Good old Fender Twin Reverb - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th April 2003, 09:22 PM   #1
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
Question Good old Fender Twin Reverb

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.
Attached Images
File Type: jpg mvc-255f.jpg (54.0 KB, 369 views)
  Reply With Quote
Old 28th April 2003, 09:44 PM   #2
Bjerre is offline Bjerre  Denmark
diyAudio Member
 
Join Date: Mar 2003
Location: Aalborg, Denmark
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
  Reply With Quote
Old 29th April 2003, 02:39 AM   #3
Joel is offline Joel  United States
diyAudio Member
 
Joel's Avatar
 
Join Date: Oct 2002
Location: Brooklyn, NY
The pot underneath is a bias pot (!!), and the bias is way outta wack too - since you can see the plates glowing red. Don't run the amp this way.
  Reply With Quote
Old 29th April 2003, 07:13 AM   #4
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
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
  Reply With Quote
Old 29th April 2003, 05:16 PM   #5
diyAudio Member
 
Magnetmaz's Avatar
 
Join Date: Nov 2002
Location: Pittsburgh, PA
Send a message via AIM to Magnetmaz
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
  Reply With Quote
Old 12th January 2004, 09:32 PM   #6
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
Today I got another two Fender Twin Reverbs coming in.
Schematic here: http://www1.korksoft.com/~schem/fend..._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
  Reply With Quote
Old 13th January 2004, 04:48 AM   #7
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
Wow what a good pic! Those are some tortured tubes

Sometimes the old coupling capacitors in it go bad and cause the anodes to glow. You might want to test these. Spending $10 on new caps could mean years and years of trouble free service.
  Reply With Quote
Old 13th January 2004, 03:21 PM   #8
diyAudio Member
 
Join Date: Jan 2004
Location: Stratford, Oklahoma
Quote:
Originally posted by Netlist
[B]Today I got another two Fender Twin Reverbs coming in.
Schematic here: http://www1.korksoft.com/~schem/fend..._100_schem.pdf
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...

Quote:
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.
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.

Quote:
On a sidenote I compared the sound of two identical Fenders.
I have to do some more listening but they sound rather different.
Are they really identical?

There are several different circuits in "silverface" Fenders.
  Reply With Quote
Old 14th January 2004, 12:11 AM   #9
Zoran is offline Zoran  Serbia
diyAudio Member
 
Join Date: Jan 2004
Location: Belgrade
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...
__________________
###
  Reply With Quote
Old 14th January 2004, 01:55 AM   #10
svokke is offline svokke  Belgium
diyAudio Member
 
Join Date: Apr 2003
Location: Leuven
nice article on how to measure bias: http://www.aikenamps.com/Biasing.html
see the "Plate/cathode current method" section of the article.
__________________
In illusion comfort lies
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fender twin reverb farts out after switching to standby frickecello Instruments and Amps 2 19th July 2008 06:52 AM
Fender twin Reverb theriffa Instruments and Amps 1 11th May 2008 02:15 PM
Fender super twin reverb jazz Instruments and Amps 1 4th September 2007 08:30 PM
Can you run a Fender Twin Reverb with only two 6L6's? garrett_92exp Tubes / Valves 5 3rd February 2006 11:34 AM
Building a fender twin reverb in the UK bigparsnip Tubes / Valves 11 5th June 2004 10:56 PM


New To Site? Need Help?

All times are GMT. The time now is 08:59 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2