Critique my first layout? (Magnatone pitch shift vibrato)

[Diverted]

A month ago or so a friend of mine gave me two old varistors of the type used in the early Magnatone vibrato circuit, so I decided to build a Magnatone 410. Only change is I am using a 6SF5 for the first half of the 12DW7 and a 6J5 for the second half as I have some nice ones laying around.
Anyway it gave me a chance to play around DIY layout creator. I've gone over this many times and all looks good per schematic, but extra eyes never hurt. Any problems noticeable in my layout?

Low res jpegs attached and also higher res pdfs on the links.
Thanks.

[sluckey]

6J5 gain is 20. And 6SF5 gain is 100. You need to swap those tubes in your layout. The oscillator needs that high gain tube.

[Diverted]

Oh, thanks.  From what I read about the 12DW7, the first stage (pins 1-3) were the ax and the second (pins) 6-8) were the AU. So looking at the circuit, I put in the 6SF5 for the pins marked 1-3 and the 6SF5 for those marked 6J5. Thanks for the correction.
That layout creator is a lot of fun to use. Only problem I've had with it is that saving does not work; when I try to select a destination folder for a file it starts getting flaky. So to this point I have been making screen captures of the layout as I go.

Thanks again.

[sluckey]

12DW7 section 1 (pins 6,7,8) is the high gain triode and must be used for the oscillator.

12DW7 section 2 (pins 1,2,3) is the low gain triode and must ***NOT*** be used for the oscillator.

Look at your schematic.

[Diverted]

Ok. What confused me was that I assumed the first section of the tube would be the lower numbered pins, and the second section the higher numbered pins. Thanks.

[sluckey]

Yeah, I always wondered about the section numbers too and usually get it wrong. The important thing to remember is that pins 6,7,8 (high numbers) is also the high gain triode. Let'um call the sections whatever they want. A rose by any other name is still a rose. 

[Diverted]

Good rule of thumb, thanks. And thanks for the heads-up... will make those changes.👍

[Diverted]

Totally unrelated question here.
Since I started trying to work on this layout and build this amp I've been thinking in the back of my mind about making it a parallel single ended output, which I've never done before. I'm wondering if it would be as simple as running the .002 coupling cap out of the varistors into the first grid of the 6V6s, obviously with larger transformers. Is that the case?

[PRR]

> assumed the first section of the tube would be the lower numbered pins

Sounds logical, don't it?

The sheet can be deciphered but I needed a lot of crayon.....

As always, Slucky has it right.

[Diverted]

Yep, that's confusing alright.

[shooter]

Is that the case

yup, easy peazy 
I like keeping the cathodes separate like you show, allows for "tweaking" current to sudo balance the tubes.  you can get by with tying both G2's together and use 1 R, but they're cheep. your impendence load will be ~ 1/2 what a single tube would be.  You can cheat that with speakers, or if buying new iron

as long as you're still drawing, see if you can move the filter caps closer to where they do work.  flying long power wires over the board can be problematic.

[Diverted]

Thanks shooter. I’ll move them around. That brings up a ground question: would you suggest I run the first filter cap to power ground at/near transformer, and other filter caps’ ground at preamp buss?
Thanks!

[shooter]

If you go here;
http://sluckeyamps.com/index.htm
you can steal him blind 

look how I didmodified one of Steve's and brought it down to "back-yard" hack 

I like a "buss" system on the board.  The + side gets drops n caps close to their use, the - side is 2 separate, pre connects to chassis at input, PA stuff (including PI) to mains power ground

[Diverted]

Thanks.

A few questions about power out and output transformer specs.

The data sheet for 6V6 says max plate current in Class A operation is 47ma. With two, that'd be 94. Adding in the ma from the other tubes, would I be wise to use a power transformer rated at 100-120 or more as the tubes will be running at 100 percent all the time?

And on the output transformer, I'm looking at the one below. Which would be better; the 3K tap on the primary or the 2.3? And I'm assuming this output transformer will be overkill given the max output power rating of 20va?
Any insights would be appreciated as I've never spec'd out transformers for a build like this before.

