A novel guitar amp ! - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 1st Jul 2017, 2:42 pm

This interesting and unique combo turned up in the shop, a 'Sears Silvertone' guitar and amp - built into the guitar case

We reckon it's from around 1964, it runs from 120V mains, and I'm told it works ! I've never seen such a thing before, a neat idea but I'm guessing it never caught on

1st Jul 2017, 4:19 pm

Rather 'blingy' shown off valves (tubes I guess given it's American) that can't be seen when closed. I like it.

Restoration73 · 1st Jul 2017, 4:21 pm

Sears was a US chain store. You'll have to use an isolating transformer as the valve
line up indicates the chassis may be live to one side of the 110 volt supply.

ITAM805 · 1st Jul 2017, 5:09 pm

Restoration73, it came with a small 240/120V tx, but I dont know if it's isolated?

The schematic shows the chassis to be connected to the mains via a 68K and 0.05uF, which I think they call in the US, a "death cap"

1st Jul 2017, 7:21 pm

Quote:

"death cap"

Not too bad on 115V, like the term "that cap"we use so frequently here.

TonyDuell · 1st Jul 2017, 8:33 pm

Quote:

Originally Posted by ITAM805

Restoration73, it came with a small 240/120V tx, but I dont know if it's isolated?

The schematic shows the chassis to be connected to the mains via a 68K and 0.05uF, which I think they call in the US, a "death cap"

That schematic seems to show one of the valve heaters going to the chassis, so presumably there is a low resistance path from the chassis to whatever powers the heaters. If it's not an isolating transformer, watch out!

I guess this is the other meaning of 'live perfomance'

ITAM805 · 1st Jul 2017, 8:48 pm

Tony, yes the the mains is connected directly to the cap so the potential (pun intended) is there !

Quote:

Not too bad on 115V

Well, if the cap was leaking, let alone short, then having one hand on the guitar strings while ones lips touches the earthed pop shield of an SM58

That said, it has survived for 50 some years without a fatality (although I dont know that for sure !)

ColinTheAmpMan1 · 3rd Jul 2017, 12:13 pm

Sears, Roebuck & Co., who sold Silvertone guitars and amps were not only a chain-store in the USA, but did mail-order. This was about the only way you would get a guitar and amplifier if you lived in some rural place tens of miles or more away from any city that might have a music-store. The "amp in the case" idea was, I think, unique to Silvertone and was quite a good idea, so long as you weren't looking for an amp that you could gig with well. It caught on, all right!

The "death cap" (nothing to do with poisonous fungi) is present on just about every US-made amplifier manufactured up until recently. It was a small capacitor wired from one mains lead to the chassis of the amplifier. Its purpose was to provide some kind of "earthing" to suppress RF interference and thus audio breakthrough, when there was no true earth. Just about all early amps only used a two-wire mains connection, the notion of a safety earth, either a power-station earth or a local earth-rod, came in some time later. There was often a switch associated with the "death cap" which enabled its connection to mains to be altered between the two mains wires, the idea being that the suppression of interference would be better in one position compared to the other. The description "death cap" comes from the use of guitars and amplifiers separate from microphones which are properly earthed via a PA system. The microphones are safe, but the guitar strings are connected to the amplifier chassis, which could be "live", especially if the "death cap" is leaky. If the microphone is also connected to the guitar amplifier, there is no such voltage difference.

All of the circuits that I have for Silvertone amps (I should probably call them "schematics" as they are American) show a mains transformer with isolated primary and secondaries, so no live chassis, apart from the "death cap". Of course, that doesn't mean that the OP's amp has an isolating transformer. I would most certainly have a check on the possibility of the chassis becoming live when the "death cap" is removed from circuit. It can be just snipped out, de-soldered or whatever and kept if originality is necessary at some point in the future. If this indicates that the mains transformer gives isolation from the mains, then take out the old two-wire mains cable (keep it for originality) and wire it up with a proper "mains and earth" three-wire connection. That should make it safe.

I should put in a caveat here, in that if you sell this on after doing the modifications I have stated, then some proper PAT testing needs to be done, too. If someone buys this and then suffers a shock, you could be liable, having sold something that has been modified. If you leave it original and sell it on, the responsibility remains with the original manufacturer, I think. An isolating 240V-to-120V transformer separate from the amp gets you off that hook, I hope.

There is an amusing story about a relatively modern manufacturer who incorporated the "death cap" in a recently-made amp, but the voltage-rating of the capacitor was only about 150V, to suit US mains. The amp was sold in the UK, however, and the 240V mains resulted in the demise of the "death cap" with a bang and the release of "magic smoke". While the loss of the cap was of no real significance, as the amp will function (safer) without it, the event rather startled those who were present.

