5F6A - Nasty Distortion when Pushed - Local chassis grounds everywhere

[cabkab]

I'm planning to implement the Hoffman recommended grounding scheme on my 5f6a clone however I don't  understand the remedy for this statement.

"If you use two 100 ohm resistors as your heater center tap, do not use the power transformer heater center tap wire and visa versa."

I do have these two resistors in by build. They are on the pilot light to chassis ground and the green heater twisted wires connect to that point as well.

Furthermore,

I've got local chassis grounds everywhere right now and when I turn the amp past 5 I get a nasty distorted sound in the amplified signal. Its a very unpleasant harsh clipping sound. The amp sounds fantastic at lower volumes. On the oscilloscope the nasty Distortion shows up on the very first preamp stage. You can see the waveform freak out as you turn up the volume past a certain point. I've shielded the preamp input wires but no difference.

Could a horrible grounding scheme cause these typed of issues?

[EL34]

I'm planning to implement the Hoffman recommended grounding scheme on my 5f6a clone however I don't  understand the remedy for this statement.

"If you use two 100 ohm resistors as your heater center tap, do not use the power transformer heater center tap wire and visa versa."

I do have these two resistors in by build. They are on the pilot light to chassis ground and the green heater twisted wires connect to that point as well.

If your PT has a center heater center tap, you can use that
If not, use two 100 ohm reistsors

[cabkab]

I'm planning to implement the Hoffman recommended grounding scheme on my 5f6a clone however I don't  understand the remedy for this statement.

"If you use two 100 ohm resistors as your heater center tap, do not use the power transformer heater center tap wire and visa versa."

I do have these two resistors in by build. They are on the pilot light to chassis ground and the green heater twisted wires connect to that point as well.

If your PT has a center heater center tap, you can use that
If not, use two 100 ohm reistsors

Gotcha!

Any thoughts on my issue and if grounding technique could be a cause?

[plexi50]

Does the amp hum at idle? (no signal) What type tube do you have in V1? Any pic's of your amp build?

[cabkab]

No hum at idle. V1 is a12ay7. All of my pics are over 1mb :(

[plexi50]

Go to / http://www.freeimageconverter.com/downloads.aspx
Center of page click download. Super easy and a must have. No adware or crap. Use advanced install and decline any offers. It will still install only the converter. I cant live without it.

[Ambugaton]

www.picresize.com

and put in 800kb as the max size

[cabkab]

Does the amp hum at idle? (no signal) What type tube do you have in V1? Any pic's of your amp build?

Here are some pics. I've since shielded the inputs with the shielding grounded only to the input.
There is one pic in here showing the amp in a bigger chassis before I rehoused it in a real 5f6 style enclosure. The problem existed before I rehoused the amp. I include this pic because the roomier layout shows the circuit a little better in a picture.

https://m.flickr.com/photos/58607046@N02/

Here is a video recording of the issue:

That sound that sounds like a mic is clipping is what I hear in the room.

[Ambugaton]

Video is private

[cabkab]

Video is private

It's public now

[Ambugaton]

Doesn't sound like a grounding issue... I would personally swap out speaker/guitar/cables/tubes before diving into the amp itself.

[cabkab]

Doesn't sound like a grounding issue... I would personally swap out speaker/guitar/cables/tubes before diving into the amp itself.

Tried all that first.. You can see the chaotic Distortion on an Oscope on the first preamp stage.

[Ambugaton]

Yeah... I forgot you already scoped it out. My bad.

This is beyond my knowledge... which isn't too much. It sounded like the problem was specific to the lower frequencies. I am sure its not just fret buzz on the low E... Seemed to be the only thing affected. I couldn't listen to the video on very high volume so I could be wrong. Does it sound similar on all inputs? Might be a simple deoxit clean of the inputs/sockets. But please wait until someone who knows something chimes in. I am just bored at work and wasting time...

Nice looking amp though!

[Willabe]

That sound that sounds like a mic is clipping is what I hear in the room.

I'm not positive but that buzzy noise at the very beginning sounds like something in the cab vibrating or a bad or loose speaker or 2.

Later there's a distortion sound that I think is being caused by something else.

Have you tried changing the tubes with know good tubes? If not try that 1st.

