Master Volume 70's Fender - Convert to LARMAR Help

[Yeatzee]

Howdy folks! A while back I worked on a MV Bassman 50 for a local guy and he's brought it back for me to swap the OT to a multitap OT, and to convert to a LARMAR master volume setup. I discouraged both, but it's what he wants so the MM OT is in and now I need to tackle the new master volume setup. Now, I've never worked on modifying or adding a master volume before so this is completely new and foreign to me... hoping for you kind knowledgeable folks to school me on what's what and how to get the job done!

YT series here for reference:

This particular amp does not have a schematic available, as I understand it, but I've heard someone say it's similar to the bassman 70 MV which does (though I see a lot of differences)

https://schematicheaven.net/fenderamps/fender_bassman70.pdf

Here's the regular Bassman 50 schem, non-MV so I guess it's a hybrid between these two schematics..

https://schematicheaven.net/fenderamps/fender_bassman50.pdf

Here's a schematic of the Larmar from robinnette.



SO, I don't fully understand the stock master volume setup but the goal is to replace that pot with apparently a 250ka dual pot. And then, for the safety resistors noted in the schematic, on the Bassman 50 it's got 100k instead of 220k resistors in that spot of the circuit so if the 250k and "safety" resistor are in parallel that'd mean a 220k resistor would be fine for safety instead of the 1.8m in the schematic so that it nets out roughly 100K? Do I have that right?

Here's a screenshot from one of the videos showing the 100k resistors (grid leaks are what they're called?) where you'd normally see 220k's in BF era.



The bias balance setup also makes the larmar install even more confusing to me, so really any insights I can get dumbed down for someone very new to this sort of thing would be greatly appreciated! My assumption is that I won't be able to simply disconnect the existing MV and the wires coming off of it onto the board as well, so what do I do there? Are there resistors needing to be installed on the board in place of the stock setup? It's all very confusing to me.

[Yeatzee]

It's crude, but I don't really know how else to show where my head is at.



Is this correct? What am I missing? And then what do I do about the wires running to the stock 1meg master volume pot? There's no way it's just a disconnect and you're good situation, unclear what to do there.

[neskor]

I will convert  this amp to AB165 in the front and the PI and output part to AA864.
That way you will have almost perfect Bassman amp.
Remove the MV and put PPIMV.

[Latole]

I often put MV on few amps.
The type 2 Lanmar you show is not recommended. Don't do it
The one I built few times. I use 500K or 1 Meg pots, type A

From the Ultimate Tone by Kevin O'Connor page 5-9

[pdf64]

For a type 2 / LarMar to work, the bias balance will need removing and converting to the previous type of shared bias adjustment. 

Which seems a beneficial thing to do anyway?

With that done, I suggest to scale the LarMar values down, so use a dual 100k pot and 1M safety resistors.

Does that seem feasible to you?

[Latole]

He have just to follow the Ultimate Tone sketch

[passaloutre]

Type 2 Larmar is fine, and what I would recommend. Also convert the bias circuit to a proper 60s black panel adjustable bias.

[tdvt]

I had a similar scenario come up with the balance circuit/LarMar & from looking around (& I may have asked but can't find the post), I came to believe that you could keep the individual feeds from the balance pot & just land them on the individual pots, as opposed to jumpering one bias supply to both pots. 


In my case, I had converted a '79 UL Pro Reverb to a bias & balance circuit & wanted to add the LarMar. That amp is now undergoing a board replacement (side-project) so I can't really report how well all of that works .


But, I HAVE put the LarMar in a few amps & find it pretty useful. There are some tonal changes as you dial it down, but they can be compensated for with the tone controls.



Haven't tried the KOC circuit that Latole linked.



Regarding values: https://el34world.com/Forum/index.php?topic=30920.msg340610#msg340610


Circuit-wise, I am guessing that that amp is close to the other higher-wattage late-70's amps that have the nightmare schematics. I pretty much used the ones' that 2deaf drew up, as they were close enough & WAY easier to read.

https://el34world.com/Forum/index.php?topic=24638.0

 

[Latole]

Haven't tried the KOC circuit that Latole linked.

