Can an OT be slightly bad?

[iknowjohnny]

In other words, can they seem to work fine but not be working at thier optimum due to a defect of some sort ? Is there any way to check them to be sure they are 100%?

[tubeswell]

Have you tested (i.e. measured) the Pri:Sec VAC ratio?

[xtian]

Yes. That nasty Band Master I had had a bad OT, but it only exhibited nasty distorion in circuit. Out of circuit, it tested OK.

[iknowjohnny]

Have you tested (i.e. measured) the Pri:Sec VAC ratio?

How would i do that?

Oh, and the title was supposed to be "slightly", not "lightly".

[Stevem]

Yes, even testing a OT with the neon bulb/ lanturn battery set up is not 100% as I just found out with a Marshall major rebuild.

John the best way with the least pain in the ass involed is to wire in serise to ground a 1 ohm 5% or better, 2 watt resistor on each output tubes cathode to ground, assuming the amp is not cathode biased.
This wil let you hook up a test meter set for dc volts across the resistor and read the tubes current draw directly.
A short in the OTs primary will make the ouput tubes work like hell to drive the OT with very little audio getting out to the speaker.
So for example, you do this check and at idle you find that your output tubes are each idling at no more than .050 amps, or .050 vdc as seen on the meter, you than pump in signal with your amps volume set for what would normally be loud and you then find that the meter is reading .100 vdc or higher, this would indicate a bad OT.
The volatge/amperage examples I am using here would be for a 6l6, el34, 6550 tubed amp, if your amp is powered by smaller tubes, like el84s, or 6v6s these voltages will be smaller.
If you go to the Duncan amps site, he has a area where you can get tube specs by punching in your tube type, and in the specs you will find for your tubes how much volatge/current they will pull for the wattage output listed.
For example, from that sites tube data specs for 2 el84 tubes in class AB1 PP, with fixed bias, the tubes would output 17 watts total, and each tube would pull .092 amp.

[martin manning]

Good suggestion, Steve. Lots of folks build-in bias test jacks, and they are useful for more than setting bias. Connecting a scope channel to each will let you observe current and balance of the output stage under signal conditions.

[Stevem]

Thanks Bro!
Hope it helps him out.

[iknowjohnny]

Very simple, Thanks ! I already have 1R test points on each cathode as dual bias so i'm good to go. I'll give that a shot in a few, thanks...

[iknowjohnny]

Well, that was easy. At max volume with a 1k .1vac tone at max volume the bias jumped to around 200mv. So i guess it's good to go. Thanks much for the info.

[Phil_S]

Re: what Stevem says, why can't you do it with a cathode biased amp? Just put the 1R between the cathode and the big Rk bias resistor. It should still act as a sense resistor, shouldn't it? I recognize this is not commonly done, but I don't see the problem with it. It still gives you the opportunity to measure mV=mA. Maybe I'm missing something?

[Stevem]

John, what output tubes does the amp use, and about how many watts?
I did not mean you can not do it with a cathode biased section, I just did not have time on hand to post what you did.

[iknowjohnny]

el34 50w