Help me Hoffmanize this MV on 5F6a

[waldner]

Here is a very simple master volume mod for the 5f6a that I would like to try on my Hoffman 5f6a.  All that is altered from the stock circuit is the MV pot and the two purple wires.  Can someone help me understand where to insert this into the Hoffman layout and how and why this works? 

Thanks in advance for any help.

[eleventeen]

The MV pot is nothing more than a variable "short circuit" across the two drive signals created by the phase splitter (one is "push", one is "pull") You can either connect such a pot to the two pin 5's on the 5881's, or, look for the same physical structure of parts on the Hoffman board. You'll be connecting the wires from the pot to a "node" that is in both cases the junction of a 220K and a .1 ufd. They are most typically arranged in that "V" layout you see. There is no other place in the amp where you'll find those twin nodes where a 220K and a .1 uf connect. They should be sort of near the power tunes, the 5881s.

Why does it work? Because the rest of the amp (the preamp & tone controls) is busy shaping tone and amplifying the basic guitar signal. By the time your git signal gets to the grids of the 5881's it is a 25-40 volt or so sized signal. Out of your guitar, you have 1/4 volt (.25 volt) so the signal has been amplified by 100x to meet the drive req'ments of the 5881's. Later on in the amp, just before that 100x signal gets shoved into the 5881's, you choke it off by partially shorting it out. Drive to the 5881's is reduced. That means you can crank up the normal volume knob a lot, or all the way to 11, and choke off most or all of the work the preamp is doing. Of course, the preamp will be overdriven at that point.  ---> preamp crunch, but the amount of drive to the power section is choked off so the thing isn't crazy loud. 

Incidentally but importantly, in the diagram you supplied there is a yellow wire (which is electrically the same as one of the 220K - .1 uf nodes to which you attach the MV control) and you have an orange wire which is the same thing, except the opposite side, the opposite phase, for the other 5881. The yellow wire and the orange wire pass through a hole in the parts board north of the speaker jacks in your dwg. The yellow wire correctly connects to that 5881's pin 5. The orange wire looks like it connects to PIN 4 of the other 5881, which is not correct. Both those wires connect to pin 5, yellow to one tube, orange to the other tube. The orange wire is misdrawn. I *think* you could do some damage if your amp was miswired that way...if the MV pot was really choked off, it would fry.

[AZJimC]

That master volume can go anywhere from the board, to the grids of the output tubes. It is known as a cross line master volume. It uses the pot to reduce resistance between the grids of the output tubes, effectively shorting the signal sent from the Phase Inverter to output tubes. That type does indeed change frequency response as it is turned down, and some folks don't like that. Have a look at the LarMar master volume. It uses a dual pot and actually replaces the two resistors on your diagram that come to a vee where the purple wires are connected to the board. It replaces those resistors with one pot each, and seems like the most transparent master to me

One thing to be aware of with nearly any master vol, is that as you lower the vol, it drops signal to the output tubes, while you can overdrive the preamp, the distortion changes from output tube distortion to preamp distortion. As signal level to output drops, they clean up and stop distorting. If quiet output tube overdrive is what you're after, look at power soaks, or variable voltage to the output tubes. The output will distort more, at a lower volume, as voltage drops on the output tubes.

[waldner]

Ok, thanks guys.  I did think there must be an error in the pin wiring on the power tubes given that they didn't match on Rob's layout.

So it seems this is what I can try on the the Hoffman layout.  I have drawn in the 1M pot with the two purple wires.   I like that it requires about 60 seconds to rig this MV up to play with.  Even if it ends up being a bad MV, it's a learning experience to try it out.

[Willabe]

Can someone help me understand where to insert this into the Hoffman layout and how and why this works? 

It's a lot easier to understand what a circuit is doing/how it works if you look at the electrical circuit not the layout drawing. And much easier to see where the signal is going from input to output, often very hard to see in a layout drawing.

If you don't have access to the circuits schematic then draw out the portion of the circuit from the layout drawing that you want to figure out.

Guys keep posting a layout drawing and want to understand the amps circuit(s). The layout drawing is to find    where parts of a circuit are located in an amp after looking at the amps circuit schematic.

