Epiphone Blues Custom Rectifier prob

[ehedwr]

I've recently done a few mods to my BC30 like fitting a bright switch and converting to EL34 output tubes. Whilst playing the other day the mains fuse (F9) blew on me. Thinking it was because I had changed tubes I put the 6L6 tubes back in. I tried a few more fuses and they kept on blowing when I switched from standby to play. I then pulled all the internal fuses and tried it by putting em back in different stages to try and isolate the fault. Guess what? From then on it worked fine.

Today I had the chassis out taking notes of different voltages at various points. Everything was hunky dorey until I plugged in the guitar and started giving it a bashing. Pop! The fuse blew again! I replaced the fuse and connected my multimeter inline with the power supply to see what was happening. Don't compare my readings here in the UK to yours elsewhere. US currents would double what I have. On switching on the power switch the current quickly settled at about 300 mA. When I switched on the standby switch it rose to 500mA. The original fuse was rated at 1.6 amps. I tried this a few times when I noticed the rectifier tube would sometimes arc whilst switching on the standby switch. The reading on my meter spiked and a crack could heard coming from my mains transformer. I had a Harma GZ34 fitted and only a few weeks old too! I put the original Chinese rectifier tube back in. Now there is no spiking, arcing or cracking and with Channel 2 cranked right up, full volume, full gain, full sag, it's drawing 650 mA.

I'm hoping that this is a just a case of an iffy valve and not some other problem causing the rectifier to go bad. And I hope I caught it in time to save my power supply TX from blowing. As for the EL34's? They'll be going back in soon as soon as I'm absolutely positive they didn't cause this. Though I don't see how they could have.

Anybody have any insights on this?

Ok. I just tried this again to see what current readings I would get with the EL34's fitted and the good rectifier tube installed. At first all was well. The plate voltage for some reason had risen from 420 VDC (with 6L6's) to 430 VDC. I turned off to measure mains current, started it back up and POP! Another fuse gone, the power TX farted and a flash from the rectifier tube. What next? I know the heater filaments draw more current with EL34's but the 6.3V winding is rated at 5 amps and only supplies the two output tube filaments. The preamp filaments are fed from a seperate DC 12V supply. Maybe it's time to look for a new power transformer and fit a solid state rectifier.

Oh and finally I must add that I uprated the filter caps from a pair of 100mF/400V electrolytics to a pair of 220mF/400V electrolytics. I did this to reduce sag. Do you think the larger caps are sucking too much current on startup?

[techineer]

It's possible that the caps are pulling too high a surge current for the rectifier tube....

Try pulling all the tubes except the rectifier tube. Be sure the caps are fully discharged, then watch what happens when you take it out of standby. If you get the tube flash, then I would suspect that the caps are the issue.

To repeat, discharge the caps again (through a high wattage resistor) before retesting.

However, I wouldn't expect the caps to cause the fuses to blow after all had settled down and while you were playing the guitar. That suggests that something else could be wrong. (also?)

[Enzo]

The amp works with one of your rectifier tubes in the socket, and it doesn;t work with a different tube in the socket. Sure sounds like the tube is bad to me.

[ehedwr]

I had to put the original caps back in the larger ones were increasing the inrush current. This fault has fried my stock of GZ34 tubes so I fitted a solid state rectifier. The amp is working fine now however B+ has increased from 420VDC to 450VDC. The power amp section is cathode biased with 30 VDC across the 250 ohm cathode resistor. That works out at 60mA of cathode current through each EL34 tube at zero zignal. Anybody think this is a bit warm? I am considering adding a fixed bias circuit, there is an unused 50V tap on the main transformer. Any tips on this would be appreciated.

As for the tube rectifier? If I go back to it I'll either fit a couple of current limiting resistors or fit another octal base and share the load through a pair of GZ34 tubes or even just ditch the standby switch altogether. It's just not needed with a tube rectifier in my opinion. As for current limiting resistors I tried a pair of 7W 100 ohm carbon resistors but they ruined the amps tone.

[Steve Conner]

The GZ34 is rated for a maximum smoothing capacitance of about 60uf. So yes, you probably fried them by using oversized caps.

[MWJB]

"That works out at 60mA of cathode current through each EL34 tube at zero zignal. Anybody think this is a bit warm?" Yes, it's a bit warm at that voltage, try 330-470ohms 10W for the cathode resistor.

A standby isn't needed with a GZ34 or 5V4 rectifier, but it's a good idea with 5U4G, or SS as these do not have a slow start up. Do your main filtercaps connect to the upstream side of the standby, or the downstream side? They should connect to the upstream side. A GZ34 will take 100uf at the main filter node.

"or fit another octal base and share the load through a pair of GZ34 tubes" Really, are you sure that the PT will stand the load, it may only have a 2A winding if the GZ34 was specified and you intend to put 4A on it?