Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: newguitarsmell on March 09, 2019, 07:08:06 pm

Title: Converting a guitar amp to a Bass Amp
Post by: newguitarsmell on March 09, 2019, 07:08:06 pm

https://el34world.com/schematics.htm#Hoffman_5F2A_Princeton

Is it possible to adapt this so I could use it for bass? I love these 5f2a amps for their simplicity and tone. Are there any 5W bass amp schematics/layouts? mods?

Thanks for your help!

Title: Re: Converting a guitar amp to a Bass Amp
Post by: shooter on March 09, 2019, 07:48:02 pm

put a bass speaker in, test. change the OT to something 2wice as big, test. change to SS rectifier, test, oh, use a bass guitar  :icon_biggrin:

start over with a KT88, or 2, or maybe  start with this fender;
https://el34world.com/charts/Schematics/files/Fender/Fender_59_bassman_manual.pdf
just breakoff the gender part of the name  :icon_biggrin:

Title: Re: Converting a guitar amp to a Bass Amp
Post by: PRR on March 09, 2019, 10:05:48 pm

It is hard to make bass waves. A 5-watt bass amp is kinda like a 50 horsepower sedan--- not satisfying. I had a 90HP wagon, yawn.

Of course it depends. If you have a 6-foot horn speaker, you can make big sound with small power. If you play in a hard closet, 5 Watts might be ample even with a Ten.

But yeah. Simple guitar amps "are" bass amps, if you use a bass-size speaker, and a not-tiny OT.

Title: Re: Converting a guitar amp to a Bass Amp
Post by: newguitarsmell on March 09, 2019, 11:34:24 pm

I tried the bass speaker part already in the amp. It sounds GREAT but not much output for sure so I'm game to change the OT and the rectifier - which do you recommend?
Should I boost the PT too?

Title: Re: Converting a guitar amp to a Bass Amp
Post by: John on March 10, 2019, 03:56:05 am

If you pop in a 10 or 12 watt SE output transformer that should make a big difference. Like Shooter says, if it's got a rect. tube change that to diodes- quick n easy. For a small practice bass I think you'll be happy. An efficient speaker will make a big difference too.


I wouldn't bother with the PT unless you plan on changing over to a 6L6.

Title: Re: Converting a guitar amp to a Bass Amp
Post by: jjasilli on March 10, 2019, 11:07:35 am

You might also try boosting bass response within the amp:


*  increase the value of the 1st 2 B+ filter caps.  With SS rectification, this can be quite large.
*  increase the value of the bypass cap on the power tube.
*  try a .1 or .22 coupling cap between the tone recovery stage > power tube.  Try a paper & oil cap which favors bass.




Boost signal strength:
*  double the value of the power tube grid leak resistor.  (Torres Mod)
*  try 220K plate resistors in the preamp.  (Torres Mod)
*  try large value K bypass caps in the preamp.  (For the 2nd stage use the VibroChamp typology to preserve NFB)
*  eliminate or fine tune NFB for better bass
*  alter or eliminate the tone stack for more mids/bass (Duncan Tonestack calculator)


I would try such things one at a time.

Title: Re: Converting a guitar amp to a Bass Amp
Post by: newguitarsmell on March 11, 2019, 12:58:54 pm

what’s the largest OT I can use with a single ended PT?

Title: Re: Converting a guitar amp to a Bass Amp
Post by: jjasilli on March 11, 2019, 01:24:50 pm

Electrically there is no upper limit to the size of the OT.  I agree with John: 10W; if you hotrod the amp with a big bottle power tube then maybe 15W. 


IMHO the real issue is bass response in the OT.  Fender considered the 5F2 a "student" amp.  I doubt the standard tranny has much bass response below 100Hz which is passable for guitar, but not great for bass guitar. 


Personally I consider cloning a 15W Ampeg bass amp; at least the preamp section.