With regard to the heater wiring, my Silvertone circuits show some with one side of the heaters going to chassis, some with an "artificial centre-tap" formed from two low-ohms resistors and some with a genuine centre-tap on the heater winding.

I hope this is all useful.

Colin.

ColinTheAmpMan1 · 3rd Jul 2017, 12:21 pm

Quote:

Originally Posted by ITAM805

The schematic shows the chassis to be connected to the mains via a 68K and 0.05uF, which I think they call in the US, a "death cap"

If you have a copy of the complete schematic (or circuit diagram), I would like one, please. This model of amp isn't in Aspen Pittman's "The Tube Amp Book" which I have. Those tubes (valves) look really unusual.

TIA, Colin.

ITAM805 · 3rd Jul 2017, 1:05 pm

hi Colin

thanks for the extra info on Silvertone. A copy of the schematic is attached

Regarding the safety issue, I will certainly pass on the comments here. I believe the amp is being sold on behalf of a 3rd party

3rd Jul 2017, 2:47 pm

The mains switch in the neutral (good idea hum wise) makes it more "lively" when off. For conscience I would supply/recomend a good quality isolating transformer.

G8HQP Dave · 3rd Jul 2017, 3:57 pm

Quote:

Originally Posted by ColinTheAmpMan1

Those tubes (valves) look really unusual.

Maybe a bit unusual in a guitar amp, but fairly common in AA5 AC/DC radios - apart from the 12AU6 (straight pentode) would probably be a 12BA6 (remote cutoff pentode) to get IF AGC.

ColinTheAmpMan1 · 4th Jul 2017, 11:32 am

Thanks for the circuit diagram and for the heads-up on the valves.

Now that I have had a longer look at the circuit, I see some (to me) unusual features, maybe even "shocking" ones. The circuitry around the pre-amp valve is all taken to chassis as its B-. This is then referenced to earth, or something approximating to it, by a parallel R/C combination of a 68k resistor and a 0.05uF capacitor - the "death cap".
The output valve (50C3) circuitry is referenced to a B- which is actually one side of the mains! Perhaps those folk more au fait with live-chassis circuits can enlighten me, but this looks dodgy to me. Remember that the guitar will be connected to the chassis through a screened cable going to the input of the amp. Even if the strings are not connected to the screen of this cable, the metal part of the jack-plug will be. The only part of the output circuitry which goes to chassis "earth" is capacitor C6 (0.01uF) which goes from the 50C3 anode, presumably some kind of filter?

The heaters for the half-wave rectifier (35W4; 35VAC) and the output valve (50C3; 50VAC) are derived from the mains using a dropper resistor, while the preamp valve (12AU6; 12VAC) gets its heater supply from a transformer in the same series chain.

Personally, I wouldn't go within a mile of this amp when it is powered up unless there was a reliable isolating transformer supplying it and some kind of bond to mains earth on the chassis. Even then, I would be nervous. This is a very penny-pinching circuit and I hope that not many people have been bitten by it in the past.
Colin.

AC/HL · 4th Jul 2017, 1:36 pm

Are there windows over the valves? If not you can add burnt fingers to the list, on a darkened stage.

ITAM805 · 4th Jul 2017, 2:14 pm

Hi Colin, you're welcome. Out of curiosity I tested the mains transformer at the shop and it is indeed isolated. Beyond this though, I advised the shop to pass on the caveat regarding the cap business to the seller and would be buyer.

AC/HL, this wasn't intended as a stage amp but more a bedroom-come-practice amp I would have thought.The valves aren't covered, in it's day (my day) if you touched a hot valve you went "ouch"

but in these over zealous H&S times ...

Nuvistor · 4th Jul 2017, 8:21 pm

I found this in the USA mag Radio Craft Jan 1946 for a mains battery guitar amp. Apart from a small resistor and capacitor, that's all there is to stop one side of the power connecting to the guitar pickup. Again it's not a stage amp but still not the best way to do the job.

Frank

ColinTheAmpMan1 · 5th Jul 2017, 12:07 pm

Just as an aside, I would put money on the guitar having actually been made by Danelectro. They had some unusual build methods including using "Masonite" (hardboard) tops and backs and the "lipstick-tube" pickups, which were really mounted in lipstick-tubes. Having said that, several famous guitarists have performed with them, including Jimmy Page in Led Zeppelin and Syd Barrett in early Pink Floyd.
Colin.

snufkin · 14th Jul 2017, 10:43 am

these sound great and were indeed dano guitars

kevinaston1 · 14th Jul 2017, 11:31 am

Hi Colin,

Yes, it is a 1962 Danelectro, see https://en.wikipedia.org/wiki/Danelectro_Amp-in-case

The current Danelectro (Steve Ridinger / Evets Corp) do a copy of this guitar, but not the amplifier.

