Another Amplifier

[Destructor]

I finally purchased a Latino ST70 kit. I’ve built two kits already, a AES SE1 300B amp and a AES Superamp, both of which are outstanding quality but I wanted an amp in the 30 watt range to be able to rotate. I currently have a 90’s production ST70 Series 2 but it just doesn’t cut it when compared to the sound quality of my two AES amps. While the Series 2 is a nice dependable amp and generally sounds very good I wanted something in the same power range and with the better driver circuit. Every review of the Latino amp I have read has been very good. I bought the stock kit. I haven’t decided yet on coupling caps. I’ll go through my stash of tubes and see what I have for 12AU7’s and I’ll start off with an old set of unused Sovtek EL34’s for its first start up. The kit arrived last night. I unpacked it and laid out the parts on my dining room table.

 

[Squidward]

Sounds real neat, I love a build thread!

 

[Lavane]

I like mine!

 

[Destructor]

I just found in a partial search of my tube stash one Eico labelled 12AU7, made in Great Britain, two International Servicemaster labelled large plate 12AU7's USA and two older stock Chinese AU7's. I know I have one or two RCA's around somewhere too. I'll use the Chinese tubes for first start up. I have a couple others in a Scott 130 that I am not currently using.

 

[Blast]

I've been kicking around one of those, or the ST-120, for a couple years now. Just haven't pulled the trigger yet. Good luck with your build!

 

[AlTinkster92]

Been thinking of if I got a new amp what would it be? More than likely one of Bob's ST 120's. He'd have to build it though. Good luck with your ST-70!

 

[Destructor]

Thanks, I got a 5AR4 and some other supplies today. A guy at work has some NOS 5AR4's that I told him I'd buy. I've used some 30 watt/channel tube amps and receivers so I know that my speakers and room size will be fine for a modest S T 70. It will be interesting comparing to my Series 2 amp.

 

[Destructor]

Bottom shelf, my last kit build.

 

[crispycircuit]

Very nice gear. I have a Sound Values preamp also. Let us know what you find out?

 

[primosounds]

Thanks, I got a 5AR4 and some other supplies today. A guy at work has some NOS 5AR4's that I told him I'd buy. I've used some 30 watt/channel tube amps and receivers so I know that my speakers and room size will be fine for a modest S T 70. It will be interesting comparing to my Series 2 amp.]

I am not sure if the st70 will equal the sound quality of the AES Super and especially compared to the 300B amp. Unless you have a mismatch with your speakers and the 300B amp. If your speakers are hi efficiency they should sound best with the 300B amp. The st70 and AES super are both PP amps that can use el34 which may make an interesting shoot out but i believe the AES is hard wired vs. the circuit board on the st70. Also 6sn7 front end is a plus as it is widely known that the 6sn7 is one of the most linear tubes ever made.

 

[Destructor]

I suspected all along it won't beat my Superamp or 300B amps. I've been itching for another build and wanted something better than my Series 2 ST70 In that power range. I also suspect My favorite will always be my 300B amp even with its mighty 8 watts. From my research I also am sure I won't be dissatisfied with my new updated ST70, especially since it will have cost less than 1K when I have it up and running. My speakers are Klipsch Forte's with rebuilt crossovers.

My Superamp and SE1 are both point to point. I'm currently using EL34's in my Superamp. My 300B amp uses Raytheon 6SL7's for drivers. I built both amps with oil coupling caps.

 

[Destructor]

Update, my ST70 went up in smoke. The choke burned up while taking voltage readings. My other two builds turned out nice and are used regularly. Out of sight out of mind. Everything has been cleared off my bench, the amp put in a box and it now sits under the eaves.

 

[eickmewg]

Talk to Bob about your problem. Seems a shame not to resolve the problem.

 

[KlipschFan61]

Ouch.

 

[kward]

Update, my ST70 went up in smoke.

Okay. why not fix it? I'd start by debugging the power supply independently of the audio circuit, and you really don't even need the choke for this level of investigation. Curious why the fuse didn't blow long before burning the choke out. That's the detective work I'd start with.

 

[Destructor]

When the choke began to smoke, I shut it off. Bias climbed well above .4 volts with the pots turned down. The high voltage readings were well below spec. I didn't have a chance to write them down before spotting smoke. Switched all tubes around etc. Everytime I turned back on, same problem. No wide area red plating, required lights off at night to see red plating only at welds on all 4 EL34's. Have't found a wiring error and no funny smells or discolored resistors, no rectifier arching or blown fuse, tried 2 new rectifiers.

 

[Shadowdog]

Start by checking voltages in the PS with all the tubes out! Choke could have shorted out internally for sure so maybe unhook it and put as close to a 50 ohm resistor or combination of some (higher wattage types) in place and you can buy another from Dynakit parts for not too much $ (under $25) or maybe it's still under warranty.

The choke shorting out could be bringing the spec'd High Voltages down a lot. Maybe the fact that the fuse didn't blow is because the short isn't total yet.

 

[kward]

Right, take the tubes out and power it up. Check the voltage on pin 5 of the output tubes. That voltage should be negative with respect to circuit ground. I don't have one of these amps so I don't know exactly what voltage you should be reading but I'd guess about -20V or so. If it's higher voltage than that (meaning less negative, like -10V or -5V or even a positive voltage reading), that would certainly explain why the tubes are starting to red plate. Also the B+ voltage will read high without tubes in sockets, but that's okay at this point.

 

[Destructor]

Will do, haven't checked with tubes out yet. I'll have to unpack the amp. I checked all connections, I also checked every solder joint as I assembled the amp, nice shiny smooth joints.

 

[nj pheonix]

I'd bet more than one of us did a build, repair, restore, or mod, quite confident everything was exactly right only to find out otherwise. I would take the assembly manual on visually do the assembly step by step. You might surprise yourself .
Otherwise I'm on board with the troubleshooting path suggested. That amp while a little snug is pretty simple.