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]

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]

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.

 

[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]

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]

....6973 For A Listenning Experience!

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.

 

[Zekk]

.... do not wish to miss the boat.

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.😕

Boat will depart anytime without me, so, I jumped in without checking for holes in the bottom hull! Get onboard first and than ride out that sinking feeling.

Highlighting a 6973 tubed amp where it does not belongs is an act of deception.

If you're used to buying expensive, that's cheap.
If you're used to buying cheap, that's expensive.

Zekk

 

[Eli Duttman]

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.

You will have to work out appropriate values for the cathode bias RC networks. The values shown work for any member of the 6V6 family, including the 'AQ5. The circuit has been executed with EL84s. From my glance at the 6973 data sheet, its drive voltage requirements seem quite similar to other "12" W. multi-grid O/P types. So, no changes to the 12AT7/ECC81 splitter/driver. The requirements of the small signal circuitry rule the roost. The 'T7 sounds good with 200-220 on the plate and I_B = 3 mA.

Look here for a satisfactory voltage multiplier topology. Eight stages of Schottky diodes, followed by the paralleled sections of a 6AL5/EB91 (to slow rail rise) as a 9th stage, should prove quite adequate. Good sized caps., say 100 μF., are fine with the SS diodes, but hold the vacuum stage cap. down to 10 μF. BTW, don't forget to stand things on their heads, as the linked diagram is for a positive rail.

 

[Poindexter]

Yah, the 6973 looks pretty close to a 6AQ5, char and rating wise. Pinout is different, though, so it's not plug'n'play.

Poinz

 

[Cassiel]

If you have a tube rectifier, why not use it?
Don't complicate things, any PP el84 or 6v6 circuit is good for the 6973. Excellent tube, use it!

If you have sensitive speakers go for triode connection without feedback. It will sound great. Phase splitter? The one used by Eli (LTP) sounds good, not the best but is good enough.

 

[Zekk]

If you have a tube rectifier, why not use it?
Don't complicate things, any PP el84 or 6v6 circuit is good for the 6973. Excellent tube, use it!

If you have sensitive speakers go for triode connection without feedback. It will sound great. Phase splitter? The one used by Eli (LTP) sounds good, not the best but is good enough.

I like that .... " Don't complicate things,..... ".

My Altec 604E is of 101dB.
So, I'm seeking out the smallest tubed amp that"ll gives me the biggest bang!
I means .... ooomph!

Thanks.

Zekk

 

[jsax420]

Hi, I had the amp chassis you describe, I could never find out much about it. Here are a few things though:

Amp is a Symphonic brand, but I can't tell you a model number. Mine came out of a Capitol-branded console from 1958. Mine had a hum that I could never trace, but in spite of that I can tell you the amp sounded GOOD...

Transformers were made by Better Coil and Transformer Company (EIA code 831), they are good-sized, and if you look at the laminations they appear to be thinner than the lams on most OPTs of the time. I don't know for sure but it seems thinner laminations are an improvement over thicker ones? Please correct me if I'm wrong on this.

I ended up assuming the hum was caused by a bad power transformer. Since I could never find a schematic, I eventually pulled the OPTs, and installed them on a Magnavox 185/175 6v6 pp amp chassis, they are quite large compared to the original Magnavox outputs. They are playing right now, no hum whatsoever, and they sound great! I still don't think they sound quite as good as they did in the original circuit however... I wonder if the magic I remember was a result of the 6973's, which are supposed to be just excellent.

Hope this helps!!

 

[Eli Duttman]

IF the O/P "iron" is of good quality, you can try the no NFB approach. "El Cheapo" works well with mediocre O/P trafos and works VERY well with good "iron". NFB works best, when it has little to do. 😉 Small error correction signals are good. Triode wired "12" W. multi-grid power types, in particular the 6V6 family, are quite linear and the 12AT7 is particularly good in PP circuits. So, the "iron" is the limiting factor.

For a no NFB approach, I suggest a variant of Poindexter's "Musical Machine" be constructed. Use Poinz's original 5965 splitter/driver, instead of his current 6GK5 setup, for socket/chassis reasons. Poindexter's site is here.

 

[Zekk]

Hi, I had the amp chassis you describe, I could never find out much about it. Here are a few things though:

Amp is a Symphonic brand, but I can't tell you a model number. Mine came out of a Capitol-branded console from 1958. Mine had a hum that I could never trace, but in spite of that I can tell you the amp sounded GOOD...

Transformers were made by Better Coil and Transformer Company (EIA code 831), they are good-sized, and if you look at the laminations they appear to be thinner than the lams on most OPTs of the time. I don't know for sure but it seems thinner laminations are an improvement over thicker ones? Please correct me if I'm wrong on this.

I ended up assuming the hum was caused by a bad power transformer. Since I could never find a schematic, I eventually pulled the OPTs, and installed them on a Magnavox 185/175 6v6 pp amp chassis, they are quite large compared to the original Magnavox outputs. They are playing right now, no hum whatsoever, and they sound great! I still don't think they sound quite as good as they did in the original circuit however... I wonder if the magic I remember was a result of the 6973's, which are supposed to be just excellent.

Hope this helps!!

Thanks for the info ......

Based on the Ampex 6973 monobloc circuitry, I've wired it all up on one chassis ... a stereo bloc.

Changed out the 12AU7 splitter for a 6CG7 and the unit sounds gorgeous with a punch ... a deep ooooommph!

 

[widerangesound]

Me too

Darn tricksters - puttin' that junk up on eBay and then allowing us to buy them. I was going to throw mine away but then decided to see where the path might lead. First thought was convert to tubes that might be in production today. Lousy phenolic wafer sockets that wouldn't solder meant that the tubes would have to fit a socket that would fit the existing hole. So it's now on its way to being a slightly lower than usual voltage Wurlitzer 6420 clone with a choke in the PS.