Hello all -
I just finished replacing all the caps in my Fender red knob "The Twin". I also replaced a number of resistors in the power stages and across the output tubes. This corrected a number of problems (crackles, pops, hum) that I was having but not the main problem.
http://www.bnv-gz.de/~ooehmann/schematics/fender/The_Twin_Schematic.gif
The main problem is that the Amp sounds fine for about 10 minutes until it gets warmed up good and then the volume surges really loud. I can stop the surge by throwing the amp's Hi/Lo switch from whatever state it is in and then volume resets. After a short bit the volume surges again. Throw the Hi/Lo switch to the opposite setting and things quiet back down.
Here is what I have tried -
1. Have run the amp with 4 or 2 outputs tubes - just need to change the speaker impedance setting. Switched out various tubes in the pair. Does not cure the volume issue as it occurs regardless of tube combinations.
2. Remove all output tubes and install a pair of Yellow Jackets and EL84s. Volume issue still present.
I have also noticed that when setting the amp bias I have to turn the trim pot all the way to get .4 volts. Actually, I only get 3.5. I can balance the tubes no problem. I don't think the output tubes are the problem. I beginning to wonder about other things like a failing transformer, a semi-fried transistor or rectifier, or just a power diode going bad.
I am not an expert a repair but do know my way around a circuit board. Also, I only have a multimeter to trouble shoot with. Can anybody give some clues as to what might be the culprit?
Jerry
I just finished replacing all the caps in my Fender red knob "The Twin". I also replaced a number of resistors in the power stages and across the output tubes. This corrected a number of problems (crackles, pops, hum) that I was having but not the main problem.
http://www.bnv-gz.de/~ooehmann/schematics/fender/The_Twin_Schematic.gif
The main problem is that the Amp sounds fine for about 10 minutes until it gets warmed up good and then the volume surges really loud. I can stop the surge by throwing the amp's Hi/Lo switch from whatever state it is in and then volume resets. After a short bit the volume surges again. Throw the Hi/Lo switch to the opposite setting and things quiet back down.
Here is what I have tried -
1. Have run the amp with 4 or 2 outputs tubes - just need to change the speaker impedance setting. Switched out various tubes in the pair. Does not cure the volume issue as it occurs regardless of tube combinations.
2. Remove all output tubes and install a pair of Yellow Jackets and EL84s. Volume issue still present.
I have also noticed that when setting the amp bias I have to turn the trim pot all the way to get .4 volts. Actually, I only get 3.5. I can balance the tubes no problem. I don't think the output tubes are the problem. I beginning to wonder about other things like a failing transformer, a semi-fried transistor or rectifier, or just a power diode going bad.
I am not an expert a repair but do know my way around a circuit board. Also, I only have a multimeter to trouble shoot with. Can anybody give some clues as to what might be the culprit?
Jerry
You can wait for the amp to anounce its problem, or you can troubleshoot it.
Isolate the problem. Put jumper cords across any effects loop jack pair to see of the return jack cutouts are dirty.
Apply a test signal to the power amp in or effects return to see if the symtpom is caused in the power amp stage or the preamp stages. Likewise, send a preamp out signal from the effects send jack to another amp. Does the signal surge when it gets there?
I don;t have the schematic in front of me, but check the speaker out jacks for solder and and switching contacts. If there is an impeadnce switch, at least clean it, and try a different impedance.
Try different speakers plugged into the amp. Your problem could be caused by intermittent speaker in the amp.
If you can localize the problem to the power amp, then monitor the voltages at the powr tubes. Is the B+ changing dramatically? Are the screens losing voltage on any of the sockets? Does the bias voltage go sky high when it dims?
How about this: Bias supply filter cap cracked solder to board. Sometimes it loses contact and bias drops causing surge.
Isolate the problem. Put jumper cords across any effects loop jack pair to see of the return jack cutouts are dirty.
Apply a test signal to the power amp in or effects return to see if the symtpom is caused in the power amp stage or the preamp stages. Likewise, send a preamp out signal from the effects send jack to another amp. Does the signal surge when it gets there?
I don;t have the schematic in front of me, but check the speaker out jacks for solder and and switching contacts. If there is an impeadnce switch, at least clean it, and try a different impedance.
Try different speakers plugged into the amp. Your problem could be caused by intermittent speaker in the amp.
If you can localize the problem to the power amp, then monitor the voltages at the powr tubes. Is the B+ changing dramatically? Are the screens losing voltage on any of the sockets? Does the bias voltage go sky high when it dims?
How about this: Bias supply filter cap cracked solder to board. Sometimes it loses contact and bias drops causing surge.
