Toroidal PT (and new driver board?) to quiet a Stereo 70

[bsmif]

I have been bedevilled by a buzz in my Stereo 70's (I run two with Biamped speakers). One you can nly hear close to the speaker but one power transformer was buzzing so much you could feel it ( that is until it got so hot you couldn't touch it w/o leaving patches of skin stuck to it) despite tightening the screws holding it together. A late night trolling on eBay I ran across a pair of Toroidal transformers someone had made in HK that perfectly fit a 70 (with 360v 6.3 volt and 5 volt taps) and grabbed them.
Here are a few pictures of the patient and her still running sister along with the new transformer in someone elses amp.
http://www.flickr.com/photos/bsmif/sets ... 525174753/
I am leaning toward mounting them horizontally.
if I do that do I need to electrically seperate the bolt from both the chassis AND the plate on top?
Can I mount the SDS Cap board directly underneath it? The clearance next to the speakers on that side is tight.
Now I am trying to decide what else to do while I have the patient open.


IS there a way to ground the amp? I added grounded plugs but disconnected the grounds to quiet grounbd loops.
They have a pair of poorly soldered ( by me some years back) Sound Values boards in them (6gh8a Tubes) that I am thinking of upgrading to Shannon or tthe VTA or the SDS board. Will this be a big change?
Could the chokes be parts of the buzzing?
Is there a way to save old transformers or test them to see what the problem is? Im a little attached to these as my late father built one of them and its been running ever since.


Thanks for your collective expertise

Ben

[kheper]

'one power transformer was buzzing so
much you could feel it'

Sounds like the amp is oscillating.
Are the output transformers rocket-
hot as well?

'IS there a way to ground the amp?'

Connecting all ground wires to a star
ground eyelet is the best solution.

'I added grounded plugs but disconnected
the grounds to quiet grounbd loops.'

When I powered up my newly upgraded
st70, the connection to the star ground
was dodgy and the load resistors
literally smoked.

The buzzing you hear and the heat you
feel may be a ground problem.

Is there any ground from the driver
board to any part of the chassis? If no,
I suspect that is the problem.

[kheper]

You could test to see if the current
flowing thru the output transformers
is too high, by getting 2 10 W 8 ohm
power resistors. Screw one lead to
'C' and one to '8'. The resistors should
NOT become hot.

The lead attached to the '8' terminal
of the power resistor arced to ground,
when I first powered my amp up.

[bsmif]

I can try the power resistor trick on the one thats still hooked up but that one is still relatively cool and quiet compared to the one whose PT I already pulled. But occasionally I hear crackling noises coming thru that speaker during quiet passages.

THE driver board is bolted to the chassis but the circuit doesnt contact it in any way. What would you suggest connecting to the chassis?

[kheper]

I do not know which driver board you
have. Is there a ground hole on the
board? A star ground hole? Connect
the board's ground hole to the eyelet
bolted to the chassis.

[mesherm]

The power transformer should not be getting any hotter than the one on the other amp if it is an 060 tranny. That and the fact that it is buzzing probably means there is an almost certainly a short somewhere in either the transformer itself, the filament circuit or the B+. Usually if the B+ has a short AFTER the 5AR4 rectifier then either the fuse or the 5AR4 blows. Same with the filament circuit, the fuse will usually blow.
Since you have already pulled the transformer no way to narrow it down by removing any tubes. If you feel confident that you will not electrocute yourself or burn the house down, hook up a line cord to the old tranny primary, tape over all the filament and HV lead ends and plug it in. It should not buzz or get hot. If it does the tranny is indeed a goner.
As to your new toroid tranny.
It will not hurt to mount the SDS board under the toroid as long as nothing touches. I would suggest a nice piece of 1/8 inch aluminum or steel plate covering the large hole in the chassis to mount the toroid on.
The top of the mounting bolt should not touch the chassis making a loop or it is like a giant single turn of conductor with very small voltage but LOTS of current and can rob the toroid of power.
I would suggest using a current inrush limiter since toroids have lower initial impedance than EI transformers. I usually use two CL-90s in parallel (because I have bunch extra) or maybe a different CL-xx.
BTW I have mounted toroids vertically on a piece of aluminum angle without a problem. Toroids radiate less magnetic field than EIs.
I would suggest rewiring the amp or at least dressing the red wires a bit neater also paying carefull attention to proper grounding. The filament leads should be as short as possible, twisted well, and dressed as close to the chassis and away from other signal wires.
The driver board should also have a wire (just like the original) connecting the driver board ground to the star gound point near the filter cap.

