YBA - 1A Drawing too much current?

[Mr_bibbles]

Hello all- First post!

Got a Traynor YBA 1a On the bench that is giving me no end of headaches.

Recently Recapped this amp, but the Issue existed before the job was done. I doublechecked my work just to be sure anyways.

Also I'll start by saying I've swapped all the tubes out without any changes.


Issue is that one of the two tube sockets is causing red plating, and then tripping a circuit breaker. Have switched the tubes to verify it is the socket position, switched the OT leads to check if this was the cause, put brand new .1uf Coupling caps after the PI, Tightened and Cleaned the Sockets themselves, and replaced every Electrolytic cap (except the 40/40 cans, still waiting on the mail...) Bias reads healthy at Idle, but when signal is applied the my Limiter bulb gets very bright and dies back down if I stop playing. I also Installed 2x 5w 1k Individual Screen Grid Resistors (might be incorrect on the terminology, it's the cement resistors that feeds pin 4 from the 80uf cap) to feed of each tube as opposed to the single 10w 480k it came with. My understanding is that it would be a little padding for each tube.


Biased within a few MA of 70%, I didn't do dead-nuts on as I just wanted to verify it was possible.


And now today after again re-tensioning the tube sockets, I'm getting an AWFUL high pitched oscillation out of the speakers reguardless of signal. WHen I first turn the amp on it sounds like a "bomb falling to earth" sound effect or a tea kettle, and when I turn it off it "Chirps" rythmyically for a few seconds. and it will change character in tone and volume if I bypass the attenuator. Chopsticking doesnt do much. Logic dictates its the Tube sockets since it's the only change I made, but I'm thinking it's also possible that it just revealed the symptoms of a seperate disease.


So Problem 1- What is causing the redplate tube / excess current draw / general malaise

Problem 2 - Weird oscillations.

[The Dude]

On the oscillations: Did you put the OT wires back where they were? It seems you might have them reversed and have positive feedback instead of NFB.
On the red plating: Remove the tubes and measure the voltages on the sockets. Post voltages here.

BTW. Welcome to the place!

[Mr_bibbles]

F*****' A, Dude

On the oscillations: Did you put the OT wires back where they were? It seems you might have them reversed and have positive feedback instead of NFB.
On the red plating: Remove the tubes and measure the voltages on the sockets. Post voltages here.

BTW. Welcome to the place!

Nice thought Dude! Had tracked the feedback to the "frequency expander" which I'm guessing is a NFB controller a la 'Presence'. Swapping OT wires cleared it right up, strange though that the noises only appeared after socket re-tensioning.


Voltage Readings:

.3mv on the heaters.

BOTH Pin 3-541v
BOTH Pin 4-533v

BOTH pin 5- -41.7
BOTH pin 6- -41.7

Pin 8/1 - .1mv on the left socket, .3mv on the right (and the offending/redplating) tube socket. All other readings are identicle between both tubes. Is there a heater shorted to the Cathode in this case?? The Indicator light flickers with the Current limiter bulb in the same fashion....

Note: Pin 8 and 1 are tied together on each socket and then jumpered across to 8/1 of the second tube. Tied to ground w/o resistors from left socket at the same termination as the death cap.

[g1]

Agree with above about the squealing, also, it is normal for the bulb to light up when playing.
If you have replaced the coupling caps, and verified no connection problems getting to pin 5 of the iffy socket, then perhaps it is the socket itself.

[Mr_bibbles]

G1- The bulb gets VERY bright while playing, beyond what normally happens with my unit. Possible it is just the high Plate voltage of the YBA-1A though? It is also tripping the circuit breaker, which leads me to believe the brightness is abnormal current draw.

+1 on the OT swap, forgot all about OT phasing!

[The Dude]

My next thought on the red plating since your voltages all look similar would be a possible failing flyback diode. Does this amp have them? I know some Traynors do and some don't. If it does have them, I would temporarily unhook them and see if the red plating stops.

[Mr_bibbles]

F*** it Dude, let's go bowling.

There are two pair in4007's each connecting Plates to Ground, is this what you're referring to? They all test .5ish / OL, but I can try swapping them in the morning as I'm leaving shop for the day. I'll let you know what I come up with, and thanks guys!

[The Dude]

Yes, those are the flyback diodes I was referring to. The "normal" failure mode is short and they will cause the fuse to blow immediately on power up. However, I have seen them cause problems like you are having. It's simple enough to unsolder one end of each string and test. You don't need to completely remove them to see if that is the problem. You can safely test the amp without them installed and worry about replacing them if you find that is the problem.

[Enzo]

Agree^^^ just unsolder one end and lift them. They are not necessary for amp function, they are just for safety. We then find out if they made a difference or not. No point in just replacing them without checking first.

Your meter is measuring the forward voltage drop, and half a volt looks fine. However the problem Dude suspects is REVERSE voltage leakage, and that occurs at the 400-500 volts your amp uses. Your meter doesn;t remotely test for that.

[Mr_bibbles]

Agree^^^ just unsolder one end and lift them. They are not necessary for amp function, they are just for safety. We then find out if they made a difference or not. No point in just replacing them without checking first.

Your meter is measuring the forward voltage drop, and half a volt looks fine. However the problem Dude suspects is REVERSE voltage leakage, and that occurs at the 400-500 volts your amp uses. Your meter doesn;t remotely test for that.

EXCELLENT advice everybody, after lifting the Flybacks from the Redplating socket, I was no longer able to replicate the problem. Goin' to install a new pair and tubes and re-bias it up!

Enzo, is there any easy way to measure the reverse voltage leakage? It makes sense to lift the flybacks and I will be doing that as my test, but I'm eager to learn.

[Mr_bibbles]

Bummer, those Flybacks really tied the amp together dude.

[Enzo]

You just did test them.

[Leo_Gnardo]

I can't see how leaking/shorted or properly working flyback rectifiers could cause red-plating in output tubes. So I'm anxiously awaiting the results here.

[Mr_bibbles]

I can't see how leaking/shorted or properly working flyback rectifiers could cause red-plating in output tubes. So I'm anxiously awaiting the results here.

Enzo - Yes, That is the easiest test for flyback diodes for sure, and it helped solve the case, so thanks! I guess I meant is there a way to test other diodes for Reverse leakage using a DMM or such? More so curious about reverse voltage leakage itself and how to detect it without subbing / disconnecting.


LG- It was the flyback diodes. I'm guessing it was creating a poor reference to ground for the plate voltage so it backs up into the tube? One of the old hats should know.

[The Dude]

I can't see how leaking/shorted or properly working flyback rectifiers could cause red-plating in output tubes. So I'm anxiously awaiting the results here.

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.