Improve this Pilot 240 (EL84/6BQ5)

[jackinnj]

Here's the schematic for the Pilot 240 -- it has some issues so will probably get a "resistor-ectomy" -- while I am operating I would be open to suggestions.

[amperex]

Schematic is only partly displayed.

[jaudio]

Originally posted by jackinnj
Here's the schematic for the Pilot 240 -- it has some issues so will probably get a "resistor-ectomy" -- while I am operating I would be open to suggestions.

Jackinnj


I have all the parts I need to make two monoblocks,can you tell me what are some of the issues this amp has.


Thanks

[jackinnj]

issues -- some of the resistors are over 25% out of spec. low level distortion is great -- less than 0.5% but when it is cranked over about 5W it goes up to 6%

when I started the amp was distorting about 17% -- I made some power supply mods, brought most of the chasis grounds to a central point, etc. obviously a good deal of the lowered distortion is owing to a much "hotter" HT supply but this will cause premature failure of the 6BQ5's/EL84.

the design dates from 1959 --

amperex -- all the preceeding circuitry relates to the tone circuits and volume control...

I have a PDF of the original schematic -- it's ~ 250kB ~ just send me an email and I will send it along.

Jack

[anatech]

Hi Jack,
No screen resistors?

-Chris

[amperex]

AuriCap- superior coupling caps to vintage caps
GE 6BQ5 grey oval plate- superior even to '50s Mullard EL84
Mullard 12AX7- a good neutral & sweet tube in most circuits
Polyproplyene- cap for cathode bias, superior to electrolytic
Vishey/Dale 1% WW resistors- quiet & neutral sonics

[gingertube]

This quickest/easiest way to drop the distortion is to use 2 separate cathode resistors on the output tubes. Use 270R 5W on each output tube cathode and use a bypass cap across each one. Anything between about 47uF and 470uF is OK. I have an amp which looks very similar and use 270R 5W on each cathode bypassed by 470uF/50V Blackgate Electrolytics. Higher Voltage rating caps tend to have lower ESR so use say 50V or 63V rated caps for the bypass rather than 25V.

If you need new output tubes and don't want to spring for the cash for NOS tubes the modern production JJ tesla EL84's are VERY good.

Further distortion reduction could be achieved by a diffrent phase splitter driver circuit BUT that requires extensive rewiring which I gather you don't want to do.

See if you can find someone in your neighbourhood who can test your tubes. An old weak tube can degrade performance significantly.

Cheers,
Ian

[jackinnj]

Chris -- no screen resistors.

Ian -- I wrote the article on making a DIY computer interfaced tube-curve tracer but I used my Eico 666 to test the tubes

The power supply electrolytics were a mess as one should have expected.

[oldhifiguy]

Hi. I have a Pilot 240 with DC balance controls for each channel but they don't appear on any svc manuals from SAMS.
I have no way of knowing how to make those adjustments. Can I just remove them somehow and reconnect where they are(short out)?
Any help greatly appreciated, especially a schematic that shows the DC balance. It has ot be a different serial #.

thanks.