A 6BQ5 Pushpull Amplifier - Motorola HS-696C

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A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby Brik » Mon Apr 15, 2013 5:03 pm

I came into a possession of a stereo amplifier and its front panel that came from a 60s vintage stereo console.
The amplifier chassis appears to be made of zinc-coated steel approximately 12" x 8" x 1/2" in dimension.
It's in a good condition albeit with a layer of rust and dust. =:o (sick)
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Marked as HS-696C.
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It has one 5U4GB (GE), two 12AX7s (RCA/Motorola), and four 6BQ5s (GE).
Image
The multi-section cap contains four caps: 40uF/400V, 40uF/400V, 30uF/400V, and 200uF/25V.
Image
A preliminary inspection shows that the quad of 6BQ5s share a common 82-Ohm 5W cathode resistor and the 200uF cap in the CanCap.
Also, the screens of the 6BQ5s are all tied together suggesting that they are pentode connected.
(Click the photo for the higher resolution version.)
Image

All the controls are on the "front panel" assembly which is connected to the amplifier chassis via the
umbilical and two RCA audio cables.
Image
Image
The front panel assembly has two 12AX7s on board:
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All the pin and socket connections were cleaned with contact cleaners, the front panel assembly
was connected, and dummy loads and shorting RCA phono plugs were attached.

The amplifier was then gradually powered up through a variac without any magic smoke escaping.
With the signal source and the speakers connected, the amplifier sounded really well, almost as good as
a Dynaco Stereo 35 clone. [:)
Image

Reverse engineered schematic for Motorola HS-696C with actual measured voltages:
Image

(Added schematic v0.1 on 4/17/2013)
(Updated Schematic on 4/23/2013)
(Updated Schematic on 5/06/2013)
(Updaged Schematic on 4/25/2015)
Last edited by Brik on Sun Apr 26, 2015 10:42 am, edited 7 times in total.
/b
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Re: A 6BQ5 Pushpull Amplifier from a Motorola Stereo Console

Postby EWBrown » Tue Apr 16, 2013 1:22 pm

With the 82 ohm shared cathode resistor, I'd SWAG that the B+ is 300 to 325VDC, and it would be in "pentode" mode, with the screens all connected together.

OPTs primaries would be between 6.6K and 8K A-A.

The date code on the chassis appears to be 10/58 (October 1958, or the 10th week of 1958).

The WW power resistor on the can cap's underside appears to have been worked pretty hard, looks a bit discolored...

I remember seeing a three-channel version at a NEARC swapmeet about ten years ago, left and right channels, and the center one was for a bass woofer, it had a larger OPT than the other two channels.

/ed B
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Re: A 6BQ5 Pushpull Amplifier from a Motorola Stereo Console

Postby Brik » Wed Apr 17, 2013 7:43 pm

EWBrown wrote:With the 82 ohm shared cathode resistor, I'd SWAG that the B+ is 300 to 325VDC, and it would be in "pentode" mode, with the screens all connected together.

You are right on the money on both counts! (b)
EWBrown wrote:The WW power resistor on the can cap's underside appears to have been worked pretty hard, looks a bit discolored...

The actual measured resistance of the WW resistor was ~320 Ohms.
I assume it is supposed to be a WW 330 Ohm/5W resistor.

I double and triple checked the phase invertor topology with the 12AX7, and the schematic is the way it is actually wired.
I expected a simple long tail pair with a shared cathode resistor and the grid of the second half of the 12AX7 to be "grounded."
I am still not sure the phase invertor is wired as designed...

Also, both halves of the 12AX7 tube are not operating in a balanced manner that I expected.
Since all the resistors are within their specified tolerances, I have to assume that the second half
of the 12AX7 is not operating correctly.
/b
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Re: A 6BQ5 Pushpull Amplifier from a Motorola Stereo Console

Postby EWBrown » Fri Apr 19, 2013 11:55 am

The phase inverter is based on the 1940's vintage "classic" floating paraphrase, simple, but effective.

