Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: valvetones on January 22, 2022, 01:48:08 pm
-
Hello all, man there is a wealth of info on here. Thank you for all those responsible for maintaining and adding to this site.
-Alright pretty easy question here.
On the 6G16 layout , in the phase inverter, connected to ground on one side , there is a 470 ohm resistor.
- in the schematic it shows a 4700 ohm resistor.
Now I’ve looked at a few other fenders with similar set ups, and I believe that the schematic is the accurate value and it should indeed be 4.7k
However!! I’ve seen the guts of two original 6G16 vibroverbs , and both of them have 470 ohms in place.
So again this resistor , one side to ground, the other connected to the junction of the 6.8k resistor and the 10k feedback resistor.
So I say 4.7k it should be. But what say you?
And, what would the effect of having a 470 ohm in place, in the original amps, do for the feedback etc?
Thank you all.
VT
-
However!! I’ve seen the guts of two original 6G16 vibroverbs , and both of them have 470 ohms in place.
Well, that speaks loudly!
There are quite a few discrepancies between Fender schematics and layouts. I would trust the schematic. A 470Ω would give much less NFB. You may even like it. It's not a deal breaker either way.
-
However!! I’ve seen the guts of two original 6G16 vibroverbs , and both of them have 470 ohms in place.
Well, that speaks loudly!
There are quite a few discrepancies between Fender schematics and layouts. I would trust the schematic. A 470Ω would give much less NFB. You may even like it. It's not a deal breaker either way.
Thank you Slucky.
A few other curiosities.
Right where the channels mix together and head into the PI grid,
the vibrato channel hits a 1 MEG resistor and the normal channel hits a 220k resistor.
On the AB763 they both hit 220k resistors before moving into the cap, then the PI Grid.
What’s the idea here? Why the difference between models?
More importantly, is there any reason why I ought not use 220k’s for them both if the rest of the amp is wired up 6G16?
-
Right where the channels mix together and head into the PI grid,
the vibrato channel hits a 1 MEG resistor and the normal channel hits a 220k resistor.
On the AB763 they both hit 220k resistors before moving into the cap, then the PI Grid.
What’s the idea here? Why the difference between models?
Those are mixing resistors. The Vib channel has three gain stages but the normal preamp only has two. So, the Vib channel uses a larger mixing resistor to knock it's signal down to match the lower signal of the normal channel.
More importantly, is there any reason why I ought not use 220k’s for them both if the rest of the amp is wired up 6G16?
Yes. If you make them both 220Ks then the Vib channel will be considerably louder than the normal channel.
The AB763 knocks the Vib channel signal by using a 3.3M resistor at the grid of the third stage, therefore it can use a 220K resistor to mix with the normal channel. The different tremolo circuits factor into this as well.
The basic idea in the 6G16 and AB763 is to make the two channels have equal volume when the volume and tone controls are set the same. Bottom line, don't change that 1M resistor.
-
I had noticed the feedback resistor inconsistency on the 6G16 also. I decided that in this case the layout was correct! It shows a 10k feedback resistor and a 470 ohm tail. The 100:5 ratio of those values is consistent with the ratio in many other amps. Using smaller resistors to get that ratio fits with a Fender tendency to move closer to the 820:100 values in the AB763 amps that came next.
-
I agree.
The 4k7 of the schematic would seem to result in the highest degree of global NFB of any valve guitar amp, more than the JTM45, even accounting for the 16ohm sample used by the latter (compared to the 4ohm of the 6G16).
-
Thanks for clarification and input.
Im in the process of building 6G16 #2.
The one under construction now will be exactly as the schematic / layout calls for.
The first one I built will be the heavily deviated version.
Gonna run them against each other and see if the mods done so far on the first one are as nice sounding as I think they are.
Alright though, 470 ohm it is, and sticking with the 1 Meg mixing resistor.
Thank slucky , AC, and PDF for your input here. This has been a helpful discussion.