I try fews , this is the best one.

[sluckey]

Treat this as two small projects... 1) remove original MV and 2) install new MV. Since we have no layout or schematic to go with, we need to see a clear pic of the original MV pot with wire connections and where those wires connect to the board. 

I suggest the circuit Latole posted because you won't have to mess with your bias balance mod circuit. I'll see if I can draw a simple layout.

[Yeatzee]

I will convert  this amp to AB165 in the front and the PI and output part to AA864.
That way you will have almost perfect Bassman amp.
Remove the MV and put PPIMV.

Is this drawing basically exactly what I drew in the post above yours? Just with the bias balance it gets a little wonky/confusing. Owner likes the amp as is just wants larmar added

[Yeatzee]

For a type 2 / LarMar to work, the bias balance will need removing and converting to the previous type of shared bias adjustment. 

Which seems a beneficial thing to do anyway?

With that done, I suggest to scale the LarMar values down, so use a dual 100k pot and 1M safety resistors.

Does that seem feasible to you?

Are you sure? I've heard of larmar's being done on bias balance setups before. The amp has already had a bias adjustment mod added to the bias balance setup with that mini trim pot added you can see in the photo, back when I had it originally. So it's got both. It'd be great to maintain that if possible! Thanks!

[Yeatzee]

Treat this as two small projects... 1) remove original MV and 2) install new MV. Since we have no layout or schematic to go with, we need to see a clear pic of the original MV pot with wire connections and where those wires connect to the board.

I suggest the circuit Latole posted because you won't have to mess with your bias balance mod circuit. I'll see if I can draw a simple layout.

Yes of course!





Stock MV is the top left pot

[sluckey]

OK. Disconnect the three wires from the original MV and remove the pot. Note the value of the pot. Now connect the yellow and greenish grey wires together. Connect a resistor (same value as the MV pot) between the white wire and the other two wires. Heatshrink and tuck out of the way under the pots.

Now for the new MV. Install the dual pot on the front panel. Unsolder the .047µF leads from the 47K resistors and put j-hooks on the floating leads. Install the new 0.47µF caps on the board and solder to the 47K resistors. Put j-hooks on the floating leads. Now wire the dual pot as per the attached drawing. Use silicone rubber to secure the additional caps to the original caps.

edit... replaced pics with a single pic.

[Yeatzee]

OK. Disconnect the three wires from the original MV. Note the value of the pot. Now connect the yellow and greenish grey wires together. Connect a resistor (same value as the MV pot) between the white wire and the other two wires. Insulate and tuck out of the way. You could make this neater by doing all this work on the board, but doing as I suggested will allow you to quickly and easily undo the mod if need be.

Now for the new MV. Unsolder to .047µF leads from the 47K resistors. Now wire the dual pot as per the attached drawing. Use silicone rubber to secure the additional caps to the original caps.

Could you explain a little bit of the theory, what is going on with the stock MV setup and what's going on with the new one? By doing what you suggest to the wires removed from the stock MV pot, basically I'm just using a resistor act as if the MV was set to 10?

Very different than anything I've seen before.

[sluckey]

Could you explain a little bit of the theory, what is going on with the stock MV setup and what's going on with the new one? By doing what you suggest to the wires removed from the stock MV pot, basically I'm just using a resistor act as if the MV was set to 10?

Not much to say. The original MV looks just like most any other VOLUME pot. The signal from V2 comes into the right side of the pot. Left side of the pot is connected to ground. The wiper picks off as much of the input as desired (determined by knob position) and sends that signal on to the PI tube via the PI input cap. Yes, wiring a resistor as I described is just like running the original MV wide open.

