Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tdvt on March 03, 2023, 06:26:05 pm
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Dragged home a sad & neglected Silverface Pro Reverb today, that had been in not-always-dry storage for many years.
Looking it over it appears to be a '79 70W MV, Ultralinear version. I had lost track of what Fender was producing in that era, so I have spent a little time looking for information & schematics to get some idea of what was the practice at that point in time.
I haven't done anything other than look it over & found it in far better shape inside than anticipated, in addition to looking all original.
After some reading, I understand the basics of the ultralinear (not always popular it seems) but I have no opinions either way, not ever having played through one.
So, I am not really interested in modding it into something else but would just like to get it up & running, albeit incorporating any fixes for known weak points or minor improvements, ie: adjustable bias, keep/remove the "boost", etc. (MV/push-pull pot is frozen solid)
I am wondering if anyone has any experience with/insights into this generation of the UL Fenders?
I am also not clear as to the added components in the power supply; the 100K resistors/220uf caps. Is this additional filtering? I was also wondering about the 30K/20W resistor to ground of the power rail? (missing part of the line in the digital copy) Just a massive dropping resistor? (implemented in a non-typical way..?)
>>>Never mind the "added components" Just realized I was looking at the series reservoir caps/balance resistors..... :BangHead:
https://el34world.com/charts/Schematics/files/Fender/Fender_ultralinear_pro_rev.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_ultralinear_pro_rev.pdf)
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Interior is way better than I expected
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If you remove the push pull boost, the amp is pretty much a blackface, besides the 330k resistors on the phase inverter, and a .01 cap on the negative feedback. At least that's what I remember.
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Nice! I like it......I'd try some DeOxit on that push-pull pot to see what that would do. Also new Filter caps right off the bat. Your gut shot looks good. I got a old 74 Pro Reverb I'm working on and
the guts are a tangled mess compared to yours. Have fun with it. Platefire
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If you remove the push pull boost, the amp is pretty much a blackface, besides the 330k resistors on the phase inverter, and a .01 cap on the negative feedback. At least that's what I remember.
Been sitting comparing circuits. Looks like most of the changes are in the phase inverter & the way the bias was implemented. No adjustable bias but a hum balance (hum balance on the heaters too)
Nice! I like it......I'd try some DeOxit on that push-pull pot to see what that would do. Also new Filter caps right off the bat. Your gut shot looks good. I got a old 74 Pro Reverb I'm working on and
the guts are a tangled mess compared to yours. Have fun with it. Platefire
Caps for sure.
I haven't even pulled the doghouse cover yet, but it's probably never been serviced. Compared to my older Fenders, this one looks a mess... guess it's all relative.
Does your '74 have MV? Is it a usable design?
Planning to test the PT & OT next as they have a bit of rust. Hoping it's all superficial.
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I never felt I needed to modify the bias, I think my 6l6 were running at 50-55%. I was still pushing quite high voltages at that though, so I wouldn't want to mess with it much.
The hum balance is a nice feature, maybe it could be a point of failure, I dunno. There's a lot of differences with the UL transformer with filtering and such, and on the super reverb at least, it's diode rectified, I imagine it's the same with the pro reverb.
I think the 30k resistor is just a bleed off for the caps?
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I think the 30k resistor is just a bleed off for the caps?
It works with that 2700Ω/10W to form a voltage divider to drop the 500V down to 410V at point A.
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I am also not clear as to the added components in the power supply; the 100K resistors/220uf caps. Is this additional filtering? I was also wondering about the 30K/20W resistor to ground of the power rail? (missing part of the line in the digital copy) Just a massive dropping resistor? (implemented in a non-typical way..?)
>>>Never mind the "added components" Just realized I was looking at the series reservoir caps/balance resistors..... :BangHead:
https://el34world.com/charts/Schematics/files/Fender/Fender_ultralinear_pro_rev.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_ultralinear_pro_rev.pdf)
Those are voltage balancing resistors for the 2-phase voltage doubler. (The way the rectifier is drawn, it looks superficially like a FW Bridge, but it’s actually a 2-phase voltage doubler)
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I will confess to getting used to looking at a particular style of schematic (primarily Fender) & not scrutinizing things quite enough to realize I might be looking at something familiar circuit-wise, just presented differently.
