Calling Silvertone Amp Fans, How about that 1473?

[Platefire]

I've been a 1482 fan for years as it was my first amp ever but even after all these years I never knew there was a similar matching design in a bass amp model 1473 until recently. Anybody else run across one of these? They must be kind of rare because I've never see one before and never knew they existed until I ran across one lately.  Platefire

[Platefire]

In 1473 Dano sailed the ocean blue. Well I guess this proves I'm truely an individual 

[Jack_Hester]

Do you have a schematic for one?  I made a quick search, and haven't found a free download for one.  A couple available to purchase. 
 
Also while searching, I saw one listed as a head cabinet.
 
Jack

[terminalgs]

http://www.silvertoneworld.net/schematics/1473_schematic2_silvertoneworld.jpg

check out the cathode bias resistor on the 6L6's...

[terminalgs]

http://www.silvertoneworld.net/schematics/1473_schematic2_silvertoneworld.jpg

check out the cathode bias "resistors" on the 6L6's...

[Willabe]

What the?

How exactly does that work?


            Brad     

[Platefire]

Thanks for the link to the schematic. I had found one but wasn't as good as that. Gosh 305 seems kind of low plate voltage of a 6l6?

What I don't understand is the 1472 was the forerunner of the 1482(guitar amps). They upgraded transformers I think and increased voltages a bit for higher output on the electronics plus changed to the smaller handle and gray tolex on the cab. On the 1473, the cab style is like a 1472---black tolex with the big handle. I don't find where they upgraded the 1473 bass amp when the they upgraded the 1472 to the 1482. I'm thinking that the sales may have been so low for the 1473 they may have discontinued it??? Platefire

BTW-Some info on this Link
http://silvertoneworld.net/amplifiers/1472_82/1472_82.html

EDIT: If you follow the links it says the 1473 only lasted one year 1961-1962

[jjasilli]

What the?

How exactly does that work?            Brad     

The filaments have resistance, so can be treated as resistors.  The 12ax7 filaments can be wired to run on 6.3 or 12 volts.  Wired in series for 12 volts X 2  tubes will want about 24VDC, which is close to the posted cathode voltage.  At 12V the heaters ea. draw .15 amps.  Ohms law says that's 80 Ohms per filament X 2  filaments in series = 160 Ohms total as "cathode resistance" for the power tubes.  This seems a tad high, but what the heck: http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6L6GC.pdf


Even with the preamp tube filaments run on DC, their inherent heater noise will wind up in the power tubes.  I would consider modding this part of the design and optimize the power tube bias with a "real" cathode circuit.  Not sure about the tonestacks either -- pure personal taste.


But, if you're into overdrive this power amp looks great to me.  Low voltage, high current Class A operation; hi resistance in the OT primaries; and a tube rectifier.  Might even sag without a choke.

[Willabe]

The filaments have resistance, so can be treated as resistors. 
Even with the preamp tube filaments run on DC, their inherent heater noise will wind up in the power tubes. 

Yeah I figured the filament would act like a R.

But I also wondered about noise/interaction (like you brought up) between the small tube and the output tubes including less and more current draw through the output tubes.

Seems like they really stretched to cut a corner on production cost?

                 Brad     

[terminalgs]

Thanks for the link to the schematic. I had found one but wasn't as good as that. Gosh 305 seems kind of low plate voltage of a 6l6?

What I don't understand is the 1472 was the forerunner of the 1482(guitar amps). They upgraded transformers I think and increased voltages a bit for higher output on the electronics plus changed to the smaller handle and gray tolex on the cab. On the 1473, the cab style is like a 1472---black tolex with the big handle. I don't find where they upgraded the 1473 bass amp when the they upgraded the 1472 to the 1482. I'm thinking that the sales may have been so low for the 1473 they may have discontinued it??? Platefire

BTW-Some info on this Link
http://silvertoneworld.net/amplifiers/1472_82/1472_82.html

EDIT: If you follow the links it says the 1473 only lasted one year 1961-1962

I think the lineage is 1473 begat the 1483. both were advertised as bass amps, and unlike the littler 1482, which got a fisher 12", the 1473/1483 got the Jensen ceramic treatment.   I've seen  people make the mistake that the 1483 was a 1484 sans trem & reverb.  maybe they do this because they look so similar, but the 1483 is a completely different beast,  just like this 1473 is different from the 1472 or 1474.

it is an interesting design, with the  6CG7 serving as dual post cathodyne gain stages.  that, along with the lower B+ on the 6L6s,  all look like design effort to maximize clean headroom for bass guitar.

