Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dumbmonkey on August 13, 2024, 07:06:58 am
-
Hi
I’m currently working on replacing all the electrolytic caps in a 1971 Super Reverb, and I noticed that the two dropping resistors under the doghouse are 10k and 2.2k. All the schematics I’ve seen list those resistors as 1k and 4.7k. Does that seem right to anyone? As far as I can tell, they are original to the amp, although someone did change the filter caps in 2013. I’m going to be putting in F&Ts. Should I change the resistors to the values in the schematic or leave them be?
-
I think the answer is fairly simple. Read the voltages in the B+ line with the current resistors and compare those to the annotated voltages on the original schematic. Are they in line with that, leave them in. Maybe the previous tech attempted to compensate for the higher wall voltage of our generation.
-
Oh yeah, good thinking. I probably should have thought of that first. When I get it back up and running I'll check and post if anyone is keeping tabs on this.
cc
-
Hi again!
For anyone watching this, I measured voltages using the AA270 schematic, which is what I believe is the correct one for this 1971 Super Reverb. The schematic states I should be seeing 395v at B+3, and 360 at B+4, and I'm actually getting 415v at B+3 and 339v at B+4.
I'm thinking that I should change out the resistors and see what we see, what does the knowlege base think?
Thanks in advance!
-
What's your wall ACV when you measure the amps DCV?
-
And how hot are the output tubes biased?
The hoter their biased, the more current they will draw and load the power supply down. That will cause the DCV to drop.
-
Hi again!
For anyone watching this, I measured voltages using the AA270 schematic, which is what I believe is the correct one for this 1971 Super Reverb. ...
There should be a paper label stuck to the indide wall of the cab?
This has the model code and valve type info.
The model code isn't always correct, it may have been that superseded labels were used up provided the valve info was still applicable.
Whatever, those HT dropper resistor values were used on the CFA and TFL model codes that seem to have been introduced in 1971 https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf
And yes, there's no voltage regulation in a valve guitar amp, if the wall mains voltage being fed to the amp is eg 10% higher than the nominal noted on the relevant documents, then the HT etc voltages inside the amp will be similarly higher.
And then the documented voltages are purely a guideline, they're absolutely not a precise value that must be achieved, the 'all voltages +/-20%' note needs to be considered.
Obviously the 5V and 6.3V heater voltages must be within ghe typical +/-10% specificied by valve manufacturers.