Oahu Tonemaster-Hum and 21khz distortion

[wsscott]

Thanks.  Yes 3 prong.

[wsscott]

Yes it’s 3 prong.

[wsscott]

The only place I get any 60hz signal is at the Output Jack.

The spec sheet for the OT is attached.  I have the brown, 8 ohm, wire lead from the OT connected to the Tip of the output jack, and the Black common lead connected to the GND lug of the jack.  I also have a wire connecting that jack lug to the chassis to ground the OT.

[wsscott]

The Output Jack is not physically attached/mounted to the chassis.  The owner didn't want to drill another hole in the chassis.  So it just gets stabilized with a plastic support mounted to the cabinet.  That's why there is a separate ground wire running from the output jack sleeve lug to the chassis ground point.

[passaloutre]

There’s no 1kHz signal on that scope

[wsscott]

The sine wave is the 1khz signal.

[shooter]

your scope thinks otherwise!

[wsscott]

I don't know why it did that.  But if you look at the top of the screen where the FFT is running, it shows a frequency of 60 hz.

Also on the screen that you point out, it shows VRMS of 159mv, which is the voltage of the input signal from my signal generator app.

So I'll check it out again today.  But when I probe the circuit's ground points for an AC signal, with no input signal injected since the amp has this hum even if there is no input signal, the only place in the circuit that the scope in FFT shows any AC frequency is this 60hz frequency that is at the Output Jack.

[shooter]

decrease your time/base to expand the FFT trace

[wsscott]

I was probing the circuit with no input signal, and have found that I first pickup the 60hz signal on V1, Pin 2 which is the Plate.  And it next shows up on to V2, and I guess continues until it gets to the Output Jack.

So why does it start at V1's Plate?  It's not on V1's Grid.  We did separate the Plates and also the Cathodes on the circuit.

Maybe it has something to do with that change to the Plates of V1?

[shooter]

I first pickup the 60hz signal on V1, Pin 2

repeat that only probe BOTH sides the plate R

[wsscott]

I had just done that.

The B+ to the Plate is coming from the 8uf cap.  There is no 60hz signal between the 8uf filter cap the connection to the 170K resistor that connects to the plate.  However once after the voltage goes through that resistor (actually its a 67K in series with a 100K since I didn't have anything close) then the 60hz frequency appears, and its taken to V1, P2 Plate.

So it would seem that its something about that connection with the resistor that brings in the 60hz to the circuit.

The Plates had originally been linked, but they were separated as part of dealing with the MIC channel's oscillation.

The resistors read properly.

So what's going on?

[wsscott]

I also should add that this 60hz only appears on the connection between the P2 Plate and the B+, and it does not appear on the P5 Plate even thought the resistors going to P2 and also to P5 are wired the same for each of their paths, and they originate at the same point on Pin 6, which is an unused lug on one of the 6V6 tubes.

[shooter]

So what's going on?

how close are those R's to the filament wires?  are they "running" parallel to filament wires?


got a spare tube?


for fun, tack, gator-clip a smallish cap ~~~ .001uf from the plate to ground, any change??

[wsscott]

No spare tube available.

The R's are running sort of perpendicular and they are elevated "in the air" above them. I wouldn't say close. The Pin 2 resistors start at Pin 6 and the Heater is at Pin 7 on the 6V6.  The resistor with the signal ends up at Pin 2 and the Heaters are at 7 and 8.  These resistors were originally 100K, but when the plates were split I had to increase them to 168K to get the plate voltage lowered.

I added a .001 cap, and it did reduce the hum some.

[shooter]

60hz on the plate is either "induced/coupled" by its surroundings, generated by the tube, or amplified by the tube.


me, I would simply re-build (all new) that stage then go play outside, snows coming.

[wsscott]

Sounds like good advice!  Thanks so much for your help!

[wsscott]

I've decided to replace the V1 socket and re-wire it.

I noticed that the filament wiring between Pins 7 and Pins 8 on V1 and V2, the pre-amp and PI tubes, is not wired like I've always done it.  In this case Pin 7 on V1 connects with Pin 8 on V2, and Pin 8 on V1 connects with Pin 7 on V2.

Does it make any difference if they're not connected Pin 7 to Pin 7 and Pin 8 to Pin 8?

