New tube testbed

[RWood]

I recently had the displeasure to have a new shorted 6l6 power tube almost destroy a good 50's NOS RCA rectifier. Or at least I think that's what happened.
A friend wanted me to build him a 5B6 for guitar and had said that he wanted 6l6WXT+ with the mica spacers/springs which had better harmonic dynamics than the 5881WXT. I was testing the difference between 6l6WXT+ and 5881WXT in the amp that I was going to give him. First I recorded with 5881s which had been in the amp for three weeks. Good tubes for bass.
Then I tried 6l6s which did seem to have more complex top end with guitar. Ten minutes into playing it began to cut in and out and squeel as one of the tubes began to arc badly Before I got it off the Rectifier started arcing also.
After taking the tubes out and replacing them with 5881s and another recto, everything was ok. I tried the NOS RCA again and it was ok also. I was quite glad as I only have two of them left. The power tube seemed good for a while but then the plate shorted.

This got me to thinking...
I think I'd like to build a simple circuit to test and maybe burn in tubes to try to catch the bad ones which might damage things in amp. What components besides PT, Rectifier, dropping resistors, filters, and biasing would I need short of entire circuit? Something with switched/variable power chains and loads maybe for testing different tubes?
This tube was ok for a while so I'm not sure if a tube tester would have caught it. A circuit that I could let idle for a day or so might catch such things. I have a pretty good stash of old parts, and my tinkering itch needs scratching.

I"m not after comparing tubes as the post I read earlier was discussing...Just something to identify bad ones and burn in new, as bad tubes are showing up more frequently these days.

[Stevem]

If you have the parts and time go for it, but just for the future if you place a fuse in the OTs B+ feed you can save the recto tubes from having excess current from being ripped out of it.
Just having a AC mains fuse takes too long to blow.
Some tubes are just not gonna fail until you get enough heat up and cool down sessions into them along with some vibration from being in a combo amp and or travels in the trunk of a car.

[johnnyreece]

Something like my Weber Maggie (which is basically a single power tube version of Dave Hunter's Two-Stroke) could fit the bill. It's mainly a 5f2 with a beefier OT. You could use SS rectification to keep from worrying about toasting a rectifier tube. For what it's worth, I use mine for that often with "suspect" tubes. With only one preamp and one power tube, it's pretty low-variable.

[Phil_S]

Similar sentiment here. Build a single ended amp -- Champ-ish circuit but with adjustable fixed bias. Use a multi tapped OT suitable for big bottle tubes and a PT that can deliver a range of secondary voltages (check Weber's WPTGP for $35 and WSE25 for $forty-eight [1]). Since it's a testbed amp, include noval and octal sockets to allow for EL84, 6V6/6L6, and EL34. With the multi OT and multi PT, you should be able to dial in a combination of OT impedance and plate voltage that works for the particular tube you want to test. I'm going to guess you can get this done for under $200. The iron will be the main cost. Bonus: you get an amp you might like.

[1] the numeral eight makes emoticon with sunglasses!

[armillary]

I can see building a test bed. But I wouldn't put anything on the test bed unless I ran it on the tube tester first.

[Phil_S]

So, how much will a tube tester cost and what is the risk of toasting that, dollar-wise? I'm asking because I don't know, not to be a smart-ass. Risk assessment is worth doing and that leads to a strategy.

[armillary]

Hickok testers are probably the best and you pay accordingly.

But good results can be had from the lesser brands for less money. I have a Sencore TC-28 that seems to reliably sort the good tubes from the bad ones and cost about $130. on ebay.

I agree though with tubes that they can be good one minute and go bad the next.

[RWood]

Thanks guys. I have 80w iron and was thinking of champ type circuit with ss rectifier and dummy load on ot with multiple switched socket options.

I know this is overkill but as I stated earlier, I have a tinkering itch that needs scratching, and have lots of scrap parts.

As for the B+ fuse, it seems as though I've seen that before and thought it would be a good idea. Would it be placed between recto and first dropping resistor or one after each B+ stage?
I've also heard of diodes to protect tubes but haven't found posts dealing with that yet.
I would like my builds to be as foolproof as my pedal builds without changing tone of original circuits.

Maybe a sticky should be posted with circuit protection/safety mod ideas so newby/intermediate builders could more readily find them without costly trial and error.

[martin manning]

Search "immortal amp mods" a series of articles by R G Keen that appeared in premier guitar.

Someone here built a tube matcher/burn-in thing such as you describe. I believe it was Prarie Dawg.

[ToneMerc]

So, how much will a tube tester cost and what is the risk of toasting that, dollar-wise? I'm asking because I don't know, not to be a smart-ass. Risk assessment is worth doing and that leads to a strategy.

For anybody doing this on a serious scale, this isn't bad.


http://www.maximatcher.com/maximatcher.html

TM

[Phil_S]

ToneMerc: Thanks for the link. IMO, too pricey for an amateur. Certainly within reach for someone who needs such a tool for a business purpose. My suggestion to build a $200 Champ-ish thing was meant to address the itch of someone who needs something, but not in it quite that seriously.

I suppose the Maxi Matcher has some sort of protective circuitry?

Regarding the OP's question about diodes, isn't that why KF put the 3 diodes on each power tube in the Express?

[ToneMerc]

ToneMerc: Thanks for the link. IMO, too pricey for an amateur. Certainly within reach for someone who needs such a tool for a business purpose. My suggestion to build a $200 Champ-ish thing was meant to address the itch of someone who needs something, but not in it quite that seriously.

I suppose the Maxi Matcher has some sort of protective circuitry?

Regarding the OP's question about diodes, isn't that why KF put the 3 diodes on each power tube in the Express?

Yes, I assume he followed Ampeg's lead as did Alexander Dumble following MusicMan, installing protection diodes on the 700v ODS power plants.
Off the top of my head for a burn rig I would build a 400Vish PS parallel output tube SE config with a Hammond 125E. Install fuses in the HT center tap, filaments and install 4kV diodes from the plates to ground.

TM

[RWood]

Search "immortal amp mods" a series of articles by R G Keen that appeared in premier guitar.

These are interesting articles. I can't see why the fuse and diode mods shouldn't be on all builds.

[Phil_S]

Search "immortal amp mods" a series of articles by R G Keen that appeared in premier guitar.

These are interesting articles. I can't see why the fuse and diode mods shouldn't be on all builds.

Labor cost is more than the cost of the parts....for one-off and boutique made, no problem.