Amp Experts: Fender 75 amp - Low power only 15 watts?

I love the 1x12 Fender 75 for its great reverb and superb Fender clean tone (forget the "overdrive" channel). But does the low power setting really take it down to 15 watts? I know that 15 watts can be louder than you might think, but this sure sounds like more than 15!

15 watts can be plenty loud with that amp depending on the speaker you are using. I don't have the schematic handy but I recall that the power drop was done by lowing the voltage on the plates AND changing the bias.

In the later years when Jimmy Thackery played with the Nighthawks he always used a Fender 75 combo with a 15 inch EV speaker and he always had the power switch in the LOW power position. He was plenty loud.

BTW, you need good power tubes (6l6's) and a good speaker for that amp if you use it in full power mode. The plate voltage on the power tubes is high enough to easily consume cheap 6L6's. Fender really pushed it to claim 75 watts with that amp.

-Tim Pruitt

Jimmy is my main man! Very interested to hear that you saw him playing with a Fender 75. Mine is the version with a 12" and it is the original JBL-style Electrovoice that came with the amp. You're correct that this amp would need a good speaker if you want to run it on the high power setting. I'm still amazed at that low power setting, though - plenty loud!

He's the one who recommended the Fender 75 to me back in '81 after I had lost most of my gear in a burglary. I still have the 1x12 Fender 75 to this day, but it now resides in my basement, and no longer sees any gig use, despite the addition of wheels. The speaker magnet is the size of a dinner plate, and the amp weighs just slightly less than the anchor of most battleships. :-\

Anyway, it was always my understanding that the low power switch actually cuts the plate voltage in half, resulting in approximately 37.5 watts (or half of what the amp is actually putting out on full power, anyway). Please feel free to correct me if I'm wrong. :-) CS

I don't have a 75 to measure the plate voltage, all I have is a schematic and the voltage is not indicated. I can tell you that the power switch is a DPDT toggle with one "throw" that lowers the plate voltage and the other "throw" that changes the bias on the output tubes.

Every time I saw Thackery play he used a Fender 75 with a 15"" EV and he was always switching channels to change his sound. He also appeared to have all of the knobs with pull switches pulled to boost the tone controls. He was using the V with Bardens most of the time also. I really dug his tone with that setup. Sometimes he had a Boss CE-2 chorus and near the very end of his Nighthawks days I think he used a Boss digital delay also.

The Nighthawks are still a great band but back in those days when Thackery was in the band the Nighthawks were an outstanding blues band. I get chills thinking about how great those shows were during the Jimmy Thackery era.

-Tim Pruitt

...those Fender 75's can be found used for relatively little $$$. You guys probably already know this stuff but here is some more info on these amps:

I believe Paul Rivera had a hand in the design of that series of amps. I know Rivera was involved in the next series that included the Twin II, Concert (II), Deluxe Reverb II, Princeton Reverb II, etc.

One drawback of the Fender 75 is the shared tone controls between the 2 channels. The next series of amps (Twin II, etc) had a similar design but with separate tone controls for each channel.

All of these amps were Fender's response to the Boogie craze that was going on at the time. They were also the last PTP wired amps that Fender made until recently.

-Tim Pruitt

These are very good posts regarding the Fender 75. I went ahead and removed the stock EV 12" speaker from mine and put in a Jensen RI C12N. The EV is a great speaker, but I really like the Jensens, too, and I don't need quite the level of super clean that the EV delivers; the Jensen is still clear and articulate, and a little warmer - and this speaker swap reduced the weight of the amp nicely, too.
If you skip the "overdrive" attempt that Fender made with this amp and just use it for it's classic Fender clean tones and reverb, and desirable power level and physical size, the Fender 75 is quite a little underrated gem.

I read somewhere that he likes using Deluxe Reverbs so that may explain why he liked the 75 on the low power setting. A Fender 75 on low power is kinda like a Deluxe Reverb, power wise. And for the occasion when he was playing a larger venue or an outdoor gig I imagine he could use the full power setting. Just a guess though.

