silvertone 1484 reverb pan repair notes:

[soundmasterg]

My 1484 was just as clean when I picked it up but for its blown OT.

I'm assuming you've fixed it already, but if not, this particular OT is one that Weber sells a copy of: https://www.tedweber.com/wot1484/

The original transformers were 4 ohm on one tap and 2.66 ohm on the other one. The same transformer was used in the 1484 (4 ohm tap) and the 1485, which used two output transformers. (2.66 ohm). The Weber will therefore work fine for the 1484 but for the 1485 there is a mismatch if used with the original speaker cabinet.

If anyone is interested, there is a long thread about modding one of the 1484's on the Hoffman forum from maybe 15 years ago that I did. I modded my brother's 1484 because it was missing the reverb and needed some work anyway and he said go ahead and make it better. Channel 2 I modified to make it more like a Fender for gain levels in the preamp, but still with the Baxandall tone stack, though I revoiced it to get the center of the stack up into guitar frequencies. Channel 1 is using an EF86 among other things. I added a doubler and an adjustable pot to the bias supply to get better range. I added a Fender reverb circuit in also, with a pan and made a new head for it. I elevated the heaters to reduce hum, and modified the grounding to have more control over where everything grounds. Added Fender style input and output jacks, with isolating washers so I could control the grounding. I put trem on both channels, and used the former ground switch to be able to switch the trem off for a slight gain boost. It can be either channel, or both channels, or off entirely. Built a 2x10 cabinet for it which is using Eminence Legend speakers. It sounds great and works well. A couple downsides though - due to the power transformer placement, there is only so much you can do and it will hum more with increased gain.

We used to run the amp before I modded it into an 8 ohm load for a long time with the 4 ohm tap and it surprisingly held up just fine. The mismatch made for a compressed sound that was lower volume, but more distorted and great for lead tones.

They're a pain to work on with the stupid layout and all the part leads wrapped around the terminals, but they're cool in a cheap sort of way. I have a stock one that needs a little TLC somewhere around here. One of these days I'll finish getting it working properly.

Greg

[nuke]

I took the 1484 head and a 2x10 cabinet loaded with Alltone 1030 to an open jam a couple of nights ago. The 1484 speaker was just more than I wanted to haul around. I set it up and let it rip and anyone was free to play it. So, got to play it and hear it while others played it.

Honestly, it has a really good tone of its own, mid-rangy, gets through without being crazy loud, really sits well in the band setting. Reverb is a weakness, but it worked.

I'll do some more work on it. Pulled it up and documented voltages around the whole circuit and it is fine at 121.4vac out of the wall. I need to document actual power output and I'll take some measurements of the output transformer, determine turns ratio, inductance and so on.

Need to ponder if there's anyway to get the reverb to work better. The circuit is a little weird, there's no control over the recovery amplifier, that end just runs full gain, the reverb knob controls the drive into the spring (Fender called it 'dwell' on the outboard units). The reverb knob has a switch, and turning it off opens the connection from the return end of the spring. That makes it very susceptible to noise bumping it, or worse, feedback from the speaker if played loud on the right notes.

Might make a homebrew version of it. Unique sound, vibe of its own, but robust enough to haul around.

[solderhead]

Greg, it sounds like you've done all of the mods that I was thinking about doing with my old 1484 head, but never got around to doing. I was thinking of exactly the same type of Fender reverb mod, building a little reverb box that the amp could sit atop (for holding the pan). I agree that the way the amp is wired, and the cheap corner-cutting parts (like the voltage doubler PT) never made me too enthusiastic about modding my amp. (Got a link to that Hoffman thread?)

Of course, now all of that lack of enthusiasm is changing in the light of nuke's positive reviews. I agree with this idea that the amp could really be interesting as homebrew where the amp is built with more robust components and easier to service construction methods, though I do think that the cheap non-interleaved OT likely with it's limited frequency response contributes to the signature tone. I wonder if replacing the PT with something more robust would change the character of the amp. How much does it sag?

[nuke]

I did a little more noodling on the bench with the 1484 last night.

Power output is a a very solid 29 watts RMS at the verge of clipping. About what I expected, especially with the screens being powered from a lower-voltage node of the power supply multiplier. Bias is not adjustable, measured -34.5v in mine. That set the idle current at 30 to 34 ma, or 15-16 watts.

Driving it into clipping didn't reveal any odd behavior, no oscillation or such. Just a little clipping on the top peaks comes in first, then both peaks, then flattens out more with the usual notch in the crossover region. Pretty well behaved actually.

