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'Silvertone 1344'
Author:Sean Weatherford (guest: search)
Date: Wed, Mar 08th, 2006 @ 23:43 ( . )

Very funky stuff. 10 bonus points for those who can figure out what's is going on with the preamp tube filaments.

The coolest trem circuit I think I have ever seen. It only has an intensity adjustment not a rate though. :(

[link]

Sean Weatherford
Bean's Amp Repair
Central, SC

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'Silvertone 1344'
Author:Arthur B (guest: search)
Date: Thu, Mar 09th, 2006 @ 00:25 ( . )

On 03/08/2006 @ 23:43, Sean Weatherford wrote :
Very funky stuff. 10 bonus points for those who can figure out what's is going on with the preamp tube filaments.
:
: The coolest trem circuit I think I have ever seen. It only has an intensity adjustment not a rate though. :(
:
: [link]
:
: Sean Weatherford
: Bean's Amp Repair
: Central, SC
--



(1) My guess is it's a ghetto attempt at a bias supply.

(2) It's phase shift oscillator. Replace/supplement the 560k resistors with a dual gang pot.

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'Silvertone 1344'
Author:Carl B (registered user: 144 posts )
Date: Thu, Mar 09th, 2006 @ 00:37 ( . )

Arthur,

When Bean thinks it's cool, he's not just easilly amused. Have a closer look:

Those are DC heaters on the first two preamp stages, and a bias supply (as you got whif of), all in one swell foop. And not a bit of silicon, germanium, or 10,000uF caps in site.

The designer either didn't have those, or they were too expensive to put in. Yet he still got his fixed bias output stage (he was probably thinking, "more power than the typical way"), and DC on the heaters of his preamp tubes.

Not bad, eh?

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'Silvertone 1344'
Author:Stephen Keller (registered user: 5756 posts )
Date: Thu, Mar 09th, 2006 @ 00:44 ( . )

On 03/08/2006 @ 23:43, Sean Weatherford wrote :
Very funky stuff. 10 bonus points for those who can figure out what's is going on with the preamp tube filaments.
:
: The coolest trem circuit I think I have ever seen. It only has an intensity adjustment not a rate though. :(
:
: [link]
:
: Sean Weatherford
: Bean's Amp Repair
: Central, SC
--

Gee Boss, do have to guess? I mean you're looking at 12.6 VAC on the filament winding divided by the two 6V6s and the 6Y6G and #47 pilot light. Also you have one of the 12SJ7s paralelled onto the 12.6AC filament winding. The other 12SJ7 and the 12SN7 are providing approximately a 30 volt drop (sort of like a back-bias arrangement) in the ground circuit. This elevates the filament CT by about 30 volts. Better not attach a chassis ground to this because (even weirder) the death cap is connected to the filament CT, which is also serving as the signal ground. Total current draw through the filament winding should be about 3.55 A (AC). Current through the two "back-bias" filaments is about 0.3 A. It's bad news if the pilot dies though. Suddenly, one of the 6V6 filaments gets the lion's share of that 12 volts and probably pops as well. Course, it's bad news if one of those voltage dropping filaments pops. You lose your signal ground. Very strange.

Like you, I don't think I've ever seen a trem circuit drive a separate power amp and use a separate winding on the voice coil to mix in the modulation. That is very strange. Definitely a three-martini design.

Stph

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'Silvertone 1344'
Author:Arthur B (guest: search)
Date: Thu, Mar 09th, 2006 @ 00:53 ( . )

On 03/09/2006 @ 00:37, Carl B wrote :
Arthur,
:
: When Bean thinks it's cool, he's not just easilly amused. Have a closer look:
:
: Those are DC heaters on the first two preamp stages, and a bias supply (as you got whif of), all in one swell foop. And not a bit of silicon, germanium, or 10,000uF caps in site.
:
: The designer either didn't have those, or they were too expensive to put in. Yet he still got his fixed bias output stage (he was probably thinking, "more power than the typical way"), and DC on the heaters of his preamp tubes.
:
: Not bad, eh?
--



I agree that's it's ingenious. However I said it was ghetto because it was unsophisticated and improvised.

