Basic old Magnatone 5w Mod'ing

[Yeatzee]

Howdy friends! Looking to glean some advice from the seasoned experts here! I've got an old 50's Magnatone Varsity Deluxe that we'd like to rock up quite a bit and also improve the tone. I've been having fun playing with the circuit, but I don't think it's quite nailed yet so I'd love to hear what you would do / try and how you would approach a project like this to be more efficient with time next time around.

Directions I received:
- make it much more angry and gainy
- offset some of the rattyness, avoid splatty & compression
- more open and smooth
- avoid the amp getting exceedingly right (bit gain increase with tone control up, but it gets too bright)

Here's the schematic:



Here's where I landed so far:



I had a 25uF in place of the 1uF before, but the low end was just too much even with swapping the other .02 coupling cap with another 4.7nF to cut it down more. I put a 10k thinking of the marshall cold clipper style setup to smooth things out, but I'm not sure if that was the play (or if it'd be better to swap what I did to V1a to V1b and vice versa). The NFB bypassed was way too ratty sounding, but 10k is a high value so it might be worth dropping that maybe in half? Amp is biased pretty spot on with a 470R, too hot with the stock value.

For reference, here's what it sounds like into an ox box deluxe reverb cab since I don't have the stock setup with the amp.

i=OtFFG4UTc5XwhyII

Definitely have more gain, but I think there's room for more without totally overloading the circuit. Still has a kind of harsh upper mid range to it, and the tone control turned up let's so much good gain through but it does get pretty bright way up there. It's definitely a lot better than stock, it was fairly anemic and thin / bright stock so good progress has been made but I don't have any sounding boards to bounce this sort of stuff off of so I appreciate this forum greatly! Let me know what you see / think. Thanks!

[Esquirefreak]

I'd increase the NFB resistor value.

Also I'd bias the first triode hotter. I'd try something like 2k7 Rk and 220k Ra. K bypassed with a 10μF cap.

And I'd lower Rg on the first triode to something like 33k-47k.

/Max

[Esquirefreak]

I think it's hard to squeeze alot of gain out of 2 triodes. Maybe put a Mosfet source follower in between the preamp triodes? I think you could get some nice compression from that.

[passaloutre]

I think it's hard to squeeze alot of gain out of 2 triodes. Maybe put a Mosfet source follower in between the preamp triodes? I think you could get some nice compression from that.

Most Champs I've played have more than enough gain, certainly plenty enough to overdrive the 6V6. Lots of very crunchy 70s rock was recorded on dimed Champs.

OP, I would start with the 5F2A circuit and go from there. At the very least, you're definitely going to want to bypass the cathode on that first stage.

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5f2a_schem.pdf

For a little hotter version, maybe look at the Vox AC4TV, which feels a little more saturated (I'd guess due to the 220k plate loads), smooth (some extra filtering with e.g. shunt caps, etc), and absolutely roars like a Marshall especially when you hook it up to a bigger cabinet. Though the higher sensitivity of the EL84 likely has something to do with this. You could maybe swap the output tube for a 7591 if you want something more like that.

https://el34world.com/charts/Schematics/files/Vox/Vox_ac4tv.pdf

[stratomaster]

A cold clipper is a poor choice for an input gain stage. It can result in low output and a sterile tone. The JCM800s get away with it because of the beefy output section.

You'd probably get where you're wanting to go by simply swapping the values of the two triodes, that is making the 2nd stage the cold clipper.

I also find the clipping from a cold clipper rather harsh and definitely wouldn't be a voicing choice I'd make to "smooth out" the tone.

I normally end up employing a plate bypass cap on cold clippers or some later gain stage (not applicable here).

It also makes sense to cut bass as early in the circuit as possible on designs meant to distort. Paradoxically a very large bypass cap in the first stage can help with hum, so that checkmates us into using a very small coupling cap after the first stage, and leaving the larger caps later in the amp.

Edit: See attached markup for where I'd start.

This might be a good candidate for a conjunctive filter or zobel ala DrZ Ghia

[Yeatzee]

Love it, some great ideas thank you! First time I've ever tried doing anything like this besides some minor screwing around with my old magnatone without a clear goal in mind. This is fun, lot of ways to skin the cat even on a simple circuit so it's fascinating to record loops into the amp and actually be able to side by side each step to see what does what to the output.

A cold clipper is a poor choice for an input gain stage. It can result in low output and a sterile tone. The JCM800s get away with it because of the beefy output section.

You'd probably get where you're wanting to go by simply swapping the values of the two triodes, that is making the 2nd stage the cold clipper.