Though I haven't even gotten it plugged in yet, I am wondering about the ultralinear OT & how that works.
My understanding is there are screen taps at a certain % of each winding to manage tube efficiency. Is that alone responsible for the huge increase in output wattage?
Further, could the UL be a switchable feature (using the UL OT) where those screen feeds could be bypassed & the screens connected to a B+ node for the typical lower power version of the circuit?
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the UL keeps the OT more "stable" than full Pentode, sorta like having it biased between triode and pentode, which usually yields less audio power, but a tighter, less distorted sound. most of the UL's I've used are tapped at 40%
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tdvt
My 74 Pro Reverb is not comparable to yours as it was truley a basket case that I bought in pieces in a couple of card board boxes. Someone had severely abused the circuit with all kind of mods that looked like to me were never fully executed. What was there, was poorly done. The OT was missing but the package included a brand new bassman New Sensor transformer that I used. Also it was a combo cut down to a head. So I rewired it to AA165(Blackface) except I converted the balance pot into a bias pot. Removed the old trem circuit, installed sluckey's Tremonator and installed 1Ohm(1%) resistors from 6L6's cathode to ground to check bias current with the mV setting on my MM. To answer your question, no master volume and no middle control. So what I got was something someone started but never finished! So I just want to get it back to that stock 1965 Blackface sound.
I look forward to watching your progress on getting yours up to your satisfaction. Platefire
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Did a little more poking around & things looked pretty good, especially in the doghouse (no leaking/open/shorted filter caps).
So I put it in the limiter, then added tubes, then put it on wall power & the thing basically still works.. not bad for 44 year-old caps, etc..
Trem not working & I need to connect the reverb tank to try that, but looks like all the expensive stuff is functional.
As you would expect, it is a little noisy, but not nearly as bad as it could be, so I will start with a good cleaning of tube sockets & pots plus new caps, see where it lands.
PLateFire, after I asked about your MV, I revisited your current Pro thread & saw the pic I had somehow missed showing the straight-up controls. This one has mid on both channels which I am happy with.
the UL keeps the OT more "stable" than full Pentode, sorta like having it biased between triode and pentode, which usually yields less audio power, but a tighter, less distorted sound. most of the UL's I've used are tapped at 40%
From what I am reading about the UL amps they are super clean, popular with pedal steel guys in particular.
But, I am still puzzled at the 70W RMS output rating & how that is achieved.
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I am still puzzled at the 70W RMS output rating & how that is achieved.
guessing the 500vdc and a hot bias will make the Mr. ohm math work out
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Trem not working
You must connect the footswitch to enable the trem. Or you can connect the trem FS jack tip to ground to enable.
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Those are voltage balancing resistors for the 2-phase voltage doubler. (The way the rectifier is drawn, it looks superficially like a FW Bridge, but it’s actually a 2-phase voltage doubler)
I'm not keen on the 'voltage doubler' term, there's already voltage doubler / multiplier rectifier circuits that actually do that (better to call a regular single rail 2 phase rectifier a 'voltage halver' :icon_biggrin:).
Merlin uses the term 2 phase split rail, which doesn't have that ambiguity.
Anyway, to me that Fender circuit operates as a FWB. Consider that 0 load current flows via the CT, to the point that the CT could be disconnected with no difference to circuit operation.
It's a different scenario to say the 75, in which an apparently identical rectifier really does operate as a split rail, or at least it does when the amp is switched to low power mode https://el34world.com/charts/Schematics/files/Fender/Fender_75.pdf
In that case, disconnecting the CT would stop the amp working in low power mode.
I am still puzzled at the 70W RMS output rating & how that is achieved.
guessing the 500vdc and a hot bias will make the Mr. ohm math work out
I don't think any sensible bias setting would affect power output more than negligibly?
