Leslie 147

[JAelec]

Hi,

I have a Leslie 147 power amp on the bench which supposedly has too low volume and distorts too much. It is normally used with a transistor organ of some kind and usually takes line level signals well according to the user. The amp has been fitted with a tele connection at some point instead of the original 6-pin connector

I think the hammond tone wheel organs that this Leslie was designed for has a quite hot output: 4 to 6 Volts peak to peak.

The amp has a "console load resistor" switch which was set at 16 Ohms. After switching this to "open" it sounds a bit louder. I tried hooking up a guitar through a preamp and it was now quite ok, but I'm not sure if it should be even louder.

I found this guide what voltages to check:
https://bentonelectronics.com/servic...147-amplifier/

Schematic is here:
https://bentonelectronics.com/servic...147-amplifier/

Most voltages were higher then they should according to the schematic


Voltages tested:
(Voltages from schematic or above guide in parentheses)

Cathode voltage for 6650 tubes

150 Ohm Resistor to ground 28.8V (25V)

Power supply

Rectifier, before choke 473V (430V)

After choke 457V (420)

Regulator

After OC3 regulator 341V (310V)

After 10k 1W resistor 297V (260V)


Plate and Screen voltages on 6650 tubes

Plate voltage pin 3 of 6650 tube 443V (415V)

Pin 3 to 4 on 6650 tube 110V (105V)


The standout issue was this:

12AT7 plate voltages (should be 115V and maximum 10V difference according to service guide linked above)

Plate voltage pin 1: 200 V
Plate voltage pin 6: 135 V

200-135 = 65V differance!


I'm not sure why this is bad and what effects it could have on sound.
Could it be that the plate resistors are beginning to go bad?

Do the other voltages look ok?

If i want to check the amplification level from input to speaker output, how do I do that with a signal generator? What level should I feed and what should I get at the output for it to be considered a functioning amp?

Thanks for your input!

[Helmholtz]

12AT7 plate voltages (should be 115V and maximum 10V difference according to service guide linked above)

Do you mean the 12AU7?
A 12AT7 in place of a 12AU7 will have different plate voltages.

[Helmholtz]

To measure output power connect a 16R load resistor rated for at least 50W to the outer ends of the OT secondary.
Neither side of the load and secondary must be grounded (only the secondary CT).
Connect your scope probe to one side of the secondary, probe ground to amp ground (not across the load!).
Feed a 400Hz sine to the input and increase level until you see first signs of clipping.
Connect your DMM across the load resistor and measure rms output voltage.

[g1]

Did you try a different tube where you have the imbalanced plate voltages? Did you check the 56K plate resistors?

[TomCarlos]

JAelec....

You have the Benton page- great. Terry Dayton of D-Lab also has a video you might want to bookmark for reference - Vintage Leslie 147 Tube amplifier power supply voltage level adjustment 6550 bias.

[JAelec]

Did you try a different tube where you have the imbalanced plate voltages? Did you check the 56K plate resistors?

I swapped the 12AT7 ... and now the plate voltages are:
183.5 - pin 1
180.5 - pin 2

3V difference, all good!

Tested the other voltages again and now get completely different readings:

(New reading in bold, old reading in italic, Schematic value in parentheses)


Cathode voltage for 6650 tubes

150 Ohm Resistor to ground 26V, 28.8V (25V)

Power supply

Rectifier, before choke 437 V, 473V (430V)

After choke 423 V, 457V (420)

Regulator

After OC3 regulator 309 V, 341V (310V)

After 10k 1W resistor 275 V, 297V (260V)


Plate and Screen voltages on 6650 tubes

Plate voltage pin 3 of 6650 tube 414 V, 443V (415V)

Pin 3 to 4 on 6650 tube 106 V, 110V (105V)



Tested swapping back the old 12AT7 as a long shot, but voltages were still good (except for the plate voltages on the old 12AT7, they were still off 185V and 122V each)

So good thing that the faulty 12AT7 was found and plate voltages resolved

But bad that I don't know why my other voltage readings were off by so much before...