Semi nubie here also, but.....perhaps the difficulty in setting the bias voltage is trying to tell you something...
Can you monitor (with a voltmeter on the test point) the bias voltage when the volume starts running away? My wild-*** guess is that your bias voltage is going away once the amp is warmed up.
Two members here that do a lot of Fender amp work are gingertube and trout. I'm sure there are many others also, these two just came to mind. Perhaps one of them will chime in.
Can you monitor (with a voltmeter on the test point) the bias voltage when the volume starts running away? My wild-*** guess is that your bias voltage is going away once the amp is warmed up.
Two members here that do a lot of Fender amp work are gingertube and trout. I'm sure there are many others also, these two just came to mind. Perhaps one of them will chime in.
Many times these volume swells are caused by the cathode bypass electrolytics used in the pre-amp gain stages....
These bypass caps are in parallel with the cathode resistors to avoid the cathode AC current feedback that would normaly occur, thus maximizing the gain for that stage.
When these electrolytics go bad or intermittent, they cause a similair problem to what you describe....
But then again I don't know the whole story so it could be a number of things...I would also agree that signal tracing would be the ticket to solve this problem quickly...
Chris
These bypass caps are in parallel with the cathode resistors to avoid the cathode AC current feedback that would normaly occur, thus maximizing the gain for that stage.
When these electrolytics go bad or intermittent, they cause a similair problem to what you describe....
But then again I don't know the whole story so it could be a number of things...I would also agree that signal tracing would be the ticket to solve this problem quickly...
Chris
Well, the volume swells stopped. But I should really say that the amp seems to running at low power now. Everything seems to work except at a lower volume. I have lost channel switching and it will not work with either the footswitch or the panel push pull pot. The channel LEDs do not light up on the panel either. Also, I notice that the V4 tube seems very faintly on. It's a brand new tube and when I swap it with another tube I see the same result.
So I am guessing that the lower voltage at the bias circuit is reflecting the amp running at a lower voltage overall. Everything sounds clean, reverb works (weakly), both channels work, all power tubes glowing nicely.
I have looked for loose wires, remelted several solder points throughout, and looked for fried components. All looks good
Bypasssing the preamp has the same problems. So now the real problem searching begins.
So I am guessing that the lower voltage at the bias circuit is reflecting the amp running at a lower voltage overall. Everything sounds clean, reverb works (weakly), both channels work, all power tubes glowing nicely.
I have looked for loose wires, remelted several solder points throughout, and looked for fried components. All looks good
Bypasssing the preamp has the same problems. So now the real problem searching begins.
When you say V4 is "dim", you mean that the heater is not lit up as bright? Have you checked the voltages?
Some tubes have the heater less exposed visually and they appear dimmer, this is not a big problem. However, if the heater was intermittently shorted or weakly connected at a preamp socket, it could manifest as volume swells.
What is the bias voltage on the power tubes grids? Or rather, what is the voltage between the grid pin and the cathode pin? if it only swells on signal peaks but sounds distorted and low at all other times, that could indicate that it is trying to operate in class C. It's possible that the bias measurement resistor has drifted high i suppose.
Is the speaker load attached the right one? Connected to the right jack? Is the jack and all the speaker wiring in good condition?
Some tubes have the heater less exposed visually and they appear dimmer, this is not a big problem. However, if the heater was intermittently shorted or weakly connected at a preamp socket, it could manifest as volume swells.
What is the bias voltage on the power tubes grids? Or rather, what is the voltage between the grid pin and the cathode pin? if it only swells on signal peaks but sounds distorted and low at all other times, that could indicate that it is trying to operate in class C. It's possible that the bias measurement resistor has drifted high i suppose.
Is the speaker load attached the right one? Connected to the right jack? Is the jack and all the speaker wiring in good condition?
Visually the V4 tube looks dim barely a trace of color across the grid. How do I measure the bias across the tubes as you suggest? I do not own a o-scope only a multimeter. When using it to adjust the bias pot I can not .4 volts, the best I can get is .3 volts when turning the pot max.
Yes, there is a speaker load on the proper output jack. I have two 8 ohm speakers wired in series plugged into the J6 speaker output jack and the selector switch set to 16 ohms. Everything seems to be in order.
The amp plays, but low volume and the channel switching does not work. I am no longer getting volume swells. My thought is that what I thought were volume swells was really the amp operating at full power and now something has failed or become loose or disconnected leading to volume fall off. I suspect the problem may be in the preamp stage.
JL
Yes, there is a speaker load on the proper output jack. I have two 8 ohm speakers wired in series plugged into the J6 speaker output jack and the selector switch set to 16 ohms. Everything seems to be in order.