Thanks.

[shooter]

a single 6V6 seems most happy with ~~ 8K (not sure jj's at 420vdc)
so 2 would want ~4k.  I typically cheat since I build, scape, re-build.  I go with a 5K because it's what other tubes like and go for 15-25W.  I also look for 2 secondary taps, 4/8ohm, 8/16ohm.  That way I can "cheat" with speakers to "adjust" primary load.  (Note:  these are xSE, pp is a whole other thing)

for PT if I calc 100mA of need, I go up to 150mA.  Making sure Filament specs are covered at least = to my "need" value.  so 2.3A calc = 2.5Amin

[Diverted]

Thanks. PT will be good. 140ma, plenty of heater current for everything I need.

Thanks on the output answer. I will ask if he can wind it for 4K on the primary, with multiple taps.

The 4/8/16 taps would be useful as (am I right here?) if I ever wanted to cut the wattage and run a single 6V6, I'd just need to switch impedances on the secondary, and run my 8 ohm speaker into 16 ohm tap?

[shooter]

and run my 8 ohm speaker into 16 ohm tap?

yup
I have a 2X12 set as 8ohm, when I cheat it gets moved to the 4 or 16 tap

[sluckey]

The 4/8/16 taps would be useful as (am I right here?) if I ever wanted to cut the wattage and run a single 6V6, I'd just need to switch impedances on the secondary, and run my 8 ohm speaker into 16 ohm tap?

That's kinda backwards.

When you pull one of the parallel tubes the remaining tube impedance will double. This means that the secondary impedance will also double. The 4/8/16 taps will become 8/16/32 taps. So if you want to use an 8Ω speaker you must connect it to the "new" 8Ω tap, which previously was called a 4Ω tap.

[Diverted]

Thanks Sluckey. I thought I might have screwed it up. Learning as I go here ...

[shooter]

 
reading like I was capable of it 

[Diverted]

Hi again,

I went ahead and changed the layout around to accommodate two output tubes paralleled and try to complete it. I could use a second set of eyes as I'm not clear on a few things.
1. I used the output stage schematic included to lay out the output stage. The original Magnatone does not have the 470 ohm resistor into the control grid (pin 5) but the output stage has them. I left them out. Should I put them in? Same thing with the 1K screen resistors; they are not in the Magnatone but I'm assuming I should put them in?
2. The Magnatone schematic shows the 6V6 cathode cap/resistor tied into the non-grounded side of the output transformer secondary. Is this a typo? Or am I not seeing something?

Is there anything else that looks wrong to anyone here? Thanks so much!

[shooter]

2.

doesn't look right, but also doesn't look like a typo, still puzzling on it.  not sure I'd power up til that mystery is solved 

[Diverted]

Yeah, it doesn’t seem right but you all have seen many more circuits than me. I would just ground cathode cap/resistors and one half of the secondary.
Weird.
This same arrangement is also shown in the Magnatone 210 which is the same amp apart from the bright switch.

[shooter]

from a purely DC perspective I'm guessing it's only 3.2ohms from ground so close enough 
from a "best" practice thing I haven't seen it as "normal" every day 

[Diverted]

OK then, thanks. Maybe I'll just use 473.2 ohm cathode resistors then
I'm just going to wire it the traditional way and see what happens.


Any stuff you don't like about my layout, or the way the grids and screens are treated on the 6V6s? Thanks!

[sluckey]

2. The Magnatone schematic shows the 6V6 cathode cap/resistor tied into the non-grounded side of the output transformer secondary. Is this a typo? Or am I not seeing something?

Back in February I pointed that out to you. Why do you want to change it?

If you decide to build this little amp look closely at the uncommon NFB circuit. It will be very easy to get that wrong.

[Diverted]

One other thing: I want to be able to run this either with a single or two 6V6s.

I was going to run the 4 and 8 ohm output secondary leads to a rotary switch next to the speaker jack. But then it would be easy to screw things up if anyone one day decided to pull a tube but forgot to flip the switch, or flipped the switch and forgot to pull a tube etc.

