65 Deluxe Reverb Reissue Hum Bad / Update: Found Problem

[plexi50]

What i am about to say here to my understanding is not possible. I have a bias supply voltage of 50VAC. My 470r,4007 diode and cap polarization are all correct. I can get 415VDC on the plates of the power tubes. The bias is sittiing at -42VDC bias grid voltage. The amp sounds phenominal but has way too much hum.

I have pulled all preamp tubes and have localized it to the bias supply. I have changed the bias .1 coupling caps as well as the plate resistors for the PI bias grid. I am using the listening probe and massive hum is being injected into the grid pins of the power tubes.

I suspect some part was bad in the bias supply line including the bias cap but these parts are good and have been changed several times already. Now after a 5 day hair pulling contest i am totally mystified as to what i have just found

I pulled both 6V6GT power tubes and inserted my listening probe onto each power tube grid.

THE AMP PLAYS AND SOUNDS STRONG WITH (NO) POWER TUBES INSTALLED:

This is not the Twilight Zone. What in Zeus is going on here????????????? I would not believe this if i wasnt sitting here. No drinking/

[John]

I pulled both 6V6GT power tubes and inserted my listening probe onto each power tube grid.

Your listening amp is getting the signal that would ordinarily be amplified by your power tubes. So, instead of preamp>power tubes you have preamp>listening amp. (power amp).

Unless you're saying you still have your preamp tubes pulled as well??

[stratele52]

More volume = more hum or not ? Filter caps ?  12xx7 tubes noisy ?

To much unmatched output power tubes = hum too .

Grid bias do not mean tubes are well biased , need to check Cathode current. 

[plexi50]

All preamp tubes are in the amp. Yes i understand now what your saying about the listening amp receiving the signal (grid) of the power section.

I just installed a new PT as well. So glad this is my amp and not someone elses. All bias and PS wiring is correct.

I cant figure out how to filter the bias supply any more than it already is. Many different power tubes all exhibit the same hum sympthom

Yes i will check the cathode current next. Yes the hum does increase badly on both channels with CWR. At zero volme the hum is 99% gone but faintly present. . All new filter caps

[stratele52]

Zéro volume = 99% no hum ; Problem is BEFORE volume pots . 12xx tube , filter cap, Pt ground missing / not good , input shorting jack defective .

Did you do some other mods?

 Do you have a heater center tap to ground with the new PT ?

[plexi50]

Zéro volume = 99% no hum ; Problem is BEFORE volume pots . 12xx tube , filter cap, Pt ground missing / not good , input shorting jack defective .

Did you do some other mods?

 Do you have a heater center tap to ground with the new PT ?

No heater CT. Using (2) 100r to ground off pilot light. Have only V1 and V6 preamp in place now and is humming loudly at zero volume. Increases with CWR. Input jacks and connections good

[stratele52]

2 X 100 resistor is an artificial center tap ; that is very good .

V 1 is good ?

Hum with V1 , you may have filter caps problem . Read how many DC volts AND AC ( yes AC ) on power supply test points .
" X"  first and "y " and  "Z"

Is this hum problem appear wit the new PT ?

[floyd]

I think that your first move , should be to parallel known good filter caps across all the mains.

[plexi50]

2 X 100 resistor is an artificial center tap ; that is very good .

V 1 is good ?

Hum with V1 , you may have filter caps problem . Read how many DC volts AND AC ( yes AC ) on power supply test points .
" X"  first and "y " and  "Z"

Is this hum problem appear wit the new PT ?

All preamp tubes are the best i have seen in a while. No issues. Have swapped out for numerous other as well.

I have 1 Volt AC on B+. I have .001 VAC on X. I have .002 AC voltage on Y. I have .034 VAC on Z.

I have already paralleled new filter caps on B+ and X-Y-Z. Persistent hum. I am still thinking bias supply but am at a loss on what else i can do

Back tracking as well i have discovered that the Deluxe Reverb power transformer i just received and installed is reading 343-0-343.
Not 330-0-330

[John]

Are the filaments wired, or part of the traces on the PCB? (I assume it's PCB) I remember HBP saying something about PCB/poor layout of heater traces/lots of hum.

If they're wired, how hard would it be to run them off a 6V lantern battery as a test to eliminate that part of it?

I have zero experience with bias supplies, but I don't think that's where your hum is coming from, if the hum is present before the volume pot.  (guessing only)

Using your listening amp, does the hum get progressively louder at each grid from V1 onward?

