Sudden SSE Capacitor Failure

[Zap]

I don't recall the bias drifting on mine, or the tubes going into runaway, but I doubt I ran them at more than 35 watts or so. I haven't used them in years. I don't know if there is a datasheet for the "new" TS 6550, but the real TS 6550 specs a max grid resistor of 250K for cathode bias, and the SSE is stock at 220K. You could try lowering your grid resistor value some to see if it stops the run away, or improves the tube performance, if you are inclined to tinkering with things.

I don't know anything about Tube Depots bias point ratings. Without knowing how they get this data point, it seems rather meaningless.

Win W5JAG

Win,

Thanks for the info - it looks like my new tube runs right around 32W so it should be OK. I spoke to the seller and he offered to replace the other tube since it is so far out of range. That should put me back where I need to be, and maybe there was a bad run of them.

To be on the safe side, I may try to install a few voltmeters to keep tabs on the bias voltage. Once I have a matched pair, then I'll do some burn-in testing and make sure that the amp is stable over a longer period.

Interestingly enough, the Genalex data sheet for the KT-88 states a maximum 220k grid resistor, so it ends up being lower than the 6550, though I don't recall having as much problem with KT-88's. I might have to look into this at some point depending on what I see.

I decided to also look at other data sheets. The KT-90 can tolerate a bit more dissipation (50W), KT-120 (60W) more yet, and KT-150 (70W) even more. As far as I can see, these larger tubes wouldn't exceed the filament tap's ratings. The bigger concern would be whether the HV current rating is enough. I suppose depending on the current draw, the bias components would have to be beefed up a bit, too.

 

[Tubelab_com]

Another option when the tubes run too hot and you aren't trying to squeeze every last milliwatt out of them is to swap the 5AR4 out for a 5U4 type. Make sure your 5 volt winding is good for 3 amps first though. It will drop the B+ by 20 to 30 volts over a 5AR4.

 

[Zap]

Another option when the tubes run too hot and you aren't trying to squeeze every last milliwatt out of them is to swap the 5AR4 out for a 5U4 type. Make sure your 5 volt winding is good for 3 amps first though. It will drop the B+ by 20 to 30 volts over a 5AR4.

George,

Thanks for the tip - my Edcor transformer's 5V tap is rated right at 3A. So does that leave enough room or will it hum/run hot if I start to push it? I'm not sure whether I've got one of those in my tube collection, but I'll take a look. It currently has some hum with the rectifier as it is. If I recall, when my prior board used the FRED's it made less noise than when using the 5AR4.

Would using a tube with higher ratings give a bit more headroom as well? I did some reading in the forum history and it looks like the voltage I'm sitting at would be decent for one of the larger TS KT tubes. I'd have similar or better power output along without pushing the tubes so hard.

Do you think the parameters here are too much for the 6550's or maybe just a bad set of tubes?

 

[Zap]

I'm still waiting for my 2nd replacement 6550. In the meantime I've been looking at the prospect of running larger tubes, and my understanding is that I'd need to beef up the bias resistors and capacitors.

It looks like 10W resistors are offered in large ceramic or aluminum housings. Is there any downside to using one over the other? It looks like I'd need to make some modifications either way because neither mounts cleanly to the PCB. In theory I could mount the aluminum ones to the amp's top plate and wire them in.

The 100V capacitors appear to mostly be snap-in type, so that might take a little effort as well.

I'd be curious to hear from anyone who has beefed up the biasing circuit as to what they did to make everything work out.

More importantly, if I beef up this circuit, is there any danger in using these components with "regular" tubes? Will they let the amp run away as opposed to how the 63V caps I have now have failed?

Thanks!

 

[w5jag]

You could parallel two (2) five watt resistors per channel, to double the dissipation in about the same space.

I have some 100 volt leaded 1500 uF caps; I probably got them from Mouser. I used them with sweep tubes that push the cathode voltage pretty high.

What beefier tubes are you going to use? The cathode voltage shouldn't be much, if any, higher, than normal tubes but you will be running more current.

Win W5JAG

 

[Zap]

What beefier tubes are you going to use? The cathode voltage shouldn't be much, if any, higher, than normal tubes but you will be running more current.

Win,

Thanks for the information - I'll have to order resistors one way or another, so I'll have to see what makes the most sense.

I'll also have to take a look at Mouser for the capacitors. I've blown up a few of the 63V versions when tubes age and the voltage drifts up or a tube starts to have a little red glow. I didn't know if putting a 100V cap in there will prevent explosion/mess, or if that somehow could allow for other damage if a tube runs away?

Given some of the reviews here on this forum, I was considering the KT120 or KT150 tubes. According to the data sheets they have more leeway before they hit max values, so maybe I won't see as early of a failure as with the 6550's. Plus, given my amp's higher B+ due to the Edcor Power Transformer, they should bias decently for a slight power bump and more headroom. From what I've read I should have enough capacity to run those tubes, possibly aside from the biasing components.

There was at least one post where someone dropped in KT150's in an existing amp. However, if the currents increase much, I suspect I will probably exceed at least the resistor's wattage rating, so I just want to ensure I don't blow anything else up!

 

[Zap]

Figured I'd post an update. I powered the amp to observe it at idle for a little while (it does run correctly) and after about 15 minutes, the new set of 6550 tubes show the faintest amount of red on the plate seams.

I did some digging around the web and the conclusion I came to is that the TS 6550 tubes just don't like being run this hot, in spite of whatever the published data sheets claim. Some of the commentary honed in on a bias current of about 75 mA being more suitable for their lifespan and happiness.

I've gone through a few sets of 6550s (one used, one new) in less time than my original KT88EH tubes that I started with when I built the amp, and with the commentary about the getters fading, the higher B+ in my amp due to the Edcor PT, and a really good deal on some new KT120's, I have decided to give them a shot instead. Looking at their operating point, they should bias up nicely with the existing resistors and not be pushed as hard based on their data sheet.

I thought about going back to the KT88EH's as they are cheaper, had a good sound, and also tolerated the current bias point well. I'd have to say that they lasted quite a while though I'd have to dig to find out when I first bought a set of 6550's (I actually still have one tube that is good as only one failed). If I ran them with a larger resistor, I suspect their lifespan would improve even more.

Given my issues with the PSVANE KT88-TII tubes I also tried, I am not sure whether they are simply unreliable or also suffer from the same new-production weaknesses and/or exaggerated ratings. I had one tube from each set I got fail within a fairly short period of time, so they sadly make the 6550's look reliable.

So time will tell how the KT120's behave and sound! I'll try to post some info/commentary when they arrive.

I also purchased a few LED voltmeters off the *Bay for a few bucks each. I may add a switch to turn them on/off, but this way I can easily monitor the bias voltage for excessive drifting as the tubes age.