I also find the clipping from a cold clipper rather harsh and definitely wouldn't be a voicing choice I'd make to "smooth out" the tone.

I normally end up employing a plate bypass cap on cold clippers or some later gain stage (not applicable here).

It also makes sense to cut bass as early in the circuit as possible on designs meant to distort. Paradoxically a very large bypass cap in the first stage can help with hum, so that checkmates us into using a very small coupling cap after the first stage, and leaving the larger caps later in the amp.

Edit: See attached markup for where I'd start.

This might be a good candidate for a conjunctive filter or zobel ala DrZ Ghia

That was on my list to try, swapping the V1a and V2b setup because I'm curious how that affects the final tone. Love it, thank you for taking the time to type this out and offer some ideas!

Could you walk me through a bit more the thought process on the changes you outlined? For example the 22k added before the grid of the second stage. And the 10k after the .02 going into the grid of the 6v6

[Yeatzee]

A cold clipper is a poor choice for an input gain stage. It can result in low output and a sterile tone. The JCM800s get away with it because of the beefy output section.

You'd probably get where you're wanting to go by simply swapping the values of the two triodes, that is making the 2nd stage the cold clipper.

I also find the clipping from a cold clipper rather harsh and definitely wouldn't be a voicing choice I'd make to "smooth out" the tone.

I normally end up employing a plate bypass cap on cold clippers or some later gain stage (not applicable here).

It also makes sense to cut bass as early in the circuit as possible on designs meant to distort. Paradoxically a very large bypass cap in the first stage can help with hum, so that checkmates us into using a very small coupling cap after the first stage, and leaving the larger caps later in the amp.

Edit: See attached markup for where I'd start.

This might be a good candidate for a conjunctive filter or zobel ala DrZ Ghia

As a follow up, I had a few minutes so I quickly did most of your recommendations and recorded it then compared it to where I landed last with the mods outlined in the OP.

I swapped the 10k over the V1B, and put a 250uF cathode bypass cap (don't have a 100uF but I believe the difference between the two should be fairly small in this application no?) along with a 1k resistor. Swapped the first .02 coupling cap with a 2.2nF, and returned the 1nF cap off the tone pot. Installed a 22k before the grid of V2B. Did not do the plate bypass cap because it already had very little high end. Didn't do the 10k before the 6v6 because I'm not sure what that's doing and didn't have time.

Takeaways, much less gain, much less output, much less top end. Bit confused why that's the case, I would have thought such a giant jump with the bypass cap would juice it up way more. Also surprised by the lack of top end. Not sure I fully understand why those changes resulted in what they did so looking forward to hearing from you guys! Definitely need a lot more gain than what it's got now, and it's too dark now but the harshness is gone which is nice.

[stratomaster]

Very interesting. I'm wondering if the cold clipper is acting as a larger bottom leg of the negative feedback voltage divider and cutting gain that way (increasing negative feedback).  I bet if you increased the negative feedback resistor substantially (say 47k-68k, to keep a similar feedback ratio) you'd get the grit back, plus the harmonics of the cold clipper. To test this theory, simply disconnect the NFB from the speaker jack and check the tone.  It'll be hairier than intended, but should be a significant improvement. Once the concept is proven, then change the NFB resistor to the recommended 47k-68k

There's also something to be said for the final stage being a strong one to drive the output.  You can try a basic 820//22μF instead of the cold clipper for guaranteed grit. Or put it on a switch for variety.

The 10k before the 6V6 is just a grid stopper.  If you're having harshness then dialing up the grid stoppers is a good way to tame it.

Edit: also on the bypass cap, above about 8μF in most cathode networks you stop getting boost in the guitar range. The method behind the madness of such a large value is h-K hum shunting. This gives as high a signal to noise ratio as you can get right up front.

[passaloutre]

I think you get diminishing returns going much over 25uf on the bypass cap

Have you tried removing the massive pad on the input jacks? Could also double that NFB resistor to the Champ value, or remove it altogether.

[Yeatzee]

Kids down for bed so it's back to experimentation time! Tried a bunch of stuff, editing some clips now to pick out what's note worthy and I'll report back. Playing with the NFB was fascinating, I've only ever just removed it entirely. I did also mess with cutting the input resistor to ~33k with a parallel resistor to open one input up a bit along with putting the stock 2.2k cathode resistor in parallel with the 10k cold clipper to cut that down for more gain as well as even clipping in a 1uF bypass cap. All fascinating, tried a bunch of combinations.