[jeffdavison]

Mi Gusto Mucho toroids! I liked them so much I used two in my dual mono
'70. Got them from Toroid Corp in Maryland.
Using two capacitor boards as well mounted under the output transformers, The C.Chong ones.

http://s130.photobucket.com/albums/p280 ... rent=7.jpg
http://s130.photobucket.com/albums/p280 ... rent=4.jpg

They work real nice, no heat and no buzz. They have all the zotz one could want. I'm using paralled CL90 in rush limiters becuase these things suck wall current extremely fast, so not to blow fuses they're installed along with a slo-blo 4 amp fuse.

With the Cl90's and tube rectification, turn on is very easy on the power tubes.

Jeff

[bsmif]

Yow! THat double toroid rig makes my head spin. I am amazed you managed to squeeze that stuff in there. Nice work. Since I already have two amps I'll keep it to one per.

I thought the tube rectifiation would slow the current rush enough but will add CL90's to my shopping list. WHere does it go in the circuit?

How did you isolate the transformers from the chassis? My ebay toroids didn't come with mounting hardware so I have been trying to fabricate something from the hardware section to hold it without making metal to metal contact. Nylon washers and a rubber test plug seem to be the best bet.

There is alot of old wiring in their. I am tempted to rip it all out and do it all over, but I dont know if I have the time.

[mesherm]

One of the things that makes toroids better is that they use less copper wire for the same inductance which means the primary DC resistance is lower. This is the primary (115 volt side) which is separate from the high voltage (tube rectifier) side. Since the DC resistance of the primary is lower, the toroid draws more current in those initial milliseconds as the magnetic field builds. This happens with all transformers but especially with toroids. This is why you need the current inrush limiter.
In an ST70 I recommend you use one of those 3 lug phenolic terminal strips with the center mounted lug. Mount it to the screw nearest the center holding the choke. The lug that is mounted will not be used and can be cut flush or bent over out of the way. Solder two of the CL90s in parallel to the two remaining lugs. Solder one of the transformer primary leads to one lug and connect the other lug with a short piece of INSULATED wire to the power switch. That way the current limiter is in SERIES with the transformer primary circuit. One CL90 is good for 2 amps with a cold resistance of 120 ohms so two in parallel will handle 4 amps with a starting resistance of 60 ohms. As the CL90s heat up under current load their resistance decreases until they are only a few ohms. And all the tubes are happy with a softer start.
Keep in mind that those CL90s will get hot so make sure they don't touch anything and have some space around them.
Jeff's amp is like an Chevy Aveo with a big block crate V8 under the hood. Yellow_Light_Colorz_PDT_12

[EWBrown]

The best way to mount a toroidal power trannie is to use the supplied bolt, plate and pads, and mount to the chassis. Since the ST70 has a large rectangular hole in the desired location , use a metal plate or strip to fill or cover part of the hole (leave enough space for the new trannie's leads to pass through). Just be sure to avoid creating a "shorted turn" situation with any additional brackets or trannie covers.


Grounding (and ground loops) can be a real bugaboo in these amps, the new cap board should have it's own "star" ground, as should the driver board. When all is connected and running, there should be only a single ground point to the chassis - and perhaps a safety ground, if a three-wire power cord is used.

Don't use any of those "convenient" little ground lugs surrounding the octal tube sockets. The mechanical connection to ground through the socket is iffy at best, and ground loops are a "demon" best avoided...


Phase swapping:

This one probably will not apply in your case, if the upgrade driver board was already in place before teh buzz/humming occured, but it is good general information:


Another thing to check on any upgraded ST70 is that you are not getting unwanted feedback because of swapped leads between the new driver board and the EL34s.

On some upgrade driver boards, the phase inverter curcuit is different (LTP instead of cathodyne) and the connections to the EL34 grids have to be swapped/crosswired. The "smoking" output load resistor appears to indicate that this could be happening. As the amp is delivering max output into the loads, the power supply is going to be "maxed out" and the power trannie will get hotter than usual. If there was a short, the fuse should blow (hopefully before something else in the power supply does).

If a speaker was connected in place of the load resistor, it would sound like a very loud fighorn, because the negative feedback became positive feedback, because of the swapped "phase" outputs.

A quick and dirty test is to simply disconnect the lead from the 16 (or 8) ohm output connection which goes back to the driver board's NFB connection point.

HTH

/ed B in NH

[Shannon Parks]

Hi Ben,

Just for clarity, does it otherwise bias up properly and play music OK?

I think I see 5 wires running to the star ground lug, and the SDS board ground going to the cap lug. You might want to switch the SDS ground to the wing lug instead. Could you document where all those ground wires are coming from? Like...

1) From Left OPT
2) From Right OPT
3) From Sound Values Ground
etc.

I would hold off from moving the toroid or getting a new driver until you slay this beast. It can be a bit tricky, but possible.