AMPEX  970


A project that was ignited by a spark from a friend in N.Y. who acquired a playback amplifier from an AMPEX Model 970 tape recorder.  The original is stereo but this clone is one channel.

The recorder dates to about 1960.  The frequency response is 100hz - 15khz, +/- 1dB ref 1khz.  The power output is 3 watts into 3.3 ohms, resistive.  This mono amplifier draws 0.270A from the mains, that's 32 watts.  It's fused with a 1A fuse.  Into 7.5 ohms resistive, it will deliver 1.3 watts.

It is a single ended amplifier, Class A and uses a 6AQ5 triode.  This recorder was made and sold in 1960 yet a 1961 RCA tube manual states that the tube is discontinued and listed for reference only.  The tube used in this clone is a N.O.S. (new old stock) Westinghouse.  There are also two modern replacements in my inventory, just in case I decide to go stereo.

For more info on Class A amplifiers,


Frequency response and THD+N measured on a Tektronix AA501 analyzer


f (hz) SPL (dB) THD + noise (%) Notes
50 -6.7 2.82
100 -2.3 2.76
500 0 3.07
1,000 0 2.82 2.21% THD+N @ 1W
2,000 0 3.06
10,000 -2.8 2.8
15,000 -4.3 3.02 1.73% with 30khz low pass filter ON
20,000 -6 2.75 2.72% with 30khz low pass filter ON






The amp under test using a variable transformer.  The fuse has been replaced by an AC ammeter for testing.  The rectifier plate voltages and output voltage are also monitored.

The extra chassis on the right was made to accommodate the larger power transformer but it was later found out to fit the left chassis.

This chassis was set aside and later used for the new and larger output transformer.





Full power output at 1khz into a resistive load of 3.9 ohms is 2.5W.  Total harmonic distortion was measured at 2.8%.







The little fella completed.  The wood chassis didn't present a hum problem.  Had it done so, a metal chassis would have been used and a 3 prong AC line used, grounding the chassis.  The circuit ground would have been floated above chassis ground.  This would also have grounded the metal tube sockets thus allowing shields to be used.

Since the original recorder used a two prong AC line, the metal chassis was considered unnecessary.



The data on the power transformer (top) and the output transformer (bottom)

A much better high fidelity transformer is available but the $148 price tag can't be justified.  Two would be required for stereo.




The underside.  The orange, yellow and white wire bundle is the unused outputs of the output transformer.

The two yellow wires outside the right side are the 5v filament leads and the grey is the 117v primary center tap.  The line voltage here is 122vac.

The green wire outside the right side is the center tap for the 6.3v filament leads.  It was forgotten to be connected at the time this piccy was taken.

In the lower right corner there is a resistor of 1800 ohms that has one lead covered with white shrink tubing along with the green wire coming from the output.  This is the feedback circuit, supposedly to lower distortion but in this case, it freaks out the amp, so it was discombooberated.  It was left here just in case I find out why.  Both tubes, the 12AX7 and 6AQ5 were replaced with no improvement.




Figure 1     The schematic.  For a larger file, click the pic.



Figure 2     This schematic shows my recorded notes.  For a larger file, click the pic.



New output transformer




The extra chassis mentioned earlier is now holding the output transformer.  The smaller output transformer has been disconnected. 



Figure 3

The before (black) and after (purple) response curves into an 8W resistive load.  The power transfer was just under 1 watt and the same for both tests.  The difference at the high end wasn't that prominent  but the improvement in the bass was quite impressive.  The speakers used for this are a pair of Klipsch type corner horns, wired in parallel and both driven by this amp.

Those horns can be seen HERE





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