The amp plays, but low volume and the channel switching does not work. I am no longer getting volume swells. My thought is that what I thought were volume swells was really the amp operating at full power and now something has failed or become loose or disconnected leading to volume fall off. I suspect the problem may be in the preamp stage.
JL
jl_lensman said:
Just leave the voltmeter connected to the test point for the bias pot and monitor the voltage during the volume swells. If the bias voltage is dropping when the vol surges, it could be a bias supply problem. The fact that you cannot get enough bias voltage to be within spec is suspicious, at least to me.
Here is an update on the ills of my "Evil Twin".
After a few minutes playing I get increasing tube noise that eventually goes into uncontrollable feedback. It occurs on both channels of the amp - even when I have (1) channel with all pots turned to "0".
The Amp is now running both channels in parallel mode and I can not defeat it with either the footswitch or the panel channel selector pull switch. Both front panel channel LEDs should light up when in parallel mode but neither is lit. I can not get the AMP tp run in a single channel mode.
I can not set the bias to .4v as specified. The best I can get is .3v and this does not change when the AMP goes into feedback.
The preamp out signal does not feedback but the AMP still only operated in parallel mode.
Here is what I have done so far -
1. Replaced all electrolytic capacitors throughout preamp and power stages.
2. Cleaned speaker impedence switch. Checked all cabling and substituted different cables.
3. Checked for loose wires and bad solder joints. I cleaned a few things up but nothing that would make a difference.
4. Switched out preamp tubes with old ones. No difference.
5. Switched out power tubes for a pair of yellow jackets with EL84s. Feedback still the same.
So it's still an ongoing mystery to me.
After a few minutes playing I get increasing tube noise that eventually goes into uncontrollable feedback. It occurs on both channels of the amp - even when I have (1) channel with all pots turned to "0".
The Amp is now running both channels in parallel mode and I can not defeat it with either the footswitch or the panel channel selector pull switch. Both front panel channel LEDs should light up when in parallel mode but neither is lit. I can not get the AMP tp run in a single channel mode.
I can not set the bias to .4v as specified. The best I can get is .3v and this does not change when the AMP goes into feedback.
The preamp out signal does not feedback but the AMP still only operated in parallel mode.
Here is what I have done so far -
1. Replaced all electrolytic capacitors throughout preamp and power stages.
2. Cleaned speaker impedence switch. Checked all cabling and substituted different cables.
3. Checked for loose wires and bad solder joints. I cleaned a few things up but nothing that would make a difference.
4. Switched out preamp tubes with old ones. No difference.
5. Switched out power tubes for a pair of yellow jackets with EL84s. Feedback still the same.
So it's still an ongoing mystery to me.
It's alive almost - still low bias voltage
Okay finally able to breath life into the beast. Here's what I accomplished.
1. The switching problem was caused by the power supply line to the inputs being shorted on the chassis by a screw securing the preamp stage to the chassis. Changed out the wire for a new one and now channel switching is restored.
2. As per Cerrem's suggestion I swapped out the cathode bypass caps for new ones. Haven't heard the AMP surge since doing that.
3. I had crackling and pop sounds so I replaced the resistors on the output tubes. Swapped out the 470K 2w with 1K 5w ceramic resistors. No more crackling or pops.
4. Replace all electrolytic caps throughout the amp. Been playing it for a few days all seems good.
My only concern is still the somewhat low voltage reading I am getting at the bias check point. Can only get .3v with trim pot turned all the way up. I can live with it but would like to understand why it not within the .4v spec.
My next mod is to add a vibrato circuit to the amp. I have Weber kit on the way to build and install. I was thinking of installing it to bias modulating but if my amp can't brought up to bias spec then I will probably set it up for volume modulating.
Thank you all for your input and patience.
Okay finally able to breath life into the beast. Here's what I accomplished.
1. The switching problem was caused by the power supply line to the inputs being shorted on the chassis by a screw securing the preamp stage to the chassis. Changed out the wire for a new one and now channel switching is restored.
2. As per Cerrem's suggestion I swapped out the cathode bypass caps for new ones. Haven't heard the AMP surge since doing that.
3. I had crackling and pop sounds so I replaced the resistors on the output tubes. Swapped out the 470K 2w with 1K 5w ceramic resistors. No more crackling or pops.
4. Replace all electrolytic caps throughout the amp. Been playing it for a few days all seems good.
My only concern is still the somewhat low voltage reading I am getting at the bias check point. Can only get .3v with trim pot turned all the way up. I can live with it but would like to understand why it not within the .4v spec.
