Pro Reverb Mod Ideas

[foreverstrung]

New member here. Specifically joined this forum to get some suggestions on a new project. 
To qualify, the last few years I've built about a dozen diff amps ranging from 18w to 50w.
Currently I've picked up a 1979 70w Pro Reverb that appears to have been dormant for quite awhile. Rusty chassis straps, dank grill cloth and it appears to have all of the original tubes. I don't want the 70w vanilla PA type sound we get from this amp. I'm looking for ideas to modify. I considered gutting the thing and building a 6G16 amp. I believe I can use the same PT and OT, chassis and cabinet. Or I could try to bring this 70w PA sounding amp to the 40w early 70's version with maybe more breakup at a lower volume.
I'm getting ready to disassemble it. Clean up the hardware. Grill cloth and tolex. I could just restore it as it is and resale it. I paid $600 for it and reverb and tremolo work great. Speakers are great. I really would like more of a project then just restoring.
Appreciate the feedback

[nauta]

Hey man

Thats a cool amp just a Twin with half the power but still with a huge chassis.

You didn't say what kind of sound you want? KOC has a mod for Fender amps like this that makes one channel an overdrive that you can make as mild or wild as you want  you still have a fender clean sound but both have reverb and trem. You use one jack and a panle or footswitch so your not just moving a cord from one input to the other.

peace

[K O'Connor]

Hi foreverstrung

Your amp has an ultralinear output stage if it is 70W. Many players did not like the sound of that, but the problem was not really the UL connection; rather, it is that in combination with Fender's low-value feedback loop.

Fender copied hifi circuits, as did every other early amp builder, and the goal was simply to have a portable PA for the instrument. All those inputs were meant to accommodate more than one player using the same amp. Times changed; player requirements changed; the amps stayed the same... for a while. The hifi motif imposes many restrictions and can make the sound a bit dull. Thankfully the preamps have free-running gain stages and in this amp there is reverb to liven things up.

As TUTs mention, and I have repeated in other places including here, the low-value loop is not a good choice for a musical instrument amp. Fender used 820R for series and 100R for shunt in most of its amps. Increasing the values by ten opens up the sound while retaining the same gain. That's the fist mod I would do.

Other things are pretty standard reliability improvements: Change the screen-stops to 1k-5W wire-wounds or metal-oxide as long as they are flame-proof; change the 1k5 grid-stops to metal-film for better reliability; add 1R-1W metal-oxide cathode current-sense resistors for safe idle monitoring - add meter pin-tip jacks to make this easy.

The FPM kit on our site updates the used and unused channels into a Fender clean plus any distortion you desire. Use ERK or a relay for channel switching.

If you intend to try any power tube swaps, make sure their heater current does not exceed 900mA, like the stock 6L6s. Many other types have heavier heaters and you might want to add an auxiliary PT just to power the output tubes. If you do this, you will have to hang the grid-stop off pin-5, then mount the 1R-1W between pin-1 and pin-8, grounding pin-1. This allows EL-34 and 6CA7 to work and gives a handy support for the current-sense resistors.

As with most amps, grounding is atrocious and should be rewired according to the Galactic Ground Method in TUT3. As a low-gain amp, these Fenders just had to be quieter than Fender's single-coil guitars. For modern players and if any mods are undertaken, noise needs to be reduced.

Have fun

[foreverstrung]

Hi foreverstrung

Your amp has an ultralinear output stage if it is 70W. Many players did not like the sound of that, but the problem was not really the UL connection; rather, it is that in combination with Fender's low-value feedback loop.

Fender copied hifi circuits, as did every other early amp builder, and the goal was simply to have a portable PA for the instrument. All those inputs were meant to accommodate more than one player using the same amp. Times changed; player requirements changed; the amps stayed the same... for a while. The hifi motif imposes many restrictions and can make the sound a bit dull. Thankfully the preamps have free-running gain stages and in this amp there is reverb to liven things up.

As TUTs mention, and I have repeated in other places including here, the low-value loop is not a good choice for a musical instrument amp. Fender used 820R for series and 100R for shunt in most of its amps. Increasing the values by ten opens up the sound while retaining the same gain. That's the fist mod I would do.

