Fender Twin Reverb 1976 Silverface "Restoration in progress”

[Avraxas]

Hello all!

I am based in EU (220v volts) and got a Fender Twin Reverb Silverface from 1976 that it was in storage from more than 15 years. It is the one with push pull Master Volume. From other topic members suggest me to build a current limiter bulb circuit to power it up through it as I don’t know the story of it.

So, I am planning this weekend to build a current limiter circuit, but till then I have already pull the chassis out for inspection.

From other topic we have already spot a screen resistor broken in half in one power tube and a replaced one more screen resistor in other power tube.

Here some photos from the chassis:

Some photos:



















Here is the broken screen resistor for 1st power tube and the replaced one from 4th power tube:

[





What I am planning before I do anything else is to replace the broken screen resistor and give it a power through the current limiter circuit to see at least if it’s working, or if there are more problems with this amp.

My goal is to replace all the electronic caps and anything "must to replace" part that is crucial.

I have tools. I am good in soldering, I know about safety, but I am not familiar with them.



I will need your help in my progress please.

So, I leave this here for questions I may have. :)

I start from the first question: What circuit is that, and do we have any schematics please?

Have a great day all!

[AlNewman]

https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf

This should be the schematic for a 76 Twin Reverb?
Later versions would be ultralinear.

[pdf64]

It's the 100W version, not the 135W (which is usually a good thing), so these 100W pull boost schematics should be fairly close https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf
A full restoration and rebuild would be nice, but is hopefully not essential to getting it working ok.

Why do you think it's from 1976? I'm used to the cardboard cased ecaps in the doghouse, and white plastic cased ecaps on the boards, being superseded by the mid 70s 
eg what are the date codes on the pots / caps / transformers?

[Avraxas]

Thank you both!

Here are some photos with date codes:

[pdf64]

Thanks, '76 it is then :)

[tubeswell]

There should be another date code stamped on the outside rear of the chassis.


It’s a 100W MV model. I had an identical Quad Reverb at one stage. Those white Mallory caps will probably be suspect (esp the bias supply). At least one of the main filter caps is bulging a bit.

[Avraxas]

I just replace the broken screen resistor today with same one that I had in my stock.

I test it on in tolerance and ohms. 510 ohm



Well, after I power up the amp (with current limiter) I planning to replace all screen resistors and any other in crucial place resistor with new ones.

Thinking about Vishay PR series. What is your opinion? Can this resistors placed anywhere in the amp?

https://www.vishay.com/docs/28729/pr010203.pdf

And about screen resistors. The original ones are 1W, but I read that better go to 3W. Should I go for 3W?

Have a great day!

[Rontone]

Yeah, 3W would be a good option, and should be around the same size with modern metal film too,

Any good brand will do, here is a forum topic with some screen grid resistor information from Amp Garage https://ampgarage.com/forum/viewtopic.php?t=20807

Here's a topic from this forum, going into more detail, has some good in-depth information on both screen grid resistors and 'grid stoppers', that is worth a read https://el34world.com/Forum/index.php?topic=21489.0

[Rontone]

Also, when you do come to changing the screen resistors later, think about the grid stoppers as they are mounted underneath across the socket, they do receive quite a bit of heat over time and are carbon composition resistors, so a change to metal film may be good

[Avraxas]

I do that. Thank you very much

[pdf64]

... I planning to replace all screen resistors and any other in crucial place resistor with new ones.

I suggest to replace all the 470R and 1k5 resistors fitted to the output valve socket lugs.
Allow some 'give' in the leads, to accommodate thermal expansion, lug movement when valves are fitted and removed, ie don't fit ghem tight between the socket lugs. eg add a slight dogleg bend to the lead.
Allow some clearance between screen grid resistor body and nearby wiring - they can get extremely hot.

It's extremely beneficial to add 1R 1W cathode current sensing resistors to each lug 8.

It also seems a good idea to replace all rectifier diodes; modern diodes are made much better than they were back then.

Also replace all electrolytic caps.

Thinking about Vishay PR series. What is your opinion?

They look great for power applications.

