Buzz on my first amp

[pbiagi]

Hello,

I would be grateful for some advice as a newbie.

I completed my first amp, a 5E3, last June. It was a kit from ampwares. I studied up and did my best, following Fender's layout, grounding the chassis, and pots..
It was fairly loud, broke up typically fairly early, and had a little hum.
I decided to rewire for a standby switch, and for the hum, star grounding plus twisted filament wiring.
But then things got worse.. and since I have tried many things.
Currently it has star grounding for the PT center tap, cathode and phase inventer(?) to the PT bolt. The preamp and pots are grounded per a bus to a bolt on the other side. However the jacks and pots are not isolated.

In June, I did not bring the amp up as an experienced tech would. I simply turned it on/off a few times. At first I had no sound but that was because of a bad solder connection on one of the power tubes.
The thing to note is that the first day when I was bringing it up, there was one time when a small puff of smoke came up near the power tubes area. It wasn't much, and I saw no sign that the caps were split, and thereafter the amp sounded good and loud, so I ignored it... (cold solder joint that's working out?)
I had some trouble soldering grounds to the chassis (per Fender) and brass plate - for example the .0047 cap, and sometimes had to resolder them. Otherwise the amp sounded great. That's another reason I felt I had to change to star grounding.

To reduce hum I added the 100 ohm resistors to the filament wiring to create a center type on the power tube cathode. That reduced hum a bit, but the buzz is still there. On the preamp side, the twisted filament wiring is as far out of the way as possible, however it hovers above the power tubes.
Because of the smoke (above) I assumed that there was an issue with the 1st cap and replaced it. No difference.
I've measured all resistors and they seem OK. Solder joints look good to my untrained eye. I've pushed things around with chopsticks - nothing dramatic.

The experience -
With volume off, there is almost no hum. When volume is up to ~2, there's a buzz. By moving my hand above the preamp or pots, the buzz intensifies. If I touch the chassis, it goes away. If I hold the chassis in my hand, it disappears completely.
If I plug a guitar in, the buzz changes, reduces a bit. If I touch the strings, the buzz gets very quiet. (But while playing, if I take my hand off the strings, the buzz noise is noticeable.)

Note that I live in Paris, France and use a step-down PT, 220 - 110 V. I measured it at 119 V actually.
The filaments are running at 6.3 V.
Other voltages -
off rectifier 422 V, after 5100 resistor 345 V, after 22k resistor 248 V, V2 pin 6 196 V, V2 pin 1 165 V, V1 pin 6 126 V, V1 pin 1 123 V.
The tubes are Sovtec. I have some other NOS but I believe the Sovtec 5Y3GT is giving a high voltage (422 V) and so am sticking with the Sovtec for now. I have some NOS RCA 5Y3GT tubes on the way.

With this amp I've learned a lot but please don't assume I understand all terms. Anyway I'll ask if I don't.

Thanks for your time reading this and appreciate any advise.

Best regards, Paul

[tubeswell]

Hi there Paul

Can you upload any pics of the guts of your build?

[pbiagi]

photos

Hi Tubeswell,

Thanks for taking the time to help me out.

As you can see I've tried a number of things. The setup is ever-changing..
One thing to note. In June - Aug. I was using a different step-down transformer (220-110V) which included a ground plug. However I managed to quickly blow a Mullard 12AX7 tube *expensive* and then measured the volts coming out of the step-down tranny. It was 129 Volts. So I bought a 750 W step-down tranny which measures 119 Volts out. It has a ground but uses the European female plug. So until now I could only use an adapter to plug in, which did not include the ground. Recently I decided to retry the old tranny to see what effect the ground would have. It blew the circuit breaker twice. I'm not sure but at the time the V1 pin 2 to normal channel was shorting to ground. As you can see from the photo I'm using a shielded cable and the shield shorted. That's fixed now..

Sorry for being so wordy and not as clear as I'd like. I've tried so many things that it gets confusing to explain.

Best regards, Paul

[Robert M. Martinelli]

Hi Paul,
from the symptoms you describe it definitely look as you' re having some grounding problem; If the buzz disappears when you touch the chassis, then this means the amp' s not properly connected to earth, and YOU become the chassis-to-earth connection. Does the place you live/play have proper earthing or is it one of those old places with no earthing network? Have you tried the amp in a different place with a "known to be good" earthing system ? Apart from the noise issue this raises also a safety issue, without a proper earth connection if something goes wrong with the amp while playing there' s a chance you get shocked.

As to the layout, apart from some less-than-perfect solder joints ( is your soldering iron' s wattage up to the task? If the power' s too low the solder joints quality will be poor ), it looks like you used shielded wire for signal transfer ( good ) but you earthed the shield on both sides; this usually is not a big problem when the cables are so short, but sometimes grounding on both sides leads to ground loops which can pick up noise. Also, if noise gets worse when you pass your hand close to the amp this means some sensitive stage is picking noise much like an antenna.

