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Old 25th December 2003, 08:30 PM   #1
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Default Help with integrated DPP amplifier.

Hi all

My last project is an integrated DPP amplifier, but some oscillation (I think) problems force me to ask your help at troubleshooting this amplifier.

The amplifier uses 4 PCL86´s (one triode like ½ PCC83 + one pentode like PL84 in the same envelop) per channel. The 4 power pentode´s are strapped as triodes, while 2 of the triodes are used for voltage amplification and phase-splitter (as showed on the attached schematic).

A passive pre amp should be enough, because my cousin (who’s going to use this amplifier) just uses a CD player as source. But the 2 triodes left made me decide to build an active pre-amp, using a schematic published in an Elektuur magazine (in the eighties, I think). The possibility to choose between passive or active pre would show which one is best.

After building the whole thing I tested the active pre-amp and the power-amp separately. Both where working fine, the power amp working for 8 hours without problems. The pre-amp worked fine too, but I can’t comment on the quality yet. The problem appeared when I “integrated” both… It still produced music at the same volume level (at my junk speaker), but lots of “cracks”. I measured around 30VAC at the speaker output (with a simple voltmeter) 30V in a 4ohm load is almost 100W!!!! Changed the speaker with a high wattage resistor, which got very hot in a few moments. I returned to the pre / power, and both worked fine again.

In the attached picture you can see a block diagram of the amplifier and the schematics of both pre and power amplifier. I would really appreciate any help.

And one more question… all PCL´s are from the same batch (fed from 14VAC), but some of them flash when I turn the amplifier on, while others don´t. Do I have to worry about this, or is it just a fast warm-up?

(Power amp schematic is published on the site from ptsoundlab, member of this forum.)

Edit: I will post the picture tonight because I have to go to the last Christmas celebration party now.
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Old 25th December 2003, 09:04 PM   #2
dhaen is offline dhaen  Europe
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Your "cracks" could be many things:
Component breakdown: Cap, valve, valve socket flashover, transformer flashover. Or if the sensitivity is high, it could be really bad mains born interference.

The power calculation / measurement is surely wrong. I'd expect 10 watts or so. Perhaps the measurement was off load, or the meter does not have a good frequency response.

Series heater chain valves often flash at the heater ends when powered up. If you are worried, try connecting a series resistor, and running them from a higher supply. I wouldn't worry though
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Old 25th December 2003, 11:19 PM   #3
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Default Block diagram

As promised
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File Type: jpg block diagram.jpg (14.1 KB, 101 views)
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Old 25th December 2003, 11:21 PM   #4
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Default Power amp

Power amp
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Old 25th December 2003, 11:23 PM   #5
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Default Pre-amp

Pre-amp
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Old 25th December 2003, 11:27 PM   #6
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Default Power Supply

Power supply.

Tomorrow I´m going to the beach, so I won´t be able to reply in the next 10 days. Wish you all a very good 2004.

Erik
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