New Guy Wondering How to Ground Bassman Transformers

[The New Steve H]

I am new; thought I would say hi. I've built two tube amps: a Firefly and a "Powerman." Both are small. The Powerman uses two 6021 submini tubes. I am currently working on a Bassman (5f6a) clone, from scratch. It's just a head. I also have a 1x12 cabinet I built from a plywood sign and a 12" Weber Signature ($35) speaker.

A guy I know from church works at the VA. He says they are helping a veteran get started on tube amps, and they wanted me to help him, but they have some kind of red tape policy that prevents them from introducing us. I sent word to him about this forum, and I am hoping he shows up here to get some help. He's building a Metroamp JTM 45 kit.

As long as I'm here, I got a question. I have read that it's a not a good idea to ground a transformer to a chassis at the transformer's base. If that's true, how do you avoid it? I can see how it would be easy to isolate a transformer that sits upright, but the Bassman has one transformer that is set into the chassis on its side, so you would think there would be a lot of contact, ensuring grounding. Is it okay to mount it in some kind of hardwood or plastic buffer material, or am I obsessing about something that will never make any difference?

[M Fowler]

I just bolt the transformer in without isolation and haven't had problems.

I work at the VA and sounds like this could be a Voc Rehab program so they can't release veteran information due to privacy issues. But seems odd that you and the veteran couldn't meet for training purposes?

Mark

[The New Steve H]

They didn't supply the reason, but if I can find this guy, I'll definitely give him a hand. They can't fire ME.

[tubeswell]

I came across this issue earlier this year with a Weber PT that had little nylon grommets isolating the end bells from the mounting bolts. It is not uncommon (Mesa do it on some of their amps). I then (later on) found out from someone on another forum (I think it was?) that mounting your PT this way can reduce hum.

I have tried it both ways on several amps, and can't say that it makes much of a difference (from my own experience).

[Cliff Schecht]

Hi Steve! I think you'll find this is a great resource for anything amp and tube related. You should bring up your shielding/chassis discussion here, I'm sure you could get some more great insight on what will and won't work.

I'm with tubeswell, I've never noticed a difference the few times I did try isolating a transformer from the chassis. I don't bother anymore, just bolt that sucker down tight to the chassis.

[The New Steve H]

I think I got a little paranoid about hum after I built my second amp. I put like 3,000 ground wires on the eyelet board and grounded them wherever I felt like it, and since then, I've been opening it up periodically to undo the damage.

My first amp, the Firefly, doesn't hum at all. But it does oscillate.

[David Root]

I would suspect the layout. Did you have a layout to build to that showed the lead dress? If so, did you follow it? If not, caveat emptor.

[The New Steve H]

I just took the schematic and ran with it. The hum is greatly reduced now, but I think I could improve things by doing away with my Twister-style grounding scheme.

Here's a link to the page where I found it: http://www.harmonicappliances.com/power ... erman.html

I really liked the sample sounds.

It's a neat amp. The tubes are so small, you don't have to do anything to provide ventilation. It's a neat little box the size of a large book. Throw it in a suitcase and go.

[The New Steve H]

Bought some structural aluminum today to see if it will make decent chassis. The big piece is 6" Aluminum Association channel. The smaller ones are 4" square tubing.

[overtone]

twister ground scheme, that made me laugh!

...As long as I'm here, I got a question. I have read that it's a not a good idea to ground a transformer to a chassis at the transformer's base. If that's true, how do you avoid it? I can see how it would be easy to isolate a transformer that sits upright, but the Bassman has one transformer that is set into the chassis on its side, so you would think there would be a lot of contact, ensuring grounding. Is it okay to mount it in some kind of hardwood or plastic buffer material, or am I obsessing about something that will never make any difference?

Some things cross my mind about your question, I will throw them out here purely for consideration, expansion or correction:

1. Could it be that they intended to say that connecting your grounds to the PT bolt could induce noise? You can get away with doing that, but I prefer to keep the grounds away from the PT to avoid noise around the transformer and, having seen so many loose PT bolts, I don't ground to them anymore anyway.

2. What Tubeswell mentioned. You see those isolated ferrules on the lamination bolts of most high quality transformers. They are used to align the bolt so that it does not touch the laminations. My memory is thin on this, but if I recall correctly it is to avoid the chance of the bolts damaging and shorting the laminations which could result in eddy currents, like in a rusted transformer. Telefunken used brass bolts. I forget how magnetics play a role here, but that was one reason why Telefunken used brass bolts. Usually the bolts are at the area where magnetic field is least anyway.

3. I have seen some transformers with the laminations mounted with a small gap to the chassis. There was a reason, but, today I just don't remember. I am sure that if it was anything dramatic I would be doing it, and I don't. But I could well be wrong.

4. I had another point.. oh yes, I am fairly certain that the transformer should be well grounded for safety reasons. Would there ever be an exception to that?

Apologies, it was a long day. Over.

[The New Steve H]

I am fairly sure I got my grounding concerns from this page:

http://www.aikenamps.com/StarGround.html

It looks like I overgeneralized what I read. They have specific advice for OT and PT grounding.

Samples:

Do not ground the output transformer common to the chassis and then ground the output jack to the chassis. This will create a heavy ground current path through the chassis, which may run through a preamp section, depending upon the location of the output jacks and the output transformer. Do not ground the output jacks to the chassis at all, they should be isolated from the chassis. Also, do not route these output transformer secondary wires anywhere near the preamp stages, they should be routed as far away as possible, around the edges of the chassis to the output jacks

The power transformer center-tap wire should be soldered directly to the ground lug of this cap, and a very short, heavy wire should run from there to the chassis ground connection (if there is one). Do not connect the power transformer center-tap to the chassis and the first cap ground to the chassis at a different spot; this will cause heavy ground current flow in the chassis. Also, do not tie the AC mains safety ground to this point; it should be connected to the chassis with a very short length of wire right at the chassis entry point.