Silvertone 1484 Restoration - almost there, but need assistance!

[q9522678]

Hi All

First post here, hoping you call help me with what I’m hoping are the last stages of a tube amp restoration. I’ve been lucky enough to be given a Silvertone 1484 Twin 12 piggyback head in stock condition which appears to have been stored quite well. The MDF head shell is still intact, the tolex is good and not a lot of rust or any major degradation from a cosmetic perspective.

I have been working on restoring this amp for the past few weeks. I’ve replaced the two prong power cord with a three prong cord, installed a new fuse holder, installed a speaker out jack, replaced all electrolytics throughout and replaced all out of spec carbon comp resistors with new 1w carbon films. I tested remaining resistors and caps for noise and other issues and they seem fine (not that there are many left!!! ).

I replaced all tubes with known good ones for now- simply for the purposes of testing, to rule them out as the cause of any other issues. The amp came with all original Silvertone tubes- which will be going back I once the restoration is complete (provided they are functional!).

Now the amp fires up and plays. It is very quiet , no concerning hum or anything like that. My only concern is it doesn’t sound like 25w in terms of volume and only mildly breaks up, even with a hot humbucker guitar, which is odd for a 1484.

I tested all key voltages and many seem a little low. I live in Australian and this is a US amp. I’m using a step down transformer for wall voltage. Used my DMM to confirm this is pumping out 122v AC.

In terms of amp voltages I measure the following:

Heaters - 3.2v (should be 6.3v)
Pin 3 of 6L6s - 447v (should be 475v)
Output of voltage doubler (C26) - 449v (should be 480v).
V1 - Pin 1 - 135v (should be 100v) Pin 6 - 135v (Should be 100v)
V2 - Pin 1 - 87v ( should be 93v) Pin 6 - 88v (should be 93v)
V3 - Pin 1 215v ( Should be 225v) Pin 6 - 103v (should be 100v)
V4 - Pin 1 - 56v (Should be 55v) Pin 6 - 130v (should be 160v)
V5 - Pin 1 206v (should be 215v) Pin 6 - 201v (Should be 215v)

Cap can replacement voltages:

22uf (C25C) - 329v (should be 340v)
10uf (C25B) - 173v
8uf (C25A) - 224v

My main concerns are the low heater wire voltage - at around half where it should be, the low power tube voltages on pin 4, and the high voltages on V1. I’m guessing this is contributing to the lower volume.

The amp plays fine, just quieter and less raucous than others I have played. Could this be a Power Transformer issue?

The stock reverb was dead (not a big deal...) . One of the piezo pieces had snapped off. I’ve started working on installing a new Accutronics spring reverb ( 8EB2C1B - 800ohm input impedance) using the existing reverb tubes for drive and recovery. Is working very faintly. Needs some tweaking but have shelved the reverb install until I sort the voltages in case this is affecting the strength of the signal being pumped into the tank,

Open to any suggestions? Ideally would like to get the voltages closer to where they should be - particularly the heaters.

Many thanks

Paul

[Enzo]

Hi

Your heaters have 6.3v if they light up and it amplifies. If you really had only 3v, it wouldn't work. You are taking readings to ground, which is either side of the center tapped winding. Only thing that matters is that you have 6v ACROSS the tube heater, NOT to ground.

The tube DC voltages do not bother me at all, really. This is just a guitar amp, nothing precision about it.

As to power supply, the 340v is on C25A, not C25C. And I think you have the other two out of order. I see no way for C25C to be higher than C25B. In any case, I see no problems. The fact they are out of order concerns me that you may have wired them in the wrong positions. The A, B, C sections are in decreasing capacitance and decreasing circuit voltage, in that order.

Could this be a Power Transformer issue?

No. Power transformer failure is THE single least likely failure in any amp.

[g1]

Agree with Enzo, most of those voltages are in the ballpark.
When you say other 1484's have more gain, do you mean ones you have tried with this guitar?
As far as the main B+ voltage, if the power tubes are biased too hot, that could pull down the supply voltage. Do you have a way to check the idle current of the power tubes?
And can you post voltages for pins 3 & 8 of the preamp tubes you listed?

[q9522678]

Hi

Your heaters have 6.3v if they light up and it amplifies. If you really had only 3v, it wouldn't work. You are taking readings to ground, which is either side of the center tapped winding. Only thing that matters is that you have 6v ACROSS the tube heater, NOT to ground.

The tube DC voltages do not bother me at all, really. This is just a guitar amp, nothing precision about it.

As to power supply, the 340v is on C25A, not C25C. And I think you have the other two out of order. I see no way for C25C to be higher than C25B. In any case, I see no problems. The fact they are out of order concerns me that you may have wired them in the wrong positions. The A, B, C sections are in decreasing capacitance and decreasing circuit voltage, in that order.



