Fender Twin Amp - need help

[Jodo]

Hi

I need help with a 2006 Fender twin amp. Two problems

1) Switching the reverb unit on creates a lot of static on both channels

2) The gain channel has a lot of static. Gain set to 5 is annoying. Gain set to 10 is unbearable.

I thought I could swap some tubes around into different positions and find out if any of the tubes are bad by doing this - but I have no idea which tubes are for what purpose other than the large ones are for the power amp.

Could one of you guys tell me which tubes to try swapping?


http://www.fender.com/support/amp_sc...Amp_SchE45.pdf

[txstrat]

Welcome to the forum.

V1 1st half is for both channels.
V1 2nd half is clean channel.
V2 and 1st half of V3 is gain channel.
V7 is the reverb driver
V5 1st half is the reverb recovery.
V5 2nd half is the mixing triode (to amplify dry and wet signal mix)

Hope this helps

[Twist]

Be careful. Just swapping tubes around may just move the problem from one place to the other. Try a known good set of tubes.

[Jodo]

Welcome to the forum.

V1 1st half is for both channels.
V1 2nd half is clean channel.
V2 and 1st half of V3 is gain channel.
V7 is the reverb driver
V5 1st half is the reverb recovery.
V5 2nd half is the mixing triode (to amplify dry and wet signal mix)

Hope this helps

Thanks so much for the fast reply.
Out of curiosity, what are V4, V6, V8 used for?

[txstrat]

Thanks so much for the fast reply.
Out of curiosity, what are V4, V6, V8 used for?

V4 is the phase inverter
V6 is the tremolo unit
V8 is the effects loop

Think of what Twist said. Swapping tubes around is just a compensation for known good tubes (new ones).

[Jodo]

V4 is the phase inverter
V6 is the tremolo unit
V8 is the effects loop

Think of what Twist said. Swapping tubes around is just a compensation for known good tubes (new ones).

I don't know if the loud static on the gain channel or the static I get when I turn on the reverb is due to bad tubes or not. I thought that if I moved some tubes around and the 'problems' moved then I could order some replacement tubes.

Could there be other causes for bad static sound on the reverb and gain channel other than bad tubes?

This is a great resource. Thanks

[Amp Kat]

Try using the send out to the front or return of another amp. This way you can tell if the noise is from the preamp or power amp section. Take the send from another amp and go to the return of the Twin. Sound good ? Not the power section.

[txstrat]

Could there be other causes for bad static sound on the reverb and gain channel other than bad tubes?

This is a great resource. Thanks

Hiss or static can come from avariety of sources. From bad solder joints to bad tube sockets or bad capacitors to noisy plate resistors, just to name some.
In the first place I wouldn't assume (cheers Enzo) these parts were bad since the amp is not that old.
Try pulling tube six and replace tube 2 with it. Tube 2 is only for the gain channel and since the clean channel doesn't have static it's not likely tube 1, as well as tube 3 which has also part in the clean channel.
So far for the noise in the gain channel.

After that try to replace tube 5 with tube 6 and see if the static with open reverb has gone. If not, replace tube 7 (reverb driver) with tube 6. Tube 6 is a 12AX7 while tube 7 is a 12AT7 but for testing reasons you can try that. It wouldn't harm the amp.

Anyhow, best would be to have at least two known good (new) tubes around to change with the suspected ones.

[jasoncreek]

fender twin amp tubes & functions

great discussion guys.

1. what is V3's purpose in the Twin Amp? you've done every other.
2. the problem I'm having with mine: low output level (although all functions are good). i have a new set of tubes on the way, but i wonder whether weak 6L6s can cause that.
3. also, above 6, my reverb introduces hum.
4. does the 1/4 power setting simply turn off 3 of the 6L6s? if so, which one remains in power? could be useful to know for troubleshooting.
5. lastly, does it hurt to toggle between 1/4 and full power live?

any thoughts? thanks! jc

[Robert M. Martinelli]

Hi Jason, and welcome!

V3A is the last gain stage ( and tone stack driver ) in the "drive" section, V3B looks like a Cathode Follower ( unity gain ) with a double purpose: the anode signal ( high impedance - out of phase with respect to the input ) goes to the Send/Return circuitry, the cathode signal - low impedance - in phase with respect to the input ) goes to the reverb driver ( depending on K3 A/B relay's status ).

As to the power reduction, you can't simply "take three tubes away" from a push pull design, as you need at least two for symmetry; taking two ( output ) tubes away ( and switching the output speaker tap to twice the initial impedance setting for impedance matching purposes ) would halve the output power, and lower the sound perception by a mere -3dB ( 0.707 ).

In this particular design the power reduction is accomplished by halving the +B voltage; this cuts the power to 1/4 and lowers the sound perception by -6dB ( half ) .

It should do no harm to change the output power setting with the amp on IMHO, for your peace of mind, anyway, you might want to put the amp in stand-by, operate the ( output ) power switch and then flip the stand-by switch on again.

As to your problems, it would be advisable for you to start a new thread instead of "hijacking" an existing one, this keeps the various topics well organized and makes searching on the forum easier.

