car inverters as tube power supplies

[gtrboy]

has anyone tried using a 150w 220v car inverter as a 12ax7 power supply? why or why not?

[Wilder Amplification]

I think imaradiostar did something similar with a low wattage Class A SE amp...using the SMPS from an inverter to power it. IIRC the thing only sagged about a volt under full output.

[Steve Conner]

These things usually work by converting the 12V supply up to 160V DC (or 320V for ones sold in 230V countries) before inverting it. So you can tap off this "DC bus". It's regulated, so no wonder it only sags "about a volt".

I expect it would have a lot of noise and hash on it that would need filtered.

[imaradiostar]

http://music-electronics-forum.com/t19365/

I haven't done much else with it. It's pretty fun, whatever the case.

jamie

[Wilder Amplification]

Then again on a true Class A amp current draw SHOULD BE constant from idle to full output power anyway since the power supply itself sees a constant load from idle to full power output so you'd think there wouldn't be any sag whether you ran regulated or unregulated. But having no actual hands on experience with Class A SE amps I could be wrong.

[imaradiostar]

Then again on a true Class A amp current draw SHOULD BE constant from idle to full output power anyway since the power supply itself sees a constant load from idle to full power output so you'd think there wouldn't be any sag whether you ran regulated or unregulated. But having no actual hands on experience with Class A SE amps I could be wrong.

My amp is cathode biased push-pull. The sag seemed to be mostly related to the limited current available from the 12 volt battery. I purposely wanted a push-pull output because it's a better use of available supply current.

[imaradiostar]

Then again on a true Class A amp current draw SHOULD BE constant from idle to full output power anyway since the power supply itself sees a constant load from idle to full power output so you'd think there wouldn't be any sag whether you ran regulated or unregulated. But having no actual hands on experience with Class A SE amps I could be wrong.

I thought I'd comment since I was at school before and didn't have much time- you're totally right, a class A amp shouldn't really sag. In my experience a cathode biased amp (single ended or push-pull) will often show an increase of voltage at the plate supply when heavily overdriven- I guess because of the average current reducing. Instead of two tubes biased at 70% all the time you get two tubes at 100% for half of each cycle- an overall lower current requirement. In a single ended amp it might look like one tube at 90 or 100% dissipation at idle. When heavily overdriven it might look more like full current only 50% of the time which usually amounts to overdissipation for half a cycle and being switched off for a good portion of the cycle.

I have no idea if the EMF of the speaker plays into this effect or not but some claim that it does. Kinda like an RF transmitter with a tuned output section or something like that.

jamie