Problems with tremolo and Reverb on fender bandmaster

[Lee_ranaldo]

I'm havin troubles with this bandmaster TFL5005X.

First i must clear some things.

1) New pre and power tubes, rectifier tube.
2) New coupling caps and Tube Oscilator Caps.
3) The amp sounds good, just a little hiss in the background and normal hum with single coils as any guitar amp would do.
4) Reverb Pan is New. (accutronics long delay don't remember the numbers)

The problems.

1) one power tube has a blue light inside, the other doesn't but both has the same amount of heat after using the amp like 30 minutes.
2) the reverb just hums and got just noise.., it seems that the reverb is not getting any signal.
3) the tremolo doesn't work, and i can't see the neon bulb working from the optocoupler. since i've opened the insulation. the LDR is fine.


Which Neon Bulbs will work to replace the bad one?.. somewhere i read that the sine wave produced by the tube oscilator is 200v sine wave , so any neon bulb with a voltage rate of 220 will work?..

According to some text, the original bulb is a NE-2 Model. i can't find a suitable device like an NE-2 Bulb here just pilot neon bulbs and similars. Someone said that i could use a xmas light bulb.

About the reverb. it's seems that the transformer is bad..
measuring at DC resistance one gives like 200K ohms and the another almost zero.

Somebody help me.. and send me some suggestions..
THanks!

[squidsquad]

I'm no *amp tech*...but what I do know:
I believe the 5th tube from the right is the tremelo tube...either 12AX7A or 12AT7A...check it.  Make sure your reverb tank has springs in it.  A common problem is the braided metal wire going in & out of the chassis.  The braids (ground) comes loose from the RCA pins.

[Lee_ranaldo]

Connections from RCA jacks are OK.. (measured with a DMM).

the RETURN signal from the reverb is ok, (if you shake it you can hear the springs working). as i said before. it's a new reverb tank. (accutronics).

All the tubes are brand new. (sovtek and one EI from yugoslavia)

thanks

[AL]

Read the Tube Amp Debugging page at GEOfx. It will walk you through the process. First thing to do is check the tubes. I know you said they're new but check them anyway. Just get a known, working tube and swap it out for the reverb tube. It's the easiest check point and should be the first check point.

AL

[nooneknows]

I don't think the reverb readings are that bad. My Vibroverb had the same problem, I thought it was a circuit problem but, in the end, it came out the reverb driver tube was gone, even if the voltages across it were fine. Try some tube swapping at first.
The color of the tube could be, if I remeber correctly, a different composition of the internal gas, it could be normal, try having a measure of the bias current and the voltages and see if they're right.
*keep in mind the voltages in a fender are HIGH, so be very careful*

[Lee_ranaldo]

the driver tube for the reverb is new! brand new..
it was the same putting an old tube, a working one  , and a new one..

so maybe it's a problem of the circuit.

thanks.

[Ripper]

The blue colour in the tube is nothing to worry about, it is like someone said, usually just an internal coating or a deposit in there causing it to look blue.  I have many tubes over the years like that.  I would however check your bias to make sure that both power tubes are getting the juice they need.  Does the amp sound good (excluding the trem and reverb issues) or does it sound "squashed" sometimes when you play harder?

There is a 25uf/25 volt electrolytic capacitor that is in the reverb circuit (check it), if it has gone bad as they tend to do in these older amps it would do what you say the reverb is doing.  Have you checked your reverb control pot?  Do the solder joints on it look good and what does it read with your meter?  Does the hum get louder when you turn the pot?  If it is bad that could be part of the problem.  It is possible that it is the transformer, but I would doubt if it would hum if it was gone.

The trem as has a couple of electrolytics you should check.  It one of them has gone, it would not allow the neon to work.  Again check your speed and intensitypots for bad solder connections or faulty pot.

Hope that helps somewhat.

[Lee_ranaldo]

Thanks Ripper for your info..

Yes the amp sounds like an old FENDER.. (reminds me to creedence clearwater records). Little Hum and Hiss (like any guitar amp will do). However this is rare because the owner (a friend) have this amp running with a 2x12 guitar cab loaded with celestions (the lower series G12XXX). (sure it screams loud when i engage the bright switch!) No pops or cracks just a little high pitched oscillation when i connect the reverb tank.

this is an Export bandmaster model, the changes from the original design are.

1) Blackface Specs Power Transformer
2) Bias Circuit "blackfaced" (220K resistors, different wiring in the bias adjustment pot).
3) A non-standard volume pot for the vibrato channel, no values printed on. Measured with a DMM the value is about 500K.

The mods are.

1) All Signal Coupling Caps Changed to 630 V Poly Film Caps.
2) Vibrato sections caps changed to poly-film 630 V caps.
2) bias and power supply caps are Sprague Atom.
3) Anode reference Caps are Brand New Electrolitics. (22uf/25V)

3 of the 5 pre-amp tubes and power tubes + rectifier are a year old
2 new tubes (Vibrato and Reverb Driver) are brand new.