And echoing Statetele, did this issue appear with the new PT? Also, your voltage is close enough, IMO. Variation in wall voltage can give you 15 volts swing, easy.

[plexi50]

I skipped the heater terminals on the board and went straight to the pilot light with (2) 100r to ground. I lifted the bias supply for a second and the hum got no worse but still present. 

[Willabe]

I lifted the bias supply for a second and the hum got no worse but still present. 

Then it can't be the bias supply.


          Brad     

[plexi50]

I lifted the bias supply for a second and the hum got no worse but still present. 

Then it can't be the bias supply.


          Brad     

No not the bias supply. Was looking at the .1 LT cap & 47r. Not that either. The input grids are grounded through the input jacks. There doing there job. Reverb transformer leads dissconnected and still hum. Im looking at V1 now. Sounds like an amp with bad power supply caps. Those have been ruled out. I had this happen if i remember in a Princeton reverb that i built. Same sound and tone of hum. It turned out to be the OT wires and i had to move them to get it to stop. Has not made a difference in this case.

All cathode voltages ae as the schematic. Bypass caps are all good. This is one freaky problem to narrow down.
I dont get stummped like this very often. 

[Willabe]

I dont get stummped like this very often.

No you don't. You'll find it.


          Brad          

[eleventeen]

The very first sentence in this thread was:

"I have a bias supply voltage of 50VAC"

I am going to assume that you mean 50 VAC input to the rectifier which produces the DC VOLTAGE required on pin 5 of each output tube.
If you have AC volts superimposed on your bias supply, you can bet you'd have incredibly massive hum. it would be identical to feeding the output tubes a big fat 60 Hz signal. Not good.

As I look at an AB763 Del Rev output section, I see -35 volts on the junction of the 220K's leading to the individual 1500 ohm grid stopper resistors. The thought experiment that comes to mind is, if you were feeding 50 VAC to that junction, would you have enough negative-going duty cycle to prevent the 6V6's from redplating? You just might. No doubt, some of that completely unwanted signal would find its way back through the .1 coupling/blocking caps and get whacked some by the power supply caps connected to that node (referring to the 12AT7 PI)

Can you scope your 6V6 pin 5's at idle to see if there is any AC there? There should be nada. If so...replace the bias filter cap. Lacking a scope, I see no reason why you couldn't measure for AC there with a DMM, come to think of it. Any AC there is "your signal".

Since we are going almost all the way to, but perhaps not quite into the Twilight Zone....if the bias winding of your PT (which is EITHER a tap along the HV winding of same) OR a completely separate winding somehow lost its reference to ground...you could potentially have awful AC impressed upon the DC bias that is apparently there, else your 6V6's would redplate. A very, very remote possibility.

Remember...a bias circuit supplies miniscule current compared to most everything else in tube-land. That means, among other things, that there is very little demand for a fat capacitor = storage tank as the filter in the bias supply. Indeed, I see only a single 25u/50 volt cap as the bias filter in an AB763 Del Rev. If it were me, I would suspect that cap and on a build would NEVER scrimp on such a cheapo part; installing one so close to the actual voltage and of such small ufd. I'd want dead minimum a 100/100 volt cap in there. But I am not Leo Fender and I did not build thousands of Deluxe Reverbs.

[stratele52]

 

And echoing Statetele, did this issue appear with the new PT? 

Why no answer yet ?

[stratele52]

If you remove V1 did you have hum ?

[Ed_Chambley]

Tube amp tone with no tubes.  Cool.  But seriously, check your artificial CT 100k resistors.  I have had them cause all sorts of noise and volume/tone stack issues.  Easy to  check and rule out.

[plexi50]

I bought the chassis with a bad PT so i had never heard it before stratele52.

There is hum regardless of the preamp tubes pulled except V6 PI in which all goes quiet.

Yes eleventeen that is 50VAC going into a bridge rectifier and 47uf/63V cap arraingment.
The DC bias voltage reads 57VDC after the rectification

[plexi50]

This is a diagram of the PT i bought. What does this text mean?

50 V BIAS
RED/YEL
TO BLU ????????

To me the red/yellow is the PT CT
The blue is the bias tap
So why are they saying TO BLU?

[sluckey]

To me the red/yellow is the PT CT
The blue is the bias tap
So why are they saying TO BLU?

Because if you set your meter to read AC volts and put one probe on the RED/YEL and the other probe on the BLU leads you will measure 50VAC which is the AC source for the bias circuit.