[shooter]

7to7 8to8 would be "best practice" in my book, although it shouldn't make a difference

[wsscott]

Shooter-I took your advice and replaced the V1 socket, and then wired only the V1A side according to the original schematic.  The cathodes and plates are connected as in the original with the original component values.  This 1/2 of the tube receives the signal from the instrument.

So the result is a really low volume.  I checked voltages and found something odd at the cap can. Nodes A and B read normal high DCV.  However Node C that drives the V1 plates reads 4 VDC!  Is that the sign that that cap is bad? 

Remember I haven't wired in the V1B side of the tube yet, ie. nothing going into its grid, plate or cathode.

The rest of the circuit from the PI on is wired per the original schematic.

[wsscott]

I tested that cap by connecting my meter to it and to ground with the power off.  When I turned the power on the DC voltage gradually increased to 23VDC, and then started falling off and finally stopped at 5 VDC.  The other 2 Nodes were reading normal.

I guess somethings wrong inside the cap?

[shooter]

However Node C that drives the V1 plates reads 4 VDC!

pull the tube, does the VDC go back to normal??


If So;
I suspect a wiring error or a component value error.  I ALWAYS use my meter to verify resistance before installing, old eyes n CCP colors are not compatible.

[wsscott]

Nope-I took out V1 from the socket I was working on, and checked the voltage from cold start.  It did the same thing.  Went a little higher this time, and then started going back down and stopped at 5VDC.  The other Nodes are reading properly.

[shooter]

NO POWER
disconnect the + lead from the cap, use a couple gator-clips ...clip....100K R...clip  put one free end of a clip to the +  , the other free end to ground.  This should discharge the cap without a snap.  leave connected


put your meter in ohms mode, + to + of cap, - to ground.   you should read near ZERO ohms with the bleeder in place.


now disconnect the bleeder, leave meter connected.  the meter should charge up the cap to ~~~~>500k out to infinity.

[wsscott]

I checked again this morning and realized that I had not installed a resistor between the B and C nodes.  So I installed that and get voltage, but with a lot of feedback.  I’m guessing that’s because the other half of the circuit isn’t installed yet.  I was hoping I could just wire up half of the V1 tube for the Instrument Input, and get that working, and then deal with the MIC Input for the other half of the V1 tube.  Maybe that plan doesn't work since both halves of the V1 tube go into the PI.

[wsscott]

Until I hook up the V1B side of the 6LSN7 tube to the MIC Input section, should I ground its grid to prevent the feedback I'm getting at this point in the rebuild?

[wsscott]

Yes, grounding the grid cleared that up.  So the 1st stage for the Instrument Input is running fine but with a little hum that does not vary with adjusting its Volume Pot.

The plate voltage is about 143VDC and should be around 85 or so.  I've got a 100K coming off the Node C B+ to the plate, so I think maybe I need to increase that to 150-200K?

I've also wired in the second stage for the MIC Input.  It is extremely loud at its lowest volume, but its a hotter input which I think will drop when I increase the resistor mentioned above since the two plates, 2 and 5, are jumped with each other and are each receiving the 143VDC.

[shooter]

The plate voltage is about 143VDC and should be around 85 or so.

does V1 share a cathode R or does each 1/2 have it's own?


IF.. they have individual R's, measure the VDC at the cathode of each, you'll probably find the one with a grounded grid is not drawing current which will affect the plate VDC some.

[wsscott]

It appears that there's not enough voltage drop across the 50K resistor connecting Nodes B and C to bring down Node C voltage to around 175-185 VDC.  Node B's voltage is fine, but it's reading 210VDC at Node C after the resistor.  So I guess I should increase that resistor?

The cathodes are sharing a 3K across a 10uf cap to ground.

[shooter]

wait til both haves of V1 are connected so the tube is properly biased.  then measure the VDC of the cathode to determine the actual current in the tube, remember to divide by 2 since it's shared.


typical biasing would be ~~~~ 1-2mA each 1/2, no signal.


higher plate VDC allows for a bigger "swing" before the tube goes into clipping/compression, if biased close to "center", so don't sweat a larger plate voltage than the amp had 80 years ago, you're playing it now, go by the sound/performance the amp gives you within an acceptable range the tubes operate.

[wsscott]

When I increase the nodal resistor by as much as doubling it, it brings the VDC down, but not enough to be in proper range.  And if I go farther and increase the resistor from 100K to 220K which connects the B+ to the plate, its still over 111VDC vs. 85. and the cathode voltage which has been about 4VDC is down to about 2VDC.  The schematic shows 3V.

