Leslie 147

[Helmholtz]

Gain is Vout/Vin = 24/2 = 12

That's in the ballpark of power amps at the 16 Ohm tap.

What I don't understand is... Isn't a tube amp supposed to distort at least some bit at maximum volume? Or is it just that 2V is not enough to distort this power amp?

With a 2Vpp input signal you won't get full output power.
I dunno what the rated output of the amp is, but assuming it's 40W you would need an input signal of 6Vpp (or 2.1Vrms) for full output before clipping.

And I don't get why I got distortion from the amp when playing guitar into the TC electronic preamp+Leslie power amp... I didn't have to max the TC preamp to get distortion. But maxing the preamp with the sig generator give no distortion... perhaps I'm missing something

Could it be that the speaker is the thing that distorts?

Separate preamp and line out signal levels often are limited to around 1Vrms or 2.8Vpp, so not enough to drive your 147 power amp.to full power.
Nevertheless they can distort without increasing output Vpp with a typical guitar or bass.

The preamp of a guitar amp can provide a much larger signal to the PI of the power amp.

I don't think the speaker is distorting.

How do you want to use the 147?

[JAelec]

--"Gain is Vout/Vin = 24/2 = 12"

That's in the ballpark of power amps at the 16 Ohm tap.

Is this a general principle? That a certain Ohm tap has a certain gain independent of power amplifier design? So my Peavey Classic 30 that has a 16Ohm speaker has the same gain value in the power amp?

With a 2Vpp input signal you won't get full output power.
I dunno what the rated output of the amp is, but assuming it's 40W you would need an input signal of 6Vpp (or 2.1Vrms) for full output before clipping.

Yes it's rated at 40W... how did you know the rating without checking the specs?

And how did you know that it would need a 6Vpp signal for full output before clipping.

Separate preamp and line out signal levels often are limited to around 1Vrms or 2.8Vpp, so not enough to drive your 147 power amp.to full power.
Nevertheless they can distort without increasing output Vpp with a typical guitar or bass.

You mean that the preamp can distort and then send along the distorted signal to the power amp... so there is distortion but the output Vpp of the preamp is still the same?

The preamp of a guitar amp can provide a much larger signal to the PI of the power amp.
How do you want to use the 147?

Ok, so perhaps I need a guitar preamp in front of the PI of the power amp.

It is intended to be used with different instruments in a studio... so for example:
-Nord Stage 3
-Transistor organ
-Guitar

I don't think the speaker is distorting.

I tried connecting two 8ohm speakers in series from a Marshall combo amp. No distortion. I tried changing the speaker to another speaker from a Leslie 760 and fitted it into the 147. Not sure but I think no distortion, trying with a Nord piano. Then I tried with a guitar and the TC electronics preamp... some distortion. Hmm...

I tried disassembling the original speaker and cleaning the around the magnet where the voice coil sits, and also the voice coil. I tried to align the voice coil but get buzzing when screwing the assembly together.

I'm going to order new caps for the crossover filter and see if that could be the problem. Saw on a YouTube video that the crossover caps can drift and let through more bass to the tweeter causing distortion.

[Helmholtz]

Is this a general principle? That a certain Ohm tap has a certain gain independent of power amplifier design? So my Peavey Classic 30 that has a 16Ohm speaker has the same gain value in the power amp?

No rule.
I've seen power amp gains between 10 and 30 depending on circuit and amount of NFB.

Yes it's rated at 40W... how did you know the rating without checking the specs?

Just guessing from power tube type and B+.

And how did you know that it would need a 6Vpp signal for full output before clipping.

For a 40W output into 16R it takes 25.3Vrms or 71.3Vpp.
Divided by your gain gives 5.94Vpp for input signal.

You mean that the preamp can distort and then send along the distorted signal to the power amp... so there is distortion but the output Vpp of the preamp is still the same?

Yes, that's what I think. The output voltage Vpp is limited by its circuit and supply voltage irrespective of how much gain is applied before the output stage.

[Helmholtz]

Any chance to add a tube gain stage?

[JAelec]

For a 40W output into 16R it takes 25.3Vrms or 71.3Vpp.
Divided by your gain gives 5.94Vpp for input signal.

Ok, thanks!

I'll write it out a little more detailed for future reference:

P = V² / R
40 = V² / 16
640 = V²
V= √ 640
V= 25.3

So we get the answer in VRMS

Convert to PP:
25.3 * 2√2 = 25.3 * 2.828 = 71.6 Vpp

Input signal is:

Vout / Vin = 12
71.6 / Vin = 12
Vin = 71.6 / 12 = 5.96 Vpp

--"You mean that the preamp can distort and then send along the distorted signal to the power amp... so there is distortion but the output Vpp of the preamp is still the same?"

Yes, that's what I think. The output voltage Vpp is limited by its circuit and supply voltage irrespective of how much gain is applied before the output stage.

That's very interesting. And makes sense... clipping is limiting... I seem to forget that! When the input signal get to hot the amp start to clip witch can be perceived as a volume increase, but the peak voltage is the same... just the area of the amplitude getting larger.

And I did not know that the power rating of an amp is calculated before clipping. I kind of thought it had something to do with the volume it could push...

Any chance to add a tube gain stage?

I'll try to connect it to a tube gain stage and see if I can make it clip.

[Helmholtz]

Rated amp output power is always specified "before clipping".
Sound level depends on speaker sensitivity.
A speaker having a sensitivity of 97dB SPL per electrical watt requires twice the input power of a speaker having a sensitivity of 100dB SPL for the same sound pressure level (SPL).

[JAelec]

Thanks for your insights... I had forgotten about speaker sensitivity.
I find it strange that 1watt would yield 97dB SPL at 1m distance... 1W seems so little

I haven't gotten a chance to test the amp this week, but will do tests with a tube preamp in 1-2 weeks when I get access to the amp again. I'll recap the crossover then also and we'll see if the distortion improves