1960s Ampeg Portaflex Model ST Solid State Amplifier

[AstroRock]

Hi Folks-

Been a long time browser of this forum but just made an account because I need help with a project that I just acquired. I saw there is already a thread concerning this amplifier (https://music-electronics-forum.com/...g-portaflex-st), however there isn't a lot of information contained within, and its from 2010 so I just wanted to create something with a little more concise information on this amp since there isn't a whole lot of documentation to be found on the interwebs. I hope this jives with everyone here.

This amplifier is from the ST series, which were all transistor based solid state amplifiers. It seems to come after the B series and SB series. Made in Linden, NJ, I'm guessing around the time Unimusic took over. The amp has two channels, each with a treble input and a bass input. It also includes build in Vibrato/Tremolo and a reverb tank. From what I have gathered, these amps were paired with fliptop cabinets similar to the B and SB series. I was able to find a manual for these amplifiers on the Ampeg website, once managed/owned by LOUD - now managed/owned by Yamaha (https://ampeg.com/support/files/Disc...s%20Manual.pdf). According to the manual there were 3 amplifiers in this series, the ST42L, ST25L, and ST22L - each being paired with their own cabinet (Colossus, Olympian, and Gladiator respectively).

The Ampeg site also has schematics for the three different amp versions (https://ampeg.com/support/files/Sche...Schematics.pdf). Okay, so here is where I need some help. I am questioning whether or not I have a version of this amp that predates these schematics. According to these schematics, all 3 versions of this amp contain both Ultra-Hi and Ultra-Low switches similar to the 70's blue lines. However, my version of the ST only has push pull pots for Ultra-High, nothing concerning Ultra-Low.

I did see that user Lowell had received a schematic from Loud which he mentioned in the aforementioned thread. I intended to PM him to see if he might have received a different one that the ones listed on Ampeg's page, unfortunately it seems I can't send PM's yet. If anyone sees this thread before I can get to Lowell, and has a schem for the original Portaflex ST I would be very grateful if you could send it over to me, or post it here. If anyone else has any further info on these amps it would be cool if you could post it here. These amps seem to be very rare, with very little info out there. I found an archived listing from Reverb, which I can't seem to find again, where one of these sold for around $500. It seems they are very cool looking once they are up and running, with glowing blue neon outlines on the front panel. Mine is going to need a complete overhaul so I'm hoping someone can help out with that schematic and any other helpful info. Sorry if this is longwinded, just wanted one concise place for information for posterity. Thanks in advance!

[Enzo]

Hi

Well, you have an ST, not an ST22 or ST40. It is best to just think of them as different models rather than "versions". There may be similarities, but each one is what it is. To me, a version means a revision level. I have attached a copy of the ST schematic.

Ampeg ST Schematic.pdf

[glebert]

I have an ST-25 that is in my "get to someday" pile. A member here tipped me off that musicparts.com has really nice service docs for this for $15. It looks like they have them for the ST as well. While in some ways I think it is a crappy business to charge that much to scan and send out a document that you didn't make, the schematic is much more readable and the layout drawings are indispensable. On the ST-25 type amps they used 1% resistor dividers to set bias voltages for the output driver stages, and I'm pretty sure as those values drifted the amp started having problems. I don't see the 1% callouts on the ST schematic, but the way that the bias design was done on mine was definitely crude, like a 2nd year engineering student would do.

[AstroRock]

Hey Enzo - thank you very much for that schematic. I checked it against what I have and the board #'s match up so you were right on. I sincerely appreciate that and all of the other incredibly helpful information I have seen from you over the years. Your're a legend.

Glebert - thank you also for the info on your ST-25, and the tip on musicparts.com. I just checked and they do in fact have the schematic, but it looks like there is a lead time because they have to actually physically scan it or something. I might actually try that out if I can't read some of the values on the schem that Enzo provided. It looks like some of those values are a little grainy and tough to read, although I can probably infer for the most part. Thanks for the info on the resistor dividers in the ST25, I'll let you know what's goin on in mine once I dive in.

I'll post some updates as soon as I can identify all the components I'll need to replace and get them ordered. So far I definitely have some power transistors that are bad, at least 1 burnt out resistor, all of the caps are original with bulgy electrolytics, and someone for some reason lopped off the power cord, which at some point was replaced with a 3 prong before the chopping It'll be an adventure alright...

Actually I do have one question - I was already looking for replacements for the three 2500uf 80v electrolytics, but I am only seeing 2500uf @ 75v or 100v. It would make more sense to up them to 100v, correct?

[Enzo]

It is just a guitar amp, not lab equipment. Don't sweat the exact specs on such parts.

[J M Fahey]

And yes, a modern 100V part will probably be physically smaller than the old one, so fit with room to spare.

IF mounted on clamps, you might want to wrap a cardboard or fiber strip around them for a tight safe fit, bet old ones were fatter.

Just curious, which speaker is mounted in that cabinet?

Post a picture, I guess you could have a square magnet CTS but nothing would surprise me.

[g1]

I think 2200uF would be a much more common value. Or 3300uF.

[The Dude]

I'll add: I would test the power transformer before ordering any parts. You'd hate to spend a bunch of money on parts only to find out the transformer is toast. One of the reasons power cords are cut is because the PT is shorted.

[AstroRock]

Thanks for clarification on the capacitor values, guys! I found this part from Cornell Dublier (500C252T100AA2B) which is 2500uf/100v, and they are actually the same size diameter cans as what is in the amp, so it would in fact fit in the clamp mounts. They are pricey as hell though... so I might consider going with a 2200uf/100v because that does seem to be a more modern and readily available value. Have any of you tried restuffing old cans with modern capacitors? I have seen this done and was considering it...

That said, thanks to The Dude for that recommendation...I will make sure I test the transformer first before I order these parts. I'm already looking at well over $100 for parts, which isn't necessarily a deterrent for me yet since I paid $40 for the head. Oh and for the record, JM, I only got the head unit. Which is a little bit of a pain due to the fact that the head was part of a flip top cabinet (as you well know) so there is no handle on the top, and one cannot be added due to the reverb tank being mounted upside down to the top cover inside the head . Womp Womp... Even more of an annoyance - I saw one of the Colossus cabs on Craigslist literally a week before I found this head listed on the OfferUp app, and had no idea what it was. By the time I got the head, the cab was already sold

[J M Fahey]

Cornell Dubilier (500C252T100AA2B) which is 2500uf/100v, and they are actually the same size diameter cans as what is in the amp, so it would in fact fit in the clamp mounts. They are pricey as hell though

That is an old school American made capacitor, probably so called "computer grade", and definitely expensive.

Check diameters, most probably you can fit a 4700uF by 75 to 100V (guess youīll find them in 80V flavour) there, which will be an improvement.

But if too fat, 2200uF is fine.

Personally if I had that amp on my bench here in Argentina I would not even search for screw terminal a.k.a. computer grade which IF available would be VOS (VERY old stock) BUT easily available and cheap plain vanilla radial leads (yes, the skinny wires) or "snap in" which are strong stubby sharp pointed legs, obviously both thinner than mounting screws so they would "dance" in the original PCB holes.
No big deal, nothing a little twisted and solder filled wire canīt solve.

Cardboard strips can solve sub diameter problems.

After all, THAT is the kind of modern capacitors you will find in ANY modern amp: Ampeg, Fender, Peavey, Hartke, you-name-it.

Screw terminal is so 60īs

Think CCR Kustom amps or OLD Sunn