[shooter]

I = E / R 


4 / 3000 = 1.3mA /2(tubes) = .65mA  which is about 1/2 a typical fender runs with No signal.


again, you can't set up the VDC until the whole system is operational, so get the other 1/2 V1 wired n tested.  then we can have fun with math 

[wsscott]

Sorry, I wasn't clear.  It is wired up fully--both sides. And these are the measurements I get.  Even when only 1/2 was wired up I was still getting the same numbers.

[shooter]

1st give up on trying to get 1950's voltages with 21st century grid.  the tube can handle 300VDC at the tap and outlast all of us.  your goal is "how does the amp sound".


the math says the tube wants more current, since you decided to go back to a shared plate n cathode, that make the math a 'lil more complicated since you have to divide or multiply by 2 


here's a chart for the tube with typical 6SN7 triode values for each half.


Ebb I believe is the Tap VDC, not VDC at plate, but I might be wrong there
Rp = plate R
Rk = cathode R
Ck = cathode bypass cap
VG = tubes Voltage gain


start by getting the tube to draw ~~ 2-4mA at cathode R
using I = E/R of the cathode


Changing either Rp or Rk will have an effect on the VDC at both plate n cathode.  once you get the bias set to 2-4mA then play it, or sig-gen n scope it.


post what you settled in on for component values.

[wsscott]

Have you got that chart for a 6SL7?  That’s what he’s using in it now.

[wsscott]

I think the 6SN7 draws more plate current than the 6SL7, and maybe that's why the plate voltage is higher with the 6SL7 tube?

Maybe I should just leave the components as is per the schematic in case my buddy decides to go back to the 6SN7 tube.

I don't know.

[wsscott]

Ok, I've got everything wired in, and am getting good sound.  I think I'll leave the voltage issue alone.

I returned the wiring of the Plates to the original schematic where they are jumped together, and not separated like I did in the Mod.

The only problem I've got is with the MIC Volume pot.  It's putting out full volume even when the pot is turned full CCW.  Remember this pot has been disconnected from the Tone pot, and this Volume pot is currently connecting off the plate of V1 which are jumped.  Originally this pot connected after the Mic Input jack and then to the Grid.

I think I may need to put it back to its original location before the grid.

[DummyLoad]

I checked the voltages on the amp and also checked the bias for the 6V6S output tubes.  I think it's running a bit cold on the bias, but I thought I would post the numbers and get your input.

6V6S is a 14 Watt Tube.

I'm reading 36mA for each tube.

The Plate to Cathode voltage is 250.1VDC.

The Plate Voltage is 268VDC.

The Screen Voltage is 274VDC.

The Bias resistor is 200.8 ohms, with a voltage drop of 16VDC.

Rob Robinette's Bias calculator says that for a Cathode Biased Amp the safe max is 56mA.  And the plate dissipation per tube is 9 Watts at about 66%.

So do you think I should adjust the Cathode 200 ohm resistor to increase the bias?

No, it's push pull - leave it be, it was designed as such to work with that reflected load - If you must, for hotter bias, reduce the value of the cathode R.

Was thinking perhaps g2 of the output stage is injecting +FB into gain stages leaking through PS rial, since phase inverter and 6V6 G2 share same PS node, perhaps PS could use some minor tweaks, decoupling power stage G2 and PI. See attached.

Regards,


--Pete

[wsscott]

Is this mod intended to deal with the hum?

[wsscott]

DummyLoad-What is the purpose of the 500K resistor just before the PI?

As most can tell, I'm really just a "mechanic" when it comes to working on amps, and not knowledgeable of the theory.  And at 76, I'm not really wanting to start learning theory, but I do appreciate learning how and why things work.

Thanks.

[passaloutre]

The 500k is grid leak for the first stage of the PI.

[shooter]

Is this mod intended to deal with the hum?

Was thinking perhaps g2 of the output stage is injecting +FB into gain stages leaking through PS rial, since phase inverter and 6V6 G2 share same PS node

yep

[wsscott]

Found the fix for the hum.  In doing the poke test I found that the .05uf cap connecting the Tone Pot to Ground had a bad lead.  I replaced the cap, and the hum is gone!!

Thanks for your help.