Other things are pretty standard reliability improvements: Change the screen-stops to 1k-5W wire-wounds or metal-oxide as long as they are flame-proof; change the 1k5 grid-stops to metal-film for better reliability; add 1R-1W metal-oxide cathode current-sense resistors for safe idle monitoring - add meter pin-tip jacks to make this easy.

The FPM kit on our site updates the used and unused channels into a Fender clean plus any distortion you desire. Use ERK or a relay for channel switching.

If you intend to try any power tube swaps, make sure their heater current does not exceed 900mA, like the stock 6L6s. Many other types have heavier heaters and you might want to add an auxiliary PT just to power the output tubes. If you do this, you will have to hang the grid-stop off pin-5, then mount the 1R-1W between pin-1 and pin-8, grounding pin-1. This allows EL-34 and 6CA7 to work and gives a handy support for the current-sense resistors.

As with most amps, grounding is atrocious and should be rewired according to the Galactic Ground Method in TUT3. As a low-gain amp, these Fenders just had to be quieter than Fender's single-coil guitars. For modern players and if any mods are undertaken, noise needs to be reduced.

Have fun

Good stuff. Sounds like I found the right spot. This is exactly what I was looking for
Recently I picked up a 70's Twin Reverb cab and looked around for what to put inside. I didn't really want a Twin, so I bought a blank chassis and built a 6Gi6 conversion design by a guy in Belgium. That was so much fun, but the sound, just bliss. A tad noisy, but that was likely a builder flaw, not the designer tho I know those Vibroverbs were known for being noisy.
So that kind of "chiming grit" I'm looking for. These mods you've suggested might just be the avenue I pursue.
Thx
Dave

[foreverstrung]

As TUTs mention, and I have repeated in other places including here, the low-value loop is not a good choice for a musical instrument amp. Fender used 820R for series and 100R for shunt in most of its amps. Increasing the values by ten opens up the sound while retaining the same gain. That's the fist mod I would do.

Other things are pretty standard reliability improvements: Change the screen-stops to 1k-5W wire-wounds or metal-oxide as long as they are flame-proof; change the 1k5 grid-stops to metal-film for better reliability; add 1R-1W metal-oxide cathode current-sense resistors for safe idle monitoring - add meter pin-tip jacks to make this easy.

The FPM kit on our site updates the used and unused channels into a Fender clean plus any distortion you desire. Use ERK or a relay for channel switching.

If you intend to try any power tube swaps, make sure their heater current does not exceed 900mA, like the stock 6L6s. Many other types have heavier heaters and you might want to add an auxiliary PT just to power the output tubes. If you do this, you will have to hang the grid-stop off pin-5, then mount the 1R-1W between pin-1 and pin-8, grounding pin-1. This allows EL-34 and 6CA7 to work and gives a handy support for the current-sense resistors.

Where can I get this FPM kit? I searched a bit but came up short.
Also, some of these mods, any chance I can find some sketches or schematics of these upgrades? Including the power tubes. Right now there are a an old set of Svetlana tubes installed. There vintage. IDK how strong they are. Probably good, but I've got a nice matching pair of RCA's I'd like to consider if these existing tubes end up coming up short
I can print out and refer to these mods you've noted, but I'd really be grateful to have a drawing to refer to. If possible.
Thx
Dave

[K O'Connor]

Hi Guys

To find the kits and books, go to the London Power site linked from this forum
( https://londonpower.com ), scroll to Amp Mods and you will find FPM, ERK and BMK. The product descriptions are fairly extensive. If you are building amps - and want to get rid of the noise of the kits you have built - then get TUT3 The Ultimate Tone vol.3, which explains how to layout and wire amps for lowest-noise and best note articulation. There are schematics and layout drawings and all the projects have the bias adjusts and monitoring all amps should have.

FPM is designed specifically for 2-ch Fender amps of the era yours is.

ERK is the switching to match the new circuit arrangement.

BMK is the bias mod kit with meter jacks, pots, current-sense resistors.

If you were interested in getting the cranked sound of the amp at lower loudness, then SV1+RBX would be required. Power Scaling lengthens tube life as they only produce the power needed. This is the most complex mod techs or hobbyists might undertake.

