RI 65 Twin Reverb with excessive feedback

cjlectronics · 10-06-2009, 08:52 PM

I'm working on a reissue Twin reverb Schematic is here that goes into a squealing feedback at 2 on the volume pot. Here is what I have done so far:
1. Both Vibrato and Normal do the same thing.
2. I have searched this forum up and down and performed a few of the suggested things like turning the reverb up a little and then banging on the amp to see if I can get it to slip into the feedback... this resulted in NO change. It did NOT force the amp into a feedback so I don't think I have a loose connection.
3. I have swapped out V1, V2, V3 (with a 12AVT), and V4, but no real change. The amp went into feedback with these replacement tubes.
4. Something of significance is that the feedback only happens when the treble knob is increased above about 3 ON BOTH Channels. If the treble is turned way down it will not feedback.
5. Also the bright switch when engaged causes a horrible hiss and grappling sound in both the Normal and Vibrato channel. In the vibrato channel it pushes the amp into feedback and squealing instantly.

I have poked around with my DVM using an 8 ohm dummy load and following the schematic to measure the test points. My measurements don't jive with the TP values? I have read the notes over and over again trying to verify I have the test conditions setup right. Are some of the voltage values on the schematic an AC measurement and some are DC measurements? For example, TP15 at V4B measures 250vdc, where do they get 4.2v on the schematic?

Hopefully someone can shed some light on this problem. Oh, I have also pulled the pc board and inspected the solder joints, they look good. Has anyone else seen this problem?

Thanks,
CJ

52 Bill · 10-06-2009, 10:15 PM

What you seem to be describing is not an acoustic feedback problem that would respond to a physical stimulus, but a high frequency oscillation.

Check the amp for broken ground connections, bad lead dress, and maybe even bad filter caps in the pre-amp stages.

Prairie Dawg · 10-06-2009, 10:15 PM

Quote:

Originally Posted by cjlectronics

I'm working on a reissue Twin reverb Schematic is here that goes into a squealing feedback at 2 on the volume pot. Here is what I have done so far:
1. Both Vibrato and Normal do the same thing.
2. I have searched this forum up and down and performed a few of the suggested things like turning the reverb up a little and then banging on the amp to see if I can get it to slip into the feedback... this resulted in NO change. It did NOT force the amp into a feedback so I don't think I have a loose connection.
3. I have swapped out V1, V2, V3 (with a 12AVT), and V4, but no real change. The amp went into feedback with these replacement tubes.
4. Something of significance is that the feedback only happens when the treble knob is increased above about 3 ON BOTH Channels. If the treble is turned way down it will not feedback.
5. Also the bright switch when engaged causes a horrible hiss and grappling sound in both the Normal and Vibrato channel. In the vibrato channel it pushes the amp into feedback and squealing instantly.

I have poked around with my DVM using an 8 ohm dummy load and following the schematic to measure the test points. My measurements don't jive with the TP values? I have read the notes over and over again trying to verify I have the test conditions setup right. Are some of the voltage values on the schematic an AC measurement and some are DC measurements? For example, TP15 at V4B measures 250vdc, where do they get 4.2v on the schematic?

Hopefully someone can shed some light on this problem. Oh, I have also pulled the pc board and inspected the solder joints, they look good. Has anyone else seen this problem?

Thanks,
CJ

Interesting stuff. I had a Crate a while ago that'd do the same thing, tracked it down to an improperly assembled reverb tank. Have you tried it with the reverb tank unplugged?

cjlectronics · 10-06-2009, 10:58 PM

Thanks for the replies,

The amp isn't that old so I wouldn't think the caps are bad yet???

Now bad lead dress and bad ground could be a possibility. I'll check it out

The amp is sitting on my bench with the reverb tank disconnected. So, I din;t think the reverb is an issue.

52 Bill · 10-06-2009, 11:28 PM

Quote:

Originally Posted by cjlectronics

The amp isn't that old so I wouldn't think the caps are bad yet???

