Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: proaudioguy on November 04, 2022, 02:19:08 pm
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The DR has 16µF caps in the PSU and either a GZ34 or a 5U4GB. I believe the 5U4GB drops more voltage.
Here is a link to the schematic for the DRII. Its not 100% accurate but close enough.
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxereverb_ii.pdf
I have modified the clean channel preamp to AB763 REVERB. The reverb mixer is 3.3Megs, so I may look into raising that, but I don’t think that is my issue.
The PI is modified to AB763 DR.
The PSU is stock. It has a full wave bridge recitifier. I imagine this is superior to even the GZ34, but is it the cause of my issue?
The Power supply filtering is much larger than a DR. I think this needs to change for sure.
I am trying to get not only the SOUND of the DR, which I almost have, but also the FEEL. The amp is VERY strident. I do not want to gut it which would simply things greatly, because I would like to retain the channel switching. I will play with the lead channel later. Now I just want to get the clean channel to sound like a 1967 DR I played and fell in love with and regretfully didn’t buy back in 2014. I would honestly like to make the lead channel more Tweed or Marshall or Boogie and I may try all of the above to get where I want with that, but I want the clean to be stock DR. The transformers are the same as far as I can tell so there is no reason I shouldn’t be able to get it there. There is even an unused triode so I could get some shananigans going later, on the lead channel. It actually has everything necessary just in the lead channel to be a basic Dumble, missing some features, but I’m not sure I want to go that far.
Looking at the schematic, should I swap out all that filtering to stock DR values, and will that soften the attack? Any thoughts on a SAG resister set up, and how I would impliment that to aproximate the 5U4GB’s effect on the voltages? Would that also help? Should I just scrap the bridge rectifier, cut a hole and install a tube rectifier? I am not sure if the transformer has the rectifier leads or if they are cut off, or if they are omitted from that one, which I think has a single letter difference in the part number. I’ll have to get into the amp to find out, IF I were thinking of ditching the SS rectifier. I feel like there should be a way to get the sag without doing that. Not sure if the time constance of a resister, which is pretty linear, would match a tube though.
The last thing I may do is at the Intensity pot 50K load and see how that compares to the no load. I prefer the no load on my Twin Reverb.
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You cannot use that PT with a tube rectifier.
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The FWB HT supply of the DRII is a different beast to the 2 phase / push pull rectified HT supply of previous versions of the DR.
They're completely non compatible, without a CT on the HT winding, a valve rectifer isn't a viable option, so there definitely won't be a 5V heater winding hidden away somewhere.
As you note, smaller HT caps and a sag resistor may be helpful. Try 100ohms 12W between rectifier and reservoir for the latter.
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You cannot use that PT with a tube rectifier.
Thanks for replying. You have helped me so much in the past. Part number on the Transformer is the same so I wasn’t sure. I haven’t been inside the amp in over a years so I can’t recall what else was going on. I wasn’t considering a tube rectifier at that time. I still would prefer not to do that part of it. If I do everything else and enough people tell me, doing that is what will get it there, then I’ll do it. I just may have to source a transformer at that point.
All the other questions still stand though. Is that very stiff FEEL because of all that filtering?
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... Is that very stiff FEEL because of all that filtering?
It’s a stiff HT supply generally.
All else being equal, a FWB rectifier will have a lower supply resistance than a 2 phase / push pull supply.
A silicon rectifier will have lower supply resistance than a valve rectifier.
And the higher the cap value, especially the reservoir, the higher the VDC when loaded.
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The FWB HT supply of the DRII is a different beast to the 2 phase / push pull rectified HT supply of previous versions of the DR.
They're completely non compatible, without a CT on the HT winding, a valve rectifer isn't a viable option, so there definitely won't be a 5V heater winding hidden away somewhere.
As you note, smaller HT caps and a sag resistor may be helpful. Try 100ohms 12W between rectifier and reservoir for the latter.
Doug has 50 Ohm 10W. Based on your suggestion of 100Ohm, I was thinking a pair of those in series would work. I could put together a clamp made of aluminum plates and put some heatsink schmoo between the resisters and the plates, then use the bottom plate to secure it to existing mounting points.
When I ordered the replacement cap can for this amp in 2020, I was looking to get it up to par with leaving the factory. Did not occur to me that all that filtering would not only reduce the hum, but also make it very stiff. No hum is number one for me, but if I can’t get the amp to soften up, I won’t play it. It honestly feels stiffer than my Twin.
Should I expect the reduced capacitance in the PSU to raise the hum level?
Where in the circuit would you put the SAG resister? Right after the bridge and before the first filter cap?
Does anyone know what the smal caps across the diode do? I’m assuming some kind of RFI filtering but I have never seen this before.
Edit,...Looks like if I cut the 80µF out, that gets the first stage to 40µF, so closer to 32, and the others are 20µF so closer to the 16. That at least would be an easy way to see if it will take me in the right direction before I roll my own cap board. Thoughts?
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One other question. Do you all see any benefit to splitting the first tube? It seems like its likely there to drive the 2 channels. The only place to actually split it is on the output. One channel could get one output and the other channel could get the other output, instead of both channels getting both outputs. Would this affect the sound? I always wondered if this preamp arrangement changed the frequency response a bit. I could replace the 47K with a pair of 100K plate resisters. That would isolate the channels even more. I have never noticed an interaction between the tone controls, but it seems like they would interact.
