Lafayette Bypass Option

[D0Hbert]

I have a vintage Lafayette preamp which I'd like to modify to make it sound a bit better by using an external preamp. Base on the schematics, can I bypass the tone and volume control together with the preamp tube 12ax7 by connecting one of the rca inputs, lets say for tuner, by making a jumper connection directly before the junction of r20 & r21, r56 & r57? the schematics of the tube follows:

Thanks for reading my post. Hope you guys can give me some advice. thanks
🙂

 

[Frank Berry]

That should work.

 

[SY]

If you're using this for phono, you'll also want to redo that phono stage. It's similar to the Dynaco, which is not a good thing.

 

[D0Hbert]

Many thanks for the responses. I'm fairly new to tubes and I greatly appreciate all your inputs. By the way, is it better to place the jumper before the coupling capacitor or after the coupling and grid resistor but before the grid stopper? From my recollection, the capacitor, paralleled to the grid resistor forms a high pass filter, therefore, if I place the jumper after that, will the output sound shrill/highish? Another thing, if I decide to make this bypass, can I remove all the 12ax7s from the circuit? By doing so, will it put unnessary voltage/current to the 7199s and el84s thereby affecting the bias points?
By the way, I'd like to get your honest opinion on the schematics of this amp, is the design worth tweaking, by making use of better caps or resistors? I've seen somewhere that some lafayettes were designed by the Haffler? Again, thanks spending time on my posts.

 

[Frank Berry]

I'd connect the jumper to the input of the coupling capacitor.
The capacitor will protect the 12AX7 tube(s) from any d.c. that might be present at the input.
You can remove all unused tubes.
You might want to replace all coupling capacitors as well as all electrolytic capacitors.

 

[D0Hbert]

Thanks for the replies Frank, appreciate it.

 

[D0Hbert]

I've tried placing the jumper directly at the top of the coupling capacitor, my only concern now is that the volume is a tad loud. Even it a very low signal from the preamp, the amp is playing loud already, and consequently easily distorted. I've read somewhere that bypassing the volume gives rise to 6db volume increase, is it because the volume control itself places for example a 100k load in series with the capacitor? One more, based on the schematic, the 7199 is using a 10meg ohm grid resistor, is this correct? I've checked the RCA tube specs, and based thereon, max grid resistance is only 1 meg ohm? If I bigger/smaller grid resistor were used, what will the effect be on the amplification of the 7199? Thanks.

 

[D0Hbert]

Another thing, if I removed all the preamp tubes, will the heater voltages of the tubes being used (7199s and el84s) shoot up?

 

[eds65gto]

amp

If you remove the preamp tubes V1 V2 and V6 the bias for the output tubes will be higher because you are taking a load from the neg bias supply, according to the schematic.
All that is required in this case is to reset R 79 and R 81 to the proper bias. You did measure the bias before you did this did'nt you???
Ed

 

[D0Hbert]

Unfortunately I didn't Ed, thanks for the info. I'll check the bias setting again and change the resistors. Appreciate the tip. Need to have someone do this for me just to be sure. Have to catch up on my readings on tubes, honestly, this is the first time I've encountered a negative bias supply. If its not too much, based on the diagram, where is it from, and what does it do? thanks

 

[D0Hbert]

Another question, based on the schematics, the tone control forms part of a feedback loop or is it a passive tone control setup? If its a part of the feedback loop, will that explain the high gain I got by bypassing it together with the volume control and the 12ax7 input stage? Thanks again.