Then if still not right, take voltage readings for ALL the tubes/pins, even if their 0v and post them.

[Willabe]

Where did you get that speaker cab?

I LOVE the faux ostrich and the grill cloth!   

[cabkab]

That sound that sounds like a mic is clipping is what I hear in the room.

I'm not positive but that buzzy noise at the very beginning sounds like something in the cab vibrating or a bad or loose speaker or 2.

Later there's a distortion sound that I think is being caused by something else.

Have you tried changing the tubes with know good tubes? If not try that 1st.

Then if still not right, take voltage readings for ALL the tubes/pins, even if their 0v and post them.

It's definitely something in the circuit other than Tubes. Voltages are all good. Been down that road. The Distortion is more sensitive to the lower frequencies. I got the cab from Mojotone.

[Willabe]

The Distortion is more sensitive to the lower frequencies.

Lower frequencies can cause something that's just a little loose in the cab to shake and make the noise in the beginning of the video. 

You can try using shielded for the grid wire runs from the PI to the power tubes, those grid wires can pick up stray air born stuff and mess with the tone in a bad way.
 

[Willabe]

It's definitely something in the circuit other than Tubes.

So you did swap all the tubes out with known good 1's?

[plexi50]

Sound like a bad resistor or tube to me. Do you have the listening device in Dougs library? It will find the problem every time.
Nice clean build! I see an older  Groove tube in V1. I have had those tubes do exactly what you are hearing. I have heard them hum at idle and all kinds of weird stuff with those tubes when they go bad. They will read excellent as well on a tester with no shorts.

[Willabe]

Do you have the listening device in Dougs library?

Link for Doug's library, it's way down toward the bottom;

http://el34world.com/schematics.htm

Link for listening device in Doug's library;

http://el34world.com/Hoffman/tools.htm

[PRR]

> nasty Distortion shows up on the very first preamp stage. You can see the waveform freak out as you turn up the volume

The volume control is *after* the first stage.

So I would be suspecting the B+ filter cap common to 1st and 2nd stages.

"Chassis grounds everywhere" can work, or not, hard to predict. Many designers/builders end up using some form of bus ground, with only -one- circuit connection to chassis (plus of course your wall-cord 3rd-pin safety ground). But this is hard to retrofit in a finished build.

[bsconz]

Any chance the OP can comment on whether or not he fixed this issue?

My 5F6A build has developed the exact same problem and I'm currently going through re-education and troubleshooting now!

[sluckey]

Send him a PM. Maybe he has email notification turned on. He hasn't been on the forum since January 2016.

[bsconz]

Good idea...just sent cabkab a PM.

I'm going to revive this thread since my issue is exactly the same. Weird noise when the amp is pushed with low E string notes or percussive type playing. Sounds like a demon in the speakers...same sound as the video clip above.

Some background on my 5F6A:

Hoffman board, circuit, and layout.
Hoffman/MojoTone supplied components.
MojoTone power and output transformers - basic in-amp checks done.
Weber speakers (2) 10A125 and (2) 10F150's - all checked to be wired in phase.
MojoTone cabinet.

Screen Resistors = 1k/5W for EL34 compatibility
Grid Stopper Resistors = 1.5k/1W per Hoffman schematic
Pentode/Triode Switch = noise seems to be made worse in triode mode

V1,2,3 = JJ 12AX7
V4,5 = GT 6L6GC matched set (also have a JJ 6L6GC matched set and amp is also compatible and have JJ EL34 matched set)
V6 = JJ GZ34 or Sovtek 5AR4 or SS rectifier

Output Tube Plates Voltage = average about 448VDC with SS rectifier and 435VDC with 5AR4.
Bias (cathode resistor method) = average about .030VDC across separate 1 Ohm resistors or about 30mA each => super cold (changing the bias range resistor tonight so the bias pot can be adjusted better)
 

Troubleshooting:

1) Output Tube Bias: It's way too cold at 13W and I will fix this tonight.
2) Output Transformer: I've done all the in-amp checks possible and everything looks good.
3) A/C Coupling Caps: Will unplug output tubes and check for leaky A/C coupling caps.
4) Grid Stopper Resistors: I can jump or remove them to see if the noise goes away.
5) Screen Grid Resistors: The stock 470R/2W resistors were upped to 1k/5W for the EL34 compatibility. Not sure if maybe this caused the issue?!?!
6) Cathode Resistors/Cathode Bypass Caps: I will check everything I can in the preamp section as well as the power section.

