Pultec MB-1 preamp problems

got some problems with my Pultec MB-1
is it possible I can blow a valve by connecting the output transformer backwards?
Also - does anyone have any info on how to change the gain?

what is the value range of the resistor?
is there a table anywhere?

thanks

Well, haven't really looked at the MB-1 schematic, but if it uses a stepdown transformer at the output, say a 4:1, and you reversed it, the tube would see 1/256th the impedance it expects...

Peace,
Al.

What kind of output transformer?

Search yields Rafafredd's plan for variable gain in the MB1

I built my MB-1 (a dually) with the gain set by a resistor network on a 10 position Grayhill switch. My switch is a break-before-make, rather than the make-before-break suggested by Kubi. There is a thump when the gain is changed, but as the "no resistor" setting is minimum gain, it's not too bad.

Pics and story:
http://www.frontiernet.net/~jff/pultec_mb1/DualTubeMicrophonePreamp.html

As-built schemo, layout, and construction docs:
http://www.frontiernet.net/~jff/pultec_mb1/images/DualPultecMB1.pdf

What's the symptom of your problem?

Jon

Use this schematic for gain control. It works great!

http://img56.photobucket.com/albums/v172/rafafreddy/mb1_gain.gif

I know this is an old thread, but I'm building the MB-1 preamp, and I have a few questions for anyone who has built this preamp with a solid state power supply. I should preface my post with saying that I want to build an efficient, yet simple psu for the MB-1. The fewer parts I have to buy, the happier I'll be.

What are the advantages and disadvantages of using a voltage regulator in a preamp power supply circuit? (I am only building a psu for one channel) Will having a regulated supply improve the stability and linearity of the preamp tubes?

Can I use a voltage regulator with a FWCT rectifier like in this schematic:
http://www.frontiernet.net/~jff/pultec_mb1/images/DualPultecMB1.pdf

Lastly, in rafafredd's psu schematic he uses DC Heater voltages for the ECC83&ECC82 valves. What considerations do I have to make when using DC heaters rather than AC heaters from the 6.3V transformer tap.

[/url]

It´s hard to get good regulated DC heaters from a 6.3vAC winding. You would need a low drop-out regulator. Or maybe try 4v tubes... Or getting another power transformer. I usually get 9vAC windings for 6.3v regulated DC.