Hope this helps

Best regards

Bob

[Enzo]

Hi Jason, welcome to the forum. I'll address your questions, but may I suggest for your amp, you start a new thread for it. That way we don't wind up trying to discuss two separate amps with their own problems in the same thread - it can be hard to keep them straight in our heads.

1. in post #2 tx mentioned that V2 and half of V3 are the lead channel. V3 is just one of the gain stages there. The other half of the V3 is basically the output of the preamp. We might call it the mixing tube, except the channel switching only sends one channel through it at a time. It then sends the preamp signal to the FX loop circuits (V8). It is not the amp of the preamp, the signal still goes through the loop, the reverb, and the MV.

2. Low output. QUick checks - make SURE your swpeakers are plugged into the main speaker jack and NOT the extension speaker jack. Then try plugging a guitar cord from FX send to FX return. If that restores the sound, the return jack needs service. Make sure the loop send and return level controls are up. DO the same test by plugging the cord from Preamp out jack to power amp in jack.

After that you want to isolate the problem. Plug your guitar into the power amp in jack. Is it clean, clear, and not so weak? How about plugging the guitar into the FX return. Same question. And also, use a cord to send the signal from the FX send over to some other amp to hear what the signal sounds like. Also the preamp out jack. DO either of those soudn the same weak way or are they clear and strong?

3. Does the reverb hum or does it make a WOOOOOO sound? Hum is what happens when you touch the end of your guitar cord. The WOOOO sound is the reverb feeding back acoustically. if that is the case, unscrew the reverb bag and the cable clips so you can set the reverb pan outside the amp cab to see if that makes a difference.

4. Not even close. The 1/4 power switch cuts your B+ voltage - the high voltage the tubes operate from - in half. All four power tubes remain in use.

5. Not really, but why would you want to?

[Enzo]

Oops, looks like Bob slipped one in there while I was composing mine.


The loop circuit has a net inversion of its own, so the signal phase when it gets to the reverb is the same whether it went through the loop circuits or not.

[Robert M. Martinelli]

Oops, looks like Bob slipped one in there while I was composing mine.


The loop circuit has a net inversion of its own, so the signal phase when it gets to the reverb is the same whether it went through the loop circuits or not.

Oops...another simulpost, Enzo!

Yep, of course the signal phase across K3 to the reverb driver is the same.

By saying "in-phase" and "out of phase" I was referring to the signals at V3B cathode/plate with respect to the V3B grid signal. The signal at V3B's plate is then manipulated within the S/R circuitry and comes back to K3 in phase with the signal coming from V3B cathode to be sent to the reverb driver.

Cheers

Bob

[jasoncreek]

thanks

ah, thanks, Bob. i'll see how the new tubes do and repost the problems if they persist.

new questions:

1. why do v1 and v2 call for 12ax7a whilst all others (excluding v7, the 12at7) call for 12ax7wa?

2. i just found the following at another site:

V1a: input gain stage (both channels)
V1b: clean channel gain stage
V2: lead channel gain stages
V3a: lead channel
V3b: gain stage after channel select relay
V4: phase inverter
V5: reverb makeup
V6: tremolo (no influence on sound, only trem intensity)
V7: reverb driver
V8: fx send and return
V9-V12: power amp

do you agree with those descriptions?

3. if you agree, then shouldn't V3 be a "good tube," too? i can see how 4-8 could be nothing special.

4. i'm thinking:

four GT 6L6-GE (or SED or JJ- what is warmest/old-skoolest? zeppelin, hendrix, sabbath...)
five sovtek 12ax7wa (cheap)
one GT SAG-AT7-MPI (for V4, the phase inverter)
two... of every good 12ax7 to see what all's out there. but again, should v3 be a "good tube" too?

thanks again! jc

[jasoncreek]

this is a duplicate... sorry

(guys- i'm posting this at the end in case you wouldn't see it for being a reply to one of your earlier replies. i'm a newby to the posting world, too... sheesh, you should hear me play guitar... or not).

ah, thanks, Bob. i'll see how the new tubes do and repost the problems if they persist.

new questions:

1. why do v1 and v2 call for 12ax7a whilst all others (excluding v7, the 12at7) call for 12ax7wa?

2. i just found the following at another site:

V1a: input gain stage (both channels)
V1b: clean channel gain stage
V2: lead channel gain stages
V3a: lead channel
V3b: gain stage after channel select relay
V4: phase inverter
V5: reverb makeup
V6: tremolo (no influence on sound, only trem intensity)
V7: reverb driver
V8: fx send and return
V9-V12: power amp

do you agree with those descriptions?

3. if you agree, then shouldn't V3 be a "good tube," too? i can see how 4-8 could be nothing special.

4. i'm thinking:

four GT 6L6-GE (or SED or JJ- what is warmest/old-skoolest? zeppelin, hendrix, sabbath...)
five sovtek 12ax7wa (cheap)
one GT SAG-AT7-MPI (for V4, the phase inverter)
two... of every good 12ax7 to see what all's out there. but again, should v3 be a "good tube" too?

thanks again! jc