Squeal and hum on 2stroke

[Paul]

I just built a 2 stroke this weekend sourcing the parts by myself (no kit).

It hums quite loudly. Ie, I have it plugged into a 2×8 cabinet, and sitting about 4-5 feet away the hum is about as loud as playing an electric guitar unplugged as loudly as possible.

The hum does not get louder with turning up the volume on the amp.

The amp squeals about as loud as the hum with volume from 0 to ~2, and again from about 9-12 (those are numbers, not clock dial locations – it’s a 5f2a chassis with silkscreening). between 2-9 the squealing goes away but hum remains.

I followed the book and updated instructions from this site with a few exceptions:

1)I did away with the brass plate. I could not get solder to stick to it even with the plate off the chassis and touching nothing metallic.
2)I did not directly ground the 220kohm/500ohm eyelet as shown in the layout. Seeing as it’s right next to the grounds for the filter caps, and those get grounded to the same spot, I simply jumpered that 220k/500 eyelet to the adjacent eyelet which is the 8uF cap.
3)instead of the brass plate, I grounded the filter caps (and therefore the 220k/500 resistor pair) directly to the back of the tone pot. I did not connect the pots with a bus wire since everything is bolted to the chassis and they connect that way. Is that a problem? otherwise i followed the book grounding instructions to the letter.

My PT has the center taps and they are appropriately grounded.

I’m getting 389V on the 6v6 plate and ~360 on the grid. Which is high relative to the book. I’m using a NOS Sylvania 5y3. Not sure if that has anything to do with this, probably not. But I did want to use this amp with a 6k6 but I’m scared to put that much voltage on that tube – what can I do to bring the voltage down – use the 300V taps off PT instead of 330s? Use the 125VAC tap instead of the 120VAC one?

Photo of the amp attached. Nevermind the impedance switch with only one impedance wire attached.

My plan is to rewire the ac heater circuit with more manageable wire (my PT has stranded, very thickly insulated wire that was hard to maintain twisting, hard to keep positioned where I wanted, and impossible to get into the 12ax7 socket lugs. Will try solid core wire that is easier to manage next time).

Thanks in advance for any tips, problems identified, or other help.

[Robin]

Congrats on the amp and nice work using that NOS rectifier tube. The idea is to keep the heater (6.3v) twisted pair away from the rest of the circuit, especially the signal path. So, I would fix that….
Follow DH’s recommended grounding scheme is important, not to say that it can’t be done differently, but unless you understand the how the grounds affect noise in the circuit, sticking with DH’s design is safe. The ground plate is not mandatory, but it works well for the Two Stoke (and Fender). You can use a buss bar too, as long as you avoid ground loops.
Use you meter to confirm you have good grounds where they are suppose to be. The filter caps for the power stage should not ground to the preamp side. Is you amp passing a signal? or just noise and hum?

Since you specs you own parts (I did that too), I’ assuming you are up for discovering more about tube amp circuit design…check out: http://www.valvewizard.co.uk/
Don’t give up, you are close…

[Paul]

Thanks robin. here’s a follow up after a bunch of work tonight.

I rewired the heater wiring between the output tube and the preamp tube. i also greatly shortenen one of the connectors between output jacks. see photo.

it didn’t fix the hum problem but it helped the squeal alot.

– the squeal is completely gone at all settings.

– the hum is still there, just as loud as before. BUT, when you crank volume all the way up to 12, it almost completely goes away, down to the point I would expect in a quiet amp.

– if i crank the amp to 12, and put the guitar volume down to about 1, I can get a nice clean sound with almost no hum and just a little hiss that you might expect from an amp that is all the way up.

– i tried running a test lead between the grounding terminal strip and the tone control pot body(which is where i am also grounding the filter caps and the 220kohm/500ohm resistor junction). this cuts the hum by ~30-50% but it’s still pretty obnoxious.

robin, you say “The filter caps for the power stage should not ground to the preamp side.” I think what you mean by preamp side is the brass plate? meaning the power side grounding is the terminal strip bolted to PT bolt? But the DH book instructs you to solder both the filter cap ground and 220kohm/500ohm ground to the brass plate. so by doing away with the brass plate and simply putting both of those onto the tone pot, what is the difference? anyway, i tried taking that grounding wire off the pot and putting it on the power ground terminal strip and it didn’t change anything.

other notes i forgot to mention:

– another couple differences between my amp an the spec’d parts list – I used plastic element pots (expensive!) because i thought i’d get lower noise. should i try just regular pots?

also, I added a filter cap drain resistor (220k, 3 watt) in parallel with the big 40uF cap. it drains the caps down to millivolts in seconds after turning amp off. why isn’t this part of the amp design? (hopefully that’s not the cause of my noise…).

