Which power transformer ?

[dannythetrucker]

I'm working on a guitar amp (my first build) with 2-6SN7 preamp tubes, single 6L6GB power tube, a VR-150, and GZ34 rectifier.

I had planned to use a new Hammond 272BX. It would work, but it's only 300V per side, 115mA, and is 3lb 5oz.

Now I came across a transformer out of a watch timer that is 475V per side, 5lb 15oz. Not sure the current rating, but it supplied 8 tubes and a small motor in it's native environment so my guess is over 200mA.

In short, two extremes. One would be extreme overkill for a single-ended amp, the other I could actually run a bit past it's rated current. I can adjust the circuit for either.

My question is what differences would I expect between the two ? My inner redneck tends to think more power is always better, but I've also been reading that some of the greatest guitar tones come from amps that are starving for voltage. Would I get some sag sound with the little guy ? Or is that strictly a push/pull big watt phenomenon ?

What else should I consider when choosing ?

[Tom Phillips]

...Now I came across a transformer out of a watch timer that is 475V per side, 5lb 15oz. Not sure the current rating, but it supplied 8 tubes and a small motor in it's native environment so my guess is over 200mA...

If you use that transformer then your resulting B+ will be too high for the project you describe. Probably 570 to 600 volts.

...I had planned to use a new Hammond 272BX. It would work, but it's only 300V per side, 115mA...

I agree that would work assuming that you are expecting to end up with a 6 to 10 Watt single ended amp.

[dannythetrucker]

The 6L6GB will do up to 23 watts, but I don't have any strong persuasion to make it much over 10 watts.

Here is a SCHEMATIC of the Watchmaster G7 circuit the transformer was removed from. I have the choke they used to make 330V.

[Tom Phillips]

The 6L6GB will do up to 23 watts...

I'm not sure what data you are referencing. However, one 6L6GC, which means that the amp is running in Class A, you will not be able to achieve anywhere near 23 Watts output power due to the inefficiency of class A operation. It will still be a good amp and can provide plenty of volume with an efficient speaker.
Cheers,
Tom

[dannythetrucker]

I'm not sure what data you are referencing. However, one 6L6GC, which means that the amp is running in Class A, you will not be able to achieve anywhere near 23 Watts output power due to the inefficiency of class A operation. It will still be a good amp and can provide plenty of volume with an efficient speaker.
Cheers,
Tom

Ahhh, good call Tom. I did have my data mixed up. Well, that makes my decision a little easier. You're right, I can use the smaller PT, have a nice 10 watt SE amp if I do it right. Then use the larger transformer later for something else.

[mozz]

I have one of those watchmaker units here for a project this summer. I don't think the power trans is 475-0-475, that could be a typo on the schematic, i have the trans pulled i can measure it if you want on my test chassis.

[dannythetrucker]

I have one of those watchmaker units here for a project this summer. I don't think the power trans is 475-0-475, that could be a typo on the schematic, i have the trans pulled i can measure it if you want on my test chassis.

That would explain a lot if it's off ! I suppose I should devise a way to test mine, either fasten the leads to a terminal plate or cap them off except the two I'm checking and go through it I guess ?

[dannythetrucker]

I have one of those watchmaker units here for a project this summer. I don't think the power trans is 475-0-475, that could be a typo on the schematic, i have the trans pulled i can measure it if you want on my test chassis.

Hey, I just found this. If you look on page 9 it says 475V to each plate of the rectifier which verifies the other schematic. But I still think the best would be to test them as you suggested. LINK

[Tom Phillips]

I see that the watchmaster power supply uses a choke input filter. That will result in a significantly lower B+ than you would get with a capacitor input filter. That explains the high PT secondary AC Voltage and the reasonable B+ DC Voltage shown on the watchmaster schematic.

Edit: There is a nice power supply designer freeware SW package at http://www.duncanamps.com/psud2/ . It is very useful to analyze a power supply circuit and see have configuration, parts values and load affect the voltage and ripple.

[dannythetrucker]

My input voltage was running 122.8V, but I tested at 520V/side secondary. 6.8 and 5.67 on the other two.

[Tom Phillips]

My input voltage was running 122.8V, but I tested at 520V/side secondary. 6.8 and 5.67 on the other two.

That is not surprising given the normal tolerances and that the transformer was designed for 115V line voltage.

[Enzo]

So 520vAc to ground from each end? And then that means 1040 volts end to end? Wow.

[dannythetrucker]

So 520vAc to ground from each end? And then that means 1040 volts end to end? Wow.

My meter won't read over 600V, but I assume so. I tested current at 80mA per side, but I don't know is it accurate to measure current without a load ? I wasn't sure about that.

Regardless, I think I got my $$$ worth. I'm thinking if I use the bigger transformer I can add a reverb circuit.

[Tom Phillips]

...I tested current at 80mA per side, but I don't know is it accurate to measure current without a load ? I wasn't sure about that...

I'm not sure what you mean. If there is no load then there should be no current flowing. What was your test setup used to measure "80mA per side"?

[J M Fahey]

My meter won't read over 600V, but I assume so. I tested current at 80mA per side, but I don't know is it accurate to measure current without a load ? I wasn't sure about that.

Not sure how can you measure current without a load.