Triode st70 trouble.

[dmag]

When I talked to heyboer they said they were doing a run for st70’s although he didnt say for who, and i dodnt want to be rude and pry. Im leaning to testing or replacing outright all the caps and maybe other components, but had really hoped i would be able to find a less exterme solution.

 

[dmag]

And gadget, if i pull either ef86 the noise shuts down on that side. If i pull both ( with 12au7 in) no noise at all. Pull the 12au7 with the ef86’s in, no noise either.
Say for giggles i fire the thing up and monitor for a bad cap to heat up, would that be a dumb thing to do?

 

[zackthedog]

When I talked to heyboer they said they were doing a run for st70’s although he didnt say for who, and i dodnt want to be rude and pry. Im leaning to testing or replacing outright all the caps and maybe other components, but had really hoped i would be able to find a less exterme solution.

Ha, lucky you! Might have been months otherwise. I can guess who they might be for but it doesn't matter. Glad *someone* is still making them!

I know, replacing all the caps would be a pain, but it wouldn't cost much and it probably wouldn't be a bad idea following the meltdown.

 

[Mr Boochie]

Do those black electrolytic caps on the board to the right, have slightly bulging tops? If so, they may need replacing.

FYI... here's what 60Hz and 120Hz sine waves sound like. It's important to know which is affecting your unit --- because they can require two different methodologies to fix it. Big difference in 60 versus 120.

 

[dmag]

Yea lucky me.
So I pulled all the filter caps. Except the small pair i assume were for bias. Tested them on a Peak/Atlas Esr70 device. All were within reason for capacitance. All were good on ESR, or as the tester speaks, one ding and a couple 2 dings. Roughly between .02 and .25 ohms. Then checked them at voltage on a Heathkit C3 cap tester. And repeated on the Peak. So im relatively comfortable they are ok. I had checked the coupling cps earlier. Still may replace them. Can anyone give me a suggesting of a brand/series to use? I used to by panasonic but they dont seem to be making them anymore.

Gadget, I checked the PT for continuity to the chassis and no go. So i have it removed. The bell underside are raw with what seems like a thin coating, but still ohms out at 0. Same with the stack where the bells meet. So i figure there will be continuity between the parts. But the nylon sleeves on the screws prevent the screws from tying the bells and stack to the chassis. So Im going on your thought that the shielding isnt working, and put it back together without them, ala the dynacos of old. If that doesnt work, maybe ill use the thing as a downrigger trolling weight.

 

[dmag]

Mr Boochie. They arent swollen. The creases seem crimped in. The shrink cover on the one has been like that since day one, almost. I think it just got a little over shrink and cracked. Also the links posted were the same as i used determine it was a 60hz hum.
Btw back to gadgets thought, would a hand on the transformer affecting the hum work into your idea of the shielding not working right?

 

[Mr Boochie]

60Hz is usually from the 6.3VAC heater lines or rectifier AC 5VAC taps.

Main power supply line hum in usually 120Hz (poor filtration, post rectification).



_______

 

[Mr Boochie]

I uploaded a schematic, which is hopefully close to the one for your amp.

 

[gadget73]

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

they would be, but pulling them doesn't take the supply caps out of the equation. The plate supply for the EF86 is still filtering or not, it just has no current draw from the tubes. If the noise were coming in on the supply there it should still be humming since its still tied to the 12au7 and goes on down the line to the output tubes.

it would shift the bias on the 12au7 stage considerably though, so I suppose it could be that pulling the EF86 causes the 12au7 to go into saturation and stop passing signal.


how's this for a test. Ohm check from the outer ring of the input jacks to pin 3 on each EF86. If all that path is right, it should ring out around 740 ohms.

 

[gadget73]

Btw back to gadgets thought, would a hand on the transformer affecting the hum work into your idea of the shielding not working right?

does touching it change the hum? That does make me think it may be EM field radiation, touching it would provide something of a drain path. Honestly if it were this though I wouldn't expect the hum to be especially loud, and I would have figured that moving the feedback line away from it would have made some noticeable difference. Running right next to the transformer I can believe it picking up noise but if its not next to the trafo anymore I don't know.

the other EM possibility, and I'm stretching some here, is that its induced in the output transformer and coupling back in through the feedback line but I'd expect to still have at least some hum with the EF86 out.

 

[Mr Boochie]

does touching it change the hum? That does make me think it may be EM field radiation, touching it would provide something of a drain path. Honestly if it were this though I wouldn't expect the hum to be especially loud, and I would have figured that moving the feedback line away from it would have made some noticeable difference. Running right next to the transformer I can believe it picking up noise but if its not next to the trafo anymore I don't know.

the other EM possibility, and I'm stretching some here, is that its induced in the output transformer and coupling back in through the feedback line but I'd expect to still have at least some hum with the EF86 out.

If it truly is 60Hz hum, there is an issue with repaired and rewired units. If you aren't careful how you wired the output tube heater lines, hum cancelling effect of push-pull can become hum enhancing. It's important to have the same heater lines of the PT go to each of the same heater pins.