My next mod is to add a vibrato circuit to the amp. I have Weber kit on the way to build and install. I was thinking of installing it to bias modulating but if my amp can't brought up to bias spec then I will probably set it up for volume modulating.
Thank you all for your input and patience.
any resolution to your evil twin problem? I am experiencing something I believe is similar to perhaps exactly the same problem! I haven't tried to diagnose it, just baffled at it. I thought it was tube related, but your story kinda squashes that. The amp is loud to begin with and when it surges it gets absurd. Even at less than half of one volume on the clean channel. It really bums me out, I may be the only one, but I really like the evil twin.
Just a point I noticed regarding the OP. The bias voltage is 0.04v, not 0.4v You were trying to set it too high. Sorry, only 11 years too late.
I was looking for common faults on this amp and came across this thread. I also had the fault with volume changing between high and low, which started after 10~15 mins of heating up and would keep varying every few mins or so. I found three possible causes, all mechanical. I fixed all three at the same time and this cured the fault. It was either fault, 2, 3 or 4 below. The most likely cause of my fault on my amp was the loop level switch. I say this because my amp was going from two distinct volumes, not varying slowly. If the switch goes open circuit due to bad contacts, it will set the line out to high, and line in to low level, and the amp will go very loud.
Since I couldn’t find any advice on common faults, I’ll pay it forward here’s what I found when I put in the time to investigate. These will be common faults on this amp if its never been looked at.
1. I bought mine new in 1988 and one of the front ¼” jack sockets had broken off many years ago. It still worked so I left it until recently. It was much easier now (with the internet) to find replacement ones on ebay and swap them out. The front sockets are all stereo ones and look like the replacements shown in the attachment (the one with more tabs) The original had pins where this one has tabs. Its easy enough to file the tabs so the fit the PCB, just use a fine file though. All the plastic ¼” sockets need replacing. The plastic just crumbled as I unscrewed them from the panel. You need this type so they will fit physically, although it is always possible to work round it if you are desperate.
2. The ¼” sockets on the rear of the amp all need replacing. They are all mono and look like the other attachement (the one with less tabs). The power amp in on the effects loop (rear of amp) has the preamp signal going through it all the time. Dirty contacts in here are a potential source of the volume fluxuations. You could try contact cleaner and pushing a plug in and out but the socket needs replaced by now anyway because the plastic will have deteriorated.
3. The speaker impedance selector switch on the rear needed cleaning. It is possible to undo the tabs on the switch body so you can open it up and clean it properly. This could be a source of varying volume.
4. The loop level switch needs cleaning. I was amazed how dirty he contacts were. Same as the speaker impedance switch, you can dismantle the switch by bending the small tabs and pulling the back off. The contacts will fall out so be careful, but its obvious how to put them back. The signal out of the preamp will go through this switch at all times, even if you don’t use the line out feature.
On a separate venture, I replaced all the electrolytic capacitors. At 30 years old, its time to replace the capacitors. You can buy a kit, but I went for the best quality/longest life capacitors I could get from radiospares. The original capacitors are all axial, ie they have a lead at each end. Most suppliers now only supply radial ones, with both leads on one end. Its easy enough to extend one lead (best to use the –ve one), insulate it with heatshrink, and run it to the correct hole on the pcb. I spent half what the kits cost by buying the capacitors direct from an electronics supplier. The result was pretty good. A really big difference in clarity. I tested all my old caps and they were all in spec using a low voltage tester. However , they must have been in trouble at higher volts because I got the benefit of fixing a fault which I had been ignoring. My reverb could not be turned beyond 3 without it howling. It didn’t bother me because I didn’t use it above three. After the cap change, its like new again, and sounds brilliant. I now use it at 4 and love the sound.
Just a thing about the weight. Everyone complains about it. Its due to the amount of copper used in the power supply and speaker transformer. It’s this which makes this amp sound so fantastic (and loud) Like having a big engine in a car as against a smaller high reving one – its just not the same as having lots of cc’s. Well it’s the same with copper in your supply. You’ll never get an amp with this amount of copper in it now, its just too expensive. Even at low volume, you can appreciate the ease at which this amp drives those speakers.
I still have the original valves in my amp. Don’t feel you have to change them just because everyone tells you to. Mine sounds great. I worked as an engineer in the BBC for many years. It was quite farcical the stories we heard about fables regarding audio quality. Trust your own ears. Emperor's clothing?
