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Post subject: Reverb issue with Silverface Twin from 1976 135 watt version
Posted: Sun Oct 21, 2012 5:28 am
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Hi all, I have been trying resolve an issue with my Fender Twin Reverb that I bought new in 1976. The amp is a Silverface 135 Watt Ultra Linear circuit, with the Pull Master Volume.

The problem I have is, if I turn the channel volume up above 2, the reverb sounds distorted. Not full on distortion, a sort of buzzy type of distortion, especially towards the end of a note. It sounds like it might be a tube, but it's not, I have changed all the tubes in the amp and experimented with different tubes from known good amps and it has not solved the problem. The odd thing is that if I leave the channel volume below 2, the reverb is perfect. I can plug into the second input which is a lower gain and push the channel volume up to around 4 before the problem starts; master volume setting has no effect. The amp at any master volume setting with the channel volume on 2 is perfect with the reverb, it's only as the channel volume goes up that the reverb starts to distort, it's like it's being over driven, however there is not a problem with the reverb turned off.

Here is what I have done thus far to try and resolve the issue:-
Changed Driver and Recovery Tubes - as well as played with all the other tubes
Measured the volts as per the Fender diagram with no input signal - they are as per diagram.
Replaced the 500pf Input Capacitor to V3
Replaced 0.003uF Capacitor in the recovery circuit V4A
Replaced the 0.002uF Capacitor in the recovery circuit V4A.
Replaced the 100K and 200k resistors in the recovery circuit V4A
Checked the 1M and 680 resistors in the driver circuit V3

I did find a poor connection on the red phono lead from the amp to the reverb unit, but this may have been caused by me constantly unplugging it. I fitted a new Phono plug as the existing one is a moulded type. What did fool me was at the amp end the Phono was red, on the reverb Tank end I was expecting it to be the red one, but Fender seem to have wired it to the grey one at that end. I tested all the cables and they are fine now.

I am now at a loss as to what to look for, confused because if the spring unit or transformer were faulty I would not expect the reverb to be perfect when the channel volume is on 2 or less. If I run the amp up with the master volume on full, it's very, very loud and there is still no sign of the problem, only when the channel volume is turned up do I get the problem. Any suggestions as to what to try next would be really appreciated.

One question, the reverb tank unit, can you connect these round the wrong way? It seems to have a transformer at either end, are they identical or are they different. Not tried reversing the cables for fear of damaging the unit. As far as I know it's connected as it always has been, looking at the amp,the red phono is on the right side looking from the rear of the amp. The reverb Tank is in perfect condition, I checked inside and there are no bad connections and the springs are fine - note the above comments that it works perfectly when channel volume is 2 or less.

The problem seems to only exist in the reverb circuit as the amp works perfectly at any setting without the reverb - it's been driving me mad for a while now trying to figure out what might be wrong. I don't have access to a scope or signal gen. so I have been working through the different options but now am stumped.

Cheers for any help or suggestions people provide.


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Sun Oct 21, 2012 7:12 am
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Good chance are reverb transformer is defective .


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Sun Oct 21, 2012 10:29 am
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Thanks Stratele52, do you mean the transformer in the amplifier or the reverb tank?

I did some more checks and the Input transformer on the reverb tank is reading about 1 ohm, which seems low to me. The output transformer on the tank is reading about 163 ohms.

The problem really does sound part mechanical, so much like when you have a microphonic tube, but none are in the amp. Although the reverb tank springs look ok and there is no physical obvious fault, I wonder if it could be the tank that has the fault. Does anyone know if the small transformers in the tank need any special mounting requirements, could they be loose or rattling causing a problem? Remembering that this amp and the tank are 36 years old. The amp is well looked after, used to be in a flight case, but well gigged and used. I think the problem does exist when the channel volume is lower, but not as noticeable as when you have the channel volume higher, why this should be I'm not sure, but maybe sending a bigger signal to the tank has something to do with it.

Any other suggestions?

Thanks for taking the trouble to reply.


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Sun Oct 21, 2012 10:43 am
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6BG4, I believe Stratele means the reverb tranny that sits on top of the chassis, next to the 12AT7 driver.

Since you've replaced just about everything in the circuit, it prolly is this tranny or the tank, itself. The one thing you may want to try is pressing on the breadboard, while the amp is on (CAREFUL about HIGH VOLTAGE --- so use a WOODEN chopstick).

Sometimes, esp after many techs have worked on the amp ---a lead pokes through the bottom board and shorts onto the chassis. Often, this is a periodic thing. Made worse by vibration and running higher voltages (like cranking up the amp).

Good luck! I know how these fixes can really become a pain in the butt.


Links:

http://store.triodestore.com/tf160.html

or this if the above doesn't seem to work...

http://www.classictone.net/40-18034.html


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Sun Oct 21, 2012 12:39 pm
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6B4G wrote:
Thanks Stratele52, do you mean the transformer in the amplifier or the reverb tank?



Yes In the amp . Reverb tank do not use transformer, it is a tranducer . But yes it look like a transformer .

I have a good reverb transformer here, both side read 1 ohms
AI read an Accusonic reverb tank 4AB3C1B ; 1 ohms at input and 215 ohms at output

It could be another problem than reverb transformer itself . These amp have very bad wiring dress. If you work in the amp before and just touch to some wire you can have problem. Is this amp had to fix before ?

