Silvertone 1474 Chassis Question

[nevetslab]

Complete 1474 Amp schematic with reverb circuit modified

I finished drawing up the 1474 Amplifier Schematic, merging the output stage & power supply into the preamp circuit shown a couple days ago. The power supply cap values are that of the replacements. In the Reverb circuit I adopted a Fender 037099 Drive Transformer for the primary to couple to an Accutronics 82B3C1B triple spring tank. I'd have to look up the part to see what the input and output impedances are.. I do recall tuning the bandwidth of the reverb send and return circuits by ear so it wasn't muddy…came out sounding very nice. In the actual build, the only place I had for the tank was down in the speaker compartment. That was a trial & error process to get the height & placement to minimize acoustic coupling from the speakers into the tank, causing acoustic feedback.

The front end is just like the Silvertone 1483, right up thru the phase inverter & driver stage. Just has the reverb circuit picked off after the coupling cap of Ch 2’s input amplifier.

One of the issues I had with the amp was their series-connected heaters of the two 12AX7 preamp tubes being in series with the cathode-bias resistor of the output stage. The heaters were starved….running around 16VDC instead of 25.2V. Physically, the tube sockets they used didn’t have a pin 9 in them, and they were riveted into the chassis. It was just too much work at the time to replace the tube sockets, and revised the power stage’s cathode bias, and run those pair of 12AX7’s off the heater supply as normal.

Anyway, here’s the full schematic as Silvertone built it, less the changes made in the Reverb circuit. Using NI’s Multisim for the schematic program, there were symbols I didn’t have…..that of a 6CG7, a 6AU6, a transformer with the correct secondary windings, loudspeaker symbols, heater symbols to hang on the heater winding, etc.

Also some photos of the 1474 chassis, showing the 4 preamp tubes, with the added Fender Reverb driver transformer, and the placement of the new reverb tank.

1474 AMPLIFIER-REVISED REVERB SCH-1.pdf



The AC Mains fuse in the schematic is supposed to be 3A S/B.....thought I had changed that.

The Silvertone amps never seemed to use muting input jacks. Being open circuit, if your volume controls are up, there's lots of hum from the unterminated input jacks. Easy change, time/parts not withstanding.

On this particular amp i restored, the power transformer ran really hot. Even with the tubes out, rectifier tube out so there was no load, it still ran hot. Excitation current was 220mA, 8.5W @ 120VAC, a bit high I think. I didn't have my Hypot analyzer on hand to check it. It ran VERY HOT with tubes installed (over 70 deg C surface). It hasn't failed yet, now having been in use by the client since fall '2012. The only other Silvertone I'd worked on was a 1484, and I didn't notice its' PT running hot.

[nevetslab]

By just looping the RCA connectors (Send to Return) together? For sure! The 'Reverb Level' becomes the blend control for the overdrive, though out of phase with the main signal in Ch 2.

We found after the restoration and re-capping of the 1474 I did, the absence of the old crusty ceramics in the tone circuits, and perhaps the Green Sangamo Type 33 Molded Plastic paper caps used in coupling stages, it lost the edginess it had, so I've more work to do on that amp to put the grit back into it.

[jrdamien]

So here it is in a cab I put together over the last several days. I only put one speaker in to just test the first channel and it is LOUD. It's much louder than I thought it would be. I'm almost stumped how those little trans are putting out that kind of volume.

Any suggestions on what kind of master volume to put in this and where? I want that great crunch but it's all the way at 10.

[nevetslab]

Master Volume for Silvertone 1474

Looking at the complete amplifier schematic that I posted previously, I'd put the Master Volume following V3A, as the two channel volume controls and the Reverb control sum at the grid to that tube. 500k Audio Taper, following C15 (10nF plate coupling cap of V3A), connect that to the top of the pot, Gnd the bottom, and the wiper connects to the grid of V3B by way of a coupling cap (10nF 250V) as V3B is the phase splitter to the power amp stage, and there's voltage at the grid of V3B. You'd leave the grid resistor in place R38 1M.

You can make an output transformer small if you keep the lowest design frequency high, allow a higher temperature rise and get away with it fudging the duty cycle in typical use. 82Hz is roughly Open Low E on a guitar, though they could have used a higher LF corner than that. Then, splitting the output load up as Enzo mentioned, the transformer size gets smaller. And, they may have had that transformer in their inventory for use on other low wattage products.

Loud isn't a factor regarding the transformer size, per se. Overall gain is, and efficiency of the speaker. Looks like it's coming along nicely.

Out of curiosity, does your power transformer run hot? The one I restored, after it being on for 1/2 hr or more, you couldn't put your hands on it. Their solution to the Standby Switch mutes the output, but it leaves the power supply running full current.

I's also be curious to know what the voltage is at the power tubes' cathodes, and what the voltage is at the bottom of that 39 ohm 5W resistor R55 (in my diagram). My client's amp's voltage for that series-connected pair of heaters for V1 & V3 was just mid-16V range, instead of around 25VDC as we'd normally expect. The power tubes, though, were also running a lot of current as a result. Room for tweaking in that area.

[jrdamien]

Awesome Nevs, thanks.

I'm going to take it to rehearsal tonight as is (before any new caps, etc) to put it through its paces and see if it survives 3 hours of being played. I'll let you know about the heat off the power trans then. Having owned and played the 1484/85 previously, I know they do get hot and also know the nature of the standby, which is why I always just turn the amps off if they're not going to be used for awhile.

As for volume, I do have some very efficient 10" Eminence drivers going so that's certainly a part of it. But this was a cheap build I wanted to do and what I had on hand. Ramrod and Rajin Cajin, 100.2 and 100.5 respectively.

I don't think I understand how the controls are working by virtue of the two outputs. If I'm plugged into channel one both channels are going. That doesn't seem to be so if I plug into Channel 2. Are they independent like, say, the controls on a Les Paul?

I'll take voltage after I start replacing caps and resistors and let you know.

[nevetslab]

The two speaker outputs are common to your two input channels. Those two sum at V3A, and then following the phase inverter and driver stage. The two power tube stages' outputs are independent, while sharing the common cathode biasing and grid drive. There's no feedback on the amp. I recall one of your questions was could you combine the two output transformers into one circuit to feed a common speaker. Yes. If the two secondaries were put in parallel, you'd want to use 4 ohm speaker instead of 8 ohm, while you could also put the two in series, and use 16 ohm speaker load. They would have to be phased so the signal adds, and not cancel.

I'd wonder a little about current sharing in combining the two outputs. You don't have independent biasing, so using matched 6L6GC's would be in order. With one transformer / power stage without a speaker load, I'd pull those pair of tubes so you don't do damage to the OT & tubes.

An earlier post by member 'drewl' in this thread has made use of the original Reverb circuit as a killer Overdrive Channel, looping the Reverb Drive to the Reverb Return circuit. That second channel is different than the first in how they fed the reverb circuit. That's not an uncommon trick, using the reverb circuit as an overdrive circuit, with the Reverb Level being the 'blend' control. Something to ponder, eh?

[drewl]

And its footswitchable!