Can this resistors placed anywhere in the amp?

They're not intended for applications in the signal path, so low noise types would probably be preferable eg on the main board etc.

And about screen resistors. The original ones are 1W, but I read that better go to 3W. Should I go for 3W?

I think it's beneficial to uprate them if you're also adding a HT fuse, eg at the HT rectifier output. It seems a bad idea if you're not going to do that.
Consider that 1W here can last for many decades, provided the valve doesn't short.
But with a 3W resistor and no HT fuse, a valve short might possibly wreck a transformer.

[Rontone]

But with a 3W resistor and no HT fuse, a valve short might possibly wreck a transformer.

That's a good point, I had not noticed there is only a primary fuse on these amps, so bigger is not always better on screen resistors!

[Avraxas]

They're not intended for applications in the signal path, so low noise types would probably be preferable eg on the main board etc.

I thought that metal film resistors like the Vishay I post that are low noise resistors.

Sorry I am brand new in amps. :)

So what type of resistors we put in signal path?

[passaloutre]

I use those PR resistors for everything these days. They will be just fine in your signal paths.

I love these 70s Twin Reverbs, I've had a few, and a few more on my bench. They are great sounding workhorse amps, remarkably plentiful, and offer lots of value compared to similarly priced modern amplifiers. I usually replace all the eletrolytic caps, screen resistors, any other suspect resistors, and rewire the bias circuit to the earlier 60s circuit. You should get at least another 50 years out of this one.

[pdf64]

They're not intended for applications in the signal path, so low noise types would probably be preferable eg on the main board etc.

I thought that metal film resistors like the Vishay I post that are low noise resistors.

Sorry I am brand new in amps. :)

So what type of resistors we put in signal path?

The Vishay info you linked (thanks!) doesn't note any noise date, and doesn't claim it's intended for use in low noise applications, rather power applications.
If a parameter isn't specified, then it could be anything, so it shouldn't be assumed that the PRs will always have low noise. Though obviously passaloutre's good experience with them should be taken into account. 
Personally I'm not fussy about particular brands, rather I find the minimum number of suppliers to get the parts needed, check the specifications of what those suppliers have got, and buy whatever seems good value for money. 100 packs of good quality MF resistors often work quite cheap, perhaps the same cost as 10 or 15 high end 'super duper' types that eg Dumble clone builders prefer.
So I guess that whichever supplier you're looking to use for the PR will also sell a low noise MF product line, so that's the first place to check.

The key parameters to check for in resistor product info are power rating, voltage rating, noise, flammability. 

If in doubt, ask! 

[Avraxas]

Thank you.

Can you please provide me some EU shops that I can buy parts from?

I know only Banzai Music. I am already customer there but if there is more shops i would like to know.

[Rontone]

https://gr.mouser.com/

https://www.digikey.gr/

https://gr.rsdelivers.com/

https://grobotronics.com/ This also has some useful parts and hardware

[pdf64]

For valve guitar amp specific stuff, Tubetown are great, I think they carry pretty much everything you might need for this https://www.tubetown.net/
Also Modulus in the UK is also highly recommended https://modulusamplification.com/
Tube amp doctor have a wide range of good stuff, some that (outside of USA) can't be found elsewhere, but they can be very expensive https://www.tubeampdoctor.com/en/
Beyond that, yes, manufacturer approved distributors such as RS, Mouser, Digikey, Farnell will tend to carry a much wider range of general electronics parts.
Farnell have a retail operation CPC, dunno if that's UK specific, a much reduced range, but they only require a fairly small order value for free shipping.

[Esquirefreak]

Another vote for Tubetown. Great service, good prices and fast delivery 👍

/Max

[Avraxas]

Hello all! :)

Indeed, Tubetown it’s very good site, but I am a bit confused.

Tubetown have better prices in same product (like F+T caps). Much better, but don’t have all the parts. For example what I don’t like is that the axial electrolytic capacitors like 22uf/(35v) are unbranded. This doesn’t mean that are not good but still is something I need to mention. Also Tubetown doesn’t have 3W resistors that I need for replace the screen resistors. But it’s way cheaper than Banzai shop.

