Twin reverb 1974 reserection

[Auke Jolman]

From a friend I got a 1974 Twin Reverb (Silverface), which has been very badly treated over the years. My primary concern is getting the amp to sound good again.

When I plugged it in, the amp distorts almost imediatly. The volume is also very low.

Does anybody have some pointers for me where to look first to fix this problem.

My goal here is to get the amp in perfectly working order again and in the proces blackface the amp.

Auke

[eleventeen]

Your best bet is to follow the procedure laid out in Doug's excellent section here:


http://el34world.com/schematics.htm#Servicing fender amps


It's a general, generic guide. Rather than forum members throwing 27 different ideas at you, it's much more efficient for you to go through the guide, note where you have questions and ask them here.


Because nobody here knows how much you know about Fender amps and how much test gear you have, know how to use, and how much experience you may or may not have. Many of us here have worked on dozens of Fenders and can suggest plenty of things to do/check...but having done that 25 times with new folks who come on the forum....there is always an extensive interview process.

[Platefire]

The fact that it works probably means that your transformers are good--power and output. Some good pictures of the circuit would allow an evaluation of how bad it has been abused. The first thing usually to check is tubes--if you have known good replacements, try them and see if it perks up. Platefire

[Auke Jolman]

Thanks for the replies.

I'll go through the service section and take it from there.

For the tubes I've got plenty of preamp tubes but not the power tubes. There are only 2 present, but none the less.

Is there anything to say about blowing up a tube when I replace them with a new set of tubes?

As to my experience and possibilities, I've build some amps from scratch, sourcing all the parts myself. Most of the times it's fairly easy to grt them to work (except for the plexi build, see my previous posts). I've got a cheapo multimeter and that's it.

Hopy you guys can tune in when I get started with the service of the amp.

Thanks

[eleventeen]

In your situation, I would make sure the bias is turned down (= more negative volts) rather more than what you read on the schematic. If the the schemo says "-53 volts" try to tweak the control to say "-60" or even "-62" if you can get there. For a couple of reasons:


Myself, I usually do this with with any "trainwreck" (meaning "extensive") kind of repair because believe it or not, your old 6L6 tubes are VALUABLE. They are not valuable per se, they are valuable because to the extent you can blow THEM up instead of blowing up NEW ONES, this is to your advantage. Maybe -65 is a little too cold, but do not forget that with only 2 qty 6L6 installed, your B+ is likely to be little high, right? That implies that you would want bias to be a tad colder (again, colder = more negative, more shut off) Until it really shuts off the output stage, the only thing "too-cold" bias does is to throttle down the output stage. If the rest of the amp works OK, and maybe (it's common) all the rest of the amp is fairly OK despite the outward signs of amp-abuse....then you're going to have a decent-functioning Twin Reverb that's just a little on the low-output-power side. Later on, you tweak back the bias.


Incidentally, you say you wish to ultimately "blackface" the amp after you get it working. The bias section as is in that amp (meaning stock) is almost certainly what we call a "balance" type of bias circuit whereas a blackface Twin has whatever you call "the other kind".... "master voltage level", maybe. If your intention is to BF the amp, this is perhaps the biggest difference between SF and BF. If you cannot get the bias to a suitably choked-off place with the current circuit (which might involve the bias supply, the bias cap) this might speak to tearing out the "balance" type of circuit BEFORE you get the amp "working" and rebuilding it BF style. On a temporary basis, you can use the same pot as is used in the balance circuit to make the "level" circuit, though it's probably not ideal. (You will ask about this if it seems like the way to go) Regardless, and this goes to your original question: You really want to be confident of the functioning of your bias circuit before you install new 6L6 tubes in the amp.


As for blowing up the little tubes, that's very unlikely, but if the amp is very rough and/or has not been run in quite a while, then nobody would fault you for going through the preamp section and simply taking plate-voltage readings one after the other, 12 voltage readings. It is very likely these will all be OK. Make a table on a piece of paper. Pin 1 > pin 6 > pin 1 > pin 6 etc; etc; Might as well grab those "key" readings with the present (little) tubes are in the amp while they are in the amp, it'll never be easier. While you're at it, measure some 6L6 volts. Obviously be careful, you have big volts on those, and you also have big volts on the reverb driver (the 12AT7 farther from the 6L6 tubes)

[Auke Jolman]

I'm going on a short holiday today.

Next week I'll start with the amp. I'm sure I've got some more questions than. For now thanks to all.

Auke

[6G6]

I'd try to get it working right before trying to change anything.
 

[punkykatt]

I'd try to get it working right before trying to change anything.  +1,  that is good advice.

[Auke Jolman]

Took the chassis out off the cabinet today.