On 03/09/2006 @ 00:44, Stephen Keller wrote :
On 03/08/2006 @ 23:43, Sean Weatherford wrote :
: Very funky stuff. 10 bonus points for those who can figure out what's is going on with the preamp tube filaments.
: :
: : The coolest trem circuit I think I have ever seen. It only has an intensity adjustment not a rate though. :(
: :
: : [link]
: :
: : Sean Weatherford
: : Bean's Amp Repair
: : Central, SC
: --
:

: Gee Boss, do have to guess? I mean you're looking at 12.6 VAC on the filament winding divided by the two 6V6s and the 6Y6G and #47 pilot light. Also you have one of the 12SJ7s paralelled onto the 12.6AC filament winding. The other 12SJ7 and the 12SN7 are providing approximately a 30 volt drop (sort of like a back-bias arrangement) in the ground circuit. This elevates the filament CT by about 30 volts. Better not attach a chassis ground to this because (even weirder) the death cap is connected to the filament CT, which is also serving as the signal ground. Total current draw through the filament winding should be about 3.55 A (AC). Current through the two "back-bias" filaments is about 0.3 A. It's bad news if the pilot dies though. Suddenly, one of the 6V6 filaments gets the lion's share of that 12 volts and probably pops as well. Course, it's bad news if one of those voltage dropping filaments pops. You lose your signal ground. Very strange.
:
: Like you, I don't think I've ever seen a trem circuit drive a separate power amp and use a separate winding on the voice coil to mix in the modulation. That is very strange. Definitely a three-martini design.
:
: Stph
:
--



(1) The good thing to do here would be to ground the CT.

(2) It relies on the same concept to a grid bias tremolo.

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'Silvertone 1344'
Author:Carl B (registered user: 144 posts )
Date: Thu, Mar 09th, 2006 @ 01:04 ( . )

If I'm not mistaken, the center tap of the PT's HT isn't where you'd want to ground reference. That's where your negative bias is generated from.

The ground reference (if this wasn't a two pronger) would be the long bus that, well, looks like it should be ground. In this case, it's death-capped.

But still, you could do a three prong conversion on this. I have to go back and work through Stephen's post, but I don't think this will have the problems he's thinking it will (besides the death cap one, natch).

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'Silvertone 1344'
Author:Stephen Keller (registered user: 5756 posts )
Date: Thu, Mar 09th, 2006 @ 12:02 ( . )

On 03/09/2006 @ 01:04, Carl B wrote :
If I'm not mistaken, the center tap of the PT's HT isn't where you'd want to ground reference. That's where your negative bias is generated from.
:
: The ground reference (if this wasn't a two pronger) would be the long bus that, well, looks like it should be ground. In this case, it's death-capped.
:
: But still, you could do a three prong conversion on this. I have to go back and work through Stephen's post, but I don't think this will have the problems he's thinking it will (besides the death cap one, natch).
--

Most of the problems I expect this amp to have would be related to its failure modes. Pilot lamp fails, leaving one 6V6 in parallel with the 6Y6 (call these tubes A and B, respectively) with both of those in series with the other 6V6 (tube C). Neglecting AC effects and any inductance in the filaments, you can think of the 6V6 filaments as having about 14 ohm resistance compared to the 5 ohm resistance of the 6Y6 filaments. With an open pilot light, you'll have about 3.6 ohms total for the parallel A|B filaments. 3.6 ohms in series with 14 ohms of filament C means that the bottom 6V6 would be running its filament at about 9.9 VAC. Much too high.

You have the same problem if the 6Y6 filament gives way, but I think I'd hear a trem loss before I heard a pilot lamp die. If one of the 6V6 filaments gives up, you have a similar problem, but don't get so far out of spec.

Regarding the conversion to 3-prong plug, I was misreading the schematic (thanks for the tweaks Sean). One should be able to nix the death cap and tie the chassis ground to the filament CT.

It's still a strange and interesting design with very clever use of what was available in the day.

Stph

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'Silvertone 1344'
Author:Carl B (registered user: 144 posts )
Date: Thu, Mar 09th, 2006 @ 01:23 ( . )

" The other 12SJ7 and the 12SN7 are providing approximately a 30 volt drop (sort of like a back-bias arrangement) in the ground circuit. This elevates the filament CT by about 30 volts. "

Elevates it from the PT's HT center tap, but that's not ground in this case. That CT is DC negative off-of "death-cap-ground." No more dangerous than your typical death-cap setup (although that's *definitely* dangerous enough).

"Better not attach a chassis ground to this because (even weirder) the death cap is connected to the filament CT, which is also serving as the signal ground. "

Not weird when viewed in a different light ...

"Total current draw through the filament winding should be about 3.55 A (AC). Current through the two "back-bias" filaments is about 0.3 A. It's bad news if the pilot dies though. Suddenly, one of the 6V6 filaments gets the lion's share of that 12 volts and probably pops as well. Course, it's bad news if one of those voltage dropping filaments pops. You lose your signal ground. Very strange."

No, if I'm not mistaken, the 12.6V ac heater winding is center tapped, so even if you do pop a lamp, it just puts an unbalanced load on the 12.6V winding. Should be able to handle that without a complaint.