/ed B
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Re: A 6BQ5 Pushpull Amplifier from a Motorola Stereo Console

Postby Brik » Thu Apr 25, 2013 6:24 pm

EWBrown wrote:The phase inverter is based on the 1940's vintage "classic" floating paraphrase, simple, but effective.


Simple, effective, and appears to have a higher gain than the long-tail-pair.

The coupling capacitors were leaking a small amount of DC current to the grid of the right-hand-side 12AX7' to throw its
operating point quite a bit.
Image

Replacing the coupling caps with fresh units restored the operating point of the 12AX7s to the proper range.

The output transformer's winding ratio turned out to be 10V AC in : 320mV AC out = 31:1.
Assuming the 8Ω output impedance, the reflected primary impedance is 8Ω * (31)**2 = ~7700Ω

Unfortunately, the output transformer for the right channel appears to have a broken primary winding that results in one of the 6BQ5's to not get the plate voltage. This transformer needs to be replaced. :( :'(
/b
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby 20to20 » Thu Apr 25, 2013 8:54 pm

The output transformer's winding ratio turned out to be 10V AC in : 320mV AC out = 31:1.
Assuming the 8Ω output impedance, the reflected primary impedance is 8Ω * (31)**2 = ~7700Ω


The plate to plate load Z for a 6BQ5 pair is pretty well figured in data sheets at the common plate voltages at 8K ohms. So I wouldn't assume 8 ohms on the secondary. Your turns ratio was calculated on just half of the primary so if you accept the probability that the amp engineers used 8K P-P then your calculation would be closer to 4 ohms on the speaker tap to get 4K on half of the primary. Most of those old amps had 3-4 ohm speaker taps.

20
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby Brik » Fri Apr 26, 2013 12:10 pm

20to20 wrote:Your turns ratio was calculated on just half of the primary so if you accept the probability that the amp engineers used 8K P-P then your calculation would be closer to 4 ohms on the speaker tap to get 4K on half of the primary. Most of those old amps had 3-4 ohm speaker taps.


Hi 20,

Sorry for not making it clear in my post.
The primary-to-secondary turns ratio was measured on the "good" left-channel transformer using the whole primary winding.

So, the primary-to-secondary turns ratio is indeed 31:1.

I don't know what the impedance of the original equipment speakers were meant to be, but I would guess the impedance to be no less than
6 ohms.
/b
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby EWBrown » Fri Apr 26, 2013 1:46 pm

With the relatively low B+ voltage, the OPTs could be replaced with 6.6K, (Hammond 1620s or the Triode Electronics TF-110-UL-48 6.6K PP OPTs) just change the 82 ohm 5W WW resistor to 68 ohms 5W, this would be equivalent to having a 272 ohm cathode resistor for each 6BQ5 , what this does is to increase each 6BQ5's plate current to about 40-45 mA, and still stay within the PD ratings, and thusly slightly lower the tubes' internal plate resistance. This would also give you a couple more watts output per channel. The "elegant" method would be to use individual 270 ohm, 2W resistors and 470 uF / 35V bypass caps, for each 6BQ5.

/ed B
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby rmyauck » Fri Apr 26, 2013 5:11 pm

Wouldn't Z-565 (SCA/ST-35) Dynaco clones be even better? It may not hurt to uravel the paper on that bad OPT to see if it's maybe burned off on the outside.
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby Brik » Sun Apr 28, 2013 5:51 pm

Hi Rmyauck,

rmyauck wrote:Wouldn't Z-565 (SCA/ST-35) Dynaco clones be even better?

Yes, I believe Z-565s would be a superb choice for this application.
In addition to what Ed suggested, I am looking at the following models as a possible replacement:

ClassicTone # 40-18087, Fender Deluxe Style, 20W Upgrade Output Transformer, 4/8/16 Ohms 40-18087 $43.56
Image

ClassicTone # 40-18090, Fender Tweed Deluxe Style, 20W Output Transformer, 4 / 8 / 16 Ohms 40-18090 $42.67
Image

Marshall Style 18W Output Transformer40-18037 $43.67
Image

Fender Push-Pull Princeton Reverb Style 15W Output Transformer 40-18045 $31.60
Image

Edcor GXPP15-8-8K $37.01
Image

rmyauck wrote:It may not hurt to uravel the paper on that bad OPT to see if it's maybe burned off on the outside.