[plexi50]

Thanks Steve. I suppose my next move is to get a 6 volt DC source and hook it up to the filaments to see if my hum is heater voltage related. It is the only thing left i can think of next to possibly elevating them.

[sluckey]

There is hum regardless of the preamp tubes pulled except V6 PI in which all goes quiet.

This tells me that the PI and power amp is fine. So look before the PI input. I'd pull all the little tubes except V1 (normal preamp) and V6 (PI). Still got hum?

[stratele52]

There is hum regardless of the preamp tubes pulled except V6 PI in which all goes quiet.

This tells me that the PI and power amp is fine. So look before the PI input. I'd pull all the little tubes except V1 (normal preamp) and V6 (PI). Still got hum?

Hum when V6 and output tubes are there , IMO this mean problem is between V5 ( yes circuit from V5 is feeding V6) and output tubes / OT.
 

[plexi50]

There is hum regardless of the preamp tubes pulled except V6 PI in which all goes quiet.

This tells me that the PI and power amp is fine. So look before the PI input. I'd pull all the little tubes except V1 (normal preamp) and V6 (PI). Still got hum?

Yes. Hum with only V1 and V6 PI with power tubes.

No signal at all with only V6 and power tubes stratele52

Edit: No (hum) at all with only V6 and power tubes

One more thing. As i pulled tubes V2 through V5 the hum got louder.

[plexi50]

Found this on the Vibrato channel ribbon connector. Re soldered. No change.

[stratele52]

There is hum regardless of the preamp tubes pulled except V6 PI in which all goes quiet.

This tells me that the PI and power amp is fine. So look before the PI input. I'd pull all the little tubes except V1 (normal preamp) and V6 (PI). Still got hum?

Yes. Hum with only V1 and V6 PI with power tubes.

No signal at all with only V6 and power tubes stratele52

One more thing. As i pulled tubes V2 through V5 the hum got louder.

I can't understand !!!! You write "No signal at all with only V1 and V6
No signal mean fo me ; no guitar sound . Sure with no V3,4.5 you can't play guitar

What about hum ?

___________________

More hum if you remove V2 througt V 5  !!!!  Not easy to follow your tests!!! 
May I understand you have always a hum ?

___________________

About ribbon connector , these connector if not in the right position/ bend  could induce noise , hum .

__________________

Sorry but too much experts in the forum with good ideas but tests go in too many directions in same time.

[tubeswell]

Other than the filter caps, other possibilities could be;

bad plate resistor(s) - maybe on the PI?
cold solder joint(s) - reflow
bad tube(s) - swap
imbalanced heaters - check your 100R ground reference, or elevate the heaters, or try a humdinger
bad presence pot (if its a LTP with a presence pot) - find out by shorting across the pot

[plexi50]

There is hum regardless of the preamp tubes pulled except V6 PI in which all goes quiet.

This tells me that the PI and power amp is fine. So look before the PI input. I'd pull all the little tubes except V1 (normal preamp) and V6 (PI). Still got hum?

Yes. Hum with only V1 and V6 PI with power tubes.

No signal at all with only V6 and power tubes stratele52

One more thing. As i pulled tubes V2 through V5 the hum got louder.

I can't understand !!!! You write "No signal at all with only V1 and V6
No signal mean fo me ; no guitar sound . Sure with no V3,4.5 you can't play guitar

What about hum ?

___________________

More hum if you remove V2 througt V 5  !!!!  Not easy to follow your tests!!!  
May I understand you have always a hum ?

___________________

About ribbon connector , these connector if not in the right position/ bend  could induce noise , hum .

__________________

Sorry but too much experts in the forum with good ideas but tests go in too many directions in same time.

Sorry i ment to say i still have hum with V1 V6 and power tubes stratele52
I have Normal Channel guitar signal and normal playable volume with only V1 V6 and power tubes but hum is still there

Some years ago i had an amp that hummed with only the PI and power tubes in it. That turned out to be tubes that were new but for unknown reasons were coupling with the eddy currents of that power transformer and would not work in that amp. Changed power tubes and hum completely was gone

It's pretty hard for me to not believe it is not either heater or power supply hum. The power supply capacitors are new. I have even paralleled other new caps over the ones i just installed but there is no change in hum.

I can make the hum go away if bias it very cold. Thats why i was thinking bias earlier. I am biased @ 400VDC plates with a -35VDC bias grid. The amp sounds great except for the hum which increases terrible with volume. I dont always explain things the best.

