Tung-Sol 6550 pair in Box (labels RCA & Leslie) V18 V67

Beschrijving

A matched pair of Tung-Sol 6550 tubes made by Tung-Sol Lamp Works Inc. in the manufacturing plant Newark, New Jersey USA.

Detailed description of this LEGEND

This is the original coke bottle version 6550 tube made by Tung-Sol in the 1972 (NOT a Russian reissue)!

The 6550 is a beam tetrode vacuum tube introduced by Tung-Sol in 1954 for application as an audio frequency power amplifiers. The tubes were applied in many US power amplifiers and especially famous for their performance in de McIntosh amplifiers.

The coke bottle 6550 is a highly regarded valve in the same league as GEC KT88, and was produced in the period from the end of the fifties till the end of the seventies.

The tube is a powerhouse; amplifiers that were designed for the coke bottle 6550 (like e.g. Mcintosh Mc75 and Mc275, Beard P100 and others) often will blow the modern reissues. Till today no modern tube construction can replace the original 6550.

The coke bottle 6550 creates an spacious sound image with a natural midrange and a powerfull bass. They lasts almost forever; even when the getter flash has disappeared many 6550 tubes still perform strong. Their performance was such that GEC was forced to upgrade their KT66. This eventually resulted in the KT88.

Note. Although both look new, the RCA only is assumed to be NIB.

These 6550 types are equivalents of the European designation, KT-88. These are of course vintage, US made, Tung Sol tubes and not the modern-day, re-issue/reproduction, Russian made Tung Sol tubes. RCA and Leslie at this time did not manufacture their own 6550 tube and rebranded Tung Sol manufactured 6550 tubes.  This is evidenced by the examining the construction of the tube in addition to noting the “6550 USA” etched designation at the top of the bulb.  Firm bases, solid, grey plates, excellent silvering, impeccable finish on the metal bases, bright, with no deterioration or rust. 

Test method

Mutual transconductance, plate current, gas and shorts tested with: AVO Mk IV, Utracer 3+, AVO Valve Data Manual 23rd edition 1981, Philips 1532, RFT stabilised power supply and 2x Keithley 2700. The tubes are tested analog to the KT88. Test conditions: Anode 250V, Screen 250V, Grid -20V, Filament 6,3V. Rated Ia = 57mA and transconductance is 8 mA/V.

Test results

The four tubes are used but perform as new unused tubes.

The measurement data below show that the tube set is (especially pairwise) matched nicely . 

Valve  V18

Ia (plate current): Transconductance (Gm): Gas test, Shorts test:

64  mA 6,7  mA/V OK

Valve  V67

Ia (plate current): Transconductance (Gm): Gas test, Shorts test:

61  mA 6,6  mA/V OK

*At the used test conditions (for KT88) the emission value for a new valve is: 57 Milliamperes (mA) and the Gm 8 mA/V (AVO Valve Data Manual 23th edition). A valve is assumed to have NOS specs if both; emission and Gm are 90% of the rated value or more.

Summarized description