diy 6g3 buzzes

[dwhutchens]

So I decided to build a brown deluxe from scratch. I had a 330-0-330 transformer laying around and an empty hammond chassis. I'm using a Weber Princeton OT whitch is probabley too small but I figure I can change that later. I followed the schematic but to get by with 2 12ax7 tubes I left out the normal channel and the tremolo and put in an adjustable bias circuit. The amp works and sounds ok but has a loud annoying buzz even with no guitar plugged in. I have my filaments grounded through a couple of 100k resistors and if I lift them from ground I get a louder lower freq. hum. If I pull the preamp tube the hum is still there but if I pull the PI also it is gone. With all tubes in it varies with the volume control. I'm going to attach a sound clip and some pictures. Maybe someone can spot something wrong with my layout.
Thanks,
David

[Chuck H]

. I have my filaments grounded through a couple of 100k resistors and if I lift them from ground I get a louder lower freq. hum.

Does your heater circuit have a center tap?

If it does you should not have any resistors on it. If it doesn't then I assume you used the two resistors to create a false center tap, as used in most designs that don't have a center tapped filament winding. If this is the case then your value of 100k would be too large. They should be only 100 ohms (one thousand times smaller). try that and see if it helps.

Chuck

[dwhutchens]

OK, I had a chance to work on the amp this weekend. I changed out the 100k for some 100 ohm resistors on the heater filaments. It didn't make a lot of difference but if I lift them from ground there is a much louder, deeper hum so that appears to be working. I clipped a 33 uf cap from the preamp node of the B+ to ground and it killed most of the noise but in the process I think I found a cold solder joint. I rearranged the leads and did some resoldering and took out the 33uf cap and it was pretty quiet. I played it for 30 minutes or so. It still seemed a little weak and didn't have much headroom although it sounded pretty good, kinda hard to judge haveing never heard an original. I set up my scope to set the bias and noticed the output waveform looked strange. One hump was fatter than the other and one would clip way before the other. I started scoping the signal path and found that the output of one side of the phase inverter was about twice that of the other. I discovered that I had left out the .1uf cap off the bottom grid (looking at the schematic). I put that cap in the circuit and now she screams but some of my previous noise is back. Oh well, back to the drawing board on that.

On another note, I've been haven trouble soldering on this project. My tip won't tin well. I bought some new tips recently and put another one on the iron and my solder refused to stick to it. I looked at the pack it came out of and it has "Lead Free" printed on it. Any body else ran into this?

[Chuck H]

I can't answer to all the issues. The .1 cap on the PI will increase gain, but should not have been responsible for the imbalance. I have seen builds where the PI plate Rs are reversed. One is usually 100k and the other is 82k. If your build includes this arrangement, make sure the 100k is on the grounded grid side and the 82k is on the input side of the PI. If thats not it then recheck all R values in the PI. Have you tested the PI for balance since putting in the .1 cap? Recheck all grounds. There's just not that many. Wiggle the ground leads while your doing it to detect faults. Is your layout correct?

FWIW lead free solder sucks A$$. But thats the direction everything is going for some stupid reason. You can go down to the tackle shop and buy a bag of giant lead balls that will ultimately be left on the bottom of a stream, lake or the ocean. But apparently the tiny solder joints in the electronic stuff we throw out, which is only half lead, is going to poison our landfills!?! Well, no use fighting it though. But the lead free stuff is much harder to work with.

Chuck

[dwhutchens]

Thanks for the replies guys. Yes, after putting the .1 cap in the PI it is now balanced. I think the noise is comming from the power supply. With no input signal I've got this little 120hz hump all through the signal path and a lot of staticy trash riding on that. I was suspecting the can cap I used so I built up a power supply on an external board and tried hooking it up with alagator clips but that turned into a real fiasco. It's tough to get a good conection with those things. I hope to be able to dig into it again this weekend.

On the solder issue, I'm using lead/tin solder but the tips I bought for my cheap little Weller iron say "Lead Free" on the package. The solder won't stick to them, it just beads up and rolls off. I'm using my really cheap Radio Shack iron to get by. I can't locate my roll of crappy lead free solder to try with those tips.

David

[cminor9]

Interesting layout. Any reason you decided to dispense with an eyelet board? I am curious to know what your thought process is on this.

Sorry I don't have much to add. I hope you get it sorted out, though.

[dwhutchens]

I just had this crazy idea to try a point to point amp and Radio Shack had a bunch of these terminal strips on the rack. Not such a good idea it seems. I had a power supply started on this generic eyelet board I got from Weber so I laid out the parts on it this weekend. That seems to have cured my hum, not sure if it was the filter cap can or the layout. It sounds pretty good at low volume now but when I turn it up for some distortion I'm getting ghost notes. I've read posts about that before so I'm going to search for that now.

