SP14 Power Tranny Issues

Hi guys,

I had a troubling experience this evening.

Maybe 10 minutes after powering on my system, I detected a burning plastic smell. After a minute or 2 of sniffing, I traced the smell to my SP14 preamp just as a I heard a pop and saw a puff of smoke coming out of it.
I powered everything down and popped the hood expecting to to find a black smudge around something on the pcb.

What I found was the plastic insulation around one of my power trannys is melted and there is lovely burnt smell emanating from it.

This is the small 6.3v antec tranny supplied to power the rectifier.

Nothing else appears out of place. No black smudges anywhere on the pcb.

Before I replace the tranny I feel the need to ask what may have caused this?

The current tube compliment is 4x OTK 6N8S and 1x OTK 6C5S rectifier.

The only other detail of note is that the rectifier tube is not fully depressed into the socket, being maybe 3mm out of the socket.

Surprised the fuse didn't blow - check its value.

Sadface wrote:Hi guys,

I had a troubling experience this evening.

Maybe 10 minutes after powering on my system, I detected a burning plastic smell. After a minute or 2 of sniffing, I traced the smell to my SP14 preamp just as a I heard a pop and saw a puff of smoke coming out of it.
I powered everything down and popped the hood expecting to to find a black smudge around something on the pcb.

What I found was the plastic insulation around one of my power trannys is melted and there is lovely burnt smell emanating from it.

This is the small 6.3v antec tranny supplied to power the rectifier.

Nothing else appears out of place. No black smudges anywhere on the pcb.

Before I replace the tranny I feel the need to ask what may have caused this?

The current tube compliment is 4x OTK 6N8S and 1x OTK 6C5S rectifier.

The only other detail of note is that the rectifier tube is not fully depressed into the socket, being maybe 3mm out of the socket.

Raw Opinion Here:

It would be quite unusual for a transformer to fail catastrophically without some warning. Not impossible, but unusual. And, whereas I am not a fan of toroid transformers in audio equipment, I am not Agin' 'em, either. So, before replacing the transformer, please check the following:

a) The rectifier tube - is it slagged? Are there any shorts? If you do not have a tube tester, obtain a base diagram for all your tubes and check them with a VOM on OHMS. Only the filament should show any continuity, or any pins that are tied internally. DO THIS CAREFULLY - you are out to protect the next transformer.

b) Look for a pinched wire somewhere. Such a defect may take weeks, months or even years to develop.

c) Do, as previously mentioned, check the fuse value. Even a tube pre-amp should draw only a tiny amount of current relative to a power-amp. And my 11-tube Dynaco FM3 tuner has only a 1A dual-element fuse in it. So, a fraction of an amp should be adequate. NOTE: while I am on fuses - Slow-Blow (wire-wound) fuses will permit massive over-currents for very long periods of time - easily long enough to melt a filament transformer.

d) Check for any other slagged components in the filament path. Check for shorts as well.

Good luck with it. Please let us know of any findings *PRIOR* to making the replacement. We do not want this to happen again.

hey Sadface,
I'd just like to confirm which transformer you have in your SP14, because I don't supply a small 6vac transformer.
The SP14 is supplied with a large 240-280vac/6vac combo and a medium sized 2x 7vac transformer.

tubes4hifi wrote:hey Sadface,
I'd just like to confirm which transformer you have in your SP14, because I don't supply a small 6vac transformer.
The SP14 is supplied with a large 240-280vac/6vac combo and a medium sized 2x 7vac transformer.

Hi Roy,

It is Duncan here from New Zealand. I got my sp14 kit from you back in April 2013.

What I received was:
1x triad VPT18-1390 2x 9vac @25VA
1x triad VTP230--100 2x 115VA @ 25VA
1x Antek AN-3106 (I think, the writing is a bit warped). 2x 6VAC, it looks like 2A.

I assumed the differing power transformers from the build guide was due to the 230V mains.

Problem 1) The fuse is a 1A slow blow. I'm not sure where I got this value from but it clearly won't do.
I will change the layout to individual 150ma quick blow fuses for each transformer.