From the other hand Banzai have everything I need. Many brands of capacitors, Vishay 3W resistors.

Also I need to say that I want to buy more than what I need to have stock in my place.

I want to buy from Tubetown for better prices overall, but I am afraid that that’s two things I wrote before (no resistors 3W and the unbranded capacitors), holding back….

[pdf64]

... Also Tubetown doesn’t have 3W resistors that I need for replace the screen resistors. …

The stock rating is 1W?

[Avraxas]

Yep. The stock is 1W but many recommended 3W, with HY fuse add.

Well,  I am beginner in tube amps restore, for my amps and only of course and as I am new I am also confused. :)

[Platefire]

This is what AI says:>)
While screen resistors can offer some overcurrent protection, they are not designed to be used as fuses and should not be relied upon as such. A fuse is specifically designed to break the circuit when a certain current threshold is exceeded, protecting the rest of the circuit from damage. Screen resistors, on the other hand, are primarily used to limit current flow and voltage drop, and while they may offer some protection against overcurrent, their primary function is different. While screen resistors can offer some overcurrent protection, they are not designed to be used as fuses and should not be relied upon as such. A fuse is specifically designed to break the circuit when a certain current threshold is exceeded, protecting the rest of the circuit from damage. Screen resistors, on the other hand, are primarily used to limit current flow and voltage drop, and while they may offer some protection against overcurrent, their primary function is different.

[Avraxas]

This is what AI says:>)
While screen resistors can offer some overcurrent protection, they are not designed to be used as fuses and should not be relied upon as such. A fuse is specifically designed to break the circuit when a certain current threshold is exceeded, protecting the rest of the circuit from damage. Screen resistors, on the other hand, are primarily used to limit current flow and voltage drop, and while they may offer some protection against overcurrent, their primary function is different. While screen resistors can offer some overcurrent protection, they are not designed to be used as fuses and should not be relied upon as such. A fuse is specifically designed to break the circuit when a certain current threshold is exceeded, protecting the rest of the circuit from damage. Screen resistors, on the other hand, are primarily used to limit current flow and voltage drop, and while they may offer some protection against overcurrent, their primary function is different.

No no, you miss understand. Maybe is my low English not you, sorry. I am talking about putting 3W screen resistors instead of 1W.

I ask here about that and if you read the replay#8, member told me that if you want to do that (that is an upgrade ) you need also to use/put/add a HT fuse at the HT rectifier output.

[Platefire]

Well even though I've been amp building for 25 years but still very much still learning. I have read such discussions before about holding the wattage rating down to minimum so it will act as a fuse. So this got me wondering what is correct on determining ratings of screen resistors. No reflection on you at all---I'm just wondering what is correct practice on this?

BTW I've been following your thread and welcome to the forum. Please post some pictures of the
the rest of your amp if possible. Talking about the cab and such.

[Avraxas]

Thank you for the welcome!

Sure!

Here you are!

[Avraxas]

Well even though I've been amp building for 25 years but still very much still learning. I have read such discussions before about holding the wattage rating down to minimum so it will act as a fuse. So this got me wondering what is correct on determining ratings of screen resistors. No reflection on you at all---I'm just wondering what is correct practice on this?

BTW I've been following your thread and welcome to the forum. Please post some pictures of the
the rest of your amp if possible. Talking about the cab and such.

The truth is that I want to keep as much the originality, but if there are parts the are "must" to be replaced to make this amp work for many years I am up 100% for that.

I am not up for replace parts just for the replace process.

Till now I know that "the must" to replace is for sure to replace ALL the electrolytic capacitor, the screen resistors, maybe the grid stopper resistors and inspect also all resistors.

I read about rewire bias circuit. I don’t know if it is a “MUST". If it is I will do it. If it is not, I want to keep it like that.

So I am about to click the order, but I am not sure what I need…and if I need  :D

It’s not about money. Not at all! It’s about keeping things as much in original state as possible. With the main goal of course to have a safe and healthy amp after that.