There's been a lot changed in the amp for as far as I can see. It looks like de amp is already blackfaced!. The value of the resitors for the grids of the PI (pin 2 & 7) from 330K to 1M 1W, the 33K has been changed to 22K. Pin 1 ==> 82K, pin 6 ==> 100K.

Also there's a loose wire (end is sealed of). It's near by the reverb RCA socket (I think the input). I'll take a better look this evening and try to post some pictures.

Then the brightswitches have been altered. I think the normal channel bright switch has been changed to engage the reverb on the normal channel also, at least that's what I discoverd when I played through the amp. I'm not sure what the bright switch on the reverb channel does. I'll look into this also this evening.

Further more there are only 2 outputtubes in the amp on position 7 and 9. Pins 1-8 of all the outputtube sockets are grounded through a 1 ohm resistor to the chassis.

The voltage reading are in the attachement.

Looking forward to your reactions!

[Auke Jolman]

I forgot to mention that de caps in the doghouse are the paperfoil type caps reading 2x 100 uF 350 V, 4x 20 uF 350 V. The e-caps on the board are the white ones, reading 25 uF 25V.

The bias circuit has changed also. The e-cap in the bias circuit reads 80 uF.

[sluckey]

caps in the doghouse are the paperfoil type caps reading 2x 100 uF 350 V, 4x 20 uF 350 V.

Those 20µF caps should be rated for 500VDC or even sometimes 525VDC. There will be more than 350V on them. And there should only be three of the 20µF caps.

[eleventeen]

Excellent points by Sluckey.


You are probably coming to the realization that you may very well have a "mod-fest" on your hands. To wit: I notice that the plate voltages on your V6 are pretty high. There is a semi-popular mod (it *may* be called the "Stokes* mod but I am not sure of that) that changes the B+ node (to a higher-voltage one) this tube gets its B+ from so that the voltage swings it produces will be higher, thus it will drive the output tubes harder.


You know how it is, Twin Reverbs aren't quite loud enough in their native state, LOL.


The point being, you could have mods all over the place, and you don't know if they were/are "attempts" which did not succeed...or what. Maybe 7 of them worked, the 8th one was the one that caused the "modder" to give up and he left it in an indeterminate state. You might have a lot of detective work to do. But along the lines of what Sluckey said, you really ought to get the power supply (including the bias) working right or pretty close to right before proceeding.


I also notice that your "bias balance" pot is probably thrown all the way one way. One side is -45 volts the other is -55 volts. Even those levels out on the 6L6 pin 5 lugs for your testing.

[sluckey]

IIRC the Stoke's mod increases the plate supply voltage to the split load phase inverter such as found in a PR or 5E3 type amp. I've never seen it applied to a LTP PI.

[Auke Jolman]

I will replace the E-caps in the doghouse with 500V rating, and indeed there are only 3 20uF ones (Thanks sluckey).

I looked over the inside of the amp once more, and here are my findings:

I think that the bias circuit is changed into a "normal" fixed bias circuit. There is a 22K resistor from the chassis of the bias balance pot to leg 1. The wiper goes to the junction of 2 220K resistors. Leg 3 contains the biasfeed voltage.

I discoverd a loose 12K resistor somewhat in the middle of the board coming from a 0.01 cap and i think is went to a 0.1 cap. I read somewhere on line that this disengages the boost function on the MV, hence the loos wire to the reverb RCA connection maybe?

2 e-caps where already changed to 22uF/63 V. From pin 8 V3 with a 2.2K resistor to ground and from pin 3 V1.

All the pots of the normal channel are replaced as are the 2 trem pots.

When I got the amp the 2 powertube where in pos V7 and V9. A changed them around to see if it mattered inwhich position they where. I tried V8 and V9, which resulted in redplating of V8. I quickly turned of the amp and put them back in V7 and V8. I looked over the bias wires again. Starting from 1 220 K bias resistor a wire goes to pin 1 of V7 and then to pin 1 of V8. The other 220k resistor is wired to pin 1 of V9 and then to pin 1 of V10. From every pin 1 is a 1.7K resistor connected to pin 5. I think the resistor in V8 is damaged because I did not get any reading there. According to the schematic the value of the resistor ought to be 1.5K. May be the drifted somewhat or my multimeter is slightly of.

I will post some photos of the inside of the amp as soon as possible.

[Auke Jolman]

I swapped the outputtubes for 2 JJ 6v6, according to the datasheet they can handle the voltages. Although the volume is stil way low. I had the 6v6 in a amp I build earlier with an 18W OT which is very loud. But at least the early distortion is not there anymore.

I ordered today a new tubeset along with the replacement e-caps for the doghouse and the board.

Will be continued.

[eleventeen]

You're putting 6V6 tubes in a Twin?