Problem 2) I haven't found yet. I'll examine the tubes tomorrow and see if I can't find the fault there.

Sadface wrote:

Problem 1) The fuse is a 1A slow blow. I'm not sure where I got this value from but it clearly won't do.
I will change the layout to individual 150ma quick blow fuses for each transformer.

Problem 2) I haven't found yet. I'll examine the tubes tomorrow and see if I can't find the fault there.

Quick-Blow fuses are typically not a good solution in tube equipment unless that transformer has no surge at all at turn-on. What does both serve the purpose, protects the equipment and does not subject you to nuisance-failures would be a DUAL-ELEMENT fuse, also known as a type MDQ.

https://www.eaton.com/content/dam/eaton/products/electronic-components/resources/data-sheet/eaton-mdq-time-delay-glas-fuse-data-sheet.pdf

They come in fractional-amp ratings.

yes, make sure to use SLOW_BLOW fuses, not standard fuses. The MDL rated fuse is a medium delay slow-blow.
Do not use fast blow fuses, at power on there will always be a very brief current surge.
Yes, you could also fuse each transformer, based on it's size. Those are all 25VA, so 1/4 amp MDL fuse on each should work fine.

Peter W. wrote:

Sadface wrote:

Problem 1) The fuse is a 1A slow blow. I'm not sure where I got this value from but it clearly won't do.
I will change the layout to individual 150ma quick blow fuses for each transformer.

Problem 2) I haven't found yet. I'll examine the tubes tomorrow and see if I can't find the fault there.

Quick-Blow fuses are typically not a good solution in tube equipment unless that transformer has no surge at all at turn-on. What does both serve the purpose, protects the equipment and does not subject you to nuisance-failures would be a DUAL-ELEMENT fuse, also known as a type MDQ.

https://www.eaton.com/content/dam/eaton/products/electronic-components/resources/data-sheet/eaton-mdq-time-delay-glas-fuse-data-sheet.pdf  

They come in fractional-amp ratings.

Peter, have you a URL to the seller of these?

Digikey
Mouser
Element 14
Grainger


https://www.mouser.com/Eaton/Cartridge-Fuses/MDQ-Series/_/N-1yv49r6Zba8h0Z1yzxjht

Hi Guys,

I'm yet to find a fault so far.

I've gone over all the tubes with a multi meter and I am only getting continuity over heater pins:
7-8 on the 6N8S tubes and 2-7 on the 6C5S.

I've given everything on top of the board a thorough eyeball and nothing looks out of place.

I've been putting off looking under the board.
I'm hypothesizing one of 2 scenarios: something wrong with b+ wires or a compromised joint somewhere.

There is one other detail of note.
I had been having issues with squealing recently. As far as I understand this is oscillation and not good.

The squealing only began after changing from some new production tungsol 6SN7 and 6X5 to the OTK 6N8S and 6C5S.

It seemed to be bound to the 6N8S as it moved with tubes.
I purchased 8 of them.
1 was mircophonic
1 was simply hissy
2 remain untried
The 4 tubes that were in there generally behaved apart from an occasional squeal during warmup.

The tung-sols were changed because they were beginning to hiss.

My mind had boxed this as a separate and unrelated issue but I am beginning to suspect it may have a role to play.

When the overheating tranny event occured, the right channel had squealed maybe 1 minute after startup. Otherwise the amp didn't sound off or odd at all while it was overheating (some 5-10minutes later).

I do have a question regarding fuse rating.

How much headroom is required to account for in rush current?

Assuming I was fusing the power trannys individually:
25va @230v = 108ma draw

Would a 150ma fuse be sufficient or would a larger rating be required?

seems your problem may have begun when you started using the cheap russian tubes.
You might try using the correct 6SN7 and 6X5 tubes

Sadface wrote:I do have a question regarding fuse rating.

How much headroom is required to account for in rush current?

Assuming I was fusing the power trannys individually:
25va @230v = 108ma draw

Would a 150ma fuse be sufficient or would a larger rating be required?

The very short answer is YES. You could get away with 140 ma, but I believe that is not a common value.

