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Quicksilver 300B PROTOTYPE Tube Monoblock Amplifiers - One of a Kind!

Sale price$9,999.00
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Quicksilver 300B Prototype Tube Monoblock Amplifiers - One Of A Kind! Amplifier

Quicksilver 300B PROTOTYPE Tube Monoblock Amplifiers - One of a Kind!

SkyFi 479

Pickup available, usually ready in 24 hours

479 South Broad Street
Glen Rock NJ 07452
United States


We received these amps from a serious collector that had the most amazing equipment, so we suspected they were quite special.

Since we could not find any information or pictures online that matched these amps, we ended up call Quicksilver. Yes they surprisingly answered the phone and were super helpful!

It turns out these amps were NEVER PRODUCED and they have no record of them existing. It's likely they were a prototype produced by Quicksilver but never released into the market.

So as far as we can tell from the experts themselves, these are one of a kind and are therfore priced accordingly. They are now part of our holy grail collection.

On top of that, after a lot of folks questioning if these were prototypes or not, another verified source contacted us: "I actually have a pair of those Quicksilver 300B push pull monoblocks. Quicksilver made a few of them as well as a few single ended 300B monoblocks but they never made it into the production line."

We tested them carefully and verified with the manufacture that they are configured in push-pull configuration, thus delivering close to 50W RMS into 8 ohms.

You see while the typical single ended configuration tends to deliver just a few watts per 301B, this method results in quite a bit more. Giving you many more options for speakers while still enjoying the amazing sound that this tube is capable of.

We used these rare beauties to power our newly acquired vintage Altec Lancing Magnificent speakers and received amazing results in clarity and authority.

The amps both look like new, with no scratches or pitting on their chassis or chrome.

We fitted both of these amps with brand new Sovtek NOS tubes, but if you want to take it to the next level you could match them up with one of our currently listed sets of NOS Western Electric tubes.

Click below to add our recommended matching cables from Kimber Kable, all brand new as SkyFi is an official Kimber dealer.

Kimber Kable - RCA Interconnects

Kimber Kable - Speaker Cables

Kimber Kable - Bi-Wire Speaker Cables

The SkyFi Testing Process for Tube Amplifiers:
We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect resistors and other passive components for signs of overheating. If tube arcing has occurred in the past we can usually spot discoloration on the output tube sockets. On vintage units we often spot check select capacitors for value and ESR.

If the amplifier passes visual inspection, we move on to a full test of all of the tubes. We use an Amplitrex AT-1000 Tube Tester which is capable of testing both emission and Gm with a high degree of accuracy. We document the results of each tube and replace any weak or suspect tubes before proceeding. When we power on tube amplifiers for the first time we usually use a variac and current limited AC supply and slowly raise the voltage up to nominal mains level while monitoring plate, screen, filament, and negative bias supply voltages where applicable. If everything is in order we feed a low level test signal into the amplifier’s input and monitor its output on an oscilloscope across an 8 ohm dummy load. At this point we are just looking to verify basic function and confirm that the output transformers are not damaged. Once we have verified that the amplifier is safe to operate, we connected it to full mains power. For fixed bias amps we set the bias to manufacturer spec. For cathode biased amps we monitor the plate to cathode voltage to determine if the output tubes are operating in a safe range. Once the output section is verified we move onto bench evaluation.

We start by feeding the input of the amplifier with a low level 1KHz test signal, slowly increase its amplitude while monitoring the amplifier’s output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. We finish off the bench evaluation with a 1KHz square wave check and a 20Hz to 20KHz sine sweep to assess the amplifier’s frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention.

Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.

If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours. For tube amps we like to run this test at least twice. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when it has fully come up to temperature. We find this step to be essential, especially for vintage units.



Original Box

YES Included!


Not Included


Not Applicable


Yes - Power Only

Physical Condition

8 / 10

Working Condition

10 / 10

SKU: 100419
Quicksilver 300B Prototype Tube Monoblock Amplifiers - One Of A Kind! Amplifier
Quicksilver 300B PROTOTYPE Tube Monoblock Amplifiers - One of a Kind! Sale price$9,999.00