Tweet
Fender FM 65R Reverb tank question
So I have a 8BB2C1B type reverb tank where the Input Impedance = 190 ohms and the Output Impedance = 2575 ohms. The original tank is missing from the amp and it was a 8DB2C1B tank with Input Imp. of 310 ohms. So my question is do I mess with this reverb tank and try to make it work? Right now I am still working out issues with the output section of the amp and it is currently in repair on the bench. It would be nice to be able to use the 8BB2C1B reverb tank if at all possible. I am just not sure if I need to make mods to make this tank work with this amp. So if anybody might have some thoughts on the matter it would be greatly appreciated, thanks.
-
When the going gets weird... The weird turn pro! -
You need to get the right tank. Sorry
Look at MOD very good for the $$$Experience is something you get, just after you really needed it. -
Okay I figured that might be the only solution for this particular question. I found one NOS, American made, for a good price. Thanks for the fast response nick!When the going gets weird... The weird turn pro!Comment
-
How did we determine the pan should be an 8DB? The circuit looks like an 8EB drive to my old eyes. Compare your circuit to that in a PV Classic 30 for example. The C30 uses an EB pan. Or the Fender Blues Junior, same story.
What is there to fix? There are two sections, just about every reverb pan has a B output impedance, so if you are working on the return, plug your 8BB into it and see what happens when you rub the springs.
As to the drive side, your BB would probably work somewhat, you'd have to see if that was enough, but really, the right pan is preferred.Education is what you're left with after you have forgotten what you have learned.Comment
-
Of course great catch ENZO!! Yeah the 8DB version is for FM65R and the 8EB is for Frontman 65R. Not that they are that much different but this one say "Frontman" not "FM" on the chassis.
My source was...
Reverb Tanks
If you look you will see which amps that tank fits. Right there I scrolled down to 8EB and realized you are completely correct. Good thing I did not order anything yet. I will just plug this (8BB) reverb in and see how it goes once it is fixed.
The amp had a shorted Q11 base to emitter and R76 was burnt as well an open R80. All rail voltages are where they are supposed to be and the preamp works great. When I got the amp D46 and D47 16v zener diodes were missing from the board. I have many recycled cabs laying around so I bought this head to fill one up and get it out there playing for someone again. It currently distorts on the output section right now and low volume.When the going gets weird... The weird turn pro!Comment
-
Your chart shows 8DB for both FM and Frontman 65R. FM means Frontman, I am not sure what difference there would be other than cosmetics. I believe their 8DB listing is in error, as they have both pan types listed for the same model amp.
As far as I know Frontman and FM mean the same model. Though minor mechanical differences. The circuits are the same. If I recall right, the Frontman came out first, and has sturdier construction, like nuts on the pots, then they cheapened them to the FM series and now the pot shafts just stick out the holes on the panel, no nuts. But I can't say if that difference was in all FM/Frontman models or just some of them.
I think all the FM65s were asian, but the Frontman 15 and 25 amps had differences whether they were made in Mexico or in Indonesia.Education is what you're left with after you have forgotten what you have learned.Comment
-
The Made in Mexico ones as I recall had the nuts on all the pot shafts. This one was made in China 2005. Also, just a silly note but on the schematic any component they listed as 2n4001 or 2n4003 they really meant 2n4401 and 2n4403.
Well I must have over looked R81 & R83 those fused 47 ohm resistors. R81 that is connected to Q11 was open and the other one measured 30megs or something. Thanks for the help Enzo for figuring out which tank is the right one for this amp. Should I test out that 8BB tank for $H!Ts and Giggles?When the going gets weird... The weird turn pro!Comment
-
Originally posted by DrGonz78 View Post
Much as I hate to disagree.. but... I think 8DB is the correct one. Here's why I think that. Here is an 8EB drive cct from the Blues Junior.
Note the 47 ohm resistor. This sets the drive current for the tank. On your sch this is 22 ohms indicating twice the drive current (all other things being equal). 2x drive current means less turns and so a lower impedance. An "E" tank is 600 ohms and a "D" is 250 ohms.
You can shove in the "B" tank but it won't be driven hard enough giving your a poor signal to noise ratioExperience is something you get, just after you really needed it.Comment
-
Yeah this is back and forth sort of thing going on huh? However, according to the schematic, pg2 board layout D5 D6 quadrants, the p/n = 0028055000. Sorry but I just realized it was there when I woke up today. So the part number on the schematic relates directly to 8EB type pan.
Accutronics Reverb Pan 8EB2C1B 0028055000
Which this is fine and dandy but still more convolution. Here is a link that states part number as 0062682000 and it is says Frontman 65R.
https://www.guitar-parts.com/catalog...erb-tank-62682
Only problem is 0062682000 does not confirm whether that is 8DB or 8EB. Generally Marshall is listed as 8DB and Fender is listed 8EB.
Accutronics reverb tank numberingWhen the going gets weird... The weird turn pro!Comment
-
Agree that Fender is usually "E" and Marshall is "D". If you've found the p/n on the schemo that settles it - and well doneOriginally posted by DrGonz78 View Post
.
I did look before I and didn't see it. I looked again with your directions and I still can't find it. Arrgggh!