If it uses those tiny Illinois brand 22@500v caps, I've seen a number of them go bad early on.

melvin · 10-07-2009, 02:26 AM

There are both AC measurements and DC measurements on the schematic. With your DVM, I think you'll want to check the DC or "VDC" measurements. The most telling ones can be the plate voltages. You may have to look to the reference point in the power supply to see what they are supposed to be. (For reference, the non-reissue schematics are sometimes easier to read.)

The AC voltages are usually labeled with VAC or "V peak to peak" and are really only useful when the amp is already working somewhat normally, in which case you use a waveform input and an oscilloscope at the test points. Not that the oscilloscope wouldn't be helpful in this situation, but comparing the DC voltages to the schematic may give you a clue to what's going on in the amp without the need for an input, etc.

As 52 Bill has already suggested, I would suspect a faulty ground in the preamp. On the reissues this is sometimes just a wire clipped to a connector on the board. Not to be trusted.

melvin · 10-07-2009, 02:39 AM

Having looked at your schematic, it seems all of the voltages without units (mV or V) are the DC voltages. The others may be RMS voltages because there are also pk - pk voltages labeled.

Don Moose · 10-07-2009, 04:06 AM

Think those are related to the 'lytics with the counterfiet electrolyte that buggered the computer industry for a while (3 - 5 years ago, IIRC)?

Prairie Dawg · 10-07-2009, 02:48 PM

Quote:

Originally Posted by Don Moose

Think those are related to the 'lytics with the counterfiet electrolyte that buggered the computer industry for a while (3 - 5 years ago, IIRC)?

I'm going to be real interested in hearing how this problem ends up getting itself fixed.

You've pretty much eliminated everything that I would have thought of. Only thing I can think of that you haven't done is change out all the electrolytics. My whole take on them is to establish a baseline of known good parts to work from, and you're already inside.

MWJB · 10-07-2009, 02:52 PM

voltages in the ovals are AC, voltages in the rectangles are dc.

Schem shows voltage supplied at "X" to V4b's plate is 393vdc, a 12AX7 in a typical Fender triode returns approx 66% of that on it's plate so I'd expect 259vdc...close enough.

The twin has a 4ohm load, you'll get different readings with a 8ohm dummy load.

cjlectronics · 10-07-2009, 05:52 PM

I have ordered the 22uf/500v and they should be here in a few days. I'll reply back with my findings.

I have checked the ground cables and they all look good. I've seen the IDC connectors from the potentiometer board cause problems with the wires not seated well. I've pressed all of those in and they look fine as well.

I'm starting to lean toward the bad electrolytic caps like 52 Bill recommended. I don't know if this is significant but when I turn the amp off the high voltage drains immediately. I remember on other fender amps I had to drain the high voltage with a 100k resistor to GND. Do these newer RI amps have bleeder resistors or are they designed to drain the caps quicker?

CJ

52 Bill · 10-07-2009, 06:25 PM

Simple test for the caps is to parallel a good one across the one in question and see if the problem stops. I think it's an ESR problem with the caps.

Be careful when doing this test. Attach the new one with the power off and then power up the amp. Shut down the amp and let the charge drain off before removing the new cap.

I've seen these caps fail early in a few of the Blues Series as well as some of the reissue blackface amps.

pdf64 · 10-07-2009, 09:48 PM

Don't forget the cathode bypass electrolytics on the pre-amp. The shared cathode resistors in several places can lead to this type of problem when the bypass caps go bad.
Also try some different 12AX7 tubes, they can have a weird coupling between their sections. Peter.

MWJB · 10-08-2009, 09:23 AM

"I have checked the ground cables and they all look good." Actually measure resistance from the grounded end of the component to the chassis, e.g. grounded ends of cathode resistors, pots etc.

If you leave the standby inplay mode when you power down, the 220K smoothing resistors at the first filter section will drain the caps.