Also a defective component in Phase Inverter circuit could give distortion ( in all channel ).


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 5:49 am
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Hi Guys - Thanks for the replies.

The amp has only been worked on by me, I'm an electrical Engineer working on Control systems, but have good electronics experience at component level, so whilst your comments are very valid about long leads or disturbing the tag board and cabling, I'm sure it is correct and not at fault.

Only the reverb has an issue, if the reverb is turned off the amp is perfect therefore I'm sure it's not in the PI. It could be the Reverb driver transformer breaking down with voltage/current, but I suspect not; why is it ok at lower volume settings, it has 410v across it all the time. If the transformer fails I would expect shorted or open turns, this is not a symptom the amp is suffering from as the reverb can be perfect, not saying out right this is not the problem, but it does not feel like it is. The problem really does sound almost mechanical as at lower volumes settings of the channel volume you can hear it sort of buzz, it really is like when a tube is microphonic, however if I set the channel volume to 1.5 and the Master on full, which is very loud the problem does not get worse, which I would expect if it was the speakers ratting the tank. I have tried it with the tank out on carpet and the problem does not change. I'm thinking it's the tank, but I can't see anything wrong with it and it's an expensive and difficult part to source in the UK, especially if it is not the problem.

Really got me scratching my head on this one.


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 6:03 am
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I would try the reverb transformer also. It is not an expensive part.
The replacement tranny for my '72 Super Six Reverb was only $15.
At the same time, I had my RCA plugs replaced on the tank. The connections were frayed. I'm sure you've checked that already tho.


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 6:22 am
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Not expensive in the US maybe, it will cost that plus double that amount for shipping, then nasty Import duty to the UK, could cost $80 and not fix the problem.

The same applies to the tank, reading the Acutronics web site, I wonder if it's the mounting bushes for the springs, they say they act as dampers, this thing is 36 years old and has been rattling round in vans and cars for a good part of that time. It's retired now, but I would love to get this amp back to original condition. No chance of finding anyone with a good tank and no chance of sale or return, so could cost me $100 or more for that, and not guaranteed to fix the problem, which is why I'm trying to figure out what might be wrong. Sounds daft but it's cost $25 in capacitors since you can't just buy one or two easily, always have to pay shipping which is fifty times the cost of the part, getting to be costly now.

I do have a Hot Rod Deluxe at home, does anyone know if the tanks are similar and if I could try the tank from that amp in this one?

Cheers


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 10:25 am
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How does the lammies that fit into the reverb transducers look like? Very loose? There are tricks to tighten this transducer action.


And the springs?


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 10:43 am
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6B4G wrote:
Hi Guys - Thanks for the replies.

. If the transformer fails I would expect shorted or open turns,


Wrong . Open = no reverb . Short = could give wrong signal / impedance = distortion

Hoe many ohms do you read at this reverb transformer ; You see mine; 1 ohms.

But I don't said that reverb trasnsformer is defective. I could be .
Not enough test from you AND hard to fix an amp by e-mail.


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 10:46 am
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BMW2002Ti wrote:
How does the lammies that fit into the reverb transducers look like? Very loose? There are tricks to tighten this transducer action.


And the springs?


Transducer loose in the reverb tank = less reverb , but never distortion. Springs , same.

I know Gerald Weber said to put some small oiece of wood under the transducer to keep them tight . I try that many time and reverb not working better but not worse.


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 11:15 am
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Hi Stratele52 - you are correct in that if the transformer was open there would be no reverb and if the windings were shorted there would be a miss match and who knows what might be happening, so it could be this I agree. I suspect it's not this on the basis that it seems to work better at lower volumes, but this is not necessarily proving anything as a bigger signal will draw more current and shorted windings would have a bigger effect then. I will not have a chance to look at the transformer for a while now due to other commitments, plus difficult to tell with just a simple meter if the transformer windings are good or not, clearly they are not open circuit completely, but that's not saying much.

I agree very difficult to diagnose via email, very much appreciate you taking the trouble to reply.

Interesting comments about the transducers, I'm still thinking it could be the tank because there is this strange sort of metalic buzz that comes through as well as a sort of distorted sound. As I said I would have put money on it being a tube because of the sound I hear, sadly it is 100% not a tube.

Time allowing at the weekend I may try the reverb tank out of my Hotrod Deluxe, unless any one says this is a bad idea, what's the worse that could happen, neither amps have a working reverb :oops:

I have found a suitable source for a tank in the UK, it would cost about £30 with delivery, so might try the tank first as mechanical damage/wear on a 36 year old amp is not to be ignored.

Thanks to all for your help, any other suggestions are most welcome, I have not ruled out the transformer completely as being at fault, but currently this is my second choice for the offending item...!


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Mon Oct 22, 2012 11:44 am
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I agree with you , tank look suspect


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Tue Oct 23, 2012 3:47 am
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New tank is on order, fingers crossed this is the problem - thanks for your help :)


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Post subject: Re: Reverb issue with Silverface Twin from 1976 135 watt ver
Posted: Tue Oct 23, 2012 6:02 am
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Good luck with the tank! Since the tank has a mechanical (moving springs and parts in the transducers), as well as an electric function --- they will wear out.

Keep us informed.


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