Am I measuring the output wattage correctly?

[King Fan]

I like math and all, but I love skipping it even more. If we leave out all the differences between MkI and MkII that *don't* affect output power, what's left? The PTs, obviously. Are the OTs the same? What else?

 

[joulupukki]

I like math and all, but I love skipping it even more. If we leave out all the differences between MkI and MkII that *don't* affect output power, what's left? The PTs, obviously. Are the OTs the same? What else?

The OTs are the same specs. Both from Musical Power Supplies. One is the vertical (in the Mk I) and one the horizontal (in the Mk II) flavor. The chokes have slightly different specs. The Mk I sports a 4H @ 50mA from Musical Power Supplies. The Mk II sports the one from Mojotone, 4.8H @ 75mA.

All other components are of the same value, just different brands/quality. Does this help?

 

[Bendyha]

 

[King Fan]

The OTs are the same specs. Both from Musical Power Supplies. One is the vertical (in the Mk I) and one the horizontal (in the Mk II) flavor. The chokes have slightly different specs. The Mk I sports a 4H @ 50mA from Musical Power Supplies. The Mk II sports the one from Mojotone, 4.8H @ 75mA.

All other components are of the same value, just different brands/quality. Does this help?

Thanks. I guess I'm lazily wondering how hard it is to compare their estimated/predicted rather than measured power output. I've never understood a) what amp-spec variables affect power production (OK, plate voltages? tube type, duh? some OT spec and impedance? what else?) and b) how far off predicted/estimated can be from measured.

OTOH, our wise friend @Bendyha seems to be showing me another great way to skip some math -- graphical analysis is the bomb. Is there a question above about whether your voltmeter is doing RMS correction or not?

Anyhow, given my only qualification for doing this is that I'm being lazy, if I put your ~11 and ~13V values on that chart at 8Ω, I get wattages of roughly 15 and 20W. Sorry, that's probably hilariously wrong -- I"m just trying to figure out "what we might expect."

 

[peteb]

The TRRI schematic calls out:

85 watts minimum RMS into a 4 ohm load at 5% THD.

https://schematicheaven.net/fenderamps/65_twin_reverb_%20manual.pdf

 

[bebopbrain]

a question above about whether your voltmeter is doing RMS correction or not?

For sine waves correction is not needed. Any cheapo voltmeter will report the correct sine wave RMS value.

 

[tele_savales]

You first should convert peak to peak voltage to VRMS, then square it and divide by the load resistance.
RMS is .707 of the PEAK Voltage, not peak to peak. So half the peak to peak voltage to get peak, THEN multiply by .707 to get VRMS, square it and divide by the load.

Luckily most $350 digital scopes will show you all of that right on the display, so you can go right to rms squared divided by the load. I've done this on the last 4 or 5 amps that have been on my bench including a 1970 Bassman (right around 45watts, just like it says on the back of the amp) my 83 JCM 800 w EL34's (a tad over 50 watts at clipping), and my 5E3 build which despite its meager wattage is not remotely "dime-able" in my apartment- BUT- I A/B'd it right next to the JCM 800 and it was half as loud to my ears.

Also recently the Laub amp I've been working on with 2x 7868 tubes, fixed bias, which are rated at 19 watts each. I measured it at around 22 watts.

At any rate, logic and intuition tells me any amp w 4 output tubes, whether they're 6L6s, EL34s, or 6V6s should have twice the output wattage at the speaker than something w 2 tubes. Why else would anyone build them? Or use 4 KT88's like in a Marshall Major, rated at 200 Watts.

 

[corliss1]

And, if in fact Twin Reverbs were only <insert some wattage number here> it would be *extremely* well known and this thread wouldn't be the first we're hearing about it It would then follow that every other Fender amp ever created also has the wrong wattage listed on the back.

