Hi, fellow tubers
An old Fender Twin Reverb came in with a nice but strong 50Hz hum.
I suspect 2 6L6's are end of life, as the anodes have the typical brown colour and when playing, they become red.
If anyone here would have a schematic I would be very thankful.
Yes, I reduced the hum with the pot meter on the backside.
A year ago it was still possible to get rid of the hum almost completely with that pot. Not anymore.
I replaced the 4 6L6's with others, not new but better ones and the hum is much better, but some anodes are still glowing.
Where should I start looking, any input is welcome, as I don't have the schematic.
/Hugo - has to fix it before Wednesday evening, has to party then.
An old Fender Twin Reverb came in with a nice but strong 50Hz hum.
I suspect 2 6L6's are end of life, as the anodes have the typical brown colour and when playing, they become red.
If anyone here would have a schematic I would be very thankful.
Yes, I reduced the hum with the pot meter on the backside.
A year ago it was still possible to get rid of the hum almost completely with that pot. Not anymore.
I replaced the 4 6L6's with others, not new but better ones and the hum is much better, but some anodes are still glowing.
Where should I start looking, any input is welcome, as I don't have the schematic.
/Hugo - has to fix it before Wednesday evening, has to party then.
Attachments
First try and goto http://www.ampwares.com/ffg/twin_reverb_sf.html and then you have the schematic...
This was the good news, the bad news is that the pot you have used for hum adjustment actually is the bias pot ? Please tell me that I am wrong, otherwise I think you might be in for 4 new tubes and a fresh bias adjustment.... at least the tube on the picture probably isn't a 6L6 anymore
regards
Claus
This was the good news, the bad news is that the pot you have used for hum adjustment actually is the bias pot ? Please tell me that I am wrong, otherwise I think you might be in for 4 new tubes and a fresh bias adjustment.... at least the tube on the picture probably isn't a 6L6 anymore regards
Claus
Thanks ,
Meanwhile I also could find another site with schematics.
Here it is for your info:
http://www1.korksoft.com/~schem/fenderheaven.htm
As far as I can see it must be the ‘135W’ version.
On the backside there are two pots: one marked 'Hum Balance',
second one: 'Output Tubes Matching'.
Too bad, these pots are easily accessible to anyone…
Here's what I did: I replaced the bad 6L6's with good ones, turned the 'Output Tubes Matching' pot until no hum was audible and no plates were glowing.
Yes, that pot reduces most of the hum.
Hooked up my scope to the speakerwires and reduces further hum with the 'Hum Balance' pot.
None of the plates glow anymore, the hum is completely gone and the amp is playing well.
I have now tree Radiotronic 6L6's and one Tronal tube in the amp.
Is this a problem on the long term, should I look any further, measure bias (how?) or other things or could everything be ok now?
Thanks for the input.
/Hugo
Meanwhile I also could find another site with schematics.
Here it is for your info:
http://www1.korksoft.com/~schem/fenderheaven.htm
As far as I can see it must be the ‘135W’ version.
On the backside there are two pots: one marked 'Hum Balance',
second one: 'Output Tubes Matching'.
Too bad, these pots are easily accessible to anyone…
Here's what I did: I replaced the bad 6L6's with good ones, turned the 'Output Tubes Matching' pot until no hum was audible and no plates were glowing.
Yes, that pot reduces most of the hum.
Hooked up my scope to the speakerwires and reduces further hum with the 'Hum Balance' pot.
None of the plates glow anymore, the hum is completely gone and the amp is playing well.
I have now tree Radiotronic 6L6's and one Tronal tube in the amp.
Is this a problem on the long term, should I look any further, measure bias (how?) or other things or could everything be ok now?
Thanks for the input.
/Hugo
It's a shame if its the 135watt ultralinear. The MONEY twin to have is the '65 blackface with the AA763 circuit
I've got a '68 super reverb that is the best amp for clean playing that i've ever heard....also with the AA763.
I couldn't tell you how to bias that model. I'm sure it's much easier than mine...which still hasn't been modified from the original stock bias resistor to a trimpot.
-Maz
I've got a '68 super reverb that is the best amp for clean playing that i've ever heard....also with the AA763.