About the blue light inside a power tube, it doesn't matter if i switch the tubes, it always gets a blue light the one that is closer to the rectifier tube.
The 220K resistors and 47K resistors falls into the right value measured with my DMM. one of the 1.5 resistors reads 1.3K and the other one falls into the right value. (sure that is the problem i guess).

The problem is, which neon bulb can i use as a replacement for the original one?.

is a NE-2 Neon bulb..? according to the data
the max specs are 200V at 0.01 amps. so can i use an X-mas light neon bulb as a replacement?.

If the reverb tank is not getting any signal but it i move i can hear the springs working?.. is the transformer bad? or just a circuit problem?.

thanks for your replies!

[Ripper]

Okay if you are getting a blue light no matter what tube you put in that socket, you have a bias problem .  If it is the center part of the tube that looks blue (where it should be red/orange) then that is what it is.  sometimes a tube will glow blue inside from gas or whatever.
You need to check that to see what is going on with it.  

For the trem, I know radio shack sells a neon bulb that works for this.
For the trem, have you checked the footswitch to make sure it is working to turn it on and off?


As for the reverb, you'll hear the springs by banging it or moving it around, that is just how they work.  If you had no signal or the tranny was done it would definitely cause this problem.  The transfomer should have 22k resistance on the primary and 8 ohms on the secondary.  Check that and see what you get.

[Lee_ranaldo]

the blue glow comes from the sides.. the center (the filament) looks orange, both filaments has the same glow.

if i remember correctly the transformer windings measured with a DMM, (DC resistance)..220K or more.. , and the secondary almost 0 ohms.. don't remember right.. i got a chance to compare with an twin reverb transformer.

as for the neon bulb replacement i need to know how much current the original bulb had to accept in the vibrato desing.

thanks

[Ripper]

Okay if the blue is glowing from the sides you are okay.  No worries there.

Depending on the range setting of you dmm, the 8 ohm side might appear as 0 ohms.  The 220K seems really high.

I forgot to ask if you opened the heatshrink on the neon far enough to make sure that it's wire leads aren't touching. http://www.tubetoneamps.com/productpages/semiconductors/semiconductors.htm
There is a link to buy the whole opto/ldr for $8.50 all nicely sealed like the original.  



The other thing to do is to go through the amp and resolder all ground connections, clean all the rca connections well with cleaner.

[aron]

re: the blue tube, I would put current sensing resistors in there (either 1 ohm or 10 ohm) and then I would measure the current across these resistors.

This will tell you if there's a problem with your bias on that tube.

It is a little strange that whatever tube you put in there is the one that lights more "blue".

It is possible to make a psuedo "audio probe" within the same amp. I've done it and it is a little spooky, but what you do is use the same blocking capacitor (600V .01uF cap) and a wire. You wire the capacitor+wire to the input of the phase inverter. This is assuming you have a master volume. If you don't, then you can wire a pot in there for a volume control.

Once you do this, it is possible to probe (carefully!) within your own amp using it as the "audio probe". Then you can detemine exactly where the reverb circuit fails.

Connect the end of the audio probe to a wooden stick and use that to probe within the amp.

[Ripper]

As long as the filament isn't glowing blue you are alright.  The blue can have a few causes, from not having all the air sucked out during the vacuum process etc.  It might glow more blueish on the one due to a little juice going to that tube.

Aron is right about using a 1ohm resistor to check your bias.  Lift the wire (or braided wick) that goes from pin 8 to ground, attach the resistor between pin 8 and the wire (ground).  Check your mA reading on both sides of the resistor and that will give you a bias reading.  That amp being a silverface type will have a bias level control, not a true bias adjust.  It is a simple thing to look up how to convert this to a blackface bias adjust circuit (simple to do) and then you can set your tubes more effectively.

I think that you are also at the point where you need to use an audio probe or a signal generator and a scope to trace the reverb issue.

[Lee_ranaldo]

Thanks for both replies ripper and aron!

yes i must check the bias in this amp to be sure. I guess the cause of this problem is a bad resistor. now i had a little time testing the amp (30 minutes). When i turn on the  standby switch after a 50 seconds warm up (yes i know is not too much time).. i hear a high pitched oscillation that ends as son as the voltages inside get in their nominal values.
(the same happens when i unplug the reverb cablesm but in an annoying way)

Both tubes have a little blue light inside, but i've seen a picture of 6l6's Working from "plate to plate's homepage"and one tube looks like the one in that picture, the other one seems blue but weaker. (the one with the correct 1.5K resistor).

I'm gonna make a gizmo for tube amps for checking bias and stuff like that.. since i got a chance of borrowing nice profesional DMM with scope i'm gonna put some 1% metal Film resistors in every voltage reference point (220K resistors, 47K resistors, 1,5K Grid Resistors.).

Every ground point is OK, i've used double insulation wire for the input signals and signals coming from the volume controls. (i don't know if i can do the same to any signal wires inside the tube amp, remember that cables have capacitante that acts as a low pass filter.)

The audio probe sounds nice, but i'm gonna make a safer way to check bias in tube amps, i'm not soo skilleed in tube amps, i know how they work and have some ideas but almost no experience in tube amps.

Thanks for both suggestions..

i'll keep you updated if my tube gizmo for tube amps really works.

thanks!