I'm with jjasilli on the tone circuit, although if you turn treble all the way "up" so there is 1M between C3 and ground, and Bass all the "down" so C11 is shorted, it seems relatively harmless.  I'd be tempted to lift R20 to see if you can get some high end though one channel to see how it brightens up.

[Platefire]

Thanks for the explaination on the cathode tied to heaters. I don't recall ever seeing that before. I would agree that is not a good design and if you had one, that would be a good first mod. So yeah, I guess the 4283 was the next step in their bass models but there is something to say about portability of the combo. If I had one I would be tempted to mod it to the next level the way it would  have been done, if they had maintained a matching combo for the 1482---so they didn't. 

A LITTLE HISTORY REGARDING THIS ERA: In my little home town back in the 60's everybody that played guitar or bass did it on a regular guitar. If you did play a bass part it was on a guitar with the tone settings set bassy on stings 3, 4, 5 & 6. I saw the need for a bass player with a real bass rig for a while and when I started playing in my first group with a drummer I became the bass player because the other guitar player was much more advanced player than me. I purchased a Framous Bass and a Gretsch Varity Amp(bass or guitar) and became the first bass player(that I know of) in my home town with an actual bass rig. Also there was not one upright acoustic bass player in our community. People now days don't realize the rarity of a dedicated bass player with a bass rig back in this time. It was a big city thing and rare in small communities. Now it is common place----so appreciate your dedicated bass player---they are usually a lead guitar player wanting to break out Platefire

[HotBluePlates]

The filaments have resistance, so can be treated as resistors. 
Even with the preamp tube filaments run on DC, their inherent heater noise will wind up in the power tubes. 

Yeah I figured the filament would act like a R.

But I also wondered about noise/interaction (like you brought up) between the small tube and the output tubes including less and more current draw through the output tubes.

Seems like they really stretched to cut a corner on production cost?

It's free d.c. heat for the 12AX7's. There's no "heater noise" to get into anywhere, because it's d.c.

Jjasilli is 90% correct on his explanation of the operation. If you wire a 12AX7 heater with the halves in parallel, the heater will draw 300mA from the "heater supply"; with the halves in series (using only pins 4 & 5), it will draw 150mA. Hopefully with 12.6v across each 12AX7's heater.

So the 6L6's are idling at 150mA total, or 75mA per tube, with 305v on the plate and 300v on the screen. The heaters force 150mA of d.c. at whatever voltage that lands on. So it must take 20.5v of bias when the 6L6 has 300v-20.5v = 279.5v screen to pass 75mA per tube. The heater's resistance is not constant (it has a negative temperature coefficient), the current is the essentially fixed quantity.

This trick was not uncommon in the old days to provide d.c. heat for input stages to minimize hum and noise in audio amplifiers. It only works if your output tubes pass a combined 150mA or 300mA (or whatever current your input tubes operate on) at a bias that is equal to (or close, as here) to the sum of the individual tube heater voltages.

For us, that often means lowish supply voltage and class A operation. You'll see the 6L6's are idling at 284.5v plate-to-cathode and 75mA, a hair over 21w; that's every bit of the allowed plate dissipation published in the original 6L6 data sheet (go to last page, subtract "screen dissipation" from "plate and screen dissipation").

[jjasilli]

It's free d.c. heat for the 12AX7's. There's no "heater noise" to get into anywhere, because it's d.c.


My point is this:  All resistors are noisy.  Filaments are resistors.  Therefore, filaments are noisy.


Some types of resistors are much more noisy than others.  E.g., carbon comps are very noisy even when passing only DC.  Hence audiophiles and some guitar amp builders prefer to avoid them for that reason.  Filaments are notoriously noisy in and of themselves whether passing AC or DC. 


Of course, the use of an AC filament supply would further add the issue of hum, to the inherent noise of the filaments themselves.  Hum in the early gain stages would get re-amplified downstream in the amp, and hence worsen. 


This amp's clever use of a free DC filament supply eliminates the AC hum issue, but adds filament noise. Perhaps the result is a net benefit in terms of hum/noise.


 

[HotBluePlates]

You're correct in an absolute sense. But do consider the scale of relative signals.

