The Amp Garage

Check out the AX84 HO: http://www.ax84.com/hioctane.html This plan uses 2x 12ax7 and one EL84. Has the benefit of lots of supporting materials and explanation. The biggest hurdle for me will be understanding the variables I'm throwing into the equation: the existing PT, OTs, and tube rectifier. This...

Currently, I have a Blackheart BH5H Little Giant. This is 5 watts, switchable to 3w, one 12ax7 and one EL84. I put in the three-way tone stack mod, and run a modded RAT distortion in front. I couldn't be happier with this $170 amp! My other amp is a Mesa Boogie Mark IV head, which covers a lot of gr...

I think he's checking the OT, putting the 5.57AC into the primary. So with .187 on the secondary, that's would be about 30/1 turns ratio. Assuming an 8-ohm load, that's around 7K3 primary impedance. Pretty close to the 8K book value for a pair of EL84s. Does the other OT test the same? Yes, the oth...

Phil, isn't this correct for an OT? It's supposed to convert the high voltage, low current signal from the power tubes to low voltage, high current required to drive the speakers.

Why do you expect the voltage to rise?

First photo shows 5 vac output from PT jumpered with white and yellow gator clips to input winding of OT. Red center tap is not connected to anything. Red and black gators run to the multimeter, which shows 5.57 vac. [img:1024:768]http://www.monkeymatic.com/pub/ebay/packardbell9.jpg[/img] Now multim...

From the rectifier socket, no tube in the socket, place a jumper from pin 2 to pin 7 of tube A. From the rectifier socket, place a jumper from pin 8 to pin 7 of tube B. Turn on the power. Measure ACV across the green and black secondary wires. Measure ACV across pins 2 and 8 of the rectifier socket...

To test the OT (one at a time), you'll need a low voltage AC source. Finish testing the PT and then you can use the 5V supply for the rectifier. Use a pair of jumpers with 'gator clips on both ends. One pair of EL84's is mated to each OT, so do one pair at a time. Run one jumper from pin 2 on the r...

I hooked up yel, yel+blk (5.5VAC) to rectifier pins 2 and 8 (filament), and red, red+grn (350VAC) to rectifier pins 4 and 6 (plate). Now I am reading 340-350 VDC between pin 2 OR 8 and PT's center tap (ground). Also reads 100-110 VAC. I posted a voltage range because I just received a second multime...

The remaining one is the high voltage secondary. It should not be zero. It should be about 500VAC give or take 50V. Yes! between red and center tap = 347.6 VAC and between red+grn and center = 348.4 VAC As for how the rectifier works, in this case a 5U4, the high voltage is sent to pins 4 & 6 as VA...

Structo wrote:Please be careful.

Thanks. Will do.

Here's my guess; please confirm before I try. PT wires yel, yel+blk (5.59 VAC) were connected to pins 2 and 8 of the rect tube (pinout says Filament). PT wires red, red+grn (which read 0 now) were connected to rect pins 4 and 6 (plates). So my guess is the rect tube will deliver two different DC vol...

* blush* I knew that.

orange, yel+grn = 6.76 VAC
red, red+grn = 0 (is that what you'd expect?)
yel, yel+blk = 5.59 VAC
grn, grn+blk = 7.17 VAC

I'll investigate the OTs next.

Really appreciate your expert amateur advice. Disappointment follows: PT: You need to be double sure all secondary wires are isolated from the chassis and the circuit. Just to make certain, I cut ALL 11 leads and stripped the ends. Meter shows 118 VAC on the mains (connected to power strip as you su...

Next, determine if there is continuity on each of the windings and what wires are paired. Set the DMM for ohms and using the guesses above, see if there is reading. Write it down and post the results. Phil, awesome! I was just staring at this spaghetti monster. Thanks for kicking it off! Power Tran...

M Fowler wrote:Does it have a cage top?

No. It was just bolted to the interior of the console stereo.