| ErikdeBest |
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 ˝ 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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| dhaen |
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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| ErikdeBest |
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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