# Linear Power mystery pots and identification



## B'dole (Nov 2, 2010)

Just pulled the case on my first LP, and am trying to figure out the purpose of a couple of trim pots. The right one is indicated by the pointing screwdriver in the image. They are Helitrim Model 91's, and can be adjusted through about a 200 degree range.









Also want to determine if this amp is from the IQ series or not (exterior labeling is gone). The board says Model 952 REV-G. I want to say it's not an IQ because there are no fuses and there is a mono/stereo switch inside, which I believe is handled automatically by the "I" in "IQ" series. But like I said it's my first LP. 

Any help would be appreciated.


----------



## jacampb2 (Dec 3, 2010)

Given the location, I would guess they are bias current adjustment. You probably shouldn't be turning them unless you know how to reset the output bias. Hopefully you marked them or something, because it sounds like you already turned them 

As for the rest of your questions, I have no clue, but probably someone else will chime in. Did you try the ampguts site to try to match it's picture with the pictures on there?

Good luck,
Jason


----------



## B'dole (Nov 2, 2010)

Thanks Jason.

I was thinking they might control the Left and Right channel and should therefore be even. I marked them before I set them even, though when I bench tested it this evening at even it did fine.  

The amp doesn't match the IQ's on ampguts, though there isn't a 952IQ. Mine does not have resistor arrays but rather indidividual resistors. Mine also doesn't have the fuses near the speaker inputs. It does have the more prominent Stereo/Mono switch. 

So again it sounds like a non-IQ. Incidentally, I was sold the amp being told it was the IQ. During the bench test, it did make a light pop on turn on.


----------



## ChrisB (Jul 3, 2008)

The IQ series has an additional relay for the "quiet" part. I'll post some photos if I still have them because I once had a 652I and a 652IQ. 

Edit: The switch for bridging automatically makes me assume that this is a non-IQ amplifier. The "I" stood for inverted channel and negated the need for a bridging switch in the amplifiers I owned.

Edit2: I can't find the photos of the different amps, but I did still have this photo of a 1502IQ. Notice the relay to the right of the gain knob:


----------



## B'dole (Nov 2, 2010)

Thanks! No relays here, not even a fuse. Also I see your circuit board says IQ next to the model name, while mine only says 952. Nail in the coffin I think.

Your 1502IQ has those same trim pots, in the bottom middle of your pic. Do you know what they do or how to adjust them?


----------



## ChrisB (Jul 3, 2008)

They are the bias adjustments. If by some chance you turned them and didn't mark the original positions, you are going to need access to a 12v power source to power your amplifier, a frequency generator, a digital multimeter, and an oscilloscope to get them back in check.

Also, yours isn't the first case I've seen where someone thought they had the IQ amplifier and it was the non-IQ or I version. It happened to me twice when I purchased amplifiers off eBay. As a result, I always asked for photos of the actual circuit board prior to bidding or buying.


----------



## B'dole (Nov 2, 2010)

Whoa, I hope they were right when I got it :worried:. I just turned them back to their original positions, but those positions are not symmetrical to each other. The left pot is at 1 o' clock and the right pot is at 8 o'clock. Would the left and right channels need different bias? I'm assuming it's one pot per channel.

Addendum: In reading about guitar and tube amps with bias adjustment, they often have one pot for bias and one pot for offset. May just leave these alone.


----------



## jacampb2 (Dec 3, 2010)

Each channel needs it's bias current set independently. That is why you have one pot for each channel. In my experience it is rare to find a DC offset adjustment in mobile amplifiers. 

Bias current is the base current applied to the base of the output transistors that keeps the outputs partially "switched on" in order to eliminate crossover distortion in the amps output. Too much bias current and the amp is going to dump a lot of unnecessary current through the output transistors, too little and you get a really nasty sounding distortion called crossover distortion. Crossover distortion is a type of switching distortion caused by the time it takes for the output transistors to switch when not properly biased.

You *should* have a distortion analyzer to set the bias, however, you can achieve very good to excellent results via a DVM and measuring the voltage drop across the emitter or collector resistors (depending on amplifier design of course)

So, moral of the story is, don't turn knobs that you don't know what they do 

Later,
Jason


----------



## TrickyRicky (Apr 5, 2009)

Its a 952 for sure. The IQ has a speaker relay and has an inverted channel (so no need for switch when using stereo or mono).

Oh yeah, dont mess with the bias. If you have them too high, the components will get hot and fail. Too low and you might experience distortion or clipping.


----------



## B'dole (Nov 2, 2010)

The seller agreed it was not the IQ and gave a partial refund. Thanks for all the identification help here.

On taking yet another look at the bias pots, they are closer to one another than I originally thought. The left one is turned 60 degrees clockwise from its minimum setting, and the right one is turned about 90 degrees.

I do have a test disc (Autosound 2000 #104). If I had an oscilloscope, what should I be looking for when I set bias? Is it the same as gain setting, just going for the point right before clipping?


