50w seems louder than 100w

[philbard]

Hey All,

So I’ve just finished my first 102 build from scratch, a 100w model. Comparing it to the CTone OTS 50w I finished in December, which was my first build, the 50w is louder. This with all pot settings matched, identical tubes in all the positions and using the same speaker. I know some folks have said there isn’t as big a difference between the two as the numbers 100 and 50 would suggest, but I’d still expect the 100w to be a more powerful amp. They both sound great, and very close tonally, I’m not unhappy at all but curious. I’m wondering if I don’t have something quite right.

Both amps are biased to around 55%. The CTone uses their proprietary transformers, I’ve got Magnetic Components in the 100w. Ceriatone’s PT output is 690v and the Magnetic Components is 640v, but plate voltages are similar on the power tubes, in the 440 to 450v range. Preamp tube plates are 250ish on the 100w and 200ish on the 50w, both amp’s PI plates are around 290v. I’ve got the same pots in all positions, so taper is not involved, since I’m home I don’t crank it beyond moderate volumes. With the Volume at 4 on both amps, the loudness matches when the Master Volume settings on the 100w is at 3 and on the 50w at 2. So the 50w actually seems more powerful. Both amps behave the same when OD is on.

I’ve rolled the tubes and all are working fine. Looking at the schematics there are a few differences in components beyond the trannies and the number of power tubes. Here is what I see….

V1A grid stopper: 50w 22K, 100w 33K (Jumpered a 68K in parallel to the 33K on the 100 and it made no difference in volume)
PI plates: 50w 110K and 100K, 100w 120K and 110K.
6L6 Screens: 50w 1K, 100w 470R
6L6 grid stopper: 50w 3.3K, 100w 5.1K

I’ve done a little poking around with an oscilloscope inputing a 1v RMS 100Hz signal, preamp seems to be working OK, but with the scope I have I can’t read very high signal voltages so don’t know what they are off the PI into the power section. I suppose its conceivable only half the power tubes are actually amplifying signals, but all connections seem solid. Any suggestions on how to determine if there are problems in the power section are welcome.

What am I missing here, if anything?

[pdf64]

Have you actually measured the power outputs?
I suspect you may be confusing a system’s gain with its max power output.
The pots we use are cheap and cheerful, certainly not precision devices, their tapers are typically untoleranced, ie they could be anything, such that a pot labelled as audio could have a taper closer to linear and visa versa, yet both would still be within spec.

[Chris Brown]

My 100 watt 102 is much louder, and also cleaner at a higher volume than my 50 watt. They both get really loud. I'm assuming you are saying the volumes are on 4,2, and 3, and not "oclock". Those are really low volume settings to compare the two amps. I think you might find that when you get the master up past half way that the 100 watt is cleaner and louder than the 50, or at least that's what I would expect.

[philbard]

Have you actually measured the power outputs?
I suspect you may be confusing a system’s gain with its max power output.
The pots we use are cheap and cheerful, certainly not precision devices, their tapers are typically untoleranced, ie they could be anything, such that a pot labelled as audio could have a taper closer to linear and visa versa, yet both would still be within spec.

Do you mean the output voltages of the pots at same settings for the wiper? That makes sense...

[philbard]

My 100 watt 102 is much louder, and also cleaner at a higher volume than my 50 watt. They both get really loud. I'm assuming you are saying the volumes are on 4,2, and 3, and not "oclock". Those are really low volume settings to compare the two amps. I think you might find that when you get the master up past half way that the 100 watt is cleaner and louder than the 50, or at least that's what I would expect.

Settings are by the numbers, not the o'clock positions. I'll have to check higher volumes when I get them alongside each other shortly.

[xtian]

You simply cannot rely on "settings." Measure the clean output power of both amps.

[philbard]

You simply cannot rely on "settings." Measure the clean output power of both amps.

Exactly how would that be done? Excuse the fact that I'm still fairly new at this stuff....

[pdf64]

Have you actually measured the power outputs?
I suspect you may be confusing a system’s gain with its max power output.
The pots we use are cheap and cheerful, certainly not precision devices, their tapers are typically untoleranced, ie they could be anything, such that a pot labelled as audio could have a taper closer to linear and visa versa, yet both would still be within spec.

Do you mean the output voltages of the pots at same settings for the wiper? That makes sense...

