Help troublshooting 5F6A

[jlatrace]

My 5F6A started making a rustling sound and a low
put put" sound today when it was being played hard. I built the amp in January with help from the folks on this forum.

The sound is after the first stage (I can pull the first preamp tube and the sound continues). The sound is effected by the treble, mid, bass and presence controls.

All the voltages check out fine. I have replaced the tubes, changed out the second b+ 100K resister, second stage 10K (1 watt) resistor & 8UF cap. However, the rustling & put put continue.

Does this sound like a filter cap, resistor or coupling cap issue?

[loudthud]

I don't know how close your amp is to the 1959 schematic but I would suspect the 20uF cap at the phase inverter or the screen grids. Just connect another cap you have handy (of proper voltage rating) with clip leads and listen for the put put to change frequency or go away.

[MWJB]

Double check that pots are tight in the chassis, check/remake grounds including the .1uf on the presence pot. Physically stake any soldered grounds in place whilst the solder cools.

See if you can shorten the grid wires that run to the power tubes, these should not be twisted together and should run to the tube pin by the shortest most direct route. You could always replace these with shielded cable (ground the shield at one end only)?

Do you have grid stopper resistors on the power tubes? If not, try 1.5K to 2.2K, bump the resistor body right up against pin 5. Do you have 470ohm 1W (or better W rating) screen grid resistors? If not, it would make sense to fit these when adding the grid stoppers.

The tone pots may just be indirectly affecting/exacerbating the problem whilst not actually being the root cause.

[jlatrace]

I don't know how close your amp is to the 1959 schematic but I would suspect the 20uF cap at the phase inverter or the screen grids. Just connect another cap you have handy (of proper voltage rating) with clip leads and listen for the put put to change frequency or go away.

Thanks for the help. My bassman is on spec except that I filtered the rectifier power section the same way as a Blackface amp (two 100uf 350 volt filters in seriies with 220k bypass resisters). I did what you suggested (clipped a filter across the existing filter caps in the phase invereter and screen grid sections) and didn't hear any difference. I am going to replace the screen grid and phase inverter filters this weekend just to make sure. I am alos going to re-check my grounds (again).

[jlatrace]

Thanks very much. I have done the following:

Double checked/remade all phase inverter and power section grounds and checked tightness of pots, no change.

Shortened screen grid wires, no change.

Previously added 1.5k grid stop and 470 1 w screen grid resistors to original build. I do not have the grid stop resistors bumped up against pin 5, they are looped over the screen grid resistor (should I change this?).

I am going to re-check grounds and re filter the phase inverter & screen grid sections this weekend.

New development - it no longer needs to get hot to start the ruslting. It does it from the moment the tubes produce sound (but not in standbye mode). Hummm....does this sound like a pesky cold solder joint?
Thanks again.

[jlatrace]

Found a Voltage Issue

Re filtered the amp and took voltages again and I found an issue at the spot where the noise starts to show up, V2 (tone stack). V2 voltages are as follows:
P3 = 1.2vdc (stock 1.2 vdc)
P6 = 316vdc (stock 325vdc)
p7 = 158vdc (stock 180vdc)
p8 = 167vdc (stock 180vdc)

V1 voltage readings are pretty normal as follows:
p1 = 158vdc (stock 150vdc)
p3 = 2.6vdc (stock 2.5 vdc)
p6 = 154vdc (stock 150 vdc)

Does this mean my problem is on the ground or B+ side p7 & p8 for v2?

Should I just do they following?
Replace;
.00025 mica cap (and/or resistors in path)?
Or...since the sound impacts the treble, middle and bass control, replace the .02uf cap running to treble control???


Also the sound is now a crackling that swells and the dies off, but never disapates, the put put is gone.

[MWJB]

All your voltages are low. The "stock" voltages you are quoting are presumably from the original schematic, 5F6As don't run that low in the real world. What spec. B+ secondary are you using? What plate voltages do you get at the 6L6s, what plate current do you have also?

I'm assuming you have tried different preamp tubes. I'm also assuming that the sound stops if you pull the phase inverter.

Sorry, I should have been clearer before...it's the grid wires to 6L6 pin 5, not screen grid wires, that want shortening. Yes, remount the grid stoppers.

[jlatrace]

Requested voltages

Here are my voltages:
For the Power Tubes:
p3 = 442 vdc
p5 = -36 vdc
p6 = 445 vdc

For v2:
p2 = 0.0 vdc
p3 = 1.2 vdc
p6 = 316 vdc (low)
p7 = 158 vdc (low)
p8 = 167 vdc (low)

voltage drop over 10k power resistor = 386 vdc to 314 vdc (low)

For phase inverter:
p1 247 vdc
p2 & p7 14 vdc (low)
p3 45 vdc
p6 241 vdc

Voltage drop over 1 meg resistors connected to p1 and 7 = 43 vdc to 14 vdc (low)

When I pull v1 the noise continues. When I pull either v2 or the phase inverter the noise stops.

It's a slowly building whooshing sound and a static. It starts now when the amp is turned on. Turning it on and off doesn't help, it returns immediately.

[jlatrace]

Fixed Issue for Now

After changing everything on the board, tracing every connection with a jumper re-soldering every connection and ground the noise continued and the voltages remained unchanged.

THE FIX = So for fun I decided to swap v2 & v3 and the noise STOPPED! It hasn't returned and the amp has been running on 10 for the last three hours.

I may never know what the issues were. The power filters were definitely blown and they were corrected early. Maybe there was crud in the tube socket Who knows, the gremlins may return.

Thanks for your help.
Les

[MWJB]

The Gremlins are probably just waiting round the corner with respect to yout negative voltages on pin 5 of your power tubes, "-36v" seems VERY low for a pair of 6L6s, I'd be surprised if your tubes were not pulling too much current. You need to check plate current & most likely reduce it (more negative voltage).

[jlatrace]

Thanks I generally run my amps at 65% to 70% disipation however you are right my bias was running a little too negative. I adjusted it and now the amp is running at 68% disipation (nos mil spec 5881's at about 17 watts on 25 watts).