Noise of tubes is commonly quoted in terms of an equivalent resistor attached to the input of the tube. So we know resistors have noise, and it's been long recognized. But very complete data sheets (usually for special industrial or military applications) specify a maximum noise voltage in the mV range. A resistor's noise is proportional to its resistance, so a 1MΩ resistor has more noise than a 100Ω resistor. The tube heater is on the low end of that range, and is relatively quiet (a 100Ω carbon comp will have negligible noise compared to anything other than lower-noise resistors 100Ω and less).

The signal at the output tubes will be ~20v for full output power. A several-millivolt noise will be over 70dB below the output signal level, and so essentially inaudible.

Also consider the tubes are running push-pull. The potential noise voltage is at the shared cathode connection, and common to both tubes. It should balance and cancel at the output transformer, which responds to a differential signal output from the tubes.

So it's a win-win, as long as you can afford to operate the tubes at the forced current.

[smackoj]

Thanks gents. Excellent reading for a novice ampoholic.   

[Platefire]

Yeah Thanks, I'm still trying to wrap my brain around it. So what is the consensus on the dc heater operation---HBP says it's a win win so I suppose that it's good but I'm not sure what effect a forced current would have on the sound and operation? Platefire 

[Willabe]

It's free d.c. heat for the 12AX7's.

Oh, now I get it, I thought the heaters had a voltage on them from the PT and were also used as a KR.



               Brad     

[HotBluePlates]

... I'm not sure what effect a forced current would have on the sound and operation? ...

Tube 1 Current + Tube 2 Current = Total Current (fixed at 150mA)


In a class A push-pull amp, each tube has a current swing at full output from idle to double-idle, back down to (near) zero current and back up to idle. Since each output tube is swinging same/similar current and in opposite directions, the sum of the individual currents stays essentially constant.


In a typical guitar amp using cathode bias, you might have a cathode bypass cap to keep the bias constant. In this case, the heaters will attempt to force a constant 150mA, where the push- and pull-sides have equal and opposite current changes.


So the free d.c. works well in class A, but it wouldn't in class AB (where opposing tube currents are not equal all the time, like when one side cuts off).

[PRR]

I don't know how heaters can be "noisy".

The way we usually FEED them can be HUMMY, cuz we use AC.

This amp avoids that. The 6L6 cathode current is fairly well filtered DC. From the heater point of view, it is a "cathode follower" with kinda-nasty on the plate, better-filtered on the Screens, and dead-ground on the grid. The cathode "follows" the sum: mostly G1, about 5% of G2 jiggle, and maybe 1% of plate jiggle. There's probably less than 0.1V of ripple at the cathodes.

Hiss-noise? All resistors do. But Pure Metal (Tungsten) much less than Carbon Comp, even less than Carbon Film, which we have all over. Also the voltage/current ratio of the hiss source goes by resistance. The heater is about 84 Ohms, which is much lower than any resistor in our hiss-critical stages (or the guitar self-resistance). And as HBP says, here injected into the highest-level stage. True, hiss also goes by Temperature, and the heater is by-far the hottest resistor around (except a pilot-lamp). Doing math we have maybe 0.03 microVolts of hiss across the resistor. Compare to the 0.6uV across that unbypassed 2.2K cathode resistor, or the 1uV-2uV of a clean 12AX7. It's nothing.

Also, hiss injected push-push at the cathodes will tend to cancel in the push-pull plates.

The "sweetest" hi-fi I ever met was a Fisher Console which used this exact trick on the preamp tubes, only using four 6V6 for the power stage.

The main drawback is that the rig takes TWICE as long to come to life. First the big bottles have to warm enough to flow cathode current. Then the preamp tubes *start* to heat. Net result is about ~~20 seconds before the first note comes through. Unless you are running a snack-bar call-system, that's a reasonable exchange for very quiet preamp heat without modern high-current rectifiers or added giant caps.

A side drawback is that a "dead output stage" can really be a broken preamp heater, perhaps perplexing. However a serious search for the big cathode resistor will lead to the trick and an easy try-this (change preamp tubes).

This amp shows 10.25V on the "12.6V" heaters. But note that in large-signal work the 6L6 will shift up in current, maybe 10%. Note that Dynaco ran their preamp tubes on 10.5V and sound OK. The 12AX7 uses the same heater/cathode scale as 12AT7 and 12AU7, but never passes even 1/3rd of the current that its sisters can. Moderate under-heating seems to not be a problem.