New 102 build: Relay transformer up in smoke

[mgotoo03]

Hi all,

Just (almost) finished a 102 build based on the ampgarage layout (attached). Played for close to an hour with no issue when the OD channel switched back to clean on its own, and switching became unresponsive. The voltages at the relay were normally ~5.8V but had dropped to 0.6V, and then the relay transformer let out its smoke. I am hoping I can get some info before the next attempt.

I had an LEI 41LEIA-424 (6V-0-6V 0.45A) transformer with a grounded center tap on the secondary. The voltage regulator is LM7812. It does not have an external heat-sink. The relays are Omron G5V-2-H1-DC5. The rectifier is from Manning's eyelet board layouts (attached). Was able to switch on the amp or with a DIY footswitch. The footswitch was built using an old midi cable as the cable.

Questions:
1. Does this combo seem compatible?
2. Is there a better choice for transformer?
3. Should the center tap be grounded?
4. Should the regulator have a heat sink?

I'm going to look for other issues, but want to make sure I have these answered if possible before the next round.

All the best

[erwin_ve]

Hi, Are you aware that you have a (!)5v operating voltage(!) relay, and the rectifier has a 12v output?

[mgotoo03]

HI erwin-ve,

Short answer is "I am now". I was only measuring ~5.8V at the regulator output, and made a dumb decision about which relay voltage based on not understanding relay voltages. I assumed a 12V relay needed to see 12V at the relay to switch. Would the overvoltage on the 5V relay potentially cause the stress on the transformer?

Thanks again

[mgotoo03]

Am I correct thinking that the over voltage on the relay could cause arcing across the relay which could burn up the transformer?

[Matt J]

3. Should the center tap be grounded?

Are you using Martin's boards like the file you attached? I've never ground the center tap on the relay PT when using those boards because of the rectifier design. I just cut it short and heat shrink the end. I've had issues in the past with incorrect relay voltages but nothing as serious as this. I just finished a #124 with a Hammond 166F12B Relay PT and 12V relays and have had no problem with it.
Matt J.

[martin manning]

I think there are two errors, grounding the transformer center tap, and using 5V relays.

The attached eyelet board layout is fine for a 6-0-6 VAC PT. Use the outer secondary leads to get 12VAC, but do not ground the CT.
The rectifier is a full-wave bridge, which should get 12 x 1.41 = ~17VDC
Feed that into a 12V regulator, LM7812 e.g., and it should be fine without a heat sink.
Use 12V relays, i.e. Omron G5V-2-H1-DC12 as listed on the .pdf

[mgotoo03]

Thanks everyone. I just got done reading about bridge rectifiers and center taps = magic smoke

Will make all the changes. For what its worth, that golden time with the amp working was pretty fantastic!

[talbany]

Yes you do not want to ever ground the CT. when using a FWBR.. The ground refrence is already being reflected back at the tail of the rectifiers diodes.

Your lucky it lasted that long
Tony