The new dual MV operates just like most other VOLUME pots too. Left side of the pot is grounded. The signal from the plate of the PI connects to the right side of the pot through the added cap and the wiper sends that signal through the original cap to the grid of the output tube. The added cap blocks the PI plate voltage from reaching the new MV and the original cap prevents the new MV from affecting the bias voltage. Wiring like this will not affect the bias at all and allows you to continue using your "adjustable bias plus balance mod" without having to change anything.

[Latole]

I can't find Bassman MV 50 , only the 10
Is the same MV circuit ?

[sluckey]

Short excerpt from KOC. He explains his preferred MV and also talks about why he does not recommend the LarMar mv...

[Willabe]

Latole left out the new caps C3/C4 need to be 10x of the original PI coupling caps so you don't lose bottom end.

Reason why is in what Sluckey posted.

New caps can be only 100v to keep them smaller.   

[sluckey]

New caps can be only 100v to keep them smaller.

That's true for the KOC discussion. But in my simple layout drawing the new caps are the light blue ones and they are connected to the PI plates so the voltage should be whatever the original caps rating is. I did it like this on purpose so Yeatzee would not have to touch anything in the bias circuit.

[Latole]

Latole left out the new caps C3/C4 need to be 10x of the original PI coupling caps so you don't lose bottom end.

That is what I do.

[Yeatzee]

What value is needed for the dual pot with this setup? I see from Latole's notes VR1 500k go 1meg? Is there a preferred pot type for this? Again, all new stuff to me. For example, audio vs linear?

[sluckey]

I would use 1M audio taper with 3/8" bushing. This one...

     https://www.tubesandmore.com/products/potentiometer-alpha-audio-dual-38-bushing

[Willabe]

Latole left out the new caps C3/C4 need to be 10x of the original PI coupling caps so you don't lose bottom end.

That is what I do.

But you didn't tell him that when you posted the KOC schematic.

[Latole]

Latole left out the new caps C3/C4 need to be 10x of the original PI coupling caps so you don't lose bottom end.

That is what I do.

But you didn't tell him that when you posted the KOC schematic.

You're right, I should have mentioned it.
But it's been a long time since these MV realizations.

When I get on the circuit, I immediately see what needs to be corrected.
The important thing was that you didn't make the type 2 Lan-Mar Master volume circuit you showed at the beginning.

Here some more ;

[pdf64]

I would use 1M audio taper with 3/8" bushing. This one...

     https://www.tubesandmore.com/products/potentiometer-alpha-audio-dual-38-bushing

In that scenario, with 100k grid leaks loading the pot, linear may work out better. As the heavy loading causes the effective taper of a linear track to become somewhat logarithmic. Whereas heavy loading of a log track may tend to result in its effective taper be coming 'double log',with the last 1/4 of the curve being even steeper.
Scroll down to 'which leads us to tapering' http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

[pdf64]

Short excerpt from KOC. He explains his preferred MV and also talks about why he does not recommend the LarMar mv...

Given the context, ie of the fixed bias supply arrangements of 60s Fenders, where the integrity of bias supply to the grids is already dependent on the wiper of the bias pot (both with the overall bias and bias balance arrangements), I'm not sure his argument holds much water here?
The LarMar at least has safety back up resistors in case a master volume wiper loses contact.
Whereas the bias balance (and possibly the overall bias here too) pots probably don't.
Consider that rather than the CCW pot lugs, the LarMar safety resistors could be taken back to the raw bias supply, before any wiper contact, thereby increasing the integrity / resilience of the bias to the grid over the original stock design (and over the type 1 arrangement being recommended).

Additionally I wonder if his argument that LarMar controls will have a greater tendency to become scratchy with use, due to the DC current flow, is based on experience?
I suspect it may be purely conjecture.
The DC current there is incredibly low.
I don't recall mention being made of scratchy LarMar controls being an actual thing, and I've not yet noticed it myself, in the 20+ years I've been installing / using it.
Consider that whilst a output valve's control grid current may be a tad higher than eg a 12AX7, the same argument would require an additional blocking cap (and grid leak as required) after every regular volume control, and also at amp inputs.

