AB763 Tightening and Defining Bass

[Jake1524]

I have played Fender amps as long as I can remember. I always roll the bass almost completely off due to the nature of the amp. I always felt I did not want any low end in my sound..

Well I purchased a Fuchs ODS and that quickly changed. I can roll up the bass a bit and it stays very tight, snappy, and well defined. This has been very enjoyable and eye opening.

What are some mods that that I can do to my fender to push it in this direction? I was thinking decrease the bass coupling cap value as well as reducing that cathode bypass cap values. Any other suggestions? I would love the low end of the Fuchs amp while retaining the classic fender sound.

Here is a quote from another thread I started on the gear page "The difference between booming bottom end...and clean bottom end. That is the hallmark of British amplifiers. The top end is crunchy and the bottom end is clean, that is the Marshall sound. Typical of British amp design.The Plexi, the 2204, just look at the schematic." This really captures what I am looking for. I would like to push my Fenders to have a similar bottom end of a Marshall(or i guess my Fuchs) without altering any of the other attributes of the amp.


Thanks!!

[Roe]

decrease cathode bypass cap to 2u2 on the first gainstage and 3u3 or 4u7 on later stages. decrease coupling caps a little too, cut the most bass early in the circuit

[Jake1524]

decrease cathode bypass cap to 2u2 on the first gainstage and 3u3 or 4u7 on later stages. decrease coupling caps a little too, cut the most bass early in the circuit

Thanks, these were my assumptions. What is your suggestion for coupling cap values?

[sluckey]

In addition to smaller cathode bypass caps, a popular mod is to change the .1µF couplers between the PI and output tubes to .047µF.

[Jake1524]

In addition to smaller cathode bypass caps, a popular mod is to change the .1µF couplers between the PI and output tubes to .047µF.

Thanks sluckey! Would you mind briefly explaining the effect of this mod and why it does what it does? I am trying to learn as much as possible while experimenting.

[sluckey]

The bass will sound tighter, more defined, less boomy or flubby. Marshall likes .022µF for these couplers. You may prefer .022 rather than .047, but I suggest .047 first.

[Roe]

decrease cathode bypass cap to 2u2 on the first gainstage and 3u3 or 4u7 on later stages. decrease coupling caps a little too, cut the most bass early in the circuit

Thanks, these were my assumptions. What is your suggestion for coupling cap values?

reduce the 20nf cap after the second gainstage (connected to the 3m3 and the reverb input) to circa 3n3.
after the third gainstage reduce the big 100nf cap connected to the 50k trem pot to 47nf, use a 1nf or 750pf phase inverter intro cap with 1m resistors in the phase inverter (or tripple that value with 330ks).

for the phase inverter coupling caps, try 22nf with 220k bias splitters or 68nf with 68k.

See here https://www.ampbooks.com/mobile/amplifier-calculators/

[Stevem]

Bedsides the circuit differences the low end response and break up point is no where near a Apples to Apples deal if where taking about a 2-12” Fender and a 4-12” Marshall.