With a few easy tweaks the clean channel on the Fender 75 can sound even more like a classic BF Fender. And the lead channel can be modified to sound better as well.

-Tim Pruitt

IIRC, the 75 was pre-Rivera, and it was an Ed Jahns design. Volume in the Low Power setting is nearly that of High Power, partially because of the non-linear relationship between power and volume.

In my experience, most of them can benefit from a couple new preamp tubes. They are more sensitive to adjustments than other amps, particularly in the gain channel, but they have some excellent sounds. Most people who understand the amp use the Low Power mode, and frequently they don't use the MV. Basically, set clean and like a BF Fender (no gain, no buttons, etc.), the 15 inch is pretty similar in sound to a BF Vibroverb. The head can sound very Marshally.

It's a good amp.

Tim Wrote: "All of these amps were Fender's response to the Boogie craze that was going on at the time. They were also the last PTP wired amps that Fender made until recently. "

The Rivera amps were the last PTP, HANDWIRED amps from Fender. The modern PTP amps are not handwired but wave-soldered as I understand it.

This thread just gets more and more helpful. What about that "tube matching" adjustment on the back of the Fender 75? What does that really allow you to do (or not do?).
And I'd be fascinated to hear about what the mods are that can make it's clean channel tone be even more like a BF Fender...

Good point about the handwired vs. non-handwired distinction. It is also interesting to point out that the Boogie amps that Fender was trying to keep up with were, and continue to be, PCB amps.

-Tim Pruitt

Tim, that is true about the boogie PCB's. However, one look at a Boogie board will confirm that they use the best board they can, and surely with the older ones the boards use flying leads to pots and jacks. I have a Mark IIB coming in for repairs. I looked at the board the other day, and the only board that comes close to it would be an old Marshall from the mid to late '70's. that said, that boogie will not do what a simple BF Fender will do. Granted, the Fender won't get the gain that the Boogi will, but that Fender clean and clarity is hard to find in a more complex switching, gain-ladened amp.

I would convert the tube matching adjustment to a regular BF style bias adjustment. This is a very common change that is part of most SF to BF amp conversions. Even though the 75 has a black face plate, the circuit has a bunch of SF features and the tube matching adjustment is one of them. It is pretty easy to do and involves switching around some existing parts on the existing bias pot and replacing the 82K and 100K resistors feeding the bias supply to pin 5 on both the output tubes with 220K resistors and then setting the bias. Save the 82K and 100K resistors and use them in #3 below.

After looking at the schematic here are some other changes I would do:
1. Remove the 500pF caps across pin 1 and 3 and across pin 6 and 8 of preamp tube 1, across pin 6 and 8 of preamp tube 2, and across pin 6 and 8 of preamp tube 4. These are there to reduce oscillation but they kill the high end. If the amp squeals without them then put them back in one at a time and see which one kills the squeal and stop there.
2. Change the two 330K resistors in the phase inverter circuit to 1M resistors, like in a BF amp.
3. Change the two 47K plate resistors on the phase inverter tube (tube 5) to 82K and 100K, like in a BF amp.
4. You could change the two 33K resistors on the input jacks to 68K resistors like in most classic Fender amps but it won't make that much of a difference. Two 33K resistors will give you a little more gain.

That's a start. Some people may also recommend replacing the .02 caps between the phase inverter and the output tubes with .1 caps like the BF amps use but the .02 keeps the low end from getting too flabby and farty. You could experiment with different values between .01 and .1 here and see what suites you. I use .05 caps in my Rivera era Fender Concert and I think it sounds pretty good, today anyway.

If you don't know what you are doing then don't try any of this stuff. Voltages in tube amps can kill you.

-Tim Pruitt

I wasn't saying anything bad about Boogie amps. :) I agree with you 100% about the Boogie PCB's and the Fender sound vs Boogie sound. I'm not really as hung up on the PTP vs. PCB thing like some people are, at least as far as sound is concerned. Now repairing or modifying some PCB amps, excuding most Boogies, is another story.