So, from the above, I call the real world validity of KOC's arguments (in the context of this mod to a Fender amp) there into question.
Whilst there's a grain of truth behind them, I think that in his criticisms of the LarMar, he's blowing things up out of all proportion (a tendency I may occasionally be guilty of myself  ).

[sluckey]

@Yeatzee... I just noticed the red/blue bias tap lead from the PT is connected wrong. Was like that when you first opened the amp. Also, schematic shows 1K but there is 1.2K on the board. Correcting these issues will probably require a slight bias adjustment. The attached pic was captured from your first video.

[Esquirefreak]

I had a A250k stereo pot fail a couple of weeks ago. Although, after deeper inspection I think there was some debris inside the pot that was the culprit. Thanks to the safety resistors bias stayed rock steady.

After using the PPIMV in quite a few amps without the additional caps and they have all worked really well for some years now.

/Max

[Willabe]

.... where the integrity of bias supply to the grids is already dependent on the wiper of the bias pot (both with the overall bias and bias balance arrangements), I'm not sure his argument holds much water here?

Yes, but, the bias/bias balance pots don't get turned very much. The MV pot probably will get turned a lot. So the MV pot has a much larger chance of going bad.

[pdf64]

.... where the integrity of bias supply to the grids is already dependent on the wiper of the bias pot (both with the overall bias and bias balance arrangements), I'm not sure his argument holds much water here?

Yes, but, the bias/bias balance pots don't get turned very much. The MV pot probably will get turned a lot. So the MV pot has a much larger chance of going bad.

A pot can have various failures modes.
Yes, the track completely wearing out is one of them.
But equally they can become dodgy BECAUSE they're never moved, eg surface gunk and oxides can develop over time and eventually cause a bad connection.
Switches and other contacts / connectors NEED exercising, at least once in a while.

eg the perennial 'How can the shorting contact in my fx loop return socket fail, I never use it?'

Whatever, the safety back up grid leaks of a LarMar should act to mitigate for collateral damage.

So whether LarMar or type 1, the net result of a track in the master volume control failing should be the same, ie one of the push pull output stages sides loses signal.

[Yeatzee]

OK. Disconnect the three wires from the original MV and remove the pot. Note the value of the pot. Now connect the yellow and greenish grey wires together. Connect a resistor (same value as the MV pot) between the white wire and the other two wires. Heatshrink and tuck out of the way under the pots.

Now for the new MV. Install the dual pot on the front panel. Unsolder the .047µF leads from the 47K resistors and put j-hooks on the floating leads. Install the new 0.47µF caps on the board and solder to the 47K resistors. Put j-hooks on the floating leads. Now wire the dual pot as per the attached drawing. Use silicone rubber to secure the additional caps to the original caps.

edit... replaced pics with a single pic.

Really appreciate it! .47uF is going to be a pretty beefy cap, any recommendations there? Something like a mallory 150, or a polypropylene orange drop style? Going to order the parts tomorrow and give it a whirl! Personally, I'm doubtful of any real benefit on an amp like this but if the owner is dead set on a new master volume I'll give it a whirl.

Good catch on the bias feed btw, stock from the factory wrong how funny.

[ac427v]

I use these Polypropylene because they are the smallest.


https://www.tubesandmore.com/products/capacitor-630v-polypropylene-radial-leads

I'm planning to use this circuit on my next build only all four caps will be .2 uf.  Is there any disadvantage compared to KOC using two .1 and two 1.0uf?

[sluckey]

Really appreciate it! .47uF is going to be a pretty beefy cap, any recommendations there? Something like a mallory 150...

I have used a lot of Mallory 150s. They will fit nicely on that Fender board. AES has a good supply.

     https://www.tubesandmore.com/products/capacitor-mallory-630v-150s-axial-lead

[Willabe]

I'm planning to use this circuit on my next build only all four caps will be .2 uf.  Is there any disadvantage compared to KOC using two .1 and two 1.0uf?

Yes.