Any help is good help!!!

[sluckey]

Is this the same incredible sounding amp you built three years ago? Did it suddenly develop this issue or was it a more gradual thing? Or has it always had this issue and you've just now noticed?

[bsconz]

Yes, same amp.

I need to admit that I made a huge boo boo one day putting the chassis back in the cabinet after installing the new Webers.

One of the keps nuts that hold the chassis to the cabinet pinched a 6.3VAC wire from the power transformer secondary and shorted to ground. It wreaked some havoc for a few seconds until I saw the smoke from the melting wire insulation.

I replaced the power transformer and did a health check on the amp at the time but didn't notice anything weird.

This was about 18 months ago and honestly the amp hasn't had too many hours of playing time since I fixed that giant mistake.

If I had to guess, this issue developing recently while installing the pentode/triode switch.

[sluckey]

If I had to guess, this issue developing recently while installing the pentode/triode switch.

I would remove the switch and see if things improve.

[bsconz]

Did some more troubleshooting:

The pentode/triode switch is definitely hooked up correctly. All it does is connect the screen grid to the plate for each output tube. Should work with 6L6 or EL34 output tubes that I will be running.

Even in the pentode or normal mode for this switch, I’m having trouble getting the bias set correctly. Can’t get enough range out of the adjustable bias so I’m having to drop the value of the fixed resistor quite a bit. The amp is loud and punchy but a little too sterile as the calculated plate dissipation is coming out to only around 50%...way too cold but definitely a very punchy percussive sound!

I’m not going to remove the switch just yet, not until the bias is set correctly.

[bsconz]

I figured some stuff out and now I’ve got my 5F6A dialed back in perfectly right now in both pentode and triode operation: dead silent with no hum or pops/crackling (except normal air on microphone kind of sound), running Sovtek 5AR4 rectifier, JJ EL34 outputs, and JJ 12AX7 preamps. Biased with 429V plate and 41mA across the cathode resistors (had to reduce the bias resistor in series with the pot down to 10k/1/2W to get to the correct bias)...the amp sounds incredible!

The reason for all of this is that I recently took this amp back out and installed a pentode/triode switch for the output tubes. I saw the mod on Rob Robinette's website (among piles of great information). In pentode mode, everything sounded ok except for the bias which is now fixed. In triode mode, something was seriously wrong. I'd hit loud, low notes on the E or A strings and it would sound like an awful clipped terrible distortion with both EL34’s and 6L6’s. I did a full check out on the amp and in doing so I noticed a problem with the wiring of the pentode/triode switch I had just installed.

I went with 1k/5W screen grid resistors for EL34 compatibility and I already had 1.5k/2W control grid stopper resistors but the way my amp and circuit board is configured, the grid stoppers are on the tube sockets and the screen grid resistors are on the turret board (Hoffman turret board). So in triode mode, the new pentode/triode switch was connecting the screen grids directly to the plates and bypassing the screen grid resistors! Obviously, that means the screen grids were getting full 430VDC plate voltage and why it sounded ridiculously bad in triode mode!!!

I replaced the purple wires from the switch to pin 3 in the first photo with some separate 470R/2W "triode mode screen grid resistors" so in triode mode the plate would run through the switch, the new resistors, and then to the screens! This setup has to be different than Rob Robinette's mod instructions because of the board-mounted screen resistors with the Hoffman turret board and layout I’m using. The beauty is that I can have different value screen grid resistors for pentode and triode mode...which will result in different tone.

It might be better to mount the 1k/5W screen resistors directly to the tube socket and use the same higher wattage resistors for both pentode and triode mode. Not sure yet what will be the final solution here but so far it's been good with the 2 sets of different value resistors. I've been jamming the amp HARD in both modes and it sounds amazing so it may just stay like this!

[davidwpack]

Nice! I tried the triode/pentode switch and it sounded like hell so I just removed it. I may try it again.