Altec 250su any experiences

[bieckmusic]

thanks for the tip, don't have a variac but do have a light dimmer.  hopefully will keep the cap explosions down to a minimum.  ha

btw coilaudio your work is really amazing, I checked out your websites, holy cow!!!! 

 

[emrr]

This is a great time to buy your first variac.  Very minimal cost given what you are taking on. 

I've a lot of related parts, should you find you need anything.

 

[bieckmusic]

do you think something like this will be good enough??

I'd love to use it on guitar amps too, that's the new (old) cool thing in town!! ha

Greg

http://www.ebay.com/itm/like/251826120718?lpid=82&chn=ps

 

[bieckmusic]

variac on the way!!  thanks for the suggestion, I can see how this can be useful for a lot of things. 

If I want to mix on my 250su, and want to run DAW out +4 into my 458a modules couldn't I insert a passive direct box transformer to bring down the DAW level to mic level into the 458a?

 

[coilaudio]

Thanks for the kind words Greg, always preaching the tube gospel...

For bringing in line level you need a balanced line "U-pad".  2x 10k resistors on the sides with a 250ohm resistor bridging. Balanced line input goes to the two 10k resistors, and balanced output off the 250 ohm. This provides impedance matching and a 40db pad. This way the line input side sees around 22k and the mic pre sees 250ohm. Found this "tip" in an old RCA manual from the 50s. I built a bunch of these onto empty patch bay jacks so I can use all my pres as line amps when mixing. I've also built custom boxes for clients that have mic and line pads on a rotary switch.

or you can buy one in a shiny red box from DW Fearn for $160...

You can also skip the preamp and use your "utility trays", they're made for bridging a line input to the mix bus. Then it's just a 10k to 600ohm transformer and straight to the daven. We make an exact clone of those peerless 15095 transformers. It's a different cleaner sound going this route, but less noise than running through pres.

 

[bieckmusic]

Awesome, thanks again! 

I found this cool link about u-pads...  cool idea about putting them into a patch bay.  I'm going to probably do the same and bring up all the inputs and insert points and outputs on a patch bay for easy access...

http://www.uneeda-audio.com/pads/

 

[bieckmusic]

Well, I got my vari-ac hooked up.  And started real slowly adding power.  The console started smelling bad, so at around 35-40% power I pulled out the modules and one of  the electrolytic can casp was hot and the modules transformer was pretty warm.  I don't think audio transformers should ever be warm so I pulled that module.

I'm going to do some double checking of things before I try adding the power again...  Could be all the dust getting hot on the tubes, but I never saw a tube even glowing, don;t think I got enough power going...  Looks like the heater voltage only got up to around 3v.  I never got above 50%.  My ampmeter says the powersupply was around .3 amp on one rectifier and .2 amp on the other compared to ground.  the tube tester needle would go up about 25% on all the modules so electrons are flowing. 

 

[emrr]

Heat bad.  Warm transformer is a very bad sign.  Confirm DC resistance of output transformer primary before going further with that module. 

Dust bad, good friend of heat.  Really!!?  You didn't even clean it and you powered it up?!

YOU REALLY DON'T WANT TO TEST MORE THAN ONE MODULE AT A TIME.

LONG BEFORE YOU TRY TESTING THE WHOLE CONSOLE. 

REPAIR ONE AT A TIME, THEN POWER THE WHOLE THING AS IF IT STILL HASN'T BEEN TESTED. 

You can hardly see these glow if I recall correctly; anyway that should never be a prime indicator if you aren't familiar with the tube type. 

 

[shabtek]

a known good heater voltage (auxiliary power supply)
And ammeters inserted on power feeds to monitor current draw--before b+ is ever brought up would not be a bad practice

 

[bieckmusic]

well, thank you for the feedback.  It was definitely not a great idea to start powering everything up.  :-\ :-\

The vari-ac from ebay is not working correctly and creating the horrible smell with nothing plugged into it.  So I'm sending it back. 

Looking for a small bench rig to power up modules.  I have an Ian Bell HT Power Supply Design card.

Looking at the web site, with the correct transformer, that looks like it might be able to supply the + - 275 volts DC at 40ma?

The schematic for the 459a amplifier requires a 6.3V DC however the HT Power Supply provides 6.3V Ac?  Can the 459A heaters work off 6.3 V ac or dc?? 

thanks!! Greg

 

[emrr]

They are wired for DC specifically, so there's no care taken for quiet AC usage, but you can at least test with AC filaments.  Good luck. 

