Magnatone 113 Head Build

[David Root]

This is a Maggie 113 from 1955, built as a head; original was a combo.
Built in a 12x6x1.75 chassis I got from Bob at RJ Guitars when I bought a bunch of his stuff to build an Express with. He even has a head box for it too.

This one appealed to me as I had seen a report some years ago on the Maggie forum of a repair job on one, done by CJ Poulos, who is a Maggie guru.

Its claim to fame is it's louder and prouder than any 2x6V6 amp has a right to be. And that is certainly true, especially thru my Altec 417B. Still pretty loud thru a Celestion Blue in a pine 1x12. Great little blues/blooz amp!

The circuit is a little different than some Maggies of that era. I could best describe it as a brown DeLuxe preamp into a cathodyne PI with a really odd tremolo. I should mention that it's not a varistor true vibrato type, just an amplitude modulated type. I changed the input to allow combining the two input stages with their gain pots, more flexible. Also I did not elevate the heaters, it wasn't necessary with the modern iron. And I changed the switched tremolo pot from intensity to speed.

The tremolo ticks a bit, and needs some speed and depth adjustments, but sounds nice old school with a 6201 (industrial 12AT7). The other tubes are a Tung-Sol 5Y3GT, 2 x Ken-Rad 6V6GT/G from WW2, '51 GE 12AX7 PI, and '60 RCA long blackplate 7025. Everything is crammed in as it's a small chassis for six tubes. The iron is from Matt at Musical Power Systems and works great!

[M Fowler]

Nice amp David.

Slucky just finished a Maggie M2 over at Hoffman Forum too.

http://el34world.com/Forum/index.php?PH ... ic=18444.0

Mark

[xtian]

Wow. Classy set of components!

[David Root]

Thanx guys! Yes I went overboard on the parts alright.

The chassis cost me nothing (thank you Bob!) but I figure with the head box I'll have about $1,000 (in Canadian discount 80 cent dollars) in it, and probably $150 of that is US to Canada freight and PayPal exchange rate.

Mark, I just checked out Steve Luckey's Maggie M2 build at the EL34 forum, looks great and man he does a helluva job with his documentation, I couldn't begin to do that stuff.

[M Fowler]

You did a great job on yours David.

[Bob S]

Wow - there's some good stuff in there David.
Nice one !

[David Root]

Thanx Bob!

I just added a .01uF across one existing .01uF in the tremolo circuit in order to slow it down a bit, worked just right. Also changed the tremolo tube to see if I could get rid of the ticking, no luck there. Seemed to get more gain though.

But I still have a residual ticking from the tremolo, even with both V1 gain pots at zero. I've relocated two trem circuit leads well away from the rest of the circuit but it still ticks. It's not noticeable when you're using the trem but after the final signal decay, there it is. The footswitch does cut it out though.

Anyone have any suggestions?

[trobbins]

Is the ticking related to the Intensity pot setting?

What signal current swing do you get from V3B cathode, and what related cathode bias voltage swing occurs on the 6V6's?

The trem will only pull the 6V6 cathode bias voltage up.

Perhaps try a grid stopper to V3B.

Edit: The heater to cathode voltage may be under stress when the intensity switch is off.

[David Root]

Thanx trobbins!

Yes the ticking is related to intensity pot setting. As I turn up the pot it lessens and disappears at 9-10 on the pot. Pot is wired backwards as reverse audio.

Signal voltage (not current) swing at pin 8 (assume this is your "V3B cathode") is about 165 to 180V, or about a 15V swing, with trem operating. With the trem out I measured 161V steady.

The 6V6 cathode voltage is 17.8V. Doesn't change with trem in or out.

A grid stopper to V3B would be pin 7? When you say V3B cathode do you mean pin 8 or pin 3?

BTW I changed the switched pot to the speed pot. The intensity pot is not switched. Should I switch the intensity instead? They're both 1MA.

I also changed the 22k on pin 1 to 100k, the 25uF/3k3 on pin 3 to 100uF/2k, and the 22k on the intensity and speed pots to 220k, as I thought these values would work better with the 12AT7.

[trobbins]

Ok, so the ticking is related to the cathode follower, and probably from the follower being over-driven, or receiving an overdriven waveform. The LFO output should be a sinewave, but a clipped sinewave would include a splatter of high frequencies that may be the cause of the ticking.

You may not have an oscilloscope that can check the LFO waveform - eg. by taking X to a separate test resistor to measure the voltage across, instead of the cathode bias circuit.

A grid-stopper to pin 7 (from pin 3) may achieve improvement with the simplest circuit change.

Is V3A plate load resistor really only 22k ???

[David Root]

V3A plate load is 100K, I changed it from 22K because that seemed kind of low for a 12AT7.
Maybe I overdid it and the gain going to pin 7 is too much.
I ran TubeCad on it and it gives a theoretical gain of 46 for V3A with 100K plate but reducing it makes not a lot of difference until at 22k it's 32. I'll measure it tomorrow.

So how big a gridstopper from pin 3 to pin 7? 470K perhaps?

[trobbins]

If the LFO output was clipping, then a large grid stopper to the cathode follower may provide some high frequency attenuation, although the 12AT7 is an RF device. My 12AT7 datasheets don't show a max grid resistance, but 12AU7 shows down to 1Meg, and 12AX7 shows up to 2Meg. So yes, see if there is any change going up to perhaps 1Meg max.

A small cap bypass on the LFO anode load may also help, but may also reduce oscillation robustness - perhaps worth trying as another 'simple' change.

The 12AT7 grounding should also star with the cathode bias ground.

[David Root]

Thanx for the further tips. The 12AT7 grounding is not to the 6V6 cathode bias ground, just the ground from the 220k on the speed pot wiper goes there. The V3A cathode ground is to the ground lug on the trem footswitch RCA jack.

[trobbins]

My query with grounding was that V3A cathode 0V (3k3 and 25uF), and the supply cap for V3 (20uF 'B') should star at the output stage 0V.

[David Root]

I rerouted V3A cathode ground to 6V6 cathode ground point. No change. Have not yet moved the screen/tremolo node cap ground to 6V6 cathode ground, nor tried the gridstopper between pins 3 and 7 on V3. On a second look, should that not be from pin 1 to pin 7 ??

Then I found that the ticking is increased by turning up either or both volume pots, so it is getting into the preamp circuit somehow. The lead from input jack to V1 grids is not shielded but it is not very close to the trem wiring.

Could it be getting in thru the V1 plate supply lead, which runs under the main board, or from the other end ie from pin 3 of the tremolo tube thru pin 2 into the volume pots (no gridstopper or cap on pin 2)?