Enzo · 10-08-2009, 03:29 PM

If you think your lytic filters are going soft and loss of decoupling is allowing oscillation, look at the B+ nodes with a scope. If you see signal there, then there is a loss of decoupling.

cjlectronics · 10-08-2009, 09:26 PM

Makes great sense Enzo... I will do this. However, I'm a little concerned as I just have a x10 O-scope probe and a 22?? series Tek scope. Is it safe to measure 500v with this setup? What do you use to scope the high voltages in these amps? I have a high voltage probe but it connects to a fluke DVM.

cjlectronics · 10-09-2009, 03:58 AM

Replaced the 22uf/500v caps and that didn't fix it. I still get the horrible screeching on both channels when the Bright switch is turned on (both channels) and the treble pot is increased above 3.

It sounds OK when the bright switch is turned off and the treble is turned low (both channels) Although it is noisy...lots of buzz.

I scoped the B+ and it is a smooth DC. Maybe I'll try the small electrolytics in the pre amp stages, although its got to be one that is common the both channels.

Again, the reverb is turned down to 0 when the symptoms occur.

I'll keep plugging away and eventually I'll get it.

CJ

melvin · 10-09-2009, 06:17 AM

Did you check the DC voltage at the plates? Does it still feedback with V1 pulled? What about with V1 in and V3, V4 and V5 pulled?

MWJB · 10-09-2009, 09:14 AM

You can't play an amp that has to have the treble turned down.

As Melvin says check the dc voltages especially V1 & V2 (as this seems to affect both channels).

How did the grounds check out, what ohms did your meter read?

Where is the buzz coming from, preamp or output? Pull the PI tube, still buzzing, then it's the output. Pull the rest of the preamp tubes and install one by one from v6, v5, v4 etc...at what point does the buzzing start?

Unlikely to be a layout issue otherwise they'd all do it, grounding is still the prime suspect.

Euthymia · 10-09-2009, 10:02 AM

Just a hunch: pull V4 and measure the resistance between V4 socket pin 7 and the chassis.

Looking for grid leak weirdness there.

Along similar lines, measure resistance from the low side of each volume pot to the chassis.

cjlectronics · 10-10-2009, 03:21 PM

The problem has been solved.

The problem was V4. Going on Melvin's recommendation, I performed a systematic series of pre amp tube shuffling. The problem went away after removing V4 and swapping it with V5... then I confirmed this by replacing V4 with a known working 12AX7. I've been running at 5 on all knobs on both channels for about 3 hours. So far it sounds normal.

I'm a little disappointed in myself for not doing this in the beginning, but every time I work on tube amps I learn a little more each time. Like, a systematic approach of tube swapping first, then dive into the circuit and suspect power supply caps, grounding, etc....

Thanks again for everyone's help. It has not gone unnoticed!

CJ

10-06-2009, 08:52 PM	#1
cjlectronics Supporting Member Join Date: May 2006 Posts: 182	RI 65 Twin Reverb with excessive feedback I'm working on a reissue Twin reverb Schematic is here that goes into a squealing feedback at 2 on the volume pot. Here is what I have done so far: 1. Both Vibrato and Normal do the same thing. 2. I have searched this forum up and down and performed a few of the suggested things like turning the reverb up a little and then banging on the amp to see if I can get it to slip into the feedback... this resulted in NO change. It did NOT force the amp into a feedback so I don't think I have a loose connection. 3. I have swapped out V1, V2, V3 (with a 12AVT), and V4, but no real change. The amp went into feedback with these replacement tubes. 4. Something of significance is that the feedback only happens when the treble knob is increased above about 3 ON BOTH Channels. If the treble is turned way down it will not feedback. 5. Also the bright switch when engaged causes a horrible hiss and grappling sound in both the Normal and Vibrato channel. In the vibrato channel it pushes the amp into feedback and squealing instantly. I have poked around with my DVM using an 8 ohm dummy load and following the schematic to measure the test points. My measurements don't jive with the TP values? I have read the notes over and over again trying to verify I have the test conditions setup right. Are some of the voltage values on the schematic an AC measurement and some are DC measurements? For example, TP15 at V4B measures 250vdc, where do they get 4.2v on the schematic? Hopefully someone can shed some light on this problem. Oh, I have also pulled the pc board and inspected the solder joints, they look good. Has anyone else seen this problem? Thanks, CJ