 

[bieckmusic]

Seems, I have two different types of 458a amplifier.  Some of them have a small pcb board mounted underneath the output transformer with 2 6.49 k resistors a few other allen bradley resistors an and a sprague cap on it.  Some of the amps are identical but missing this small circuit pcb board all together???  Seems like these parts are not on the alter 458a schematic.

Greg

 

[emrr]

That's a mod of some sort

 

[ruffrecords]

Looking for a small bench rig to power up modules.  I have an Ian Bell HT Power Supply Design card.

Looking at the web site, with the correct transformer, that looks like it might be able to supply the + - 275 volts DC at 40ma?

The schematic for the 459a amplifier requires a 6.3V DC however the HT Power Supply provides 6.3V Ac?  Can the 459A heaters work off 6.3 V ac or dc?? 

thanks!! Greg

My HT PCB can  certainly supply 275V dc at 40mA. With suitable resistor values it can supply up to 100mA if required.

The HT board does not supply any heater voltage. That you need to do separately, As others have said, use ac heaters for initial testing. Then ,when you get the original power supply working you can use that for the dc heaters. In fact the power supply is relatively simple so it should not be too hard to get going - you might need to simply replace all the electrolytics in it to have a working power supply.

Cheers

Ian

 

[coilaudio]

Heathkit makes great power supplies for bench top testing.  Check ebay, IP or PS series. They have regulated variable b+ up to 300v or 500v depending on model. They also have 6.3v and 12v for filaments. Plus they show current draw, which is great for trouble shooting.

Definitely replace all electrolytics, they were never intended to last this long. All the big caps in the power supply for sure. Mallory or cornell dubilier/CDE still make the big "computer Grade" cans. Might involve some rigging to get the replacement caps to fit in the same space... little glue, some mounting brackets...

Bring it up on its own/disconnected and make sure its all cool.  Before connecting it, check all the high voltage wiring in the frame and make sure all is well, no shorts, no "modifications", etc. Remove the bulbs from the meters or you'll see a short on the 6.3v.  Make sure and check across the entire back plane. The insulation here is usually dry/cracked. They used little pieces of electrical tape to cover the splice on the filament run...Had to redo all the wiring on one I restored.  Always important  to check anything that carries or distributes high voltage and current. You can then check the power supply attached to the frame and make sure the proper voltages are going only were they are supposed to on the rear module plugs...REMEMBER DEATH HANGS OUT AT THOSE PLUGS AND WITH THE POWER SUPPLY.

If the board and power supply are all good, you have a perfect test bed...

Recap all the modules, the large black 3 section is the only one in the 458. 459 also has two caps in the signal path that can be leaky. You can buy multi sections at antique electronics (pricey), or you can replace with three separate small ones underneath. You can leave and disconnect the big one on top. Definitely check all resistors  to make sure they're in tolerance. Check the modules one at a time, then slowly add them to the board to make sure all is well as you load the power supply. There are "insert sends" on the back plane they come directly off the module for checking em out.

I've definitely been  guilty of excitedly plugging some new vintage piece and firing it up immediately... But luck eventually wore out.

 

[bieckmusic]

thank you for the great post, I really love this forum!!

and I hear the high voltage warning, thanks for that

did you use old style wire to re-wire your board or modern mogami type??  My wiring harness look pretty good but I probably for safety's sake at least rewire the current carrying wiring...

thanks again!!!

 

[emrr]

Wire rated for the current demand. 

 

[coilaudio]

I believe it's 18awg rated for at least 300v (better if 600v) for the B+, and 12awg 300v for the filament. The filament run is mostly 12awg with small jumpers of around 18awg to each connector. The tube check run is 18awg/300-600v. Mogami/canare is typically for audio wiring. Wouldn't suggest rewiring the whole thing unless absolutely necessary.

 

[bieckmusic]

Ok, so I built a test station. 

I found a lot of old audio test equipment for sale.

I got a Hickok Tube Tester, this thing's awesome.  Anyone in Nashville need tubes tested, come on...  Will take Girl Scout Thin Mints in exchange for tube testing.  ha
And an oscilloscope, an old signal tracer with a magic eye tube, and an ohm commander. 

The heath Kit ip-17 I got does perfect with the dc current.  But the ac 6.3v is coming out 7.3v.  Looks like it's a tap right off the transformer with no regulator on it.  Will 7.3 damage tubes? I've read that exposing the tubes to  too high a voltage really lowers it's life.  I could wire the 6.3 volts right off the Hickok tube tester, maybe??  I tried a light dimmer on the 7.3 and it didn't do the trick...

best!
Greg

 

[emrr]

Use some series resistance to lower the voltage.  The current supplied is much higher, therefore it swings high.  Something like a 10W 0.5 ohm could be a good starting position, as a guess.