Deluxe reverb build problems...could use some help.

[Rogue]

Hi guys.

I've built a deluxe reverb minus tremolo, but reverb, and TMB controls. Pretty straight ab763 design, otherwise.

I'm having a few issues. The amp fires up and actually sounds pretty good. However, there is an underlying sort of distortion riding beneath the tone. It's not immediately obvious, but it is there. It kind of oscillates as it all fades out.

What I know....

I am using a Classictone upgraded power transformer. It is a upright, 120/240 version, but speced the same as the standard. According to the standard version, I should be getting 413 volts with a 5AR4. I am using the JJ GZ34 / 5AR4. I am getting 468 volts B+ (at variaced 120).

As a result, all voltages are about 10% higher than the voltages listed in this schematic....

http://www.thevintagesound.com/ffg/sche ... _schem.gif

with the exception of the bias voltage, which is -64 volts, as opposed to the -35 volts listed in the schematic.

I can variac it down to where the B+ is 420, and everything else comes down to reasonable values, except the bias, which is at that point -50ish.

I am having trouble with my tubes not being well matched, but I can get the bias to 21ma on one tube no problem. So the mismatch of the voltage but correct current has got me confused.

I don't know what my B+ is that high. Normal wall voltage around here is more like 125, with peaks beyond that. So this is quite troublesome.

That's the most immediate problem I have.

The second is I can bias up one tube to be 21ma, but the other would be about 15ma. I don't know if this is the source of the distortion or not, nor if the mismatch commonly produces this type of distortion.

One strange issue is that I checked the bias current with the transformer shunt method and got it to 21ma and 15ma. I since then blew my meter fuse and have resorted to getting the resistance of transformer winding per tube to center tap and then measuring the voltage drop for each side. I got 189.5 ohms for each side. .041 and .032 voltage drops. This comes to .21ma and .16ma. The right numbers, although two orders of magnitude off. So what did I do wrong here with the calculation (v/r)?

I have went through multiple times checking the circuit. I have rolled in other preamp tubes (don't have other power tubes available). I have removed the reverb from the circuit to see if that was it, as the noise is more obvious in the reverb trail. Reflowed some solder joints that I thought might could be better. None of this made any difference in the noise. I variaced it down to where all the voltages (except bias voltage as listed above) and it didn't affect the noise.

Any suggestions? Or questions?


Any help would be appreciated.

[JazzGuitarGimp]

There's a 10k resistor to ground from one end of the bias adjust pot. Try taking that down to 3.3k - make sure you've got the bias pot all the way to the most negative end before you reapply power. This should allow you to get your output tubes biased correctly. Doing so will also bring all of your B+ voltages down a bit, but I wouldn't worry too much about that: if you read note 1 on the schematic, it states that measured voltages are +/-20%. This will likely fix the distortion issue as well. Good lock!

[Rogue]

There's a 10k resistor to ground from one end of the bias adjust pot. Try taking that down to 3.3k - make sure you've got the bias pot all the way to the most negative end before you reapply power. This should allow you to get your output tubes biased correctly. Doing so will also bring all of your B+ voltages down a bit, but I wouldn't worry too much about that: if you read note 1 on the schematic, it states that measured voltages are +/-20%. This will likely fix the distortion issue as well. Good lock!

Thanks Jazz. I should probably be clear. I can get either tube at 21ma bias current, it's just each about 5ma off. If I get one to 21ma, the other is 16 ma. If I get the other to 21ma, the other is 26ma.

I swapped the tubes to sockets as well, and the difference follows the tube.

I have the range to set the bias current, so I'm not sure changing the resistor would matter. Could you elaborate on that, please? Just trying to understand.

I don't know if the bias imbalance is the source of the noise. I would be ecstatic if it were.

I can try to get another set locally, but I'm not sure what is available and concerned they might be happy with the high plate current.

[JazzGuitarGimp]

I misunderstood. I thought you could not get the bias voltage closer to ground than -64V.

