Magnatone MP-1 reverb

[StevieRayVehkakoski]

I would like to copy the reverb from this late 60's Magnatone to one of my builds but I'm not really familiar with the way the signals are mixed before the PI.

Here's the schematic:
http://www.bustedgear.com/images/schematics/schematic_Magnatone_MP1.gif

So, the dry signal is taken from the cathode of the push-pull reverb driver and the signal is fed through the LDR + some other stuff to the cathode of the mixer tube. Also the wet signal is fed to the cathode of the mixer tube. What I would like to do is take the reverb part from this amp and build it in one of my amps but I'm not really sure what resistor/cap values I should use for mixing the signals. Also in this Maggie I would guess the signals are both fed to the cathode of mixer tube because there's one totally unusable input on the grid of the mixer tube. I've seen that some Ampegs (R-12-R?) feed the reverb signal to the grid of the mixer tube and the dry signal to the cathode of the mixer tube and I guess it could be done that way here too to maximize gain?

[Willabe]

Hi SRV,

I dont have an answer for you, but someone here will. Any way, I think this Maggi schem is very interesting. How did you come to pick this reverb for your build? Have you ever played through one of these amps? KOC likes to use a push/pull verb. Smaller trans. so less noise because of less magnetic leakage, push/pull output so noise canceling is built in. Looks good on paper  -but- how does it sound? He does say the pp type is cleaner and may not be as warm sounding as a Fender single-ended type, because your not getting the "extra vail of character" that the single-ended type adds.

Never seen an LDR used in a verb before? Also cant remember seeing an LDR drawn this way before, is this a clue (how its drawn) to what its doing in there? Id like to understand what the LDR is doing/why its being used here.

Schem lists pp verb trans as - 25k pri. to 8R sec.- ? Not exactly an off the shelf trans,but maybe Im wrong. Any one know where you can buy one? KOC sells one and Im sure its a good trans, but it has PC pins on it instead of fyl leads for hook up. I guess thats OK but, would not be my 1st choice, also it aint cheap.


  Wish you well with your new build!     


        Thanks,   Brad

[sluckey]

The LDR belongs to the tremolo circuit.

[Willabe]

slucky,

So thats why the schem has the 2 arrows pointing down from the resistor part of the LDR to the bulb part of the LDR, with 2 arrows at the bulb 1/2 pointing up to the resistor 1/2.       Didnt realy look at the ocillator. Should have looked a little longer.

   Thanks,   Brad   

[HotBluePlates]

I would like to copy the reverb from this late 60's Magnatone to one of my builds but I'm not really familiar with the way the signals are mixed before the PI.

... Also the wet signal is fed to the cathode of the mixer tube. ... Also in this Maggie I would guess the signals are both fed to the cathode of mixer tube because there's one totally unusable input on the grid of the mixer tube. I've seen that some Ampegs (R-12-R?) feed the reverb signal to the grid of the mixer tube and the dry signal to the cathode of the mixer tube and I guess it could be done that way here too to maximize gain?

Well, it's done to keep isolation between the instrument and mic channels.

I'd personally copy the Maggie circuit, minus the LDR. The 47k resistor in parallel with the LDR is the dry signal path.

The mixing tube has inputs on the grid and cathode, but the mic channel is fed into the grid. That seems a little odd to me, because there is more gain available from the cathode input (grounded-grid operation yields the highest voltage gain). My guess is the other "feature" of the cathode-input is the very low input impedance, which normally knocks down the signal level a lot except for very low impedance signals.

[StevieRayVehkakoski]

The reverb is the best I've heard in a guitar amp. And the best part is that it's 100% silent, I haven't yet succeeded in making a reverb circuit that doesn't hum or hiss but this one is dead-quiet even when maxed. And when maxed the reverb is just crazy, it's like you're lost in a cave.

The tremolo is quite unusable in this amp, it's more like on-off than a real sine-wave tremolo so I want to get rid of it. My question is: what's the resistance of the LDR when not in operation, I would guess it's zero or infinite?

So, to maximize gain I should mix the signals just like in this amp, both on the cathode of the mixer tube? I'll try to mock-up a schedule tonight just to show what I mean.

[sluckey]

I would guess it's zero or infinite?

It's very high.

[StevieRayVehkakoski]

So here's the first try, pretty much a champ with the added reverb from the Maggie. I did this quite fast so there might be some stupid errors but I guess the principle should be clear.

How would you maximize gain in this circuit, I've got no idea what's the gain of the last preamp stage (the mixer tube)

[Willabe]

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Went and searched my box of trans. and found a Hammond 3w. pp OT (125 series) that has a lot of taps on it for different in/out set ups. It does have a 22.5K to 8R connection listed on the chart thats on the box. I was going to use it on a firefly build but I think it should also work for this Maggie verb ?

Any one here know of any other off the shelf 2w/3w pp OT (25k/8R) that could be used for this build ?