But yes, the 40W PR has a lower saggy HT voltage, it'll probably drop to 400V at high power output. Whereas the FWB rectification of the 70W is much stiffer and higher, it won't sag much at all at high power output.
When I check the primary taps on a late 70s Fender, they're at about 12.5%. So the amp is still basically in regular pentode mode, the anode UL curves based on 40% taps won't apply.
The taps here just make the high stiff HT feasible, by taking a bit of stress off the screen grids, by reducing the voltage differential between anode and screen grid a bit at high power output.
To achieve the 70W output though, decent valves are required, eg Sylvania STR387. Current production 6L6 probably won't be able to achieve the high peak current that's necessary.
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Studying it further, the only difference I am seeing from a traditional rectifier circuit is that the Sec CT is connected midpoint between the series reservoir caps, as opposed to ground.
In a current thread regarding a 135W Twin, there is a schematic posted there, that other than quad output tubes, appears to be the same circuit as I am looking at. Only it has been re-drawn in a much more digestible (for me), old Fender-style.
https://el34world.com/Forum/index.php?action=dlattach;topic=30023.0;attach=104276
(also 2nd attachment below)
The power rail in that drawing looks very much like the typical configuration, with the exception of the huge dropping resistor to ground (30K/20W) & the CT going to the reservoir caps.
Presented this way, it is much easier to see the voltage divider at the front end of the rail that Sluckey pointed out.
What is the net result from connecting the Sec CT to that point as opposed to ground?
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On a more basic level, as far as rehabbing, I am thinking to rebuild the rectifier/bias board as it was very poorly assembled & is also heavily coated with wax.
I understood they used wax on the eyelet boards during this time, but I didn't realize how much wax.
It looks as though it was sprayed on the populated board as a molten mist, coating everything. Quite a mess to clean up for re-soldering.
Anyway, I have several metal-case FWB units in stock (35A/1KV) that would be nice to use up. Would the little .002 caps across each point of the bridge still be needed/good idea in this instance?
My understanding was that those were added to balance loads on the individual diodes.
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What is the net result from connecting the Sec CT to that point as opposed to ground?
Connecting the CT to the midpoint of those two series connected filter caps will force the two caps to have equal voltage across them.
Anytime you have a FWB rectifier with one side of the bridge connected to ground, YOU MUST NOT CONNECT THE SEC CT TO GROUND! Look at the simple pic and I bet you will quickly realize why.
The .002 caps across each diode are there to remove any voltage spikes/noise generated when the diodes switch on and off. Good idea to keep them.
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What is the net result from connecting the Sec CT to that point as opposed to ground?
Connecting the CT to the midpoint of those two series connected filter caps will force the two caps to have equal voltage across them.
Anytime you have a FWB rectifier with one side of the bridge connected to ground, YOU MUST NOT CONNECT THE SEC CT TO GROUND! Look at the simple pic and I bet you will quickly realize why.
The .002 caps across each diode are there to remove any voltage spikes/noise generated when the diodes switch on and off. Good idea to keep them.
Thanks for weighing in & the reminder.
I was not planning on grounding that CT, but I wouldn't have even asked that if I had thought about it more. Not a lot of seat-time with FWB rectifiers.
I guess they utilized the CT for balance, since they had the series caps. Isn't the CT usually taped off if a FWB is utilized with a CT PT?
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What is the net result from connecting the Sec CT to that point as opposed to ground?
In that arrangement, a winding CT sits at a DC voltage halfway between the +ve and -ve outputs of the FWB rectifier.
It has the ability to source high current levels.
As in your amp the -ve output is connected to ground, also connecting the CT there would be a massive short circuit, things would go bang.
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In that arrangement, a winding CT sits at a DC voltage halfway between the +ve and -ve outputs of the FWB rectifier.
It has the ability to source high current levels.
As in your amp the -ve output is connected to ground, also connecting the CT there would be a massive short circuit, things would go bang.
Understood, & thank you guys for the reminder.
I am not modifying anything circuit-wise, but I will keep that more readily in mind when looking at a bridge arrangement.