Many people complain its too loud. Its capable of incredible volumes. At home, its very easy to make up a die cast box with a 250k logarithmic pot with input and output sockets. You need to feed this from the line out on the back of the amp, and feed it back into the amplifier in. This allows you to get the input volume on the front of the amp, up off the end stop. That allows you to make proper use of the tone section of the pre-amp. It improves the sound a lot, even at the same low volumes in your house. If you don’t want to make a box, have a search for “metal volume box guitar amplifier” for a relatively cheap solution to the volume, with no need to modify the amp in any way.
I was looking for common faults on this amp and came across this thread. I also had the fault with volume changing between high and low, which started after 10~15 mins of heating up and would keep varying every few mins or so. I found three possible causes, all mechanical. I fixed all three at the same time and this cured the fault. It was either fault, 2, 3 or 4 below. The most likely cause of my fault on my amp was the loop level switch. I say this because my amp was going from two distinct volumes, not varying slowly. If the switch goes open circuit due to bad contacts, it will set the line out to high, and line in to low level, and the amp will go very loud.
Since I couldn’t find any advice on common faults, I’ll pay it forward here’s what I found when I put in the time to investigate. These will be common faults on this amp if its never been looked at.
1. I bought mine new in 1988 and one of the front ¼” jack sockets had broken off many years ago. It still worked so I left it until recently. It was much easier now (with the internet) to find replacement ones on ebay and swap them out. The front sockets are all stereo ones and look like the replacements shown in the attachment (the one with more tabs) The original had pins where this one has tabs. Its easy enough to file the tabs so the fit the PCB, just use a fine file though. All the plastic ¼” sockets need replacing. The plastic just crumbled as I unscrewed them from the panel. You need this type so they will fit physically, although it is always possible to work round it if you are desperate.
2. The ¼” sockets on the rear of the amp all need replacing. They are all mono and look like the other attachement (the one with less tabs). The power amp in on the effects loop (rear of amp) has the preamp signal going through it all the time. Dirty contacts in here are a potential source of the volume fluxuations. You could try contact cleaner and pushing a plug in and out but the socket needs replaced by now anyway because the plastic will have deteriorated.
3. The speaker impedance selector switch on the rear needed cleaning. It is possible to undo the tabs on the switch body so you can open it up and clean it properly. This could be a source of varying volume.
4. The loop level switch needs cleaning. I was amazed how dirty he contacts were. Same as the speaker impedance switch, you can dismantle the switch by bending the small tabs and pulling the back off. The contacts will fall out so be careful, but its obvious how to put them back. The signal out of the preamp will go through this switch at all times, even if you don’t use the line out feature.
On a separate venture, I replaced all the electrolytic capacitors. At 30 years old, its time to replace the capacitors. You can buy a kit, but I went for the best quality/longest life capacitors I could get from radiospares. The original capacitors are all axial, ie they have a lead at each end. Most suppliers now only supply radial ones, with both leads on one end. Its easy enough to extend one lead (best to use the –ve one), insulate it with heatshrink, and run it to the correct hole on the pcb. I spent half what the kits cost by buying the capacitors direct from an electronics supplier. The result was pretty good. A really big difference in clarity. I tested all my old caps and they were all in spec using a low voltage tester. However , they must have been in trouble at higher volts because I got the benefit of fixing a fault which I had been ignoring. My reverb could not be turned beyond 3 without it howling. It didn’t bother me because I didn’t use it above three. After the cap change, its like new again, and sounds brilliant. I now use it at 4 and love the sound.
Just a thing about the weight. Everyone complains about it. Its due to the amount of copper used in the power supply and speaker transformer. It’s this which makes this amp sound so fantastic (and loud) Like having a big engine in a car as against a smaller high reving one – its just not the same as having lots of cc’s. Well it’s the same with copper in your supply. You’ll never get an amp with this amount of copper in it now, its just too expensive. Even at low volume, you can appreciate the ease at which this amp drives those speakers.
I still have the original valves in my amp. Don’t feel you have to change them just because everyone tells you to. Mine sounds great. I worked as an engineer in the BBC for many years. It was quite farcical the stories we heard about fables regarding audio quality. Trust your own ears. Emperor's clothing?
Many people complain its too loud. Its capable of incredible volumes. At home, its very easy to make up a die cast box with a 250k logarithmic pot with input and output sockets. You need to feed this from the line out on the back of the amp, and feed it back into the amplifier in. This allows you to get the input volume on the front of the amp, up off the end stop. That allows you to make proper use of the tone section of the pre-amp. It improves the sound a lot, even at the same low volumes in your house. If you don’t want to make a box, have a search for “metal volume box guitar amplifier” for a relatively cheap solution to the volume, with no need to modify the amp in any way.
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