I just think its funny that Fender was trying to compete with the features Boogie was offering (cascading/high gain, master volume, channel switching, small combos) and Fender was still hand wiring their amps PTP while Boogie was using PCB's. One look at the rat's nest of wires inside a TwinII or a Concert from the early 80's and you can really appreciate the "neatness" of Boogie's PCB's.

-Tim Pruitt

And I most definitely would not be the one to get in there and do those circuit changes (I'm just a guitar player!), but I have the great amp tech Bruce Collins here in Colorado, and can request him to do the specific steps that you listed. Before doing any of that, though, I'm just curious what that tube adjustment control on the back of the Fender 75 actually does. Do you put in a new pair of 6L6s and gradually adjust that control based on what you're hearing, or is it something completely different?

Tim. I just love to look at the wiring in my two Super Champs. Something that small that does what it does and is PTP is just marvelous, imo. IMHO, the Rivera era Fenders are the only ones that come close to doing a gain switching thing that is useful. You are right that a good PCB arangement can sound good. I particularly like Rivera amps in that regard. Rivera seems to be able to get closer to that pure Fender BF sound than other switching amps. However, as you say the repair aspect is another can of worms. I just rebuilt a potentiometer for an '88 Twin due to unavailability. Now, that is fun! Now I know what that slanted slot on the side of the switch housing is for. :)

Wally,
I like the Rivera Fenders also. The overdrive channel isn't too terrible as long you keep the bass and midrange controls low and don't go crazy with the gain control. I have better luck thinking of the lead/overdrive channel on those amps as a way to add a little grit to a basic Fender BF sound and I use a pedal for the high gain and big overdriven sounds.

The Rivera Fenders also make a great platform for mods. Some have even made Dumble clones with them since the stock circuit is about half way there already, at least as far as the Concert and Twin II are concerned.

I envy you getting paid to work on this stuff. I just mess with amps as a hobby and for friends and to keep my own equipment functioning.

-Tim Pruitt

I don't have a Fender 75 in front of me but on the other amps from this era the output tube bias "balance" or matching control is located on the bottom of the chassis.

On the back of the amp is a "hum" balance control that adjusts the voltage going to the heaters of each tube. This is NOT the bias balance/tube matching control.

The bias balance/tube matching control on the underside of the chassis balances, or unbalances, the bias going to each tube. This would be useful if you need to compensate for using unmatched power tubes but it does not provide a way of adjusting the overall bias. If you use matched tubes you are probably better off converting the bias balance/tube matching control to a BF style bias adjustment control. The ideal would be to have separate bias adjustments for each output tube or and a bias adjustment and a bias matching control.

As I mentioned in an earlier post converting the bias balance/bias matching control to a bias adjustment is pretty simple.

-Tim Pruitt

The manual on Output Tubes Matching:

"This control makes it unnecessary to use matched power output tubes for optimum performance. Best balance is achieved by setting the MASTER control to 1, and adjusting for minimum hum level. Ordinarily, no adjustment should be required unless the output tubes are changed."

The Hum Balance text says essentially the same thing- after the ground switch has been set for minimum hum, the hum balance can be used to further reduce hum, and does not normally need adjustment unless the output tubes are changed.

To be clear, there are two separate controls on the rear of the chassis.

Not that it's necessarily a big deal, but my 75 has the hum balance to the far right and the tube matching adjustment is next to the reverb pedal jack. I think I'm getting into the category of "useless information" now!

They're all like that. It got switched while rushing the editing.

The 75 is one of my favorites!
The first (good) amp I got was a 15 inch combo.
Sold it twelwe years ago but bought a head in January...
Love them.
Thank you for the mod tips tlpruitt!

The "trick" (for me) with the gain channel is to not use too much gain. I get enough sustain out of it with the gain control on 3! Would be nice with a special lead EQ though...

pEr

To answer the opening post, to switch the plate voltage to half its value does not halve the power. A quarter it's more like.

It's P = U² / R so half voltage equals a quarter the power, as ballpark value (ignoring other parameters involved).

Still I suspect the "Low power" of this amp to be more than 15W, I sold mine but remember it was plenty loud!