I guess the confusion was born out of there even being a CT shown in a FWB application.
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> confusion was born out of there even being a CT shown in a FWB application.
99% of transistor power amps have FWB with CT, grounded.
You gotta get out of the old neighborhood once in a while.
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You gotta get out of the old neighborhood once in a while.
This is sadly true on SO many levels, ...though I actually left the house to today to play an afternoon date.
I have a hard enough time finding my way around here, without getting completely lost in Silicon Valley ...just yet
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> confusion was born out of there even being a CT shown in a FWB application.
99% of transistor power amps have FWB with CT, grounded.
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Taking this to the nth degree, in that scenario, the FWB rectifier operates as a 2 phase split rail, rather than a single phase FWB :icon_biggrin:
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As I am starting to work through this (caps underway) & getting familiar with the circuit, I am thinking that I would like to remove the "line out" feature & replace it with a Pre-Out/Pwr-In as used in Sluckey's November project (snippet below) as that would be far more useful. Probably even a no-drill modification if I relocate the tube balance pot.
In Sluckey's implementation, the pre-out is post MV but I am wondering if pre-MV would be useful? Essentially just on the other side of the MV pot.
Related question about the 120pF cap that appears to be tapped in at roughly 50% of the MV pot. I haven't really studied the connections of the part in hand, but it is permanently frozen. Why was that cap implemented that way?
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I will confess to getting used to looking at a particular style of schematic (primarily Fender) & not scrutinizing things quite enough to realize I might be looking at something familiar circuit-wise, just presented differently.
Though I haven't even gotten it plugged in yet, I am wondering about the ultralinear OT & how that works.
My understanding is there are screen taps at a certain % of each winding to manage tube efficiency. Is that alone responsible for the huge increase in output wattage?
Further, could the UL be a switchable feature (using the UL OT) where those screen feeds could be bypassed & the screens connected to a B+ node for the typical lower power version of the circuit?
I agree, that schematic is particularly hard to wrap your head around. I've heard the UL fenders are tapped at about 15% and not 40% as most actual ULs would be. You could probably measure voltages and find out what the exact measurements are.
You can make anything switchable with enough contacts and ambition.
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UL does not make "more power". It does make more power for a given % THD distortion, but do we care?
IMHO the UL connection may be less liable to spew its guts when severely abused, shorts or opens. This in turn "allowed" jacked supply voltage and higher Watts number while still allowing a Warranty. (Any 6L6 amp can make 65 Watts if you can get out of town before the customer blows it up.)
Aren't these amps a dead-end since PA system prices fell in the late 1980s? Nobody rocks the Coliseum with just their own amplifier anymore. The Fender 300, the true SVTs, and all the little 135W head are now just bass-amps or time-sinks for guitarists who never did it back in the day. (Back hurts just typing about those amps.)
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Aren't these amps a dead-end since PA system prices fell in the late 1980s? Nobody rocks the Coliseum with just their own amplifier anymore. The Fender 300, the true SVTs, and all the little 135W head are now just bass-amps or time-sinks for guitarists who never did it back in the day. (Back hurts just typing about those amps.)
Let's be honest... We're all here beating dead horses. Maybe the horses from the 60's were funner to beat than the horses from the 80's.
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Aren't these amps a dead-end since PA system prices fell in the late 1980s? Nobody rocks the Coliseum with just their own amplifier anymore. The Fender 300, the true SVTs, and all the little 135W head are now just bass-amps or time-sinks for guitarists who never did it back in the day. (Back hurts just typing about those amps.)
Let's be honest... We're all here beating dead horses. Maybe the horses from the 60's were funner to beat than the horses from the 80's.
There is truth to both of these statements, as this amp would probably get you shown the door most places these days.
This amp is roughly the size of my bass rig & once it gets speakers, will likely be heavier.
But I am hoping to get it sorted & usable as a "swiss army knife" for the backline at a local theater.
I queried about the UL earlier, as 70W really is MORE than needed with present-day sound systems, I would prefer the 40W version (still more watts than required) , at the same time I would think that 70W clean watts will make a good platform for touring guys with big pedal boards.
