UREI 6500 Power Amplifier Bias Specification? - diyAudio
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Old 25th November 2015, 09:58 PM   #1
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Join Date: Dec 2014
Default UREI 6500 Power Amplifier Bias Specification?


Does anyone out there have a service manual for or know the bias specification for a UREI 6500 power amplifier?
I have two schematics that were included in the owners manual, an original and a revised schematic, neither has the bias spec.

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Old 26th November 2015, 01:35 AM   #2
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unless repairs where done there should be no need for bias adjustment
the manual covers 4 models 6150, 6250, 6300, 6500
7 millivolts across R69 or R78 depending on what channel your adjusting (this is in the manual starting on page 21) if you have a revised schematic this may not be correct.
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Old 26th November 2015, 02:42 AM   #3
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Hello Turk and thank you very much for that information.
I recently purchased the amplifier and one channel was down. The left channel had smoked bad at some point and somewhat less than appropriate service techniques were used in repairing it by who knows who. Anyhow, the right channel was operational and I was able to take a voltage drop measurement across an emitter resistor and it measured 5mv after a bit of warm-up/stabilization.
However, the output modules have obvious differences like serial numbers that are far apart and a few parts sourcing differences. The major differences were different bias pot values and resistors in the bias circuit itself. I have copies of two schematics, one module matched one schematic and the other module was from the other schematic. There were also some minor changes to the fan driver circuit and driver device. It is obvious which schematic is the later revision so I applied those changes to both modules and everything worked out just fine. I adjusted the bias to obtain a 5mv drop across the emitter resistor closest to the front panel as it was easiest to probe and the amp operates/sounds great. While I was in there I also did a bit of house-keeping for good measure, re-capped both channels, cleaned-burnished the molex connectors, refreshed compound on all output devices, replaced both bias pots with new 200 ohm values as in the updated schematic and corrected the resistor values in the older module bias circuit. I even upgraded the input stage op-amps to LM49740 for good measure. The bad channel had a 28mv DC offset after rebuilding it that was the cause of a bad input stage op-amp so I decided to upgrade them all.
I also made a small modification to the fan circuit so it runs less aggressively. I simply installed a couple 20k trim pots in parallel with R58 (UREI changed from 3k to 10k in revised schematic. I assume they had overheating issues at first) so I could adjust the fans start-up speed down a bit as they were running a bit fast and noisy for my listening room. I'm running the amp under very easy conditions. Altec 604E speakers (16 ohms) so the amp never even gets warmer than it does at idle with no load.
I'll go ahead and push the idle current up a tad to get the 7mv drop the manual suggests. I'm not far off now so I doubt it will make a temperature difference but I just really wondered where UREI had initially set the bias. Now I know, thank you again!
I sure would love to have the service manual and I even tried to buy on from a reputable manual supplier I use and they sent me a download to the owners manual and schematic which I already had. I have to guess they didn't have it or they would have sent it. Would you consider selling me a copy of your manual?

Thanks again Turk
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Old 26th November 2015, 02:47 AM   #4
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The two schematics are identical with the exception of R34, R35, R36 in bias circuit and Q10, R58 in fan drive circuit.
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Old 26th November 2015, 04:20 AM   #5
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i haven't seen a 6500 in years i had a reference to it in old service notes.
refresh my memory i remember two versions one had the left and right amp modules on vector pac style frames that pulled forward out of the main amp chassis the later version did not have the pull out feature which do you have?
no offense to the amp but if i recall correctly thermal stability was an issue with them.
also with no dc protection if worked hard (like most i've seen in PA applications) output stage faults also meant reconing or replacing speakers
check this out:

Last edited by turk 182; 26th November 2015 at 04:26 AM.
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Old 26th November 2015, 12:33 PM   #6
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Yes, the 6500 is built on a modular frame allowing everything except the power supplies (2 completely separate power supplies) to be removed for swap or service.
It appears that from the changes I mentioned in the two schematics, which are essentially identical, being idle current circuit and fan drive circuit that obviously they had issues with over heating leading to catastrophic failure.
However, I'm simply blown away at how this amp sounds. It has excellent detail, amazing bass, clear but smooth mids and very articulate highs. It is a very simple design and has huge power supplies which probably accounts for its great sound and excellent bass authority and detail. The completely dual-mono design most likely accounts for the "air" and wonderful openness it has. Like I mentioned though, I had to tame the fans as they were just too intrusive in my smallish listening room but once I slowed then down they are hardly audible even at very low listening levels. I also replaced them with new fans that ran quieter. They still move enough air to keep some flow so the amp runs cool so slowing them down was not an issue.
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Old 26th January 2016, 04:40 AM   #7
thejohn is offline thejohn  South Africa
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Location: Cape Town, South Africa
Hi bryaudioguy,
I'm about to repair a 6500 with one channel down.
Both channels seem to correspond with the older schematic (the one included with the manual at jblproservice) with 500R bias pot.
I'd like to change to the later values...
Apart from changing the pot to 200R, what were the revised values of R34 and R36?
In the fan circuit on my schematic I see R58 changes to 10k as you said and Q10 is shown to change to TIP47... is that correct?
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Old 27th January 2016, 03:40 AM   #8
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Hello John,
There were two changes in the fan circuit. Q10 was changed to a TIP47 and R58 was changed to a 10k ohm. These changes make the fan run faster at any given heat sink temperature. I installed a trimmer pot in R58 location so I could customize the fan speed as I felt it was running too fast and circuit variations due to component tolerances was making the two output modules fans run at different rates so the trimmer pots allowed for synchronizing the two modules fan speeds.
The bias circuit had three changes. Bias adjustment trimmer R36 was changed to a 200 ohm trimmer. R34 was changed to 390 ohms and R35 was changed to 120 ohms. I assume they initially had issues with the amps overheating so they made changes to the bias circuit in order to create a more stable bias setting and made the fans run faster for better cooling. And the TIP47 fan driver transistor provides for better fan operation dependability and less probability for cooling circuit failure.
The reasons for the circuit changes are all my assumptions as I was not a UREI service technician, but all seem to be related to or could solve possible overheating issues. The component values given here, however, are correct as per UREI's updated schematic and service manual and are changes I made to my 6500 amplifier.
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