Fender Twin 65 Reissue Preamp

[TomCarlos]

Friends....

I am working on a Fender Twin 65 Reissue. The very low output seems to be traced back to the preamp stage.

I see to have a similar problem in both channels, but my troubleshooting has been on the Normal channel. I have swapped 12AX7 tubes and get similar results. So here we are.

I measured the dc voltages for V1A and V1B.

VR4 (plate resistor for V1A) has 408 volts on X, 272 volts on Pin 1. The 136 volts / 100K gives us approx 1.3ma.
VR5 (cathode resistor) has appr 2.0 volts across it. 2.0v / 1.5K is approx 1.3ma.

VR11 (plate resistor for V1B) has 408 volts on X, 272 volts on Pin 1. The 136 volts / 100K gives us approx 1.3ma.
Vr23 (shared cathod resistor with V2B) measures approx 2.15 volts (2.15 v/ 820 ohms) / 2 = appr 1.3ma.

Per the schematic, I have 12mvac on Pin 2 of V1A (TP24).
I am measuring apprx 512mvac on Pin 1 of V1A (Plate).
With the volume, treble, and base set to 5, I am measuring 14mvac at Pin 7.
On v1B, I am measuring 850 mvac at Pin 6.

I do not understand what I should be seeing at TP 5. The schematic shows 2.0 vac?

On V4B, Pin 6, I am seeing approx 30mvac. TP 15 says I should see 4.2 vac?

It looks like the preamp tubes are idling ok... just not sure what is happening to my signal.

Any hints for troubleshooting would be appreciated.

Thanks, Tom

[lowell]

I'd start with disconnecting the top of the volume pot. Will tell us if its in the tone stack or proceeding stage. Most likely the tone stack. A pot or cap.

[Tom Phillips]

...I do not understand what I should be seeing at TP 5. The schematic shows 2.0 vac?
On V4B, Pin 6, I am seeing approx 30mvac. TP 15 says I should see 4.2 vac?...

I suspect the V4b stage. Please tell us the DC voltages associated with V4B just as you did regarding V1 & V2.

[Randall]

"I do not understand what I should be seeing at TP 5. The schematic shows 2.0 vac? "

That's 2.0 vdc.

[TomCarlos]

Ok... Duh.... If the TP starts with a (+) sign, that is a DC voltage.

For V4B

Plate Pin 6 - 256 Volts
Grid Pin 7 - 3.1 mVolts
Cathode Pin 8 2.25 Volts

For V4A

Plate Pin 1 - 256 Volts
Grid Pin 2 - 3.1 mVolts
Cathode Pin 8 2.25 Volts

As I increase the AC signal, I can see the output bust into an oscillation. This was well below full power. So for now, I want to try and follow the signal in the preamp.

[TomCarlos]

Ok.... making some progress....

I pulled the front PC board and cleaned the pots, check solder connections, etc.

I can now pass a signal to the Phase Inverter that is clean. I have 18 vac at TP 19. I have a clean signal on both sides of the 1.5K resistors. The problem is with the output - I can only get 9 Vac at the output. I have a 4 ohm resistor load. I will recheck the 1.5K resistors (all 4) and the 470 Resistors (all 4).

[Tom Phillips]

Ok... Duh.... If the TP starts with a (+) sign, that is a DC voltage...

It turns out to be that way by coincidence. The "code" is that voltage figures in ovals are AC signals and voltage figures in rectangles are DC readings taken with no AC voltage present.

Attached is a copy of page 8 from the service manual which shows the whole schematic. The notes in the lower left corner are important test setup information. Let's look at the whole amp circuit during the discussions rather than the peep hole views.

In your earlier testing it seemed like you were saying that you were injecting a signal into the normal channel and you were reading unusually low AC signal on V4B pin 6 (the plate). Note that the Normal channel signal does not pass thru V4B. Only the Vibrato channel passes through V4B. You would have just been reading cross-talk bleed-through signal there if I correctly understood your setup.

65_Twin_Reverb_ReissueE73.pdf

Edit

...I will recheck the 1.5K resistors (all 4) and the 470 Resistors (all 4).

And you can also measure the screen voltage at pin 4 of each 6L6 to cut to the chase. One or more of the 1.5K or 470 resistors would need to be Way Way off (or open) to cause the problem you describe.Normally the screen voltage will be nearly equal to the 6L6 plate voltages.

[TomCarlos]

Thanks Tom....

For now, I want to focus on the Normal Channel. So the controls on the Vibrato Channel are a "1". Yes, I have the full schematic- thanks.

