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Dynaco PAS-4 (yes, 4) a little bass shy, need help
Dynaco PAS-4 (yes, 4) a little bass shy, need help
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Old 25th May 2020, 01:15 PM   #1
robertb1958 is offline robertb1958  United States
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Default Dynaco PAS-4 (yes, 4) a little bass shy, need help

I recently picked up one of these, mildly moded by the PO. The pre sounds wonderful throughout the mids, very 3d. But the line section seems a little wimpy on bass. The phono section sounds just fine compared to other good phono sections.

I think the only mod that the PO did was to substitute 1 microfarad Mundorf caps for the C9/C109 ooutput coupling caps that should be 3.3uF per the schematic.

Wondering if I should put some 3.3uF caps back in? Line stage tubes are Amperex bugle boy 6DJ8s, those should be good. I could roll in some other tubes, or maybe test them on my Eico?

Would appreciate any input.
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Old 27th May 2020, 08:12 AM   #2
duncan2 is online now duncan2  United Kingdom
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Location: Scotland.
Looking at the Dynaco-PAS-4 circuit diagram it shows -


C9/C109 as =1uF- polypropylene,
this is shown also in the parts list.
Dynaco PAS-4 - Manual - Vacuum Tube Preamplifier - HiFi Engine


There is no sign of =3.3uF capacitors.
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Old 27th May 2020, 08:46 AM   #3
robertb1958 is offline robertb1958  United States
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Thank you, Ducnan.

Interesting, the online diagram I found elsewhere indicated a 3.3uF in that position.

I found that I had some 6DJ8 tubes on hand, rolled them in. I think they helped the high level section bass response. They tested a little stronger in my Eico 667.
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Old 27th May 2020, 08:01 PM   #4
dotneck335 is offline dotneck335  United States
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That preamp wants to see a high-impedance load, for sure, like ≥100KΩ. If your load is less, you may see some performance losses. Putting in a higher-value cap certainly won't hurt anything, and may improve bass response into a lower impedance load.
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