Oscillating 6SL7 ptp preamp - am I missing the obvious?

[Molwam]

Hi, I'm a new member looking for help with my amp build. For my previous builds I have found a lot of usefull information on this board and I want to thank you for sharing this impressive knowledge to the world, helping others to advance in this hobby!

I attached the schematics of the amp I'm building. This is the first time I'm using 6SL7s. I've always worked with noval tubes and wanted to try something different this time. I almost completed the amp except for the negative feedback loop and its attached controls. At lower gain settings it's working fine, but higher gain settings result in squealing. At mid gain you can also hear the oscillation creep into the overtones, resulting in a somtimes unpleasant overdrive sound. So obviously something is not right here. I tried to narrow down, when and where the osciallation is occuring with a scope:

both gain controls behave the same way: oscillation sets in once a certain point on the pot is reached. The point is roughly the same for both pots. If you turn up the gain, the oscillation frequency goes down. All four bright caps increase the freqency.

both volume controls can adjust the oscillations volume but have no other influence.

Pulling V2 and V3 I could still observe the oscillation along the signal path up to R10 at high gain settings. I completly disconnected R12 and C6, no changes. Pulling any of the load resistors of V1 got rid of the oscillation. Therefore it seems to me, that both triodes of V1 are somehow interacting. I also removed the relay, removed the fuse for the relay power supply and hard wired the first pot to be the only gain control.

Here are my voltage readings on V1 (V2 and V3 are still pulled, all pots are set to 0):

1(grid1):0V
2(anode1): 196V
3(cathode1): 1.92V

4(grid2):0V
5(anode2): 184V
6(cathode2): 1.78V

7(heater1): 70V
8(heater2): 77V

I have elevated the heaters and use dc for V1 and V2. These voltages seem ok to me (heater voltage will drop to an acceptable level once the other tubes are reconnected).

What I've tried so far (schematics show the currrent state):

• increase grid stopper from 10k to 68k on both V1 grids (oscillation is now stetting in later)
• shielded wire form input jack to V1 (no changes)
• decrease coupling cap C1 to 4n7 (no changes)
• decouple B+ for both stages (no changes)
• completly remove the volume pot and hard wire C1 with a 1M grid leak resistor to R7 (no changes)
• unmount the input jack and move it around to see if it has any influence (no changes)
• change tubes (I don't have a veryfied working tube, but I tried 4 different tubes from different brands - no changes)
• run 470pf between cathode and anode of the first stage (oscillation stops but at full gain unpleasant overtones are still present. If I disconnect the guitar from the guitar cable, the amp squeals again. Increasing the capacitance muffles the tone too much)

Using the chopstick method, the only thing I could find was that the oscillation volume increases if i poke at the tube socket around pin 2 (second stage anode). Moving other components didn't have any impact.

here are some images of the socket of V1:




The wires near the input jack connect the front LEDs, front switch and relay to the footswitch jack and the power supply. In my current setup they are unused because I removed the fuse of the relay and LED power supply. The wire running underneath the input jack connects the volume pots to the passive effects loop. It's currently unused as well.

Honestly, I have no idea what to try next. I already started to wonder, if the tube socket can cause all this. But I think it's very unlikely. Any help would be greatly appreciated!

[tubeswell]

Without  complete gut shots, it’s difficult to form an opinion about cause of oscillation. You’ve obviously laboured to get such an exquisitely tidy build. However, without a complete picture of the layout it’s hard to see where unwanted coupling might be occurring. You seem to have runs of wire going from pots across tube sockets to other parts - could be emf from high-VAC terminals and leads coupling into low-current signal wires but hard to say.

[Molwam]

Thanks for your reply, tubeswell! Here are some pictures of the whole circuit. All the wires going from pots across the sockets are ground wires. I also scoped B+ to see if the power supply is part of the oscillation, but I only get some very minor ripple (about 20mV).

[acheld]

It may oscillate,  but it is pretty!

