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-   -   Pilot 248B Amp with weak channel (http://www.diyaudio.com/forums/tubes-valves/225355-pilot-248b-amp-weak-channel.html)

ArcticBrew 10th December 2012 01:59 AM

Pilot 248B Amp with weak channel
 
1 Attachment(s)
I purchased a Pilot 248B Integrated Amp 2 years ago. I fixed some bad components re-tubed it and set the bias according to the manual. The amp worked fine and produced about 30 watts per channel into 8 ohms at clipping.

I gave it to a friend. He contacted me recently because it began distorting badly at power levels above one watt.

I can't find a schematic for the 248B. The attached schematic is for a 264 amp but is similar enough from V3 and V4 thru to the outputs for troubleshooting.

I replaced all the caps in the HV supply and the 100uf cap in the bias supply. The HV supply shows a 10k ohm 2W resistor before V3 and V4. It was bad. In my amp it is actually a 6700 ohm resistor. I replaced it with a 10k 5w resistor.

These changes brought one channel back to 30w output before clipping and the other channel back to ~12w before clipping. I can hear a low level buzz in the bad channel that does not exist in the good channel. Otherwise the sound is good until clipping.

I swapped the 7591s and the 12ax7 amp/pi tubes between the two channels and the problem stayed with the bad channel. I did other tests on the tubes and I think they are all okay.

The 248B has a 100k ohm resistor in series with the input and the grid of each of V3 and V4 audio amplifier stages. The signal before and after the resistor in the V3 channel does not clip up to full power. In the v4 channel it does not clip before the resistor but it shows clipping after the resistor at about 10v peak.

In the V3 channel I replaced the 4700 ohm cathode resistor (it was 5600 ohms on my meter), the 100k ohm plate resistor (120k on the meter), and the coupling cap (it is actually 0.1uf). This did not fix the problem.

Can someone here suggest what I should do next to identify and fix the problem.

Thanks,

Steve

dmadmc 9th September 2013 04:45 PM

There must be cold soldering joints...
Also check the input connector contact, especially the ground contact.

Does anyone have a complete Pilot 248 schematic?
I need trouble-shooting this amp as well.
By the way where did you get the aluminum can capacitor replacement?
I appreciate your support.

DF96 10th September 2013 11:28 AM

Measure DC values, and compare channels.

JonSnell Electronic 10th September 2013 12:08 PM

Measure the voltages on pins 1 and 6 of the double triodes. That should give you a clue also I would check the output valve voltages by comparison.

MelB 10th September 2013 12:53 PM

Did you wiggle the a/c balance control on the bad channel? Could be poor contact there.


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