Amp is a bit buzzy, could use some advice

[alnight]

Hi guys, I'm new here and fall squarely into the novice builder category.

I have built what is basically a copy of an Emery Sound Superbaby in a 5F2 chassis. A little single ended tube tasting amp. It started off as a Weber Maggie kit that I was never really happy with, then the output tranny in one of my superbabys went out. Fixing that and getting close with the circuit led me to "I'll copy this circuit into that chassis and have a cool little gig and jam friendly combo."

It doesn't sound as good as an amp built by Curt Emery, but he's an engineer and I'm some guy with a soldering iron and a "best guess" power transformer. Still has been pretty cool, and reliable as a handy low-ish power stage amp.

There was always a touch of buzz on the notes when the amp started breaking up which I initially chalked up to a speaker characteristic and just lived with.

Then I got bored with it, decided it was a bit too stiff and needed more gain, so I add Ken Fischer's universal gain stage out of the Trainwreck pages. Once I got the oscillation under control, that pretty much did everything I wanted it to. Except that the mild buzziness is much more pronounced and I can't seem to get rid of it.

I have been through all the power filter and coupling capacitors. Tried different tubes. Chopsticked the amp like a bowl of vermicelli to do what I can with my lead dress. Tried every type of fizz cap configuration I can think of. Grid stoppers, grid leaks, various values, etc. Voltages check out ok.

I even unbolted the additional gain stage and reverted the amp to it's original configuration, only to find that I can't unhear that annoying bit of zzzzz now that I know it's there.

Interestingly enough, the largest effect on the buzziness seems to come from changing the values of the plate load resistors in the preamp section. This can make it slightly better, like a balanced square wave 'fuzz face' kind of texture underneath the note. Or much worse, like out of phase fuzz textures overlapping. This leads me to suspect that the issue may be originating in the power section. Increasing filter capacitance has no positive effect though, so I am at a loss as of how to possibly address this.

Might I need a choke?

A different transformer?

Am I looking in the wrong place entirely?

Does any of this make any sense at all?

Feel free to straighten me out on the path here.

All this being said, I've learned a LOT about voicing an amp from this project already, and how moving which component value in what direction influences the sound and response. So even if I never get this thing the way I envision it, it was still good education.

Your assistance is appreciated!

[Colossal]

Do you have a schematic for what you've built? Without diving into the weeds, SE amps do not enjoy common-mode hum cancellation so using a C-L-C pi filter (cap-choke-cap) BEFORE B+1 (which the OT is connected to) is needed to get the AC ripple WAY down. Using a CLC makes for a very quiet SE amp.

[alnight]

Pardon my crude drawing. I believe this is what you're asking for, correct? I can do the whole thing, but it will take a minute. P/T is a Hammond 272 BX. 600VCT

[Colossal]

Yes, that is correct, although I would change that 200R for a choke for a much greater reduction in ripple. But barring that, a CRC will be better than just a C. You can also bypass your standby switch with 100k/100n or just remove it altogether.

[alnight]

Thanks! That's just what I needed to know, I think. It never occurred to me that the standby switch might add to the issue.

Now, having never built an amp with a choke before, could you point me in the direction of how to select a properly sized one? I don't speak Henries yet, and I am reasonably confident that if I just try to go by resistance and max voltage I will end up with way more than I can actually use. Or am I outsmarting myself here?

[Phil_S]

Please confirm if single ended or push pull and what power tube(s). The choke must be rated for the current draw of the whole amp.

[alnight]

The amp is single ended, cathode biased, designed to run with most octal power tubes and rectifiers. I have run it with everything from a 5y3 rectifier and an el84 in a yellowjacket, to a gz34 with a kt88, and most combinations in between. That's part of the fun.

I am just now reading the choke article over at Aiken Amplification. It seems to be telling me that I'm going to need more choke than I had at first supposed.

Power Transformer is 600VCT at 115ma. Similar to what one would find in an AC30. If I'm understanding what I just read, I'm probably looking for a choke that can take every bit of that while not having too much resistance.

Or am I wrong?

[Colossal]

Yes, because the choke is filtering 100% of the amp's current draw, it must be rated for all of the expected current, plus some for added safety. You need to anticipate MAX current for the largest power tube you plan to run, and by max, I mean if you plan to smash the amp into overdrive. Look at the datasheet for the max conditions for your tube(s) of choice and plan accordingly.

[ViperDoc]

I just reduced some buzz in a new amp I built by eliminating weak solder joints at each preamp grid ground reference.

[alnight]

Ok, I am confident enough to order parts. Experimentation shall recommence upon their arrival. AES loves me lately.

Thanks guys! I shall post results when I have some.

There's nothing like the smell of hot solder and the tingly tase of fresh electricity!

[Colossal]

Thanks guys! I shall post results when I have some.

There's nothing like the smell of hot solder and the tingly tase of fresh electricity!

Excellent!

And if the tingle tastes like bacon, you're doing it right

Additionally, I looked up some notes on a SE amp I built. Using a 40uF - 10H choke - 40uF pi filter (after SS rectification), the AC ripple at the first cap was 9.7VAC. After the choke and second cap, the ripple was 330mVAC, a 29x reduction. The OT center tap was fed from that second cap. Very quiet amp. A choke will produce a much lower ripple voltage than a resistor. Look for something with low DC resistance and reasonably high inductance (5-10H). The choke I used was only 73Ω at 10H, but was quite physically large because of the large gauge wire used (low DCR). A typical Fender choke is about 90-110Ω and 4H, but you've gotta watch the current handling, so that may limit your power tube choices, especially if you want to run a KT88. Any of the Vox type chokes 10H/280R or 20H/460R will be suitable for your build and will provide 200mA handling, easy. Check out the Hammond chokes; lots of options there for reasonable prices.

[Phil_S]

Hammond has several chokes that I think will be appropriate:
https://www.hammfg.com/electronics/tran ... eferer=788
Typical prices range from around $18 to about $33, so it won't break the bank. There are two 5H and one 7H that look more than adequate to me. You could look at the 30H, but the 40H is only 50mA, which isn't enough. You could also look at the 193 series but these will be physically larger. Make sure you have space on the chassis.

[Mac Dillard]

I have built several SE amps with a resistor in place of a choke for the power supply. No problem.. What ViperDoc said, check your solder joints.

Is the first Capacitor in the drawing 0.47 or is it 47 ?

[Lynxtrap]

I'm not sure how AC ripple would cause this problem or how a choke would cure it, but I'm ready to be enlightened. The issue is not really noise, as I understood the description?

[alnight]

Ok, so I got some chokes in the mail today. A 5H and a 7H. Both 150ma. Initial experimentation has led me to a few conclusions.

1. This is definitely moving in the right direction. The amp is much smoother with a choke.

2. I'm going to need a bigger choke. The 5H doesn't do all that much on its own. The 7H makes a big improvement, but isn't enough. Using the 7H in the power section and then plugging the 5H in between the power tube and the preamp gets it pretty close. It behaves well until the volume knob starts closing in on max, at which point it gets a bit buzzy but is not absolutely awful. I've got a couple of 10H chokes on the way with higher power ratings. Hopefully one of these will get me where I'm trying to go.

I'm getting ready to bolt the universal gain stage back in and light the second preamp tube back up, then see where it goes from there. Hopefully I'll learn something useful.