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Chepo 4th June 2009 08:05 PM

Figuring out how many watts I need

I've wanted to delve into the world of tube amps for a long time now and since I'm making a decent wage for a change this summer I'll have the time and money to do it. It also helps that the previous tenant before I moved into my apartment left a bunch of tubes. Right now I'm looking to replace my 100w sherwood stereo amp with a tube amp. Now from what I've read chances are that I'm not using anywhere near 100w in my small living room. I figure finding out how many watts I'm actually using would be a good place to start.
At the absolute loudest the amp is at 35 (assuming the number the volume control gives means anything). At this volume the 16in woofers (big old speakers I found at a yard sale. All I know is fisher made them) in the speakers are moving A LOT and there is a nice breeze coming from the port. Its as bout as loud as most people can comfortably take.
At 40 is when your ears hurt. Movies and music are usually played at 20~25. Games 25~30. Call of Duty 5 is played at 35 (makes it feel like you are actually there...kinda...minus the part where you're not actually dying...).

I have no idea how sensitive the speakers are but given their age I guessing not good but not bad. If you know of a way to measure the output please share.

dsavitsk 4th June 2009 08:30 PM

There are a number of ways to measure. The easiest, and cheapest, is to download a 60Hz tone and play it at the loudest volume at which you'll listen. Put a DMM set to AC volts across the speaker terminal and measure the voltage. Then, using the impedance of the speakers, you can get an approximation of the watts you need.

Assume your speakers are 4 Ohm and you measure 10VAC. Then, using V=IR, the current draw would be ~ 10/4 = 2.5A. Watts = VI, which in this case would be 10 * 2.5 = ~25W.

This only gives you a rough estimate, and there are lots of other parameters that are important in determining whether particular amps and particular speakers work well together, but it should get you started.

cuibono 4th June 2009 08:47 PM

I've been thinking about using one of these to try and measure something similar. Might work well...

rman 5th June 2009 12:43 AM


Looks like that thing will tell you how many watts your amp is pulling out of the wall socket, which has absolutely nothing to do with how many watts it is pushing into your speakers.

whitelabrat 5th June 2009 02:35 AM

I think it may be more worthy to consider that in the world of tubes, one of the greatest limitations would be how much transformer can you afford. :)

I think that anything beyond the ballpark of 30w begins to get a bit expensive where you'll need big beefy transformers for the job. There are folks here who are saavy enough to use unpopular tubes for audio which can save some money.

A Push Pull EL84 amp can provide modest volume levels without blowing your budget. An EL34 Push Pull amp may give you around 35 watts which you may find satisfactory. A KT88 amp will get you around 60ish watts and clean your wallet out good.

Consider this though. To get twice the preceived volume level you will need 10 time the power to get there. For example, if you want twice the output of a 10watt amplifier, you'll need 100w. So really I'd say a 15w to 30w design would just about cover most listening needs. It just won't take you to "my amp goes to 11" though.

A good first step would be to determine how efficient your speakers are. Get the model number of those Fishers and google 'em to see if you can find the specs on them.

ChrisA 8th June 2009 09:12 PM

Re: Figuring out how many watts I need

Originally posted by Chepo

I've wanted to delve into the world of tube amps for a long time now and since I'm making a decent wage for a change this summer I'll have the time and money to do it. It also helps that the previous tenant before I moved into my apartment left a bunch of tubes. Right now I'm looking to replace my 100w sherwood stereo amp
I'm just getting started too. Here are some ideas..

1) You do not need as many wats with tunes as with SS. The reason is that with SS you never want to drive the amp into distortion as it will clip. But tube amps don't sound so bad when driven hard. So you can use all 30 wats of a 30 watt tube amp but you'd never use all 100W of a 100W SS amp.

2) start SMALL. Build a low power amp. You need to develop skills in building and debugging and don't want to have to learn on a complex amp. You will also need to collect some tools and test equipment.

3) By FAR the most expensive part of a tube amp are the transformers. You need a power transformers that converts AC mains to 400 or 600 volts or even more for a large amp. (These are lethal voltages, be careful.) You will also need an output transformer for each channel. For a small first amp expect to pay about $60 each ($60 x 3 = $180) Larger transformers can run $150 to $250 each.

The the characteristics of a tube amp are defined by the transformers. Basically all the amp does is this: (a) step up mains power to high voltage, (2) control high voltage current with a valve, (3) step down high voltage. Buying new transformers for a tube amp cost double to price of a new solid state amp.

If you are cheap or just like building stuff by hand. You can salvage transformers from microwave ovens and re-wind the secondary. Or do what I just did and look in Crags list for a broken tube guitar amp. Buy them for "cheap" and salvage the parts. it's about 1/10 the price of new.

If you are learning don't learn on expensive new transformers, use "salvage" parts.

4) You built this for education and experiance and because you want a unique product. Not to save money. It's like fishing. It's cheaper to buy fish at the super market then to make a trip to the lake, but you don't go to the lake to save money.

m6tt 9th June 2009 01:43 AM

Just guessing, but having heard a number of "100w" solid state amps, you will probably be more than satisfied with something like 6L6 or EL34 push pull, around 30-50 watts. Most solid state amps (especially more recent ones) aren't really rated RMS anyway. 100w of EL34 is audible *outside* my house and makes my roommates unhappy near clip. 6L6/el34 p-p will be able to be heard clearly over a small party or gathering, el84/6v6 might* be straining.

For what it's worth, I've seen a number of solid state amps that claim to output more power than they draw from the wall, and many that are very close. This is impossible or unlikely respectively, except with Class D. Frequently 50w per channel is listed as 100w on the amp as well, which would be in the 6L6/EL34 ballpark.

*I guess it depends on how loud your friends talk when they start drinking wine, assuming standard inefficient speakers as well.

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