Remember, I’m just a hacker, working on my own stuff. Trying to catch a dragon. Once caught, I’ll be done.
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Does anyone know what the smal caps across the diode do? I’m assuming some kind of RFI filtering
These caps are a way of protecting the diodes from an over voltage condition. As you can see, they really cheaped out and spec'd 1N4006 diodes in the rectifier. The caps tend to spread voltage over the two diodes in series, much like what you would see when using two caps in series (and using resistors to balance the load).
These caps do not have a role in RFI reduction AFAIK.
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Does anyone know what the smal caps across the diode do? I’m assuming some kind of RFI filtering
These caps are a way of protecting the diodes from an over voltage condition. As you can see, they really cheaped out and spec'd 1N4006 diodes in the rectifier. The caps tend to spread voltage over the two diodes in series, much like what you would see when using two caps in series (and using resistors to balance the load).
These caps do not have a role in RFI reduction AFAIK.
Awesome. If they ever fail, I have a pretty good stock of the big ones I put in the Twin.
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Appears the OT, RT, choke are all the same as the DR. Looks like the PT is the same as the Super Champ. That probably means its the same as the PRII as well.
Should the sag resisters go between the secondary winding and the rectifier diodes? Apparently VOX does this.
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Should the sag resisters go between the secondary winding and the rectifier diodes?
That's one way to do it.
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Should the sag resisters go between the secondary winding and the rectifier diodes?
That's one way to do it.
OK, how would you do it?
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That's how I would do it. :icon_biggrin:
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See 80’s AC30 with sag resistor https://el34world.com/charts/Schematics/files/Vox/Vox_ac301986.pdf
Does anyone know what the smal caps across the diode do? I’m assuming some kind of RFI filtering
These caps are a way of protecting the diodes from an over voltage condition. As you can see, they really cheaped out and spec'd 1N4006 diodes in the rectifier. The caps tend to spread voltage over the two diodes in series, much like what you would see when using two caps in series (and using resistors to balance the load)…
I can’t make sense of the above, sorry, are you sure you’re referring to the DRII schematic?
800V rated diodes are fine in a FWB rectifier with a 400VDC output, as per the DR II. The max peak voltage across the diodes, eg no valves fitted and assuming 10% regulation and mains voltage 10% higher than nominal, won’t be over 500V. So that’s 1.6 x the minimum necessary, actually 1N4005 (600V rated) would’ve been fine.
See the section under the ‘required diode rating’ heading http://www.valvewizard.co.uk/bridge.html
The caps in parallel with the diodes might be referred to as snubbers, their purpose is to damp down the PT HT winding circuit ringing on each switching cycle, caused by the slow switch off and consequent reverse current in the diodes.
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I can’t make sense of the above, sorry, are you sure you’re referring to the DRII schematic?
I was referring to the rectifiier diodes in the DR-II schematic -- those were the only diode/cap combinations I saw in the schematic. Maybe I misunderstood what he was asking about. If so, my bad.
As far as the rectifier diodes and their associated caps, I believe my explanation is correct. See Merlin's new Designing Power Supplies book, pages 44-50. (Great book, by the way.)
OK, maybe my comment that Fender cheaped out with 1N4006 diodes did not take into account the costs of diodes back in the 1980's -- I have no idea what the cost differential was back then. Now, of course, there is no cost advantage to using a 1N4006 over an -07.
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... The caps tend to spread voltage over the two diodes in series ...
I was referring to the rectifiier diodes in the DR-II schematic -- those were the only diode/cap combinations I saw in the schematic...
But the rectifier diodes aren't connected in series :w2:
It's a full wave bridge rectifier, not a 2 phase rectifier.
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Update. I had not worked on the amp since I posted this thread. It just sits in the corner. Today I split the first Preamp tube and that has made a rather pleasing differece in the sound. I think I will do the same to the Concert. I have a 100 Ohm and a 200 Ohm 50W resistors on order to try between the rectifier B+ and the first cap. I completely forgot it was later suggested to place before the bridge, which I think would require 2 resistors, one on each transformer wire. Hopefully placing 1 between the FWB and the first cap will do the trick. I disconnected the 80mfd on the cap can.
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... I completely forgot it was later suggested to place before the bridge, which I think would require 2 resistors, one on each transformer wire. …
Still only 1 resistor required in the AC side of the rectifier.
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... I completely forgot it was later suggested to place before the bridge, which I think would require 2 resistors, one on each transformer wire. …
Still only 1 resistor required in the AC side of the rectifier.
Not one on each leg?
Or can it be implemented on the primary side?
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Yea, I assumed one on each leg since its a full wave bridge rectifier. I’d love to know.
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Every bit of current that flows out of the top leg must flow in the bottom leg. A single resistor will work exactly like two separate resistors. Or, you could put two resistors in the top leg and none in the bottom leg. Makes no difference to a FWB rectifier.
It would make a difference for a two diode conventional full wave rectifier because only one leg is working at a time.
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It would make a difference for a two diode conventional full wave rectifier because only one leg is working at a time.
Right...I missed the "bridge" part.
Amp I am fooling with at the moment has a 2-diode full wave, I was in that head.