Add more gain to Home built Guitar Amp

[john65b]

Hello all, after listening to my home built 6SL7 /6SN7 / 6L6GC PP amp at practice, I noticed my friends Fender Hot Rod Deluxe is quite a bit louder than my home built amp...I understand the HRD has 12AX7 (100mu) and I have 6SL7 (70mu),and that is part of the reason...

But after looking at the HRD Schematic I noticed that it also has two more gain stages than mine...

So I am looking to add another 6SL7 (2 more gain stages) in between the existing 6SL7 and 6SN7 as shown in the red box. Can I drop in DC Coupled as I show it? Or would I be better off with cap coupled stages?

Quite happy with the amp as is (Schematic) but I have the room in tranny for heater, and a empty octal socket...and I would mind being a bit louder to keep up with the other guitarists...

 

[JonSnell Electronic]

DC coupled will be a problem as it is not solid state. It will not work properly. Use capacitor AC coupling and bias the valves properly. The phase splitter leaves a bit to be desired, unbalanced drive from that circuit. It is not solid state!

 

[john65b]

AC Couple before new stage, or between new 6SL7 stage, or after? Or all three?

 

[JonSnell Electronic]

As it was, removing the 470k after the gain control will double your output.
I would use a similar circuit to the Fender Blues Junior front end. Plenty of decoupling and all AC coupled.

 

[JonSnell Electronic]

The first part of the 6SN7 is not giving you any gain at all. If you use it as a gain stage, similar to the 6SL7, that may give you enough.

 

[john65b]

Considering the following (without adding a stage):

Raising Rp of both 6SL7 stages to 220k and dropping first 6SL7 Rk to 1.5k
Dropping grid stopper from 470k to 10k on second 6SL7 stage as advised by johnsnell

 

[danFrank]

You want more gain... OK, so why do you have a 6SN7 in your amp? Change to 6SL7. Change the P.I. 22K resistors to 47K since that will now be a 6SL7. Better yet, change to a Marshall style PI as that will give you more gain.
AC couple all stages. Quit thinking in solid state terms; you're working with tubes here.
Add the 2 extra gain stages if you want more overdrive sound but your original schematic has enough stages to fully drive the 6L6 tubes if you change the P.I. to a Marshall style PI, using 2 triode sections.

 

[danFrank]

Considering the following (without adding a stage):

Raising Rp of both 6SL7 stages to 220k l

This will give extra gain but higher than 100K tends to "thin out" the sound.

dropping first 6SL7 Rk to 1.5k

This is a good idea for more gain. Add a cap across 1.5K resistor for even more gain.

 

[Ketje]

Making the anode resistors 220k yes but not lowering the cathode resistance.Like that there is allmost no -Vg left.I say put 4k7 in for -Vg=2.3V.
Shorting the 470k is a simple manner to get more gain and make the other 470k a 1M to keep the load the same.
With positive feedback you can get much more gain but if you overdo it oscillation starts .To do that I propose a trimmer of 2k in, adjust at personal taste
Mona

 

[john65b]

OK, here is where I am with this amp after toying with it much of last night - with two 6SL7 in the amp, way too much gain and phase inverter voltages were way off, so popped in the 6SN7 and all seems much better...The phase inverter voltages are pretty much spot on...quite a bit more gain now - we will see how she sounds at practice...

 

[DUG]

The tone stage is designed to work with a low impedance source...hence the buffer stage to drive it.

You now have a much higher impedance source...this will affect the tone controls.

If you like the results then it does not matter.

 

[john65b]

The tone stage is designed to work with a low impedance source...hence the buffer stage to drive it.

Yup, realized that...put back the CF buffer...still better than before.

Still may add another 6SL7 tube (2 more gain stages)

 

[gingertube]

Just for your possible interest.
Here is the 2 channel 6SL7 preamp that is in my own homebrew git amp. Actually it is KOC's LPSP preamp with an few component changes which I found suit the 6SL7.
Sorry about the scrappy drawing. The "corrected" component values are what I changed to when doing the "set to work"/"fine tune"
Note that as you add gain stages you need interstage attenuators so as to not overdrive the following stage by too much.
Just follow the LEAD channel path to see how to add your extra gain stages. The J201 JFETs can be replaced by a switch.
Note the channel switching control signals labelled barred LEAD and barred CLEAN. The bar meaning "NOT", that is the barred CLEAN is active when LEAD channel is selected and VV.
I run a 5E3 looking power stage which consists of another 6SL7 as common cathode gain stage and concertina splitter into a pair of 6V6.
Cheers,
Ian