No. Power transformer failure is THE single least likely failure in any amp.

Hi Enzo

Thanks for the prompt and detailed reply.

Reference the heater voltages, that’s a relief. You are correct - I was measuring to ground and not across the pins. Slipped my mind to measure that way, first amp I’ve working on in 4 years (had previously given up fixing amps as preferred spending my limited time playing rather than fixing).

With the cap can replacements caps, yes, the 22UF is “A” but you may be right in that the smaller two are wired back to front as I recall the 8uf (C) measuring higher than B. Will need to check will I get home. I thought I’d traced the wires and checked the circuit previously but may have missed it.

Good to know that nothing stands out as significantly amiss with voltages.

[q9522678]

Hi g1

I played a couple of 1484s a few years ago- different room, different guitar. Appreciate there are many variables both internal to the same tube amps, let alone considering the guitar and other environmental factors.

I remember those amps have quite a bit of grit and dirt when dimed. This one sounds almost glassy - with more headroom. It doesn’t sound bad. Just different to previous ones I’ve played. If that’s the character of this amp - all good. I can certainly live with it!

I send through pin 3/8 voltages when I get home.

[q9522678]

Hi g1

I played a couple of 1484s a few years ago- different room, different guitar. Appreciate there are many variables both internal to the same tube amps, let alone considering the guitar and other environmental factors.

I remember those amps have quite a bit of grit and dirt when dimed. This one sounds almost glassy - with more headroom. It doesn’t sound bad. Just different to previous ones I’ve played. If that’s the character of this amp - all good. I can certainly live with it!

I send through pin 3/8 voltages when I get home.

Pin 3 and 8 voltages respectively:

V1 - 1.16v and 1.45v
V2 - 0.67v and 0.56v
V3 - 117v and 3.32v

V4 - 0.69v and 0.9v
V5 - 7.06v (pin 3 and 8 jumpered)

[q9522678]

I just checked the power supply caps.

The circuit looks wired ok - however I did note that one the 68k resistors that bridge the power supply caps was in the wrong place - 20uf and 8uf were wired together instead of 20uf and 10uf which would explain the whacky voltages here and likely V1. Haven’t had a chance to rewire and retake voltages across the as yet, but will post back when I do, but suspect this will bring most back into the advertised range in the schematic.

Next challenge is the new reverb tank. I’m using a proposed schematic I found online to convert V4 and V5 to drive a reverb high impedance reverb tank. Whilst it seems to be working - is quite faint. If I turn the channel 2 volume down and turn the reverb up, it’s certainly there. And with channel 2 mixed in you can here the reverb but certainly not loud or washy. Seeming like the front end being driven enough. I’m by no means a reverb expert (this is my first attempt) - but what are the standard ways for boosting the input?

[q9522678]

This is the reverb circuit I am currently using for inspiration:

https://www.dropbox.com/s/nufi88kd1k...%2049.jpg?dl=0

Note I have removed and jumpered R40 (560k).

The tank I am attempting to mate with it is an Accutronics 8EB2C1B. Whilst there appears to be quite a bit of discussion online about replacing the stock piezo reverb with a non piezo tank but no real detail regarding how this is done. Many appear to go with Ampeg VT style circuits.

I think the circuit I am using was drafted but not necessarily tested. Open to any suggestions / observations.

[g1]

What do you get with R40 (560K) in circuit? I think shorting it out makes the reverb tank output transducer almost short that V1B grid to ground.
Also, not sure why they didn't just use standard parallel triode to drive the tank (tie plates together instead of running to 2 separate coupling caps). The way it is, any cap differences can lead to phase cancellations. With plates tied together you would need to adjust the plate resistor value.
But first, just try with the 560K in there, and maybe adjust its value to taste.

[Enzo]

Take a step back and look. Removing 560k doesn't load the grid, with the 560k, there is a voltage divider. The signal from the reverb device was piezo crystal, fairly hot. Signal level divides down via 560k over 68k R41. Short 560k, and now full device voltage hits grid. Probably a good idea as I suspect a real pan produces a lot less signal than the old piezo.

I agree, the whole double V2 thing seems pointless. It is just the original circuit bent to fit...poorly. If you want to try the original circuit, just eliminate V2b, R46 and C19. C18 to the drive end of the pan, and ground the other lead from the pan drive. Putting the extra triode in parallel with the first ought to be a good idea too.

And do yourself a favor and move the depth control over to where R40 is in the return circuit. Then just wire the drive circuit input up full on, and important: move the source to after the volume control.


Alternative: consider the original drive was a push pull. R47 provides out of phase signal for lower triode. If you use a reverb pan with isolated input jack, you could wire it up like the old device. No ground connection to drive side. Signal might be too hot for the pan, but is easily reduced with a resistor.

