Testing used up looking Tung-Sol 6550 Type one

[kcline]

See pictures; This was in my Dynaco Mk-3 Paoli mod amps. I was testing today using my Maxi-Matcher. This tube had been in amp for 4 years, I would guess around 1000 hrs and this was used already. A few weeks ago I retubed with GE 6550 and today I decided to retest the worst looking of the four. Current tested at 39.8 mV transconductances at 4.60 mS . The numbers from years ago were 40.0 at 4.61. I have found that these mid 1950,s Org. T-G construction, black solid plates flashing on top only, hold up very well even if they look like this one.

 

[arts]

Unfortunately,the Maxi-Matcher does not provide the current levels necessary to actually measure the tubes output capabilities.

Many tubes,while still showing good Transconductance at low current levels,fall completely flat when asked to produce higher output levels. While the Maxi-Matcher provides good low-current Transconductance measurements,it doesn't test for output.

It takes some serious current to properly test a 6550,and even more if you want to get into the KT120 & KT150 types.
Virtually everything I work on is 6550 or KT120 based,which require at minimum a 300 to 500mA @ 400V plate supply for proper testing. The screen supply,at 300V,requires in the under 25mA range. All supplies,including the Heater,are fully regulated to ensure accuracy and repeatability of testing.

My own power output test rig uses a Sorenson DCS 600-1.7E power supply as the plate current source,and an old (modified) Lambda for the screen and bias supply. Heater supply is a modified high-current 5V supply from the early days of digital.
Also used is a high-output generator to provide the needed grid drive,and lots of metering to keep an eye on everything.

I guess what I am trying to say (while at the same time bragging me arse off about how magnificent my own setup is) is that it takes a lot of large,heavy stuff,and a lot of time and effort to set it up,if you really need to fully test power tubes.

 

[maxhifi]

^^^ Arts is right on all counts. A simpler test is to simply put a load bank on your amplifier, and see if it still meets the power spec with the old tubes. Then try it with new tubes and compare values. I've had tired old 6L6s for example which test good on my tube tester, but struggle to put out 15W in an amplifier. This is actually part of the scam of some people selling use old tubes on eBay. Everything "tests good" in a tube tester unless it is outright defective.

 

[gadget73]

+1 on power output testing. If you've got new tubes installed, thats a perfect situation for this. You have a reference for what new tubes will do. Compare that to the old tubes and see how they do.

I have a set of 6CA7 that test acceptable but not wonderful on my tester. In the amp they make less than 10 watts where good tubes make 45 in the same amp.

 

[dcgillespie]

Absolutely agreed. Power Output is the only way to really test power output tubes, which I've done now for nearly 30 years.

Dave

 

[kcline]

I agree with everyone who has posted so far. I have 30 years + of using Maxi-Matcher this is my 2nd one I worn one out. Of course it has issues, but I very rarely had any problems with the thousands of tubes I matched. I did lots of testing for jukebox and guitar amps mostly 6L6-GC types, never had any issues with jukebox folks and very few with guitar folks, a tube would red plate almost every time bad caps. Had cases of GE 6550-A retubed two ARC D- 250 amps those run the tubes very hard but never gave me problems, amps ran for years. Back in the day I would of liked to have a high power test rig, I did try to buy one from a ham radio estate that had it setup for transmitting tubes but could not afford. The Maxi-Matcher does place 400 volts on the plates just about double what a Hickok type does. I tested a collection of well used TG 6550 type three, all had tested good on Hickok 539-C,all failed on matcher