Yes, that’s kinda the definition of taper

[philbard]

Have you actually measured the power outputs?
I suspect you may be confusing a system’s gain with its max power output.
The pots we use are cheap and cheerful, certainly not precision devices, their tapers are typically untoleranced, ie they could be anything, such that a pot labelled as audio could have a taper closer to linear and visa versa, yet both would still be within spec.

Do you mean the output voltages of the pots at same settings for the wiper? That makes sense...

Yes, that’s kinda the definition of taper

I do understand taper, was just thinking out loud. Thx!

[xtian]

To measure output wattage, you need a signal generator (can be mobile phone), scope, and resistive load. Should I continue?

[talbany]

Imo..This why it's imperative that you measure and check every single component and know all the pot tapers before installing them. (especially volume and gain)

If I took an amp and switched back and forth between the 50 and 100w settings The one thing you will notice in the 100W should be a slight increase in low mid content or a slightly thicker bass (for a lack of a better word ). You should be able to notice a small increase in volume levels.The 100 should of course have more headroom so the 50w should enter output tube distortion earlier..If you are only going by your ears only then the difference in volume between a 50w and a 100w should only be around 3 dB or barely audible
The other thing to consider is frequency response!. Since the most sensitive part of our hearing is around 1k area an amp that is heavy in mids (500hz) will always seem louder than one that is scooped so your tone stack and tone capacitors can play a role in frequency response.

The only way to positively confirm the problem is in the output section is with a signal generator and scope shoved in the effx loop access points and watch for just below clipping..The other thing you can do is take the signal out from the 50 and run it into the return of the 100 and see if the volume levels come up..If they do then you know it's in the preamp section.. (I would still keep a load on the 50)

BTW..I make sure and measure each component including output transformers and and still have slight variations in volume levels at the exact same settings amp to amp
BTW#2. A 35% taper will act similarly to a linear pot and the volume will come up much faster with this type of taper than say a 15% taper which will be slower
Good Luck!
Tony

[Reeltarded]

match all settings by ear and compare control positions

[philbard]

OK, after pdf64's suggestion about checking output voltages at the volume and gain pots, it looks like they are the reasons. Both the volume and gain pots on the Ceriatone (1M Audio Alpha) are outputting much higher voltage at 9 o'clock and noon wiper positions than the ones in the 100w self build (1M Audio CTS). This would explain it. Both sets of pots are consistent within the brand from what I can see, it sounds like their definitions of "Audio" are perhaps different(?).

Tony, I did know to measure all components before installing, even when building my first amp, but didn't apply that to the pots. Live and learn and leave it to you guys to supply the proper tests. Appreciated.

I won't subject my neighbors to any tests at high volume right now, that will have to wait until I get it out somewhere that I can crank it, but I have a Dumblator and could probably use that to get a sense of the amps sounds at higher volume settings.

[Chris Brown]

Even if you measured the pots it might just be the taper... they aren't always exactly the same. 20% tolerance allows for a lot of variances from part to part. I think at low volume 50 or 100 watt versions of the same circuit will sound about the same depending on where you set the controls. My 100 watt 102 has a bit more low end in a beautiful way than my 50 watt 102, even at low volume, but I can dial them to sound exactly the same. When you get master up past halfway, high enough to start pushing the power section is when you can really tell the difference in headroom...

[philbard]

Even if you measured the pots it might just be the taper... they aren't always exactly the same. 20% tolerance allows for a lot of variances from part to part. I think at low volume 50 or 100 watt versions of the same circuit will sound about the same depending on where you set the controls. My 100 watt 102 has a bit more low end in a beautiful way than my 50 watt 102, even at low volume, but I can dial them to sound exactly the same. When you get master up past halfway, high enough to start pushing the power section is when you can really tell the difference in headroom...

Yeah according to my voltage measurements it seems to have something to do with the way Alpha and CTS define audio taper. I get that there may be part to part variations as with most components.

My 2 amps are also remarkably similar tonally at lower volumes, the edge may go to the 100w, but not by much. Just tried them both cranked to 3 o'clock on volume and gain through a Dumblator (did not touch the settings on it between amps), and surprisingly the amount of gain from the 2 amps is similar at high volumes. However, there was no comparison on the headroom, the 100w won hands down.