Kevin

ColinTheAmpMan1 · 14th Jul 2017, 12:23 pm

That's really interesting info, Kevin, thanks. The item about Steve Ridinger/Evets Corp. sparked my interest, because I thought that I recognised Evets Corp.. The reason for this is that there is an English actor and musician called Steve Evets (he was in "Looking for Eric") who was born Steven Murphy, but when he joined Equity, he had to change his name, so he chose a palindrome - Evets is Steve backwards).
I love trivia like that!
Colin.

1st Jul 2017, 2:42 pm	#1
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	A novel guitar amp ! This interesting and unique combo turned up in the shop, a 'Sears Silvertone' guitar and amp - built into the guitar case We reckon it's from around 1964, it runs from 120V mains, and I'm told it works ! I've never seen such a thing before, a neat idea but I'm guessing it never caught on Attached Thumbnails

1st Jul 2017, 4:19 pm	#2
Guest Guest Posts: n/a	Re: A novel guitar amp ! Rather 'blingy' shown off valves (tubes I guess given it's American) that can't be seen when closed. I like it. Last edited by Guest; 1st Jul 2017 at 4:19 pm. Reason: splelling

1st Jul 2017, 4:21 pm	#3
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: A novel guitar amp ! Sears was a US chain store. You'll have to use an isolating transformer as the valve line up indicates the chassis may be live to one side of the 110 volt supply.

1st Jul 2017, 5:09 pm	#4
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: A novel guitar amp ! Restoration73, it came with a small 240/120V tx, but I dont know if it's isolated? The schematic shows the chassis to be connected to the mains via a 68K and 0.05uF, which I think they call in the US, a "death cap" Attached Thumbnails

3rd Jul 2017, 12:13 pm	#8
ColinTheAmpMan1 Octode Join Date: Jan 2012 Location: Wimbledon, London, UK. Posts: 1,465	Re: A novel guitar amp ! Sears, Roebuck & Co., who sold Silvertone guitars and amps were not only a chain-store in the USA, but did mail-order. This was about the only way you would get a guitar and amplifier if you lived in some rural place tens of miles or more away from any city that might have a music-store. The "amp in the case" idea was, I think, unique to Silvertone and was quite a good idea, so long as you weren't looking for an amp that you could gig with well. It caught on, all right! The "death cap" (nothing to do with poisonous fungi) is present on just about every US-made amplifier manufactured up until recently. It was a small capacitor wired from one mains lead to the chassis of the amplifier. Its purpose was to provide some kind of "earthing" to suppress RF interference and thus audio breakthrough, when there was no true earth. Just about all early amps only used a two-wire mains connection, the notion of a safety earth, either a power-station earth or a local earth-rod, came in some time later. There was often a switch associated with the "death cap" which enabled its connection to mains to be altered between the two mains wires, the idea being that the suppression of interference would be better in one position compared to the other. The description "death cap" comes from the use of guitars and amplifiers separate from microphones which are properly earthed via a PA system. The microphones are safe, but the guitar strings are connected to the amplifier chassis, which could be "live", especially if the "death cap" is leaky. If the microphone is also connected to the guitar amplifier, there is no such voltage difference. All of the circuits that I have for Silvertone amps (I should probably call them "schematics" as they are American) show a mains transformer with isolated primary and secondaries, so no live chassis, apart from the "death cap". Of course, that doesn't mean that the OP's amp has an isolating transformer. I would most certainly have a check on the possibility of the chassis becoming live when the "death cap" is removed from circuit. It can be just snipped out, de-soldered or whatever and kept if originality is necessary at some point in the future. If this indicates that the mains transformer gives isolation from the mains, then take out the old two-wire mains cable (keep it for originality) and wire it up with a proper "mains and earth" three-wire connection. That should make it safe. I should put in a caveat here, in that if you sell this on after doing the modifications I have stated, then some proper PAT testing needs to be done, too. If someone buys this and then suffers a shock, you could be liable, having sold something that has been modified. If you leave it original and sell it on, the responsibility remains with the original manufacturer, I think. An isolating 240V-to-120V transformer separate from the amp gets you off that hook, I hope. There is an amusing story about a relatively modern manufacturer who incorporated the "death cap" in a recently-made amp, but the voltage-rating of the capacitor was only about 150V, to suit US mains. The amp was sold in the UK, however, and the 240V mains resulted in the demise of the "death cap" with a bang and the release of "magic smoke". While the loss of the cap was of no real significance, as the amp will function (safer) without it, the event rather startled those who were present. With regard to the heater wiring, my Silvertone circuits show some with one side of the heaters going to chassis, some with an "artificial centre-tap" formed from two low-ohms resistors and some with a genuine centre-tap on the heater winding. I hope this is all useful. Colin.

