Voltage Measuring Question

[ValveBytes]

I have checked the voltages in my amp and they are roughly 25% over spec across the board...

The transformer has a sticker on it saying 250-0-250 on the output, but when I measure the voltages before they go through the two diodes that are being used for rectification I get 695 volts once it stabilizes, that would suggest to me that the transformer is more like a 350-0-350!! and that is 28% more output voltage than the circuit is designed for!

anyway just wanted to check that my methods of coming to this conclusion aren't flawed, before I order up a new 250 0 250 transformer.

[Tom Phillips]

Comments, Things to think about and things to check:

1) The 250-0-250 is the voltage you would expect under rated load with the proper line voltage. Make sure those are your measurement conditions.

1a) If you are measuring the open circuit voltage, it will be higher.

1b) If you have an old amp it could have had a 117V, 115V or (if way old) a 110V line input rating. The current line voltage could be 120V or higher. I often measure 125V. That could account for a significant portion of your “over voltage”.

2) Is your meter accurate? If the calibration is off, that could account for some or all of the higher readings.

3) Transformers often have a wide tolerance rating. That could account for up to 20%. Note that you often see a note on old schematics that say the voltages shown are “within 20%.

4) It’s quite common for the voltages to read high in old guitar amps when they are run off of modern line power. What’s important is that your B+ is acceptable when everything is connected and the bias is set correctly. What’s the B+ reading? What’s the amp model anyway? Any sign of mods being done?

5) You are reading 695V where you were expecting 500 so by my way of calculating, your secondary voltage is 39% too high not 28% (195/500). That could be caused by one or more of the things I covered above. Maybe all the specs lined up on the plus side in our amp. Could also have had a weird/bad PT from the beginning.

HTH,
Tom

[ValveBytes]

Thanks for the help some good pointers

I haven't checked my AC line voltage we have 230v in the uk, I'm pretty sure it is set to 230 or 240 so that should be ok but I haven't checked what's coming through.... its a late 50's watkins amp the HT(B+) reading across the first smoothing capacitor is 444v no mods have been done but the original transformer and full wave rectifier have been replaced with a huge transformer compared to the original and two diodes.

Need to test my tester actually have to find a good reference voltage or another meter to compare the readings.

my reference is the original schematic and someone else's example voltages here's how they compare, first is my voltage then the schematic rating and so on...

On the ECL82 output valves

Mine Rated Example
Pin2: 29.4v :22 : 26.8
Pin6: 393v :280: 289
Pin7: 377v :270: 260
Pin8: 2.4v :1.7 : 2.06
Pin9: 163v :130: 140

And on the ECC83/12AX7:

Mine Rated ex.
Pin1: 198v :150 : 149
Pin3: 2.46v :1.8 : 1.75
Pin6: 276v :215 : 199

[Tom Phillips]

If I understand correctly the origianl had a tube rectifyer installed. Now you have solid state diodes and a different transformer. That would seem to explain your non-stock voltages. You may be able to get back into reasonable range by switching back to a tube rectifyer if your existing transformer has a 5V winding. If not, there are other solutions. Were you thinking about replacing the power transformer? If so, maybe you can procure readings from the other amp example that you mentioned.

[ValveBytes]

yeah that is exactly my plan I have sourced a similar transformer to the original and a 3 part 20uF x3 450v smoothing cap can. the replacement transformer has 2x6.3v windings 1 for the EZ80 rectifier and one for the other heaters should do the job.

this is a diagram that someone else made I was using their readings as a guide>

[Tom Phillips]

Interesting amp. Sounds like you are on your way to a fix. The tubes are a little rare but I see that they seem to be redily available although a little more expensive than the more common types used for new equipment today.
Have fun,
Tom