Finally, i measured the ac lines coming into the amp and I get 123.6V. I guess I should switch over to the blue 125V tap on the PT? that might lower my high tube voltages, right?

thanks again!

[beelzebum]

Well, the squeal sounds like positive feedback caused by your outputs being too close to the preamp side. It seems you fixed this by shortening your leads. As for the pot cases, you should have a ground wire attached to the back. Trusting that the chassis will ground it is not the best, and also the back of the potentiometers are usually just crimped on. This will not always sufficiently ground the pot. It is not good to attach the filter caps ground too far from the main chassis ground. Since the chassis has resistance every fraction of an inch you get away from your main ground will increase noise due to voltage dividing. If you see a reduction of noise by running a jumper from the back of your tone pot to ground, then hard wire it, and try moving your filter ground away from your pots. As for the power tap, the main thing you want to make sure is that your filament voltage is as close to 6.3v as possible. if you lower or raise it too much, you will shorten you tube life drastically, and also you may get more noise. I personally like keeping my tube voltage lower than the recommended specs for any given tube. That being said, I have never seen a professional guitar amp manufacturer that thought the same. Most of them run voltages way over tube specs, and everyone seems to like doing the same. It will shorten tube life also but not as much as you would assume. Many people seem to prefer the sound that this causes. Same as you, I also like running a bleeder resistor off the filter caps, and it should have no effect on the noise. Lastly, a good way to see where your noise is coming from in your amp is to take a guitar cord, shrink wrap the end that is not plugged in, and use the amps input as a signal tracer. this way if you hear a hum, and think that it is coming from a specific component, then you just move the end of your cord closer to the suspected area. If the sound gets louder then you are in the right spot, and find a way to eliminate that sound from affecting your preamp. Remember to shrink wrap the tip so you don’t accidentally dump 300+ volts through your input.

[Robin]

With the amp on and humming, use a chopstick (or similar) to move the leads around a bit and see if you can reduce the hum. Lift the 5.3v twisted pair away from the components and see if that helps.

Confirm that you only grounded one end of the shielded cable(s). Like the one going from the input jack tip lug to pin 2 on V1.

Beelzebum’s simple signal tracer sounds good, be sure to heed his warning about insulating the cord tip. Use one hand to trace the signal and keep you other hand away from the amp chassis.

I did my 2Stroke heater circuit like this and there is no hum at all from it. https://www.tubeampnetwork.com/tan-community/74-robin/photos/photo?albumid=4#photoid=97

[Paul]

Thanks all. I did indeed try moving wires around with a non conductive stick to no avail. My heater circuit at this point seems as tightly wound and away from signal wires as any other example i’ve seen.

I’m no expert but it would seem to be a grounding or pot thing, right? Why would the hum go away at 12? If it were heater circuit, wouldnt the humming either be there or be worse at 12?

Any reason not to star ground everything? It would not be difficult to add 4 wires to individually ground each pot, the input jack, and the filter caps/220kohm-500ohm resistor junction.

Any reason to suspect the pots? I bought both regular 1M pots and these expensive plastic element ones. I could try the regular ones for kicks.

Also, I’ll switch over to the 125V power tap and see if that lowers my voltages any, and double check the heater circuit voltage before/after.

Thanks again for all your help – seeing the hum almost disappear at full volume makes me think i should be able to get it there at all volume levels, and then this would be one hell of an amp.

[Robin]

A star ground scheme will work fine, be sure to follow the guide lines here: http://www.valvewizard.co.uk/Grounding.html

The pots you have should work fine, be sure the connections to the pots are correct and that there is no open grounds. Soldering to the cases (or a brass ground plate)is no problem with a properly cleaned and tinned 40w or more iron. 60w + is better.

[Paul]

Just so I’m clear here – am I right in saying that the brass plate grounding described in the DH book ties together:
1) the pots
2) input jack
3) cathode bias resistor
4) filter cap negatives
5) 220kohm resistor

And this brass plate grounds these items to the chassis by the fact that it’s bolted between the pots, jack, and chassis. And the chassis is therefore used as the conveyance to connect these all back to the main grounding terminal strip?

And, it would therefore be in theory better to either individually run each of these 5 items to the main grounding terminal strip, or at least run the filter cap negatives/220kohm resistor to a lug on the strip, and connect up the pots and input with a bus wire and then directly wire them up to the main grounding strip?

Thanks

[Robin]

I ground the main filter cap and the screen caps (40uF, 16uF and 8uF), AC ground to a lug on one of the PT bolts, ground the heater center tap there too.
I ground everything else (cathode, preamp filter, input jack, and pots to another ground point on the other side of the chassis, a brass ground plate in the case of my first two 2Strokes or a buss bar on others.

