Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ronquest on May 12, 2024, 07:56:58 am
-
Picked up a Fender 75 from 1980. Interesting amp, some of the last handwired Fenders, so I had to get it. Doing my typical after purchase inspection, I found the bias to be pretty cold at 23/24mA @ 500V B+. This amp has a 75 watt and a 15 watt setting that cuts B+ to 250V and bias accordingly. This amp runs 6L6GC with an UL output trans.
I noticed a large hot cement resistor (20 watt 30K) so after looking at the schematic and lots of wire tracing found it tied to B+ and straight to ground. It drops the voltage a considerable amount and pulls over 6 watts. Question, what is it's purpose here? Voltage drop, filtering, chassis heater?
Looks like the reverb circuit draws it's B+ (390V) from the same node as the phase inverter?
Few things on my list of potential mods will be eliminate the 15 watt cluster, setup proper bias supply, up the reverb drive and tone, input jack setup, and add an IEC jack. I've already pulled the ground switch and associated wiring. Any suggestions are welcome. I'm ok with the core tone of the amp, but the 2 channels do open up a ton of options and I'd be willing to mess with the OD channel.
-
That 30K/20W and the 2.7K/10W work together to make a voltage divider to drop the voltage to 410V for the little tubes. A general rule of thumb states the bleeder current (current flowing through the voltage divider resistors) should be about 10 times greater than the current needed for the load (little tubes). This high bleeder current provides decent regulation (low sag) for the load. That's why the voltage divider resistors are high wattage. Yes, they do get hot.
This high current bleeder type voltage divider was more common in days gone past.
-
Voltage divider you are talking about :