But the amp is what it is, & presents a good opportunity for some tweaking without worrying about undoing something hardcore (& $$) "vintage".
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If the high power output is a problem, the 75 schematic I linked to earlier has a 1/4 power switching option that you might want to consider :think1:
It should be fairly easily transferable to your amp, though may require a degree of head scratching.
And it’d make a very big heavy 10-15W amp :icon_biggrin:
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If the high power output is a problem, the 75 schematic I linked to earlier has a 1/4 power switching option that you might want to consider :think1:
It should be fairly easily transferable to your amp, though may require a degree of head scratching.
And it’d make a very big heavy 10-15W amp :icon_biggrin:
Good thing it has wheels... :icon_biggrin:
I did check out that other amp you linked. Had to look at that schematic for a while to get an idea of the power switching, not the most user-friendly or intuitive layout.
Looks like B+ node shifting, am I reading it right?
That said, I will just get this up & running as-is, see how it works out.
I am thinking it could always be de-tuned (power-wise) if needed by defeating the UL, a different type of rectifier or even swapping in a regular BF Pro PT.
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The UL here isn’t UL in the normal sense of the term.
To all intents and purposes, the output stage is operating in regular pentode mode.
The UL taps are just taking some stress off the screen grids.
If the taps are dispensed with, the new screen grid arrangement will need to take account of that. The standard Fender arrangement would allow them to over dissipate.
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If the taps are dispensed with, the new screen grid arrangement will need to take account of that. The standard Fender arrangement would allow them to over dissipate.
I am not sure I follow.
If the taps were disconnected & taped off, would it not be as straight-forward as moving the screen grids to a B+ node?
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by connecting the plate and G2 together you've created a triode. by adding a 2nd tap for screens and a current limiting resistor for G2 you now have the "classic" pentode setup.
KISS
you have the transformer, wire it, play the amp for a month til you're bored, read for a week about cool things to do to boring amps, modify
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If the taps were disconnected & taped off, would it not be as straight-forward as moving the screen grids to a B+ node?
That would allow the screen grids to overdissipate, unless higher than normal value screen grid resistors were used.
The HT is 500V.
The g2 limit is 450V.
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I think we are all talking bout the same thing.
I meant an appropriate B+ node, downstream of the plates, not the same point as the plate/OT connection.
But it's going back together stock anyway, other than some physical reconfiguration with the power supply. The way that section was wired seemed like a whole lot of afterthought & drunken implementation.
Waiting for caps & new power resistors.
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If I were you, I would try a PPIMV between the 6l6 plates. I did this and I can easily play at low volume. In fact, it seems to take off too early, like 3 at the volume dial it is already beyond driving where I would normally play. Of course I'm not roasting the power tubes at that volume, but I'm also not playing at arenas. It's a pretty easy thing to alligator clip in and test.
I used the original 1M value, but next time I have it open, I'll probably see if I can reduce the resistance to about 250k, there just isn't much sweep on the pot. I also turned the pullout switch to a 330k/1M grid leak resistor on the PI, and did the Rob Robinette Switchable feedback mod, except instead of having 1500R/820R/500R, I went 1500R/820R/and 820R with a .68uf cap to ground. They're all very subtle changes to the tone, but very usable.
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I have an earlier non-UL silverface Pro Reverb that has been converted to AA165 specs and it is one of the best amps I've ever heard. If I picked up an ultralinear in that nice of shape for not much money I would buy a replacement blackface power transformer and blackface it.
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It’d need a choke and additional screen grid HT node.
And as well as the screen grid taps, the UL OT primary impedance may be a bit higher than previous versions, and it may be far beefier, higher spec than the 125A6A / 022848.
https://el34world.com/charts/Schematics/files/Fender/Fender_pro_reverb_aa165.pdf
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I will admit I am pondering all of these ideas...
but I am also trying to temper my impulse to tinker & start out with putting it back into working order in it's original configuration, as I have found there is real value in having a good solid starting point.
