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Post subject: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 12:02 am
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Live, off-beat version of Stone Poney's one hit. Are the amps late 1960s solid-state variety?

Thanks!



https://www.youtube.com/watch?v=TGZznJXY1Xc


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 1:38 am
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Location: Province de Québec, Canada
It reminds me of something but I do not know wich amps there are.
Yes, solid state


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 1:42 am
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Various transistorized models made by Acoustic Control Corporation. The tall one with the horn tweeter is a 260.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 1:50 am
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Retroverbial wrote:
Various transistorized models made by Acoustic Control Corporation. The tall one with the horn tweeter is a 260.

Arjay



+1

Acoustic


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 2:01 am
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Their bass amps were legendary and played by many of the notable bassists from the '60s and '70s. My brother owns several 370/371 sets......

Image

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 6:09 am
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Roger, thanks! That horn in the cab looks very intimidating. I don't know if I'd like to be standing (or singing) right in front of it.

:)


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 7:14 am
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The 260 could really cut through a mix with its horn complementing a pair of Altec 418's. I remember seeing Albert King play one at a small club in Belleville, Illinois and there was no sonic ambiguity whatsoever when he ripped off even the most subtle of riffs.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 9:43 am
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.
That second amp from the left is a model I've worked on. The amp has a top control panel and a power stage on the bottom with wires running between.
Inexplicably, the 120VAC power line was stapled together with the shielded audio cable for its full height. I moved the audio cable to the other side of the cabinet and reduced the hum by 1/2.
Then I replaced all the tired old 2.2uF coupling caps with poly versions. That woke up the dynamics and detail.

The sound coming out of these old SS amps makes them worth the effort to keep them going. But make no mistakes, they are tricky to work on.
Some have no emitter resistors. That resistance is calculated into the wire they use and if you "upgrade" the wires, it will blow the amp.
Bias is measured as a DC voltage in the driver stage. It is based on the Beta of the transistors in the preamp staged. All these old transistors have drifted into higher Beta; sometimes by more than 100%, unbalancing the drive voltages.
Replacement transistors are seldom available and also have higher Beta than the old ones.
I have never been able to find the right parts to bring one back to life after it blew.
If it runs, cherish it.


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 10:04 am
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Tim, Where do you find DC offset information for each amp?

Thanks!


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 8:41 pm
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The DC offset for a modern complemantary amp with dual supplies is zero volts.
The DC offset for these old quasi-complemantary, capacitor-coupled amps with a single supply is 1/2 Vcc.
Back in the day, I fixed these in old Marantz and Pioneer receivers. Back then, it was mostly just blown outputs. Now, they are 50 years old and have a host of age related problems
The hfe of each transistor balances the preamp. With age, the transistors drift and unbalance the offset. Besides junction measurement, sucessful troubleshooting requires hfe measurement of all transistors from the differential input pair to the outputs.
The differentials must match hfe within 10, preferably 2. Overall gain can vary by more than 100 from the original without much penalty. Actual hfe is not as important as a good hfe match.

Next, the class A phase inverter should be close to the same DC voltage as the printed spec on the schematic. It is usually between 2 and 10 volts
My best trick to rebalance the offset is to find the Class A phase inverter transistor after the differential input stage. Changing the base-emitter voltage there by adjusting the resistance can help the offset. Sometimes just a few ohms can make the difference..
That Class A transistor usually runs hot and is the first to drift out of spec.

The bias transistor is critical and should be the original number so it can track properly through the heating cycle. If you have to substitute, monitor bias on the emitter resistors as you heat up the heat sink. Then monitor it as it cools. The heating of the bias transistor changes its characteristics and it turns on a little more as it warms up. This lowers the bias drive to the output transistors to keep them under control as they get hotter.
Collector-emitter voltage on the bias transistor is usually 1.2v and -1.2v. This sets up the driver and output to turn on a bit. This is usually the easiest place to evaluate DC offset and bias characteristics.

The driver transistor circuit also has a gain dependent circuit on the high side. If that transistor has drifted, it throws the offset off.

The outputs are not sensitive to gain. Any similar volt-amp rating will operate. They all have relatively low hfe.

The output coupling cap to the speaker almost never fails. I replace them on a ,"just because its 50 years old" basis. That also applies to all the electrolytics in the power stage..
On a capacitor-coupled amp like these Acoustics, without loads connected, you will measure about 1/2Vcc DC voltage on the speaker terminals. This is normal. Connect loads and the DC goes away..
The transistor numbers used and the state-of-the-artof semiconductor technology of the 1960 makes thes old gems very difficult to rebuild

The virtues of the Acoustics was the capacitor coupling to the speaker that was tube-like at clipping levels.
They also had a very short audio path. There are fewer transistors in the entire unit than in one dual op amp. The discrete transistors have a faster slew rate than op-amps for better articulation
The SAP reduces intermodulation distortion and keeps noise low in the primitive circuits used.
300 watts in a 40 pound head was as attractive then as a Tiny Terror is now. Its not tube, but Hey, my back's not steel.


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Tue Apr 11, 2017 9:05 pm
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Tim, thanks, for explanation. The level of complexity is another order of magnitude, over simple tube circuits. Prolly, why I avoid restoring vintage solid-state amps. There are many ppl that are fans of 1970-80s solid-state hi-fi amps. These things often need a lot of TLC, after being bought off EBay or Craigslist. I won't touch them, except for the most minor of fixes.


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Wed Apr 12, 2017 9:11 am
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These are tough amps to repair. A little knowledge going in helps. If the offset is more than a few volts off 1/2 Vcc, I bail right there.
Bias drifts high over time and needs adjusting. Coupling caps are easily upgraded. tighten HS screws. and call it good.


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Thu Apr 20, 2017 12:03 pm
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Tim, have you ever worked on any vintage Kustom/Kasino amps?

If so, would those same protocols apply?

Thanks

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Thu Apr 20, 2017 1:53 pm
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Our guitar player in the 70-80's used an Acoustic Control 150 or 160, (I can't remember), through a 6-10 cab, and got some really great sounds!
It fell off a semi bed we were using for a stage at an outdoor party once, and quit. I opened it up, and found the reverb tank had broken the (internal) wires, and shorted the signal. I taped off the rca's and removed the tank, and we finished the weekend. I later fixed the tank, and tried to reinstall it, but he didn't want it. (I've still got that tank somewhere!)


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Post subject: Re: O/T: what are the amps in this YT video?
Posted: Thu Apr 20, 2017 3:04 pm
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That would've been their 150 guitar head paired with the 104 guitar cab. With 125 watts, it was plenty loud and quite punchy for a solid-state amp.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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