And we wouidn't lose signal ground, right? We would lose the return current path, much like you would if you had an HT fuse blow.
:
: Like you, I don't think I've ever seen a trem circuit drive a separate power amp and use a separate winding on the voice coil to mix in the modulation. That is very strange. Definitely a three-martini design.

Not the voice coil. Think back. Farther back. It's modulating the field coil. Back before permanent magnets were available or cheap, you needed a feild coil as an electromagnet. With all the experience going before us, we can look back and wonder about his tremelo drive, but he may not have had the implementations that we have at our fingertips available to him.

If I'm wrong on this, just let me know ... But this just seems like a *really* cool - and well though out - circuit.

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'Silvertone 1344'
Author:Stephen Keller (registered user: 5756 posts )
Date: Thu, Mar 09th, 2006 @ 12:37 ( . )

On 03/09/2006 @ 01:23, Carl B wrote :
: No, if I'm not mistaken, the 12.6V ac heater winding is center tapped, so even if you do pop a lamp, it just puts an unbalanced load on the 12.6V winding. Should be able to handle that without a complaint.

Hmm.... I've got to think about this for a bit. My other comment about neglecting AC effects was overly broad, I think. I was considering it as a straight 12.6 vac supply. The DC resistance of the two filament windings though is much lower than any of the filament resistances. This makes for an overall balancing effect doesn't it. You'll see a mild imbalance on the filament winding, but unless they shaved their tolerances way too close, it should not be an issue.

Thanks, for helping me understand this better. My earlier comments were misguided.


: And we wouidn't lose signal ground, right? We would lose the return current path, much like you would if you had an HT fuse blow.
: :
: : Like you, I don't think I've ever seen a trem circuit drive a separate power amp and use a separate winding on the voice coil to mix in the modulation. That is very strange. Definitely a three-martini design.
:
: Not the voice coil. Think back. Farther back. It's modulating the field coil. Back before permanent magnets were available or cheap, you needed a feild coil as an electromagnet. With all the experience going before us, we can look back and wonder about his tremelo drive, but he may not have had the implementations that we have at our fingertips available to him.
:
: If I'm wrong on this, just let me know ... But this just seems like a *really* cool - and well though out - circuit.
--

I think the tremelo is very cool too. So that's how the old speakers worked: the field coil provided the magnetic field the speaker needs? So in this case, the trem varies the intensity of the magnetic field which in turn governs the overall efficiency(?) or volume the speaker can output, right?

Stph

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'Silvertone 1344'
Author:Sean Weatherford (guest: search)
Date: Thu, Mar 09th, 2006 @ 16:09 ( . )

Yes that is the way the trem worked. PM speakers were not readily availible or affordable until roughly 1954-5. By 1958 they were a thing of the past.

Sean

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'Silvertone 1344'
Author:Sean Weatherford (guest: search)
Date: Thu, Mar 09th, 2006 @ 10:24 ( . )

The amp design dates to 1951. No solid state rectifiers availible other than copper oxide. This is the earliest example of fixed bias operation in a guitar (or instument) amplifier I have run across. The only other fixed bias amps with earlier build dates used a 6H6 Duodiode to obtain bias. That was in some Western Electric (i think) theater amps. This of course excludes the 1930's era radios and phonographs with battery bias. I thought it was a very interesting and innovative way to obtain a DC bias voltage. Especially in light of the devices availible.

The Trem design is unique in that the feild coil of the speaker was always used as a filter choke. This is a very novel concept and probably done to get around some patent. The original drawing shows one of the Resistors in the RC oscillator in the upper control panel. Nate realized the need for a rate control, perhaps Sears and Roebuck would not pay for it. The early fifties silvertones were not the cheapo models of the the middle to late sixties. They were still respectable player in the very young electrified instument market.

Nate Daniels nearly always had DC heating filament of at least the first preamp tube. Usually it was in the cathode circuit of the power tubes. That option was not availible here without the additional current through the feild coil of the speaker. He often used 12 Volt filament trannys with split loads as you see here.

Also of interest was the split plate load on the 12SJ7 preamp tube.

Chassis was not grounded as grounded outlets and supplies were not availible. Anyone got a early sixties Gibson with the clip on ground lead?

I know of two these amps with original tubes that still work after nearly 60 years. I have just not heard one in person.

