martin manning wrote:12AX7's can use either 6V or 12V heater power. To wire them for 6V you connect one lead to pins 4 and 5 (tie them together) and the other lead goes to pin 9.
I powered up without any tubes in and tested for AC voltage at the socket pins that I just strung together (pins 2 & 7 of power-tube sockets and pins 4/5 & 9 of the pream-tube sockets). It shows 3.2 V on all of them. Am I doing okay?
If you are measuring pin 2 to ground, and pin 7 to ground, yes, you should see 3.2vac. But if you measure pin 2 to pin 7, you should see 6.3vac. For preamp tubes, measure from pin 9 to pin 4/5 and you should see 6.3vac.
I mention this because there was just this guy over at TGP who had one broken heater wire, and he was measuring 3.2vac to ground, but the heaters weren't lighting up!
I build and repair tube amps. http://amps.monkeymatic.com
xtian wrote:If you are measuring pin 2 to ground, and pin 7 to ground, yes, you should see 3.2vac. But if you measure pin 2 to pin 7, you should see 6.3vac. For preamp tubes, measure from pin 9 to pin 4/5 and you should see 6.3vac.
I get 3.22 VAC measuring against ground, but 6.45 VAC measuring between pins 2 & 7. Is that within a healthy boundary?
Yes good. Since the filament circuit is grounded at the center tap of the PT filament winding you see half of the voltage when measuring to ground. I guess you are ready to put the tubes in!
martin manning wrote:Yes good. I guess you are ready to put the tubes in!
Have to build a quick mount for the chassis that allows me to fiddle with all the controls and take internal measurements. Should take 30 minutes tops...
Update: making a few physical adjustments before mounting tubes... max 15 minutes, hopefully!
Just discovered that I'd broken one of the terminals off of the back-panel right bias control (going to V5 & V6). I must have nudged into it while fiddling around inside.. I'm going to see if it's a simple repair involved ...
Last edited by Mousey Dung on Mon Oct 06, 2014 5:41 pm, edited 1 time in total.
martin manning wrote:Take your time... Stable bias voltage is essential.
Looks like I will have to replace the pot. In the mean time, I have some spare trimpots. The 25 K ones I have are tiny. Those that are larger and similar to those on the preamp board and closest to 25 K are over-spec'd at 5.9 K. Is it alright to use these?
UPDATE: I found a spare bias control, probably came with the mod kit. It's overspec'd at 100 K. I think I'll go ahead and use this.
Mousey Dung wrote:Do I bias with the bulb limiter at this point?
No. The bulb limiter is a tool to prevent damage or burnt fuses as you trouble shoot the amp for first power up. Now that that's done (right?) you can take it out of the system.
You cannot use the bulb limiter while you're trying to set the bias and power tube dissipation.
I build and repair tube amps. http://amps.monkeymatic.com
You have to get close to 25k with the pot to get the right range for the bias voltage. If you have a 100k in there put a 33k shunt resistor across the outer lugs (the ones with the 15k resistor and the purple wire) to make it look like a 25k. Check the voltage and range on pins 5 to be sure you have it working correctly.
martin manning wrote:You have to get close to 25k with the pot to get the right range for the bias voltage. If you have a 100k in there put a 33k shunt resistor across the outer lugs (the ones with the 15k resistor and the purple wire) to make it look like a 25k. Check the voltage and range on pins 5 to be sure you have it working correctly.
Just to be sure, let's do it like painting by numbers
Between which points do I put the 33 K resistor exactly? Do you mean point 6 & 7?
You do not have the required permissions to view the files attached to this post.
Last edited by Mousey Dung on Mon Oct 06, 2014 7:39 pm, edited 1 time in total.
Structo wrote:Looks like you are getting the help you need.
For my first build I chose the Brown Note D'lite 44 watt.
I received the amp about three weeks after I ordered it.
Too bad you didn't come here first before you bought the kit from BN.
Many people have not received their kits or other promised parts.
It's really a shame because he had the corner on the market for these
amps.
Good luck with the power up and I expect to read that you have a functioning Blues Master amp.
Good luck!
Yes, everything was so promising. We had a good rapport to start with, so I placed my confidence in his reliability. I was even looking forward to doing a heck of a lot of business with him, as in the longer term, I plan to build several more of these and I've had friends asking if I can help them build one too. The trust factor has rapidly dwindled to near zero as M. acts erratic and undependable, coming and going as he pleases, fully aware about the fix I'm in, yet not replying or selectively ignoring the critical issues I must resolve to move on with my build.
However, you guys are a wonderful bunch! I'm eternally grateful for your kindness -- all of you!
Last edited by Mousey Dung on Mon Oct 06, 2014 8:51 pm, edited 1 time in total.
Each pot has one lug connected to a lead from the rectifier board (purple in the layout drawing), and also a 15k resistor from another lug going to ground. The 33k resistor goes across those two lugs, which will put it in parallel with the pot's element. 100k in parallel with 33k is 24.8k. The third lug is the wiper, which is where the bias voltage is taken from.
... So yes points 6 and 7 if that is the location with the 100k pot.