If you were to add a second bias pot, you could bias the tubes independantly of one another.

[Rogue]

I misunderstood. I thought you could not get the bias voltage closer to ground than -64V.

If you were to add a second bias pot, you could bias the tubes independantly of one another.

No, you are still correct. As I said, I am somewhat confused as to what is going on. My measured bias voltage was -64 volts (with 470 plate volts). Yet I can still bias the tubes to 21ma, although they are far from "matched".

According to the schematic, my bias voltage should be -35ish volts, but I'm nowhere close to that....yet, I can get bias current in line....although tubes aren't very matched.

Anyways, I'm not sure the difference in the two are the source of my noise. Nor do I have a way of checking it.

Still, any advice is appreciated.

[martin manning]

Might you actually be idling at 0.2mA? Are you sure that your meter is undamaged and giving you good information? Try measuring a new 1.5V battery and see if you get a reasonable result. I suggest putting 1 ohm resistors between the power tube cathodes and ground so you can measure idle current there as mV, numerically equal to mA.

[Stevem]

This type of distortion can come from two different things, one is called crossover distortion which takes place due to the output tubes being biased too cold and cutting off as they get back down to idle current.
This type of distortion sounds worse as you strum harder!

The second type of distortion you really only hear at very low volumes and it kind of fades in and rides in back of the note being played and I have found this to be a bad preamp tube, or a bad speaker with a voice coil rub.
Does both channels in the amp have this distortion issue, if so the only tube that could do that would be the PI tube, if both channels do not do this than your issue is the cold bias setting or the speaker!

Unfortunately your output tubes are very unmatched and even if you where to go thru the trouble of also installing a bias balance pot to get them idling the same they will never really sound all that good when playing because they are also not linear Enough with each other for them to react the same to signal and different frequencys being drivin into them.
The louder you play with tubes mis- matched this far the more your notes will loose sustane and the distortion you do have will harsh and grainy!

[pops]

I was just working on a deluxe reverb with high plate voltage 452 and my line is 125 also replaced the rectifier tube with a Ruby 5Y3 and voltage dropped to 408. Was great, the Ruby drops near 50 volts like a NOS 5Y3. Tried a GZ34 and a Sovetec 5Y3 and both had high plate voltage. Been using the Ruby's for years and they are great if you want lower plate voltage and a warmer sound which they seem to give.

[Rogue]

Might you actually be idling at 0.2mA? Are you sure that your meter is undamaged and giving you good information? Try measuring a new 1.5V battery and see if you get a reasonable result. I suggest putting 1 ohm resistors between the power tube cathodes and ground so you can measure idle current there as mV, numerically equal to mA.

Meter reads 1.552 volts on a fresh C battery.

I suppose it is possible it's idling a .2ma, but I would guess it wouldn't sound as it does. Like I said, it sounds good with the exception of the underlying noise.

I'll check again here shortly and see what values I get.

[Rogue]

This type of distortion can come from two different things, one is called crossover distortion which takes place due to the output tubes being biased too cold and cutting off as they get back down to idle current.
This type of distortion sounds worse as you strum harder!

The second type of distortion you really only hear at very low volumes and it kind of fades in and rides in back of the note being played and I have found this to be a bad preamp tube, or a bad speaker with a voice coil rub.

It sounds more like the second thing you describe. You have to listen for it, it's very "behind" what you are playing. Sometimes I can't pick it out. At the same time, it's most obvious the louder you strike a chord a let it ring. It "warbles" a little, and fades as the chord fades.

I have removed the reverb from the circuit, and it still makes the noise. This gave me the ability to roll the reverb 12at7 into the phase inverter slot to check that tube. Still made the noise. I had some extra 12ax7s to try in the other two slots, but no luck, it still made the noise.

It could be the speaker cab (one of the speakers), but I played another amp through the same cab and I couldn't identify the same noise...however, it's a marshall type amp, so even set clean it has some dirt, so it might be harder to identify. Unfortunately, I don't have another 8 ohm cab to try, but I could wire the 16 ohm tap up and try another cabinet.