[Helmholtz]

I think shorting it out makes the reverb tank output transducer almost short that V1B grid to ground.

No problem with that. The V1B grid sits on 0 DCV anyway. It will load down the output transducer via R41 (68k), though, causing reverb treble loss with an inductive transducer.

...any cap differences can lead to phase cancellations..

Hardly. While there may be a small signal current phase difference at low frequencies, "phase cancellation" (meaning destructive interference) would require a phase difference of more than 90° which doesn't seem possible with this circuit. Up to 90° phase difference the 2 currents positively add.
Don't know why they choose this driver circuit. But the 2 triodes are actually wired in parallel for AC signals. Maybe the reverb transformer was not suitable for primary DC current (no air-gap) and/ or connecting the plates would have caused instabilities.

Bypassing R42 will increase reverb return level.

[q9522678]

Hi

The grids (2 and 7) and cathodes (3 and 8) are tied together. That’s how I read the schematic and both are jumpered. However separate caps come from each tube pin per the original design. Can certainly simplify this by placing a single larger value cap in there attached to the jumpered plates) - I was simply minimising the work involved and working with what was already in there. Noting the plates are tied together at the grid is there still likely to be phase issues with rather two parallel capacitors? In any case will experiment with this.

Why remove the 560k resistor? This was included on notes that accompanied the schematic - suggested jumpering this resistor (unsure why this wasn’t amended on the schematic). Apologies, should have mentioned that.

I also found this blurb where it appears an amp tech had successfully installed the same tank using what appears to be a similar circuit but driving as a cathode follower (NB I tried this also with same results).

"Silvertone Spring Reverb tank mod:
Use a short 3-spring reverb with 800 ohms input: 8EB2C1B.

First build a tank driver:
Re-wire the reverb driver tube as a follower: remove connections to Pin 2 and wire Pin 2 to Pin 7.
Connect one end of a 1.5uf 250V cap to the cathodes. the other end goes to the reverb tank input.

Now fix the reverb recovery circuit: Jumper the 560K resistor and remove the resistor (and sometimes capacitor) that connect to Pin 2 of the reverb recovery tube. Reason is, these parts make a voltage divider that kills the signal. Install a 220K resistor from Pin 2 to ground as was done in AB763."

Again I have tried driving from both the grids or cathodes with similar results. The reverb is there but very quiet.

Will experiment with 560k resistor back in there as can’t hurt..

[q9522678]

Enzo,

Will give your suggestions a shot and report back. Will seek to simplify the front end and move the input and depth connections and let you know how it goes.

[g1]

Sorry, I stand corrected on R40.
As far as the parallel triode, I did not realize this was the original way the circuit was, and thought it an odd way to do it rather than the standard paralleled triode like used in standard Blackface fender. So easiest to just stick with the stock configuration.

Agree with Helmholtz that cap bypass on R42 will help, and I'll ask the others if something can be done in the R26 mix area to increase recovery signal.

[HaroldBrooks]

Just a general statement about the same amp having a different level of gain... I have amps with very close circuits, and sometimes the gain is radically different from amp to amp and it's usually a big combination of things, not just one thing.

I've toyed with raising the gain at various statges in all my amps, by biasing hotter and increasing plate voltages, and raising the value of the grid leak resistors, and tube rolling to find the most powerful V1 or V2 preamp tubes and had some good results overall, but I've backed off on changes once I saw what other non-gain related "Changes" could acheive.

Through all that I've come to one overriding conclusion (for me that is), I would rather spend time to change the filtering of the signal first, by trying different size coupling caps, cathode bypass caps, and ensure the filter caps are working properly or new, and adjust the pentode screen voltages high enough, while using screen resistors. Gain at different stages is important to strike a type of harmonic tonal balance, but perhaps I overrated it's importance early on.

I've had the best tonal changes by concentrating on the above filtering and amp health factors first, and not just on boosting the amp's internal gain, and then finally feeding the amp a high frequency and tailored signal up front, furnished by an EQ'd and clean boost pedal. Once you nock back any offending frequencies in the guitar input, and in the amp circuit itself, and deal with any parasitic losses from bad wiring layout, hum, resistors, caps, and bad tubes, most amps will sound great boosted, even without coming to the perfect balance between preamp and power amp gains.

Speakers and cabinets are very important in the quest for a great tone, once you have your amps bigger issues in any amp sorted out, but it pays to work on both things at the same time, that is try different speakers WHILE you are making other tonal mods. A very Complicated process for sure, but you gain insight because our final tone and response is all going to be a blend of the parts, and just trying to overcompensate for defeciencies in a cabinet or speakers is a pyric victory.

Just my limited observations, so take what I said with a grain of salt, as always !