Power supply question about a Gibson GA-25RVT Hawk

[tkreeve]

Hi all. Long time lurker, first time poster.

I'm seeking some advice from the more experienced and educated than myself - schematic attached. The voltages on my unit around 10% of the schematic, which is good - I had it with a tech a few years ago, before I got into electronics myself. It took him 2 years to restore it, and by the time I'd gotten it back I'd learned a little bit. I do not know why, but he'd replaced R42 (last dropper) with a 47K, which about halved the specified voltage on that node. I changed it back to 4.7K and the voltages returned to as specified, but then I noticed R43 starting to burn up (it was a 3 watt wirewound).

I temped in a 5 watt wirewound and measured the dissipation across it about 2 watts - I guess my question is does this appear normal? From what I added up of all the valves downstream of R43, the total dissipation would be about 5.8 watts, unless I did it wrong. Is this just a badly designed power supply, or am I missing something?

Couple other things I found was that V4 (12au7 interstage driver) gets quite toasty compared to the other 12au7 on the same node (have tried other valves with no change - components, voltages around them in spec.) The other thing was the reverb circuit is pretty hummy when engaged, irrespective of tank placement or valves used. I thought about trying to measure the turns ratio of this to see if there was a shorted turn maybe - but I haven't been able to find the original specs, so I subbed in a fender reverb driver briefly and it didn't appear to make any changes to the dissipation.  I would appreciate any insight, thanks so much.
TK

[sluckey]

According to the voltages on the schematic R43 is dissipating 3.6 watts and R42 is dissipating .3 watts. You need to double those actual wattages for a safety factor. I suggest using 5000Ω/10W and 4700Ω/2W.

[kagliostro]

I agree with Sluckey

May be they tried to go on the cheap with components or used a stock they had

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About to verify the transformers integrity see here




Franco

[tkreeve]

Thanks guys. Lots of good information in so few words. Will also go back to the books and figure out why my numbers were so wrong - I used the schem volts too. Cheers

[sluckey]

P=E²/R

The schematic shows the voltage on each side of the resistors. Subtract those two numbers. Square the result then divide by the resistor value. That's POWER.

[PRR]

> V4 (12au7 interstage driver) gets quite toasty compared to the other 12au7 on the same node

"same node" but different bias. 10mA at 250V is near 12AU7 max. How about V7? The reverb driver side runs a similar condition.

> reverb circuit is pretty hummy

I also note that V7 is real near the choke, and the small side of the 12AU7 is a voltage gain stage. This looks like a rushed design.

[tkreeve]

P=E²/R

The schematic shows the voltage on each side of the resistors. Subtract those two numbers. Square the result then divide by the resistor value. That's POWER.

And that result of the differences is the voltage drop measured across the resistor - understand why I went wrong now, was doing the power in 2 steps, getting current v/r, then power i v - but using the wrong number as the voltage drop. That’s much simpler. Thank you!

[tkreeve]

> V4 (12au7 interstage driver) gets quite toasty compared to the other 12au7 on the same node

"same node" but different bias. 10mA at 250V is near 12AU7 max. How about V7? The reverb driver side runs a similar condition.

> reverb circuit is pretty hummy

I also note that V7 is real near the choke, and the small side of the 12AU7 is a voltage gain stage. This looks like a rushed design.

Thanks. V7 doesn’t get quite as hot to touch - I tried swapping their places as well. I’ll compare the dissipation with the schematic once I order the correctly rated droppers and go from there if needed.

Rushed design - yeah, I’m no expert but it’s very messy in there, and hard to work to on - L-shaped chassis with terminal boards pushed right into the corner. Was not built with service in mind either.

Thanks again