Both are good to have but they are 2 different things. If you want to repair/build amps then you really need to learn to be able to read and use both.



                         Brad          

[eleventeen]

"I like that it requires about 60 seconds to rig this MV up to play with.  Even if it ends up being a bad MV, it's a learning experience to try it out."


That's exactly right. It is less than optimum in a few ways, but hey, maybe you like it. In this position, by the way, it could be more useful to have  a LINEAR pot instead of an audio pot (which every other pot except trem speed is) An audio pot might have only a very small range or rotation where it is useful. This will not change the freq rolloff nature of this MV mod, it will just make it a tad easier to use.


One thing that confuses people at first; I said the MV wires connect to pin 5 on either output tube. Here you have them connected to pin 6 on each tube. Huh?
A 6L6 or 5881 generally has pin 6 missing from the base. Same with pin 1. If those pins *are* present on the tube, they connect to nothing inside the tube. Hence, we can use pin 6 or pin 1 as a tie point and that's what your dwg shows. The wires feeding the 5881 tubes (the "drive" signals) have a 1500 ohm R inserted, and the oppty is taken to install that 1500 R in between pin 5 and pin 6 on both/either tube. Make sure you have that straight in your thinking and do not think "Hey, no big deal if I get the wires 1 or 2 pins off on the tube socket.

[Willabe]

If you had the schematic (or drew it up) you should be able to see that the 1M pot is between the push/pull feeds to the output power tubes.

Those 2 feeds/lines are 180 degrees out of phase, they come from the phase inverter (PI) which is what the PI does.

So you have 1M of resistance standing in between them or isolating the signal lines from each other. As you turn the pot you will have less and less resistance between them and they will start to cancel each others signal out.

With the schematic you can see this.


                        Brad   

[Willabe]

The yellow wire correctly connects to that 5881's pin 5. The orange wire looks like it connects to PIN 4 of the other 5881, which is not correct. Both those wires connect to pin 5, yellow to one tube, orange to the other tube. The orange wire is misdrawn. I *think* you could do some damage if your amp was miswired that way...if the MV pot was really choked off, it would fry.

Good catch, the orange wire is drawn going to the screen, pin 4 and like you wrote it should go to the grid, pin 5.

[waldner]

Can someone help me understand where to insert this into the Hoffman layout and how and why this works? 

It's a lot easier to understand what a circuit is doing/how it works if you look at the electrical circuit not the layout drawing. And much easier to see where the signal is going from input to output, often very hard to see in a layout drawing.

If you don't have access to the circuits schematic then draw out the portion of the circuit from the layout drawing that you want to figure out.

Guys keep posting a layout drawing and want to understand the amps circuit(s). The layout drawing is to find    where parts of a circuit are located in an amp after looking at the amps circuit schematic.

Both are good to have but they are 2 different things. If you want to repair/build amps then you really need to learn to be able to read and use both.
                         Brad          

I hear you Brad.  I'm just starting to learn to read schematics but they are more confusing at this stage for me.  I think the reason rookies always post and ask about layouts is that the layout represents the actual amp they are looking at/working on, and it's the only tangible thing that will get a result.  I would love to be able to look at a schematic and draw it into a layout at some point or understand how to translate back and forth between the two.  I will try to study at the schematic to better understand what the MV is doing though.  Thanks.

That's exactly right. It is less than optimum in a few ways, but hey, maybe you like it. In this position, by the way, it could be more useful to have  a LINEAR pot instead of an audio pot (which every other pot except trem speed is) An audio pot might have only a very small range or rotation where it is useful. This will not change the freq rolloff nature of this MV mod, it will just make it a tad easier to use.

One thing that confuses people at first; I said the MV wires connect to pin 5 on either output tube. Here you have them connected to pin 6 on each tube. Huh?
A 6L6 or 5881 generally has pin 6 missing from the base. Same with pin 1. If those pins *are* present on the tube, they connect to nothing inside the tube. Hence, we can use pin 6 or pin 1 as a tie point and that's what your dwg shows. The wires feeding the 5881 tubes (the "drive" signals) have a 1500 ohm R inserted, and the oppty is taken to install that 1500 R in between pin 5 and pin 6 on both/either tube. Make sure you have that straight in your thinking and do not think "Hey, no big deal if I get the wires 1 or 2 pins off on the tube socket.