[Yeatzee]

Here's the NFB experimentation

00:00 - 10k NFB (stock)
00:06 - 56k NFB
00:11 - NO NFB
00:15 - clipping in the variations

[stratomaster]

Could you walk me through a bit more the thought process on the changes you outlined? For example the 22k added before the grid of the second stage. And the 10k after the .02 going into the grid of the 6v6

They're grid stoppers. They're used primarily to stabilize the circuit in the ultrasonic range.  But if you use higher resistances then it begins to roll off the highs in the audible range.  I added these as this design didn't have grid stoppers, and you wanted to juice the gain and said the amp was harsh.  Adding gain can add instability and oscillations, so this should remedy that.  If you don't want the highs affected audibly then stay below 10k for preamp stages and ~2.2k for power tubes (rough rule of thumb).

The resistors on the input jacks behave similarly, but shave the highs very early in the circuit.  If you want more highs, then replace the 100ks with say 22k-33k.

[Yeatzee]

Could you walk me through a bit more the thought process on the changes you outlined? For example the 22k added before the grid of the second stage. And the 10k after the .02 going into the grid of the 6v6

They're grid stoppers. They're used primarily to stabilize the circuit in the ultrasonic range.  But if you use higher resistances then it begins to roll off the highs in the audible range.  I added these as this design didn't have grid stoppers, and you wanted to juice the gain and said the amp was harsh.  Adding gain can add instability and oscillations, so this should remedy that.  If you don't want the highs affected audibly then stay below 10k for preamp stages and ~2.2k for power tubes (rough rule of thumb).

The resistors on the input jacks behave similarly, but shave the highs very early in the circuit.  If you want more highs, then replace the 100ks with say 22k-33k.

I'm stupid. What is the 470k doing?

[Yeatzee]

00:00 - No NFB
00:04 - 1.8k cathode resistor no NFB
00:09 - 10k cold clipper NO NFB input resistor 32k
00:13 - 1.8k cathode resistor 56k NFB
00:18 - 1.8k cathode resistor no NFB 500pf plate snubber
00:23 - 10k cold clipper NO NFB input resistor 32k
00:27 - Input 2 10k cold clipper NO NFB

These were my favorite of the tests I did, all landed in a fairly similar place. I think I lean so far towards the 10k cold clipper, no nfb and the 32k input resistor on input 1. Might drop the 10k down to a 6.8k or something. All of the variations I tried with a 1uF bypass cap on V1B just didn't sound good. I think there is plenty of gain in all of these options. Let me know your thoughts!

[Yeatzee]

And single note stuff:

And just realized I put in the quick title the wrong note. Using input 2 doesn't mean it's got a 100k input resistor. Input 1 is a shunting type while the other two are not. I'm still confused on how input wiring works, everytime I think I understand it I lose the plot.

So plugging into input jack one your signal goes through a 100k resistor to the grid of V1a. Also connected to the grid of V1a are two other 100ks going to input jack 2 and jack 3. Neither have a path to ground because they're both open jacks.

Plugging into input jack 2 your signal goes through a 100k resistor to grid of V1a. Also connected to the grid of V1a are the two other 100ks. One (on jack 3) does not connect to ground but the 100k on jack 1 does because of the shunted tip. So what does that mean exactly as far as what the signal sees? I don't quite understand.

[stratomaster]

I'm stupid. What is the 470k doing?

 Grid leak.

Grid stoppers go to the grid (preferably right at the socket), and grid leaks go to ground AHEAD of the grid stoppers. If the order is reversed (like the input jacks) then it results in a voltage divider.

Speaking of the inputs, the resistors attached to the two unused jacks form a lower leg of a voltage divider with the resistor off the active jack being the upper leg.

This opens you up to a lot of tonal variation by putting different resistors to each jack.

The stock config trims the input signal by about a third.

If you replaced them with say 33k, 68k, 220k you could have three distinct levels of gain/feel just by which jack you choose.

00:00 - No NFB
00:04 - 1.8k cathode resistor no NFB
00:09 - 10k cold clipper NO NFB input resistor 32k
00:13 - 1.8k cathode resistor 56k NFB
00:18 - 1.8k cathode resistor no NFB 500pf plate snubber
00:23 - 10k cold clipper NO NFB input resistor 32k
00:27 - Input 2 10k cold clipper NO NFB

These were my favorite of the tests I did, all landed in a fairly similar place. I think I lean so far towards the 10k cold clipper, no nfb and the 32k input resistor on input 1. Might drop the 10k down to a 6.8k or something. All of the variations I tried with a 1uF bypass cap on V1B just didn't sound good. I think there is plenty of gain in all of these options. Let me know your thoughts!

