3 output-tube schematic analysis ?

[tdvt]

A friend I play with, built a harp amp (Lone Wolf 3X6V6SE) from plans that were once available online, including a circuit description, BOM, layout, & schematic. It seems to be gone from the web now, I don't have any digital copies, all I have is the paperwork that he printed out at the time.

He never got it running & it has sat on his bench until last week when I took it to see if we could get it running. I rebuilt it as-drawn over the last week & while it works, there are some issues, which I am wondering are baked-in in some manner, due to the design.

It is essentially a pretty simple SE amp, but complicates the equation as it uses 3 6V6 output tubes, fixed bias & a really low-gain 1st stage. Looks like the tone control is copied from a Kalamazoo Model 2 & is placed after both gain stages, immediately before the 3 power tubes. 


At first test, the hum was just beyond tolerable, but I have temped in some parts & think that adding a choke & additional cap will get things to a usable level.

The issue I can't really solve or explain, (but can work-around) is a medium trem-speed oscillation that occurs when the volume is nearly at full, & it kind of sounds like trem. Might qualify as motorboating. The bias voltage is steady when the oscillation occurs.


The oscillation does not occur if:

• the V2B bypass cap is switched out
• the V1A cathode resistor value is lowered (jumping one of the 2 resistor stack)
• Use a 12AY7 instead of a 12AX7 (12AY7 on the schematic, 12AX7 in all the other printed material)
• One of the 3 output tubes is pulled

Really no information out there on the web, other than a few mentions from 10 years ago. Guessing not many were built.

Anyone have any insights?

[WimWalther]

Very briefly..

It does sound like motorboating. Try the following:

1) Cut C4 to half present value, see if it improves.

2) At least double the cap values in the power supply. It's a SS rectifier, so feel free to more than double..

Btw, I assume that pulling any (1) of the (3) 6V6 will produce improvement? Yes?

[tdvt]

Thanks,


The filtering seems inadequate to me, & temping in a 47uF helped with the hum as well as lessening the onset of the oscillation, so I figured I was on the right track.

I don't have a choke on-hand but have worked out how/where to fit it.


You are correct, pulling one power tube eliminates the oscillation.
 

[WimWalther]

I'd go with motorboating for now. And inadequate PSU bypass / excessive LF cutoff in the power section are assignable causes.

If you have a .22uF in C4, your LF cutoff is like 3.2Hz, way too low. Going to 0.047uF will raise it over two full octaves.. and you'll still be around 14Hz. That's plenty low for a gitamp that needs to cover like 80-320Hz (plus harmonics).

Anyway, you know what to try for now. And we agree that the PSU seems to be insufficiently bypassed. Where are you adding that 47uF btw?

[dwinstonwood]

FWIW, I did find a layout...

[tdvt]

Had a paper copy of that layout, the rebuild was pretty much along those lines.

The other part of the circuit that I am not clear on is the V1A cathode set-up which sets quite a high cathode bias voltage.

The amp calculators indicate that reduces gain considerably, which is what the harp guys like, so I suppose that was the goal.

But the board-mounted 1M grid leak is also tied there, & wired as drawn (as well as an alternative physical layout), it introduced DC to the input jack.

I bagged the fancy switching jack (it switched the V1B bypass cap in/out) & went straight Fender with 1M at the jack. No more DC.

[kagliostro]

For those who are interested to have digital copies

From my archive (see attached files)

Not saying they are correct, only they were saved some time ago


Franco

[tdvt]

Thank you Franco, I will file those away for future reference.

You must have many hard drives-worth of archives...

[astronomicum]

You may want to correct the schematic typo on the PS from 680V to 380V. I don't think a 450V cap will last that long there 

[tdvt]

So the large value coupling caps seem to have been causing the problem.

The first version had .22uF, I knocked them back to .1uF in my preliminary rebuild, but both builds had issues with the LF oscillation. 

Changing out all 3 caps to .047uF has eliminated the problem & I may be imagining it, but the overall hum level also seems reduced. I have a choke coming which I hope will lower it even further.

Interestingly, or not, the design overview included in the paperwork I had was somewhat more detailed than the sheet Franco shared & regarding coupling caps, the designer specifically states that ".1uf is the minimum, .22 is a lot better an .33 is a little better still..."  Not sure how that would pan out.


Hats off to WimWalther for pointing out the likely culprit, I appreciate the help.

[WimWalther]

What still puzzles me a bit are the effects of changing C1 & C3, or subbing 12AY7 / 12AX7. I mean, the same basic principles hold, but the driver tubes use only tiny fractions of the current used in the output section. How they are able to modulate the PSU (cause motorboating) I don't quite get.

[PRR]

The power tubes modulate. The little tubes let that back in the INput of the amp.

The nodes with 1Meg grid resistors should use 0.01u caps, NOT 0.22u. Apply the RC formula.

[tdvt]

This is what I ended up with for the preamp at this point; some value changes, a deletion & component swap.


I also reworked the bias circuit (as it was way out of range), changed the first resistor (R13, 50R to 500R) in the power rail to lower the B+ a little, used a 40/20/20/20 cap can & added an artificial heater center-tap.



Otherwise built as drawn in the original schematic.

[tdvt]

There was also a Lone Wolf single 6L6SE design & I had forgotten that I had saved a digital copy back when it was still posted online.

The V1A mods I made to the 3X version are nearly exactly what the 6L6 version uses, my added pot replaces the V1B input voltage divider.


Also has the big coupling caps... 

[kagliostro]

Other documentation about the version of the Lone Wolf 6L6 SE

Franco