Stromberg-Carlson Signet 70 (SAU-70) Amplifier
Archived listing from the original vintage-amplifier.com. Prices, seller details and purchase links have been removed; this page is a reference record, and nothing here is for sale. About this archive.
Rare and superb sounding 70 watt Stromberg Carlson SAU-70 dual 6550 Tube amplifier.
2) 6L6GC (subs for the 6550 tubes)
1) 5Y3
1) 5U4
2) 12AX7
1) 6U8
The front panel has separate volume control knobs for MIC-1, MIC-2 and Phono plus a Bass/Treble/On/Off control. The back panel features a Speaker output socket plus a 1/4″ speaker output with a wire that allows you to choose 4, 8, or 16 ohms speakers. There is an RCA plug for Radio/Phono/CD Input, a 1/2″ MIC input and a 1/4″ guitar plug. There is also a Hum Adjustment on the top of the chassis.
We tested this amp with a microphone, CD player and a guitar using an 8 ohm speaker. Everything on this amp works perfectly and it sounds wonderful! It’s very loud (rattles the windows) with a great vintage tone and no background hum or hiss what-so-ever! All the transformers and capacitors stayed nice and cool during our testing and all the knobs turn freely and quietly. The chassis does have some minor surface rust but the cage is in great shape as you can clearly see in the pictures. The power cord was taped by someone else and I believe the amp is all original except for the 1/4″ input and speaker sockets. It measures approximately 16 1/2″ x 9″ x 7 1/4″ and weighs about 29 lbs. unpacked.
All tube amplifiers are brought up to full power slowly with a variac to avoid damage and to reform the capacitors. Although I test them to the best of my ability, I am not an expert!
About this model
The title says Signet 70 and the text says SAU-70: a 70-watt Stromberg-Carlson amplifier built around two 6550 output tubes. This one arrived with two 6L6GC tubes in their place, which the seller fitted because he had no 6550s; the remaining tubes were a 5Y3 and a 5U4 rectifier, two 12AX7s and a 6U8. Treat that substitution with caution. The 6550 and 6L6GC are not equivalent, and running 6L6s at the plate voltage meant for 6550s can shorten their life.
The feature list is pure public address: separate volume controls for two microphones and phono, a quarter-inch guitar input, a hum adjustment on top of the chassis, and a speaker lead selectable for 4, 8 or 16 ohms. Size per the listing: about 16½ × 9 × 7¼ inches, 29 pounds.
| Maker | Stromberg-Carlson, Rochester, USA |
|---|---|
| Model | Signet 70 / SAU-70 (listing) |
| Rated output | 70 W (listing) |
| Output tubes | 2 × 6550 (6L6GC fitted as substitutes, per listing) |
| Other tubes | 5Y3, 5U4, 2 × 12AX7, 6U8 (listing) |
| Speaker taps | 4, 8 and 16 Ω (listing) |
Questions collectors ask
Can 6L6GC tubes replace 6550s?
Not as a general rule. The 6550 is a larger tube with higher ratings; a 6L6GC may be over-stressed at the voltages and bias set for 6550s. Check the amplifier's operating conditions first.
Is the Stromberg-Carlson Signet 70 a guitar amplifier?
It was built for public address, with microphone and phono inputs, but it also has a guitar input, and the original site filed it under guitar amps.
Sources: Wikipedia: 6L6; Wikipedia: KT88.