I tested the Leslie with the transistor organ that it is supposed to connect to:

It sounds louder, like it should with the new tube but still distorted.

A 12AT7 has louder output then a 12AU7 like in the schematic, could it be that this is making it distort?

To measure output power connect a 16R load resistor rated for at least 50W to the outer ends of the OT secondary.
Neither side of the load and secondary must be grounded (only the secondary CT).
Connect your scope probe to one side of the secondary, probe ground to amp ground (not across the load!).
Feed a 400Hz sine to the input and increase level until you see first signs of clipping.
Connect your DMM across the load resistor and measure rms output voltage.

I disconnected the speaker leads and measured the speakers (both the big one and the horns connected together). Speakers measure 10.3 Ohms. I think the big speaker is supposed to be 16Ohms and I think the speakers together should read 10 Ohms like in my reading but cant seem to find the source of that info right now.

So shouldn't I connect a 10 Ohm 50W resistor then, not 16 Ohms?

What would knowing the first signs of clipping tell me, what should I look for? When is the signal supposed to clip, I mean what would you expect from a normally functioning amp in your experience Helmoltz?

[Helmholtz]

High PI plate voltages mean low plate currents and reduced headroom.
As the amp seems to be designed for a 12AU7, you should use one.
A 12AT7 needs to be biased differently.

Scoping will show if there's another problem.

[JAelec]

High PI plate voltages mean low plate currents and reduced headroom.
As the amp seems to be designed for a 12AU7, you should use one.
A 12AT7 needs to be biased differently.

Scoping will show if there's another problem.

So the plate voltages become higher when a 12AT7 is substituted because the bias was set for a 12AU7?

[Helmholtz]

So the plate voltages become higher when a 12AT7 is substituted because the bias was set for a 12AU7?

Yes.
A 12AT7 requires a lower value cathode resistor for same plate current.

[JAelec]

Helmholtz, about the speaker impedance... I see now that the schematic says "To dividing network Z=16 ohms". So that would mean that the amp is expecting a 16 Ohm load right?


This site says the overall impedance of the speaker system should be 16 Ohm.
http://www.highontechnology.tech/201...er-system.html

Bentonelectronics says the 15" Woofer is 16 Ohms but does not mention the smaller speakers impedance.
https://bentonelectronics.com/servic...147-amplifier/

This site...
http://theatreorgans.com/hammond/faq...y/mystery.html

...Says that "The stock Leslie crossover is a 12 dB per octave, 800 Hz unit requiring both 16-ohm high- and low-frequency drivers for correct performance. Most "mondo-power" 15-inch woofers have an impedance of 8 ohms, which tends to extend the reponse a bit higher than 800 Hz. Although this shouldn't cause any difficulties, it will make an 8-ohm high-frequency driver extend a bit lower, which will definitely cause excursion problems. The answer is to either buy a 16-ohm replacement driver, or perform the following modifications. "


Why am I only measuring 10.3 Ohms? A 16 ohm speaker should measure between 12-14 Ohms on a multimeter. I realize now that by measuring the terminals connecting to the amp I am measuring before the crossover, don't know if that matters? Isn't 10.3 ohms a bit low?

Edit: The original speakers for the 147 should be a Jensen V21 driver 16 ohm for the highs and a Leslie/Utah 15" 16 Ohm speaker for the lows.

Still don't understand how two 16 Ohm speaker would be wired to get 16 Ohms as required by the amp.

[Greg Robinson]

DC resistance (what you are measuring with your multimeter) ≠ impedance.
The passive crossover network for the woofer/tweeter maintain the nominal impedance at all frequencies - at lower frequencies the high-pass to the tweeter shows an increasingly high impedance in series with the tweeter, and at higher frequencies the low-pass to the woofer shows an increasinly high impedance to it. So the low and high bands only ever "see" the woofer and tweeter respectively, maintaining the nominal impedance throughout the whole audio range.
The DC readings that you are measuring are fairly typical, being 60-80% of nominal impedance.