10-08-2009, 03:29 PM	#15
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,366	If you think your lytic filters are going soft and loss of decoupling is allowing oscillation, look at the B+ nodes with a scope. If you see signal there, then there is a loss of decoupling. __________________ Education is what you're left with after you have forgotten what you have learned.

10-09-2009, 10:02 AM	#20
Euthymia Senior Member Join Date: May 2006 Location: Alameda, CA, USA Posts: 185	Just a hunch: pull V4 and measure the resistance between V4 socket pin 7 and the chassis. Looking for grid leak weirdness there. Along similar lines, measure resistance from the low side of each volume pot to the chassis. __________________ -Erik

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10-06-2009, 10:15 PM	#2
52 Bill Old Timer Join Date: Jun 2007 Posts: 1,253	What you seem to be describing is not an acoustic feedback problem that would respond to a physical stimulus, but a high frequency oscillation. Check the amp for broken ground connections, bad lead dress, and maybe even bad filter caps in the pre-amp stages.

10-06-2009, 10:58 PM	#4
cjlectronics Supporting Member Join Date: May 2006 Posts: 182	Thanks for the replies, The amp isn't that old so I wouldn't think the caps are bad yet??? Now bad lead dress and bad ground could be a possibility. I'll check it out The amp is sitting on my bench with the reverb tank disconnected. So, I din;t think the reverb is an issue.

10-07-2009, 02:26 AM	#6
melvin Member Join Date: Aug 2009 Location: Seattle, Washington Posts: 86	There are both AC measurements and DC measurements on the schematic. With your DVM, I think you'll want to check the DC or "VDC" measurements. The most telling ones can be the plate voltages. You may have to look to the reference point in the power supply to see what they are supposed to be. (For reference, the non-reissue schematics are sometimes easier to read.) The AC voltages are usually labeled with VAC or "V peak to peak" and are really only useful when the amp is already working somewhat normally, in which case you use a waveform input and an oscilloscope at the test points. Not that the oscilloscope wouldn't be helpful in this situation, but comparing the DC voltages to the schematic may give you a clue to what's going on in the amp without the need for an input, etc. As 52 Bill has already suggested, I would suspect a faulty ground in the preamp. On the reissues this is sometimes just a wire clipped to a connector on the board. Not to be trusted.

10-07-2009, 02:39 AM	#7
melvin Member Join Date: Aug 2009 Location: Seattle, Washington Posts: 86	Having looked at your schematic, it seems all of the voltages without units (mV or V) are the DC voltages. The others may be RMS voltages because there are also pk - pk voltages labeled.

10-07-2009, 04:06 AM	#8
Don Moose Senior Member Join Date: Jul 2007 Location: Indianapolis Posts: 676	Think those are related to the 'lytics with the counterfiet electrolyte that buggered the computer industry for a while (3 - 5 years ago, IIRC)?

10-07-2009, 02:52 PM	#10
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	voltages in the ovals are AC, voltages in the rectangles are dc. Schem shows voltage supplied at "X" to V4b's plate is 393vdc, a 12AX7 in a typical Fender triode returns approx 66% of that on it's plate so I'd expect 259vdc...close enough. The twin has a 4ohm load, you'll get different readings with a 8ohm dummy load.