A 6 volt DC supply for the filament voltage test is needed before i can really go any further to figure out what the problem is.

Maybe i should have kept that old 6 volt battery from the 1949 Chrysler split windshield flat head i used to own/

[stratele52]

I can make the hum go away if bias it very cold. Thats why i was thinking bias earlier. I am biased @ 400VDC plates with a -35VDC bias grid. The amp sounds great except for the hum which increases terrible with volume. I dont always explain things the best.

 

Grid voltage on ouput power tube do not tell how tubes are working a this bias voltage , need cathode current . If too much difference between tubes you'll could have some hum.

Grid voltage is the way to bias ( it is not the bias ) the tubes but how is the bias on each tube . ?

[plexi50]

Here are my findings on the bias current. 400VDC plate. 6V6GT #1 (16.2 ma) and  #2 (17.6 ma).
Grid now reads -36 VDC

I have tried multiple different 6V6 tubes all with the same hum and the presence of it (tone) and level

[stratele52]

Here are my findings on the bias current. 400VDC plate. 6V6GT #1 (16.2 ma) and  #2 (17.6 ma).
Grid now reads -36 VDC

I have tried multiple different 6V6 tubes all with the same hum and the presence of it (tone) and level

Fine.
You can go to 21 ma but if you like the sound live there at 16/17 ma.. Those tubes are fine.

[plexi50]

I bought a 6 Volt DC battery but it evidently does not have enough amps to supply a good strong 6 volts due to the amps current demand. I was reading 4.0 volts after i turned the amp on. It was quiet but i could not get enough volume and filament power to really achieve any kind of accurate results.

It sounds like a bug lantern (buzzzz) and worse as the volume is turned up. Dam i have seen a lot of amps and they all had some issues but nothing as crazy as this bugger. This has got to be some kind of oscillation that is coupling with the preamp. The amp was free so i dont feel to bad.  Just dissapointed that it is not revealing itself by now. 1 week + is a lot of eye ball strain.

[plexi50]

Just finished replacing (all) coupling capacitors and plate resistors. Sounds like heater of power supply issue. Power supply and all capacitors were changed this morning as well again. The old ones which were new checked out good anyway on my analog and cap tester. Changed slope resistors and treble peak caps. Got a good DC 6 volt supply today and it still hummed. Maybe i just bought a bum transformer. One things for sure,the amp is completely rebuilt. New choke,OT but new PT is in question. That all i am left to believe. I will measure the transformer leads tomorrow after i remove them from the board and rectifier. Tried several different rectifiers as well

[DummyLoad]

hum is probably from bad filter cap or broken trace to a filter cap. follow the traces with an ohm meter. with PCB amps it easy to pull a though-hole out with old part and that though-hole could have been the continuity to a trace on an opposite layer.

--pete

[Willabe]

hum is probably from bad filter cap or broken trace to a filter cap. follow the traces with an ohm meter. with PCB amps it easy to pull a though-hole out with old part and that though-hole could have been the continuity to a trace on an opposite layer.

You've already changed the filter caps, so....

The dreaded......    cheap......      PCB!


               Brad       

[plexi50]

I thought of that yesterday morning. I pulled the dog house and tested each power supply lead back to their board terminals. Then i checked the board traces for continuity after installing each new cap and resistor. I suppose i need to check each wire coming from each tube to the board. What a dick this board is being. It looks great and all the work is nice and clean.

It's been a while but in the past i have found cracked PCB traces (Crate BV120) (Fender 90 DSP) that you will not see with the naked (eye) some times

So far which has been i think (5 dozen times) and as many continuity checks i have failed to find any loose,intermittant,or other isssue other than both channels are effected

Power supply hum is pretty noticeable and straight forward in it's pitch (tone) I will find it soon enough. Then after i find the problem i will take a hammer to the board and just put in a hand wired board which i am planning to do any way. I must love pain/ 

[plexi50]

Done,finished sounds wonderbar. Here is what the problem was. This is a great study in ground loops and the problems that they can cause in hum and oscillations. Like an antenna. I love and hate myself. My error,though:

I had wired and grounded the entire power supply ground to the chassis on it's own dedicated ground lug because i did not like the long wire length of connecting the preamp power supply ground C19 seperatley to board grounded terminal and to preamp PS cap. ( These amps are wired screwy)

I had left C19 preamp power supply ground connected to the main board and power supply board under the dog house not thinking that it would cause any harm

If you look at the schematic you will see that the power supply capacitor grounds themselves are all grounded on a single buss except for the preamp PS capacitor (which is C19)

I know this sounds very confusing but this is just the way they made and wired the reissue Twin & Deluxe Reverbs

How it came to me: I woke up and said (this makes no frickin sense) I looked at C19 preamp PS ground and pulled it off it's terminal

Flipped the amp on and silent. Plugged in git fiddle and angels landed on my bench.  

