Red Knob "The Twin"

[spacemanspiff]

hy guys

i have searched for this on the forums but haven't found anything this far.
i have a fender "the twin", with the red knobs. unfortunately the sound is WAYYY to loud.
i've read on the internet, that you can pull two tubes and set the impedance to 8 ohms.
i did that, but the amp started to distort quite early even on the clean channel. so i put those two tubes back in and tried to set the bias and balance. i got values which were way above the desired 0.4V, and i could set the balance even close to 0.

i noticed, that only two of the powertubes (6l6GC) have a "blue glow" inside.

what can i do to get a clean sound again?

replacing preamptubes?
replacing powertubes?

another question is: what is meant by "pulling the two inner tubes" in the manual?

is it (from left to right): 1001 or 0011? (0 being no tube in the socket)
i tried 1001 and when i switched to "High" the amp started to make terrible noises and the left tube started flickering like hell....?

thanks

[Alex R]

That's a whole lot of questions. Basically it sounds like you have an amp with some problems I'm sorry to say.

'Pulling the two inner tubes' means going 1001. That's how this amp is laid out - the two left and two right tubes comprise the two push-pull sides. 'High' or 'Low' should work fine with that.

The blue glow is ok and neither here nor there, but it sounds like you have at least one bad power valve. There may be associated issues inside the amp like open screen resistors. Would you be confident checking/replacing those if we could identify them for you? As I recall they're the big fat resistors that sit between the valve bases on the board.

[Amp Kat]

Couple of the things you can try is back biasing which would require a zener diode (preferably heat sinked to chassis if over 1 watt )from HV CT winding to ground to reduce the B+ voltage to the plates of the power tubes. You could also try 12AT7's in the preamp slots and also maybe a 12AU7 in the PI slot only.

[Enzo]

I don't think the amp needs modification, it is just broke.

Pulling two tubes will cut the power in half, yes, but that is only a 3db drop in loudness. If the problem is the amp is WAY too loud, this won't solve it. The amp already has a high/low switch which selects between 100 watts and 25 watts.

Like KB says, the blue glow itself is not really an issue, but if two glow and two do not, it sounds like one side of the circuit is not operating. And that will indeed sound like crap. The fact you cannot bias is up right is further evidence it isn't right.

ANy tube flickering, sparking, flashing, or otherwise putting on a light show is almost certainly defective. I don't know how old the power tubes are in your amp, but they probably need to be replaced.


The proper bias reading is NOT .4 volts, it is .04 volts - that is 40 millivolts - 40mV. Each test point covers two tubes, so that is 20mV for each. Since the test point is measuring across a 1 ohm resistor, each millivolt equates to a milliamp of current, so that means 20ma per tube.

If you were to remove half the tubes, then the proper bias would no longer be 40mV, it would become 20mV.

And that suggests the poor man's tube tester. Take all four power tubes out. Then put one back in a socket, powr it up and see if you can bias the test point for that socket to 20mV, or something close. Now remove that tube and stick another one in the same socket. See if it will bias out at more or less the same setting on the adjuster. By trying all four tubes one at a time, you can find out if any of the tubes are not working right.

Once you know if any tubes are bad, take a working tube and stick it in the socket next to the one we just used. Does the tube work in that socket too? Now move the tube to the other sockets. This way we verify that each socket will function with a good tube in it.

It is possible you have a bad tube AND some damage under a socket, such as a burnt out screen resistor. Also possible some problem exists with the bias for two if the sockets. The sockets are wired together in pairs, so if bias fails for one socket (or is dragged down by a bad tube) it will also be gone from the neighbor socket.

[spacemanspiff]

thanks!

i think i'll try the "poor mans tube tester..." :-)
since the tubes are probably up to 20 years old, i'll have to buy new ones anyway.
will buy 2 tubes, which is at least budget-wise better and run the amp at 8ohms. i've read a bit about sovtek, fender, Groovetubes etc. but this subject is making me crazy. i think i'll go with the groovetubes.

There may be associated issues inside the amp like open screen resistors. Would you be confident checking/replacing those if we could identify them for you? As I recall they're the big fat resistors that sit between the valve bases on the board.

can i access them without opening screws? if so, i'll look for them the next time i'm in our practice-room.

another thing is, that the channel-switching isn't very fast (takes like 0.25 seconds). can this be tube-related (pre-amp)?

thanks anyways! GREAT support on this board...! (way better than any support in the whole of zurich...)

[stanner]

ummm.....i used to have a few of those redknobs-loved them.
mine had a rocker switch on the front panel that allowed for full/quarter power. that is i think 100W or 25W. i always found that useful for maintaining the same tones at lower volumes.
never pulled a tube.
s

[spacemanspiff]

Each test point covers two tubes, so that is 20mV for each.

sorry, another question, but what do you mean by "each test point" ? as far as i know, there is only one test point for all the tubes...?

[Enzo]

My The Twin schematic shows test point 201 and 202 - one for each side. There are two cathode resistors, R215,R216, one per side - one ohm resistor with diode in parallel. If they changed that, I am not aware of it. If there are more than one version of the schematic, I don't have but one.

The screen resistors are inside, but if you remove all four power tubes, you can check for B+ voltage at pin 4 of each socekt. If it is there, the screen resistor is intact. Alternatively, power the amp down and let it fully discharge. Now measure resistance between pin 4 on one socket and the pin 4 of each of the other sockets. SInce the screen resistors are 470 ohm, there should be about 940 ohms between any two pins 4. Those two methods can be used without removing the chassis.

[spacemanspiff]

My The Twin schematic shows test point 201 and 202 - one for each side. There are two cathode resistors, R215,R216, one per side - one ohm resistor with diode in parallel. If they changed that, I am not aware of it. If there are more than one version of the schematic, I don't have but one.

The screen resistors are inside, but if you remove all four power tubes, you can check for B+ voltage at pin 4 of each socekt. If it is there, the screen resistor is intact. Alternatively, power the amp down and let it fully discharge. Now measure resistance between pin 4 on one socket and the pin 4 of each of the other sockets. SInce the screen resistors are 470 ohm, there should be about 940 ohms between any two pins 4. Those two methods can be used without removing the chassis.

thanks man! i have that schematic too, but it's a jpeg and the resolution is crap, so i can't distinguish the testpoint from another. plus i'm too whimpy to open up the amp...so i'll try the socket-trick. thanks again!

[Enzo]

Remember, when looking at a socket from above, you count the pins in the other direction. In other words, from underneath we count pins clockwise, but from the top we count them counterclockwise.