Could I use a heavy duty DPDT switch instead?
1. One half would feed the correct impedance to the speaker;
2. The other half would cut one half of the heater wiring to one of the 6V6s.

It seems to me like this would work fine but am wondering what you all think?

[Diverted]

2. The Magnatone schematic shows the 6V6 cathode cap/resistor tied into the non-grounded side of the output transformer secondary. Is this a typo? Or am I not seeing something?

Back in February I pointed that out to you. Why do you want to change it?

If you decide to build this little amp look closely at the uncommon NFB circuit. It will be very easy to get that wrong.

I'm sorry, I must have missed that earlier post. I'll go back and review, thanks.

[shooter]

Back in February

solve that mystery b4 messing with the secondary 
since NFB is implied , changing output impedance will have an effect on NFB

1 tube 2 tube, personally I'd "pull" one tube, forces your brain to remind yourself, ahh, and speaker!   

[Diverted]

Yes, thanks to you and Sluckey, who reminded me of a thread I had totally forgot I even posted. Thanks!

[Diverted]

Updated layout to reflect schematic, thank you.
Should I add the screen and grid resistors to the output stage as shown in the small schematic a few posts above?

[sluckey]

As long as you're deviating from the original, go for it.

NFB... Keep in mind that even though this is an uncommon NFB, there's still a 50/50 chance the phase will be wrong. Don't cut your OT leads yet. You may have to swap the primary leads.

[Diverted]

Thank you! Still have a ways to go before I get to that point, but really, really looking forward to it. I have heard videos of these old Magnatone true vibrato amps and they sound amazing, but have never heard one in person. So I'm looking forward to it and hope I've come up with a stable clean layout; we will see! Thanks for the help.

[tony321owen]

Although it doesn't seem to specify in the 210 or 410 schematic here is an excerpt from someone who studied those circuits and described the unusual design of the negative feedback circuit. It seems trustworthy since it is found here: magnatoneamps.com/schematics/magnatone_210+.pdf.
the cathode bias cap is apparently flipped so the negative side goes to the cathode of the 6v6 which is also connected to center tap of the 6 volt heater winding.  This is for sure unusual as well as injecting NFB on a power tubes cathode.

Im building a 210/410 inspired amp and am curious about what is described below and whether I should wire it like that or try to use a more conventional approach. Not sure of the pros and cons yet. Is there another logical place to insert nfb into the circuit?

Below is only part of the knowledge from the pdf mentioned above. check out the rest at magnatoneamps.com/schematics/magnatone_210+.pdf
 
"Negative Feedback
The cathode circuit of the 210’s single­ended 6V6 is very unique among guitar amplifiers. Rather
than connecting the 330Ω cathode resistor to ground, Magna engineers connected it to the
positive side of the T­2’s secondary speaker winding. Furthermore, the 25uf/50v is installed
backwards, with its (+) positive lead connected to the speaker winding (+) side, and its (­)
negative lead connected to the cathode. This was done to provide negative feedback to the
amplifier.
In audio amplifiers, Negative feedback (or NFB) is the re­application of some portion of the output
audio signal to the input of the amplifier in opposite phase potential such that part of the input
signal is cancelled out due to the opposite phase relationship of the two signals (the two signals
being the input signal and the feedback signal). This serves the purpose of reducing non­linear
harmonic distortion. It results in an amplifier that is more stable (in terms of clean audio signal
amplification), more clean headroom, and less distortion.
On similar Magna single­ended amplifiers that preceded the 210, namely the Magnatone 108,
NFB was provided by connecting the output transformer’s (T­2) (+) positive speaker to the
cathode of the preamp gain stage that preceded the 6V6. This cathode is a convenient place to
inject NFB because of the out of phase relationship of the NFB signal and the input signal as it
appears on the plate of that triode (This is the same NFB design used on most single­ended
guitar amps, such as the Fender Champ).
So when it came to designing the 210, why didn’t Magna engineers use the more common
108­style NFB circuit? For one, they couldn’t use the preceding triode’s cathode because it was
already in use in the phase­splitting duty of the vibrato circuit (See below) schematic. For many
reasons (all vibrato related), this would not work as a place to attempt to inject NFB.
So, if NFB was to be applied, it had to be done elsewhere. Magna engineers chose to do it at the
6V6’s cathode. The signal from the speaker can be directly applied to the 6V6’s cathode (without
attenuation) and still maintain a desirable NFB ratio of 2% to 4%.
But why the backwards installation of the electrolytic? Instead of thinking of this cap as the
cathode by­pass cap, it must be thought of as a coupling cap between the speaker and the
cathode. The potential of the AC signal at the speaker is positive in respect to the AC potential at
the cathode"

