"Buzzing" sound behind the note

[AcePepper]

Hi,
Well I've finished my first D-type, based on the '97 Overdraft Specimen schem. It's not exactly a "clone", different layout, dual bias adjust w/ test points, 6V6 tubes. The biggest change I made was to use a dual 50-50uf cap for the plate and screen filters, 50uf for each. The amp works fine for the most part, no hums or relay pops, but I'm hearing a "buzzing" noise behind the note (in clean and OD modes.) Could this be caused by the 50uf screen filtering? Should I change to the 20uf in the schems? I've moved wires around and changed some parts, nothing makes any difference so far. I hear a really good amp in there somewhere, but the "buzz" behind the notes is driving me crazy. Suggestions?
Thanx!
Ace
PS: I can post a layout drawing and pics of what I've done if it would help.

[Normster]

Cone cry?

[dogears]

Check your grounding. Search the forum for tips. The buzz could be related to the filtration.

Definately use 20uf or 22uf on the screens. No brainer.

Also the 97 scheamtic is chock full of errors. Hope you scoured the forum for the corrections.

Lastly, I feel strongly that 6V6 tubes are a poor match for thses amps.

[AcePepper]

Thanx for the tips!

I've checked grounds and they seem real good (0 ohms.) Tha amp is really quiet when nothing is played, no hums. The relay is really quiet too, no pops or hums in either mode (clean or OD.) I've tried moving some grounds around (relay power supply, switching) to no avail. Still buzzes.

I'll try changing the screen filter to 22uf. I thought it'd be cool to have more filtering (50uf) and save some space with the can cap up front. I could be wrong;-)

I've made some part/tone corrections based on stuff I've read here. The tone is actually really smooth and nice, except for the damn buzz behind the note;-) I figured I'd add/try stuff once I had the basic thing working.

Why do you feel 6V6s aren't so good in this amp? I've seen 6V6 versions on this forum and I really like 'em in most other amps.

As for "cone cry"; I've heard the term, but not sure what you mean. I'm testing the head thru my trusty GenzBenz 2x12" that sounds fine with everything else. Can you describe "cone cry"? Why would this head do that when others I've built don't? Color me puzzled;-)

I'll keep searching the forum too. I can't be the only one who's had this;-) Any answers/suggestions are most welcome! I'm diggin' learning about these Dumbley-things!
Thanx!
Ace

[skyboltone]

Also check the quality of the cap. It's been mentioned on the forum and I've also recently read in Gerald Webber's collumn in Vintage Guitar about one particular brand, (sorry, I can't remember which) that causes the symptom you are talking about.

Use Sprague, you can't go wrong.

YMMV
Dan

[groovtubin]

Hi,
Well I've finished my first D-type, based on the '97 Overdraft Specimen schem. It's not exactly a "clone", different layout, dual bias adjust w/ test points, 6V6 tubes. The biggest change I made was to use a dual 50-50uf cap for the plate and screen filters, 50uf for each. The amp works fine for the most part, no hums or relay pops, but I'm hearing a "buzzing" noise behind the note (in clean and OD modes.) Could this be caused by the 50uf screen filtering? Should I change to the 20uf in the schems? I've moved wires around and changed some parts, nothing makes any difference so far. I hear a really good amp in there somewhere, but the "buzz" behind the notes is driving me crazy. Suggestions?
Thanx!
Ace
PS: I can post a layout drawing and pics of what I've done if it would help.

Try biasing the tubes colder, also look at the output on a scope, you may have problems w/feedback arrangement, Look for a SMEARING on the peaks, also distorted lines. I built a 4x 6V6, not based on any schematic here , had SEVERE prob`s w/feedback, the feedback arrangement is NOT always cut and dried, you NEED to try diff values( id suggest adding MORE neg feedback first) Not all transformers are the same, and if you follow a CUT and DRIED schematic, you may LOSE big time. hth`s

[AcePepper]

Here's a pic of what I'm up to. I've got the filter section layed over the amp section, all on one board. All the grounds from the PI forward are on a buss, grounded on a screw near the input (preamp ground.) The relay is on this board also. The relay is powered by a bridge rec/cap board on one end of the chassis, near the PT. This is grounded on one of the PTs mouning bolts (power ground) along with the cts and the can cap for the plate and screen filters (50-50uf)

I've used most of the circuit values from "Hybrid Amp Rev A" (no HRM), along with suggestions from here. I'm getting about 212vdc the plate of CL1. I tried the 150k-to-ground to drop some voltage, which it did, but made no difference in the buzzing. No Local feedback loop and no 350pf plate-to-cathode on OD1 or 2. As I said, tone is actually really good now, except for "buzzing-behind-the-note" issue. I do not have the trim pot across the PI plates, 82k/100k instead.

I did try changing the screen filter from 50uf to 22uf. No difference in tone. Buzzing still there, as before.

Perhaps I've strayed too much in my layout. I'm thinking about building a new board closer to the layouts posted on this forum. But the thing is sooo close to being right as it is;-) And it seemed to make more sense to condense the power supply and tone circuit, less wire flying and all that. Whaddaya think?

[groovtubin]

Here's a pic of what I'm up to. I've got the filter section layed over the amp section, all on one board. All the grounds from the PI forward are on a buss, grounded on a screw near the input (preamp ground.) The relay is on this board also. The relay is powered by a bridge rec/cap board on one end of the chassis, near the PT. This is grounded on one of the PTs mouning bolts (power ground) along with the cts and the can cap for the plate and screen filters (50-50uf)

I've used most of the circuit values from "Hybrid Amp Rev A" (no HRM), along with suggestions from here. I'm getting about 212vdc the plate of CL1. I tried the 150k-to-ground to drop some voltage, which it did, but made no difference in the buzzing. No Local feedback loop and no 350pf plate-to-cathode on OD1 or 2. As I said, tone is actually really good now, except for "buzzing-behind-the-note" issue. I do not have the trim pot across the PI plates, 82k/100k instead.