One tip on the reverb: If you get hum in the recovery side, swap the leads from the reverb return. Turns out, the reverb spring is completely symmetrical. Either side is send or return, and the leads are not shielded or polarized at all. The return side is right next to the power transformer (duhh). But just swapping the leads quieted the hum.

It turns out the reverb spring got dislodged a little bit from transporting it to the jam and back. I have had some conflicting reasearch about how tight to tie the hold-down thread in the middle of the spring. I set it kind of loose, with just a little tension on it. Some say to tie it all the way down to touch the masonite. Also, the kapton tape I used to insulate the tabs stayed stuck, but it is pretty slippery and the spring doesn't hold on to it well. I may revert to paper, at least for the end supports.

I would suspect the failure mode is the hold-down thread breaks, leaving the spring to fly around inside the box and coming off the end-tabs. The inside of the aluminum cover on mine bears witness to a lot of rubbing by the spring.

Gotta figure that back in the day, Sears was shipping these things all over the place both to stores and tons were sold via the Sears catalog. They would have been built well enough to survive that far.

Maybe paper offers more friction to hold the spring and I'm going to swing by Bass Pro for some heavy duty thread for fly fishing to re-tie the middle thread. I may tie down the ends of the springs too for good measure.

I always thought the output transformer was puny, but it actually has a lot of iron stacked up in it. It is just an odd shape, but it is a little beefier than a Pro Reverb or Vibrolux transformer for the same power/4-ohm application.

[soundmasterg]

Greg, it sounds like you've done all of the mods that I was thinking about doing with my old 1484 head, but never got around to doing. I was thinking of exactly the same type of Fender reverb mod, building a little reverb box that the amp could sit atop (for holding the pan). I agree that the way the amp is wired, and the cheap corner-cutting parts (like the voltage doubler PT) never made me too enthusiastic about modding my amp. (Got a link to that Hoffman thread?)

Of course, now all of that lack of enthusiasm is changing in the light of nuke's positive reviews. I agree with this idea that the amp could really be interesting as homebrew where the amp is built with more robust components and easier to service construction methods, though I do think that the cheap non-interleaved OT likely with it's limited frequency response contributes to the signature tone. I wonder if replacing the PT with something more robust would change the character of the amp. How much does it sag?

Solderhead, you know I looked over there and went back and looked at all my posts about 1484's back to 2010 and I couldn't find the thread in question. Here is a basic rundown of what I did though.

I did pretty much whatever you could imagine to a 1484 and documented the pros and cons pretty thoroughly, but I can't find that thread right now. You can boost the power of the amp a little by adding a filtering stage and connecting the screens to the stage in the doubler after the power tubes....so they would have three stages instead of the stock two. It will take the RMS power to 30-35 watts from the stock 25-30 watts. The OT is very small on these amps....if you go with something larger and bump the screens up you can likely get to 40 watts or so with much better overall frequency response, but the mid focus is part of the character of the 1484 too. On my brother's I added an EF86 to channel 1, added a Fender reverb circuit and tank, put verb and trem on both channels, bumped up channel 2 for more gain, and redid the whole preamp layout so the signal didn't cross back on itself. I also redid the grounding and isolated it from random chassis grounds. Due to the increase in gain, it hums more than stock and that can't be avoided due to the very poor PT location. The amp sounds good, with more power and better reverb, but still some Silvertone character since I stuck with the stock OT and stock phase inverter. I had to revoice the tone stack in channel 2 as when I increased gain the previous stock circuit was now affecting bass frequencies. I left it as the stock Baxandall tone stack but I shifted frequencies by changing parts values. I have pics but they are blurry....but I can share if you want. I can also get some new pics as my brother wants me to mod something on it again. The never ending project! YMMV.

The Silvertone 1484 sags a bit with the stacked doublers. I modded a Bogen CHB100 which also uses a doubler. (just one doubler on that one) I feel like an amp with a doubler in the power supply probably sags as much as say a power supply with a 5U4 maybe. I could be wrong as I haven't measured it, but it feels like a squishy tube rectifier when you're driving it hard. You can use bigger caps too since you don't have to worry about exceeding a tube rectifier limit, so you can get a squishy but firm power supply if that makes any sense? I will say that my modded Bogen CHB 100 is the most touch responsive amp I've ever played, and I own a Vox AC30 too which are known for being touch responsive.

