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Triode st70 trouble.

dmag

dmag

AK Subscriber
Subscriber
My first ever tube build was a Triode Electronics ST70 kit. It went pretty well for a newbee. That was several years ago. Never had a problem with it. Until about a year ago. I fired it up and it died. As best I can recall it blew fuses , and arced the rectifier. I replaced the rectifier with a diy ss rectifier, which i had used successfully on that and other st70's. That overheated and smoked (literally) the power transformer. Im guessing i simply had a catastrophic transformer death. Dunno who made their transformers then or now. But as I recall I had a similar one crap out some years back on another amp.
It sat since then. I just got a replacement transformer from Hayboer. On the stock transformer wiring came out of both sides, on the new from only one side. Similar to the new replacements from Dynakitparts. It meant the wire run would be a bit different. No big deal, I hope.
So it is now done, biases up ok, most voltages are not insane, except on the 12au7. (It runs 2- ef86 and 1-12au7.) Pins 4 and 5 show 3.2vdc and pin 9 shows 3.4vdc. Triodes chart shows them at N/A, However a 12au7 pinout shows pins 4 and 5 as heaters and 9 as the tap.
There is also a wicked 60hz hum that first got my attention. i dont know if it is related to the 12au7 voltages, or if the different wire dress is causing that. I have checked the coupling caps on a heathkit cap checker at 450 volts. They show ok. Transformer wiring will be double checked.
I have no idea where to begin trooubleshooting this. Triode is known for less than stellar tech support, so I dont expect any help there.
Image 6032 are the voltages. 603 the original transformer wiring per their instructions back in the day, 8 or ten years ago, 604 wired with the new style transformer, and 606 with Triodes new instructions. Rev2 2011. So I was using Rev 1, even older instructions.
 

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those will read about half heater voltage to chassis ground. Measure from 4 to 9 and it should read about 6.3 vac.

the DC voltage on those pins really doesn't matter.

do you have the input jacks shorted? Not too uncommon to get hum with open inputs on tube gear.

If they are and it still hums, pull the EF86 tubes and see if it stops.
 
Gadget, The inputs are shorted. Pullung either the EF86's or the 12 au7 quiets the hum. Ive rewet the chassis ground, found and rewet poor solder joint on one of the el34's as well.
 
Sorry for the confusion. The hum goes away completely when the EF86's or the 12au7 is removed. Ive done all the tube swapping etc to see if it could be the tubes. And the hum isnt a light hum. It is pretty darn loud. Not something that could be covered with a music source.
On a similar situation with a Mac240 i was able to find a bad filter cap by jumping a good cap across each on the Mac to isolate the bad. Im wondering if I could try the same on this, if it is an issue on the cap board. But id have to flip it upside down.
 
It's possible it's something physical with the new PT. The low hanging fruit is to double check the bolts and nuts that fasten it in place....can't hurt to check the OPTs too.
 
Ive checked the hardware. Especially the ground, which is attached to one of the PT bolts. Why would you say the OPT's , and what check would you suggest?
 
and just to beat the dead horse a smidge more, if you remove just one EF86, does it shut up only one channel or both?

final un-organized question, is the new transformer the paper bottom or the black metal bottom one?
 
Whichever ef86 is removed, that channel becomes quiet. The other remains noisy.
 
dmag said:
Ive checked the hardware. Especially the ground, which is attached to one of the PT bolts. Why would you say the OPT's , and what check would you suggest?
Click to expand...

Just that the hardware is all snug, which it sounds like you've done.
 
Are the RCA jacks isolated from the chassis? A couple of hires pictures of the wiring from different angles might be helpful.
John
 
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dmag said:
Whichever ef86 is removed, that channel becomes quiet. The other remains noisy.
Click to expand...

ok. Gut feeling is that its either something at the input wiring between the jack and the board, or *maybe* coming in through the feedback line. The feedback wire is the one from the 16 ohm transformer connection back up to the board. If thats run somewhere that it can pick up a lot of hum it dumps into the amp's input and will hum. Not completely sure where that wire is, maybe the yellow one that comes up next to each bias pot? If so its running right next to the transformer and a bunch of wiring. Maybe as a quick and dirty test, try moving it away from the other wires and see if anything changes.

another random thought, does the new transformer have a shield wire that needs to be grounded? If not, is the transformer itself making good electrical contact with the chassis? Might be worth an ohm check. The transformer needs to be grounded in order for the shielding to work correctly.
 