I live in *Floridah* At least a good start to the day. Wow! Thanks all*

[John]

Cheers to you for not giving up, and for sharing the solution! 

[plexi50]

There would have been no problem if i had not wired the PS grounds to a single chassis lug. I was think old school and missed the original ground schem for the preamp PS in this amp. Man it sounds great. 

[Willabe]

A ground loop.   

Glad you found it.


            Brad     

[Platefire]

Ye haw! Now lets Jam----

[Slimtim]

very good news,congrats.

[plexi50]

Ok all i really screwed up in the posting of this thread. This was a 65 Twin Reverb that i converted to a 65 Deluxe reverb using a DR PT,OT,Choke using 6V6GT tubes.

The 65 DR RI has a different power supply wiring than the 65 Twin RI so i may have you all really scratching your heads. Steve?  

The 65 Twin Reverb schematic does not show the preamp PS cap where it is really located on it's own separate ground terminal and mounted on the main PC board. The preamp PS caps ground is also the ground for the OPTO


I'll just show pics to try and clear up any confusion i have created.  

[sluckey]

The 65 Twin Reverb uses a bridge rectifier for the B+. I'm guessing you figured that out?

[plexi50]

The 65 Twin Reverb uses a bridge rectifier for the B+. I'm guessing you figured that out?

It was a little work but not bad. I had to have a lower wattage amp with a rectifier tube. No way can i do much tonally with 100 watts

This was the amp in an earlier post a few months ago that had the charred OT & PT. That's why it was given to me

What could cause such catastrophic transformer combustion like that? It was a Busch Gardens amp used for shows here

[ajeffcote]

Your deserve a cold beer for sticking with it. But, honestly, don't you just hate to work on PC boards?

[plexi50]

Sort of dont like working on PC boards. All depends on the amp. Like anything else you just have to get them apart and go.
It sounds phenominal. The deluxe reverb OT that i used has something wonderful going on inside of it.

I pull it every time i sell an amp that i have it in before selling an amp. Im glad i have kept this OT. The sound is a cross between a Deluxe and Vibrolux reverb.

I found a 1972 Twin reverb cabinet with casters and tilt back legs to put the chassis in. Im using a 1965 Gibbs reverb tank and bag.
The new aged grill cloth came out nice on a new grill frame. The old grill frame had turned to dust

The 12" speakers make the difference in headroom. The woody scoop is to die for. No beer these days. Have gravitated towards dark rum*

The Genesis 3 on the floor is not used as it is too complicated an not needed for my tone. Just another toy collecting dust

[ajeffcote]

What kind of OT are you using? Sounds like what I'm looking for.

[plexi50]

Long shot on failure of P/T and OT. 

Just a little over a year ago, I made arrangements to have a power plug put in at a stage, so a DJ could run a remote control board. One of the options, was three phase power. output 120v. (208v between legs).    As some know, you can get three phase power in delta and wye.  (three wire and four wire).  For example if you assume that you have wye three phase power and you actually have a delta system, then the output voltages are 120-120-208.  If you plug in into the 208 leg, voltage is too high, and you fry your P/T, and possibly your O/T.
   Actually ran into this, while working for a general/electrical contractor.  The plans called for a wye system (stamped by an electrical engineer).  Power company provided delta system.  a few battery chargers and radio were fried, as well as an expensive control board for a HVAC unit.  As general and electrical contractor, one third of the electrical distribution panel was useless.   (The HVAC contractor was warned that power was delta, rather than wye,  Long story short, HVAC contractor didn't change jumper, and fried a control board). 

Since you stated the amp was used at Busch Gardens in Florida, (Tampa, I'll bet).  Three phase power is probably a common distribution. 

Just a possible explanation how to fry a p/t and o/t.

That would be Tampabay,Florida. That is a good explanation for the transformer failures. Never seen both toast at the same time.  The OT came from M Components a little over a year ago ajeffcote.