 

[tubedude]

And, if in fact Twin Reverbs were only <insert some wattage number here> it would be *extremely* well known and this thread wouldn't be the first we're hearing about it It would then follow that every other Fender amp ever created also has the wrong wattage

Luckily most $350 digital scopes will show you all of that right on the display, so you can go right to rms squared divided by the load. I've done this on the last 4 or 5 amps that have been on my bench including a 1970 Bassman (right around 45watts, just like it says on the back of the amp) my 83 JCM 800 w EL34's (a tad over 50 watts at clipping), and my 5E3 build which despite its meager wattage is not remotely "dime-able" in my apartment- BUT- I A/B'd it right next to the JCM 800 and it was half as loud to my ears.

Also recently the Laub amp I've been working on with 2x 7868 tubes, fixed bias, which are rated at 19 watts each. I measured it at around 22 watts.

At any rate, logic and intuition tells me any amp w 4 output tubes, whether they're 6L6s, EL34s, or 6V6s should have twice the output wattage at the speaker than something w 2 tubes. Why else would anyone build them? Or use 4 KT88's like in a Marshall Major, rated at 200 Watts.

More tubes of the same type do mean more power, all else being equal, but intuition fails here as certainly not any amp with four tubes will have twice the power of any amp with two.
Two KT-88's will provide way more power than 4 6V6's. And double the wattage doesn't mean double the SPL. The real benefit of more power is greater headroom. To double the volume of a 50W amp you need 500W.
The well regarded Kelley amps used 4 6V6's instead of 2 6L6's. They generate roughly the same power, but he preferred the tone and dynamic response with the 6V6's.
As an aside, the speaker efficiency is a great variable in the mix. If anyone feels their amp is too loud, or not loud enough, selecting speakers with different efficiency ratings will help. Speakers are available from mid 80's decibels to 105dB.

 

[joulupukki]

As an aside, the speaker efficiency is a great variable in the mix. If anyone feels their amp is too loud, or not loud enough, selecting speakers with different efficiency ratings will help. Speakers are available from mid 80's decibels to 105dB.

I can attest to that, for sure. This same amp is drastically louder with the 70W Mojotone Greyhound 12" (Sensitivity: 101.3dB) vs the 60W Celestion Creamback Neo (Sensitivity: 97dB). Both of these speakers are an excellent pairing with these amps.

As an aside to this aside ... the Greyhound has a lot more bass response and less high trebles vs. the Creamback Neo. The Neo performed extremely well the other day here in a jam with a bass, another guitar, and drums. I didn't hook up the Greyhound in that night, but I have no doubt it would have easily held its own.

 

[joulupukki]

I built a switchable resistive load today using some 50W wire wound resistors and Aiken’s design. Not sure why the Mk I output seems so low on the 4 Ohm setting. But here are some measurements I took with the scope and a 1kHz sine wave:

 

[2L man]

My quess is that OT secondary coil outputs are connect wrong? Easiest is first to compare that clean signal anode voltage sweeps are about the same. If they are then measure OT winding ratio.

If you have any transformer you can feed its output to power tubes anode sockets and measure all AC voltages. Three primary voltages which two are CT to anode. OT secondary which has 4, 8 and 16 output there come six measures. Then you know what the OT truly is

You can feed filament AC if you cut HV hot out of circuit.

I always measure OT winding ratios before installing because manufacturing is prone to mistakes and it take only about five minutes.

 

[joulupukki]

Ah, yup. Looking though my pictures of the amp it looks like I have the 4 Ohm and 16 Ohm taps on the wrong lugs of the switch! I’ll get that taken care of and re-measure. Thanks for that idea @2L man!

 

[joulupukki]

Ok, after a re-wire and re-test, this looks a lot better (thanks again @2L man for the tip):

The difference in power between the two amps is because of the power transformers. Everything else about them is the same (circuit-wise).

 

[2L man]

If you did just use the same input signal amplitude they are very close each other. Turn powers to db and the difference is very small.

Mechanism for MKII 8 ohms higher output voltage interest me if you have energy to ponder it?

 

[joulupukki]

The main reason for the MK II's higher output is because it uses a more beefy power transformer, the Deluxe Reverb PT from Mojotone which is giving a plate voltage of about 450V compared with 410V on the Mk I.