Some ppl use two different colored heater wires, to help keep the connections correct.

 

[dmag]

Yes, touching the transformer does change the hum.
Yesterday I pulled the transformer. Gadgets early idea that the shielding might not be working had merit. There was no continuity between it and the chassis. Ive removed the plastic sleeves and have good continuity to the chassis now. Ill probably reassemble the thing today. As for the heater wiring, Ill ring them out, but they had been fine before. No hum. What is odd though, is that the old transformer had no bell on the bottom.
Also, looking at pics from disassembly, the brown/yellow and green/yellow wires had been twisted. Not so on the replacement installation. Ill do that as well.
Ive been putting together an order for new filter caps as well. I posted a thread on the diy forum. Gadget gave my some insight how to choose. Only trouble is one size is not available for 6 months. 390uf/250v 22mmx25mm snap-in. The physical size cant change due to the board layout.
Mr Bochie,it looks like the cap board section of the schematic is different. It calls for (8) of the above listed caps in positions c3-8, where i have only two , in c3 and 4.
Ll see if there is a schematic in my original paperwork.

 

[nerdorama]

I've built an SDS cap board for the ST70 and a couple for Mk3's. It appears that Triode Electronics has upped the values from Sheldon Stikes original design which didn't reconmmended such large capacitance values. You can use other values and probably get them quicker and the amp will work just fine. They aren't magic numbers except maybe a bit for the first pair off the rectifier.

 

[Mr Boochie]

Yes, touching the transformer does change the hum.
Yesterday I pulled the transformer. Gadgets early idea that the shielding might not be working had merit. There was no continuity between it and the chassis. Ive removed the plastic sleeves and have good continuity to the chassis now. Ill probably reassemble the thing today. As for the heater wiring, Ill ring them out, but they had been fine before. No hum. What is odd though, is that the old transformer had no bell on the bottom.
Also, looking at pics from disassembly, the brown/yellow and green/yellow wires had been twisted. Not so on the replacement installation. Ill do that as well.
Ive been putting together an order for new filter caps as well. I posted a thread on the diy forum. Gadget gave my some insight how to choose. Only trouble is one size is not available for 6 months. 390uf/250v 22mmx25mm snap-in. The physical size cant change due to the board layout.
Mr Bochie,it looks like the cap board section of the schematic is different. It calls for (8) of the above listed caps in positions c3-8, where i have only two , in c3 and 4.
Ll see if there is a schematic in my original paperwork.

Weren't you saying that the noise is 60Hz, not 120Hz? 60Hz is un-rectified AC. Like from the primary side of the PT, secondary before the rectifier, or the heater line (usually the culprit --- in my experience). I've had poor connections to the pilot lamps or fuse inject 60Hz into the amp.

Filter capacitors for the power supply are for smoothing out 120Hz (rectified AC).

Anyhow, prolly good to replace the filter caps (I would do it, for sake of completeness). But, this may not eliminate the noise issue --- if it truly is 60Hz.

 

[gadget73]

I've built an SDS cap board for the ST70 and a couple for Mk3's. It appears that Triode Electronics has upped the values from Sheldon Stikes original design which didn't reconmmended such large capacitance values. You can use other values and probably get them quicker and the amp will work just fine. They aren't magic numbers except maybe a bit for the first pair off the rectifier.

well if big caps are good, huge caps are better right?

honestly it really doesn't need to have cap section values grossly in excess of the stock 4 section can capacitor. If these are in series, and it appears they are, the effective value is 1/2 of whatever you put in, so a pair of 100uf at 250v makes for 50uf at 500v. I'm just assuming its reasonably designed with resistors across the caps to divide out the voltages properly and all that.

 

[Mr Boochie]

well if big caps are good, huge caps are better right?

honestly it really doesn't need to have cap section values grossly in excess of the stock 4 section can capacitor. If these are in series, and it appears they are, the effective value is 1/2 of whatever you put in, so a pair of 100uf at 250v makes for 50uf at 500v. I'm just assuming its reasonably designed with resistors across the caps to divide out the voltages properly and all that.

From the schematic, looks like they put 330K voltage leveling caps across the series pairs. So, all should be fine.

 

[dmag]

I put it back together. Admittedly wire dress was nothing to be proud of. No change.
I was going to attach a audio file buc cant figure out how. But it does sound like the 60hz file above.

 

[Mr Boochie]

I put it back together. Admittedly wire dress was nothing to be proud of. No change.
I was going to attach a audio file buc cant figure out how. But it does sound like the 60hz file above.

Hopefully, something in putting it back together solves the 60Hz issue. These can be crazy things to find and fix.

Just outta curiosity, you or some tech, didn't rewire the heater lines? Thanks!

 

[dmag]

There is no change.
And no, i didnt rewire the heaters. I simply installed a new transformer.
It had no issues before the old one died. Ive swapped out all the tubes and checked everything i can think of.

 

[Mr Boochie]

So, 60Hz?

The new transformer has center taps for the heater lines? They are grounded?

Volume control changes hum volume or not?