Rectifier tube arcing.... fuse blows

[Garydean]

1964 B-15 Ampeg

I'm getting further with this amp and still finding problems. After putting in new tubes and testing the caps, all tested fine 25M resistance to chassis, I fired up the amp. It sounded great, no hum, hiss or crackling. I played it for about 10 minutes, shut it down, plugged back in, hit power then flipped the standby switch after warm-up. The gz34 arched internally then the fuse blew. I removed the rectifier, placed in a new fuse, did the start-up and the fuse was fine. I should suspect a bad tube or filter caps? The output tubes could be bad too, but would that make the rectifier arch internally?

Thanks!!

[stokes]

"all tested fine 25M resistance to chassis" That "test" means nothing.If those caps are original,it is time to re-cap.That doesnt mean they are the cause of your present problem.It is more likely you have a bad rectifier tube,but a shorting input filter cap could cause it as well.What kind of GZ34 do you have?I would recommend finding a good NOS GZ34 the current production arent as tough.

[trevorus]

Probably bad rectifier. My Vox AC30 did the same with a Sovtek rectifier.

[Garydean]

Thanks for the response. The tube that was in when the fuse blew is a JJ.

[clyde1]

Ditto on the bad rectifier, esp. if it's a JJ. They're no Mullard. I had the same
thing happen to me, a new GZ34 right out of the box and another after about 8 months on the road. I'm now going with Sino, hope they're better.

[52 Bill]

Yes, the quality of modern GZ34s is really spotty. I've had rejects from all the current makers.

Gary- When you retubed, what output tubes did you install, and at what level did you bias them?

[Garydean]

I put the original 5AR4 back in and it's stable.

The output tubes are 6L6GC from TAD. I measured the plate current resting at 58mA. That's too high no/yes. The plate voltages are just under 400v.

thank you,

Gary

[clyde1]

Gary, leave the amp on for awhile to see if the rectifier misbehaves. Don't be surprised if it arcs again. This was my experience.

[Bruce / Mission Amps]

I put the original 5AR4 back in and it's stable.

The output tubes are 6L6GC from TAD. I measured the plate current resting at 58mA. That's too high no/yes. The plate voltages are just under 400v.

thank you,

GaryDo you mean 116ma for both power tubes?
That is pretty high but that isn't really "too" high for a good pair of 6L6GCs.
Have you replaced the bias supply filter cap yet? If not, do it now.
You might find the power tube's idle current drop a bit.

Along with everyone else, I'd suspect the rectifier tube.
Most modern GZ34/5AR4 tubes, although rated at 250ma@full voltage, probably are closer to a 175ma tube.
I can't prove that but, I have seen too many of them fail in 35-50 watt amps.
If you can afford a NOS one, you might find that after a few years you are still enjoying the long life of a NOS GZ34/5AR4, vs being on your third or fourth or maybe even fifth modern made one.

Now, for me I have found that the newest Russian SOVTEK GZ34/5AR4 has been better then any of the SINO 5AR4/GZ34s.
I stopped buying the JJ GZ34 for a while because I was blowing them up at an alarming rate.
I would not be surprised to find out the most current ones are better now as JJ seems to make adjustments as they go while other tube manufactures don't react as fast to public pressure.
Another simple mod you can try is to use a solid state, FW rectifier plug in there instead of the tube rectifier and install a 10 watt, 100 ohm resistor from the rectifier tube socket lug 8, to the first filter cap.

[52 Bill]

I'd be reluctant to send out a customer's amp with each TAD 6L6GC drawing 58mA. I just know that it would be back in no time, with either a failed tube or transformer.

[Garydean]

Thanks guys,
I replaced the bias supply filter cap last week. The bias now is at 58mA each. The 5ar4 is a GE and is holding steady. Ideally, what do I need to change to get the bias at 35-40mA each?

thanks,

Gary

[52 Bill]

You need to change a resistor in the bias circuit to allow more negative voltage to hit the grids. Check the schematic and you'll find a bleeder resistor at the output of the bias supply. Increase the value of this resistor or replace it with a trimpot to get the bias level where you want it.