[bmccowan]

I am jumping in here, not with solutions but with questions. I note that Tubeswell is one of the experts here, but there are many others (I'm not one of them, just a confused hobbyist) that have not replied. I wonder if that is because they are thinking, like I am, that the workmanships is really nice, but man-o-man you sure have a lot going on in that amp. The more components and the more connections the more things can go wrong.
Random thoughts:
You say you can hear the oscillation up to R10 - does that mean you are using a listening amp? If not, I would suggest that so you can better identify where in the circuit noise develops. Removing components and testing gets tedious after the first 40 hrs, eh?
Are you sure you are not overdoing the gain in the amp? Off-hand, with 2 preamp tubes feeding one 6V6 I do not see much to attenuate the gain.
Is this a known circuit? Can you describe what you were going for?
My apologies if I am asking stupid questions, but I am trying to stir up some interest from the experts on the Forum. Good luck.

[Molwam]

It may oscillate,  but it is pretty!

Right now I wish it was messy but working, but thank you!

[uki]

Very nice work ! Usually providing the schematic helps a lot to troubleshoot !

This work reminds the BC Audio amps, is your amp inspired on BCAudio amps?

[sluckey]

Very nice work ! Usually providing the schematic helps a lot to troubleshoot !

Schematic is in first post.

[mresistor]

schematic helps a lot to troubleshoot !

here is schematic 

[Molwam]

I am jumping in here, not with solutions but with questions. I note that Tubeswell is one of the experts here, but there are many others (I'm not one of them, just a confused hobbyist) that have not replied. I wonder if that is because they are thinking, like I am, that the workmanships is really nice, but man-o-man you sure have a lot going on in that amp. The more components and the more connections the more things can go wrong.
Random thoughts:
You say you can hear the oscillation up to R10 - does that mean you are using a listening amp? If not, I would suggest that so you can better identify where in the circuit noise develops. Removing components and testing gets tedious after the first 40 hrs, eh?
Are you sure you are not overdoing the gain in the amp? Off-hand, with 2 preamp tubes feeding one 6V6 I do not see much to attenuate the gain.
Is this a known circuit? Can you describe what you were going for?
My apologies if I am asking stupid questions, but I am trying to stir up some interest from the experts on the Forum. Good luck.

Hi bmccowan, thanks for your input 

I'm using an oscilloscope to trace the oscillation. I have disconnected R12 from the second triode and removed V2 and V3. So the only active part in the signal path is currently from the input jack to the anode of the second gain stage. The rest is disconnected. I can observe the oscillation at both grids and anodes of V1. Disconnecting any one of the load resistors of V1 gets rid of the oscillation, so it seems to me that there is a positive feedback loop  within V1a and V1b. I can add a second schematic which shows the amp in it's current state, if that's helpful.

This isn't a known circuit, but I've already built almost the same amp with 12ax7s and a 6aq5 (that one doesn't have switchable gain and volume controls, lower B+, no gyrator and is cathode biased). You are right about the gain, it is too much for a 6V6. One goal of this build is a single ended power amp that can take a 6v6, 6L6GC or an EL34 because I wanted to see if I'm able to hear differences between them. My OT has a 5k primary tap which I use for the bigger tubes and a 10k tap which I use for the 6v6. The bigger tubes need more gain. So my plan was, to tame the gain down, once the amp is up and running, to find a good compromise for 6v6 and 6L6/EL34 operation. The power amp is already working. I've tested it by playing trough the effects return input when all the tubes were pluged in.

[bmccowan]

I'm not exactly oscilloscope friendly, but given your description I would have to agree that if you disconnected R12, you know where the issue occurs. Scope or listening amp, I would think it possible to follow the signal chain and find where the signal deteriorates. Others here will know how to interpret scope signals. I would likely cut out all the extra switches and such, gut that part of the circuit down to the bare minimum and then if the signal is clean, start adding things back in, testing with each step.

[Williamblake]

How does the V2b bias work?
// Nevermind, i can remember now.
https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.valvewizard.co.uk/dccf.html&ved=2ahUKEwjxo4DN6Zr3AhW5R_EDHaJQBwQQFnoECBAQAQ&usg=AOvVaw0I5qeCC-7s7FpTWTrKPT6M

[tubeswell]

… I can observe the oscillation at both grids and anodes of V1. Disconnecting any one of the load resistors of V1 gets rid of the oscillation, so it seems to me that there is a positive feedback loop  within V1a and V1b…

This isn't a known circuit, but I've already built almost the same amp with 12ax7s and a 6aq5 (that one doesn't have switchable gain and volume controls, lower B+, no gyrator and is cathode biased).

