Converting 6V6, 6cg7 & 5y3 phono amp into guitar amp...Is my PT Bad??

[tubeswell]

Ok, continuing the process of this conversion. I have ordered a tranny out of an amp that used a 6V6 and 5y3 and successfully installed it. Everything is done and all is left is to hook up the tranny, however this is where I run into probelms. Availble is the color coding for the wires (http://www.nostalgiaair.org/PagesByM...3/M0008183.pdf)
, however the wires are so old that I can barely identify the wires i.e.

http://www.auctiva.com/hostedimages/...0,0,0&format=0.

My question is how do I figure out which wire is ground and which would be the B+. After that I can figure out the others via multimeter I already ruined one tranny and at this point would probably smash the amp if I had to get another.

Any help would be wonderful, I am stuck between a rock and hard place. Many thanks!!!

(going by that information) for the secondaries, Red = HT (with a red-yellow centre tap), Yellow = 5V winding for rectifier, and Green = heaters 6.3V winding (no centre tap)

The other wires are your primaries (one for each end of the primary winding) and your shield. The shield won't be connected to any other wire. The primaries should have a low-ish DC resistance across them.

If you can't tell the colour of the secondaries, very well. The pairs of wires on the windings will all have a low DC resistance across them. The HT will have twice the resistance from end to end as it does from each end to the centre tap. Carefully hook-up the primaries to a wall socket VAC supply, with the shield going to the mains earth, and the HT centre-tap going to the mains earth. Make sure none of the secondaries are touching each other or shorting on anything, then switch it on and (very carefully) measure the VAC across each corresponding pair of secondaries.

[skydogda]

Once again your great. With the knowledge provided, I was able to connect the power supply to check the windings. With the center tap and shield wire grounded I started by testing the HT winding and got 312 VAC and was enthuised, however my enthuisiasm quickly died. The next two windings read roughly 80 and 140 VAC. What the hell? At first I thought it was maybe because I used the same ground point as a filter cap, however that was not the case. I even disconnected the shield ground wire completely and still got about the same reading.
Should I blame the ebayer who sold it to me? Or did I do something wrong?
I was also thinking that possibly I connected the shield to the power supply (although I did test them) instead of the primaries, would this explain the extremely high VAC's?
-Once again I am stuck with no measureable results (ie nice guitar amp) and a heap of frustration.

[tubeswell]

Hi skydogda

All I can think is, either maybe the PT isn't what it is proported to be, or you have the wrong wires connected as the primary, or maye you didn't measure the correct 'pairs'? (How badly discoloured are the wires?)

Did you have the shield connected to the (mains) earth? (important for safety)

How did you measure the VAC on the HT? (you can either measure it from one end of the winding to the other, or from each side of the winding to the centre tap. The former should be twice as much as the latter)

[skydogda]

Tubeswell,
Everything is fine and dandy now. I figured out that I was just testing the voltages wrong. I was testing grounded, instead of winding to winding as you described. The HT winding tested to ground was right, so I figured the rest would be tested to ground, but I assume since the HT had a center tap testing to ground didn't matter? I don't know how that works, but the good news is I HAVE A FUNCTIONING AMP! There is only one more problem with it though; when the volume is past about 7 it does this weird tone scream thing. I am going to go ahead and replace the last filter can since the amp doesn't make that sound when flipped over and the can is depressed back into the amp. Hopefully, after tonight I will finally have myself a cool handmade amp.
One last question, Which resistor/capacitor relates to the 'feedback' of the amp? I would like to play around with the sustain, etc. A link to another thread would be great as well, since I know others have asked.
Thanks so much once again to helping establish my newfound tube amp obsession!!

[tubeswell]

Tubeswell,
Which resistor/capacitor relates to the 'feedback' of the amp?

You can have different feedback circuits in different locations.

The type of overall negative feedback loop in a fender champ comes from a resistor between the OT secondary winding back to the cathode of the driver stage.

In many LTP PIs you get a small 'feedback' cap across the plates of the two sides (look at the tweed bassman schematic for example)

Local feedback can be designed into just a single stage with a resistor from after the coupling cap back to the signal grid - the valve wizard site has an excellent article on local feedback resistors here:

http://www.freewebs.com/valvewizard/localfeedback.html

Take a look at a PV classic 30 schematic - R21 is one such local FB, but it also has an NFB loop using R56, R57 and R70 and C36, C37, C53 and C54.

[skydogda]

Ok, more than 3 months later...I have stopped working on this project due to lack of time but am ready to finally finish it. The last thing I had to do to this puppy was replace the finally filter can. I removed it and replaced the four cap's with the correct ones and grounded them all and rehooked the wires but now no sound. I have tested all the cap's and their connections and they all have power running through them. This is the cap can that runs to the speaker out. Any suggestions on how to get sound again??