Interesting on the linear pot for this application.  And i was wondering why on the original Fender 5f6a drawings they go to pin 5 and Doug's layout has them going to pin 6.  So he was using the empty pin 6 as a mounting point to then allow a cleaner and more stable connection for the resistor which links to pin 5, rather than having it hang in the air.  Makes sense.  Thanks.

[Willabe]

.

[Willabe]

Link to Doug's page on amp current flow, it's a high lighted layout with text explanation;

http://el34world.com/charts/currentflow.htm

And here's the link to Doug's library of tube amp information (you can find the link at the very bottom of any page your on in his site), LOT'S of great info in there;

http://el34world.com/schematics.htm

[eleventeen]

There is very little that is obvious or intuitive about "translating" from layout to schematic or vice versa. I don't know where you go to learn that (other than right here)


For example, you see a typical Fender preamp and look at the tone stack. There is *nothing* at all that tells you those pots are on the front panel and the R's and C's are (generally) on the parts board. But wait, this cap *could* be on the volume control. On a Fender (blackface) power supply there is *nothing* that tells you that those e-caps are under the doghouse, on the TOP side of the chassis, and that their ground has to (well...it should) come back and attach to the power transformer.


It is only through experience that you can look at a Fender amp schematic and know that this part is on the front panel, that part is on the back panel, we don't want this part near that part because it will probably create hum/noise, we should probably use shielded wire for this connection, I can't mount 6 parts on one parts board turret, and on and on.


Meanwhile, if you're old school like me, we *never* had layout dwgs and always worked from schematics. Layout dwgs *are* very useful and they allow less geeky types to build amps which are not that complicated as long as you deal with one wire at a time. But most of the time, they make little sense to me unless I really stare at them.

[sluckey]

Well, it's been over 60 seconds. What's your opinion of the crossline MV in a 5F6A?

[waldner]

Funny, I just broke out my soldering iron and tried it.  It's actually pretty good at a glance.  I should be working right now:)  The only thing is that I didn't have any linear pots to try (as was suggested above in this thread) so I used the 1M Log pot I had laying around.  There is a tiny range where the MV is effective and it's a bit hard to control.  Makes me wonder what the log pot would do to help? 

I also always wonder about whether there is a way to custom taper pots or create one somehow.  Maybe measure the resistance right in the usable range and then create a pot of sorts that would expand that range? 

The other thing is that the MV pot works backwards, so that as you turn the pot up the volume goes down, which makes sense given what it's doing.  It's a bit weird but fine. 
I also like that when it's all the way up, it's like it was never there.

[sluckey]

The other thing is that the MV pot works backwards

Move the wire that's connected to the outer lug of the pot to the other outer lug.

[eleventeen]

" Makes me wonder what the log pot would do to help?  "


Assume you mean "lin". The critical point where the pot does its voodoo would take up more degrees of pot rotation. Easier to "dial in"


"I also always wonder about whether there is a way to custom taper pots or create one somehow.  Maybe measure the resistance right in the usable range and then create a pot of sorts that would expand that range?  "

Sure. You would do this measuring the linear pot to make life easy. (with the amp on, turn it as low at it is useful to you, power down the amp, measure. Turn the amp back on, twist the pot as high as it is useful to you, power down, measure) Suppose (after you swap in the 1 MB lin) and find that you like the 1 M pot traversing the range from 10K to 280K and outside of that range, it doesn't do anything for you. Now you go get a 250K pot and create a series string consisting of a          -----10K-----250K lin pot----22K----     and now you connect the outer wires of that string to where your wires go now. Your pot by itself goes from 0 to 250K and the R's just add. When the pot is zero, the string is 32K, when the pot is at max, the string is 282K. Simple. Usually takes a little bit of futzing around to get where you like it. 

"The other thing is that the MV pot works backwards, so that as you turn the pot up the volume goes down, which makes sense given what it's doing.  It's a bit weird but fine.  "

Change the outer wire to the other side of the pot, it will rotate the right way.

"I also like that when it's all the way up, it's like it was never there.