There is some info on that OT out there but not a lot. According to Tube Doctor (Euro) it is a 4.4KΩ/2&4Ω configuration with the addition of the UL taps @20%.
Another chart listed that part number as an 8Ω sec.... but I am thinking that the 2/4 is probably correct.
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And it’d make a very big heavy 10-15W amp :icon_biggrin:
:laugh:
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Still plugging away on this & I have completed the unofficial "phase 1", the of rebuilding the power section.
This section was wired with strings of back & forth, from the doghouse to the transformer area, with related components separated in an illogical way, so I decided to do some heavy editing.
I had remembered a photo of an AB763 that Doug rebuilt from a parts amp & he had mounted the filter caps inside the chassis which inspired me to go that route, but converting things to radial caps at the same time.
The power section is pretty much re-built as-drawn on the Fender schematic, but all inside the chassis, the doghouse is no more. The only real mod was to add adjustable bias to the tube balance circuit & I have accumulated a small pile of wire in the process.
At this point, I have also replaced all the by-pass caps, installed new power tube sockets & did some other editing; removing the drive circuit, removing the odd CBS-added caps & basically blackfacing the front end of the amp, up to PI
It fired up fine after the smoke test, has super deep reverb, still no trem (LFO blinking/changing speed, but intensity pot is partially frozen/defective) & is quite loud.
Replacing all the pots is the next phase, along with new shielded grid wires & reconnecting the master volume, bypassed at the moment.
Beside the massive amount of wax to be removed, I have found many instances of broken wires, right at the eyelet, held on by just one or two strands. I am not sure why this amp ended up abandoned in a shed the way it did, but I am guessing it became unreliable due to this kind of stuff.
The cabinet has also been getting some attention with all the loose Tolex being re-glued, deep cleaned & will get new hardware.
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looking really good! Nice looking work
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Replacing all the pots is the next phase …
Unless you can source J taper pots for the relevant positions, be prepared for using higher volume / tone settings than you might be used to with black or silver panel models.
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I have no history with this amp, so I am not accustomed to anything in particular regarding the pot taper, but thanks for the reminder.
Audio taper on the TS(s) & volume(s) should bring it inline with the blackface specs near as I can tell.
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Audio taper on the TS(s) & volume(s) should bring it inline with the blackface specs near as I can tell.
But which audio taper?
Standard CTS and bourns is 10%, Alpha Taiwan is 15%.
Original 60s spec as far as I can tell was about 25 to 30% for volume and treble, 10% for bass and mid.
J taper is about 35%.
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Audio taper on the TS(s) & volume(s) should bring it inline with the blackface specs near as I can tell.
But which audio taper?
Standard CTS and bourns is 10%, Alpha Taiwan is 15%.
Original 60s spec as far as I can tell was about 25 to 30% for volume and treble, 10% for bass and mid.
J taper is about 35%.
That's getting into the weeds for me.
I will turn the knob until it sounds good.
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That’s fine.
It’s just lots of people seem to fixate on setting the knobs to particular values.
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It’s come to my attention that AmplifiedPart/AES/CE carry J taper pots, the Fender branded ones actually specify a 25% taper https://www.amplifiedparts.com/products/potentiometers_0?filters=1902a1910c147a1902
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It’s come to my attention that AmplifiedPart/AES/CE carry J taper pots, the Fender branded ones actually specify a 25% taper https://www.amplifiedparts.com/products/potentiometers_0?filters=1902a1910c147a1902 (https://www.amplifiedparts.com/products/potentiometers_0?filters=1902a1910c147a1902)
Thanks for the link.
I had a few Alpha pots in some correct values, the rest are incoming. Based on your "%" comparison earlier, I will land somewhere between vintage audio & current standard, which sounds workable to me.
Tone settings never seem a big deal as far as more or less knob, it's abrupt volume that I find annoying, we'll see how these pan out.
The one mod I DID already complete, was to add a dwell control (on the back panel) which makes the reverb much more flexible to dial in & get a nice splash that's not overly saturated.