I did try changing the screen filter from 50uf to 22uf. No difference in tone. Buzzing still there, as before.

Perhaps I've strayed too much in my layout. I'm thinking about building a new board closer to the layouts posted on this forum. But the thing is sooo close to being right as it is;-) And it seemed to make more sense to condense the power supply and tone circuit, less wire flying and all that. Whaddaya think?

since you are doing a 97 OVERDRAFT WITHOUT the HRM, i`d HIGHLY suggest the local feedback loop and adding pf`s, more than likely the prob will go away..

[Bob-I]

Groovetubin is right. Without the post OD tone stack, and I don't see one on the pics, you'll get terrible high end buzz. I did that and dogears straightened me out.

I'd start by making the changes you'll see on the Hybrid-A schem. 100k OD volume instead of 1M, 270pF to 390pF "snubbers" from plate to cathode of OD1 and OD2, experiment with the value.

One issue is that the OD output needs to see a 100k load, therefore the 100k pot, but the PI needs to see a 1M load. The best way to do this is to have a master volume that adjusts both channels and an OD output volume to balance the 2 channels. Again use the Hybrid schematic.

Good luck, keep us posted.

[Normster]

That's an amazing looking build! Mighty tidy.

Cone cry is a little buzzing/sizzling noise that seems to come from the edge of the speaker. Dumble amps seem to be great at causing it to happen.

After reading your description of the problem, and what you've omitted from your amp, I think you can probably tame the buzz somewhat by installing the 270pF snubbers on OD1/OD2. If it's better, but still there, try raising up to 330pF. If you go too high you'll lose the "hair" off the note and it'll start to sound lifeless.

As for the 6V6 debate, my favorite build to date uses 6V6s and a 5AR4 rectifier, but it's NOT a Robben Ford tone. I also used the preamp values from #124 which are almost Fender. When you push the amp you can hear a little bit of power tube breakup with the preamp distortion which sounds pretty cool. I got the inspiration for the amp after hearing a 20 watt amp that Gary (Glaswerks.com) built for Harry McGraw.

Edit: Never mind...looks like Groovetubin and Bob have got you covered.

[AcePepper]

You might be onto something with that - f/b-thing. I changed the f/b resistor to 10k (was 4.7k), now note-buzzing goes away when presence knob is turned up. A bit better now. Question: to get "more negative feedback" should the f/b resistor be higher or lower in value?

I put in the 390pf caps across plate/cathode of OD1 &2, it does smooth it out a bit. I've got the 100k OD Level comin' off OD2 and 1M Master vol (w/22pf bright cap) as per Hybrid A. 100k Drive after OD1, feeding OD2. I'll need to score a terminal strip to hang local f/b loop on CL2. Got a lot of bass now, almost too much. Maybe CL2 cathode bypass (now 5uf) should go down to 1uf. Might also try changing mid cap in tonestack from .047 to .01uf as suggested elsewhere in the forum. Any more suggestions? It's gettin' there!
Thanx!
Ace

[swt]

lower resistor values means more neg feedback. Have you tried changing the leads of the ot to the plates?. I had an amp with that problem, and it turned out that i have some positive feedback...but don't know why it didn't scream as usual, but did the problem you have...if it goes away by going up on the feedback resistor...that might be the problem. also...try to identify if the problem is in the power section or the pre. do you have the grid resistors in the sockets?. what value?

[jelle]

Hi,

I agree with swt. Find out if the source is in the pre or poweramp.
FWIW, I build a 97 hrm that also had a buzz after the notes in both clean and OD mode. I had to rebuild the thing to find out what it was....crosstalk in the preamp...Now I use a strict layout and shielded wire.
I hope you find out what it is in your amp.

Good luck,
Jelle

[groovtubin]

You might be onto something with that - f/b-thing. I changed the f/b resistor to 10k (was 4.7k), now note-buzzing goes away when presence knob is turned up. A bit better now. Question: to get "more negative feedback" should the f/b resistor be higher or lower in value?

I put in the 390pf caps across plate/cathode of OD1 &2, it does smooth it out a bit. I've got the 100k OD Level comin' off OD2 and 1M Master vol (w/22pf bright cap) as per Hybrid A. 100k Drive after OD1, feeding OD2. I'll need to score a terminal strip to hang local f/b loop on CL2. Got a lot of bass now, almost too much. Maybe CL2 cathode bypass (now 5uf) should go down to 1uf. Might also try changing mid cap in tonestack from .047 to .01uf as suggested elsewhere in the forum. Any more suggestions? It's gettin' there!
Thanx!
Ace

Whatever it takes to get the buzz gone is good, you just have ta experiment w/ more/less feedback ta find the sweet spot...

[AcePepper]

lower resistor values means more neg feedback. Have you tried changing the leads of the ot to the plates?. I had an amp with that problem, and it turned out that i have some positive feedback...but don't know why it didn't scream as usual, but did the problem you have...if it goes away by going up on the feedback resistor...that might be the problem. also...try to identify if the problem is in the power section or the pre. do you have the grid resistors in the sockets?. what value?

I hadn't thought of the OT leads on the plates. It's not screamin', so I figured it was OK. I'll try swapping 'em. I do have the grid resistors on the sockets, 3.3k. I'll try the 5.7k seen on some schems. I thought the buzz problem was in the CL part of the preamp, but now think it might be in the power section. Will also try going down in value on the -f/b resistor (more -f/b), maybe 2.2k?
Thanx!
Ace!8-)>