[tony321owen]

wait does anyone know if this is actually safe to try? I always thought that electrolytic caps in guitar amps would explode if wired backwards!

[sluckey]

The potential of the AC signal at the speaker is positive in respect to the AC potential at the cathode

This sounds like pure bullshit to me. The guy that wrote that whole blurb had my attention until this closing statement.

I don't see anything on the schematics to indicate the cap is actually installed backwards. If I were building this from the schematic I would install the cap with the positive lead connected to the highest DC voltage. That is the 12 or 14VDC on the cathode.

[shooter]

had my attention until this closing statement.

missed THAT boat completely 

connected to the highest DC voltage

+1

[PRR]

> the 25uf/50v is installed backwards, with its (+) positive lead connected to the speaker winding (+) side, and its (­) negative lead connected to the cathode. This was done to provide negative feedback to the amplifier.

Someone has confused DC polarity with how we track signal phase.

But a 25u in a tube cathode will not blow up bad. Don't stand there with your mouth open, but I doubt much harm can come, just a blown cap or tube.

[Diverted]

Hi all,

I've suddenly had plenty of free time on my hands so I built the amp, but in the standard single 6V6 configuration as the larger output transformer good for two 6v6s still hasn't arrived. I'll build another when it comes as I have four more varistors.
Anyway, the amp plays fine, but the vibrato speed and intensity are low. I have to turn both up past 8 to get a solid vibrato signal, and even then it's not very pronounced.
Any one have suggestions to help oomph it up?
Voltages are all dead on EXCEPT for the grid on G4, the oscillator. Spec calls for 22v, I'm only getting 1vdc. I'm using a 6Sf5 there which is equivalent to half a 12AX7 and also equivalent to the high gain section of the tube that originally ran in this amp, 12DW7/7247.

I'm wondering why I have such a low grid voltage here, seems like it's barely oscillating. All tubes are good, swapped out several NOS 6SFs with the same result. Any ideas? Schematic and voltages below:

V1 6SF5 (originally was 1/2 a 12AX7)
P 83, K 1

V2 6SF5 (originally was 1/2 a 12AX7)
P 131, K 1.3

V3 6J5 (Originally was low gain side of 12DW7/7247)
P 297, K 67, G 30

V4 6SF5
P 190-200 or so, K grounded, G 1 (Originally was high gain side of 12DW7/7247)

V5 6V6
P 328, K 12 (300 ohm resistor), G 241

B+ 338
B+1 306
B+2 241
B+3 138

Thank you!

[PRR]

{withdrawn}

[Diverted]

Thanks.
I tried shorting that 820k to ground and a 108 plate voltage resulted. The 270k resistor ohms out at 262k so no problem there, and I’ve swapped four or five NOS tubes in with no change in result.
Something else is up?

[sluckey]

That 22v on the schematic is a mistake. I have an M10-A that has the exact same oscillator. My measured voltage is -0.5v.

If you have a good DMM, measure the ***AC*** voltage on the plate. What have you?

[sluckey]

Here's everything I know about that Maggie oscillator...

[Diverted]

Thanks Sluckey. I’ll give it another once over.

Still getting weak weak vibrato. I tried grounding the oscillator cathode via an LED, and while it pulsed in time, it had no effect either way on the strength of the vibrato. So I went back and just hardwired the cathode back to ground.