Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Throstur on June 23, 2011, 06:01:57 am

Title: Bassman 5F6A low voltage on V1 and V2
Post by: Throstur on June 23, 2011, 06:01:57 am

I have a Bassman 5F6A built with the original Fender schematic on a fiberboard (the original Fender layout). The volume is "weak"/low, and when cranked, it gives a rather harsh ugly distortion.

First thing I did was to register all voltages. It seems to me that voltages on V1 an V2 are to low, see voltage chart http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11288 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11288)

Can I have your opinion this matter?

Title: Re: Bassman 5F6A low voltage on V1 and V2
Post by: DummyLoad on June 23, 2011, 06:39:21 am

pin 7 of V2 should measure ~~140V - make sure pin 7 of V2 is tied to pin 1 of V2.  B+ is little too hot. the PT you have is probably wound for the 5U4GB.

Title: Re: Bassman 5F6A low voltage on V1 and V2
Post by: Throstur on June 23, 2011, 07:25:33 am

My mistake, pin 7 on V2 measures 140V and is tied to pin 1 f V2. Must have forgot to register it and I think it´s not possible to edit the chart. First time I use this wonderful tool.

Title: Re: Bassman 5F6A low voltage on V1 and V2
Post by: sluckey on June 23, 2011, 07:33:52 am

V3 pins 2 and 7 cannot be zero volts. It's nearly impossible to get an accurate reading on the grids of a LTP circuit, but logic says the voltage will be slightly less than the cathode voltage. Your meter will load the actual grid voltage, causing your reading to be lower than the actual voltage, but never zero.

Title: Re: Bassman 5F6A low voltage on V1 and V2
Post by: Throstur on June 23, 2011, 08:17:01 am

Found one "wrong" resistor in stage four of the power supply. It´s  8.2K but should be 10K according to the schematic. I have changed it to 10K and made a new Voltage Chart for this amp. Since I was in there I also changed the 8uf in power stage four to 20uf. Please take a look at the chart for me. If I compare my chart to other 5F6A charts in the database, it looks that the voltage at pin 1, 7 and 8 on V2 is considerably lower on mine, than the others. http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11293 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11293)

There is something wrong that I can´t get my hands on. The sound is very dull, the volume is very low and weak, I have to take the volume up to 10-11 to make it overdrive, and then the overdrive is harsh and ugly.

The Hoffman Bassman I made few years back is a fantastic amp that has a lot of power and good headroom and punch.