A beer for the 1st person who can solve this DR reverb issue!

[taddy]

Hi everyone,
My first build. A Fender DR without the trem/vib (v similar to the sluckey tweed deluxe). The dry sound works fine - great tone but maybe a little hissy. However when I turn up the reverb something strange happens. For the first quarter of the reverb pot turn the reverb seems to be coming through and weirdly the hiss on the dry sound disappears. In fact the dry sound seems to be completely eclipsed. However at about 40% the reverb cuts out and there is just hum and no guitar signal whatsoever. This continues up to max.
I've tried replacing the rev pot.
I've tried changing the tubes
I've tried replacing the RCA connectors. I've tried them insulated and non insulated from the amp chassis
I've tested the resistors throughout the reverb circuit.
All to no avail.
One weird thing is that if you put a chopstick/finger near but not touching the junction where the dry and reverb channel mix (i.e. 10pf,3m3, 470k,220k) it induces a massive hum like this junction is massively oversensitive.
I don't have a reverb pedal on this.
If you can solve this I'll send you a warm ale from England!
Thanks for all your help in the past

Tom
UK
 

[TIMBO]

Reduce the 3m3 resistor to 100k and see what that does.

[purpletele]

Hi everyone,
My first build. A Fender DR without the trem/vib (v similar to the sluckey tweed deluxe). The dry sound works fine - great tone but maybe a little hissy. However when I turn up the reverb something strange happens. For the first quarter of the reverb pot turn the reverb seems to be coming through and weirdly the hiss on the dry sound disappears. In fact the dry sound seems to be completely eclipsed. However at about 40% the reverb cuts out and there is just hum and no guitar signal whatsoever. This continues up to max.
I've tried replacing the rev pot.
I've tried changing the tubes
I've tried replacing the RCA connectors. I've tried them insulated and non insulated from the amp chassis
I've tested the resistors throughout the reverb circuit.
All to no avail.
One weird thing is that if you put a chopstick/finger near but not touching the junction where the dry and reverb channel mix (i.e. 10pf,3m3, 470k,220k) it induces a massive hum like this junction is massively oversensitive.
I don't have a reverb pedal on this.
If you can solve this I'll send you a warm ale from England!
Thanks for all your help in the past

Tom
UK

I just came from that road.  I had just finished 3 amps with reverb.  I had an event that took out the primary side of the reverb transformer on one amp.

The other one which was a Deluxe gave me fits for a couple of days until I realized I had left off a plastic ring on a gold reverb jack.

I'll have a beer anyway!  Anxious to see what your issue turns out to be.  I think I mixed up the 3.3 Meg at one point and used 3.3 K, but I found that before it was an issue.

Your description of the massive hum would make me think you have an incorrect component in that circuit.

Can you post photos?  Don't know that I can help, but I have certainly spent time in the reverb circuit recently.


BV

 

[sds1]

Replace the 10pF cap if you have an extra handy.

[sluckey]

the junction where the dry and reverb channel mix (i.e. 10pf,3m3, 470k,220k)

Did you forget to connect the 220K to ground? With amp off, measure resistance from that junction to chassis with the reverb pot set to zero. Then, measure resistance from that junction to chassis with the reverb pot set to max. What are the two resistance readings? Should be about 150K and 159K.

[taddy]

Hi everyone,

Thanks for your suggestions.

I've tested the resistance as sluckey suggests (thanks Sluckey!)  but it is exactly as you say it should be.

PURPLE TELE: Thank you for the reassurance. Its definitely a 3m3 resistor as I've measured it.

SDS: I don't think I've got a silver mica 10pf cap handy although I might have a ceramic one. Would that really explain the behaviour? I thought the 10pf cap is just letting through some high frequencies of the dry signal. Maybe I've misunderstood the circuit.

I could try the 100k res instead of a 3m3 but I'm worried that's gonna make the dry signal very loud. What does everyone think?


I'm using a 4ab3c1b Accutronics tank and a Hammond 1750A reverb transformer. At the mo I have both the IN and OUT rev sockets non-isolated wrt the chassis and I have the green rev trans wire connected to the input and the black connected to the 'sleeve' of this input jack (i.e. grounding at the chassis on the input.) The output signal goes to the grid of v3 (7) has 220k resistor to ground (pre-amp bus bar - should it be grounded to the chassis next to the rev sockets?)   

[sds1]

SDS: I don't think I've got a silver mica 10pf cap handy although I might have a ceramic one. Would that really explain the behaviour? I thought the 10pf cap is just letting through some high frequencies of the dry signal. Maybe I've misunderstood the circuit.

Wild ass guess mostly targeting the noise issue you reported when getting your hand close. I guessed the silver mica because you read about how the cheaper SM's can go bad. I agree it seems to be only a bypass for treble. You could probably just lift the 10uF and see if it has an effect.