DO NOT discard your tubes. They are on their tone plateau and will not change for many decades - only mechanical upset will harm them. This is discussed many places on this forum and in our books. Anyone encouraging you to change tubes every six months does not know anything about tubes and/or wants to sell you tubes you do not need. There is a thread regarding power tube tone.
https://theultimatetone.com/thread-345.html

Best to peruse the forum and read what is here - there is a tonne of information for individual projects, say, that pertains to much wider application. Similarly, read the full product descriptions for the books and kits - especially TUT3 - as you can learn things just from those.

[foreverstrung]

Hi Guys

To find the kits and books, go to the London Power site linked from this forum
( https://londonpower.com ), scroll to Amp Mods and you will find FPM, ERK and BMK. The product descriptions are fairly extensive. If you are building amps - and want to get rid of the noise of the kits you have built - then get TUT3 The Ultimate Tone vol.3, which explains how to layout and wire amps for lowest-noise and best note articulation. There are schematics and layout drawings and all the projects have the bias adjusts and monitoring all amps should have.

FPM is designed specifically for 2-ch Fender amps of the era yours is.

ERK is the switching to match the new circuit arrangement.

BMK is the bias mod kit with meter jacks, pots, current-sense resistors.

If you were interested in getting the cranked sound of the amp at lower loudness, then SV1+RBX would be required. Power Scaling lengthens tube life as they only produce the power needed. This is the most complex mod techs or hobbyists might undertake.

DO NOT discard your tubes. They are on their tone plateau and will not change for many decades - only mechanical upset will harm them. This is discussed many places on this forum and in our books. Anyone encouraging you to change tubes every six months does not know anything about tubes and/or wants to sell you tubes you do not need. There is a thread regarding power tube tone.
https://theultimatetone.com/thread-345.html

Best to peruse the forum and read what is here - there is a tonne of information for individual projects, say, that pertains to much wider application. Similarly, read the full product descriptions for the books and kits - especially TUT3 - as you can learn things just from those.

Good stuff
Thx

[foreverstrung]

So I ran into a shipping issue ordering and their working on it, so perhaps tomorrow I can get the parts I need out the door. I do have in my shopping cart the SV1+RBX and the FPM and ERK. I didn't add the BMK tho I think I want to. I def want to have easier simpler access to set the bias. Is this BMK a good fit for this amp? There is not a lot of extra real estate in this chassis.
I started disassembling everything and I'm really happy with the condition of this amp. The chassis tho their is pretty much no room to spare. Am I going to have room for these PCB boards? I can only guess their sizes. I guess I'll see. I'm sure I can make room. 
Last question, for now, after these mod/upgrades, will these stock speakers still be preferred? What other speakers would pair nicely with this amp now with these changes?
Thx
Dave

[foreverstrung]

Got a little snag. I started really messing with the inside of this today and found my power tubes getting way too hot. My plate voltage is 500 DCV and using a bias meter i'm getting 70/74 mA when I dial the balance trim pot on the back of the chassis to the lowest setting. (when I turn it in the opposite direction, one tube will go down into the 60's, but the other will get up to 80 mA). One tube was red plating. I did not leave it on, of course. I purchased the BMK bias upgrade for this amp, but while I wait, any suggestions to bring this bias down? I'm getting -58.4 at pin 5.

[K O'Connor]

Hi Guys

The control on the rear panel is called "Output Tubes Matching" and just alters the balance of current NOT the median value of current. There is a 3k3 to ground off the wiper of this control. You could change this to 4k7, say, (which might be too cold), or lift one end and add a 10k pot in series with it to give actual variable bias.

Good luck

[K O'Connor]

Hi again

Speakers are expensive, so it is best to voice the amp for what you have. There is a wide range of tones possible with the mods. Install each mod one at a time and see what it does. Evaluate the stock values of the kits and then consider in what direction you need to move the tone?

You will have to move something to have everything fit in. Like every other manufacturer, Fender's circuits fill the whole chassis despite their simplicity. It i rare to find a lot of open space in a guitar amp chassis.

Have fun

[foreverstrung]

Hi Guys

The control on the rear panel is called "Output Tubes Matching" and just alters the balance of current NOT the median value of current. There is a 3k3 to ground off the wiper of this control. You could change this to 4k7, say, (which might be too cold), or lift one end and add a 10k pot in series with it to give actual variable bias.