I think you’ve answered your own question

[Molwam]

I'm not exactly oscilloscope friendly, but given your description I would have to agree that if you disconnected R12, you know where the issue occurs. Scope or listening amp, I would think it possible to follow the signal chain and find where the signal deteriorates. Others here will know how to interpret scope signals. I would likely cut out all the extra switches and such, gut that part of the circuit down to the bare minimum and then if the signal is clean, start adding things back in, testing with each step.

This is the exact approach I took and I came up with this minimal circuit, which still produces the oscillation (see attached schematic). I've now removed the gain pot again, and changed it for a 470k grid leak right at the socket, simulating a dimed gain pot:


Still the oscillation remains. I can't see any faults on the wiring, chopstick poking and bending components around does't change anything.

I haven't tried to remove the cathode bypass, which might fix the oscillation but will also reduce the amount of distortion I can get from the preamp. If possible, I would like to avoid that.

… I can observe the oscillation at both grids and anodes of V1. Disconnecting any one of the load resistors of V1 gets rid of the oscillation, so it seems to me that there is a positive feedback loop  within V1a and V1b…

This isn't a known circuit, but I've already built almost the same amp with 12ax7s and a 6aq5 (that one doesn't have switchable gain and volume controls, lower B+, no gyrator and is cathode biased).

I think you’ve answered your own question

But unfortunately this is where I'm stuck right now. I'm pretty sure about the positive feedback loop within V1a and V1b, but I've tried everything I know to avoid positive feedback (except for reducing gain) and nothing seems to help. Or do you suggest, that the oscillation might stem from the differences between the amps?

Another thing I have noticed is, that I can see the frequency of the oscillation on the heater supply. Once I stop the oscillation, it also stops on the heater supply:

heater:


oscillation (measured at the coupling cap):


Is this behaviour to be expected? I've never examined the behavior of the heaters in a working amp. Could it be that it is simply influenced by the other parts of the tube and acts as another electrode, or is this part of my problem? I've tried to remove the heater elevation and tied it to ground, without visible change.

[John]

When it's giving you the "squeal" or whatever, have you tried gently gripping the tube? I doubt it's that simple, but 6SL7s are often noisy. Just suggesting because I was chasing a whistle and finally thought to dampen the tube, and problem solved. Beautiful build, btw!

[tubeswell]

I'm pretty sure about the positive feedback loop within V1a and V1b, but I've tried everything I know to avoid positive feedback (except for reducing gain) and nothing seems to help. Or do you suggest, that the oscillation might stem from the differences between the amps?

Another thing I have noticed is, that I can see the frequency of the oscillation on the heater supply. Once I stop the oscillation, it also stops on the heater supply

The scope seems to be showing slew distortion in the pre-amp, and as its also evident in the heater winding, you seem to have a feedback loop coupling probably from the signal path (in your tone control section) into the HT rail - being induced into the heater winding through the PT. At a guess, (from what you've reported to-date about disconnecting V1), it may be coming either from the odd RC network you have after V1 (which could be producing a phase shift at HF), but could be a combination of that and your layout. Its an unusual set of tone controls, and the layout is a bit mixed up with pots near tube sockets etc, which puts low current signal path components in proximity to high current parts (without spending too much time on it and not having it in front of me to much around with).

[Molwam]

Very nice work ! Usually providing the schematic helps a lot to troubleshoot !

This work reminds the BC Audio amps, is your amp inspired on BCAudio amps?

Thank you! As the others said, schematic is attached as a .pdf to the first post. Is it better practice to post schematics as an image within the posts body? Yes, construction wise I took some inspirations from BC Audios builds (which wires to bundle together, tubes on pots side), I think they look great.

[uki]

Is it better practice to post schematics as an image within the posts body?

I just missed it because it is right above the picture, I though was the picture file. 

[Molwam]

When it's giving you the "squeal" or whatever, have you tried gently gripping the tube? I doubt it's that simple, but 6SL7s are often noisy. Just suggesting because I was chasing a whistle and finally thought to dampen the tube, and problem solved. Beautiful build, btw!

Thank you! I haven't thought of this and I tried it today. The oscillation amplitude goes down, when my hand approaches V1. If I touch it, it goes away completely, except if I dime the amp. I tried a russian metal base 6H9C (almost a 6SL7 equivalent) in V1 and it showed the same behaviour as if my hand touched the tube all the time. No oscillation except when dimed. This doesn't seem to be a physical problem with the tube, I think, so dampeners wouldn't have an effect. If it was physical, simply approaching the tube would have no effect. I've never seen something like this. Is this a typical symptom of an oscillating amp, or is there something wrong with my tubes? Should I invest in a new tube selected for V1 position?