Like I said just a guess, but you had already done some guessing yourself and hey... Chance at a free beer.

[taddy]

Hi SDS. Thanks for the suggestion. I tried lifting the 10pf cap but no difference I'm afraid.

This is driving me crazy!

I also tried reversing the outputs on the rev transformer to see if that made a difference, thinking that there was some out of phase cancelling going on when I upped the reverb level but with the black to tip and green to ground it produced exactly the same problem.

What is stumping me is that at about 50%-100% on the reverb the dry signal is completely cancelled and all you here is hum. from 20-50% you get some reverb but there is a quite a bit of hum although the background noise is reduced.

[taddy]

Just tried putting the 220k ground resistor next to the reverb sockets as opposed to on the board. No difference there either!

[sluckey]

Take some voltage readings. Show us some pics. Show us your actual schematic. Compare your schematic to your layout to your actual amp.

[92Volts]

My guess is DC voltage on the reverb pot.

Assuming your amp is similar to this schematic: http://fenderguru.com/wp-content/uploads/2010/04/Deluxe-Reverb-AB763-bright-cap-Schematics.png

There is a cap listed as .003u between the plate of the 7025 reverb recovery tube, and the 100k pot. If it's leaking/shorted or missing, this would pass DC voltage and screw up the bias where reverb is re-injected. The shared cathode resistor of that 7025 and the 470k/220k voltage divider keep things together for a bit, but eventually the tube's biased fully on (saturated) and can't amplify.

[jjasilli]

Take some voltage readings. Show us some pics. Show us your actual schematic. Compare your schematic to your layout to your actual amp.

A beer for the 1st person who posts an actual schematic with voltages

[taddy]

 
Thanks guys. I'm at work now in Limey land but will post the voltages later on then we can all trade beers.

[sluckey]

Thanks guys. I'm at work now in Limey land but will post the voltages later on then we can all trade beers.

Voltage readings will be much more useful if we have your schematic. Then we don't have to guess what V3 is used for.

[taddy]

Hi everyone,

Thanks for all the suggestions. I've attached a photos of the build. The schematic is the same as Steve Luckey's Tweed Deluxe Reverb so the reverb circuit is essentially the same as a Blackface Deluxe Reverb.

This is a link to me explaining the layout wrt the reverb part of the circuit. https://www.screencast.com/t/6SJIvpRm5
https://www.screencast.com/t/6SJIvpRm5

I've measured the voltages throughout the circuit and there seemingly no DC leaks across coupling caps.

In terms of plate voltages etc Valve 2 (the reverb tube into transformer is showing 8.5, 0, 440 (cathode, grid, anode), Valve 3 is showing 1.7, 0, 207 on the reverb pickup (pins 8,7,6) and 1.7,0,223 on the dry, reverb mix boost stage before the phase inverter (pins 1,2,3)

Sorry for the rambling nature of the commentary in the video above

Thanks again for all your help!

Tom

[jjasilli]

At this point I would:


1.  Disconnect input signal to & output signal from the reverb circuitry.  How does the rest of the amp perform now?  If OK, then
2.  Re-attach reverb signal input only.  Use a listening amp or 'scope to check reverb output.


BUT:  Sluckey's Tweed Deluxe:  where is this schematic???  schematic??? schematic??? schematic??? schematic??? schematic???


No vibrato??? Is there a "vibrato" or 2nd channel?  Or just one channel? Where is your reverb sourced?  Is your reverb signal out of phase with your dry signal?

[John]

http://sluckeyamps.com/tdr/tdr.pdf

[jjasilli]

Thanks! Reverb phase is OK, assuming that taddy's amp matches sluckey's schematic.  What taddy actually says is: A Fender DR without the trem/vib (v similar to the sluckey tweed deluxe).  (emphasis added)

[mat janssen]

When I looked good I see NO filaments wired !!

[John]

Pretty sure that pic was taken before he wired them up. Otherwise, he'd have no sound of any kind- hissy or otherwise.

[taddy]

Hi guys.

Yes. Everything is wired up with heaters etc and I say the dry sound is superb if a little on the loud side. Sorry about the photo. My phone died at the weekend and so have not been able to take an up to date pic for you guys. The circuit is identical to the sluckey apart from

On my amp c16 c17 are .1uf as the DR not .047
I don't have a master vol control so c13 connects direct to r23. I might chuck in a resistor to attenuate the signal at this point a little as the sound is pretty bloody loud!
I have one input with 1m to ground and 33k between input and grid of first stage.
I don't have the c5 bright cap and switch
I don't have the 220k resistor to ground on the rectifier 5VAC heater
There is no reverb footswitch.
I have added a 100k RAW pot at the bottom end of the tone stack (plus kept the 6k8 resistor in series). This works very nicely giving a bit more body if needed.