Sean Weatherford
Bean's Amp Repair
Central, SC

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'Silvertone 1344'
Author:Newt1 (registered user: 248 posts )
Date: Thu, Mar 09th, 2006 @ 11:40 ( . )

"The amp design dates to 1951. No solid state rectifiers availible other than copper oxide"

When did the selenium rectifiers appear? [link]

This display would have gotten my attention at the local electronics store: [link]

And the star of the movie: [link]


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'Silvertone 1344'
Author:Balijukka (guest: search)
Date: Thu, Mar 09th, 2006 @ 11:56 ( . )

The B+ at the first pentode is shorted to ground.
Jukka

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'Silvertone 1344'
Author:Balijukka (guest: search)
Date: Thu, Mar 09th, 2006 @ 11:58 ( . )

as are the utput tube screens and the tremolo B+
Jukka

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'Silvertone 1344'
Author:Doug H (registered user: 5397 posts )
Date: Thu, Mar 09th, 2006 @ 12:06 ( . )

On 03/09/2006 @ 11:40, Newt1 wrote :

: And the star of the movie: [link]
:
:
--



I think I got shocked from a selenium rectifier when I was a kid. Looking at those photos now, I believe that's what it was. I had an old tube phonograph that had an input jack on it. I plugged my guitar into it and plugged the headphone output into my Silvertone amp-in-a-case (imagine that, in *this* thread...) for some monsterous distortion and feedback. There was a problem with the phonograph cutting out, and being the inquisitive yet non-electronics savvy 12 yr old that I was, I reached in there with it powered on and received a strong jolt from what now appears to have been a blue selenium rectifier. (Note to self: Turn things *off* before you touch them.)

After all these years... I knew never knew *what* that part was. Go figure...

Doug

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'Silvertone 1344'
Author:Balijukka (guest: search)
Date: Thu, Mar 09th, 2006 @ 12:20 ( . )

And still you keep sticking your fingers at live crcuits :)
Jukka

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'Silvertone 1344'
Author:Newt1 (registered user: 248 posts )
Date: Thu, Mar 09th, 2006 @ 12:25 ( . )

My family's first multi-speed record player with changer's circuitry consisted of a single tube - a 50C5. The 60-volt turntable motor was in series with the tube filament. It had a big blue selenium rectifier rectifying the AC for the tube's plate. Phonograph needle connected to the grid. And a big decal on the front saying "Hi-Fidelity".

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'Silvertone 1344'
Author:Doug H (registered user: 5397 posts )
Date: Thu, Mar 09th, 2006 @ 14:23 ( . )

On 03/09/2006 @ 12:25, Newt1 wrote :
Phonograph needle connected to the grid. And a big decal on the front saying "Hi-Fidelity".
--



Hey, sounds like a pretty pure signal chain to me...;-) LOL!

Doug

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'Silvertone 1344'
Author:Carl B (registered user: 144 posts )
Date: Thu, Mar 09th, 2006 @ 14:20 ( . )

Hey, at least I used mom's yellow latex gloves!

It was a blackface Tremolux, about as boring to a 13 year old as plaid golf pants, and it needed "more power!"

Looking back, what I did (had no clue at the time, just sat there with the jumers seeing what made things "Louder!") was wire the bright caps always on, and use those switches to short-out the screen stop resistors. You could choose: short the left and/or short the right grid.

Dumb as cracking open a thermostat to get at the mercury (that was cool stuff). But boy, that sucker sounded *a lot* better!

Not that anyone should *try* this - ever - but my fav setting was one switch on, one switch off. Kind of that, "in-between hot-rodded" mode as I thought of it.

Used some radshack 20 gauge solid core in ... green if i remember, to "point-to-point" the mod. As you can imagine, the implementation was as exceptional as was the design.

And the rest is history.

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'Silvertone 1344'
Author:Carl B (registered user: 144 posts )
Date: Thu, Mar 09th, 2006 @ 14:04 ( . )

Wow! A murder mystery with vacuum tubes and radios.

"True Geek Confessions."

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'Silvertone 1344'
Author:Doug H (registered user: 5397 posts )
Date: Thu, Mar 09th, 2006 @ 14:26 ( . )

On 03/09/2006 @ 14:04, Carl B wrote :
Wow! A murder mystery with vacuum tubes and radios.
:
: "True Geek Confessions."
--



LOL! Well, for me the mystery of "What the heck was it that shocked the crap out of me?" has been solved.;-)

Doug

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'Silvertone 1344'
Author:Sean Weatherford (guest: search)
Date: Thu, Mar 09th, 2006 @ 16:12 ( . )

Seleniums were availible (they have been around since the 1930's or maybe before) but are bulky and expensive. May have even cost more than a 6H6 and a socket. Surely cost more than the two filament you had already in the preamp tubes. The size varied more with Inverse Voltage Rating that with current handling IIRC.

Sean

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