Cool stuff. I'm partial to the 1.8k cathode, 56k nfb combo. Keep in mind that the feedback ratio is determined by both the cathode resistor and the feedback resistor.  With the 1.8k in place you'll need around a 10k NFB resistor (stock value) to keep that constant vs the 10k cold clipper.  Nothing says it has to be constant, but just fyi.

[Yeatzee]

I'm stupid. What is the 470k doing?

 Grid leak.

Grid stoppers go to the grid (preferably right at the socket), and grid leaks go to ground AHEAD of the grid stoppers. If the order is reversed (like the input jacks) then it results in a voltage divider.

Speaking of the inputs, the resistors attached to the two unused jacks form a lower leg of a voltage divider with the resistor off the active jack being the upper leg.

This opens you up to a lot of tonal variation by putting different resistors to each jack.

The stock config trims the input signal by about a third.

If you replaced them with say 33k, 68k, 220k you could have three distinct levels of gain/feel just by which jack you choose.

00:00 - No NFB
00:04 - 1.8k cathode resistor no NFB
00:09 - 10k cold clipper NO NFB input resistor 32k
00:13 - 1.8k cathode resistor 56k NFB
00:18 - 1.8k cathode resistor no NFB 500pf plate snubber
00:23 - 10k cold clipper NO NFB input resistor 32k
00:27 - Input 2 10k cold clipper NO NFB

These were my favorite of the tests I did, all landed in a fairly similar place. I think I lean so far towards the 10k cold clipper, no nfb and the 32k input resistor on input 1. Might drop the 10k down to a 6.8k or something. All of the variations I tried with a 1uF bypass cap on V1B just didn't sound good. I think there is plenty of gain in all of these options. Let me know your thoughts!

Cool stuff. I'm partial to the 1.8k cathode, 56k nfb combo. Keep in mind that the feedback ratio is determined by both the cathode resistor and the feedback resistor.  With the 1.8k in place you'll need around a 10k NFB resistor (stock value) to keep that constant vs the 10k cold clipper.  Nothing says it has to be constant, but just fyi.

That was my #2 favorite. There is some value to keeping the NFB circuit I think, smooths things out a bit. I'll likely end up going that route, particularly because the stock wiring is already there going to the output / speaker.

Yeah the grid leaks just confuse me, we typed at the same time it looks like if you look at my previous post. I don't get the input wiring, it's really unintuitive to me.

With all 3 in their stock 100k config, what exactly is happening? Is there a difference using input 1 with the shunting jack or does that not matter at all? I like the idea of having 3 variations a lot, I'm definitely going to do that. I just wish I understood how it worked. Going into jack 1 with the 33k resistor, what affect does jack 2 and 3 with the 68k and 220k have on jack 1? Is there an affect at all?

Edit: Thank you btw, I really appreciate you taking the time to help educate me. Much appreciated!

[Yeatzee]

And here's the two favorites next to eachother, vs what I originally had done mentioned in the OP vs stock.

i=G3uZqgF1GWrcAZHJ

I think either of these with the input resistor mod(s) will be perfect.

[stratomaster]

I don't get the input wiring, it's really unintuitive to me.

With all 3 in their stock 100k config, what exactly is happening? Is there a difference using input 1 with the shunting jack or does that not matter at all? I like the idea of having 3 variations a lot, I'm definitely going to do that. I just wish I understood how it worked. Going into jack 1 with the 33k resistor, what affect does jack 2 and 3 with the 68k and 220k have on jack 1? Is there an affect at all?

Edit: Thank you btw, I really appreciate you taking the time to help educate me. Much appreciated!

The schematic shows something different from what you're describing.  On the schematic all jacks are the shunt type.

I'd recommend putting a dummy plug into the jack with power off, and the 12AX7 removed for good measure, then measuring resistance across the terminals.  The results should be around 150k according to the schematic. Any deviation from this will be revealed with this measurement.

[Yeatzee]

I don't get the input wiring, it's really unintuitive to me.

With all 3 in their stock 100k config, what exactly is happening? Is there a difference using input 1 with the shunting jack or does that not matter at all? I like the idea of having 3 variations a lot, I'm definitely going to do that. I just wish I understood how it worked. Going into jack 1 with the 33k resistor, what affect does jack 2 and 3 with the 68k and 220k have on jack 1? Is there an affect at all?

Edit: Thank you btw, I really appreciate you taking the time to help educate me. Much appreciated!

The schematic shows something different from what you're describing.  On the schematic all jacks are the shunt type.

I'd recommend putting a dummy plug into the jack with power off, and the 12AX7 removed for good measure, then measuring resistance across the terminals.  The results should be around 150k according to the schematic. Any deviation from this will be revealed with this measurement.

