Fender Deluxe Reverb Reissue Hoffman Board Conversion - The Basics

[sluckey]

I got some "topic reply notification" emails this morning but don't see any replies since Paul's from the other day???

I made a reply this morning and as soon as I posted I realized I had made the same reply earlier. So I deleted my new post but apparently even though the post had only been live for a couple minutes, that was long enough to trigger the email notification.

So what do you guys think about a ground being off in one of the transformers (maybe the PT center ground tap)? Is that something that could cause this hum? And if so, could I open up the PT and fix it or is it dead?

If the PT center tap was broken you would have no B+ and the amp would be completely silent. Do you have the heater wires connected to the board in order to use the 100Ω artificial center tap?

[davegardner0]

I got some "topic reply notification" emails this morning but don't see any replies since Paul's from the other day???

I made a reply this morning and as soon as I posted I realized I had made the same reply earlier. So I deleted my new post but apparently even though the post had only been live for a couple minutes, that was long enough to trigger the email notification.

So what do you guys think about a ground being off in one of the transformers (maybe the PT center ground tap)? Is that something that could cause this hum? And if so, could I open up the PT and fix it or is it dead?

If the PT center tap was broken you would have no B+ and the amp would be completely silent. Do you have the heater wires connected to the board in order to use the 100Ω artificial center tap?

Ooh that makes sense as far as the email notification!

Yes, I do have the heaters connected all in parallel and also connected in parallel to the two 100 ohm resistors.

What about the ground on the reverb transformer?

Or....what if there was some corrosion on that chassis grounding screw for the power amp ground? Maybe that connection isn't too good?

[sluckey]

What all is humming? Everything? Normal channel only? VIB channel only? VIB channel only when reverb is turned up? Divide and conquer...

Remove all tubes except power tubes, rectifier, and PI (12AT7). Does it hum? If so, the problem is in the power amp or power supply. If not, the power amp is clean. Now plug in the normal channel tube only (probably V1). Does it hum? If so, problem is in V1 circuit. If not, normal channel preamp is OK. Next, plug in VIB preamp tube V2. Does it hum? If so, problem is in V2 circuit. Next, plug in reverb recovery tube V4. Does it hum? If so, problem is in V4 circuit. Half of V4 is the reverb recovery amp and the other half is the wet/dry mixer amp.

Does the above help you narrow it down a bit?

[davegardner0]

What all is humming? Everything? Normal channel only? VIB channel only? VIB channel only when reverb is turned up? Divide and conquer...

Remove all tubes except power tubes, rectifier, and PI (12AT7). Does it hum? If so, the problem is in the power amp or power supply. If not, the power amp is clean. Now plug in the normal channel tube only (probably V1). Does it hum? If so, problem is in V1 circuit. If not, normal channel preamp is OK. Next, plug in VIB preamp tube V2. Does it hum? If so, problem is in V2 circuit. Next, plug in reverb recovery tube V4. Does it hum? If so, problem is in V4 circuit. Half of V4 is the reverb recovery amp and the other half is the wet/dry mixer amp.

Does the above help you narrow it down a bit?

Next time the amplifier is humming I'll try this line of troubleshooting, seems pretty straightforward to narrow down the issue. The only tough part will be catching the amp humming consistently for long enough. Sometimes it does it for 30 seconds and then won't do it for a while. Either way when I can I'll do this troubleshooting and report back with my results...

One question, will channel 2 work with V2 and V6 only? Don't I also need V4 for the vibrato channel to work at all?

[sluckey]

One question, will channel 2 work with V2 and V6 only? Don't I also need V4 for the vibrato channel to work at all?

V4 must also be in the circuit for the VIB channel to work. But I'm not interested in hearing a guitar thru the amp. You already said the amp works fine. I'm just trying to find out where the hum is coming from. In the troubleshooting flowchart above, you do need to plug in V4 BEFORE you plug in V2.

[davegardner0]

One question, will channel 2 work with V2 and V6 only? Don't I also need V4 for the vibrato channel to work at all?

