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Old 7th April 2013, 04:11 AM   #1
Bugnuts is offline Bugnuts  Canada
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Default Help Identifying a Console Pull Amplifier

Hello! I hope I am posting in the proper area.
I am a "Toob noob" and have just acquired my first tube amp. It was removed from a console years ago and the gentleman I got it from knew nothing about it. I am hoping someone can help me identify it and point me in the direction of a schematic.
Any help or comments are appreciated!
2 6BQ5, a 12AX7 and a 5U4.
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Old 7th April 2013, 07:05 AM   #2
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Given the 5U4GB's 250 mA. capability, the rectifier also powered the console's tuner. That's good, as the power trafo has some "cojones".

Don't worry about a schematic. Rebuild the "beast" as a RH84. The RH84 setup does not place the O/P trafo inside a global NFB loop. Given the mediocre trafos found in inexpensive console amps, you avoid the issue of insufficient magnetic headroom and obtain maximum bass extension.

Single ended pentode mode EL84/6BQ5 amps yield approx. 5 WPC. You need mid 90s efficient speakers in combination with that sort of power.

BTW, ECC81 = 12AT7.
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Old 7th April 2013, 09:52 AM   #3
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Quote:
Originally Posted by Bugnuts View Post
Hello! I hope I am posting in the proper area.
I am a "Toob noob" and have just acquired my first tube amp. It was removed from a console years ago and the gentleman I got it from knew nothing about it. I am hoping someone can help me identify it and point me in the direction of a schematic.
Any help or comments are appreciated!
2 6BQ5, a 12AX7 and a 5U4.
Hi Bugnuts,

Interesting piece of nostalgia you inherited there. I don't know the exact identity of your amplifier, but from the tube lineup and tracing a bit of the circuitry in the photos you posted, I'm pretty sure it's a 2-channel Single-Ended amplifier.

The 5U4 is the rectifier tube for the power supply. Common as dirt, and a pretty gutsy tube as Eli points out.

The 12AX7 is a dual triode - one triode functions as the input amplifier for each channel (the inputs appear to be the blue and white wires that attach near the 12AX7).

Each of the 6BQ5 tubes is the power amp for each channel.

The referenced schematic (see EDIT below) is not exactly your amplifier, but it is very close. The tube lineup and topology are the same. The major differences are:

1) the actual component values in your amplifier are different (the color coding on the resistors tells me they aren't the same as in this schematic, for instance), and

2) the output transformer you have doesn't have the extra winding for the feedback (shown in the referenced schematic) - your feedback is the white wire that comes off the output side of the transformer, from one of the speaker wires, and connects back to the 12AX7.

Maybe you can use that schematic as a template to create your own by filling in the values from your amp if nothing better comes along.

You will also probably need to do the "usual" list of things if you want to revive this amp. Those are

1) replace all of the capacitors - they dry out over time (especially the electrolytics), and the ones in that unit look well past their use-by date.

2) Add a 3-wire power cord, securely bolting the green safety ground to the chassis with a lug. The green wire could save your life, especially if the old power transformer decides to short to ground.

3) Add a fuse (not more than 2 amp) and good power switch as well - those were probably on the console, again the fuse could prevent a serious disaster if something in that amp goes sour.

Highly recommended that you read the Safety sticky in this forum - the high-voltage present in these amps is dangerous and can be lethal. If you're just getting into tubes it's important to play safe! That 6BQ6 is running at several hundred volts - well into the danger territory.

If you can find somebody, an old Ham or other electronics enthusiast, that knows how to safely work on tubes it might be good to spend some time with them to get oriented. They will also have the equipment you will need to work on this amplifier - meters, oscilloscopes, etc. and can show you how to safely use them.

Good luck, play safe!

~ Sam

EDIT - I just noticed that the owner of that schematic had a copyright notice on it... Rather than posting it, here's the link to his site - he has a link to the schematic, you can review it and decide what you wish to do.

http://www.angelfire.com/vt/audio/se6bq5.html

He changed the original 5U4 rectifier tube in his amplifier to a 5Y3 in the schematic - but the hookup and functionality is the same (just provide your own pin numbers).

The topology on the referenced amplifier is pretty common - this one has the same tube lineup as yours, so I think it's just a matter of substituting the parts values (and making any other minor changes) for you to come up with your own schematic.

Best luck!
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Last edited by rfengineer2013; 7th April 2013 at 10:02 AM.
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Old 7th April 2013, 10:30 AM   #4
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Quote:
Add a 3-wire power cord, securely bolting the green safety ground to the chassis with a lug. The green wire could save your life, especially if the old power transformer decides to short to ground.
100% correct! Part of the rebuild process is adhering to today's safety standards. Do not attach any other wire to the safety ground mounting setup. FWIW, I like a ring terminal (crimped and soldered to the green wire) and a lock washer, along with the screw and nut. As Sam said, you are protecting your very existence. Take no prisoners.