Does both channels in the amp have this distortion issue, if so the only tube that could do that would be the PI tube, if both channels do not do this than your issue is the cold bias setting or the speaker!

It's a single channel amp. A deluxe reverb with no tremolo, basically.

Unfortunately your output tubes are very unmatched and even if you where to go thru the trouble of also installing a bias balance pot to get them idling the same they will never really sound all that good when playing because they are also not linear Enough with each other for them to react the same to signal and different frequencys being drivin into them.
The louder you play with tubes mis- matched this far the more your notes will loose sustane and the distortion you do have will harsh and grainy!

I'm going to buy some more tubes locally, as soon as the half inch of ice leaves the roads, and see what happens. I think it's entirely possible the unmatched tubes could be the issue, but at the same time, the high plate voltage and strange bias voltage has be befuddled.

The transformer specifies -50v for bias, but I'm getting well beyond that.

[Rogue]

I was just working on a deluxe reverb with high plate voltage 452 and my line is 125 also replaced the rectifier tube with a Ruby 5Y3 and voltage dropped to 408. Was great, the Ruby drops near 50 volts like a NOS 5Y3. Tried a GZ34 and a Sovetec 5Y3 and both had high plate voltage. Been using the Ruby's for years and they are great if you want lower plate voltage and a warmer sound which they seem to give.

Thanks. I am considering trying a another rectifier to bring the voltage down, but I would like to eliminate any errors on my part before doing so.

[Stevem]

The match on the tubes will not make for your issue as it would always be there if the back ground distortion was due to the output tubes, but the amp could also be oscillating due to a number of things.
The first thing to try out is swapping the blue and brown plate wires on the OT, if the and then howls like mad then you had the right phase to begin with!
The other cause of this problem could be just due to the amps build and lay out.
The most critical thing/ wires in the amp are the grid wires on the V1 tube and the PI plate wires to output tube grid.
All the grid wires on that first tube should be shielded and grounded only at one, and that grounding point should be where that tubes cathode picks up its ground.
If these PI wires are more than some 3 inches long then that can make of oscillations.
There is a whole list of things to do to kill this issue, so let us know what you find!

[Rogue]

Thanks Stevem.

The inputs are good and as you say.

One of the PI plate leads might be 3-4 inches in total length. I might can get it a little shorter, but it's nearly 3 inches straight to it. So, that might be an issue. Any solutions for that under that scenario?

I'll try swapping the transformer leads, but first I was going to try another cabinet to eliminate that as the problem and see if I can get the bias voltage down.

[Rogue]

I got it fixed! The difference in yellow and orange in NOT a color! lol

Instead of the bias range resistor being 10k, it was 100k. Embarrassingly, I looked over this half a dozen times while trying to track down the problem. A mental block...of course it was the right value! Ugh. Live and learn.

Oddly enough I noticed the error when I was trying to adjust the bias voltage by changing resistor values. Once I actually laid eyes on the multiplier, I thought, "wait a minute, wth?"

So, getting the right value in there and adjusting the bias brought the B+ down to a comfortable 432v and the bias voltage is now at -41v. It's amazing the brain farts that occurred in this process. But hey, all is good in the world again!

Tubes are still not matched well, 22ma and 16ma. I guess I'll grab a new set of tubes. Got these from tube depot and they've always been consistent for me. There's always a first for everything.

So, this is my first build, and now that is resolved I'm pretty danged happy! Crappy pic, but here it is...

[img768]http://i1209.photobucket.com/albums/cc3 ... ni0a0l.jpg[/img]

Ultimately not as clean as I was shooting for, but overall I'm okay with it. Still for some cleanup to do after trying to figure out what the hell was wrong with it.

One single 0. lol

Thanks for you help guys!!!

[martin manning]

So the idle current really was 0.2 mA then?