V4 must also be in the circuit for the VIB channel to work. But I'm not interested in hearing a guitar thru the amp. You already said the amp works fine. I'm just trying to find out where the hum is coming from. In the troubleshooting flowchart above, you do need to plug in V4 BEFORE you plug in V2.

Ok understood!

[Mike_J]

I would suspect the heater wires are not balanced if not for the intermittent nature of your problem. I put a humdinger pot and DC voltage on top of the heater AC wires by injecting it at the wiper of the humdinger pot. Can dial out all the hum with the pot. The humdinger pot is a 250 ohm five watt wirewound. You could try purchasing one to replace the 100 ohm resistors and run the wiper to ground. Wouldn't need to install it permanently until you find that it works to cure your problem. Just one possibility I thought of. Sluckey would know if it would be worth trying.


Thanks
Mike

[davegardner0]

I was able to do some troubleshooting yesterday. Lately the amp has been humming, but pretty infrequently, which makes catching it difficult. Sometimes it will randomly hum for maybe 20 seconds and then stop, and some times it just "clicks" every few seconds. The clicking sounds like 1 or 2 cycles of the hum. I was able to catch some of the humming on video:

!Hum

Next I tried pulling certain tubes like was recommended and then listening for hum. Since the amp doesn't hum all the time it's possible to get a false negative here - the amp just happened to not hum in the 30 minutes or so that I had a certain tube configuration in. However it if hums with a certain tube configuration there's no disputing that! So here are my results so far:
V6, power tubes, rectifier: No hum
V1, V6,  power tubes, rectifier: Hum!
V2, V4, V6,  power tubes, rectifier: Hum!

Next I want to try out V6 + power tubes config more, since I heard hum through both preamp channels. Then I'll move on to the reverb circuit. Is V4 + V6 + power tubes also something worth checking?

[davegardner0]

Tried out the Deluxe last night with a full band, and man does it sound good! It really cuts through nicely. Of course after 4 songs the buzz/hum started so I had to switch to a different amp but while it worked it was amazing!

I will continue with the tube-pulling tests tonight...

[eleventeen]

Have you seen if you can start the odd noise by tapping on a/the tubes? Maybe a smallish screwdriver handle.


Sometimes a tube can go microphonic but does so in a "resonant" way where the speaker-driven vibration of the cabinet induces the microphonic and then that gets amplified in a sort of feedback loop. Clue: Did this ever happen when the chassis was outside the cabinet?

[davegardner0]

Have you seen if you can start the odd noise by tapping on a/the tubes? Maybe a smallish screwdriver handle.


Sometimes a tube can go microphonic but does so in a "resonant" way where the speaker-driven vibration of the cabinet induces the microphonic and then that gets amplified in a sort of feedback loop. Clue: Did this ever happen when the chassis was outside the cabinet?

Tapping on the tubes doesn't seem to do anything. Back when the amp still had the PCBs (and it had the EXACT same issue) the issue did happen quite a few times on the bench outside of the chassis. I couldn't get it to go away by moving any wires, tapping on anything, etc...

[sluckey]

Back when the amp still had the PCBs (and it had the EXACT same issue)

Sounds like you may have to consider the parts that are still original as the culprit.

[davegardner0]

Back when the amp still had the PCBs (and it had the EXACT same issue)

Sounds like you may have to consider the parts that are still original as the culprit.

Thinking about it a little further, in the video above I got hum through BOTH channels and BOTH volume controls affected the level of the hum. So does that mean the issue is in the power supply to the preamp? Maybe the low voltage secondary coil has an intermittent short to the high voltage secondary coil? I noticed that one of the wires on the low voltage coil goes straight through the rectifier tube so if there were actually in that wire it wouldn't necessarily be rectified, right?

Has anyone successfully dissected a power transformer to look for shorts or would I not be able to get it back together?

[sluckey]

I noticed that one of the wires on the low voltage coil goes straight through the rectifier tube so if there were actually in that wire it wouldn't necessarily be rectified, right?

I don't understand what you are saying. Please explain.

