YBA - 1A Drawing too much current?

[Leo_Gnardo]

I can't say 100%, Leo. I only know that I've seen it before. My only thought is that the diode breaks down when enough signal is added to the B+ causing the tube to draw excessive current. It's no longer driving just the OT, but also a load caused by leaky diodes. And, apparently not enough leakage to radically effect B+. Speculation, but I have seen this in the past. In most every case, the diodes check good on a DVM diode test. Sometimes if you check the diode both directions on a high resistance scale, you can see that there is some reverse leakage as both directions will read similarly.

On another note, if this does prove to be the problem, I might consider adding another diode to each chain to up the PRV.

I got hats that were old when Bartholemew Cubbins started wearing his first one, and still can't see how leaky diodes would cause excess current to pass thru the tube, rather than pass thru their leaky selves. Nonetheless, if replacing the flybacks, and even adding a fourth (typically we use 3 x 1n4007 in series, according to Saint Ken right?) solves it then so be it. Or, leaving out the flybacks.

FWIW I can see how, like some flaky capacitors, leaky rectifiers won't show up until stressed with sufficiently hi voltage. How this would encourage extra charge to move along a parallel path? 'Tis a mystery.

[The Dude]

...... still can't see how leaky diodes would cause excess current to pass thru the tube, rather than pass thru their leaky selves......

Well, if the diodes are only "slightly leaky", when signal is added to the B+ on the plate(s) and the diode begins to give way, would that not abnormally load the plate? I'm thinking, as a comparison, of say a 1 ohm load on a solid state amp. The more signal that is applied to the tube, the more the diode tries to shunt the output to ground. I've not tried it, but my bet is that if you scoped the B+, you could probably watch it move up and down with signal if this is the case. (Thinking out loud).

[g1]

I think you are both right, there is extra current through the diodes via the OT, but also the tube is trying to drive a much lower impedance than it should, so it redplates early. In the same way that a mismatched load (too low impedance) will cause early redplating.

[Enzo]

I have no idea.


There is no way to test for leakage at voltage with a hand meter, unless you also use a high voltage power supply and some sort of resistor to protect things. Set up a test circuit, in other words. Generally the functional test is good enough for me. If I disconnect a leaky part, and the leakage stops, I don't need to go back and test it anyway.

[Leo_Gnardo]

I think you are both right, there is extra current through the diodes via the OT, but also the tube is trying to drive a much lower impedance than it should, so it redplates early. In the same way that a mismatched load (too low impedance) will cause early redplating.

Well I'm thin'in' along these lines: movement of charge, you have parallel paths possible, thru the tube and thru the rectifier string. If the rectifiers are OK, not leaky, electrons pass thru the tube cathode to anode like usual, no problem. But if rectifiers are leaky then some electrons pass to the plate connection point with transformer winding via the rectifier string, that's fewer electrons thru the tube, not more, so how do it red plate?

And of course Enzo's "fix the symptom, problem solved" is practical. But it still leaves me scratchin' my head under that old hat. Hellp, mister wizard, hayeelllp!

[TomCarlos]

Gents.... Just curious and trying to follow along. Is this the schematic for the YBA-1A you are referring to?

[Leo_Gnardo]

Gents.... Just curious and trying to follow along. Is this the schematic for the YBA-1A you are referring to?

OP will have to answer. Note that the earlier versions (not MkII) had G.E. 'whatsistors' (my name for the device) between output tube plates. Mr-b's amp apparently had a rectifier string (3?) from plates to ground as suggested by Ken Fisher, an item included in some form in many MusicMan amps, also built into millions of Peavey and Fender amps in the last 25 years.

[Mr_bibbles]

Gents.... Just curious and trying to follow along. Is this the schematic for the YBA-1A you are referring to?

Hey Tom, that is indeed the amp. The offending parts were flyback diodes installed by another tech that began to fail, two strings of two IN4007 added from plates to ground, So it wont show up in the schem. Was able to crank the amp up to 10 with my attenuator, no redplating, everything looks healthy, so hopefully this is the last I'll see of the beast.

Thanks again for everyone's input, and for explaining theoretical information and test methods to help me understand this stuff. Have followed many threads before and am looking forward to expanding my knowledge, here as I'm a self taught working tech with no one to help me with things over my head!

[DrGonz78]

I always see 2KV PIV 250mA fast recovery diodes (or something of the sort) in those locations for flybacks. Could this be a problem if you just use plain old vanilla IN4007 diodes instead?

[52 Bill]

Did the power tube in that one socket ever actually redplate or did it just draw a lot of current and light up the bulb limiter?

I always see 2KV PIV 250mA fast recovery diodes (or something of the sort) in those locations for flybacks. Could this be a problem if you just use plain old vanilla IN4007 diodes instead?

Yes, the modern diodes are better, but in the olden days, 2-3kV diodes were not that easy to find, so three 1N4007 were often substituted.

[Enzo]

Many amps have used 1N4007 strings for decades. HV diodes might be better, but the plain old plain old have worked fine in the meantime. I am not aware these amps have a reputation for eating them.

[The Dude]

Yes, 3 1N4007 should be just fine. I would not go with only 2 as the OP's amp was configured.

[Mr_bibbles]

Bill- It was redplating.

I ended up replacing the guy's flyback's with two IN4007, but I will start using three from now on especially on anything with monstrous voltage like the traynors, cause what's the harm in having more protection??

[52 Bill]

..cause what's the harm in having more protection??

It's not really more protection, it's just a better rating for the part so that it doesn't short as easily.

The three series connected diodes create the equivalent of a 3000 volt part, versus two making for a 2000 volt part.

[Jazz P Bass]

I don't like series diodes.

Maybe it's just me.

I buy the SR2873 diodes from Peavey.