Generally, the rule-of-thumb for resistance (filament) loads is 1.3 x measured steady-state draw. For motor-loads, that goes to 1.5.

tubes4hifi wrote:seems your problem may have begun when you started using the cheap russian tubes.
You might try using the correct 6SN7 and 6X5 tubes

Roy is right on this one, that I can attest. Using "post-blight" junk tubes in any VTA/Tubes4 hifi/Dynaco variant does a grave injustice to these fine circuits. Even legendary cheapskate David Hafler didn't skimp on tubes in his product line. You'll see plenty of old Dynaco labeling on Tung-Sol 6550s. They are and will remain tied for first with the British KT-88s.
Good 6SN7s are thick as as fleas. Try some old Tung-Sol, RCA, GE or Sylvania and the like in those holes. Anything less is like putting water in your Jaguar's gas tank.

I was under the impression that late soviet era tubes were of some of the highest quality?

Sadface wrote:I was under the impression that late soviet era tubes were of some of the highest quality?

Sadface, there are certainly exceptions, and old Sov tubes are among them. The Melz ladder-plate 6H8C (40s-50s-60s) substitutes nicely for any 6SN7 and will give them a run for their money, at least in the Chrome-dome league. The Sovs also made some good 6922 variants. Problem with Cold War-era tubes is poor quality control. Half the time the leads have become un-soldered from the pins, generating a sound like strafing on a flock of parrots. I'm speculating here that the bottle and innards themselves are lab-grade, but it was left to unskilled Zeks to do the non-critical finish work, i.e. soldering the pins with solder no doubt made from melted beer cans.
It's easy to undertake a pre-emptive fix, at least on the 6H8Cs. With heat and a solder pump, de-solder all eight leads from the pins and remove the base. Ream out any residual solder from the pins, then re-tin the leads with Cardas or equivalent solder and re-install the base with the pins and leads correctly aligned. Something that will make life measurably easier is to use a grease pen mark aligning the base with the glass before you take it apart.
The trouble is worth it. You've got a long-lived, sturdy tube with great high-end authority without sounding shrill.
Nothing wrong with some of the newer stuff. Some people swear by the new Chinese CV-181 6SN7 replacements. The Gold Lion KT-88 is well thought of. But only time will tell if you can sweat 10,000 hours out of the new stuff; it simply hasn't been in the field that long.

I'm with deepee on this (except for the part about taking tubes apart and re-soldering pins).

I have to assume NOS USA tubes are about as easy to source in NZ as Soviet tubes.

Go with some old GE or Sylvania NOS 6SN7. There are plentiful and since they are rarely used in guitar amps are fairly inexpensive. Ditto the 6X4.

Good luck.

I had experienced shorting issues in both my preamp and st-120 power amp when I brought them first on line (not to mention something dumb I did with the bias). I was in a clumsy phase at the time. I am in the habit of replacing fuse holders with expensive and properly rated circuit breakers since then. If it is rated for a 150ma slow blow, I usually use a 120ma breaker and have no problems. They pop in plenty of time and I don't have to wait for more to arrive when I run out of fuses. I know the purists push them away but I have done extensive bench testing and CBs work very well when you spec them correctly

G'day Guys,

I finally had the courage to pull the board out for inspection.
No smoking gun yet. The worst I could find was a corroded joint on the left channel heater decoupling cap beside V3.

Nothing funny looking in the B+ string anywhere. The only thing of potential note is that the wire runs I used for the B+ are longer than necessary.

My plan right now is to reflow all the joints. I plan to redo all my mains wiring also with individual fuses on each transformer.

I have a question regarding replacement transformers. How much difference does the 0.3V make between 6v and 6.3v for the 6x5 heater?

I am finding 6v transformers readily available and 6.3v transformers scarce.

actually almost ALL 6v transformers are spec'ed at 6.3vac, as that's a standard voltage, for tube usage, the nominal spec is 6.3v +/- 5% so anywhere from 6.0 to 6.6v is fine.
On the SP14 there is also a 1 ohm current limit resistor in series with the 6X5 filament so as to prevent a turn-on surge, and that 1 ohm resistor will drop about 1v anyway,
so the tube will last for many many years running at slightly low voltage.