I will post a video tonight using my wife's phone of the reverb/mix prob. This will show what happens when I turn the reverb on and up. Basically It becomes hummy and, worse, it seems to be killing the dry signal feed which is why I previously thought there was some kind of phase cancellation thing happening. I tried reversing the reverb transformer output wires but no difference.

One thing that seemed weird to me is that when I turn the reverb pot up this affects the plate voltages - they drop by about 10-15V . Is that normal!?? I can't see why this would happen.

I can't detect any DC leaks across coupling caps.

I don't have an oscilloscope but I'm going to make some kind of testing lead to check the signal at various points in the reverb circuit. Will keep you posted.

[sluckey]

I don't have a master vol control so c13 connects direct to r23. I might chuck in a resistor to attenuate the signal at this point a little as the sound is pretty bloody loud!

You need a resistor to ground there to tame the gain and make it sound like an AB763. Hoffman uses a 47K and I chose to use a 100K MV pot for the same reason.

What is the measured resistance between V3 pin 3, 8 to chassis?

[taddy]

Its reading 1.7V to ground.

[sluckey]

What is the measured resistance between V3 pin 3, 8 to chassis?

[taddy]

Sorry Steve.

I can't do it at the moment but it was nothing unusual as I checked all the cathode resistances prior to turning it on for the first time. It would be 820R within acceptance range.

[taddy]

Here is the video showing the mixing issue when the reverb is turned up. Basically it kills the dry signal (I think): -

[sluckey]

I asked about the resistor because from the pic you posted I cannot make out the colors of that resistor. I'm looking for 820Ω, grey, red, brown, but don't see that.

One thing that seemed weird to me is that when I turn the reverb pot up this affects the plate voltages - they drop by about 10-15V . Is that normal!?? I can't see why this would happen.

Which plate voltages are affected? There is no way that turning the reverb pot should change the plate voltage of any tube. Look for a wiring error. Disconnect one end of C11. Does the reverb pot still change plate voltages?

[taddy]

Good idea. I'll try that asap.

I didn't check all the plate voltages but I think it was at least V1 and V3. I'll get back to you on this with a def answer later.

I was thinking the same thing in that the rev pot shouldn't affect it at all. But then there is seemingly no DC leak as far as I can detect.

[taddy]

So the plate voltage drops when you turn up the reverb pot seem to happening on all the valves (or at least valves 1-3, which were the ones I tested).  So pins 1 and 6 on valve 1 dropped from 207 to 192. Pin 1 and 6 on V2 436v to 404v. Pin 1 on V3 was 224 to 207 and pin 6 on V3 was 206 to 192.

I disconnected C11 by disconnecting the top end of the reverb pot and there was no drop on any of these valves when the reverb pot was turned up.



Could it be the C11 cap?

I'm also a bit confused about the pin 1 voltage on V3. It has the same 100k resistor as pin 6 (both read 99k) and both sides of the valve share the same 820R cathode resistor (which is reading just under 820). So why is pin 1 at 224V and pin 6 at 206V? Sluckey has them much closer on his reads on the schematic (206 and 209). If the C11 cap is leaking slightly, could this be affecting pin 1 of V3 by messing with the grid of that stage? And could this explain the massive hum that is induced when I put anything near but not touching the R20, R13, C8, R21, V3pin2 junction?

Thanks again for all the help everyone and to Sluckey for his design and the schematic.
 

[sluckey]

I disconnected C11 by disconnecting the top end of the reverb pot and there was no drop on any of these valves when the reverb pot was turned up.

While C11 is disconnected from the pot measure the voltage on each end of C11. What have you?

[taddy]

206V on the plate side and a small voltage on the pot side (disconnected) which oscillates around 0.35V. This is with nothing plugged into the input of the amp.

[taddy]

Hi guys

I changed the .033 cap. No change unfortunately.

I did notice that that if I tap the chassis around the v3 area it the noise was amplified - kind of live sounding in comparison to the rest of the chassis.

Could I ask your opinion on the best way to ground the reverb jacks? At the moment they are not isolated from the chassis and I have the black output lead from the reverb transformer (Hammond 1750A) going to the sleeve lug of the reverb return. The 220K resistor from the tip of the return is grounded to the sleeve  lug of the send jack.

[taddy]

Hi people,

Just a quick update if anyone is still there!

It seems more or less ok now. The HT wire feeding into the reverb transistor primary (red wire) I had running on the floor of the chassis under all the wires to and from the pre amp tubes and connected to the 470R resistor on one of the power amp screens. This seemed to be creat3ing a lot of hum so I rewired last night and now it runs to the filter cap. Its the same locus on the circuit but a much better route and seems to have solved the issue.

When I last tested it I was still getting that weird plate voltage drop of around 10v when I turn the reverb up and I still can't figure out why exactly but the reverb is sounding good even if the dry/wet mix needs a bit of balancing.

I'm gonna install a 47k resistor as mentioned previously to curb the loudness.

Thanks for everyone's help on this!!!