Good luck

t's actually a 33.4k resistor. Still a 10k pot in series will work as a variable bias? 
Thx
Dave

[foreverstrung]

I started with a 10k pot in series and it wasn't enough so I bumped it up to a 25k pot and it got close but not enough so I changed out the power tubes with a matched pair of NOS RCA tubes and it got to a perfect spot. Plate voltage at 510 DCV with the bias set to 40.5 mA. I can live with that for now until the BMK kit arrives.
Thx
Dave

[makinrose]

Hi Guys

To find the kits and books, go to the London Power site linked from this forum
( https://londonpower.com ), scroll to Amp Mods and you will find FPM, ERK and BMK. The product descriptions are fairly extensive. If you are building amps - and want to get rid of the noise of the kits you have built - then get TUT3 The Ultimate Tone vol.3, which explains how to layout and wire amps for lowest-noise and best note articulation. There are schematics and layout drawings and all the projects have the bias adjusts and monitoring all amps should have.

FPM is designed specifically for 2-ch Fender amps of the era yours is.

ERK is the switching to match the new circuit arrangement.

BMK is the bias mod kit with meter jacks, pots, current-sense resistors.

If you were interested in getting the cranked sound of the amp at lower loudness, then SV1+RBX would be required. Power Scaling lengthens tube life as they only produce the power needed. This is the most complex mod techs or hobbyists might undertake.

DO NOT discard your tubes. They are on their tone plateau and will not change for many decades - only mechanical upset will harm them. This is discussed many places on this forum and in our books. Anyone encouraging you to change tubes every six months does not know anything about tubes and/or wants to sell you tubes you do not need. There is a thread regarding power tube tone.
https://theultimatetone.com/thread-345.html

Best to peruse the forum and read what is here - there is a tonne of information for individual projects, say, that pertains to much wider application. Similarly, read the full product descriptions for the books and kits - especially TUT3 - as you can learn things just from those.

Good stuff
Thx

Cool amp!  I work on lots of vintage Fender and the speaker vary quite a bit since they used lots of manufacturers.  I'd be interested to see what is in there---I've play with most of the speakers and might be able to give you some insight.

[foreverstrung]

Hi Guys

To find the kits and books, go to the London Power site linked from this forum
( https://londonpower.com ), scroll to Amp Mods and you will find FPM, ERK and BMK. The product descriptions are fairly extensive. If you are building amps - and want to get rid of the noise of the kits you have built - then get TUT3 The Ultimate Tone vol.3, which explains how to layout and wire amps for lowest-noise and best note articulation. There are schematics and layout drawings and all the projects have the bias adjusts and monitoring all amps should have.

FPM is designed specifically for 2-ch Fender amps of the era yours is.

ERK is the switching to match the new circuit arrangement.

BMK is the bias mod kit with meter jacks, pots, current-sense resistors.

If you were interested in getting the cranked sound of the amp at lower loudness, then SV1+RBX would be required. Power Scaling lengthens tube life as they only produce the power needed. This is the most complex mod techs or hobbyists might undertake.

DO NOT discard your tubes. They are on their tone plateau and will not change for many decades - only mechanical upset will harm them. This is discussed many places on this forum and in our books. Anyone encouraging you to change tubes every six months does not know anything about tubes and/or wants to sell you tubes you do not need. There is a thread regarding power tube tone.
https://theultimatetone.com/thread-345.html

Best to peruse the forum and read what is here - there is a tonne of information for individual projects, say, that pertains to much wider application. Similarly, read the full product descriptions for the books and kits - especially TUT3 - as you can learn things just from those.

Good stuff
Thx

Cool amp!  I work on lots of vintage Fender and the speaker vary quite a bit since they used lots of manufacturers.  I'd be interested to see what is in there---I've play with most of the speakers and might be able to give you some insight.

Look to be 12" 1980 CTS ceramic. I'm going to pull them and list them. I picked up a pair of mid 70's Alnico Eminence speakers I'm going to put in there and see how they sound once I get things together. Waiting on a few ordered parts right now.....I also have a set of Jupiter 12LC's I can put in there too. I know some like these CTS's, I'm not a big fan.