Did you read reply 17 from Sluckey where he posted a link to what KOC said about what sized caps to use and why?

 https://el34world.com/Forum/index.php?action=dlattach;topic=31710.0;attach=113681

[sluckey]

I'm planning to use this circuit on my next build only all four caps will be .2 uf.  Is there any disadvantage compared to KOC using two .1 and two 1.0uf?

I don't see any disadvantage. Makes perfect sense to use two .2µF in series. The total capacitance is .1µF, same as the original cap.

I chose .47µF for Yeatzee's circuit because his original cap is .047µF, so total capacitance is .043µF, close enough to the original .047µF. But if I was scratch building this circuit I would have chosen to use four 0.1µF caps.

If Yeatzee is willing to change the original blue turds, I would suggest using four 0.1µF Mallory 150s. They would accomplish the same thing as using the original .047 plus a .47, and would look much nicer on the board.

[pdf64]

A 2 pole HPF isn't the same as a single pole HPF with twice the corner frequency.

[ac427v]

A 2 pole HPF isn't the same as a single pole HPF with twice the corner frequency.

Hmmm. I had to ask
I have no theoretical training to help me understand why two capacitors in series would affect the signal differently than one electrically equivalent capacitor.
So I am asking: What principal is at play here and what is the sonic effect?

[pdf64]

A 2 pole HPF isn't the same as a single pole HPF with twice the corner frequency.

Hmmm. I had to ask
I have no theoretical training to help me understand why two capacitors in series would affect the signal differently than one electrically equivalent capacitor.
So I am asking: What principal is at play here and what is the sonic effect?

The caps aren't in series per se. There's a shunt resistor / resistance after each.
A cap and resistor arranged like that forms a single pole RC high pass filter, cuts low frequencies at 6dB per octave.
A series pair of them creates a 2 pole filter, cuts at 12dB per octave.

[ac427v]

Still trying to understand the sonic difference.

A lone .1uf cap forms a 6 db per octave high pass filter having a certain -3 db point. For sake of this discussion, say 100Hz.

A pair of .2uf caps in series each create a 6 db per octave high pass filter having their -3 db point one octave lower. For sake of discussion, say 50Hz. So wouldn't their combined -3 db point be the same as the single .1uf cap? 100Hz?

Wouldn't both circuits sound the same?

[pdf64]

Still trying to understand the sonic difference ... Wouldn't both circuits sound the same?

Everyone hears things differently. It'll be quicker and more effective to tack something together and test that for yourself :)

I think it's safe to state that if the frequency response plots were compared, they'd be a little bit different, even around, and beyond a decade above, their corner frequencies.

EDIT - Also it doesn't seem good practice to introduce additional phase shifts inside a negative feedback NFB loop.
With most valve guitar amps, there's not much NFB, it's more of a hifi thing. But some eg JTM45 use a lot of NFB, they can be touchy regarding stability, and another 90° phase shift may not play nicely.

[Willabe]

I don't see any disadvantage. Makes perfect sense to use two .2µF in series. The total capacitance is .1µF, same as the original cap.

Size could be a disadvantage. But if you have the room, ..... Guess it would be which type, poly prop, polyester, film and foil, and brand of cap you use?

[stratomaster]

EDIT - Also it doesn't seem good practice to introduce additional phase shifts inside a negative feedback NFB loop.
With most valve guitar amps, there's not much NFB, it's more of a hifi thing. But some eg JTM45 use a lot of NFB, they can be touchy regarding stability, and another 90° phase shift may not play nicely.

I'm hearing implications of a hybrid master where once the phase shift approaches a critical point the function switches to cross-line--or even pre phase inverter. 

Triple gang pot with custom wafers ala TBX control?

Very intriguing.

[Yeatzee]

Alright, parts arrived and I gave it a go following Sluckey's diagram which made it very easy.