I know that I am fresh new on that, and with no good knowledge. Or I need to lock the amp for one year and start reading before do anything, or I can get help from you all and slowly slowly make this beauty sing again. :)

[Platefire]

Your starting out on a pretty complex circuit. I started on a simple 5F1 champ. Thanks for posting the extra pictures. Looks like your have a pretty well preserved amp that don't look like it's been tampered with very much. I would recommend you only doing whats necessary toget the amp up and running performing like it was designed to work. Then after it's working as it should, then decide what you want to change.One big thing in your learning process you need to get the schematic(if you haven't already) and study it. So if you don't know how to read a schematic, you need to learn---very important. In changing out your parts, I didn't see you mention coupling caps? Those are those big black capacitors on your board. Those block your DCV and allow ACV to pass. If any of those are leaking(letting DCV Pass) DCV, they need to be replaced.

On the bias you probably have a bias balancing circuit, I haven't looked at your schematic, so I'm not for sure. Most people like to modify the balancing circuit into a bias adjustment circuit which is a pretty easy mod. Also on bias, another popular mod is installing a precision 1 Ohm resistor cathode to ground on your power tubes that allows you to read mA as mV on your multi-meter that will help you precisely adjust your bias once you've modified you bias balance circuit into bias adjust circuit, if you should decide to do that. I think Leo Fender had his people adjust the bias by reading/adjusting the negative bias voltage to a per-determined bias voltage on every like amp. The modified circuit allows you to fine tune your bias to how you want your amp to respond/sound based on your tube voltage readings and doing the math.

[Avraxas]

The only experience I had last year it was on my Silvertone 1331 amp.

I replace almost everything.

[Platefire]

I've always liked that model Silvertone the way the front of cab is designed. Your work looks nice on the circuit, how does it sound? I have a coupleof Silvertone 1482's. One all stock, one modded.
Your 76 twin makes me think of my 74 Pro Reverb except mine is not complete and original like yours. On mine someone cut the combo cab down into a head cab plus installed snake skin tolex. I call it my Sneak Head Pro:>)--an odd one indeed!

[Avraxas]

Nice beauty’s you have!

Silvertone is great one! I have also a pro reverb from 1978. Same thing as twin reverb. It’s next in line after I finish with twin. If I make it! I hope I will! :)

[Platefire]

Yeah if you like fooling with amps like all of us do here, you'll find a way to make it work. Amp building and repairing is kind of like the guitarbug, once you get it, hard to get rid of:>)

[Avraxas]

Ok I read some stuff today. Can you confirm the following please?

For Screen resistors I go for 2W Metal Oxide ones.

For Grid stopper resistors I go for 1W Metal Oxide ones.

Overall in eyelet if I find any damaged, I go for 0,5W Metal Film ones.

I recognise some on eyelet that are 1W carbon (100K). If I find any damaged can I go for 1W Metal oxides there?

[Platefire]

Since nobody has responded to this, I will give it a shot.

Regarding resistor wattage ratings, you could install as high wattage as you want to and I don't think it would effect the operation of the amp as long as the desired resistance is used. You yourself have to determine where wattage ratings in abundance is overkill or not needed.
 
As a base line I use the old mid sixties Blackface AB763 schematic as a guide to keep things reasonable. That schematic is off of Doug.s Misc schematics here:

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_aa763_schem.pdf


First regarding the wattage ratings and type resistor used at the time of this schematic in Approx 1965, a note on the schematic says "All resistors are 1/2 (.5) watt unless otherwise noted". So if you look at the schematic you'll see almost all the resistors with the exception of screen resistor is 1/2 watt. Also all the resistors were carbon composition(CC) resistors. So in spite of this being mighty dated by today's standards, these amps performed and sounded great for years and years. Well for me the exceptions to the schematic referenced is this:

1-On new builds I do not use CC resistors. Even repairing an old amp I wouldn't use them only if for some reason I wanted to preserve the originality as much as possible

2-I feel perfectly comfortable using .5 watt metal film resistors replacing all the designated 1/2 Watt CC resistors with a few exceptions as follows in item #3.