The RCA jacks are isolated. I can ohm them anyway. The transformer is bolted to the chassis with LK nuts that have an integrated star lockwasher. So I think Im good there. I dont know about a shield ground. But the screws have those nylon sleeves that center the screw in the holes in the core stack. ill do an ohm check there. And check the feedback wire too. Ill loosen everything up and run a different feedback. . That will keep me busy for the day. Thanks for the ideas.
 
If I had a catastrophic rectifier and transformer failure, one thing I'd be sure to check is the power supply electrolytics. If the Heyboer is supposed to be an exact match for the failed transformer, then your lower AC tap and supply voltages would seem to indicate excess current draw somewhere, and a bad decoupling cap could make the amp very noisy. Are you sure it's 60Hz hum and not 120Hz with harmonics?
 
a shield lead would be a single wire that comes out that isn't part of any winding. Some transformers have it, most do not. If there ended up being a single "extra wire" though, that would probably be it.
 
The jack wiring has been unchanged. There was no hum before. But the jacks are isolated. There is no extra wire in the transformer. No continuity from the bells or core to the chassis. Ive jumped both to the chassis with no effect. I moved the feedback clear of all wiring, and moved the rectifier wiring as well so everything os separated as well as possible. I lifted the cap board. To further separate things. Nothing helps.
I assume the hum is 60 hz. I listened to tones online to make that assumption. I wouldn't know if it is 120 with harmonics. I don't know what that sounds like.
 
dmag said:
The jack wiring has been unchanged. There was no hum before. But the jacks are isolated. There is no extra wire in the transformer. No continuity from the bells or core to the chassis. Ive jumped both to the chassis with no effect. I moved the feedback clear of all wiring, and moved the rectifier wiring as well so everything os separated as well as possible. I lifted the cap board. To further separate things. Nothing helps.
I assume the hum is 60 hz. I listened to tones online to make that assumption. I wouldn't know if it is 120 with harmonics. I don't know what that sounds like.
Click to expand...

Find an online tone generator on your computer and listen to the difference between 60Hz and 120Hz.

Did Heyboer build you an exact replica, in terms of secondary voltages? If they did, something is amiss in the power supply, because the HV secondary voltages are getting pulled down quite a bit at the rectifier. Again, a rectifier arc-over followed by a smoked power tranny followed by loud-ish hum and low voltages with the new tranny all says bad PS cap to me, but I could be wrong. If the hum is 120Hz that's an indication that the PS has problems. It's a pain but you might want to test the PS caps, or just replaced them if you have no way to adequately check them.
 
its also just possible the new transformer is lower voltage output. I worked on an ST-70 with a reproduction trafo from an unknown source and it's voltages were just like OP's. Everything was fine in it, can cap was brand new.

all of the voltages look proportionally right, if there was any significant extra load it seems like it would have to be the first cap off the rectifier. I'd expect it to be getting noticeably hot if that were the case.

also that doesn't really explain why it shuts up when an EF86 is pulled only on that side.
 
gadget73 said:
its also just possible the new transformer is lower voltage output. I worked on an ST-70 with a reproduction trafo from an unknown source and it's voltages were just like OP's. Everything was fine in it, can cap was brand new.

all of the voltages look proportionally right, if there was any significant extra load it seems like it would have to be the first cap off the rectifier. I'd expect it to be getting noticeably hot if that were the case.

also that doesn't really explain why it shuts up when an EF86 is pulled only on that side.
Click to expand...

You could be right, gadget. Though the EF86 would be especially sensitive to PS noise if a filter cap was bad, would it not?
 
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