Lead Dress

[minions]

Is there a rule-of-thumb so to speak concerning lead dress? What I mean is, are certain leads coming close together going to cause more oscillation than others? How should I know which ones will do this?

Thanks.

[stevo58]

Hi.

Think about it in terms of current. Leads with strong current are going to have a bad effect on leads with weak current.

Heater leads carry very strong current. If you look at transformer specs, you will see that the high-voltage windings are usually rated at hundreds of milliamps, while the 6.3V winding is rated at anywhere from 2-5 amperes. So, you want your heater leads as far as possible from other leads.

The power tubes are also major current consumers. The power tube anodes are _the_ major current consumer on the HV winding.

The anodes of the preamp tubes consume far less current, on the order of a couple of milliamps.

BUT that is only part of the story. The high-current leads have an effect on low-current leads. So, the, lead to the anode of the first stage of the amp, while the current is 'only' a few milliamps, is close to the input lead from the jack, which has a extremely low-level signal, easily overwhelmed by the current in the anode lead.

Practically, this means: keep hi-current leads away from low-current leads. Where they have to be near each other, cross them at right angles to minimize interference. And keep air between them. And, the longer a lead, the greater the chance it will either pick up from or cause interference in another lead. So keep them short.

Here's a good diagram showing current flow:

www.el34world.com/charts/currentflow.htm

The other absolutely critical issue is your grounding scheme. This actually relates back to current flow - the high current runs to ground, too, so keep the high current ground as far away as possible from the low-current ground. Grounding is a religious issue, so in the end you will have to try a few different schemes and find out which works best for you, or for a particular amp. For example, some people swear by, others swear at start grounding. The Hoffman grounding scheme works well for me. Others like to ground a stage at that stage's filter cap.

steven