I can get you some pics of the amp if you like and show the headbox that I had to make in order to add the Fender reverb. I have the amp here right now and it will be here for awhile still as I need to fix it and mod it again and I don't have the time right now to do the work it requires. The schematics and layout for what I did is all on paper so that isn't easy to share, and it will be changing anyway once I get time to work on it. I plan to move the EF86 to the second stage on channel 1 instead of the first stage. I think it will work better. It never had a problem with microphonics, but that could be because I accidentally spilled red loctite in the tube socket back when I first did the mods and the EF86 is basically welded in place LOL. I need to figure out how to get that out somehow so we can reuse the tube in the other socket. Let me know if you would like me to share pics.

Greg

[solderhead]

I understand the problem with paper documents. I prefer to work on paper as well.

I always love to see amp porn photos. I'd be especially interested in the reverb mods.

[nuke]

Some interesting notes.

The thread holding the reverb spring is critical and very very tweeky.

I got some braided fishing line to tie it down. I think this would be best thread. It is non-stretchy, doesn't easily abrade and strong. I'd suggest 20lb-30lb test. The 50lb is a bit thick.

Tried it yesterday tying the spring down tight and contact with the masonite.

That produced a very strange effect. The reverb basically became a 3rd channel on the amp. Literally I thought I had shorted something in the circuit.

But what happens when the spring is tied tight, the piezo sender vibrates the masonite strip it is mounted on, transmits the sound through the masonite to the receiver piezo.

The electronics of the 1484 are not like Fender. The reverb knob controls the sending strength, with a tap before the volume control. The recovery signal is simply injected back into the amp circuit, *after* the volume control.

The 1484 drives the snot out of the sender piezo crystal, you can easily hear the reverb unit make sound on the bench, with the standby switch in the standby position and a dummy load on the speaker output. (ie, no speaker, no output transformer ringing.)

It seems there is a happy place with respect to how to tie the spring in the middle of the reverb pan. Too loose and the spring flops around and is very susceptible to acoustic feedback and howling in loud settings. Too tight, and all the sound couples through the masonite board on the bottom of the pan, not through the spring.

I think about 1/8" to 3/16" of an inch are about right. I'd suggest putting a 1/8" spacer in place, tie the spring down snug to that and remove the spacer and that should do it.

In that position, the amp produces a usable level of reverb, doesn't howl or get acoustic feedback, and it doesn't transmit sound directly through the masonite.

I've searched a bunch of pictures and schematics, Danelectro used this kind of reverb in a number of amps, and all of the photos of the spring unit show the same deal, either a thread, the remnant of thread, or at least the razor cuts in the masonite and the duct tape over the middle of the tank where the thread goes.

The circuits are varied. The 1484 and its big brother, the 1485 are nearly identical, but the reverb send is drawn from the wiper of the volume control, versus top of the volume control. That's a one-wire that would probably be better, but it would weaken the reverb a bit.

I think the customers were asking for that "wet surf sound" and Danelectro obliged best they could in the 1484. I've heard sound clips from the 1485 and the reverb is not nearly as strong.

The piezo thing I suspect may have been to get around patents on electro-magnetic reverb springs, as used on Hammond organs and Fender amps. Or perhaps, the piezo was just a lot cheaper, no transformer to deal with.

My best effort yields usable reverb from this amp. I'm gonna stop with that most likely. I may move the pick off to the volume wiper to see how that works out, but this is probably what it is good for.

All in all, the amp sounds really good actually. It's crisp and tight in the bass, overdrives nicely actually and is reasonably loud enough to take out and play with a band.

[soundmasterg]

I understand the problem with paper documents. I prefer to work on paper as well.

I always love to see amp porn photos. I'd be especially interested in the reverb mods.

Here are some pics. For the reverb mods, as I said, it is just a transplanted Fender circuit, and key to that is to rearrange the tube order so the signal doesn't cross over itself or you might get oscillations.

Forgot to mention a few things; I added a NFB switch and some NFB. I also added an IEC socket for the plug. Elevated filament center tap with associated circuitry, though I forget how many volts I elevated it. Voltage doubler on bias circuit and bias adjust pot.

