Deluxe Reverb Clone ..reverb pot acting like a volume control

[ctcpete]

Have a problem with my blackface Deluxe Reverb clone. It had been under the cover for a few months. I broke it out the other day and it has an issue:
The reverb pot is acting like a volume control.
•With the reverb ctrl at 0, the output volume of the amp is like 90% less than normal (at a volume of 10, it sounds like it is at 2) on both channels, so I'm assuming it is circuitry common to both channels.

•If I turn up the reverb pot, the volume comes up with a very strong reverb / distortion type sound. Once again, occurs on both channels.

•I tried replacing each pre amp tube with a known good spare, one at a time, no change. I have no spare power tubes other than for my Twin.. 6L6's ..have no 6V6's and no rectifier tube to try. I was going to order spares but have not yet.

When built, This amp was modded for the reverb to work on both channels with a little more gain on channel 1

Thanks for any advise

[Tom Phillips]

As you describe it your problem is most likely NOT caused by a tube so I wouldn't worry about procuring tubes to substitute at this time. Since you have both a clone and a "mod for the reverb to work on both channels with a little more gain on channel 1" I would start the troubleshooting at the point the two channels join. Did you build this clone? What is you level of expertise and do you have basic test equipment?

[soundguruman]

Basically whoever modified it did it wrong. Sounds like a waste of a blackface deluxe.
And we can not possibly know "what?" they did to it.
You will need to document with pictures, and or schematics what has been done, or nobody would have a clue.
OR you can take it to somebody qualified to work on it.

[Bruce / Mission Amps]

Check to see if the guy forgot to put the 470K "isolation" resistor on the wiper of the reverb circuit's output, ...used to separate the reverb pot from the dry signal.

[Tom Phillips]

Pete,
Based on your original post I am assuming that the amp did originally operate correctly and then developed the problem. Is that right?
Tom

[ctcpete]

Pete,
Based on your original post I am assuming that the amp did originally operate correctly and then developed the problem. Is that right?
Tom

Yes Tom, it did work fine for about 1 year. A friend of mine from a guitar forum built it for me. Basicly channel one is the normal Deluxe AB763circuit and channel one was modded for a little more gain and ability for reverb to operate. It functioned very well.

I have a little experience on amps, did a few projects with the kind help of members here. I fixed industrial size printers and other circuitry for years as a field tech. Very limited tube circuit experience and what I did learn was from a few years back and is not fresh in my mind.

I'll pull the chassis hopefully in the near future and have a look. Was just looking for some starting points / tips or hoping that this was a common problem

Thanks

[ctcpete]

ok, I cleared some space in my garage and set up a little work area. Got the chassis out. Using the layout referenced below, I started checking voltages on the pre amp tube sockets. One thing that I found was on V5 Vibrato tube the voltage on pin 1 is listed on the layout as being 210 VDC (little blurry maybe 270VDC) is at 430VDC. If i turn on the vibrato with the foot pedal I get a flucuating voltage of like 250 to 285VDC. That point connects to a 220K resistor which then connects to a junction point with a 10 meg and a 100K resistor. At that junction point of the 3 resistors there is 440VDC. I ohmed out that 220 k resistor thinking it was the problem but it ohms out fine. Pulled it from the circuit anyway and replaced it , no change.

Does anyone know if the voltage test points listed in the layout for the vibrato circuit are for the Vibrato on or off? Should I have that pin 1 voltage of 210V either way?

I also noticed on V6 phase invertor tube I have some voltages not as listed on the lay out: layout calls for170V on P1, I have 230 at P1 and P6..60V at p3 / 8

Here is the schematic and layout. My friend did change the circuit to have reverb on both channels and for more gain on channel 1. I am trying to find out how he did those mods now. Here are the links

http://www.thevintagesound.com/ffg/s...b763_schem.gif
http://music-electronics-forum.com/a...763_layout.gif

[Bruce / Mission Amps]

Don't get distracted and waste time with the vibrato circuit.... I don't think there's anything really wrong there.
Look at the actual reverb pot too and see if there is a short somewhere... also, look over the mixer stage of the 10pF and 3m3 resistor and don't forget to check that 470K resistor I mentioned previously.
And of course, just disconnect the reverb pot and see what happens to the volume.

[ctcpete]

Thanks Bruce, I'll try all that next crack I get at it and report back, tomorrow probably. I'm kind off a tube amp newbie, as you can surely tell. The 470k resistor is the one that connects to V4 pin7along with a 220 k and a 4.7Meg that has a 10pf cap across it? I'm getting this from the layout. By 3m3 resistor you mean 3.3Meg I assume?

Here are some pictures of the chassis FWIW
Amp pictures by ctcpete - Photobucket
Thanks again.

[pontiacpete]

There's a discrepancy between the layout and the schematic. On the schematic of the AB763 circuit a 3.3meg resistor is used and in the layout there's a 4.7meg.

[ctcpete]

There's a discrepancy between the layout and the schematic. On the schematic of the AB763 circuit a 3.3meg resistor is used and in the layout there's a 4.7meg.

Thank you, had me scratching my head for a few minutes, found the 3.3meg resistor and 10pico cap on the schematic.

[ctcpete]

Don't get distracted and waste time with the vibrato circuit.... I don't think there's anything really wrong there.
Look at the actual reverb pot too and see if there is a short somewhere... also, look over the mixer stage of the 10pF and 3m3 resistor and don't forget to check that 470K resistor I mentioned previously.
And of course, just disconnect the reverb pot and see what happens to the volume.

ok, I checked the reverb pot, ohms out at 100k, resistance varies as expected. Disconnected it (removed the 2 leads that go to it and left the ground connection) no change in my volume problem (volume on both channels very low..with vol pots on 10, sounds like they are on 2 1/2) reverb still comes on and off with the pedal. Before disconnecting the pot I checked continuity to where the leads connect on the chassis.