I have reservations about installing a 5Y3 in place of a 5U4. While its 125 mA. capability is sufficient to support a 12AX7 and 2X 6BQ5s, a 5Y3 exhibits a much larger forward voltage drop than a 5U4. The B+ rail voltage could easily come in too low.

The O/P "iron" looks reasonably massive. Perhaps a few dB. of global NFB will not lead to low frequency error correction signal induced core saturation. A saturating O/P transformer core sounds FUGLY.

Bignuts, do you have an o'scope and signal generator on your bench? Part of a well executed global NFB setup is phase compensation and those instruments are the tools you use to get phase compensation right.
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Old 7th April 2013, 01:47 PM   #5
Bugnuts is offline Bugnuts  Canada
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Wow! Thanks for the replys! Hard to believe you can tell all that from a few pictures. My head is spinning and I will have to read the replys a few more times to get it all.
I have an oscilloscope and I am still leaerning about that as well.
I plan to install an IEC plug socket and a power switch. Then maybe add a little juice with an adjustable wallwart I have lying around.
Will reply back once that is done!
Thanks again!
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Old 7th April 2013, 05:04 PM   #6
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Bignuts,

Do NOT apply power until all electrolytic capacitors are replaced! The can shaped object next to the rectifier is a multi-section 'lytic. The 'lytics have literally dried out, with the passage of time. Applying power in the unit's present state could easily lead to the destruction of irreplaceable "iron".

While a "super/duper" end product is not in the cards, you can, with some care, end up with something quite nice. The key is mating the proper speakers, after refurbishment is completed.
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Old 7th April 2013, 07:45 PM   #7
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I agree with Eli - the amplifier should not be powered-up until at least the electrolytic capacitors have been replaced. You don't need to get exactly the same physical can-type capacitors as originally used in the amplifier - they are old-school, hard to find, and anything "old-stock" that has been sitting on a warehouse shelf for 40 years is going to be just about as bad as what you have.

Best thing is to go through the amplifier, identify the values and the voltage-ratings and order new caps from any of the vendors (Digikey, Mouser, Newark, Allied, etc.).

Be sure the caps you order have the correct voltage rating. Voltage ratings are usually stamped or marked somewhere on the capacitor.

The old carbon-composition resistors would probably be the next candidate to replace. They are the smaller dark-brown-body resistors with the color bands on them. Carbon resistors absorb moisture and values can change over the years, especially for the high-value resistors. And as you read through the threads in this forum, you will find many arguments that carbon-comps should be changed to metal-film resistors anyway...

IMHO, you can't go wrong replacing the old carbons with metal-film, 1% resistors and be done with it. They cost about the same, and have much better performance.

For the resistors, wattage rating is important. Carbon resistors are power-rated according to their physical size.The diameter and length determine the wattage-rating, it's standard for those resistors (they were built by the bazillions from the 1930s on, commonly used in electronics even today although they are slowly phasing out in favor of the film-type resistors).

You can find many references to the color codes for the resistors. "BBROYGBVGW for GSN" has a particularly useful, if tasteless, mnemonic for remembering the color codes . It is still used even on the smaller metal-film resistors being built today. Good to know.

If you have access to a tube tester, check the tubes - it's possible you will need to get new ones, although it's hard to tell until you actually turn them on. The tubes have a silvery splotch on the sides or top that is called a "getter" - if it is not whiteish, then at least the tube hasn't cracked or leaked gas and will probably still work even if it is not at full-performance.


In the end, it is likely that you will need to replace most of the parts in this amp. Not a show-stopper, as long as the power transformer and output transformers are still good.

Which raises the question, "Do you have a way to safely test the power transformer?"

At a minimum, you should check continuity of the windings with an ohmmeter. And also, check each winding with the ohmmeter on its highest setting to make sure there are no shorts to the frame of the transformer.

It needs to be unplugged, of course, and you can just pull the tubes to check continuity and voltage. The high-voltage (HV) winding has a center-tap that connects to ground - find that wire and disconnect it from ground to do continuity and voltage testing.

The transformer has at least three AC outputs: 5VAC (to the 5u4 rectifier tube heater), 6.3VAC (to the 12AX7 / 6BQ5 heaters) and a High Voltage (HV) output (something around ~300VAC) to the 5U4 rectifier plates. The HV output will be center-tapped, with a third wire (from the transformer) attached to ground (it looks like this amp uses the chassis as ground... Another thing to address once you get it running).

For the output transformers, the easiest thing to do is a simple continuity check. It's not likely that the output transformers have a problem, but it's easy to check so why not? Same idea as the power-transformer continuity check - the windings have continuity and no shorts between any winding and the transformer frame (disconnect the output connection soldered to the chassis for this test, too)

On the topic of inputs/outputs, you will need to add RCA input jacks and some kind of speaker-connection (probably isolated binding posts) - both available and inexpensive, although you will need to drill some holes for them because the console used internal wiring (that big mess of wires attached to your amp) instead of "normal" input/output connectors.

