silvertone 1484

[johnfromcyrene]

I came across a silvertone 1484 that mojotone put out. they eliminated the trem and reverb.

Can someone confirm that they modified the voltage doubler? Im not to familiar with this type of circuit.

Did they just use a different power transformer?

[sluckey]

The original weird stacked voltage doublers have been replaced with a more common power supply using a standard power transformer.

[johnfromcyrene]

Would it have any effect on the sound? was just thinking of building this one instead of the doubler one.

[passaloutre]

This Mojotone kit was the first amp I built, 14 years ago.

The preamp and phase inverter are true to the original Silvertone design, without the effects of course. The power section, including the power transformer, is very Marshall-inspired. The PT is the same unit they sell for their JTM45 kit.

It’s a great sounding amp, but different from the originals that I’ve played. Cleaner and punchier, which I attribute to the Marshally power amp. I think the low screen voltage from the Silvertone voltage doubler contributes to the originals’ signature tone, which is a bit mushier.

[johnfromcyrene]

Do you need a special transformer for the voltage doubler? or do u just use a lower powered one.

[passaloutre]

I think it requires a different transformer spec.

It may be possible to use a more traditional power supply and just drop more voltage to the screen node for the same effect? I’ve never tried.

[johnfromcyrene]

which one sounds better then?

[kagliostro]

Do you need a special transformer for the voltage doubler

When you use a voltage double you double the voltage at the expence of the current

Si if you require a 100mA you must use a doubled current transformer (200mA)

Franco

[passaloutre]

which one sounds better then?

Well that's pretty subjective. I may be one of the few who've played both, and I'd say they both sound great. The "tone" is quite similar, as the preamp hasn't changed, while the changes to the power supply affect more the "feel" of the amp. I would say the Silvertone has a spongier, compressed feel with less headroom, and the Mojotone version is more punchy and dynamic, loud.

It was my first amp build, so I followed Mojotone's circuit faithfully. Knowing more now, many years later, the Mojotone design almost seems a step between the original Silvertone and something like a 1970s Orange amp: https://el34world.com/charts/Schematics/files/Orange/Orange_graphic_mkii_or120.pdf, the chief difference being negative feedback.

[johnfromcyrene]

I think the mojotone uses diodes instead of a rectifier. what about adding a gz34 to create sag for the amp?

[passaloutre]

You can definitely use a tube rectifier to get more sag, but you'd be building yet a third different version of this power supply, so no one here can tell you what it will sound like. It won't be a 1484, but it may be something you like

[johnfromcyrene]

I believe you can also simulate sag with a resistor as well.

[johnfromcyrene]

Also I took a look at the schematic again and to my eye it looks like all they really changed other then deleting the trem and reverb was change those diodes for the doubler circuit.

[bmccowan]

It’s a great sounding amp, but different from the originals that I’ve played. Cleaner and punchier, which I attribute to the Marshally power amp. I think the low screen voltage from the Silvertone voltage doubler contributes to the originals’ signature tone, which is a bit mushier.

the changes to the power supply affect more the "feel" of the amp. I would say the Silvertone has a spongier, compressed feel with less headroom, and the Mojotone version is more punchy and dynamic, loud.

I suspect these comments hit the nail. Overall higher voltage has those effects. If wanting the sound of the original I would seek out a lower power PT. For example Mojo has a lower power JTM45 PT, and Hammond has many options. It'd be interesting to compare the B+ voltages of originals vs the Mojo . 
Further, I see that the difference between plate and screen voltage is dramatic. I didn't know this when I owned one - I was a Freshman in High School 1968! At the time I was looking to trade it for a solid state amp! Who knew - not me.

[passaloutre]

Yes! I think it's not just the overall B+ plate voltage, which shouldn't be that different between the original Silvertone and the Mojotone (the Silvertone schematic says 475 and my Mojotone had 465), but the *difference* between the plate and the screen.

[johnfromcyrene]

Are you saying just to use a different power transformer then? change that and nothing else?

[passaloutre]

I'm not saying that. I'm just discussing the differences in the circuits. It's up to you which one you want to build.

If you want to stay true to the original, Mercury offers a "ToneClone" power transformer here, for $200: https://www.mercurymagnetics.com/model/1484/. Looks like it has two 91V taps for the voltage doublers, and another 26V tap for the bias supply.

Otherwise you can use a more traditional power supply, and choose to keep the screen voltage high (as Mojotone did), or find a another means to drop the screen voltage.

For the record, here is the original schematic: https://el34world.com/charts/Schematics/files/Silvertone/Silvertone1484.pdf

[bmccowan]

You might want to look at this thread https://el34world.com/Forum/index.php?topic=16698.msg165443#msg165443
and contact Tubegeek - he built one and was pretty happy with it.

[johnfromcyrene]

So if you wanted to lower the voltage you would increase the 1k 5 watt resistor in the mojotone layout?

[passaloutre]

Yes that would drop the screen voltage and all the preamp voltages. You’ll have to experiment with dropping resistors to get it right. My estimate is 15k, but I could be way off.

[bmccowan]

Alternatively you could use a parallel power supply scheme similar to what most Matchless amps use. That would allow the selection of individual dropping resistors without a significant effect on voltage delivered to the preamp stages. Sorry if that just complicates the discussion.

[SoundmasterG]

I think the mojotone uses diodes instead of a rectifier. what about adding a gz34 to create sag for the amp?

The stock Silvertone 1484/1485 amp with the multiple voltage doubler stages creates sag all by itself, even with the silicon diodes. A voltage doubler doesn't have much sag until the current demands get high, like when you have the amp way up loud. When the demands get too high, the doubler circuit will sag even more than many tube rectifiers. In the 1484 and 1485, they used two doublers stacked on top of each other, so you have even more sag than usual when the current demands go up. Add to that a really small output transformer that doesn't have much low or high frequencies in comparison to a larger transformer, and the fact they are using a 6CG7 for the phase inverter in a cathodyne circuit, and preamps with lowish gain, you end up with a unique amp. The fact that the screen voltage is using only a couple of the doubler stages instead of all of them limits the power and makes the tone a bit muted and soft. Moving this to the same tap as the B+ like a Fender does ups the power, but not much. These amps are really only about 30 watts RMS in stock form. Moving the screen tap gets you to about 35 watts, maybe, but it increases the risk to blowing the stock output transformer. That said, an amp I modified for my brother years ago does this along with many other mods, and it is still working on the original transformer.


I'd guess the Mojotone circuit was likely using different phase inverter tubes than the 6CG7 since no one makes it these days. You could sort of duplicate it with a 12AU7 but it doesn't have as much current capability. I couldn't find the kit on Mojotone's site, and their link for their schematics went nowhere also.


Greg

[johnfromcyrene]

I posted the mojotone schematic and layout at the top.