Sears Silvertone 1474

[CJ]

why not get after a classic Silvertone  1474 guitar amp?

cool website-
http://silvertoneworld.net/amplifiers/1474/1474.html

comments>

"Our finest Silvertone Amplifier" was the phrase used in the catalog listings, and it was the truth. The 1474 was top of the line for its year of availability, and was the first Silvertone amp to feature reverb.

The 1474 welcomed the 60s with a new sound - reverb - and a new look - the suitcase amp. This look would carry through the early 60s in the 1471/81/72/82. They looked kinda like a TV, kinda like a suitcase, but completely like a Silvertone. One oddity, I've never seen the 'triple 3' controlled model shown in the catalog illustrations, every example I've seen has a '2, 3, 4' knob configuration."

 

[CJ]

notice that there are two output transformers, these drive one Jensen Special Design each,

 

[CJ]

weird chokes down near the speakers, the one that came into the shop had these missing,

 

[CJ]

circuit>

 

[CJ]

can't explore an amp without taking apart the OPT,

so we take it into the pre-op room,

might be the same as another Sears we did, looks familiar,

http://groupdiy.com/index.php?topic=59899.msg760499#msg760499

right on, same core but this guy has a different DCR and ratio,  good news, we get a new print for a 6L6 OPT,

 

[CJ]

bracket and end bells removed,

 

[CJ]

this is an easy structure, pri-sec

secondary is 4 layers like this, about 30 turns per layer of #24

 

[CJ]

KP between layers,

 

[CJ]

3000 turn pri  looks like it got a little warm,

 

[CJ]

here is the print,

these transformers are a little small for 6L6 tubes, but if you jack up the core size you end up blowing the old Jensen single 12 attached to each one, so we keep the stock core and try to get wire on there that will run cool, a nylon bobbin will allow more turns as the margins can be tightened,

this transformer is different from the 1484 OPT , 3000T instead of 1500T, and a different ratio also,

 

[CJ]

we did a rewind on one of the 1474 output transformers and the inductance jumped up, even after torturing the lams with the heat gun/dis-assembly process,

so we probably had some shorted turns either in the pri or sec, the enamel on the sec wire could be scraped off with a fingernail, so there is a good chance that the problem was there,

also, the sec wire is #23, not #24, so we changed the print up above,

here is the inductance chart before and after the rewind>

 

[CJ]

with two output transformers for the price of one, you can do winding experiments and compare,

we wound one 1474 OPT just like the original, and took a shot at a new version with a split bobbin, winding one side of the p-p coil on the left and another on the right,


see here>

 

[CJ]

here is the results for upper freqs, dual bobbin is the flat line,

do not know if this will be audible or prevent HF osc, this amp had a 6L6 socket catch on fire, this is common when you turn the amp into a microwave, HF arcs over the tube socket and once you get carbon, it starts a nice meltdown, we have ceramic sockets on there now as this is the second Sears amp to come into the shop with the exact same problem,

 

[CJ]

if the outputs are sketch, time to take apart the choke also,

same lo-tek wire so why not do some preventive service work, don't want to think about the PT, 

 

[CJ]

choke print>

 

[alexc]

Fascinating thread again CJ 

The commentary, the hi-qual pics and the discussion of some of the underlying. Great stuff.

I, for instance, never gave a thought to tubes catching fire, let alone the reasons why!

And tailoring the characteristics ..  like you do...  So interesting to read a bit of the whys and wherefores.

I've thought a lot about having more than one output traffo.

I also use an old Bell and Howell 12" fullrange (with wizzer) box - it has a decent sized  'speaker-lump'.

I remember them a lot as a kid but didn't understand it - I guess it is to provide a series inductance to the speaker?
To relieve the cone voice coil from having to provide a high inductance ?

 

[CJ]

limit the bass to the speakers, a pair of 6L6 tubes are a bit much for an old Jensen 12 inch, so plate volts are dropped to about 360, small core outputs are used, and chikes are added to the speaker line,

they are not on the schematic,  probably about 1 Henry, that would yield 6.28 x 1 x 10Hz = 63 ohms at 10 Hz, double that to 2 Henries and you get 120 ohms, which is 240 ohms at 20 Hz, etc.

 

[CJ]

got this thing restored, sounds great!

a few notes, it is not a stereo amp, both channels get mixed together before the phase inverter,

i bet Sears got a deal on surplus output transformers, but they were too small, so they split the load,

gives the amp a little redundancy, if you pop an OPT, you get to keep playing,

check the heaters out on the preamp tube, powered off the 6L6 cathode bias! seen this before, a transistor circuit being powered off the cathodes, might have been a Sears amp, might have been the same engineer,

only problem is that you need to have tubes that will bias up to 24 volts,  but it provides a nice DC voltage to keep hum down, have to make sure the bypass cap does not open up or you will have audio on the heaters, might not matter,

this method also provides a nice ramp up for the heaters, the pwr tubes have to conduct before the 12AX7 heats up, it also helps regulate the cathode voltage by means of a constant load on the cathodes, about 150 ma which is about equal to the bias voltage, this might be good for bass as there is less degeneration, some folks use inductors in the cathode circuit for the same reason,

actually, i see no bypass cap on this thing, maybe the tech before be stuck one in there, might not need it with the heater load,

 

[PRR]

Audio on the heaters is no-problem.

(Also as long at the 6L6es stay class-A, there is no audio in a push-pull cathode.)

The "downside" is the amp takes twice as long to start playing. First the 6L6 has to warm up. Then the 12AX7 has to warm up. If you lived with tubes all the time, the slow-to-play is noticeable.