Twin Reverb - help with troubleshoot

[überfuzz]

I'm trying to fire the engine of a twin reverb. A summary of what I've got so far:

Power switch on - silence
Power switch, stand by on - evident ground hum
NORMAL channel - silence
VIBRATO channel - squeaks if I turn the volume. Starts around 2-3.
Plate voltage is 360 V on all tubes, measured on pin 4 on the socket.

First of all, plate voltage is set to ~460 V on the circuit scheme. Is 360 V a problem? Second, any suggestions of where I should start looking for glitches?

[Enzo]

Your terms are confusing. None of the sockets has a plate on pin 4.

Pin 4 of the power tubes will be almost the same voltage as the plates. The plates are pin 3 on the power tubes. but on all the small tubes, the plates are on pins 1 and 6.

First make sure ALL the tubes have heaters glowing inside.

You want to find about +460v as you said on pins 3 and 4 - more or less, doesn;t have to be exact - on each power tube. 360 is not good, it means the amp is not healthy in one of several ways. Set your meter to AC volts and measure the same pins 3 and 4. If you see something like 80-100vAC there, you have lost a filter cap or two.

Also VERY important to get something like -50v on pin 5 of each power tube. If that is missing, your power tubes will soon get red hot, and that will drag your 460 volts WAY down.

[überfuzz]

Sorry about my confused post...

Here's what I have, after redoing the probing with your input in mind.

Pin 3 - 360V - all tubes
Pin 4 - 360V - all tubes
Pin 5 - -50V - all tubes

All 10 tubes are glowing.

Why should I measure with AC? Would I get AC if a cap has gone? And 0 AC there if all caps are healthy..?

With multimeter on AC
Pin 3, pin 4 ~780 Vac

[überfuzz]

Seems I called 50 Hz hum, ground hum. I believe the sound is from 50 Hz... Thats the frekvency we're using in Sweden - Europe.

[Enzo]

Why should I measure with AC? Would I get AC if a cap has gone? And 0 AC there if all caps are healthy..?

Exactly so. The purpose of filter caps is to smooth the DC power supply into a steady DC voltage. If the filter is not doing its job, then the resulting DC is very bumpy - unfiltered. The result is a LARGE amount of ripple, which can be measured as AC voltage.

[überfuzz]

I did some further measurements. I can't find any bad grounding.

Rectifier output:
Power switch + Stand by switch
ON + OFF : 300V
ON + ON : 360V

[überfuzz]

Hope you're indulgent with my noob questions. Heres more:

1. Is it safe to unplug the speakers during the troubleshoot?
2. Is it safe to unplug the reverb tank during the troubleshoot?

[Krwkka]

Disconnecting the speaker load with the amp on is a definite no-no unless you have a dummy load on the output.

If you remove the third tube from the right (12at7 reverb driver) it's safe to disconnect the reverb tank.

_

[überfuzz]

Hi all!

I just replaced the filter caps and fired the engine. At the moment the amp is behaving nice at idle. However, there are two issues now. Might be more but when I tested all the features like vibro, reverb etc I could only find two things.

I used a guitar to test the functions described below. The amp is now properly biased yet. It's turned down quite low.

1. NORMAL channel
   Seems to be working like a charm.
2. VIBRATO channel
   The volume in low, no power what so ever. (There are some some squeaks in the pots. It won't keep me awake at night but still, it's there.)
   1. Reverb on
      Seems to be working.
   2. Vibro on
      The tremolo is oscillating nicely, but there is a tapping sound. I believe it's each time the volume peaks, at the end of each wave.
      If the volume is over 6-8 theres a LOUD squeaking sound if the guitar is played.

Voltages that used to be a problem are now. No VAC could be find.
B+ 461 VDC
pin 3 459 VDC - all sockets
pin 4 459 VDC - all sockets
pin 5 -59 VDC - all sockets

[Ted]

[LIST=1][*]NORMAL channel
Seems to be working like a charm.[*]VIBRATO channel
The volume in low, no power what so ever. (There are some some squeaks in the pots. It won't keep me awake at night but still, it's there.)

Try swapping putting V1 in the V2 position. If that doesn't make a difference replace V2 and try V1 in the V4 position.

[überfuzz]

You're spot on regarding the bad channel. I shuffled around a bit and it seems that one, perhaps two of the preamp tubes are bad. A bit annoying since they're brand new.

I still get a faint wet tapping sound while the tremolo is on. (Hope my way of describing the sound isn't to confusing...) I read about this issue and the Fender response to it. However, could this be something else?

[überfuzz]

Now the amp is howling out brilliantly. It's a beast! However, there is an issue with the tremolo.

Issue - VIBRATO
When the tremolo is on there is some noise. It doesn't matter what the volume is, the sound stays the same. It's kind of a blowing noise, almost as if you're trying to whistle in tremolo but you can't whistle. At the end of each wave there is a faint 'tad'. Furthermore, there seem to be two different waves in the tremolo at different speed.

Measurements - multimeter
All resistors and capacitors in the tremolo circuit seems to have the right values. The opto-thing shows 47,6 kOhm when the amp is off.

I'm in the dark now, don't know what to look for... Anyone that can point me in the right direction?

[überfuzz]

This is a file of the tremolo. Tremolo or vibrato if you like. Sorry about the format on this file, [amr] sony ericsson phone recording. I'll get a amr to mp3 converter as soon as I have time to do so.

Settings on the vibrato channel for this recording:
Speed = 1
Intensity = 9
(Volume = 0)

[Ronsonic]

I can't get your file to play without work, so...

I will ask if your expectations of quiet from that circuit are reasonable? It is very common, even for pro's who have an amp on the bench listening intently to lose perspective on what it does and what they can expect. Put the amp on the floor, turn up the amp and guitar to playing volume and get perspective on the loudness of the noise.

This circuit can be a bear to troubleshoot.

[überfuzz]

I can't get your file to play without work, so...

I will ask if your expectations of quiet from that circuit are reasonable? It is very common, even for pro's who have an amp on the bench listening intently to lose perspective on what it does and what they can expect. Put the amp on the floor, turn up the amp and guitar to playing volume and get perspective on the loudness of the noise.

I did test the amp at different volumes 1 to 10. Like I said, it's a beast. ;-) There is a problem that's not in the realms of OK background noise. Furthermore, a thing I maybe forgot to mention. Besides the noise, the tremolo has the strange 2 wave shape when I play a guitar through the amp.