Tube amp project

[gui_tarzan]

It's been a busy few nights, here's what I have done so far. I am getting a lay-down style bell set for the PT and should be able to start wiring this weekend.

I'm using a Yamaha B100ii solid state head cabinet because in actuality, I like it and I got it dirt-cheap. The Tolex is in great shape but I will be replacing the grille cloth.



I cut out the area of the chassis that had all the funky stamping and holes that weren't going to work and replaced it with a piece of aluminum sheet, then drilled the tube mounting holes.

[g1]

Before even reading the post I thought "hey, that's Yamaha grille cloth!", so I'm sorry but I'll have to vote for keeping it . It would certainly be unique.

[TomCarlos]

So it looks like you kept the original chassis and cut out some big rectangles to fit in the new pieces? Can't wait to see your progress. Good luck!!

[olddawg]

You need to be careful of ground loops. It will probably work as is....... but if it were my project I would use a steel plate if it is a steel chassis. Funny stuff happens when aluminum meets steel like that.

[gui_tarzan]

You need to be careful of ground loops. It will probably work as is....... but if it were my project I would use a steel plate if it is a steel chassis. Funny stuff happens when aluminum meets steel like that.

Yeah, I thought about the potential for the dissimilar metals reacting, I'll probably put something in there to protect from that and make sure the grounds are solid. It's a heck of a lot easier drilling aluminum for the tube holes than steel too.

[gui_tarzan]

So it looks like you kept the original chassis and cut out some big rectangles to fit in the new pieces? Can't wait to see your progress. Good luck!!

Yes, I thought about making a new chassis but this is a low $ project just for me so I went the cheap & easy way.

[gui_tarzan]

My replacement bell cover set arrived today from Heyboer (right here in Michigan) and they fit perfectly. Back to work!

[gui_tarzan]

Well it's all together and I have no sound. None. No buzzing, no hums, nothing. Time to start looking for missed connections and voltage levels.

[gui_tarzan]

I am supposed to have around 415v on pin 3 of the power tubes but I have - and this seems highly unusual to me - 1100v. Is that even possible?

[g1]

Doesn't seem very probable. Try measuring across the first filter cap.

[gui_tarzan]

580v and it fell to 535v in about 30 seconds, and it's dropping 1uf about every four-five seconds now.

[gui_tarzan]

Looks like it's stabilizing around 500v.

[edit] Oh geez, I've got 1192 on the second cap. I bet I wired one of them backwards.

[edit 2] No, according to an AB763 schematic they're right.

[g1]

Which AB763 is this based on? What meter are you using and do you have a good ground?

[gui_tarzan]

It's actually based on the 5F6 schematic except I used 80/80/16/16/16 for caps instead of 20/20/20/20/8. I got the voltage specs from the SR AB763 schematic but now I see the 5F6 schematic shows 425v on the first two caps where the choke connects. Yes, I checked the grounds and they all look & feel good & tight.

I checked with two different meters, one is a Commercial Electric and the other is a cheapie Cen-Tech, both show the same readings.

[Enzo]

OK, your B+ wwinding has a center tap that is grounded. Make sure you didn;t ground one of the winding end wires instead of the center tap.