I couldn't tell you how to bias that model. I'm sure it's much easier than mine...which still hasn't been modified from the original stock bias resistor to a trimpot.
-Maz
Today I got another two Fender Twin Reverbs coming in.
Schematic here: http://www1.korksoft.com/~schem/fenderamps/twin_reverb_sf_100_schem.pdf
Till now I was stuck with the strange bias setting of Fender amps.
I found this site: http://www.hoffmanamps.com/Biascircuits.htm
It gives a good explanation of how things work.
On a sidenote I compared the sound of two identical Fenders.
I have to do some more listening but they sound rather different.
I'll report later when I found more.
/Hugo
Schematic here: http://www1.korksoft.com/~schem/fenderamps/twin_reverb_sf_100_schem.pdf
Till now I was stuck with the strange bias setting of Fender amps.
I found this site: http://www.hoffmanamps.com/Biascircuits.htm
It gives a good explanation of how things work.
On a sidenote I compared the sound of two identical Fenders.
I have to do some more listening but they sound rather different.
I'll report later when I found more.
/Hugo
Netlist said:
The "silverface" 100 watt Twin. Not a real jewel!
It's from the period when the CBS engineers were, at the same time, trying to make the amp more stable and fighting the "more power" wars with other manufacturers. As a result, the classic Fender Twin tone fell by the wayside...
The bias circuits in Fender amps can be strange to say the least.
In many of the "silverface" era amps, this one included, you really only have a bias balance adjustment. And it's further complicated by the vibrato (really tremolo) circuit which pulses the volume by varying the bias voltage on the power tubes. The pot only serves to allow different bias voltage for each half of the push pull power stage. The idea is to adjust out any hum created due to mismatched power tubes. The actual bias voltage is not adjustable other than by changing values of the fixed resistors ahead of the pot.
It's a silly circuit when you consider the fact that a twin uses four power tubes. The bias balance pot only addresses the pairs of tubes, so if you don't have matched tubes within each pair, you can still have tremendous problems -- such as red plates on only one tube.
The common solution is to rewire the bias circuit as in the "blackface" era amps, i.e., AB763, and use matched quads of power tubes. And a twin puts quite a lot of stress on the power tubes, so you need to use really good ones. The Phillips STR tubes of that era are a great match if you can find them. Oddly, the new Shuguang 6L6GC holds up pretty well, sounds great in a Fender, and is really inexpensive. The JJ and Svetlana 6L6GC are also good choices. The Sovtek 5881 will hold up very well in this amp but, IMO, doesn't sound good at all.
Are they really identical?
There are several different circuits in "silverface" Fenders.
Be extreme careful...
You do not need a schematich at first
to get firs aid for the twin...
1. do not put hi voltage for a long time in that condition of device... (stan-by on)
2. becouse if You do that You expose twin to permanently damage the output transformer I persume this is an original
and that is the unchangable part...
3. tru to change the position of 6L6 tubes
and if the one previously fo into red heat the same one glows imeadetly change all
pair with a new...
4. if the other tube is going red and very hot at the position that means that maybe something is wrong inside the twin...which is pretty unlighltly...
5.anyway measure the curents throught the all tubes as well as for the G2...
And finaly be very careful there You have very hi tension...expetialy when You geting out and putting bac in combo chases turn the plug out of main
.becouse of the power supply switch which is at very dangerous possition for fingers...I have personal expirience about that...Good luck...
You do not need a schematich at first
to get firs aid for the twin...
1. do not put hi voltage for a long time in that condition of device... (stan-by on)
2. becouse if You do that You expose twin to permanently damage the output transformer I persume this is an original
and that is the unchangable part...
3. tru to change the position of 6L6 tubes
and if the one previously fo into red heat the same one glows imeadetly change all
pair with a new...
4. if the other tube is going red and very hot at the position that means that maybe something is wrong inside the twin...which is pretty unlighltly...
5.anyway measure the curents throught the all tubes as well as for the G2...
And finaly be very careful there You have very hi tension...expetialy when You geting out and putting bac in combo chases turn the plug out of main
.becouse of the power supply switch which is at very dangerous possition for fingers...I have personal expirience about that...Good luck...
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