Hope this helps

Best regards

Bob

[pbiagi]

earth

Hi Bob,

Really appreciate your advice and comments. You were right on.
The power chord's ground was attached to the chassis.., BUT I had switched step-down tranny's (220->110) because the original one was putting out 229 V. That was causing the amp voltages to be elevated (and I believe partially responsible for blowing a good 12ax7 tube?).
However the new tranny (output 119 V) has an earth, but I had to use an adapter to plug in the amp. The *adapter* did not make the earth ground connection.
I ordered a female U.S. power plug (w/ ground) and it just arrived yesterday.
After replacing the plug on the tranny to the U.S. power plug w/ ground, now the buzz is entirely or nearly gone! Finally. Amp sings great!

Question: the deluxe did not originally have an earth ground. Shouldn't the amp not make a buzz if setup correctly?
Is the problem still there, but we're just sending the ground noise to earth?
Or is that how it's suppose to work?

Per your other points - (the pictures are not too clear..)

For soldering I used a Weller WECP-20 (I borrowed from my office), which has a 50 W transformer. I used the pointy tip. Should I have used the flat-head screwdriver-type tip?
Usually I heated up the connection about 20 seconds before applying solder, and quickly removed heat once the solder melted.

The shielded wire is only grounded on the plug side, not at the tube side.
I used electrical tape to hold back the shield since I didn't have any shrink wrap.

The amp is working out well now, however it is somewhat sensitive to parasite noise from external electrical devices. For example turning on a dimmer light in my home introduces a small buzz - not near as loud as before, but it's there. Walking near the cathode TV does too. Maybe this is expected.?
I don't remember having this issue with the Fender Super Reverb. (Need to check.) (If I keep the dim light and TV off, there's virtually no hum or buzz noise.)

Best regards, Paul

[Robert M. Martinelli]

Hi Paul,
first of all let me say I' m truly glad to help, this is what this great forum' s all about.

As to the soldering iron, 50 W should be enough for soldering components, tube sockets and wires, they' re probably a little low if you need to solder something directly to the chassis without using bolted lugs....

As to your questions about proper grounding on old amps, I found a lot of old amps were missing proper earthing, in that case I add it immediately, as this is a big safety issue IMHO... as to hum/noise, it can be that a well built amp has no/little hum even if not earthed, but I would not feel safe playing through it...whatever leak you might have, the player becomes the easiest way for electricity to reach a lower potential. I just restored an old Italian amp ( I also started a thread about in in the mods and tweaks section ) and it had no earthing system whatsoever, though it was very quiet, I earthed the chassis and now it's both quiet and safe.

Sorry if I thought you connected the shields on both sides on signal cables, you did it the right way but, seeing the green/yellow tape on both sides I got that wrong. At least we double checked this...

Best regards

Bob

[Tom Phillips]

For soldering I used a Weller WECP-20 (I borrowed from my office), which has a 50 W transformer. I used the pointy tip. Should I have used the flat-head screwdriver-type tip?
Usually I heated up the connection about 20 seconds before applying solder, and quickly removed heat once the solder melted.

Hi Paul,

I'd like to offer some soldering technique comments.
20 seconds is way too long to heat the joint. You need a larger tip and maybe the soldering station temp control turned up higher. (As Bob said, a 50 watt unit should be large enough) What is the temp setting you are using? If it were me, I'd be using 700 degrees F with a large tip. The goal is to heat the joint faster, apply the solder, and remove the heat as soon as you have a good solder melt. The whole process takes only a couple of seconds with a properly sized soldering iron. One thing that most people don?t realize is that faster work using more heat at the joint actually imparts less heat to the components. This is because there is less time for the heat to migrate down the component leads.

A long time ago I saw a film clip of an assembler at Fender soldering components in an eyelet board. She was using what looked to be a big 250W soldering iron and was flying though the task at a rate of about 1 eyelet per second. They were soldered from the back which explains the large convex shape of the solder blob on original vintage Fender eyelet boards when viewed from the top.

Cheers,
Tom

[pbiagi]

Hi Tom and Bob,

I'm with you per earthing an amp. I remember vividly well being shocked often enough back in the 70's..

For soldering, I used between 625 - 800 F, depending on what I was doing.
It often took 20 seconds because I'd turn the iron off between solders, so most of the time it had to heat back up. From what I gather it's better to keep the iron on during a session and keep going.

Thanks for the great advice.
The amp is sounding great and I've definitely learned a lot and got the bug. Don't know what my next project will be but there will be one.

Best regards, Paul