I looked at the area you mentioned (3.3meg resistor and 10 pf cap) the resistor ohms out ok (in circuit). I have 0VDC at one end of the 3.3Meg the other end goes to ground..see my note below. I did not remove the cap to test it, I can if advised to.

I checked the 470k and 220k resistors that connect with the 3.3M resistor. The 470k is labeled as a 470 k yellow violet yellow but it only ohms out as 195k (varies from 195k - 150K when the reverb pot is turned when I had the pot connected) this seems like a major deviation to this newbie. I have no spare of that value though. The 220k pot is only ohming out as 160k. Significant?

I took some voltage measurements:
on V4: pin 1 is 158VDC, pin 6 is 145VDC pin 3+8 are 2.3v pin 7 (grid of section 1 )is at 0VDC it connects to the junction of the 3.3meg, 220kmeg and the 470K (which is really around 200K) resistors. I read that the grid voltage should be around 0 though, frequency adjuster??

Very sorry for my ignorance, I worked on my old Twin back in 2008, read Ted Webers book but did not use any of it since. Went to electronics school back in the early 80s. Worked as a field tech in the printing industry for 25 years, printing is in the crapper. Currently in the midst of a career change..RN school. My old brain is kind of toast I fear.

Let me know what other info I can provide. Kind of enjoying this. And thanks a bunch.

[pontiacpete]

The 470k is labeled as a 470 k yellow violet yellow but it only ohms out as 195k (varies from 195k - 150K when the reverb pot is turned when I had the pot connected) this seems like a major deviation to this newbie.

Measured in circuit, this is normal.

[ctcpete]

Measured in circuit, this is normal.

I measured it again yesterday and it is 470k (the 470K "isolation" resistor on the wiper of the reverb circuit's output), not sure if something changed or what checked it multiple times, but I was sure it was like 200 k the other day, even remember confirming the color code..weird.

I also heard from the guy who built it, this is how he accomplished reverb working on both channels (he had a marked up schematic lost in a hard drive crash though): This might be best to explain as looking at the circuit layout -
http://music-electro...b763_layout.gif

"Take a look to the left of the octocoupler/roach, on the layout you'll see a spot where there are two 220k resistors with an X and a y marker above them; this is the spot where the two channels merge and go into the power/driving stage.

Normally at this spot, the respective X and Y connect to these resistors - in the case of this amp, the X resistor for the normal channel is removed (or at least not in use).

Now look over to the right on the layout, I am talking about the area between the Z and D - look for the string that goes 1 meg resistor, 500pf cap, .022 cap (below the D). a 220k resistor was added between the 500pf and .022 cap, X from the normal channel (the X as seen on the layout to the far right) connects where the 500pf and 220k resistor meets, then this same spot also connects to Z on the layout. Finally, the x on the far right from the normal channel, a 220k resistor was also added at this point as well, this resistor essentially sits between and separates the two channels."

So in effect, my point Z is where the added 220k resistor and 500 pF cap meet. This is also point X on my amp..I'll call it point XZ. When I tested continuity between point XZ and Z (where the 3.3mg resistor (labeled as a 4.7meg on the layout) is I have no continuity. When I measure I have 3Vdc at point XZ and 0Vdc at point Z and when I touch the probe to point z I get a loud hum through the speaker. I can see two wires coming from under the turret board at point XZ (I assume one goes to X and one to Z) I resoldered that point as well as point z. Before this problem occured the amp was moved and got bumped a little, maybe I lost the connection to point Z? From Bruce's previous post he was pointing me towards the point Z area. I am assuming I should have a voltage at the grid on the other side of the 3.3meg res.

I have two ideas:
1. run a temporary jumper between point XZ and Z
2. desolder point Z and try to fish the wire through the turret board with tweezers etc..maybe enlarge the hole a little if needed. I'm leaning towards this. Assuming a bad connection. Tried to remove the lock washers holding the board down but the board is held down by various wires etc.
Once again thanks for the help..I've been trying this week to learn as much as possible about this stuff. Been trying to find additional V test points other than what is on the layout with no luck. Had a look at the Weber's book I bought years ago not much help for this.

[52 Bill]

So in effect, my point Z is where the added 220k resistor and 500 pF cap meet. This is also point X on my amp..I'll call it point XZ. When I tested continuity between point XZ and Z (where the 3.3mg resistor (labeled as a 4.7meg on the layout) is I have no continuity. When I measure I have 3Vdc at point XZ and 0Vdc at point Z and when I touch the probe to point z I get a loud hum through the speaker. I can see two wires coming from under the turret board at point XZ (I assume one goes to X and one to Z) I resoldered that point as well as point z. Before this problem occured the amp was moved and got bumped a little, maybe I lost the connection to point Z? From Bruce's previous post he was pointing me towards the point Z area. I am assuming I should have a voltage at the grid on the other side of the 3.3meg res.

I can't get your link to work so a lot of your description is too confusing for me to follow. Before you do any rewiring check the other side of the 3M3 resistor for continuity to XZ.

The 3M3/10 pf cap combo is the basic mix resistor for the straight and reverb signals. So the straight signal from both preamp outs should be connected to one side of this res/cap and the reverb return signal should connect to the other side of the res/cap.

There should be little or no voltage on either side of the 3M3 resistor. If there is, then you need to look for a leaky coupling cap.