I wonder if this might be a good choice (since its so quiet and has great depth) for a stand alone verb not only for guitar  -- but --  for an FX for recording ?     
Is it less "boingy" than a Fender ? maybe a little smoother ?

SRV did you get a chance to look at the verb tank up close/the guts ? how long ? how many springs ? was it just a standard accutronics type ?
 
Im also wondering how much the signal being k feed to the mixing tube stage (low impedance ?) is adding to the " lost in a cave" like sound ?

       Thanks,   Brad         
 

[HotBluePlates]

Without looking at a data sheet, I'd guess the 6SN7/6SN7 can't dissipate much over 1w per plate. Class A has 50% theoretical efficiency, but real-world rarely gets much over 25-35%, especially with triodes. Push-pull nets you a hair over twice what a single tube would get you, and so we're looking at about 2 * 1w * .3 =  0.6w.

It is worth noting that O'Connor's push-pull reverb transformer is *tiny* being smaller than a preamp tube. Pic here

One of the key features of any push-pull stage as compared to a single-ended version is the much smaller transformer that can be used due to the low-to-zero standing current in the primary; push-pull features balanced idle current, which could otherwise saturate a transformer core.

So your 2-3w OT is too-good for what is shown here. But if you already have it, and no home for it, then I suppose that beats buying another transformer.

[HotBluePlates]

So here's the first try, pretty much a champ with the added reverb from the Maggie. I did this quite fast so there might be some stupid errors but I guess the principle should be clear.

How would you maximize gain in this circuit, I've got no idea what's the gain of the last preamp stage (the mixer tube)

I didn't realize this would be a single-channel amp, so no other signal path is utilizing the grid. It may be worthwhile to switch to typical grid-input.

However, let's assume there is magic in the Maggie plan. Note that the second resistor in the T-attenuator is 82k instead of the 47k you have drawn. Also, there is an additional triode gain stage between the mixer tube and the phase inverter. Just from eyeballing it, you will likely need the gain of that extra stage.

Are you able to breadboard this to try before a full amp-build?

[StevieRayVehkakoski]

I've got one amp that I could scrap just to try this reverb but let's see.

I guess it would make sense to take the signal from the reverb pan straight to the mixer tube grid but feed the dry signal to the cathode? Some Ampegs feed the wet signal to the mixer tube grid and the dry signal to the grid of the mixer tube but they have the cathode connected only with a 22k resistor but this maggie has 82k+47k between them cathodes and 2,2k to the ground between the two resistors so the signal is knocked down quite a bit.

[Willabe]

So, all we need is a 1w pp @ 25k/8R (or does that schem read 23k? I cant realy see it.) I only mentioned the Hammond because of its turns ratio and its easy to get a hold of one. Id rather save the 3w. Hammond I have for a Firefyl build.

I thought I'v read here before if you 1/2 the sec. you 1/2 the pri. (is that right?) to keep the turns ratio the same?  So we would now have a 12,5k/4R as an option?
I seem to remember reading here that any pp OT trans over 10k on the pri. would be hard to find? (I thought it was PPR) Maybe I have this wrong.

Which brings me back to, does anyone know where to get an off the shelf pp OT (25k/8R @ 1w.) trans for this Maggie verb build? I think I'd like to build one too, only maybe as a stand alone unit.


       Thanks,    Brad

[Willabe]

SRV,

I forgot to ask you! Its your build and idea.          Have you found your OT for your build? If so can you point me in the right direction?

     Thanks,    Brad   

[StevieRayVehkakoski]

Hammond 125A:

http://www.hammondmfg.com/125a.htm

I'll try to sketch the plan so that the dry signal would be fed to the cathode and the wet to the grid, maybe somebody could make a educated guess on how it should work..

EDIT: here's the sketch. One thing that I really would like to figure out is how to calculate gain on these cathode-connected things?

[davohilts]

Hello,
I'm working on a Magnatone MP1 which has a bad LDR.  The incandescent bulb works but the photocell is hit and miss and I want to replace the photocell.  Can anyone help me with what the values might be for it?
My OT was bad and I looked around to find an 8k to 8 ohm replacement which put out around 24 watts and most of the manufacturers didn't have any.  I found a supplier on EBay that sells one and bought one and installed it and it sounds real good. Here is a link to the auction, he had them made for a marshall 18 watt clone project I think.  http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=160434574056&ssPageName=STRK:MEWAX:IT

Anyway if anyone knows magnatone amps and could fill me in as to what photocell I might use to replace my uncertain one I would really appreciate it.  When the circuit works the tremolo sounds just the way I like it.
Dave

[sluckey]

...the photocell is hit and miss and I want to replace the photocell

RadioShack has a 5 pack of LDRs for 2.99.  Catalog #: 276-1657
One or more should work just fine.

[davohilts]

just that easy? it hardly seems fair.  thanks man!
Dave