Tricked On Ebay!

[Zekk]

Hi Folks!

Following is my recent Ebay encounter :

Bait: " Old Tube Amp 6973 ".

Looks like an Ampex 6973 Stereo .
The 5U4 and 2x 6AN8 were not mentioned at all!

"Buy-It-Now" .. Do not wish to miss the boat .. so ...... jumped in!

Upon receiving, wiring not in order of an 6973 circuitry.
Pin #1 connected to B+, #2 and #8 not connected!?

There's no marking what so ever on the chassis. Doubted the quad RCA 6973 black plates meant for this amp!

Initially felt cheated but after re-accessing what I've on hand, beefy irons, nicely built chassis, not too bad ya!?

Making the best out of this prevailing situation, I've decided to build a clone copy of the Ampex 6973 15watts amp on the existing chassis.

Problem is, are the OPT with inscription TR 159-831-839, compatible or not? Quite heavy and weights 1.8kg each.
Have 5 wires sticking out, 2x for spks, one for B+, and the other 2 individually connected to plates. From the schematic, needs a connection for the screen.
How do you figure out an unknown OPT?

PLEASE HELP!!

Thanking all for looking & Wishing all a happy holiday,
Cheers!

Zekk

[wa2ise]

Originally Posted by Zekk

Problem is, are the OPT with inscription TR 159-831-839, compatible or not? Quite heavy and weights 1.8kg each.
Have 5 wires sticking out, 2x for spks, one for B+, and the other 2 individually connected to plates. From the schematic, needs a connection for the screen.
How do you figure out an unknown OPT?


Zekk

One way is to figure out the turns ratio. Apply some low AC voltage, say 12V AC to the primary. Using an AC voltmeter that can measure mV AC, see what you get on the secondary. Once you have that, divide the voltage you applied to the primary to what you measured on the secondary. That gives you the turns ratio. Impedance ratio is turns ratio squared. If 8 ohm load on the secondary gets you twice too much primary impedance, use 4 ohm speakers. This will, IIRC, double the bass response at the cost of the treble end.

[poynton]

Why the title " Tricked on Ebay"?




.

[Tom Bavis]

Pin 1 is the screen, could be connected to B+ or the OPT tap. 2 and 8 would not be connected. The stereo Ampex amp I've seen has 7199 drivers and a solid state rectifier - perhaps this is an earlier one?

[Eli Duttman]

Originally Posted by Tom Bavis

Pin 1 is the screen, could be connected to B+ or the OPT tap. 2 and 8 would not be connected. The stereo Ampex amp I've seen has 7199 drivers and a solid state rectifier - perhaps this is an earlier one?

Zekk,

I think that Tom may be on to something. Using a 6AN8 in Scott/Dyna style circuitry is common enough and the type is available, while the 7199 is fast becoming "unobtainium".

You indicate that the "iron" lacks UL taps. So, triode and full pentode modes are your choices. A glance at the 6973 data sheet shows the type to be "12" W. that uses the same sort of trafos the EL84 and 6V6 use. 6 WPC is a realistic expectation for triode mode.

Trace the circuit out and post your findings.

[Zekk]

wa2ise
Thanks for that invaluable guidance to a newbie like myself.

poynton
.... that's my feeling!

Eli Duttman
I've cleaned out the chassis to bare metal. A postmortem is not possible.

Wish to start anew.

The two existing Ampex 6973 schematics available runs in UL mode.
When switching over to triode, do I needs any modification besides connecting plate to screen via a 100R resistor ?

Or perhaps, someone out there have a similliar 6973 circuit to share?

Read so much favourable comments on various forums, that I'm obsess to building one for a listenning experience .

Hey Folks ... keep them coming ... THANKS!

Zekk

[Eli Duttman]

Zekk,

Does it have to be an Ampex circuit? You have the makings of an "El Cheapo" variant. Switch to SS rectified B+ and use the 5 VAC winding to energize a voltage multiplier to get B-.

FWIW, I'd use 1 KOhm parts to tie g2 to the plate. Using 100 Ω parts may not provide sufficient protection against excessive g2 current.

[Original Burnedfingers]

I don't quite understand how you were tricked on ebay. Isn't it your responsibility to ask any and all questions that you need answered in order to make an informed decision to buy? Sounds like it was still worth the money.

[vinylkid58]

Originally Posted by Eli Duttman

FWIW, I'd use 1 KOhm parts to tie g2 to the plate. Using 100 Ω parts may not provide sufficient protection against excessive g2 current.

I found that out the hard way.

Jeff

[Zekk]

Originally Posted by Eli Duttman

Zekk,

Does it have to be an Ampex circuit? You have the makings of an "El Cheapo" variant. Switch to SS rectified B+ and use the 5 VAC winding to energize a voltage multiplier to get B-.

FWIW, I'd use 1 KOhm parts to tie g2 to the plate. Using 100 Ω parts may not provide sufficient protection against excessive g2 current.

Looks interesting but can 6AQ5 be sub with 6973?

As mentioned earlier, wish to make full use of componants in hand.

Eli .... appreciated your contribution.