[Helmholtz]

This ^^^^.

Helmholtz, about the speaker impedance... I see now that the schematic says "To dividing network Z=16 ohms". So that would mean that the amp is expecting a 16 Ohm load right?

Don't overthink things.
As nominal load impedance is 16 Ohm, the test load should be 16R.
But using a 12R resistor wouldn't make much difference.

Scoping will show if output is reasonable and the signal looks good.
You could also determine gain.
Please exactly follow the instructions of post #3.

[JAelec]

Sorry I did not see that there were new posts until now... Thanks for picking up this thread again, I'm very grateful for you help and knowledge!

I see about the speaker impedance... thanks for explaining Greg Robinson!

Now to the amp test...

I have connected a 16R50W resistor across the output terminals. My sig generator can only output 200mVpp so I have a bass amp with a line out on the back. Using that as a preamp I'm able to get a maximum of 2Vpp from the line out. Outputting more then 2Vpp gives me distortion in the signal coming from the preamp so that's how far the preamp can go without effecting the measurements.


I have attached some pictures:

Picture 1:

Shows unity gain. External bass preamp is outputting 2Vpp into the input of the 147 and with the amp volume pot at 66% there is unity gain between preamp and power amp output. Volume pot measures 1.62 kOhms between cold lug and wiper when at this position. Total pot resistance is 9.29kOhms. Resistances were measured in circuit with probes and preamp disconnected.

I tried measuring with the preamp connected but then I would get close to zero ohms between cold and hot lug, why is that? ...Just curious. Perhaps the preamp has an output transformer on the line out to balance the signal?




Picture 2:

Lower sine is preamp at 2Vpp. Upper sine is power amp output into the 16R load. Here we get 12 Vpp. No distortion yet even at maximum volume. When connecting a guitar to the bass preamp there is considerable distortion




To take the test further to find the clipping point I think I need a stronger signal then 2Vpp. I have one more of these bass preamps..(TC electronic RH750 is the model).
Can I daisy chain them together or would it hurt the input of the bass amp to get 2Vpp into the input of another bass amp?

I think a hammond preamp can output 4-6 Vpp with the pedal floored. That's double of what my preamp can output.
Reference: http://www.dairiki.org/HammondWiki/P...evel?version=2

[Helmholtz]

First of all your signals look perfect, no signs of a problem. (Scope settings indicate a frequency of 100Hz?)

To measure gain the 10k volume pot should be fully up.
Gain is found from Vout/Vin, where Vout is twice the Vpp of one secondary half (where the scope should be connected) and Vin is the Vpp at the vol pot wiper.

I tried measuring with the preamp connected but then I would get close to zero ohms between cold and hot lug, why is that? ...Just curious. Perhaps the preamp has an output transformer on the line out to balance the signal?

What preamp are you speaking of ? Is there a schematic?

Yes, for full power you will need several volts at the 147 input.
You can't wire 2 preamps in series for more signal as this would result in shorting one of the outputs.

[JAelec]

Scope is set on 0.5ms/div so 400hz. It is hard to see on the picture because of the angle plus it is a bit blurry so the dot is easily missed. But thanks for pointing out any possible issues!

Ok so to calculate the gain of the amp:

Vin = 2Vpp
Vout = 2*12Vpp = 24 Vpp

Gain is Vout/Vin = 24/2 = 12



I don't have a schematic for the preamp but I see now in the manual that the line driver on the back is transformer balanced, perhaps the preamp output is connected to this transformer as well.





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? What is a normal tube preamp output level? I think it is designed to receive 4 to 6.5 Vpp from a Hammond AO-29 pre amp. Perhaps if I could get up to those levels I could see some distortion in the sine?

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?