I see that the normal channel uses both sides of V1. I have a clean signal at the Phase Inverter and am measuring 20 Vrms at the 1.5K resistors. The problem is that I am only getting 10 VRMS at the output. I have a 4 ohm load.

The Screen Voltages on all 4 tubes (Pin 6) is at 434 volts dc, the same as the plate voltages.

[g1]

The schematic shows 9V output when there is approx. 20VAC at the PI plates. That sounds like what you have described?

[TomCarlos]

Yes it does... the problem I was having is that I could not increase the output beyond that point without massive distortion and oscillation of the signal. I did some socket cleaning, some extra spot soldering, etc etc etc and now I get 80 watts rms before clipping. I'll take that.

I''m gonna fire the amp up again tomorrow to make sure it's working.

Stay tuned....

Tom

[TomCarlos]

Ok.... I abandoned this amp for a bit but had to come back to it. It's the same problem... distorted output.

I went back to check the dc voltages on Tubes 1 - 5 just to make sure I did not have a problem there. Every looks ok. I switched the Tremolo on and off and can see the tube voltages changing as per the schematic (and I can hear the Tremolo working).

For the phase inverter, Tube 6,

V6A
Y = 427 vdc (dead nuts as per the schematic)
Pin 1 = 227 vdc (~2.43ma through R42, 82K)
Pin 2 = 56 vdc
Pin 3 = 90 vdc (a bit short of the schematic's 99 vdc)

V6B
Y = 427 vdc (dead nuts as per the schematic)
Pin 6 = 240 vdc (~1.86 ma through R43, 100K)
Pin 7 = 59 vdc
Pin 8 (90 vdc - connected to pin 3).

R41, the 470 ohm has 2 volts across it- ~ 4.25ma.

Does this seem right for the phase inverter?

[Enzo]

DC voltages tell us generally that stages are working. VOltages can vary quite a bit. There is a limit to what they tell us.

If you have a relatively clean signal at the grid of the upper triode, is it still clean at the two plates? And then at the four power tube grids?

Have you made a transformer tester? GO the RG's Geofex site and find his tech tip of a simple transformer tester. A shorted turn can kill your signal, and a meter wil never detect it. Likewise I keep an old Bassman OT on the bench as a universal OT sub. You might unhook the plate wires - blue and brown likely - from the power tube sockets, and clip in some other OT. The impedance doesn't matter much for this test, we just want to see if a different transformer wakes it up.

Of course make sure the speaker is plugged into the correct jack - the one with the shorting contact.

I would also use a signal tracer. It is a handy thing to have on your bench, and easily made, but a small cheap amp with a probe for an input makes a fine one. Listen to the signal through the signal path. let your ears work instead of your eyes on the scope.

[TomCarlos]

Thanks Enzo.... I've got a list of items I will be checking. Without having a line out, it gets a bit more tricky to test the midway point of an amp.

I have tested the speakers with another amp- they are solid. Yes, I am plugged into the switched jack. I do have an extra OT from a Traynor that ran 4 5881's so that would be a good test. But before doing that, I want to eliminate the other possibilities.

Using a 1 khz signal, the amp will put out 72 watts rms before clipping. The signal looks clean. The amp sounds ok if I am hitting the high strings. The problem is with the lower stings and notes or if I hit a chord. It almost sounds like speaker distortion but I know the speakers are good.

I might pull 2 of the 4 tubes and run either tubes 1 and 4 or 2 and 3 with an 8 ohms load. Maybe this will tell me if there is a difference in tubes or the sockets. I have multiple sets of 6L6 tubes to test with. But I tried that before- I think the problem is elsewhere.

Thanks...

[Enzo]

You don't have to install the different transformer, just clip the three primary wires into the circuit, and connect a speaker directly to its secondary. Don't even worry about NFB.

[TomCarlos]

I used another transformer and connected as Enzo suggested in #14. I could still hear the problem. So we should assume the Output Transformer is ok.

Here is how I am testing.....

Problem is occurring on both channels
Reverb tank disconnected
All controls on the Vibrato Channel are at "1"
All controls on the Normal Channel are at "5"
Testing is being done on the Normal Channel

I have 12mvac at grid of V1A.
I have 512mvac at the plate of V1A (a gain of ~43)
I have 26mvac at the grid of V1B
I have 662mvac at the plate of V1B

At the junction of C18 (.047uf) and R35 (220K), I am seeing 320mvac.
Even when turning up the volume control, there is no way I can get to 5.8V pk-pk at TP16.