Indeed, I had already stated peeling off the paper insulation layer to make sure it was not a superficial broken wire/connection at the outermost layer. :/
/b
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby rmyauck » Mon Apr 29, 2013 12:07 am

The guitar trans. would limit freq. response and have more distortion as they wouldn't have the interleaving a HIFI OPT would have. The Edcor should be the best of that bunch I would think.
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby EWBrown » Thu May 09, 2013 9:23 am

FWIW, this "floating paraphrase"circuit could be built upon a Budgie SE PC board, with a couple of minor resistor "jumper" modifications, with no etch hacking required.

I had tried this earlier, on a Clementine board, with 12V6GTs, and it worked, but with low output power. I later realized that I had made a minor design error, the 12AX7 cathode resistors should have been of a higher value, 1.8 to 2.2K, and the NFB resistor value adjusted accordingly. I since reverted it back to the original design, and it is serving as a "plump" Clem right now.

[:) ;) (lol)
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby Brik » Tue May 14, 2013 5:41 am

EWBrown wrote:FWIW, this "floating paraphrase"circuit could be built upon a Budgie SE PC board, with a couple of minor resistor "jumper" modifications, with no etch hacking required.


Hi Ed,

Interesting idea making Budgie SE board into a mono-block P-P driver board.

In the similar vain, but in reverse, I could make the HS-696C into a 4-channel clone of the Budgie SE amp by replacing the pair of P-P OTs with 4 SE OTs and making minor mods such as "un-sharing" of the cathode resistor/bypass cap.

After finding out how fragile the primary winding of the original OT was, I decided to replace both OTs.

While I am at it, I am thinking of making this amp into a test-bed for squeezing-out more power from a pentode-connected 6BQ5 P-P with 700V B+ and 350V screen, with 15mA on each 6BQ5.
http://www.diytube.com/phpBB2/viewtopic.php?t=3235#p24155

I am thinking of using a pair of TF-110-UL-48 6.6K PP OPTs utilizing the 4-Ohm secondary connection for the reflected primary impedance of 13K Ohms.

I can replace the full-wave valve rectification with a solid-state full-wave bridge rectification to generate the 700V B+ voltage while the power transformer's center tap can provide the 350V screen voltage.

I am hoping that TF-110s can handle the 700V primary-to-secondary voltage potential gap. :P :[ p[
Converting the biasing scheme to fixed-bias is straightforward but not trivial. *)
/b
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby Brik » Sun Apr 26, 2015 11:26 am

Rebuilding the amplifier was straight forward, although all the tube sockets and speaker terminals were riveted on, and these had to be drilled out.

Also, the original 9-pin tube sockets had 10 soldering lugs, a configuration that I could not find the replacement parts for.

Some aggressive sanding and metal polishing resulted in a bare-metal, mild steel chassis.
Image

After nickel plating at a local plating shop.
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Top view, before and after.
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Bottom view, before and after.
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Frontal view, before and after.
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Minimal circuit changes were made.
A power switch, a fuse, and a bleeder resistor were added, and the 330-ohm 5W B+ step-down resistor was replaced with a 330-ohm 10W variety.
Image

The original output transformers were marginal at best, so they were replaced with the ClassicTone 40-18087s.
The can cap was replaced with the Multi-section, 40/40/40uF 525VDC, CE Manufacturing unit.
Last edited by Brik on Mon Apr 27, 2015 7:06 pm, edited 2 times in total.
/b
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Re: A 6BQ5 Pushpull Amplifier - Motorola HS-696C

Postby EWBrown » Mon Apr 27, 2015 1:27 pm

Lookin' good!!! (love) [:) (y) (b) (banana)

The transformation is truly amazing!

/edB
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