Has anyone successfully dissected a power transformer to look for shorts or would I not be able to get it back together?

There is no bell cover on the bottom of your PT so you can already see everything you need to see.

Are you using the same tubes since you rebuilt? Same sockets?

[davegardner0]

I noticed that one of the wires on the low voltage coil goes straight through the rectifier tube so if there were actually in that wire it wouldn't necessarily be rectified, right?

I don't understand what you are saying. Please explain.

Ok I'll admit I don't fully understand the internal wiring of the rectifier tube so my theory might make sense but here goes:
According to page 4 of the layout: http://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf
The power transformer has a red secondary coil, a yellow one, and a green one. The red coil is rectified through the tube. But the yellow coil connects directly to pin 8 (which is the "output" of the tube, right?). So, what if red has an intermittent short to yellow inside the transformer? Then a high-voltage AC signal could be reaching pin 8 of the rectifier tube, and thus going into the amp. I'm guessing it would be too much for the filter caps to smooth out on their own.

Again this is all just a theory so let me know if it doesn't make sense. I'm just trying to think of what could cause intermittent on/off humming on BOTH preamp channels that is affected by BOTH preamp volume controls. Also I'm trying to think of which of the original components still remaining could cause this.

Has anyone successfully dissected a power transformer to look for shorts or would I not be able to get it back together?

There is no bell cover on the bottom of your PT so you can already see everything you need to see.

Are you using the same tubes since you rebuilt? Same sockets?

I have all brand new tubes and new tube sockets.

I see a metal cover on the bottom of the transformer, is that what you're referring to? (looks like the photo below) The top of the transformer just shows a bunch of wires sticking out. I'm assuming all 3 coils are wound together on the secondary side, though? So if there was an issue with the wire's insulation...

[mresistor]

Pardon my interjection - but that sounds like a high voltage arc. If it were my amp I would hook the chassis up on the bench circuit side up and fire it up at night and turn off the lights to check for arcing. If no visible arcing anywhere I would be inclined to think the power transformer has something going on internally.
Additionally as Sluckey has stated if it did this before then it would be logical to think that one of the original parts is causing it, and the PT is one of them.

[davegardner0]

Pardon my interjection - but that sounds like a high voltage arc. If it were my amp I would hook the chassis up on the bench circuit side up and fire it up at night and turn off the lights to check for arcing. If no visible arcing anywhere I would be inclined to think the power transformer has something going on internally.
Additionally as Sluckey has stated if it did this before then it would be logical to think that one of the original parts is causing it, and the PT is one of them.

The arcing thought is a good one. My bench is currently occupied with a bass finishing project and I don't think it's safe to fire up the amp out of the cab anywhere else. But when I'm able to I'll try out the "in the dark" idea. I can imagine something arcing in the power transformer, for instance... And even though I had it on the bench and turned on plenty of times I never turned out the lights. Good thinking!

[mresistor]

The chassis needs to be out of the cab, upside down and a load hooked up to the speaker jack. I use a spare 8 ohm Jensen 12" in it's shipping box with a cable and connector on it for my load when testing amps on the bench. Your load could be better. Also you can use the speaker that is in the cabinet if you have room on your bench, to set the cabinet alongside the chassis that is out and upside down resting on it s transformers.
The arcing sound I have heard before in radar systems.

[davegardner0]

So I haven't been able to work on the amp too much lately but I thought I'd give an update on the troubleshooting. My workbench has been occupied with a bass guitar finishing/build but a few days ago I got the chassis set up to run outside of the cab, to check for any arcing visible around the power transformer. Unfortunately I haven't been able to "catch" the amp misbehaving yet (figures, right?). However based on how tightly built the PT is, it doesn't seem to be certain that I'd see anything even if that's what is happening. Therefore I'm thinking of just buying a new power transformer since it seems likely to be the culprit.

So, do you guys have a good source for a power transformer that will fit in the DR Reissue chassis? Most I see for sale online are for the original amps and they have a note saying the transformers won't fit in the reissue. Of course the wiring is correct but I guess the square hole / screw locations won't line up.



p.s. slightly off topic but here's what's been occupying the workbench in the mean time. It's an aniline dye burst finish with french polish clear coat over the top. Almost done!

