This is my first post in this group -- looks like a nice place to share and learn.
I am providing tech support for a friend who is building a JTM-45 Tremolo "Bluesbreaker" amp.
The goal is to match the tone quality as much as humanly possible to the amp(s) used by Clapton, but to eliminate as much hum and buzz and other problems that accompanied the amps of that day. We are using NOS GEC KT66, NOS Mullard 12AX7, NOS mustard caps, etc, etc. Due to the expensive tubes, I intend to employ either dual bias or bias + balance controls on this amp. We plan to bring test points to the rear panel and remote the bias and/or balance pots to the rear panel as well, so the bias can be checked and adjusted without removing the back cover of the case. Maximum reliability to protect the output tubes is desired.
Due to the low cost of 1N4007 rectifiers today, I cannot understand why the bias circuit doesn't use full wave rectification instead of 1/2 wave -- 2 diodes instead of 1.
As I see it, there are many advantages:
1. Significantly lower ripple due to 100/120 Hz charging of the 10uF caps instead of 50/60 Hz with resulting lower hum.
2. Faster charging of the bias circuit when the amp is first turned on which can only help.
3. Failure of a single rectifier (admittedly unlikely), will still result in some Bias to prevent tube destruction.
4. Very slightly less DC in the PT due to balanced "withdrawal" from the bias circuit.
The more frequent charging of the caps will likely result in a change in the voltage, so some values may have to be adjusted.
Can anyone help me understand if there is a down-side to this approach.