So, in the face of all of the foregoing work, perhaps it's better to just rebuild this into a better amp using your chassis and transformers?

Perhaps - it's certainly a thing to consider. Eli has posted a good candidate-design, and there are other similar amplifiers "out there" that could be built on your chassis. The chassis you have would be a good starting point for any of the many different flavors of SET amplifier designs floating around this forum and elsewhere.

OTOH, simply rebuilding this amplifier could be a valuable learning tool for you. You could just rebuild this amp as it has been designed and get it running, test drive it for a while, and then see if you want to upgrade the performance.

Once you get the existing amp running (assuming you go that route), it's highly likely you will have hum issues since the amplifier appears to use chassis as "ground". This means that when you hook up an external source, and if that source is also grounded to the power mains, you will have ground-loops a-plenty!

However, if you hook up a battery operated device (such as a iThingie) or any device isolated from power-mains ground (such as a laptop running off a 2-wire charger or batteries) it shouldn't be a problem.

Hum itself is a whole 'nother topic, again I'd look at it as another "learning experience"

Lots of work ahead on this project, but it's a good chance to learn a LOT about tube amps! And once you get it running, it's gonna be Big Smiles (and all the hair you've torn out along the way can begin to grow back!)

Again, I cannot emphasize safety enough! The voltages inside this amplifier can be lethal and need to be respected.

The "Safety" sticky in this forum, at a minimum, is a must-read before you plug anything into the power-mains! Nothing is worth getting hurt over, and these devices can hurt very badly if not treated with proper respect!

Good luck

~ Sam

BTW - hope you have a decent soldering-iron!
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Last edited by rfengineer2013; 7th April 2013 at 07:56 PM.
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Old 7th April 2013, 08:37 PM   #8
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BTW - your chassis already has holes punched for a full-blown push-pull amplifier if you decide to go that way. It's quite outside the current discussion, but if you decide to completely rebuild the amplifier into a new, modern design your chassis has the holes for it and the power supply (using your 5U4 rectifier tube) will likely support it.

A big plus, IMHO, and makes the chassis quite valuable all by itself!

You would need new output transformers (the single-ended transformers won't work in a push-pull design) and it's a lot of "surgery". Still, it's another option for consideration.

~ Sam
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Old 7th April 2013, 09:08 PM   #9
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Looks like a Stromberg Carlson chassis to me, they produced many chassis of this ilk in both single-ended (yours) and push-pull, variants (see the additional covered punch-outs in your chassis intended for two more EL84s (6BQ5) and one other ECC83 (12AX7)?)

One of our members, Tom Bavis, is a resident Stromberg Carlson expert so I'm sure he could identify whether or not this is the case. Not of essentially vital import, but if you're like me always nice to know the history and origins of a particular machine particularly with respect to its iron...and if you are a beginner, the original schems would definitely help you sort out the wiring tentacles attached to this particular chassis which at one time was connected to some kind of rudimentary pre-amp section.

As Eli and Sam have pointed out install a three prong AC plug and wire securely grounded at the chassis via lug so it is safe! to proceed with anything.

As to re-doing all the caps prior to adding some juice to it I would concur with the above suggestions unless you have a variable power transformer that you can use to slowly inject some AC at lower voltage increments to test the stability of existing caps etc. as you bring it up to line voltage.

Being a minimalist, I've monkeyed around with dozens of the old console amp pulls. Some are surprisingly well engineered and implemented while others are underwhelming to say the least. But, I have yet to have one implode on me using a variac to slowly...and i repeat...SLOWLY!...bring up to line voltage.

If you are able to sort out where to inject some variable source signal (ie: a CD player with variable output control) and where to connect some inexpensive and expendable test speakers (both easy enough to do even without schems) you might have a reference point to use to decide whether to re-cap and re-do the existing chassis or cannibalize the existing iron for an entirely new build.

Either way...these old console pulls provide an invaluable learning tool for beginners contemplating building, restoring or tweaking tube amps.

Have fun...but play it safe!!!!!!!! Leon
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Last edited by devilsindetails; 7th April 2013 at 09:12 PM.
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Old 7th April 2013, 10:01 PM   #10
Bugnuts is offline Bugnuts  Canada
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All great posts full of information! Thanks so much for taking the time to help!
I have heard your words of wisdom and have begun sorting out the values of the capacitors and resistors. The giant can has me a little concerned, but I will continue to research.
I was going to ask about the extra plugged holes and assumed that they were for a more advanced amplifier. I think I will try and get it running in the original design and then weigh my options for further modification. The push/pull configuration looks really interesting, but I'll need alot more help on that project!
Once again, thanks for the helpful suggestions and advice! It will be headed!
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