late '70s 2 input preamp tagboard question

[alschnier]

hey guys,
I've been working on my FL112. ordered the recap kit here & just fin. the filter caps (no one told me mil. spec meant they tied them in knots before soldering! )

I'm on to the preamp tagboard. just want to bring this back to orig. specs. there are a few sections (both ends) that had a little work done. my knowledge is limited, & I've been trying to piece together what I can from mark's site, other pix, & my cursory knowledge. some things are easy for me to spot & replace (ie. replace orange drops w. same value sozo's, as noted on schem.). others not so much...

here are 3 pics of my tagboard.




here's a pic of a late '70s 2 input tagboard that appears to be stock:


link to GIF on mark's site:
http://www.hiwatt.org/Layouts/Tag3.gif

when you compare my board to the other examples, it appears that a few things are amiss.
- specifically, the 1st 2 positions on the L (cathode resistor/cap?) seem out of order.
- also the last position on far R, should mine have the resistor & cap? (you'll not mine has both in the full pic, but the resistor leg broke. I wasn't sure where to go from here, as info seems to vary...)

I know all of this may be hard to tell from this limited info, but if you can point me in the right direction, I'd appreciate it.

still learning,
al

[Dr.HI-TONE]

I am pretty sure that the first two turrets are linked, so it shouldn't be an issue whether the cap or the resistor come first..... However Mark or Mikhailwatt are more advanced Techies than me, hopefully they will chime in!

It looks like the cap on turret #1 is a 250uf ? It should be a 100uf. I would replace the carbon comps with more appropriate Carbon film resistors.

[OldSchoolDave]

Fender guys love those blue molded caps. So, even though they aren't stock, I wouldn't be in a hurry to change them out.

Dave

[mhuss]

Take out the cap and sell it to a Fender guy .

On the left side, it depends which resistor is hook to which cathode on V1. Perhaps someone wanted more gain on the Normal channel and moved the cap.

On the right, some amps have caps here, some do not. Having the cap adds more gain, your call.

--mark

[alschnier]

this is great help. thx. will check continuity on those 1st 2 turrets & see where they go. where are they supposed to go (on V1)?

sorry for spreading this out over several forums, but fig. the info would be helpful once it came...

[Dr.HI-TONE]

Mark, please correct me if I am wrong, (I am much more familiar with 4 input HIWATTs) but I think it should be the yellow wire on V1.

ps. I am pretty darn sure, but I hate to give info unless I am extremely sure of my answer.

[mhuss]

Well, duh, that resistor and cap are in parallel, so electrically it makes no difference which one is on which side. This is where that 'late 70s layout' would come in handy.

+1 on what the doc said earlier, I'd replace the carbon comps with carbon films as per the originals.

--mark

[alschnier]

- OK, so after some quick testing, turns out the Dr. was right. those 1st 2 tags are connected to pin 8 of V1 (yellow wire).
not sure why these were reversed (& why the last soldererer chose a 250uf instead of the 100uf), but I'll put it back to stock value (but I'm not sure what that is. I know I'm looking for a 100uf axial elec. cap, but not sure what V I'm looking for).

now, more of a general question (although it does pertain to this amp too) - the resistors on 1st & last tags have changed a bit over the years (see the 3 diff. GIFs on mark's site). is there any reason why I can't use the 2.2K instead of the 1.5K? I'm assuming it'll reign in the gain a bit, closer to early/mid '70s?

thx for the help.
al

[mhuss]

The resistor you're moving over is lower in value because it's feeding both cathodes of V1. The original design has a separate 2k2 resistor for each cathode. The 100uF cap can be as low voltage as you can find it (it will have ~2v across it typically). 16v or 25v are normally used values.

Using a 2k2 cathode resistor for V2A and V3A (if not there already) will decrease the gain and clean things up a bit.

--mark

[alschnier]

think I'm all set. hope to fire it up later today...

[alschnier]

no sound...

checked all variables. used known tubes, speaker, cable, etc.

here are a few pix of my work. anything look suspect? (recapped & addressed the preamp tagboard that had been worked on)





thx,
al

[mhuss]

I don't see anything suspicious in those pictures. Did you check the HT fuse?

--mark

[alschnier]

fuses are fine. I did notice that when I crank the volumes, I can just hear the signal coming thru. also, one of the power tubes (far L if tubes are facing up) glows a little brighter than the others when not in standby (running). these power tubes are brand new, winged c's.

I'm sure I must've missed something.

[OldSchoolDave]

I'm sure I must've missed something.

Make sure the contacts on your impedance switch are clean and making good connections.

Dave

[loylo]

Well (excuse my bad english), as far as I can see on your last pic, there should be a resistor (1K5) in parallel with the 47n cap on the last turrets (at the right of the picture). Without that resistor, the bias of V3 is completely off. That may be why you have no sound.

By the way, I've taken picture under the turret board of my 78 DR504. It's messy, a lot more than the typical layout of early 70's preamp as it seems to me.
I'll try to post the layout I have drawn.