----------



## Oliver (Jun 25, 2007)

Bias FAQ

G"Luck

http://www.ab4oj.com/quadra/sshfamp.html



> Introduction: A solid-state HF amplifier differs fundamentally in its design concept from a tube amplifier. Vacuum tubes operate at high anode voltages and moderate anode currents; typically, a tube PA (power amplifier) stage requires an RF anode load resistance of approximately 2000Ω for optimum efficiency. Modern tube amplifiers are single-ended, with one or more tubes. Multiple devices are connected in parallel; a tuned output tank circuit, such as a pi (or pi-L) network, is used to transform the load resistance to the nominal 50Ω load. A working Q of 12 ensures adequate harmonic suppression.
> 
> The RF input circuit of a vacuum-tube amplifier may be either broadband or broadly-tuned. Grounded-grid configurations are most often encountered in modern tube amplifier designs.
> 
> *The topology of a solid-state PA stage is quite different. The relatively low voltages and high currents required by the RF power transistors (typically 50V at 40A for a 1 kW amplifier) dictate a low collector (or drain) load resistance, of the order of 3Ω for a 250W PA module. *The difficulty of matching the low impedances involved here to a 50Ω resistive load over a wide frequency range has dictated a broadband, base-driven (or gate-driven) architecture, using ferrite-cored input and output matching transformers. A working Q of unity or less is not uncommon; these matching networks thus offer no harmonic suppression.


----------



## TrickyRicky (Apr 5, 2009)

Hey oliver isn't your banner RACIST???? I just see those two "SS" and vomit a little.


----------



## Oliver (Jun 25, 2007)

TrickyRicky said:


> Hey oliver isn't your banner RACIST???? I just see those two "SS" and vomit a little.


*better ?* :santa2:


----------



## TrickyRicky (Apr 5, 2009)

Yeah, a whole lot better.


----------



## LinearPower (Apr 16, 2009)

You will need a 12 volt regulated power supply, a signal generator, an oscilloscope and a load resistor(4 ohm) to set the bias properly. You really need a set of 2 load resistors and load each channel even though you will be setting one channel at a time. The load resistors do not have to be of high power since this test is done at very low signal, this is not a power test where you would need load resistors rated to handle at least full power of the amp per channel. A pair of 15 or 25 watt resistors will work fine. For full power testing you would need to have at least 50 watt power resistors. The resistors would need to be non-inductive load resistors if possible, these have less change with temperature rise. This test should not go long enough or at a volume level that would heat the resistors to any dangerous level. 

You will need to send a very low voltage 10k signal into the amp and monitor the speaker level output under a 4 ohm load resistor per channel. Do not use a speaker, it changes impedance with temperature and frequency and does not provide a stable platform for setting bias. Bias can not be set correctly with no load applied either!

The gain on the amp needs to be set to unity gain (OFF, or full counter clockwise)

You should set the freq/div on the o'scope to view the 10k signal (around 10 uS) and have it set to the lowest volt/div to still see a clean sine wave (around 10mV). Sine wave viewed in the screen should be a large as possible for the volts/div you are using (around 75 percent the total size of the screen) You will probably need to set the signal generator at its lowest setting (-20 dB attenuation) and turn the amplitude up enough to get the desired waveform height on the o'scope screen. The waveform should be a clean sharp line(set focus and intensity on o'scope correctly), the only thing you might see is the crossover notch distortion (glitch). Even if you do not see it you need to adjust the pot to the point that you do see it and then rotate the pot until it disappears again. The pot should adjust toward the clockwise rotation to clean it up and provide more bias. Counter clock wise reduces bias. 

The two pots are going to be set so they are nearly mirror image opposites of each other when they are set correctly. Most of these pots fell in the 10 to 2 o'clock position when looking at the pot from the respective side for that channel. If viewing both channels from one side only, the near pot might look to be set a 10 o'clock and the opposing pot would appear to be set at 2 o'clock, but because of the orientation on the board they would actually be set nearly the same position on each pot. They may not be exactly identical but in most cases are very close to identical. 

As a side note most all larger Linear Power amps had DC Offset pots too, due to the large power output of the amplifier can make terrible pops when turned on and off due to DC offset variances. The smaller Linear Power amps did not have DC offset controls.

Once you have this set you should be able to turn the bias pot to see the waveform get a glitch or start cleaning itself up, once you see the small glitch in the waveform and you turn the pot so that the glitch goes away (clockwise) stop turning the pot, do not over bias the channel, repeat the same procedure for the opposite channel. A improperly biased audio channel can have some very bad and audible distortions, it also can pick up high frequency RF oscillations that can de-stablize the channel causing excess heat, distortion, and in severe cases output or driver transistor failure. Oscillation eats up usable power and causes excess heat, it can also cause one half of the output pair to pull excess current and go into thermal runaway and destroy itself. Bias is important not only to sound quality but to efficiency and longivity of the amplifier.

Linear Power amps for the most part will not kill themselves from improperly set bias, they just won't sound as good as they should, unlike some amps that have a wide bias pot resistance range and if set wrong will blow the outputs up almost immediately. I do not know how many Rockford amps I have repaired over the years, that people thought they found another gain control in the amp and turned the pot wide open, this results in HUGE current draw and almost immediate blowing of the output fets.


----------



## B'dole (Nov 2, 2010)

Fantastic info, LinearPower! Thanks for taking the time to post that procedure. I was not comfortable using the FAQ's on setting bias outputs for guitar tube amps on this solid-state device. Exactly what I was looking for. 

Will locate a 'scope and a couple of dummy loads, then post back how it went.


----------



## TrickyRicky (Apr 5, 2009)

Very nice info. We need more educative information like the one above. Welcome back Ray.


----------



## jimmy2345 (Jul 12, 2010)

TrickyRicky said:


> Very nice info. We could use more educational information similar to the post above. Welcome back Ray.


Fixed...


----------