3rd Jul 2017, 2:47 pm	#11
Guest Guest Posts: n/a	Re: A novel guitar amp ! The mains switch in the neutral (good idea hum wise) makes it more "lively" when off. For conscience I would supply/recomend a good quality isolating transformer.

4th Jul 2017, 11:32 am	#13
ColinTheAmpMan1 Octode Join Date: Jan 2012 Location: Wimbledon, London, UK. Posts: 1,465	Re: A novel guitar amp ! Thanks for the circuit diagram and for the heads-up on the valves. Now that I have had a longer look at the circuit, I see some (to me) unusual features, maybe even "shocking" ones. The circuitry around the pre-amp valve is all taken to chassis as its B-. This is then referenced to earth, or something approximating to it, by a parallel R/C combination of a 68k resistor and a 0.05uF capacitor - the "death cap". The output valve (50C3) circuitry is referenced to a B- which is actually one side of the mains! Perhaps those folk more au fait with live-chassis circuits can enlighten me, but this looks dodgy to me. Remember that the guitar will be connected to the chassis through a screened cable going to the input of the amp. Even if the strings are not connected to the screen of this cable, the metal part of the jack-plug will be. The only part of the output circuitry which goes to chassis "earth" is capacitor C6 (0.01uF) which goes from the 50C3 anode, presumably some kind of filter? The heaters for the half-wave rectifier (35W4; 35VAC) and the output valve (50C3; 50VAC) are derived from the mains using a dropper resistor, while the preamp valve (12AU6; 12VAC) gets its heater supply from a transformer in the same series chain. Personally, I wouldn't go within a mile of this amp when it is powered up unless there was a reliable isolating transformer supplying it and some kind of bond to mains earth on the chassis. Even then, I would be nervous. This is a very penny-pinching circuit and I hope that not many people have been bitten by it in the past. Colin.

4th Jul 2017, 1:36 pm	#14
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: A novel guitar amp ! Are there windows over the valves? If not you can add burnt fingers to the list, on a darkened stage.

4th Jul 2017, 2:14 pm	#15
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: A novel guitar amp ! Hi Colin, you're welcome. Out of curiosity I tested the mains transformer at the shop and it is indeed isolated. Beyond this though, I advised the shop to pass on the caveat regarding the cap business to the seller and would be buyer. AC/HL, this wasn't intended as a stage amp but more a bedroom-come-practice amp I would have thought.The valves aren't covered, in it's day (my day) if you touched a hot valve you went "ouch" but in these over zealous H&S times ...

4th Jul 2017, 8:21 pm	#16
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: A novel guitar amp ! I found this in the USA mag Radio Craft Jan 1946 for a mains battery guitar amp. Apart from a small resistor and capacitor, that's all there is to stop one side of the power connecting to the guitar pickup. Again it's not a stage amp but still not the best way to do the job. Frank Attached Thumbnails

5th Jul 2017, 12:07 pm	#17
ColinTheAmpMan1 Octode Join Date: Jan 2012 Location: Wimbledon, London, UK. Posts: 1,465	Re: A novel guitar amp ! Just as an aside, I would put money on the guitar having actually been made by Danelectro. They had some unusual build methods including using "Masonite" (hardboard) tops and backs and the "lipstick-tube" pickups, which were really mounted in lipstick-tubes. Having said that, several famous guitarists have performed with them, including Jimmy Page in Led Zeppelin and Syd Barrett in early Pink Floyd. Colin. Last edited by ColinTheAmpMan1; 5th Jul 2017 at 12:36 pm. Reason: Correction

14th Jul 2017, 10:43 am	#18
snufkin Triode Join Date: Nov 2008 Location: London, UK. Posts: 47	Re: A novel guitar amp ! these sound great and were indeed dano guitars

14th Jul 2017, 11:31 am	#19
kevinaston1 Hexode Join Date: Aug 2014 Location: Featherstone, West Yorkshire, UK. Posts: 386	Re: A novel guitar amp ! Hi Colin, Yes, it is a 1962 Danelectro, see https://en.wikipedia.org/wiki/Danelectro_Amp-in-case The current Danelectro (Steve Ridinger / Evets Corp) do a copy of this guitar, but not the amplifier. Kevin

14th Jul 2017, 12:23 pm	#20
ColinTheAmpMan1 Octode Join Date: Jan 2012 Location: Wimbledon, London, UK. Posts: 1,465	Re: A novel guitar amp ! That's really interesting info, Kevin, thanks. The item about Steve Ridinger/Evets Corp. sparked my interest, because I thought that I recognised Evets Corp.. The reason for this is that there is an English actor and musician called Steve Evets (he was in "Looking for Eric") who was born Steven Murphy, but when he joined Equity, he had to change his name, so he chose a palindrome - Evets is Steve backwards). I love trivia like that! Colin.