Lar/Mar PPIMV

[BarryW]

are you kidding?? Thanks to the boat a yas . . .I really appreciate the help. I still have a lot of learning to do. But I always will! Really, thanks.

[sepulchre]

Yeah, I don't think that ever stops. I've got a long way to go (obviously).

[loicdulux]

Hi guys,

I'm reading that thread because I'm planning to implement this PPIMV in my fender bassman 50 ('74, AA371).

I will first convert the balance bias to old fashion fixed bias (as per EL34world schematics).

Now I have some doubts whether I should order a 100k dual pot or 250k, as I could read that on the fender most schematics, it should be 100k only.

What do you think?

Thank you!

[martin manning]

Since the bias feed resistors are 100k, I would use a 100k dual pot, but keep the safety resistors on the Lar-Mar at 2M2, which will land you at ~96k with the MV up all the way. That will preserve the LF roll-off from the PI coupling caps as it is now, and maintain the ~100k grid leak resistance. You could add a bias level adjust and keep the bias balance feature as shown below.

[loicdulux]

That is a killer answer Martin.

I think I have found THE forum with THE guys here, I'm so happy!

I will do your mod then, and order a "pot" for this bias.

When I will be more advanced on my mods, I will open a thread showing pictures of my project.

[martin manning]

SInce this is a Lar-Mar thread, here's the rest of it... (see later post with improved circuit)

[loicdulux]

Thank you so much!!

[loicdulux]

Hello there, Hi Martin,

Me again

Now the bias mod is (hopefully) finished and the power plug simplified, the next step is to wire the master volume.

Before doing so with the non most adequate parts, I would like to check what should I use:

- at the pot, I see the 2.2M on the schematic: is that resistor? doesn't look like with that shape?

- "going to the grids": same pin on socket than described in the wiki?

[martin manning]

Those are 2.2M resistors on the MV pots. They are there to keep negative voltage on the grids in the event that the pot wiper loses contact.

Those two wires labeled "to grids" go to the power tube sockets, pin 1, in place of the two leads from the 0.047uF coupling caps on the board that I X'ed out. Fender put the grid stopper resistors on the sockets, from the grid pins (5) to the unused pin 1. You have to maintain the same cap-grid relationship as the original leads, but the signal will pass through the MV pots. I suggest twisting the leads together in pairs- the ones from the 0.047uF caps to the MV, and the ones from the MV to the tube sockets, and you might as well twist the ones from the balance pot too.

[loicdulux]

Hi Martin!

Here some news! I couldn't wait buying wire with another color so... But now I need to wait for the 2.2M and another color for grids.

As you can see, I had an issue with one pin wire... The solder meets it... S****... So I must replace that one. It goes from pin2 on one tube to pin 7 of the other. And vice versa. I can't see that wire on AA371 official layout, strange.
I notice the section was bigger than the other: is there specific spec I need to look for? Isolation?

Edit2: found out!
http://www.el34world.com/charts/fenderservice4.htm

Make sure that the heater wires on the power tubes stay in phase. Pin two should go to pin two and pin seven should go to pin seven. If you see that they are crossing over from pin 2 on one power tube to pin 7 on the next power tube, you need to unsolder them at one of the power tubes and reverse them. This has the effect of canceling hum in the power amp. You will find them wired wrong all the time.


So it means I need to rewire them. If they have 6.3v, what type of wire should I take then?

[img:200:150]http://s20.postimg.org/4276ovgpl/image.jpg[/img]

[img:200:150]http://s20.postimg.org/m3qbmoaqh/image.jpg[/img]

Ps: I didn't notice any shielded wire to ground, should I? I have some speaker wire with shield if needed..

[martin manning]

Twisting the wires should keep you out of trouble, if problems occur maybe you could try shielded wire. I might have twisted the wires from the board to the pot a little bit tighter, but try it as it is. I wouldn't worry about that burned heater wire as long as it isn't going to touch any other wires or ground, and I don't think it will. Did you test the bias voltage and range? If that looks good you could test the MV without the 2M2 resistors installed. They are a safety feature only. The unknown at this point is if there will be any hum or feedback induced by the new wires. Only one way to find out...

[loicdulux]

Exciting, will try tomorrow hopefully then! Thank for the info. I'm learning a lot! Yes, the MV to board wires were my first try, I tightened bias to MV a bit more. Easy to re-adjust, I may do so tomorrow.

(Would you re-wire in phase pin2->pin2 and pin7->pin7)

Time to sleep here

[martin manning]

The heater wires are usually left out on the layouts and schematics for clarity. I would rewire pin 2-pin 2 and pin 7-pin 7 to help minimize hum. Heater wires carry more amperage than most others, so they might be 18AWG (~1.0mm). Again, you can worry about that later.

You will have to set the bias to test the amp. To make it a bit easier you could meter across the outer lugs of the balance pot and adjust the balance for zero volts. Then use the DC resistance of the OT and the voltage drop across it to calculate plate current and set the bias level.

[loicdulux]

You will have to set the bias to test the amp. To make it a bit easier you could meter across the outer lugs of the balance pot and adjust the balance for zero volts. Then use the DC resistance of the OT and the voltage drop across it to calculate plate current and set the bias level.

Ok so if I understand well, I can see with the bias probe the current at the tubes, and I should set it equally and around 30/40mv (70% of the plate voltage).

Can I check the plate voltage with the bias probe, the one that should be around 425V/450V? Or should I go with the OT shunt method (which worry me a bit)?

edit 3:
After some reading I found out that: I just need to plug red and black cables for amp measurement of my multimeter and then I can adjust the current with the bias pot.

To use the formula I need the plate voltage, that can be measured in volt using pin3 (so I guess with alligator clip of the multimeter) and ground.

[martin manning]

I wasn't sure if you had your bias probe up and working yet. Better make sure that it is working, or be very careful.

You will want to set the current such that the plate dissipation (the energy flowing into the plate, V x I) is about 60-70% of the maximum limit for the tubes. For 450V and 30W 6L6GC tubes, that means I = (30 x 70%)/450 = 0.046 or 46mA. The plate voltage will vary a bit with the current, so you may have to iterate.