I'm pretty sure about the positive feedback loop within V1a and V1b, but I've tried everything I know to avoid positive feedback (except for reducing gain) and nothing seems to help. Or do you suggest, that the oscillation might stem from the differences between the amps?

Another thing I have noticed is, that I can see the frequency of the oscillation on the heater supply. Once I stop the oscillation, it also stops on the heater supply

The scope seems to be showing slew distortion in the pre-amp, and as its also evident in the heater winding, you seem to have a feedback loop coupling probably from the signal path (in your tone control section) into the HT rail - being induced into the heater winding through the PT. At a guess, (from what you've reported to-date about disconnecting V1), it may be coming either from the odd RC network you have after V1 (which could be producing a phase shift at HF), but could be a combination of that and your layout. Its an unusual set of tone controls, and the layout is a bit mixed up with pots near tube sockets etc, which puts low current signal path components in proximity to high current parts (without spending too much time on it and not having it in front of me to much around with).

When I reconnect the gain control, dime it and turn it down a bit, the waveshape of the oscillation goes from triangular to a sine function, which probably confirms that that we are seeing slew distortion. I have a few questions about the other things you said, if you don't mind:

• If the HT rail is part of the oscillation, I should be able to observe the frequency (probably with lower amplitude) on HT, right?
• Right now the oscillation occours around V1. I disconnected the circuit around V1 from rest of the circuit. I was wondering how the tone control could still have an influence on the oscillation (as you suggested), because the only common connection to v1 I see is ground. Is there anything else or did we just talk about different scenarios? Btw the tone control is supposed to be switchable between a AB763 and a 6G5 type. Do you have a suggestion to make it less odd? Or is it just uncommon?

Anyway, thanks for the good hints! I have a few things I'm going to try later.

[tubeswell]

Right now the oscillation occours around V1. I disconnected the circuit around V1 from rest of the circuit. I was wondering how the tone control could still have an influence on the oscillation (as you suggested), because the only common connection to v1 I see is ground. [/l][/l]

Both Slew Distortion and Parasitic Oscillation are complicated to analyse and I'm no expert.
Parasitic Oscillation can cause problems resulting in unusually high distortion and can be triggered by positive feedback via stray capacitance between early and late stages of an amp, which means better layout and/or shielded cables may help.
Even a single stage by itself can oscillate due to impedances at the grid and plate (which if the impedances are both inductive, or both capacitative, they can combine to form a negative resistance) and if a capacitor is connected in parallel with this between plate and grid, this = an oscillator.
Parasitic Oscillation can be caused by inductance in the grid and plate circuits, and as negative input resistance is proportional to Gm, high Gm tubes are more likely to oscillate - and you're using 6SL7s. You could try grid stoppers and plate stoppers or 'build-out' resistors to dampen this.
Slew Distortion is another problem (and you may have more than one problem going on).  When driving capacitative loads, there's a maximum frequency able to be delivered into a reactive impedance at a given amplitude because current demanded by a cap depends on how fast the voltage is changed across it. (So the louder you go, the more likely it will slew at HF.)  In slewing, the input signal is not 180 deg out-of phase with the output signal (but rather some other degree of phase lag, and this can disrupt the load line where the load appears almost completely capacitative). If the resulting phase shift results in positive feedback, we have an oscillator (which could be enhanced by the particular series of C/R filters associated with the stage. That's my hunch. Try eliminating one of your tone stacks - You've got 2 x .0047UF in series (C1 and C5), with resistor networks between these, which (seem to) have an assortment of wire runs associated with these going through an assortment of switches and Brite caps etc - and these might be acting as antennas if not switched to ground etc)

Anyone else care to chime in?

[PRR]

> Is this a typical symptom of an oscillating amp

Yes.

[shooter]

... along with design electrical design considerations above, layout and build considerations are important.


are sensitive signals crossing noise makers?
are grounds proper and secure?
were parts damaged by heat from soldering?
are the parts bad, correct value, or sloppy fitting?
did you build as fast as you can so you could hear results faster?