Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: delux1959 on March 17, 2018, 12:01:38 pm

Title: bogen chb14a to 18 watt conversion
Post by: delux1959 on March 17, 2018, 12:01:38 pm

So, I have this head laying around waiting for me to do something with it. I was curious to see if anyone thought it would great/horrible idea to convert it to an 18 watt lite. Or to convert it to Doug's 18watt stout? I plan to convert the 6au6 to a 12ax7. I'm don't know if the output tubes (6gw8) would make for a good Pi and out put section.

Title: Re: bogen chb14a to 18 watt conversion
Post by: sluckey on March 17, 2018, 01:07:32 pm

That's not a good candidate for an 18 watt amp. I suggest you evaluate the amp as is first. Plug a guitar into the mic input and see what you have.

Then if you want to modify it I would leave the power supply and power amp alone. Modify the 6AU6 circuit to be like an AC-15 preamp. Or if you really want to replace the 6AU6 with a 12AX7 go ahead. You'll need a bigger socket hole but then you will have a bigger variety of preamp circuits to build... Stout, 18W Lite, Fender AB763, plenty to choose from. Just connect the new preamp output to the Bogen MV control.

Here's a very similar amp that I modified with an AB763 preamp. Sounded pretty good. Schematic at bottom of page...

     http://sluckeyamps.com/PeeWee/peewee.htm

Title: Re: bogen chb14a to 18 watt conversion
Post by: delux1959 on March 17, 2018, 02:47:02 pm

 I know it worked. I believe i tried making some modifications to it in the past, that didn't improve it at all. I don't believe I messed with the power section or Pi. Ill look into the AC15 like you suggested. Thanks

Title: Re: bogen chb14a to 18 watt conversion
Post by: DummyLoad on March 17, 2018, 04:48:07 pm

built something similar within a pair of Lafayette 612 PA - built one, loved it so much we found another and built the second. this is similar. 6AU6 is high-er gm pentode than EF86 so more challenging  to tame. see attached - should work well. try using a switchable network for bass cut: copy the matchless DC30 EF86 channel bass cut switch - you burn more caps but don't have insertion loss of a tone stack.


--pete

Title: Re: bogen chb14a to 18 watt conversion
Post by: delux1959 on March 18, 2018, 06:47:33 pm

So, i plugged it in today, and I forgot how nicely it grinds and sings when cranked. Thou it doesn't get very loud. If the volume control was swapped to a 1 meg could i expect move volume? I believe i disconnected the volume and the mixer, and used only the master volume.

Title: Re: bogen chb14a to 18 watt conversion
Post by: sluckey on March 18, 2018, 07:11:46 pm

I would not expect an increase in volume by changing the volume pot. However, the schematic shows there is a 1M pot in the circuit already.

Title: Re: bogen chb14a to 18 watt conversion
Post by: shooter on March 18, 2018, 07:41:32 pm

try taking out the NFB and see :dontknow:

Title: Re: bogen chb14a to 18 watt conversion
Post by: DummyLoad on March 18, 2018, 08:14:28 pm

if the output tubes and power supply filters are working well, then 14W amp driving a guitar speaker should be fairly loud. if it's grinding, it's got enough gain - subjectively speaking.


--pete

Title: Re: bogen chb14a to 18 watt conversion
Post by: shooter on March 19, 2018, 08:54:33 am

YUp
my 14W definitely drives everything outta the house when I plug in, the neighbors comment they heard me playing :icon_biggrin: