I have a basic book (a little too basic actually) on tube amplifiers and I wanted to try building the guitar amp project from it. Most of the example circuits are hi-fi but I want to try a 5W guitar amp.

This circuit is the power supply for this amp directly from the book. When I simulate it the choke current goes up to a peak of 1A and RMS of 314mA in the first second before the caps charge up. I'm curious because the author suggested specific models of transformers and filter choke, and this choke is only rated to 200mA.

I don't have any experience with power supply chokes like this. Is it normal for them to exceed the rated current at startup, or is this a poor suggestion on the part of the author?

And while I'm at it, the power transformer suggested is rated to 60mA, 500V CT. My simulation shows a steady state current of 73mA RMS. Is RMS the correct figure to use in this case? The model doesn't specify inductance or DC resistance so I didn't include any in the simulation.

I estimated the current draw of the tubes in the simulation with resistors, based on the info from the book. About 33mA for the power tube and 9mA for the gain tubes. The circuit has (1) 12AX7, (1) 6SN7, and (1) 6V6GT.

 

Look ma, no inductors!
470 uf seems way too large for this application.
I think you found a bad example.
Show me the schematic and I can probably find the equivalent in something commercial.

 

I don't have any experience with power supply chokes like this. Is it normal for them to exceed the rated current at startup

Yes.
The choke current rating is a continuous rating based upon the heat build-up due to the winding resistance.
You can greatly exceed this rating momentarily during startup without any problem.

 

I am agreeing with crutschow, but I'm pointing to the 470 uf capacitor as the cause. 470 uf @ 500V wasn't even available when these amplifiers were invented. Look at my example schematic and see 8 uf to 16 uf with no inductors required.

I have designed and built my own guitar amp with (2) 12AX7's and a 6V6.
This is entirely within my scope of experience.

 

Yeah the 470uF is definitely the reason. I just don't know how much plate voltage ripple it takes to be audible, and this circuit got it pretty darned low in simulation.

When I get home I'll post some pictures of the circuit in the book.

 

The ripple is mostly irrelevant in this circuit because the 6V6 is a current amplifier, not a voltage amplifier.
The 4.7K resistor cleans up the ripple for the voltage amplifier stages.

 

Look ma, no inductors!Show me the schematic and I can probably find the equivalent in something commercial.

The attached pictures show the circuit. It's not showed all in one schematic, it's explained one section at a time.

If the choke isn't necessary I'd be happy to save the money.

 

Yes.
The choke current rating is a continuous rating based upon the heat build-up due to the winding resistance.
You can greatly exceed this rating momentarily during startup without any problem.

Thanks. I figured there was some leeway for momentary conditions, but it must have some maximum current that will burn up the winding or something even in a short time, right? If the datasheet doesn't say, I try to err on the side of caution.

 

That's pretty much the same amplifier I made for myself, except the tone control is in a different place and I ran the voltage amp tubes under 1 ma per plate. You can see in my photos that I used (3) 22uf/450V and I was really over doing it. (That 4th cap is the cathode bypass for the 6V6.)

I don't think I've seen a commercial version with the extra 2 stages of gain, but I'll look after I get back from my chores today.

 

Here's one with more tubes than you're using and the caps are (2) 20uf and (2) 10 uf.
No inductors.

 

Thanks!