[Garydean]

thanks Bill.

Might this be a 1.5k 3w (?) resistor that is shared by both 6L6's off pin 4?

[52 Bill]

That doesn't sound right. What model is this, an B-15N? I'll have to look up the schematic.

Look at the negative voltage bias supply on the schematic. It will supply the two grid bias resistors that each attach to pin 5 of the two 6L6's. There will be a resistor that goes from the bias supply output to ground. I want to say maybe a 3.9K 1/2 watt. Whatever the value, you need to increase the value of this resistor so that the bias voltage increases.

EDIT-OOPS! Is this a B15-NC?

I'm sorry, based on the 1K5 screen grid resistor that you asked about, I'm assuming that you have an NC model, which is cathode biased, and not fixed bias. MY BAD!

You will need to change the cathode resistor to adjust the current draw. Also, did you change the cathode bypass cap?

[Garydean]

yes Bill, it's a 1964 B-15N.

off pin 5 of each 6L6 is a 1k resistor which are each connected to 270k resistors.

thank you

[Garydean]

I did replace the cathode bias cap. Is the cathode resistor the 250ohm 10w wirewound resistor that goes to ground?

250ohm 10 w

sorry

[Bruce / Mission Amps]

I did replace the cathode bias cap. Is the cathode resistor the 250ohm 10w wirewound resistor that goes to ground?

250ohm 10 w

sorryOK... I didn't read this the first around so if you have the cathode biased version the first thing you need to do is remember that the idle current is determined by using the voltage from plates to cathode not to ground.
Assuming you know how to measure all this with that in mind, if that is still 400vdc, then I would replace the 250 ohm cathode resistor with a 330 ohm 10 watter.
I have run the Chinese TAD 6L6 tubes at around 48ma-54ma in my Tweed 6L6 amps with fixed bias and plate voltages of around 400vdc-410vdc.
They seemed to be OK with it.

[Garydean]

I just measured the voltage from plate to cathode and it is 355vdc. The line was at 120vac drawing 1.25amps. Does all this sound OK? The tubes look fine. I am going to suggest new filter caps.

Side note on the rectifier arching. I read that these amps tend to blow fuses on a warm restart, which is what happened earlier. Correcting this problem involves installing a 10w resistor off the cathode of the rectifier, to the rest of the amp. I forgot the value but it was low.

comments welcome!!

[52 Bill]

Did you check the value of the 250 ohm 10 watt cathode resistor? They will drift.

The rectifier arcing was a bad tube, does it blow fuses when you warm restart it now?

[Garydean]

Hi Bill, Yes that resistor checks out fine. I did another warm restart with the NOS rect tube in, and everything seems stable, fingers crossed. I played thru the amp again this morning for about a half hour at all volumes and everything is fine so far. Plates are fine and the amp breaks-up when it should. The plate voltages on the output tubes are about 10% lower than what's shown on the schematic. That I can live with.

With a self biasing amp, how reliable is that design, with respect to the tube setting/finding it's own bias point, once it gets cranking? Am I understanding this correctly?

Thanks again,

Gary

[52 Bill]

Very reliable, but it allows for less power output, which I think was the main reason for switching over to fixed bias designs.

[stokes]

Now that it seems you have the fuse blowing sorted out,I'd like to offer a suggestion.I recently built a bass amp for someone and used the B-12 and B-15 as a base.I put a switchable cathode/fixed bias set-up in it.I found the cathode bias mode broke up too quick,even the fixed bias mode was not to my liking,so I upped the filtering and it helped a lot.Unless you are concerned with keeping the amp stock for collector value,using 100uf on the main and screen taps really tightened things up a lot.That 10% lower plate voltage could be due to old caps as well.