supro got the bright idea to make it seem louder by selling it with 6L6GB!

[EFK]

Trying to map out this preamp is a nightmare. One thing I can tell you (meaning anyone actually reading this) is that the front end of V1 seems to be grid leak biased - cathode is tied directly to ground...... that's as far as I've gotten...

[custom_amp_30]

Can you post some pictures of your progress. I'm still a bit new at giving advice, but a picture will peak the attention of all the folks with great advice. Positive or Negative, the information given may prove helpful.

[EFK]

No pictures right now, but I've managed to sketch out the preamp/PI (sans tremolo circuit, which I don't understand at all) and the power amp, and I will post those pretty soon once I clarify a few things and scan them. Might help someone out - they are VERY weird.

[EFK]

This is the best that I can do for now. Please bear in mind I am not a pro, but I think this is accurate. Just about all of the resistors and caps are original, although some of the larger .05 caps are replacements. The electrolytics were all original - the power amp had not been touched at all. I did not worry about the trem circuit because I didn't feel like it (it works very well) and I don't understand them anyway! The trem speed pot has an on/off switch stock (pot is original) and the trem channel (channel 2) actually seems to have a little more gain than channel one (which is grid leak biased?). I have made some REVERSIBLE changes: new baffle board (grillcloth is independent of the baffle) w/ a 12" speaker, and I went w/ a much larger OT, a radiospares JTM45 OT for 8 ohm at 6.6K. I mounted this to the replacement baffle. I also added a cathode bias cap on a switch (also mounted on the replacement baffle), 10uf, but honestly it does not seem to make very much difference; just the most imperceptible bit of added gain, hard to notice. Maybe if I went with a larger cap... The tone knob has a VERY dramatic effect. One thing: this thing is extremely touch sensitive and has a LOT - a HUGE amount - of harmonic feedback. Think Clapton's 12th fret g string note on Steppin' Out off the Beano album, and then some. Better than a Marshall in this respect. Overall, the amp is not terribly loud though; even w/ the changes I made, it is definitely not as loud as my blackface deluxes. I'm going to guess that it maybe only makes 15 watts currently? Very odd for a (2) 6L6GB amp. This is with a Weber blue dog but I might swap in a celestion blue or gold to get a bit more volume. The amp sounds like a cross between a deluxe and a JTM45, not as flubby in the bass as the deluxe. There are notable differences between all 4 inputs. I'll bet dropping those 100K grid resistors on the normal inputs down to 34K or 47K would open the those inputs up even more. Probably could also get more volume by popping some 7581A tubes in there and biasing the holy hot crap out of them. Right now, w/ the JAN 6L6WGB tubes I have in there, the 250 ohm resistor only has them dissipating 19watts by calculation, but I would think it would be louder. The 5V4 rec is RCA but otherwise unmarked and very old, so maybe it is weak and maybe not even a 5V4 - I wouldn't know. I'm going to get a new one to try out as well.

Anyway - here is the best I can do w/ schematics, please pardon the unprofessional drawings. I was sketching them out as I went along, so some of it is a little wanky. If anyone sees things that just can't be right, PLEASE TELL ME! I love this little sucker! GREAT AMP!

[custom_amp_30]

nice effort, Thanks for sharing.

[EFK]

Oops, fixed one mistake and found another; V3(PI) cathodes pins 3/8 are tied to ground via 2.2K resistor.

[EFK]

Fixed!

[EFK]

A zombie returns!!!! After a few years of Valco-based obsession, and after going though a bunch of 1690T amps as well as Vega V70s, a 1615T, multiple 1695s and a weird-a** Oahu version, here is a much more readable schematic which tries to cover pretty much all the variations I have found. And there is a LOT of variation. Nevertheless:

http://www.dirtygirlamps.com/Dirty_G...ro%201690T.pdf

[nickb]

+1! Thanks for doing that!

Do you know anything about the National Professional? I've been longing to build one of these for years.


1957 National Professional Amp
5U4
4 x 6V6,
2 x 5879
2 x 12AX7
Speaker: 1 x 15 Jensen Alnico 5“Concert Series” P15N.

That's about all I know. I'd love to see a schematic for one of these babies. Any clues?

Nick

[ron vogel]

You might want to try Magnetic components in Schiller Park, IL. They're still around...and manufactured the trannies for all the different branded Valco amps in the 50's/60's.

I just happen to work directly across the street from them. I know for certain they had the drawings for the transformers still...I saw them with my own eyes when I was in there.