You have the center taps for the 6.3v pair and the B+ (high voltage) pair grounded, right? If the PT has any other center taps, they should be capped off and not grounded. Does your amp hum with nothing plugged in the the input jack?

[Paul]

PT is the weber model DH specs – I am pretty sure that the only center taps it has are the heater and high voltage and those are the two that I grounded. will check tonight.

i will also try your grounding scheme minus the plate (using a bus bar).

And yes, the amp hums with guitar in or out, it starts as soon as the tubes heat up and start amplifying.

plugging in the cable and guitar does not change the hum at all – tone wise or volume wise.

only cranking up to 12 makes the hum almost go away, but a white noise hiss also come on – but again, exactly the hiss you’d expect out of a cranked amp.

I appreciate all these continued responses!!

[Paul]

Success!

I’m not 100% sure what did it, but I did the following:

– moved the ground wire from the AC mains off the terminal strip and onto it’s own dedicated ground (my chassis has a welded on bolt next to the PT mounts for exactly this purpose, i think.

– with the now freed up 3rd lug on the terminal strip, i ran a 16 ga insulated copper wire to the back of the tone pot. ran the same wire type from back of the tone pot to the ground lug of the vol pot. ran yet another from there to the ground lug of the input jack.

– i took the ground wire from the cathode bias resistor/filter caps off the tone pot and put it onto the terminal strip ground.

Voila! a *tiny* amount of hum, about what i expected there would be from the onset – it’s pretty quiet. There are 3-4 leads in the amp i can shorten if I try, and i’ll do that over the next couple weeks while I build the cabinet.

I also switched to the 125V tap and that brought the plate voltage down to about 369-370 from 389-390.

That made me ready to try a NOS 6k6 in there, which worked great, although it was not as big a volume drop from 6v6 as I expected (I have a small PP amp that will take all the same tubes as this one, and the difference between 2 6V6s and 2 6K6s is noticeable.

between that and a NOS British made 12ax7 (mullard? who knows) I bought for it this thing is ready to go.

Thanks all!!

ps – anyone tried a pentode/triode switch on this? that’s the first modification i’ll make. i want this thing to be as low volume as i can make it…

pps – Robin, what’s that black box with 2 knobs and a jack on the back of your red two stroke?

[Robin]

Congrats on getting it sorted. The black box is a solid state “Stage Center” reverb unit I built, I think I got the schematic from the General Guitar Gadgets site. It works well, but the Two Stroke is so harmonically rich, I pretty much stopped using reverb with it at all. The Two Stroke is a real stomp box killer…you just don’t need them.

[Paul]

Once I got the amp working with a cheap chinese 6v6 I had lying around, I tried other output tubes. I’m having problems with several tubes. Note I have not hooked up any impedance taps save the 8 ohm one, so I know I am running the amp with some mismatches, but that doesn’t explain the problems I’ve had.

Chinese 6V6 new production: sounds great. very quiet, no squealing.

JJ 6V6 new production: squeals at low and high volume settings. Sounds thin relative to the Chinese 6V6.

NOS 6K6 tubes (National and RCA): sounds great. very quiet, no squealing. A little less bright than the chinese 6V6, and a little less volume/earlier breakup, but it’s a bright amp and it can be made to sound like the chinese 6V6 with a tone knob increase.

Tung Sol Reissue 5881 new production: HORRIBLE. squeals terribly and is totally unuseable. the brief moment I tried playing I thought it sounded like a promising setup but the squealing is prohibitive.

Valve Art Chinese EL84 new production: Somewhat like the JJ 6V6. Squeals at the volume extremes and doesn’t seem to sound good.

I’d attribute the problems to impedance mismatches except that this makes no sense, because the single 6V6 setup should be a perfect match and the JJ sounds awful and squeals. And the 6K6s, which are mismatched impedance, sound great with no squealing.

Any ideas?

Thanks!

[Robin]

You mean a EL-34 (rather than EL-84), right? I’ve never seen an impedance mismatch make squealing noises. Obviously there is a problem with stability. Assuming V1 (the preamp tube) is not monophonic and causing the problem, I’d still look for a lead dress issue.

[Paul]

34, yes!

Strangely, I can’t seem to get stable performance out of a single speaker connected to this amp.

my 2×8″ cabinet works fine. my 1×12″ cabinet makes it squeal. either 8″ speaker alone seems to make it squeal.

if I have the 1×12 in on one speaker out jack, it squeals, but stops as soon as i plug the 2×8 into the other (both are 8 ohm cabs). but the 2×8″ cab goes almost completely silent and i only hear the 1×12 – which doesn’t squeal.

eventually this amp is going in a 1×8 combo so i need to solve this problem…

i’ll try some other 12ax7s tomorrow and see what happens.

[Author]

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