[sluckey]

Mercury Magnetics has a 'drop-in' replacement PT for the Deluxe Reverb Reissue, part number FDR-PRI. I have one in my amp. Kinda pricey. Here's a link...

     http://www.tedweber.com/fdr-pri

     http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_fenderA-D.htm#Deluxe_

[davegardner0]

Mercury Magnetics has a 'drop-in' replacement PT for the Deluxe Reverb Reissue, part number FDR-PRI. I have one in my amp. Kinda pricey. Here's a link...

     http://www.tedweber.com/fdr-pri

     http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_fenderA-D.htm#Deluxe_

Wow definitely pricey. Are they an "upgrade" though? Maybe less voltage sag at high volumes?

[davegardner0]

I found what looks to be a drop-in replacement for the DRRI power transformer here:
http://www.angela.com/fenderdeluxereverbpowertransformer.aspx

It's on order, so hopefully it fixes my hum issues!I'll post an update once it's installed...

[davegardner0]

Ok so an update on my amp...it's been a while but I got the new power transformer installed. So far, no issues! We'll see if it returns, fingers crossed!!

I mentioned a few posts ago, right when I first powered up the amp, I'm getting some hum that increases as I turn up the reverb, and it was suggested that I isolate the RCA reverb jacks from the chassis to try and fix this. I'm thinking I can move the jacks from under the board to the back of the chassis (actually leaving the old jacks in place and installing new ones). First I'll try disconnecting the reverb tank and/or pulling V4 to see if the hum goes away, as was suggested.

On another note, the vibrato footswitch seems to be not working quite right, the function is reversed. By this I mean that with no footswitch plugged in, or the switch in and the switch set so that it breaks the tip-sleeve circuit, the vibrato functions. This is backwards to how the amp used to be before I rebuilt it. Is the Hoffman vibrato design backwards, or have I wired something wrong? Additionally I get a medium volume "pop" when I turn the vibrato off, not sure if that's normal?

[sluckey]

The trem footswitch is working properly. Hoffman's circuit uses an older style trem circuit that operates quite differently from a full blooded AB763 amp. You may notice that the trem works on the normal channel also.

[davegardner0]

The trem footswitch is working properly. Hoffman's circuit uses an older style trem circuit that operates quite differently from a full blooded AB763 amp. You may notice that the trem works on the normal channel also.

Ah that makes a lot more sense...

One other thing I noticed is that, at least on the vibrato channel the volume control is a little weird. There is no sound up to about 2.5, then the amp gets loud pretty fast. Is that normal?

[sluckey]

Not normal. Could be a weird pot. Swap between channels to confirm.

[davegardner0]

Haven't had a chance to swap out the vibrato channel volume pot yet, but I noticed a different "issue" when the amp is cranked. On the vibrato channel only, I seem to get a harsh buzzing overtone when digging in on notes. As the note decays, the overtone stops abruptly. It seems to happen only above a certain level which can be dictated by guitar volume or pick attack. It happens with both reverb and vibrato turned off using the footswitch. I tried swapping out V2 and V5 with new tubes but that didn't help. I also tried running the amp into a 4x12 cab instead of the internal speaker and the issue was the same.

Here's a video where you can hear the issue - I have the amp set at 9 on the volume:
video

So is this normal? Have you guys run into this before?

[davegardner0]

Anybody have any thoughts on the nasty overtone I asked about in my last post? I bought an attenuator and confirmed yesterday that the amp does the same thing when attenuated. Along with the evidence that the overtone is speaker-independent, it would seem it's inherent to the amp itself. One thing I did noticed is that if I turn the bass control down to 0 the problem goes away. Does this mean I should tweak something in my tone stack or coupling caps? (and if so, where do I start?) And, is this normal for the deluxe reverb.

I can make a better video now that I can replicate the problem at reasonable volumes (thanks to the attenuator). Would that be helpful in diagnosing?

Thanks!!