Yeah, you want that.

[waldner]

Ok, I'll try to measure and put together the right value pot and string of resistors.  It sounds like I'll be able to trial and error my way to a good result. 

Also, it's nice that I can switch to the other tab on the pot and have it not backwards:)  Thanks again.

[waldner]

Ok, after work I got a chance to really play with this new MV and it's not great.   Turning down the MV results in additional distortion even when the regular channel volume is set fairly clean, and the distortion is kind of gritty and not very pretty. 

With the 1M pot still in place I measured where the lower and upper end of the useful range was and I got about 5k to 45K.  That was measuring the resistance between the two MV pot connections being used.  I found a 250K linear pot laying around from a guitar project and that gave much better range of control over the volume.  The 250K pot seems to get completely out of the way when turned all the way up, so its easy to play around with the before and after. 

This MV is fine if you just want to be able to still use the amp and turn down with some dirt, but the sound quality is not great. Might have to look in to that Lar/Mar master volume.

[Ed_Chambley]

I have never liked this type of MV.  On most amps I perfer the Dual 250K LarMar except on this circuit.  Here I use Ken Fishers MV-1.  I am a huge fan of the LarMar on Marshall circuits and also Cathode Bias amps, but I have tried both on a 5F6a.

The 250K Dual LarMar seems to prefer EL34 as it doesn't change the distortion characteristics much.  On the 5f6a the LarMar seems to choke the amp down and makes it impossible to get power tube distortion.  I think it is because of the 6L6 vs El34 or El84.

Just my experience, but the 5f6a circuit only sounds good to me when in breakup area where you roll back just a touch to clean up.  This is where it really shimmers.  The LarMar kills this but with the MV I have attached you can still achieve that tone.

Still some tweaks may be in order to get the presence you desire.

[AZJimC]

Ed,
I have seen this MV but never used it. I've also seen the "extra" coupling caps used on the LarMar type as well, but I've always left out the second pair of caps, and go straight from the pots to the output tube grids. I don't see a need to use them when you've already dumped the DC from PI with the caps normally in place.

Am I missing something, or are the caps leaving the MV pots actually not needed?

Jim

[Ed_Chambley]

Ed,
I have seen this MV but never used it. I've also seen the "extra" coupling caps used on the LarMar type as well, but I've always left out the second pair of caps, and go straight from the pots to the output tube grids. I don't see a need to use them when you've already dumped the DC from PI with the caps normally in place.

Am I missing something, or are the caps leaving the MV pots actually not needed?

Jim

I believe you would have DC on the pots without the caps.  This MV does not replace the resistors like the LarMar does.  I think this is correct.

[sluckey]

The caps are not needed. If the amp is cathode biased with zero volts on the tube grids then connect the 'ground' side of the pots to ground. But if the amp is fixed bias with a negative voltage on the grids then connect the 'ground' side to the negative bias. If this doesn't make sense to you then use the extra caps.

[Ed_Chambley]

The caps are not needed. If the amp is cathode biased with zero volts on the tube grids then connect the 'ground' side of the pots to ground. But if the amp is fixed bias with a negative voltage on the grids then connect the 'ground' side to the negative bias. If this doesn't make sense to you then use the extra caps.

This is the same as the LarMar isn't it except the resistance from the pot is greater.  What is the purpose of the additional caps.  I used them on my 5f6a and liked the outcome, but it is adding series capacitors isn't it?

Maybe why I prefer the additional caps.

[AZJimC]

In the type 1, they're adding 1m to ground on each leg. The original resistors that carry bias to the OT grids remain in place.

Yes, they're adding series capacitance, cutting effective capacitance by half. That is why I've never used them, plus, as stated, they've already dropped the DC component from PI plates with the original capacitors.

The LarMar type II is replacing the bias resistors with the pot, adding big value resistors across the pot as a safety, so bias is not lost even if the pot fails. The point of this type is that, when it is turned all the way up, the circuit is the same as if there were no master volume at all. In the type 1 there is always that added 1M load. Granted, that's going to be an undetectable difference, but you know how anal tube guys can get.... 

[waldner]

Very cool and interesting MV possibilities to try.  Thanks guys!