I'm trying to track down (and eliminate) the source of hiss in a Conrad Johnson PV8 preamp. It occurs in both channels and is affected by volume. I grounded the grid of V6 with 1uF film caps, but no change. So it would seem it's either coming from V6 itself, solder joints in the area, the cathode resistors, or the power supply. I did try tacking in some extra capacitance across the plate supplies, but no change. Many different 5965s have been subbed in without effect. The cathode resistors are all RN65--unlikely that one would go noisy, right? The tube socket has been cleaned. I'll go in and examine the solder joints next, but let me know if there's something (or many somethings) I've overlooked.
I have no experience with this preamp, but this hiss suddenly came from nowhere? Or has it been there all the time?
I see lots of zeners in series in the power supply. Perhaps they cause noise / hiss?
Regards, Gerrit
I see lots of zeners in series in the power supply. Perhaps they cause noise / hiss?
Regards, Gerrit
Usually when there's hiss like described in my experience it usually end up being leaky coupling caps, bad grid leak resistors, heater cathode breakdown, or a combo.
Start pulling tubes to narrow it down to which section is what I would probably do to start. The schematic doesn't look like anything will blow up if there's a tube missing.
Maybe @wiseoldtech has worked on these and knows a known issue?
Start pulling tubes to narrow it down to which section is what I would probably do to start. The schematic doesn't look like anything will blow up if there's a tube missing.
Maybe @wiseoldtech has worked on these and knows a known issue?
As both channels are affected the problem has to be the power supply. . The pass transistors are high voltage n-types like BUX 85 (tip 50) except Q1 which a p-channel tor. I once had a similar problem with a cj- preamp and the problem appeared to be Q1. I replaced it with a BF 762 a 350v, 10w type. Any p-type with similar properties can be used here.
To prevent hiss zener strings are usually bridged by a capacitor of anywhere from 0.1 mu- 0.47 mu so you might try that as well.
To prevent hiss zener strings are usually bridged by a capacitor of anywhere from 0.1 mu- 0.47 mu so you might try that as well.
I agree that the power supply seems most likely. But if Q1 (or some other component in the supply) were the source of the hiss, shouldn't temporarily adding extra capacitance across the plate supply lessen it?
If the zeners cause the hiss, extra capacitance across the plate supply will not work.
Try one or more small capacitors across the zener string (0.1 - 1 uF). See if it helps.
Regards, Gerrit
Try one or more small capacitors across the zener string (0.1 - 1 uF). See if it helps.
Regards, Gerrit
If Q1 were the source of hiss extra capacitors would help a little but are not the cure.. Apart from this, as zeners always produce hiss to bridge them with capacitors won't hurt.
To find the cause replace parts of the stabilized supply by a resistor - cap combination fe 100mu and 4k7 and start backwards. You can also disconnect the stabilization part altogether en replace it temporarily with let's say 4 elco's and 3 r's of 4k7 just to be sure it is the pwr supply.
To find the cause replace parts of the stabilized supply by a resistor - cap combination fe 100mu and 4k7 and start backwards. You can also disconnect the stabilization part altogether en replace it temporarily with let's say 4 elco's and 3 r's of 4k7 just to be sure it is the pwr supply.