Bassman Micro B+ Voltage Issue

[cusebassman]

Hey all - I'm doing a build of Rob's Bassman Micro, and I've gotten as far as wiring up the AC in to the power transformer, out to the lamp and heaters, and the rectifier. I'm following the first schematic and diagram here:

https://robrobinette.com/images/Guitar/Bassman_Micro/Bassman_Micro_LTP_Schematic.png

https://robrobinette.com/images/Guitar/Bassman_Micro/Bassman_Micro_LTP_Layout.png

I'm using the Hammond 270AX. The only difference between the wiring diagram and my build is that I ran the artificial center-tap off of the lamp assembly rather than the first preamp tube socket.

https://www.hammfg.com/files/parts/pdf/270AX.pdf

Here's the issue (see attached image for measurements I took with a Fluke 115). I've tried two different brands of rectifier tubes, and in both cases, I am getting 245VDC off of pin 3, rather than the 350-400VDC that the wiring diagram and schematic call for. I first tried it with the diodes in place, and then without, and got roughly the same reading in both cases.

I'm hoping someone here can help clear up my confusion. Any additional info needed, just let me know. Thanks!

[PRR]

> I am getting 245VDC off of pin 3

You *need* a capacitor to get the 300V of DC which you expect.

[cusebassman]

Well, I learned something new today. Apologies for the amateur question, but thanks for answering!

[Latole]

That is why diagram show the 300 volts AT the rectifier/ capacitor connection.

 

[tubeswell]

...I am getting 245VDC off of pin 3, ...

That 245 'VDC' is your DC voltmeter struggling to 'average out' the DC pulses at the rectifier cathode that appear without the reservoir cap connected. The cap's job is to charge up to a peak positive VDC charge - as electrons are pulled out of the cap's positive pole with each successive positive DC pulse from the rectifier - and store this positive charge (hence the term 'reservoir' cap). The charge in each pulse is parabolic over time, starting at zero VDC climbing to a peak charge of about 368V*, then going back to 0 again before the next pulse, with each pulse occurring 120** times per second (at the rectifier cathode). This is too fast for your meter to follow, so (in the absence of the reservoir cap) it tries to read the 'average voltage' of the pulses - which are less than the peak voltage.

*assuming the HT winding is 260-0-260VAC

**or 100 times per second if its a 'euro mains' power supply.


More reading: http://www.valvewizard.co.uk/bridge.html

[acheld]

Just out of curiosity, the layout .png shows pre-amp out and in, but the schematic does not.  Are you planning an fx loop?

[cusebassman]

That 245 'VDC' is your DC voltmeter struggling to 'average out' the DC pulses at the rectifier cathode that appear without the reservoir cap connected. The cap's job is to charge up to a peak positive VDC charge - as electrons are pulled out of the cap's positive pole with each successive positive DC pulse from the rectifier - and store this positive charge (hence the term 'reservoir' cap). The charge in each pulse is parabolic over time, starting at zero VDC climbing to a peak charge of about 368V*, then going back to 0 again before the next pulse, with each pulse occurring 120** times per second (at the rectifier cathode). This is too fast for your meter to follow, so (in the absence of the reservoir cap) it tries to read the 'average voltage' of the pulses - which are less than the peak voltage.

Thanks for the explanation, tubeswell! This thread was a total brainfart moment - I should've realized that without anything downstream of the rectifier, the output signal is not yet smoothed out, so the meter wouldn't be seeing what I thought it should.

[cusebassman]

Just out of curiosity, the layout .png shows pre-amp out and in, but the schematic does not.  Are you planning an fx loop?

Both the schematic and the layout diagram are from Rob's page on the Bassman Micro:

https://robrobinette.com/Bassman_Micro.htm

I intend to add a passive effects loop, but I do want to try it in its original position and in the position currently denoted on the wiring diagram. I've never added a passive effects loop to an amp build before, so I want to try it out here, and possibly play with a buffered loop as well.