First play test and.... well it's not good enough to give back to the customer. The volume drop off is massive as soon as you're off 10, and the tone loses a lot of top end and is all bloated in the low mids. Big shift from 10 to 9 and so on... probably worse than the stock MV. I'm guessing it's the added cap value?

I used this pot and these caps:
https://www.tubesandmore.com/products/capacitor-630v-polypropylene-radial-leads (0.47uF)
https://www.tubesandmore.com/products/potentiometer-alpha-audio-dual-38-bushing

I did also buy a linear 1m dual gain pot per PDF's suggestion to try, but I didn't catch the change in bushing size and it's got a thick metal spike on the edge that isn't bendable/removable like all other alpha pots I've used are. I'd need to dremel it off to try... which I might do if it's required.

https://www.tubesandmore.com/products/potentiometer-alpha-linear-dual

Quick audio sample, first with the levels from the amp and then I normalized the two different settings to take out the fletcher munson affect.

[pdf64]

Big shift from 10 to 9 and so on... probably worse than the stock MV. I'm guessing it's the added cap value?

I don't think so.
Rather the cause seems more likely to be having a 220k load on a 1M 10% taper pot.

the tone loses a lot of top end

When high value pots are turned down a bit, the effective source resistance feeding the grid Millar capacitence rises up to 1/4 of the track resistance.
In this regard, the 220k grid leaks are actually beneficial here, as by being in parallel with that, they'll limit the max value to about 110k.
But that's still a lot higher than the total effective source resistance without the type1, which was probably less than 10k.

all bloated in the low mids

That may just be due to the negative feedback losing its action as loop gain reduces.

[sluckey]

Sounds disappointing but at least now we know. I wonder if KOC ever tried this circuit. This is the first time I've ever seen this MV implemented. I purposely avoided the more popular LarMar MV because of your bias circuit which would have had to be converted to the old AB763 (or Plexi style) bias circuit. (still an option, although more involved).

That "pot metal tab on the pot will simply snap off with a needle nose pliers. No Dremel needed. I've done plenty of them.

[Yeatzee]

Sounds disappointing but at least now we know. I wonder if KOC ever tried this circuit. This is the first time I've ever seen this MV implemented. I purposely avoided the more popular LarMar MV because of your bias circuit which would have had to be converted to the old AB763 (or Plexi style) bias circuit. (still an option, although more involved).

That "pot metal tab on the pot will simply snap off with a needle nose pliers. No Dremel needed. I've done plenty of them.

The regular alpha pots yes, but not the linear one linked I bought. It's very thick, not flexible at all.

[pdf64]

I'm pretty sure that like sluckey notes, it should snap off easily if you bend it with some suitable pliers.
It's just a die cast part, zinc or similar, not steel.

[Yeatzee]

Sounds disappointing but at least now we know. I wonder if KOC ever tried this circuit. This is the first time I've ever seen this MV implemented. I purposely avoided the more popular LarMar MV because of your bias circuit which would have had to be converted to the old AB763 (or Plexi style) bias circuit. (still an option, although more involved).

That "pot metal tab on the pot will simply snap off with a needle nose pliers. No Dremel needed. I've done plenty of them.

Took a beefy set of plyers to get the tab off but it eventually snapped off. Nothing like any other alpha pot I've used, but either way yes it's possible sans dremel.

While I have the amp still on the workbench I decided to install the linear pot instead and see if it helped. The drop off a cliff in volume definitely happens later now, around 7 or so vs 9 so that's an improvement. But it's still not great, and the tone change is of course still very much there. I did another set of recordings with the new pot hooked up, and tried using the bright switch at the lower settings to see if that was a quick fix. Definitely helped, but still I'm not too happy with the end result. I haven't tried it through a cab yet, I'll do that later.

2nd half I raised in the output volume of the OX Box to keep the levels closer to eachother when on 10 vs 4 so that I'm not having to digitally boost the gain 20dB to match.

[sluckey]

Put a bright cap across the two wires at the mv pot. I'd try 47pf, 100pf, 120, just whatever is handy. Need one cap on each pot. Any better?