1980 Fender 75 Rehab

[twilightofthedogs]

Hello all. I recently picked up a fender 75 amp from 1980 on a whim. Its got some funkiness that I am trying to work out. This is the 12" speaker versions. I'll probably post a few questions/document the process here. First question is regarding the output transformer. This one is loaded with a fender 022889 OT, which i believe is the OT from a twin reverb or similar. My understanding is the original OT is Ultralinear. This one has a 4.7k resistor and an inductor of some sort the is connected to the screen grids of the power tubes as sketched in the below photo. I assume this is a modification due to the dissatisfaction with UL OTs. However, there are a few photos of these amps on line with an inductor shown in the same position, so it was either a very popular mod, or some of these amps came stock like this? Anyone had any experience with doing a mod like this? Just curious. Also, any weirdness about using a Twin OT with only 2 6L6s? I assume that means this amp wants a 4 ohm speaker?

Fender 75 by Michael Lovato, on Flickr

Fender 75 by Michael Lovato, on Flickr

[The Dude]

They just used the resistor and choke to create a power supply screen node where there was none because of the original ultralinear transformer. I can't imagine you'd replace the transformer rather than rewire it if you didn't like the ultralinear thing. My guess is that the original one went south and somebody couldn't find, or didn't want to pay the price for a replacement. As to your impedance question: If the OT was meant for a twin (4 tubes), you'd double the speaker impedance to match if used for a 2 tube amp.

[twilightofthedogs]

Yes, if the OT was meant for a twin, you'd half the speaker impedance to match if used for a 2 tube amp.

Thanks! So forgive me, but if this OT is designed for a twin reverb, which wants 4 ohms impedence, then using it in this 2 tube configuration would mean that 2 ohms would be the appropriate load?

[Enzo]

I don't know, but I tend to doubt any of them came from Fender like that. I would expect Fender circuits to be a little more polished than that.

[The Dude]

Thanks! So forgive me, but if this OT is designed for a twin reverb, which wants 4 ohms impedence, then using it in this 2 tube configuration would mean that 2 ohms would be the appropriate load?

I corrected my post. Had a short mental breakdown. Sorry. Actually you'd double the speaker impedance to match- not half. So, in this case, you'd want an 8 ohm speaker.

[twilightofthedogs]

I corrected my post. Had a short mental breakdown. Sorry. Actually you'd double the speaker impedance to match- not half. So, in this case, you'd want an 8 ohm speaker.

Ha. perfect. 8 ohm speakers are way easier to come by than 2 ohms! Thanks. Next task, replacing the baffle. It's like the grill cloth shrank and warped the flimsy particle board baffle. The "drip edge" appears to be made out of glorified laminated tin foil that is completely failing. Can't decided if I should try and find some extruded aluminum, use some sort of silvery marshall style piping, or just leave it off. Otherwise I am going to attempt to extract the existing black grill cloth off the baffle and reapply to a new plywood baffle. There are about 1000 staples.

20210816_211443 by Michael Lovato, on Flickr

20210817_194513 by Michael Lovato, on Flickr

[The Dude]

............just leave it off............

That would be my vote. If it were me, I'd just remove the speaker and logo from the baffle, measure it, toss the whole thing in the bin, and start from scratch. You can get hunks of speaker grill cloth on eBay for reasonable prices. Just measure it and find a piece that will fit. There are lots of styles.

Edit: I see now the speaker doesn't attach to the baffle. It's a separate grill. Even easier!

[Helmholtz]

Make sure there's a decoupling/filter cap from the choke end which feeds the screens to ground.

[twilightofthedogs]

Make sure there's a decoupling/filter cap from the choke end which feeds the screens to ground.

Thanks! It appears that it's connected just like i've drawn it. Do you recommend just adding a filter cap there between the choke and the 470 ohm screen grid resistors. Something like 20uf/500v? Or is there a better way to connect how the voltage gets to the screens? And while I'm asking what problems can the lack of the decoupling cap/filter cap at that location cause? Thanks!

[Helmholtz]

20µ/500V should be fine (except it B+ is higher than 500V). Connect as explained above.

Without the cap the choke should cause heavy screen NFB, which lowers power tube gain and available power.
Might even cause instability.

[Helmholtz]

Do you recommend just adding a filter cap there between the choke and the 470 ohm screen grid resistors?

Yes.

[twilightofthedogs]

Awesome. Thanks. Better approach to use an F&T 20uf 500v cap there and cross my fingers, or (2) 47uf @350v caps in series? If I go that route would I need to create some sort of voltage divider circuit, or will simply having two caps in series suffice to get a better voltage rating?

[Helmholtz]

Awesome. Thanks. Better approach to use an F&T 20uf 500v cap there and cross my fingers, or (2) 47uf @350v caps in series? If I go that route would I need to create some sort of voltage divider circuit, or will simply having two caps in series suffice to get a better voltage rating?

First measure B+ with tubes out. If higher than 500V, use the stacked caps with 220k/2W resistors across each.

[twilightofthedogs]

And just for fun, here's this week's haul. Top left: the 1980 75. Bottom left: 1975 Super Twin. Top right: 1980 Super Reverb. Bottom right: 1978 Super Reverb. That's a lot of late 70s fender spaghetti!

70s Fenders by Michael Lovato, on Flickr

[Leo_Gnardo]

And just for fun, here's this week's haul. Top left: the 1980 75. Bottom left: 1975 Super Twin. Top right: 1980 Super Reverb. Bottom right: 1978 Super Reverb.

Hey you're a glutton for punishment, especially with the Super Twin and not far behind, the 75. I'm diggin' the test instruments built into your workbench's back wall. Are they functional? Way kool whether they are or not.

That's a lot of late 70s fender spaghetti!

To go along with that, you need-a spicey meat-a ball.