6L6 5F2a/Maggie/Two Stroke project

[capohk]

I have got to a point with this project where I think I have most of the pieces in place.

The plan is to build a super-compact/portable “lunchbox” head based on Dave Hunter’s Two Stroke and the Weber Maggie amp for one 6L6 or EL34 in a 5F2a type circuit. Aim is to be simple, reasonably robust, and as short a signal path as possible. PtP wiring in compact Hammond 1550Z124 enclosure.

I have drawn out a schematic based on the titular amplifiers. I have omitted the boost switch as I would typically use a pedal to achieve that in actual use:

As I am using a solid-state rectifier, I have added an extra filter capacitor before the load resistor but I am not sure that it is necessary. I am also not sure about the values of each stage. For reference, the Ampmaker WF-55 is SS rectified and uses 20/20/20/20. Weber Maggie uses 20/8/8 or 20/16/8 depending on whether you look at the layout or the schematic. Two-stroke uses 40/16/8. I think I'd like to use a can cap for the power filter stages, and then put the preamp filter cap inside the box to separate the ground, but the multi-can cap values are limited.

Looking on Amplified Parts, CE have a 20/20/20 @525V

JJ have a 16/16 @500V and a 40/20/20/20 @500V

Obviously not all sections need to be used.


I am still reading Merlin's book and Jack Darr, but in the meantime, I am impatient to begin soldering...

Here's a gut shot of progress so far for interest's sake:

All comments and advice, as always, are gratefully received.

 

[Lynxtrap]

A grid stopper on the output tube would not hurt, especially if you are going to drive it hard.
56k in the power supply seems a bit high.

 

[chas.wahl]

Interesting, but your title is a little confusing: "5F6a"?

 

[D'tar]

Interesting, but your title is a little confusing: "5F6a"?

How about a "2A"

 

[2L man]

If there is a possibility to waste some B+ voltage I recommend a series resistor before the first filter capacitor and if you plan to parallel two 20uF you could install one between them as well.

Silicon diodes are very efficient and the current peaks thru power transformer come much more violent than when a tube rectifier is used. Then there is a possibility that PT electromagnetic pulse effect the OT. Series resistance smoothen PS operation. Technically voltage loss does not get completely wasted because it is part og the filtering

You already seem to have a bridge rectifier installed but for future builds you should use fast recovery diodes for example UF4007. They function better and they are almost as cheap as typical 1N4007.

 

[Lynxtrap]

If there is a possibility to waste some B+ voltage I recommend a series resistor before the first filter capacitor and if you plan to parallel two 20uF you could install one between them as well.

What value would you recommend for that resistor? I figure it will affect the response of the output stage already at quite a low resistance?

 

[capohk]

Interesting, but your title is a little confusing: "5F6a"?

Whoops. Yes, a '2' would be better there.

 

[capohk]

How about a "2A"

It sounds better already!

 

[bebopbrain]

The first stage grid resistor should be 33K to be a 5F2A (which has 68K resistors in parallel). I mount it as close to the grid as physically possible.

 

[bebopbrain]

The way to do NFB is to have 68K hard wired and not switched.
Then have a 33K resistor on a SPST switch that gets connected in parallel.
This won't be as noisy switching as what you have shown and doesn't need SPDT.

I would make the wire from the NFB resistors to the triode as short as possible. The signal from the speaker will not pick up noise; the signal from the resistors to the cathode might.

 

[bebopbrain]

The power tube cathode bias and bypass is a little weird. What bias current do you want? A typical number is 60ma. I * R = 0.06 A * 510 ohms = 30.6V which exceeds the rating of your bypass cap. Granted, your power tube may run colder than that.

 

[capohk]

The power tube cathode bias and bypass is a little weird. What bias current do you want? A typical number is 60ma. I * R = 0.06 A * 510 ohms = 30.6V which exceeds the rating of your bypass cap. Granted, your power tube may run colder than that.

I think that I carried that voltage over from the 6V6 version - I will change that to 50v

 

[bebopbrain]

Maybe make the bleeder resistor on the reservoir cap 2W and it will run cooler. 400V * 400V / 220,000 ohms = 0.73W.

 

[Esquier]

I made mine with two output sockets, each with their own cathode cap/resistor set up, like a two stroke. Also octal and noval sockets in parallel for V1 as per Steve Ahola's "Angela Super Single Ended Amp". No NFB on it, and used a Hammond 125ESE OT and a big Hammond choke. There's a Chinese Weber Deluxe size transformer with a lower set of secondary taps that I'm using. I use a 5Y3 and get 365VDC on pins 3

 

[2L man]

What value would you recommend for that resistor? I figure it will affect the response of the output stage already at quite a low resistance?

I can't say what value to use. Ihave test installing even CRCRCRCRC filtering and mains hum has got significantly lower in single ended amps. I have used many different values smallest I remember were 22 ohms and highest at least 120 ohms. In SE amps PS resistor voltage drop comes multiplying resistance using bias current and 2mA for each pre amp tube. B+1 has small effect to bias current so that math is only approximation but so is installing a cathode resistor value schematic calls!

 

[capohk]

I finally got round to going to the electronics shop down in Sham Shui Po and picked up what I needed to finish this. Spent the afternoon wiring it up as it was a typhoon day here in HK and schools were closed. It was this or reruns of Brooklyn 99 with the kids...

I had big ideas about making it beautiful on the inside, but in the end, pragmatics have somewhat won out. I ended up with a filter stage of 47uF > 10k10 > 22uF > 56k3 > 8uF as that was what was available at WECL. I paralleled two 68k resistors to make the 33k input resistor as they didn't have any 33k of any wattage and it's a general mismash of brands and colours as I used what I had in the drawer as well.

So far I have left out the tone pot, the NFB and the boost switch as I don't have all the parts yet. My vol pot is only 500K.

Anyway, it works. At least at bedroom levels. There is some hum when I turn it up full so I suspect my grounding is not optimum.

Voltages at the filter nodes are 348.3, 316.6 and 263.6 which I think are in the right ballpark for 6L6, but maybe could go higher. I have options at the PT to raise voltage but not lower.

I installed the ammeter in line with the cathode from the OT as in the schmatic above. It serves the primary function of lighting up to show the amp is on. It does not however show any needle movement. I did wonder if I had wired it in backwards, but reversing the wires made no difference. So not sure about that.

Next I need to clean up some lead dress and then take it to band practice tomorrow and let it rip.

I've just found this thread which I think will prove useful reading.

Cheers

 

[capohk]

Did some biasing today. Ended up pushing the voltage up by ising the 220V primary tap s othe B+ is around 376 today and with the 510R cathode resistor, the bias was way too cold so I added another 510R in parallel to bring up the current. I think that is the limit of the PT - no more voltage available so the only option to bias it hotter is further reduce the cathode resistor.

From Robrobs site this looks fine for 6L6WGC and also leaves room to try a EL34 (which I don't have at the moment).

So far there's no bells and whistles. I wanted to get it working first. Next will be NFB. At the moment it sounds pretty good up to about 12 o'clock then gets pretty hairy and will feedback my 6" test speaker. I think NFB will clean this up a good bit. I've used 68K to the 16ohm OT primary. Still not had a chance to crank it up properly but this weekend hopefully.

 

[2L man]

High B+1 shift Operating Point away from center of a loadline. Operation comes very unlinear and there comes lots of 2nd harmonics. If OT can take higher bias current you could drop B+1 and increase bias current if you want more cleaner headroom. But loadline does look now about what typical Champ has and it sound good so perhaps NFB will smoothen it enough?