Motorola HS-701A audio amp (in a powered speaker circa 1959)

Occasionally I enjoy working on smaller tube gear such as an audio amplifier. This Motorola HS-701A amp was mounted on a shelf inside a large speaker cabinet. Amp controls are On-Off, Volume, Treble and Bass. Tube complement consists of two 12AX7 as preamp and phase inverter, two "Made in Holland" 6BQ5/EL84 in push-pull output, and a 5Y3GT rectifier in a cathode-bias class AB1 circuit.

HS-701A audio amp feeding a Pioneer speaker

Motorola HK-33 logo

HK-33 cabinet as pictured in Sams Photofact

HK-33M cabinet inside

Repairs
The HS-701A amp had needed very little repair. After safety checks including checking for any possible line to chassis leakage, for the proper level of resistance in the B+ lines and a check of the output transformer and speaker connections, I powered the set slowly, checking for any leakage at the input grids of the amp tubes. I did not found any leakage but replaced all of the coupling capacitors anyway to avoid any future leakage that could affect the bias. A bit of deoxit was applied to all the controls. That and replacing the pilot light completed the repairs.

Performance
The little amp performed very well after repairs, a very desirable piece for anyone who prefers tube sound. The amp is conservatively rated and designed. The 6BQ5/ EL84 output tubes run with a plate voltage of 290 and screens at 295 as shown on the Motorola schematic. That is close to the 300 volt side of tube manufacturers' recommended specifications but does not exceed those specs. Here is a snapshot of the GE tube specs for the 6BQ5 in push-pull AB1. Note that the power output ranges from 11 to 17 watts depending upon tube voltages and currents. The 17 watt figure is based on 300 volts on the plates and screens with a cathode bias resistor of 130 ohms. The Motorola comes with a bit more conservative cathode resistor of 150 ohms.

GE tube specs for 6BQ5 in push-pull class AB1

Motorola to Marshall?
The very popular Marshall 18 watt simple guitar amp has a very similar output stage. The Motorola could be easily rebuilt as a copy of that Marshall amp. The tube complement is identical with the exception of the rectifier. Let me know if you tried modifying it to a Marshall clone.

The Motorola power transformer is conservatively rated and stays relatively cool. A solid state plug-in replacement for the rectifier could be as simple as using a tube base and a four 1N4007 diodes (two in series for each leg to double the PIV) with an added resistor in the output under the chassis. That mod would save 10 watts of 5Y3GT filament power and allow for a bit of higher B+ voltage to match the Marshall 300 volt spec.

What about the big HK-33 speaker cabinet?
The HK-33 cabinet and its speakers are in decent condition as well, but I do not wish to hide the amp in the cabinet again.

HK-33M cabinet

The cabinet takes up considerable floor space. I am debating whether to use just the cabinet and speakers directly for sound from a heavy boatanchor radio, either placing the radio on top or possibly modifying the cabinet with a new shelf inside and cutting out a rack-mount size face opening for the radio. Granted that modification will ruin the originality of the cabinet, but the result would be more useful, justifying the floor space.

Manual source
Schematic and information can be found in SAMS Photofact 464-13, dated 11-59. A copy of the Sams info and the schematic can be found at this public link to my Dropbox folder.

More Audio systems
I have repaired and documented a number of other audio amplifiers and equipment. A list and links to that audio equipment can be found here.

9-30-12