Yes, JJ 6V6's can probably handle it....though you *are* taking some risk of them arcing. But 6L6 bias levels will probably almost cut off 6V6's. 6L6's usually take (crudely) -52 volts or so. 6V6's usually take -35-40 volts. So they will be noticeably choked off. This is not necessarily a bad thing at all for testing purposes; but unless you get the bias to an appropriate range, that amp won't even put out as much power as a nominal dual 6V6 amp.


Incidentally, I believe a Twin Reverb with 4 qty 6V6's installed could be an interesting amp (if you have good help available for moving it around) and I've wanted to build a 4 x 6V6 amp for a while.

[Auke Jolman]

I had to change the bias quit a bit to get the little tubes put out anything. That might explain why the amp was still not very loud.  I believe the JJ's can handle the voltage. My intention was not to keep them in there, just for testing.

I'm expecging the parts witin a few days and take it from there.

Thanks for all the support.

[sluckey]

That might explain why the amp was still not very loud.

There's another factor too. The TR OT is a good match between four 6L6s and a 4Ω speaker load. But it's a poor match for two 6v6s and a 4Ω speaker load. A good match gives more power transfer. A bad match gives less power transfer.

[Auke Jolman]

Got the parts in today.

I replaced all the caps in the doghouse and also all the 25 uF caps and the bias cap. After that I put in 2 new Tung Sol 6L6 tubes and set the bias at 70 % (30/490*0.7=42 mA). The amp is still very quiet. Furthermore I found that the reverb channel is not very clean.

I went over Doug's service manual and will follow it up, but if somebody can give me some pointers how I can proceed, I would appdiciate that!

[DummyLoad]

sound clips.


detailed close ups shots - 1024/768 resolution with good lighting.


voltage chart - all anodes & cathodes & screens & power rails.


fender ckt number. you amp may have mods; we need to see if any have interacted negatively and we need a reference point.


--pete

[eleventeen]

"some pointers how I can proceed..."


I pointed out during an earlier moment in your assessment of the amp that it *appeared* that the B+ of the 12AT7 phase splitter (last tube in the chain driving the 6L6's) was moved to a higher voltage node in the power supply. This (I have heard) is a mod done to various Fender amps to increase the voltage swing applied to the 6L6 grids---


I would get that back to stock, indeed, I would in your place try to get the amp back to stock in the other places---in other words "demod" the amp. This will require some study on your part. For example, you have to decide whether you wish to retain the push-pull volume control, if present. If you look on a Twin Reverb AB-568 schematic, on the 6L6 tubes you will see some parts that are not there on AB763 models---there are a pair of 2000 pf caps, and some non-polarized caps lashed up to the cathodes of the 6L6 tubes. Most people would call those the "irritating" SF changes and want to eliminate those parts. Fenders of the particular era your amp is from have some odd changes in them, and while you probably want to eliminate those parts, only you can determine which of those "odd" changes are there. I am just using that particular circuit revision as a particularly bad example of the goofy things Fender threw into their amps. Moving forward to model AA270, some of those changes went away.


But it sounds you are doing things in a logical manner, certainly getting the power supply & bias straight is a good thing.


I am assuming you wish to end up with or at least shoot for an AB763 blackface Twin. 


Next: Keep or kill the push-pull master volume (if present) 


Straighten out the bright switch changes.

[Auke Jolman]

Replaced the 1.5K res to pin 5 off the power tubes with 1.5K carbon film res. One old resistor was broken in half, so I guess that takes care of the redplating (did not test it yet). I checked the value of the 470R res. to pin 4, they all checked out ok (aprox. 500R) and they are all rated 3W.  Is it wise/important to replace them anyway?

Switch it on, still low volume. Went back to the PI and checked all the resistors there and found that the 470R res. had been replaced by a 47K res!  Replaced it and voila.. the problem of the volume loss is solved!

There's still a lot of work ahead, the amp hisses quit a lot and there's more hum there than I like it to be. The added gain function of the MV is disengaged and I think I'll leave it that way.

The humbalance pot is still in place. Is it better to replace that with 2 100R res. attached to the lightbulb - ground?

As for the hiss I'm thinking of replacing the 100K res to pins 1 - 6 of the preamp tubes with metalfilm ones. Is it wise to replace the res. in the doghouse also for metal film ones?

When I want to blackface it all the way, should I replace the res in the doghouse with the values according to the AB763 values?

What are your thoughts about leaving/take out the MV

I'm glad to have solved a few problems and that's for a great part due to the help I got here! It helped me to go through it systemmaticly. Thanks for that all.

As I said there's still a lot to go over, but I'm sure I'll get there.

[eleventeen]

"Replaced the 1.5K res to pin 5 off the power tubes with 1.5K carbon film res. One old resistor was broken in half, so I guess that takes care of the redplating (did not test it yet)."


Thing of beauty, yes?  With the idea that there are no cheaper parts than resistors, (well, OK, maybe diodes, but usually you have cruddy problems when they go bad) it is GOOD NEWS (in my book) to discover bad resistors!