I'll try it tomorrow, but yeah the schematic is wrong. I've had 3 of these 108 varsity deluxe's and all were the same. Only shunting on jack 1.

[bmccowan]

This is not meant to disrupt your experimenting, as you are having a blast. Just some ideas from my working on similar circuits. Just about everyone made a version of this circuit. To my tastes, the Gibson GA-5 is the best of the bunch, and hits on most of your want list. You have been trying many of the elements of that circuit. I'd be tempted to duplicate it in that amp as a starting point.
Also - you do not mention trying different tubes. IME, 12AX7s can vary a lot in tonal character and with a simple circuit the preamp tube flavor plays a big role. I'd roll a bunch of tubes - 6V6s too.

[passaloutre]

That Gibson circuit looks like a good recipe to me!

[scstill]

Why are these designs using 220k for 12ax7 plate resistor? Old school maybe?
100k is more typical right? Unless I missed and this was already discussed.

If not already done, Might be a good idea to remove the death cap and put the switch on the line side of AC.

[passaloutre]

The bigger plate load, in conjunction with proper biasing, provides higher voltage gain.

I think a lot of these older designs were expecting weaker input signals, though some makers (e.g. Vox) continued using higher plate loads well into the 60s.

I find it to be a more saturated, smoother sound

[scstill]

Thanks, The resistive coupled amp booklet is new info to me, and much appreciated

It makes sense that pickups in the day with a lower output voltage need more gain.
And as pickups evolved to became hotter they needed less voltage gain.

So to the original request wouldn't "more angry and gainy" suggest to move possibly from 220k to 470k
or is it too much of a "good" thing

[Yeatzee]

This is not meant to disrupt your experimenting, as you are having a blast. Just some ideas from my working on similar circuits. Just about everyone made a version of this circuit. To my tastes, the Gibson GA-5 is the best of the bunch, and hits on most of your want list. You have been trying many of the elements of that circuit. I'd be tempted to duplicate it in that amp as a starting point.
Also - you do not mention trying different tubes. IME, 12AX7s can vary a lot in tonal character and with a simple circuit the preamp tube flavor plays a big role. I'd roll a bunch of tubes - 6V6s too.

I happen to own this identical model Magnatone, so when I'm done with this one I think I'm going to mod mine as well. I'm not super familiar with the ga5, I do own a 53 GA40 but it uses some of the less common tubes in that one. I'm assuming theres a later ga5 with the same tube compliment? I'll check it out!

Re tubes, I'm using what the owner supplied which are all vintage. I've got modern stuff on hand, but no vintage tubes to offer.

[bmccowan]

A pdf of the GA-5 version I'm referring to is included in my post. Gibson used that model # on multiple, and quite different, amps. Jokers apparently. And, yes the GA-40 is a different animal using pentode preamp tubes. Early used the 6SJ7, later the 5879. They are pains to work on but sound great.

[Yeatzee]

A pdf of the GA-5 version I'm referring to is included in my post. Gibson used that model # on multiple, and quite different, amps. Jokers apparently. And, yes the GA-40 is a different animal using pentode preamp tubes. Early used the 6SJ7, later the 5879. They are pains to work on but sound great.

Woops, sorry missed that!

Ended up going with the 56k NFB, 1.5k cathode resistor for V2B and for the inputs 33k, 56k, 100k resistors. I tried 220k, 100k, 33k but the latter two had such a big drop off in gain that it didn't seem like the play when the owner was specifically wanting more and more drive out of the amp.

Quick little comparison loops if you're curious, nothing fancy. The ox always rolls off the edges / homogenizes the sounds a bit but I'm happy with the results and I just need to do the final install on the parts and call it good. Thanks again to everyone that offered some ideas and I definitely will circle back to the Gibson concept with my personal one when I get time.

[bmccowan]

When watching the video I found myself yelling at the screen "stop turning the knobs!"
Glad you got it to where you want it. I think you are likely close to the Gibson circuit, anyways, with the changes you made. I would guess that the biggest change was upping the NFB resistor from the stock 10K to 56K. Gibson value is in the same ballpark 47K.

[Yeatzee]

When watching the video I found myself yelling at the screen "stop turning the knobs!"
Glad you got it to where you want it. I think you are likely close to the Gibson circuit, anyways, with the changes you made. I would guess that the biggest change was upping the NFB resistor from the stock 10K to 56K. Gibson value is in the same ballpark 47K.

Haha owner wanted to hear it through the knob range since the other video covered it static. Pretty happy overall, learned a lot for sure! Got another 50s amp, this time a valco / Oahu with a similar goal. Should be fun!