3-The preamp plate resistors, I might raise to 1 watt. The power rail resistors 2 watt minimum Metal Oxide and screens 2 watt minimum Metal Oxide.

I'm going from memory on this because my building new amps is pretty much finished last few years and my memory might be faulty. So I stand to be corrected. I just now mostly repair my existing amps and keep them going. So I hope this may help! Platefire 

[Avraxas]

Thank you!

[Avraxas]

Maybe that’s why 2 screen resistors was cracked and one had replaced in the past.

The speakers wiring is 16 ohm…

Crossing fingers for no more surprises…



I made the order from Tube-Town.

Instead of 5uf cap I buy 4.7uf and instead 25uf I order 22uf. And 2W screen resistors

[pdf64]

Maybe that’s why 2 screen resistors was cracked and one had replaced in the past.

The speakers wiring is 16 ohm…

Crossing fingers for no more surprises…

Your amp is intended to have a 4 ohm load. At higher power output levels, a 16 ohm load will greatly increase the current drawn by the screen grids (hence the damage to the screen grid resistors), and will significantly reduce the amp's power output. And will alter the amp's tone and response.
Assuming those speakers are 8 ohms each, I suggest that they're rewired to be in parallel, to result in a 4 ohm load to the amp.

I made the order from Tube-Town.

Instead of 5uf cap I buy 4.7uf and instead 25uf I order 22uf. And 2W screen resistors

The values of electronic components are nominal, the actual value might be up to +/-20% different.
Hence for our purposes, 4.7=5, 22=25

[Avraxas]

Yes, they are 8 ohm and yes I will bring it back in parallel wiring (4 ohm)

[pdf64]

... And 2W screen resistors

Are you also adding a HT fuse?
The rear panel hole for ground switch could be repurposed for this (I assume that the amp will be used with grouded power outlets, or power outlets that are protected by a safety trip device).

[Avraxas]

What ground switch?

Yes there is 3 prong chord already installed from previous owners. You can see it photos from circuit.

If you tell me were to put the HT fuse (show me better), what type of fuse should I put and how many Amps… I will sure do it. :)

I will add also new 1W grid stopper resistors.

[pdf64]

What ground switch?

Oh yeah, it wasn't included for the export version of your model revision.
I think there will be still a hole in the chassis for it, to the left behind the '50/60Hz' note on the rear panel plate.
So it would be easy to drill a suitable hole for a fuseholder through the thin aluminium rear panel.

If you tell me were to put the HT fuse (show me better), what type of fuse should I put and how many Amps… I will sure do it. :)

I will add also new 1W grid stopper resistors.

A good place in the circuit for a HT fuse would be between the rectifier output and the rest of the amp.

So remove the wire linking the eyelet on the rectifier board to the eyelet on the main board, and wire in the fuseholder between them. Rectifier output to the end lug on the fuseholder, main board eyelet to the side lug.
Fit a T time delay fuse, somewhere in value between 500mA and 1A. 630mA and 750mA are typical values that may be ok.

[Avraxas]

Thank you very much Sir!

[Avraxas]

Just arrived!



I am not very familiar with this "Suntan" brand, that the small capacitors are… but they measured and they are with less than 2% deviation.

[passaloutre]

I wouldn't worry much about the brand on those small cathode bypass caps. They're probably all made in one factory with a 100 different names stamped on them. They will suit your purposes just fine.

[Avraxas]

I am so slow! No time at all! I work so many hours…

Just finished my "COHIBA" current limiter! 

Just the case. Tomorrow the wiring…I hope!

[Willabe]

What's the wattage on that bulb?

A 100w bulb will work well with that 4 x 6L6 amp, no larger than 100w. Lower wattage gives more protection.

[Avraxas]

It is 100W :)

[Avraxas]

Done.





Now it’s amp time!

[Avraxas]

Project started!

[Willabe]

You don't want any carbon comp R's in the power supply, they can catch fire if a power supply ecap shorts or something else shorts and causes massive current draw from the power supply.

Use flame proof metal oxide power R's for all the power supply R's. Same for the power tube screen grid R's too.