Help requested with tube amp troubleshooting

[figuringstuffout]

Ok, so I feel like I am getting much better at this building tube guitar amp thing, but am stuck on a fender 5e3 delux build currently.  I am following the triodestore layout but with a completely different preamp section (one gainstage with a cathode follower into a vox tonestack and into another gainstage.  from there it hits a split load PI and into the 6V6 output)

Well, I currently have two issues.....
1.  Upon initial start-up, the B+ on the 5y3 was 367v until i turned off the standby switch.  At this point the B+ jumped to like 470v or so before I switched the amp off.  Concerned, I checked everything, re soldered lots of connections, didnt find anything wrong and tried it again.  Low and behold, things seem to be working now.  So I'm concerned that I may have an issue that will crop up later....any ideas as to what happened (is happening?)  Not sure if it is relevant but the DC resistance from B+ to ground (post 5y3) is infinity (or at least higher that my multimeter goes - 2M).

2.  I have gain, high, low, and master volume controls for the preamp and for some strange reason when I have both the gain and master down all the way I get some super annoying static noise (not too loud but not quiet either) that goes away when either volume raises a tiny amount.  Don't know if it is relavent but I do have a bright cap (180pf) on the master volume pot.  I have checked the grounding (using bus into a star) and can't find any issues.  I will also add that when I shunt the last gainstage (going to the master volume) to ground I get a seriously loud squeal for some strange reason.  I imagine the two of those issues are related but can't figure out why either would be happening.  Ideas?

Thanks for the help!

 

[gemini86]

Double check your grounding layout, and that it doesn't pass closely to any large voltage swings, you may be suffering some HF to RF oscillations. With the volumes down, check your bias voltages at each cathode, make sure they are what they should be (do some maths)

A schematic would really help, given all the different circuit bits you've grafted together.

 

[CJ]

ok the voltage jumping is normal,

power tubes take 90 percent of the current in the whole amp.

so when you hit standby, the load drops by 90 percent,because it breaks the cathode circuit so no plate current can flow,
so you get a voltage rise due to less series drop in the transformer winding and other things which really are not important,

static noise is tube noise, move the tubes in the sockets to see if it is a dirty or loose socket,

microphonics from bad preamp tubes is my first guess,

sometimes with a little burn in time, the noise goes away slowly,

otherwise try tube substitution

when you ground out your master, are you going from the wiper or some other leg?

you might be removing negative feedback somehow, need a circuit diagram to fix this problem,

 

[figuringstuffout]

Thanks for the replies,

gemini86...
attached is the preamp schematic.  The rest of the schematic is the same as the Fender Deluxe, though with some B+ resistor mods in order to account for an extra tube for an effects loop that is currently disengaged in order to figure out this problem.  The problem remains the same whether the loop is on or off (I unplugged the tube running the loop and disconnected it from the circuit).
- I have triple checked the grounding layout and am not finding an issue...though that being said, a number of my problems do end up being a bozo on the ground...i'll continue to check the ground every time i'm in front of the amp.
- Cathodes were the first thing I checked, but the volume was not all the way down with the loud noise happening.  When all was quiet the voltages looked right.  I'll check with the noise happening.

CJ...
- I'm not sure we are together on the B+ jumping issue.  While standby was on, the B+ off the 5y3 was normal.  When I turned standby off to engage the amp, the B+ everywhere was somewhere in the neighborhood of 475v....from preamps to poweramp to OT.  The voltages didn't drop from any of the resistors and I about wet myself thinking i was going to blow my tubes and such.  The second time I turned it on, all voltages were normal and has been since.  I was just wondering if you think there might be some sort of issue that might crop up later.
- Tube sockets are fine
- I have found tubes to cause  some noises like you are describing as well, though this noise only comes when I turn both volume and gain down all the way....otherwise the amp is dead silent even with gain and volume up to where the guitar is really loud.
- I have a star ground that the preamp connects to (volume goes to this master), which then connects to the filter caps (B+1 and 2), and then busses to the PI ground, and then to the star ground on the chassis.
- Forgot to mention, I am not using any negative feedback....I hate what it does to the sound of an amp


**Further Development*
- I removed the bypass cap on the volume control and the problem went away.  I used the amp for two hours last night and everything sounded fantastic, but when I ground the signal from right before the volume control, after the 22n cap off the plate, I still get the loud noise.  So whatever the issue is, it is still there, but now that I don't have the bypass cap on the volume control anymore, the effects of the issue are gone.  I can live with it now (sounds awesome) but really want to figure out why this issue is there..........uuugghh.....

 

[gemini86]

Those bright caps over the gain pots are probably allowing some rf to come through, I don't know if it's the problem, but maybe try some NF through about a 68pF cap just to limit the bandwidth to audio frequencies.

 

[CJ]

try and  swap the output transformer wires that go to the tube sockets,
might be pos feedback problem,

voltage jump could have been ultra sonic osc getting rectified by your volt meter,

 

[figuringstuffout]

Thanks again for the responses.

As for an update....all voltages holding steady at normal for several days now so whatever the original high voltage problem was, it seems to be sorted out.

CJ...
good thought on switching the OT wires, I'll give that a shot tonight and see how it goes.

gemini86...
will give that a shot as well

 

[lassoharp]

good thought on switching the OT wires, I'll give that a shot tonight and see how it goes.

Be sure and keep monitoring volume low if using a speaker or other sensitive input.  If the NFB is connected it can squeal violently when the polarity is in the wrong position.