"I checked the value of the 470R res. to pin 4, they all checked out ok (aprox. 500R) and they are all rated 3W.  Is it wise/important to replace them anyway?


Those should be OK, but once you are putting in a parts order---nobody can criticize you for blowing $1.20 replacing 4 old R's with new, esp if you know they are flameproof. (When you say "3 watt" that is a sort of contemporary size/rating (how do you know they are 3W?)


"Switch it on, still low volume. Went back to the PI and checked all the resistors there and found that the 470R res. had been replaced by a 47K res!  Replaced it and voila.. the problem of the volume loss is solved!"


I love bad resistors. 


"There's still a lot of work ahead, the amp hisses quit a lot and there's more hum there than I like it to be. The added gain function of the MV is disengaged and I think I'll leave it that way.


"The humbalance pot is still in place. Is it better to replace that with 2 100R res. attached to the lightbulb - ground?


They are, or should be entirely equivalent.


"As for the hiss I'm thinking of replacing the 100K res to pins 1 - 6 of the preamp tubes with metalfilm ones. Is it wise to replace the res. in the doghouse also for metal film ones?


Yes and yes and yes.


"When I want to blackface it all the way, should I replace the res in the doghouse with the values according to the AB763 values?


Yup, I didn't think those were capable of producing hiss but there is a recent post about a fellow who was having hiss probs on a brownface, he changed the doghouse R's, and voila. Change 'em.


"What are your thoughts about leaving/take out the MV


Most people do not like it, meaning, they do not like how it works, but that prejudice is maybe part of the old dislike of silverfaced amps in general. If it is otherwise disengaged, I would tend to leave it as is.


"I'm glad to have solved a few problems and that's for a great part due to the help I got here! It helped me to go through it systemmaticly. Thanks for that all.


Chase those boogers down!


"As I said there's still a lot to go over, but I'm sure I'll get there.

[Auke Jolman]

As promised here is a link to all the pictures I took https://cloud.odin-ict.nl/public.php?service=files&t=175c4394c72311d72d4521a9cc3cef05

I replaced all the 100K res with metal films. This took care of a great deal of the hiss, but I'm nog quit there yet. Can I improve things by using shielded wire and if so, which wires?

All the pots on the normal channel have been replaced, as are the tremolo pots. Yesterday I orderd the remaining pots for the reverb channel and the MV. I'm still not sure wether to leave the MV pot in or not. For now I think I leave it be. Does anybody have pro's or con's?

By turning the humbalance pot I was able to reduce the hum. The hum increases quit a lot when I turn up the reverb pot. I must look into that

By the way, I replaced the recoverytube with an ECC81 tube. This gives me more control over the reverb. I found that with the ECC83, turning the reverb past 2.5-3 was already to much.

As you can see, I also have a lot of work with the cabinet  The same applies to the faceplate which has also been painted.

[sluckey]

By the way, I replaced the recoverytube with an ECC81 tube. This gives me more control over the reverb. I found that with the ECC83, turning the reverb past 2.5-3 was already to much.

One of the most useful things you can do to a Fender reverb circuit is replace the 100K-L Reverb pot with a 100K-A pot. Makes it much easier to dial in the amount of reverb you want.

[Auke Jolman]

I've been playing through the amp for some days now. I like the possibility to switch the normal channel through the reverb. I've replaced the switching wires with shielded ones. This took care of most of the hum. Tweaking the hum-balance pot and lifting up the heather wires helped also.

Today I noticed some crackling on the volume pots. From what I've read this is likely caused by leaking caps. I think I'm going to replace them all. What are the best caps to use? And how can I find the leaking caps?

Thanks.

[Ed_Chambley]

I've been playing through the amp for some days now. I like the possibility to switch the normal channel through the reverb. I've replaced the switching wires with shielded ones. This took care of most of the hum. Tweaking the hum-balance pot and lifting up the heather wires helped also.

Today I noticed some crackling on the volume pots. From what I've read this is likely caused by leaking caps. I think I'm going to replace them all. What are the best caps to use? And how can I find the leaking caps?

Thanks.

Rarely will caps cause the bacon frying sound if that is what you mean.  Plate resistors usually are the culprit.  Cheaper than caps.

I have found the best caps are the ones I have the correct value in.  Mallorys are what I use in Fenders.  Others use Orange Drops.  Either are good.

Bad caps leak DC.  Lift one end and see if you get DC across them or simply do a complete service and replace all of them.  That is what I usualy do if an older amp and I am not troubleshooting.

[Auke Jolman]

Today I replaced all the caps. Did not want to bother with trouble shooting which cap was bad. Did use Orange Drops.

Amp sounds okay now. I'm really pleaced with the end result.

Thanks to all who put me in the right direction!

The only thing left to do is to fix the cabinet.