[dwhutchens]

I had some more time to fool around with the amp. I hooked up the scope and the signal looks good up to the input of the PI. I put the scope on the .1uf caps going to the 6v6 tubes and when the signal starts to clip it grows this little wart on one side. Could this be my "ghost note". It sounds kinda like somebody playing a kazoo in the background. Here are some pics of the waveform. 1khz input signal at 100mv peak to peak. First pic volume was about 6 and volts/div was 1, next pic volume was at about 8 or 9 and volts/div was also at 1, last pic volume was at 12 and I had to change the volts/div to 2. I think the probes were set to x10.

Thanks for looking,
David

[dwhutchens]

Presence question

I swithched out the speaker and it sounds much better. I found a cut in my origianl speaker cone, looked like from a razor knife. Don't know how that happened.

With most of the bugs out of the original 6g3 circuit minus tremolo and 1 channel it seemed to have way too much gain. It was like full blast on about 2. I decided to use up some of that extra gain and replaced the tone pot with black face Deluxe treble and bass controls. That seems to work well so far.

Now I decided I wanted to try a presence control. I replaced the 1k5 resistor in the feedback circuit with a 10k pot and .1 uf cap from the 5F6a Bassman circuit, I know the Bassman has a 5k pot but I had a 10k handy. It works, I guess, I've never used a presence control. The sound goes from sorta dull to almost shrill. The questions I have are:

What difference will a lower value pot make?

Should I add another resistor in series with the pot to get back to the 1k5 value?

The only problems I have noticed are when I turn the pot up it really boosts any noise from the guitar and also turning the pot results in a scratchy noise. I've read about DC on the pots causing this. I checked and I have about 20vdc on the top end of the pot. Any suggestions what to do about that?

Thanks for reading this.

D Hutchens

[dwhutchens]

Forgot one question, should the presence pot be audio or linear?

[txstrat]

With most of the bugs out of the original 6g3 circuit minus tremolo and 1 channel it seemed to have way too much gain. It was like full blast on about 2. I decided to use up some of that extra gain and replaced the tone pot with black face Deluxe treble and bass controls. That seems to work well so far.

Just to make sure with the gain problem. Did you change the value of the cathode resistor on the first preamp tube? It should be double in value when using only one channel.
There is a 1.5k resistor with both channels, so it should be around 2.7k or even a little higher to decrease gain when using only one channel.
FWIW the brown deluxe has a different sound than a blackface deluxe partly BECAUSE of the different tone arrangement. If you go for the brownface sound I would check all parts values again and try to get the sound I was looking for.
Wanna listen to a brown deluxe? Theres a vintage shootout of Fender amps on youtube: http://www.youtube.com/watch?v=2m4enz7nQJo at around 2 minutes they fire up the brown deluxe. Even my 8 year old son said the brown one sounds best. As a matter of taste, though.

Hope this helps

Matt

[dwhutchens]

Hey Matt, thanks for the reply. I've got the amp on the back burner right now. I can't get the overdrive sound right. I've ordered a Deluxe OT from MOJO. I've got a Weber Princeton OT in it now and I don't think its big enough. When I get that installed I'm going back to the drawing board on the tone stack. I'll try your resistor suggestion. What about the dropping resistor in the power supply. I'm thinking with one tube missing I'm not drawing enough current and the voltage on the first stage may be too high compaired to the PI. Also I see that the plate resistors on the first stage are 220k and I'm used to seeing 100k.

David

[txstrat]

Hi David,

I would assume with a 2.7k cathode resistor, the plate resistor of 220k is ok. Otherwise, if you have a look on the Princeton 6G2 you can see a 100k plate with a 1.5k cathode resistor. I think this is partly a reason for the different sounds of both amps. I would try both configurations and then decide what I like more.
Regarding the voltage on the first plate. If you stay in reasonable voltage areas not too far from the original I wouldn't see that as a problem.
The 100k is what I'm used to see as well. With the 220k the tube provides less headroom. I think that's a try to make the 7025 circuit act like the former 12AY7 circuit in a 5e3 (could anybody correct me if I'm wrong).

Matt

[dwhutchens]

OK, I got the Mojo Deluxe Reverb transformer installed and have been playing around with the amp. It sounds pretty good but I still have the ghost note problem. If I put in a 5y3 rectifier it's hardly noticeable but with a GZ34 it's pretty pronounced. Ive got around 350vdc with the 5y3 and just over 400 with the GZ34. Is it possible the PT can't deliver enough current with a GZ34? Would that cause ghost notes? It came from an old record player power amp that was running a 5y3 and 2 6k6 power tubes. I've tried paralleling a 33uf cap across the second filter cap, replacing the 10k resistor with a choke and clipping a 100uf cap into the bias circuit. I've adjusted the bias up and down. Its worse on the neck pickup on my Ibanez LP. Ive lowered the pickup down even with the trim ring and its still there.
David

[Chuck H]

What kind of speaker/s are you using?

Chuck