Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dearborndave on January 30, 2019, 01:40:49 pm

Title: Blues Jr. Output Transformer Upgrade
Post by: dearborndave on January 30, 2019, 01:40:49 pm

I'm doing the Hoffman Amps point to point conversion of the Blues Jr. amp and I'm also upgrading the transformers at the same time. I purchased an output transformer from Allen Amplification (part #TO20), and it came with instructions from Bill M on how to install it. Included with the transformer was a 100pF capacitor. In the instructions for the install it says to solder the capacitor across R30 (cream board) or R19 (green board) to prevent phase inverter oscillation. My question is do I need to use this capacitor in the build I'm doing , and if so, where does it go? Thanks in advance for any help you all can give me! :help:

Title: Re: Blues Jr. Output Transformer Upgrade
Post by: sluckey on January 30, 2019, 02:04:15 pm

Quote

I'm doing the Hoffman Amps point to point conversion of the Blues Jr.

Best to follow Hoffman's instructions and forget about BM. Hoffman's version is not an exact copy.

Title: Re: Blues Jr. Output Transformer Upgrade
Post by: dearborndave on January 30, 2019, 02:21:13 pm

Thanks Sluckey, you are the man! :worthy1: I studied both the fender schematic and the Hoffman conversion schematic, and I found the resistor in question in both, so if I have any problems with phase inverter oscillation, I'll know where to look!

Title: Re: Blues Jr. Output Transformer Upgrade
Post by: sluckey on January 30, 2019, 02:58:00 pm

Yeah, that will be easy enough to add later. Another popular way to address that same issue is to put the 100pF cap directly across pins 1 and 6 on the PI socket.

Title: Re: Blues Jr. Output Transformer Upgrade
Post by: dearborndave on January 30, 2019, 04:07:39 pm

Yep, that'll work too! Both easy fixes. I'll leave it off to begin with, but if I have a problem, I already have the solution. Thanks again!