'50s Bell amp power output

[Randall]

I have restored two Bell Sound Systems Hi-Fi amps. They are running American RCA 6V6s. One measures 16v p-p and the other 18v p-p into 4 ohms. By my math of 16v p-p / 2 = 8v peak. 8v-p x .707 = 5.65v RMS. 5.65v x 5.65v / 4 ohms = 8 watts.

It seems that is quite low to me, but they both are acting pretty much the same, so I don't know. Can anyone shed any light on this? Do I have a problem or not?

[Enzo]

Probably not. They are not guitar amps, they don't drive the piss out of their tubes like we do.

And are you fully driving it? Are you feeding it through jack #3 up top right into the PI?

[Randall]

I tried all four inputs, the two before the pre tubes and the two after. The results were the same, I could drive it to clipping with any of them. That was with a sig gen, I am now listening to a Miles record going into the Mag1 inputs and it sounds good. Noise floor is kinda high on higher volume setting with no input, But maybe that is how Hi-Fi amps were back then?

[Enzo]

Oh, we lived with a lot more hum and noise than we would put up with today. And some hifis were more hifi than other hifis. HAving a crystal cartridge input limits how hifi it is going to be.

[Randall]

In this case it's a $1K Thorens turntable with a hot rod cartridge. Not mine, an vinyl junkie friend. But, it just doesn't seem loud enough to me. Most voltages are considerably higher than the schematic calls for, 145 on the pre plates for example, but the same for both amps. I have combed through this several times, and I just cannot find anything other than high plate voltages. For this I have been advised here previously not to worry about.

?

[Enzo]

You have the hot rod cartridge, but the amp includes an input designed for old crystal pickups. To me that is like going to a restaurant and finding a bottle of ketchup and mustard on the table. They might make a great burger, but it probably is not a gourmet feed trough.

And that brings up a thought, some modern high end pickups have pretty low level outputs, expecting a high gain preamp. Perhaps a lower end magnetic cartridge like on a typical DJ table might be hotter?

Run a line level music signal into the RADIO input, how does that sound?

Power supply? DO you have 620vCT on the PT secondary? Is the cathode of the 5Y3 way over 320vDC?

[Justin Thomas]

Might your high voltages be partially explained by the date on the schematic? Wall voltage wasn't the same in 1951 as it is now. This amp was probably expecting 110, and might be getting as much as 125. That'll make a hefty increase...

Justin

[Leo_Gnardo]

I tried all four inputs, the two before the pre tubes and the two after. The results were the same, I could drive it to clipping with any of them. That was with a sig gen, I am now listening to a Miles record going into the Mag1 inputs and it sounds good. Noise floor is kinda high on higher volume setting with no input, But maybe that is how Hi-Fi amps were back then?

I remember this project from a couple months ago. Some things I see: cartridge loading, for most modern moving magnet cartridges, is 47K. Both your mag 1 & mag 2 inputs load the cartridge with a lower resistance. Mag 1 is 47K paralleled with 303K, and mag 2 is 33K paralleled with 303K. Hows about you pick one input to be THE input, say Mag 1, and disconnect the 270K resistor. For now just lift one end of it. The end "nearest" the 47K. You should hear a bit more volume & clarity.

I think last time around we were examining cathode bypass caps on the triode preamps. Are we sure they're all up to snuff? Let's not trust half century + old caps. Cheap enough to replace so we're sure they're OK.

Noise with no input? What's the condition of that input when you're listening for noise? If left open, nothing plugged in, yes you should expect a fair amount of hum/buzz. With a shorting plug at the input does the noise drop? I'm sure it would. The cartridge coil looks a lot more like a short to the preamp input. Like guitar pickup coils, you have to keep magnetic cartridges away from stray AC magnetic fields.

Hot shot cartridge hmmmm.... what does that mean? It's expensive??? Has dazzling specifications? What's more important, is it a moving magnet cartridge, or moving coil? There's lots of moving magnet cart's, from $20 to hundreds, maybe thousands of $$, quite a large variation in kwality. But moving coil, those tend to ALL be expensive. Also, they have a fraction of the output of moving magnet, and require either a transformer or pre-preamp (geeze more expense!) to get their output level in the range of the more common moving magnet. But those moving coils, ah they're s'posed to be magic, with kwality to make audiophiles swoon! So let's make sure we're not dealing with an mc cart, or if we are, understand that it needs an extra stage of boostage to drive your preamp inputs.

[Randall]

Enzo, I forgot to mention I am running both amps on a variac set to 117 vac. I have 645 vac on the secondary and 345 vdc on the rectifier. Line level through the radio input sounds good, but he doesn't want to use Solid State in the signal path, if using a preamp is where you are going. But I will offer it as an option.

Leo: It's a Grado Prestige moving coil cartridge, so it seems you are onto something. Input load is indeed 47K, and output is rated at 5mV @1K. I think you have Mag1 and Mag2 reversed? Lifting the 270K disables Mag 1 input signal path, and leaves Mag 2 with 47K to ground, is this what we want? If it's hard to read Mag 2 is on top.

All caps are fresh and tested before installed. Some of the original carbon resistors remain if they were within spec, but pretty much all caps and resistors are fresh.

Regarding noise, it's with nothing plugged in. It didn't occur to me to shunt them. In that case, shouldn't any inputs that are not being used be grounded?

[Enzo]

Line level through the radio input sounds good, but he doesn't want to use Solid State in the signal path, if using a preamp is where you are going. But I will offer it as an option.

I wasn't going anywhere. I was suggesting you test the amp with a line level signal there so as to compare to the phono signal. If the line level signal comes through OK, that is another finger pointing at your phono cart producing super low levels of signal.

[Randall]

I think that is correct. I listened to a record going straight in Mag 2, and then into radio using a ss preamp. Preamp is definitely louder, but sounds not as warm to me. I guess it is what it is. Thanks once again fellas!

[Jazz P Bass]

Something that you may want to keep in mind: As Enzo stated, the input was designed for a "crystal" cartridge.
The physical construction of a crystal cartridge has the benefit of having a built in RIAA curve.

The schematic as posted does not have an RIAA circuit, so you can basically forget about expecting a magnetic pickup to sound 'proper'.
#1 you will not have enough bass frequencies
#2: it will be awfully trebly.

Check online for an RIAA input converter.

Note: The RIAA circuit is employed when 'cutting' the master platter.
It lowers the bass & increases the treble frequencies.
(you have to lower the bass or the cutting head will move too far.
Likewise, the treble must be boosted or the cutting head does not move far enough)
So to hear what was actually recorded, before the master, you have to increase the bass & cut the treble.

I guess you could call it an anti RIAA circuit.

[nosaj]

I think that is correct. I listened to a record going straight in Mag 2, and then into radio using a ss preamp. Preamp is definitely louder, but sounds not as warm to me. I guess it is what it is. Thanks once again fellas!

One thing I haven't seen is what speakers you are using. Efficency of speakers can make a difference in perception of loudness.

nosaj

[Leo_Gnardo]

Leo: It's a Grado Prestige moving coil cartridge

Moving coil, I suspicioned as much. So it needs a step up transformer or pre preamp. Case closed.

edit: OH WOOPS MAYBE NOT! Bulletin straight from Grado's website:

All Grado cartridges use a derivation of the moving iron principle

Iron, as in mag net. So much for claims of moving coil. Rats, throw that idea away. Goddam! How I luv disinformation ! ! ! ! NOT.

Yes I swapped Mag 1 for Mag 2, woops! Small matter, you really only need one input. How many turntables does anybody run at once, unless you're a dee jay at the dance club? And even that guy has separate preamp channels for each one.

Yes it makes sense to shunt any unused inputs. But in the case of your mag inputs there's no point in having two. Use the one with 47K loading R & simply disconnect the other by removing the 270K resistor. If the Bells are to be used with turntable only, heck you might as well remove the connections from crystal & radio inputs to the preamp output, where they connect just before the "#2 blue" jack, then you wouldn't be adding any noise that input jacks might pick up, also would not lose any signal by shunting the radio input. As the circuit shows, you would lose half your signal strength by applying a shunt to the radio input with the circuitry standing as-is.

True there is no authentic RIAA eq on the mag input preamp. But there IS some sort of built in EQ, a vague attempt at a low pass filter, AKA hi frequency rolloff, following the 1st stage preamp and before the 2nd. If you're feeling energetic you could substitute a more acceptable RIAA type filter, cob it from some standard manufacturer's preamp.

Lastly, there's a glaring problem I may have mentioned a few months ago, and it really should be looked after. Take a look at the 3rd triode, labeled "2nd AF Amp", there's no grid leak resistor anywhere in sight! How the Bell did that happen? Its grid can't be left just floating aimlessly, can it? 1: that stage can't operate properly if there's no grid leak resistor OR B) there is one, it just doesn't show up on the schematic. So - let's find out what's going on there for real. And if there's no grid leak, please do add one, a simple cheap 1 Meg half watt carbon comp costs-pennies resistor will do just fine.

[Enzo]

I listened to a record going straight in Mag 2, and then into radio using a ss preamp. Preamp is definitely louder, but sounds not as warm to me. I guess it is what it is.

Was it a solid state PHONO preamp? Phono preamps have the RIAA response, which emphasizes bottom end and cuts high end. A plain old preamp will be flat so the sound tends to be tinny.