10-07-2009, 05:52 PM	#11
cjlectronics Supporting Member Join Date: May 2006 Posts: 182	I have ordered the 22uf/500v and they should be here in a few days. I'll reply back with my findings. I have checked the ground cables and they all look good. I've seen the IDC connectors from the potentiometer board cause problems with the wires not seated well. I've pressed all of those in and they look fine as well. I'm starting to lean toward the bad electrolytic caps like 52 Bill recommended. I don't know if this is significant but when I turn the amp off the high voltage drains immediately. I remember on other fender amps I had to drain the high voltage with a 100k resistor to GND. Do these newer RI amps have bleeder resistors or are they designed to drain the caps quicker? CJ

10-07-2009, 06:25 PM	#12
52 Bill Old Timer Join Date: Jun 2007 Posts: 1,253	Simple test for the caps is to parallel a good one across the one in question and see if the problem stops. I think it's an ESR problem with the caps. Be careful when doing this test. Attach the new one with the power off and then power up the amp. Shut down the amp and let the charge drain off before removing the new cap. I've seen these caps fail early in a few of the Blues Series as well as some of the reissue blackface amps.

10-07-2009, 09:48 PM	#13
pdf64 Senior Member Join Date: May 2006 Posts: 613	Don't forget the cathode bypass electrolytics on the pre-amp. The shared cathode resistors in several places can lead to this type of problem when the bypass caps go bad. Also try some different 12AX7 tubes, they can have a weird coupling between their sections. Peter.

10-08-2009, 09:23 AM	#14
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	"I have checked the ground cables and they all look good." Actually measure resistance from the grounded end of the component to the chassis, e.g. grounded ends of cathode resistors, pots etc. If you leave the standby inplay mode when you power down, the 220K smoothing resistors at the first filter section will drain the caps.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

10-08-2009, 09:26 PM	#16
cjlectronics Supporting Member Join Date: May 2006 Posts: 182	Makes great sense Enzo... I will do this. However, I'm a little concerned as I just have a x10 O-scope probe and a 22?? series Tek scope. Is it safe to measure 500v with this setup? What do you use to scope the high voltages in these amps? I have a high voltage probe but it connects to a fluke DVM.

10-09-2009, 03:58 AM	#17
cjlectronics Supporting Member Join Date: May 2006 Posts: 182	Replaced the 22uf/500v caps and that didn't fix it. I still get the horrible screeching on both channels when the Bright switch is turned on (both channels) and the treble pot is increased above 3. It sounds OK when the bright switch is turned off and the treble is turned low (both channels) Although it is noisy...lots of buzz. I scoped the B+ and it is a smooth DC. Maybe I'll try the small electrolytics in the pre amp stages, although its got to be one that is common the both channels. Again, the reverb is turned down to 0 when the symptoms occur. I'll keep plugging away and eventually I'll get it. CJ

10-09-2009, 06:17 AM	#18
melvin Member Join Date: Aug 2009 Location: Seattle, Washington Posts: 86	Did you check the DC voltage at the plates? Does it still feedback with V1 pulled? What about with V1 in and V3, V4 and V5 pulled?

10-09-2009, 09:14 AM	#19
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	You can't play an amp that has to have the treble turned down. As Melvin says check the dc voltages especially V1 & V2 (as this seems to affect both channels). How did the grounds check out, what ohms did your meter read? Where is the buzz coming from, preamp or output? Pull the PI tube, still buzzing, then it's the output. Pull the rest of the preamp tubes and install one by one from v6, v5, v4 etc...at what point does the buzzing start? Unlikely to be a layout issue otherwise they'd all do it, grounding is still the prime suspect.

10-10-2009, 03:21 PM	#21
cjlectronics Supporting Member Join Date: May 2006 Posts: 182	The problem has been solved. The problem was V4. Going on Melvin's recommendation, I performed a systematic series of pre amp tube shuffling. The problem went away after removing V4 and swapping it with V5... then I confirmed this by replacing V4 with a known working 12AX7. I've been running at 5 on all knobs on both channels for about 3 hours. So far it sounds normal. I'm a little disappointed in myself for not doing this in the beginning, but every time I work on tube amps I learn a little more each time. Like, a systematic approach of tube swapping first, then dive into the circuit and suspect power supply caps, grounding, etc.... Thanks again for everyone's help. It has not gone unnoticed! CJ