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Airline 62-9052

The Airline 62-9052 was the premium Airline amplifier. This one was a 1969 model. It is part of a family of amps that Valco made from 1966 to 1970 for Airline, Gretsch, Supro, National, and Harmony. Many of the amps in this family used this dashboard style control panel. Some were combo amps, some were of the head/cabinet arrangment made popular by Fender.

The basis of all of these amps was a 35W dual 6L6 cathode-biased power section. The 70W models (like this one), used two of these power sections in parallel each with its own speaker network.

Build quality was high, and engineering improvements were made all the way through 1970. As a result of Valco's bankruptcy, the amplifier divison was likely sold off in 1969, but the new ownership continued to build amplifiers for Airline with the same build quality and with an engineering staff that continued to develop the circuits (This new company produced Airline amps for about one year).

The 62-9052 is the same schematic as the GVC-9052 ("62-" was the catalog product prefix, "GVC" denotes a Valco build of the same product).

Amplifier features

The two input channels are quite different, both in circuitry as as in the sound produced. Channel One has a tone stack with a fairly low insertion loss due to the presence of both treble and bass controls. Two 12AX7 triodes are used, one for initial gain, and the second for make-up gain. Channel Two has three 12AX7 triodes in addition to reverb and tremelo.

Both channels are mixed together and sent to the amplifier's single phase-inverter (contrary to what some characterize as the design of this amp). In the case of the 9052, a para-phase type inverter is used.

Original Catalog

Schematic

The original 62-9052 schematic had an error on it that I corrected here (Rp R9 was corrected to ground instead of B+). Lots of Valco builds of the 66-70 era are similar to this.

click to view in full-size

The best Airline Amp schematic repo I've come across are these two:
Black Magic Amps schematic collection.
Old Fret's Valco schematics

Variations/Running Changes

The early versions have a Stand-By switch and lamp, and a some point before production was halted, the 62-9052 used that switch for a HIGH/LOW switch that increases gain in the preamp for both channels.

Also, my later version example has a slightly different power supply layout. On the schematic shown above, all 12AX7 B+ supplies come off the same string in the power supply from R78 to R82. On the later version, the phase inverter B+ is at the tail end of the R74 screen supply node for V7 and V8.

Dual Output Transformers

Dual Output Transformers (OT), each with a pair of push-pull 6L6GC's. A stereo plug was required for the output (TRS plug), with Sleeve being the negative for both OT's, and Tip being the positive for one OT, and Ring being the positive for the other OT. The Speaker cabinet had 6x 10" speakers, three for each OT. the Extension Speaker jack required the same stereo plug (the Supro used 2x 12" speakers).

This is probably the reason both of my OTs and power circuits were extensively damaged! No doubt, stereo speaker jacks got seperated from their heads, and with no warning of special output 1/4" speaker jack requirements on the back of the amp, these amps, no doubt were run with shorted OT's (for a while...).

Replacement Parts: Power Transformer

The filament winding requirement is a minimum of 5.5A @ 6.3V:

  • 4x6L6GC @ 900mA (each) = +3.6A
  • 1x6973 @ 450mA = +0.45A
  • 5x12AX7 @ 300mA (each) = +1.5A

If built with two 6L6's instead of four, you'd need 3.75A @ 6.3V

The schematic for the S6699 indicates a 320VAC secondary non-center tapped winding.

Replacement Parts: Output Transformer

The Classic Tone 40-18061 is still produced and is a very close match of the original made by Triwec (The OE supplier to Valco).

Reverb

The reverb circuit is straight forward. The 6973 driver and 4F hammond tank are an upgrade from the light-weight reverb designs used by Valco (and Danelectro) in earlier Airline models.

see Valco Reverb.

The Story of my 62-9052A:

I got this amp cheap sans tubes and transformers. The OT was easy to source, but the PT proved to be a challenge. All suitable PT's I found were too big to fit in the cabinet. I had a PT from a 2x6L6 1990 Fender that I thought work work. The replacement PT didn't have enough amerage capacity for the massive 6.3A heater requirement, so I installed a small 1V 6.3V Hammond filament transformer for V4,V5, and V6.

2x6L6 instead of a 4x6L6.
Since my amp didn't come with any PTs, I decided to build it with only one of the OT/6L6 pairs. The circuit is pretty simple to omit, or to add back the other OT/6L6 pair. This makes the amp a 35W instead of a 70W amp.

Heater supply.
Since the replacement PT didn't have a center tap, I created an artifical CT with two 100Ω resistors. The center tap was elevated to the cathode of the 6L6GCs.

Reverb 6973 converted to 12AT7.
See Valco Reverb.

Missing C6 capacitor.
Mine did not have C6. I don't know if came that way or if it was removed at some point. I added a .1uf cap as the schematic calls for, but I found the sound to be too dark and muddy. removing it brightened up the amp. If I were to put it back, I'd use a lower value, like C21 on Channel One (or increase R4 to 220K).

Power.
Of course, the addition of a three prong plug was obligitory as was the removal of the "death cap". The original power switch and the line reverse (death cap) switches were by-passed in favor of a 3 position OFF-ON-ON switch on the rear of the chassis. The two ON-ON positions allow the switch to be wired as OFF-STANDBY-ON. I also added a Blown Fuse Indicator as a gimick.

Power Supply Mods

The new transformer produced a lot more B+ than the amp was designed for (487VDC vs. 360VDC!!). to handle this, I installed several axial 500V capacitors in the circuit and disconnected some of the old cap-cans. This amp is full of 20uf-10uf-10uf/450V cap-cans and those are simply not made anymore. Valco would double two 20uf caps to provide 40uf, or two 10uf caps for 20uf. I also had modify the power supply node chain to lower the B+ for the reverb driver.

This amp is perfect foddler for some future tech to complain about a hack-job on a circuit. Just in case, here are updated schematics:

Photos

Power tubes are show here. The filament wiring needed undoing in many places, so I took the opurtunity to twist the pair of wires and route them a bit more favorably all the way across V1 to V3.

 

Here you can see the two holes were the other 6L6's used to live. Black plugs cover the transformer holes, and a plate covers the socket holes. It would be easy to install the transformer and sockets, but the power switch would need to be moved.

 

The same side of the amp is pictured here. the small transformer is an extra 6.3v filament transformer to power V4,V5, and V6 (My replacement PT didn't have the capacity). You can also see the OFF-STANDBY-ON power switch relocated to the back.

 

It is amazing how big this chassis is, yet the 6L6s are less than ½" apart at the base. I measured 320°F between the two tubes during operation! These are TAD 6L6's. They are the shortest 6L6 I could find. I went for short and fat over tall and skinny for clearance and heat purposes (I hope I choose correctly).

more Airline Info

 
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