At present, Channel 1 is EF86 gain stage > 1/2 12AY7 cathode follower > FMV tone stack but slightly changed values > 1/2 12AY7 gain stage. Channel 2 is 1/2 12AX7 gain stage (similar to Fender BF first gain stage) > Baxandall tone stack with modified values to move the center frequency up into the guitar's range (if you increase gain, the center frequency of the tone stack goes lower so need to re-voice) > 1/2 12AX7 gain stage similar to BF Fender. Those channels mix like a Fender and the reverb adds back in here also. Then on to the stock phase inverter > stock power section, except screen supply is moved to third node and filter stage is added for screens. Moving to the third node increases overall power of the amp and brightens it up a bit. The trem goes into each preamp after the first stage but is otherwise almost the same as a BF Fender. Injecting it earlier gives more depth, and this is how Silvertone did it, and it sounds fantastic so I left it there. I did hook it up to a switch however and I can have trem on either channel with the normal gain reduction that you have with a roach trem.

The reverb tank is mounted above the amp head on a steel platform in a new, taller head box. I forget which way it is turned around but one way hums a lot and the other doesn't. The reverb circuit is identical to a BF Fender with the same transformer, a 12AT7, a 12AX7 recovery stage, etc. I made the 2x10 also, but my brother chose the cabinet color. I would have used the same red as on the head, but he decided to reverse it on the speaker cabinet.

I plan to change Channel 1 around and have a 1/2 12AY7 gain stage > EF86 gain stage > 1/2 12AY7 cathode follower > FMV tone stack but slightly changed values. I need to look closely and see if I can accomplish this without having to change the tube locations in the chassis. As you can see, it is a serious rats nest.

Greg

[soundmasterg]

And some gut shots.

Greg

[nuke]

Wow, that's a huge amount of work that went into that! Bravo! So many mods in a point-to-point chassis must have been many, many hours of effort.

I'm trying to keep the one I have as true to its nature as I can, while getting it to perform as well as possible.

Now that I have it working, I like the sound of the amp and it performs well in a live band setting with a variety of players and guitars. The basic circuit is good and it is unique, doesn't sound like another Fender or a Marshall (same thing? lol) or a Vox.

The reverb is a weakness, but Danelectro made a number of amps with reverb of the same type, and several variants of the circuit. So, it must have worked enough to sell the amps. Danelectro even made a stand-alone reverb box, with a single long spring. The solid state amp version of the reverb has an inductive sender and a piezo receiver on the spring.

The amp's reverb as it exists, has plenty of gain and could support an inductive spring reverb. The drive circuit in the 1484/1485 and similar amps, uses the 6CG7 as a floating paraphase inverter and drives the piezo directly. That's clever and neatly avoids the need for a transformer as found in Fender amps. I'll have to work out the impedance of the 6CG7 drive, but I suspect it is possible to drive a high-Z coil spring directly, or perhaps in conjunction with a matching transformer with the existing circuit. The recovery amplifier has loads of gain already.

I think the easiest solution for deep reverb is just to use an outboard unit. Just insert it between channel 1 and 2, in the same manner one might put a jumper between channels. An outboard spring reverb unit could be constructed with solid state and a short-style spring for under $100 not counting whatever housing you want. Heck, even a digital reverb pedal connected the same way would be very good.

[soundmasterg]

Yeah you got that right Nuke! It was a TON of work, but not just the reverb. I did so many things to this amp. I think it took me around 9 months to do everything. It's not just any point to point chassis - Danelectro made these hard to work on since they wrapped all the part leads around and through the terminal strips. Makes it robust though. It does sound better than stock, and I wouldn't have bothered if the amp was completely there, but since the reverb tank was missing I figured why not. It has worked well for around 15 years but he wants to make some changes to it now so it is back for round 2! I have a stock one myself that I recapped, and everything works but it has issues and likely some of the 20% resistors have drifted. It's on my pile of things to do so I'll get to it eventually. Thankfully the reverb works fine on that and nothing is warped with the tank so 'knock on wood' hopefully that will stay working well.

If I wanted to add better reverb to another one of these, I have the blueprint from what I did to this amp and know how to do it and what to do, but not sure I'd go to that effort again as it is a LOT of work. There are likely easier ways and probably most people would just use a pedal these days. That said, I learned a lot with the project and I made it better so that's all you can ask for sometimes.

Greg

[B Ingram]

... If I were smart I would have tuned it up and sold it when the White Stripes exploded on the music scene and Twin Twelves skyrocketed in price. I was just too busy to bother. The heads are now worth about 1/3 of what they were worth at their peak. I keep the amp around as an example of the era when American manufacturing with PTP construction was actually inexpensive enough that it could be used for bottom-tier amps that you'd find in the Sears catalog.

I never understood why people liked the amp. ...

These things were never "great amps." They were department store budget alternatives to better-grade instruments & amps sold in music stores.

