5F6A troubleshooting intermittent oscillation

[jlatrace]

Just compeleted a 5f6a build. I followed the traditional schematic except that I changed the NFB resistor value to match the value in the low power twin (56K). When I cracked the amp, I was using a hotplate into a 2 ohm speaker load and it worked fine. Power tube pin 5 voltages were -48vdc and pin 3 voltages were 441 vdc, seemed normal. I played the amp noting no usual signs intially. Very quiet but at the same time a dirty, moderately high gain, blues monster.

However, when I switch the attenuator off or disconnect the attenuator the amp has a low frequency oscillation/hum, not as loud as when the OT primaries are reversed. If I turn the amp off and turn it back on with the volume down, it stops. But after playing it briefly, it returns. I switched the NFB resister to 27K and it made it worse, happening also at lower attenuation levels, but going away if I increased attenuation.

As I said I can play the amp attentuated. In doing so I have noticed that with certain notes there is a non-musical distortion. I think it's related to vibration but I mentioned it since it maybe related to this issue.

I plan to troubleshoot this this weekend and am hopeful this group can help me. Several years ago Bruce Collins got me started with his 5E3 kits. Now I'm up to seven various builds but with little issues, so I'm still just learning how to troubleshoot.

Thanks for the help.
Les Alexander

[MWJB]

Without knowing what plate current you have, power tube bias cannot be ruled out as a contributary factor. Usually, I would expect to see more like -55v on pin 5, or maybe more depending on the brand of tube? Are there any signs of redplating?

Poor layout & grounding can cause all sorts of problems. Check for overly long grid wires, shield the ones you can't shorten (inputs), use a wooden chopstick to move wires around and see if this affects the problem.

Check the integrity of the grounds with an ohmeter, clip the black lead to the main filter ground, the meter should default to lowest restance reading. Describe your grounding scheme, the original schematic & layout are not detailed enough in this respect.

Posting some detailed photos might help identify potential causes?

[jlatrace]

Thanks. I will post some pics tonight.

I used the grounding scheme from the hoffman site running the input section grounds to a bus bar soldered to the back of the pots and terminating on one of the input jacks and bolted to the chassis near the jack. I grounded the HV caps for the Rectifier & Output Tubes, Output Tube Cathodes, PT Bias Tap and PT Center Tap to a star bolted to the PT bolt. I grounded the AC power safety ground to a separate PT bolt where the power enters the chassis. I also ran a safety ground from the OT ground at the speaker jack to the buss bar where the NFB loop is grounded.

I have used this approach with JTM45's without issue. The amp is very quiet and doesnt seem to exhibit grounding issues.

Thanks alot for the help.
Les

[MWJB]

Grounding plan mostly sounds cool, I don't know about the OT secondary being grounded to the preamp buss bar though.

[jlatrace]

I'll try disconnecting the ground but I don't think its the issue. Could be the routing of my NFB wires. My input section grid wires are very short and do not cross any other wires so I think tha they are okay too.

My hunch is its either the routing of the NFB (I did it differently on this amp trying to run it to a switch so I could switch between 56K abd 26K NFB), the routing of the Outrput Tube plate wires, Output Tube grids or something to dao with routing the B+ lines from the topside capcan (although I twisted the wires B+ send and return wires pretty tightly and routed the same way my blackface super reverb routesthe wires).

[jlatrace]

OK I have jiggled and moved wires but I cannot induce the squeal. Only two things do it everytime.

When I use 56k as the NFB resistor, I can play the amp and when I hit a power chord or bass harmonic it starts a low register squeal.

When I switch the NFB resistor to 27k and turn on the amp it squeals as if I have the OT primaries switched. I rechecked my readings on my power tubes pin 5 reading and they are reading -34vdc......hum.......low. Can an amp with the OT primaries reversed play at times but at others squeal?

Pics will follow soon but now its time to spend some time with the family.

Thanks for everyones help with this issue.
Les
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[TD_Madden]

did you try reversing the primaries "just to see"?

[jlatrace]

That's the first thing I plan to try tomorrow and thanks for staying in touch. I'll post the results.

[jlatrace]

[dai h.]

Grounding plan mostly sounds cool, I don't know about the OT secondary being grounded to the preamp buss bar though.

yes, I would group the OT secondary with the power tube cathodes, etc. since the OT secondary is a high current return (might be fine from what I see in the pic if the jack is directly connected to the chassis).

also, it's hard to make out what is in the pic, but if that pot next to what looks like the spk. out is the bias pot, the bias circuit ground shouldn't be grounded to the same spot as the spk. ground also since the bias circuit is very low current.

[jlatrace]

I will move the cathode ground to route with the OT primaries. Right now they circle around the opposite way.

The bias pot feed and return is the blue twisted wire running to and from the pot. The bias circuit is grounded to a satr at the PT bolt.

Here is a better pic of the power section which shows which wires are elevated and which ones are routed together. The heater wire is green from the PT to the power tubes and brown leaving the power tubes to the phase inverter tube.

[dai h.]

I will move the cathode ground to route with the OT primaries.

maybe you meant to say secondary but please don't get the OT primary and secondary confused. The primary side of the OT has the center tap and two connections to the power tube plates (all high voltage and none go to ground!). The secondary of the OT is your "speaker out" (hot and ground).

I would leave the speaker output jack as it is (grounded directly to chassis at that spot). The bias pot ground (not the casing of the pot but the ground connection--I am assuming here that it's hooked up as in most Marshalls from C- to ground) however I would connect to the preamp grounds side. The speaker ground is a high current AC return and the bias pot ground is low current, so they should not connect together like that.

[jlatrace]

Switch the OT primaries and it worked. Thanks.

Also I learned that an amp can be played when its OT primaries are switched and you attenuate the amp's output.