I wonder why they doubled the drive current?Experience is something you get, just after you really needed it.Comment
-
These guys think it is an 8DB too.Originally posted by DrGonz78 View PostExperience is something you get, just after you really needed it.Comment
-
An examination of the impedance of a 4700 ohm resistor in parallel with either a 127.3mH coil (8E) or a 49.3mH coil (8D) at increasing frequencies may be of some use in deciding which tank to use. At 1KHz both tanks have a parallel impedance that is very close to their nominal impedances. At 5KHZ, the 8E tank has a parallel impedance that is significantly lower than the 4000 ohm impedance it has by itself. This means the gain of the opamp is significantly less than the gain would have been with the tank in the NFB loop without the 4700 ohm resistor. The 8D tank at 5KHz in parallel with the 4700 ohm resistor is not significantly different than the 8D tank by itself. The 8D parallel impedance doesn't have any significance until around 10KHz.
The 4700 ohm resistor in parallel having the desired effect at the desired frequency with the 8E tank and not with the 8D tank argues for the 8E tank. Doesn't mean that they actually used an 8E tank, though.Comment
-
A valid point. It's very odd. They doubled the drive current so I'd expect then to half the parallel resistor to stop the opamp from hitting the rails at higher frequencies. This implies than the tank is a "D" to stop that from happening. Of course the input to the drive stage drive may be half of the FBJ implying a 'E'. We just can't tell.Originally posted by 66 Kicks View Post
The other thing I thought of was to take one of each off the shelf and measure the insertion loss for each and compare with the schematic. But the numbers in the schematic are useless as they simply state >5mV at the output of the recovery stage. I think that figure belongs on the output of the tank. Also they show the output of the driver opamp as 191mV. I don't believe that either it so think that TP is meant to be the input to the driver stage.
I took at look at the Frontman 212R schematic. It is same at FM65R and with equally useless voltage of >5mV.
The only to way settle this is for someone to actually look at one. Surely someone out there must have one?Last edited by nickb; 08-18-2016, 08:10 PM.Experience is something you get, just after you really needed it.Comment
-
My internal voice is saying to me, "I bet it doesn't much matter, and either one will probably work."Education is what you're left with after you have forgotten what you have learned.Comment
-
Stopped reading all the maddening contradicting posts guessing things, comparing different schematic versions, Forum posts and "I read it on the Internet" and thought it as a designer would.
You canīt just say "Fender uses this, Marshall uses that" because as any Tech knows, "brand means nothing".
Point is that reverb tanks are basically a small speaker, driven by a small power amp, vibrating a spring, with an electromagnetic pickup (magnet and fine copper wire wound on a bobbin) at the other end.
"Power amp + speaker" get you straight away into "impedance" and that is the main point here defining compatibility (or lack of it).
Old fender amps used a 1 W power amp (parallel 12AT7) transformer coupled into an "8 ohms speaker" , and get killer reverb.
Dedicated fender reverb used an even higher power pentode (2 or 3 W?) for even better one, again with a plate to speaker transformer.
Enter SS: a *few* designers (Roland, Yamaha, etc.) drove their 8 ohm tanks HARD using a small chipamp but MOST got cheap, very cheap.
Accutronics started making high input impedance tanks which could be (poorly) driven by a single transistor, we are talking a few milliwatts, check old Traynor SS PA circuits , old Gibson, etc.
Some did it right, think Lab Series, and used a real few watts power mp, typically an Op Amp boosted with TO220 or TO39 transistors, what we use as power amp drivers.
99% others use a single cheesy Op Amp, which isnīt even a Power Op Amp but a signal rated one, a glorified 741: RC4558 / TL072, etc. ... what we have here by the way.
From what I see (just read the datasheets), standard Op Amps can drive full swing (almost rail to rail) down to 2k loads and below that fail miserably, barely driving a 600 ohms load.
Back to driving reverb tanks:
1 Op Amp can drive a 600 ohms tank , not balls to the wall (of course) but passably well, and thatīs the most popular combination.
2 or 3 Op Amps in parallel (Marshall loves doing that) drive a 300 or 200 ohms tank respectively , and of course somewhat harder than the modern (cheesy) standard.
Now, single Op Amp driving a way lower than 600 ohms tank? (what you have) ..... well, it will somewhat work, sort of, but will clip earlier and drive it less than even the cheesy ones, because Op Amps have poor output curret capability.
Now if you put a NE553x there, with guaranteed 15 mA output current (vs 2 or 3 mA from a TL072) then things might improve
FWIW Marshall often uses LM1458 in reverb driving duty, in an amp otherwise chock full of TL072 or the switching version, M5201 ... I was always intrigued by that and *guess* it must have a somewhat looser output stage and in practice be able to supply a couple mA above the official ratng.
I can not find another explanation to their complicating supply by stocking a different Op Amp while the assembly machine dispensers are already filled with tons of general purpose TL072.
FWIW PHATT, our Australian friend in SS Guitar , has designed the MaxiVerb circuit, which takes a regular cheap reverb tank as found on 99% amps and drives it with an Op Amp boosted with 1A TO220 transistors and fed from hairy +/- 35V rails, results are *impressive* , once again proving that sound/tone is not in "the parts" but on "the design" .
Hereīs a free sample of his very simple rig, the amp itself is a Laney 60W keyboard amp.
No, itīs not a Digital reverb nor even a Delay unit, or a Studio type one, just a cheap plain vanilla reverb tank .... properly driven.Juan Manuel FaheyComment
Comment