The original jump in their collectibility in the 1990s going into the 2000s was nostalgia-driven, as guys who got them for Christmas when they were 11 were reliving their youth now they were in their 40s and 50s with disposable income.

The 2nd period of collectibility around the White Stripes was about creating the image of "I'm not wealthy; I'm poor & can make music with junk."

[nuke]

After thoroughly going through this one and making it work they way it did new, I have to say, it is a surprisingly useful and good sounding amp. It breaks up nicely at a loud but reasonable volume, and it works well in a live setting keeping up with bass, drums and other guitars playing through smaller amps like Blues Junior. I like it a lot with a 2x10 cabinet loaded with the old Alltone 1030 speakers. I loaded some modern Jensen C12Q in the 1484 cabinet (the original C12Q were long gone), so it is a bit brighter with those.

The reverb isn't going to win a prize, but at lower volumes and reasonable settings, it really isn't bad. It took a lot of dinking with the spring unit to get it working well. There's just not any info on how it is supposed to work, and I guarantee you, every one of them on earth has a broken thread, unless recently repaired. After fooling with the tie-down thread, I think I got the trick and the way the spring has to be stretched just right and to use something with friction to insulate the metal supports from the spring. Tacked the spring ends down with a small drop of titebond to acid-free art tape.

I kinda set this one aside 20+ years ago. Pretty much always heard the same opinion that it wasn't much of an amp, figured it was mod fodder. But I have to say, pleasantly surprised by it. I'm glad I decided to make it work right before chopping it up.

I'll post some more tech nerd stuff from measurements and notes I took along the way. I plotted out a lot of the load lines and other calculations, since it isn't designed like most other guitar amps. It is its own bird, with some interesting design choices. The voltage multiplier power supply is an interesting choice technically, and I don't think it was because they were being "cheap" in choosing it.

The really bad part is how fragile the cabinet and chassis are. Literally, about as structurally sound as a tater-tot. It is also not a convenient size for grab-n-go jams and gigging. But sound-wise, it is an interesting and useful amp, especially if you want to sound "not the same". The authentic "relicking" job that years of abuse and neglect left, you couldn't pay even Murphy Lab to do as well. It just looks like "the blues".

I'll study it some more, because I feel some inspiration for a home-brew derivative of it.

[soundmasterg]

The voltage multiplier power supply is an interesting choice technically, and I don't think it was because they were being "cheap" in choosing it.

The stacked voltage doublers is actually based in cheapness. If you use the voltage doubler power supply, then the caps you need to use are lower voltage, and at the time good caps were very expensive. So they saved money there. Also using a transformer meant for a voltage doubler (or in the case two stacked doublers) is a bit cheaper to make than a standard power supply. They weren't thinking it at the time, but doublers don't regulate that well so they have some sag characteristics, which we guitarists happen to like. They tend to keep it together pretty well for most things, but when you really stick the beans to it and demand high current, they sag out a bit due to their poor regulation. A lot of organs and cheaper hifi stereos or console stereos used doublers as most of the time they were in a steady operation so the doubler worked fine. They tend to hum more than a standard supply also, which is a bit harder to weed out, but it is doable.

Greg

[nuke]

The stacked voltage doublers is actually based in cheapness. If you use the voltage doubler power supply, then the caps you need to use are lower voltage, and at the time good caps were very expensive. So they saved money there. Also using a transformer meant for a voltage doubler (or in the case two stacked doublers) is a bit cheaper to make than a standard power supply.

Greg

I'm not sure that is necessarily all there is to it.

I looked up capacitors and rectifiers in a 1965 parts catalog. The 100uf at 150v electrolytics are actually more expensive than 20uf at 500v. ($1.05 each vs. $0.96 each in the 1965 Mallory book). The 20uf at 500v are actually a little bit smaller too. The high voltage electrolytic capacitors were ubiquitous at the time, I mean just about everything was built from tubes and had high voltage. The silicon rectifiers are $0.80 each, nearly as much as the caps! (mallory price list for qty 1)

The transformer in the 1484 has two secondary coils, which is in reality, exactly how a center-tapped winding is made and they just connect the windings. Just a few more turns on the secondary coils with a lighter gauge of wire. I'd say the cost would be a wash.

But I suspect they had a couple of goals in mind, namely, getting rid of the tube rectifier, and I think they were intent on running the 6L6GC on lower screen voltage for longer life. Those two goals probably lean more towards reducing warrantable failures than reducing BOM cost.

The voltage multiplier stack up in this case, provides a really well situated node for the screen supply, about 130v lower than the plate supply.