Scratch build amp wiring issue

[See_Mos]

Disconnect pin 4 of the 6L6 and connect it to the top end of the output transformer via a resistor of about 390 ohms and all should be well. The extra 10K and 47uF should not be needed but worth a try.

[kalee20]

That could work, but there would be more hum (injected via g2).

(It's a 6V6 not a 6L6 by the way!).

The 50k resistor feeding g2 and the previous stages is too big - the 6V6 will be running at half-cock or less - but that's a matter for the future, as is a redundant capacitor in the tone control. Let's help the OP to get it operational first, and then optimise it!

[See_Mos]

50K is quite high so with G2 connected to the left side of the 50K resistor could it be possible that G2 is modulating the HT supply to the lower signal stages?

I just had a look at one of my single ended home brew amps that uses a 6J5 and a 6V6. It follows a design printed in Radio Constructor magazine and has negative feedback tone control so designed more for quality rather than gain / power.

G2 of the 6V6 is connected direct to the HT side of the output transformer and the primary has a 10nF capacitor across it to help stop HF oscillation so adding one here might help with the instability problem. The cathode resistor is also lower at 220 ohms but that isn't going to make much difference.

[RogerLLL]

I modified the circuit as you suggested Kalee20, and and immediately the motorboating disappeared. I didn't have a 47uF but fitted a 56uF instead.
I made a few mods in-line with some of the comments and suggestions, as in the attached schematic. These included:

1. Added a 470k grid leak "safety" resistor to the 6V6. I didn't want to lose the MV and totally get the potential issue of the wiper becoming disconnected, so this seemed like a good option.
2. Changed the 50k resistor post OT to a 22k, in-line with the Fender schematics and suggestions

I also calculated plate dissipation which came out at 9.2W which is 66% of the maximum of 14W. I tried a 350 ohm resistor in place of the 500 ohm to hit the max dissipation, which produced a very slightly higher volume, but reverted to the 500 ohm rather than running at max dissipation. I've ordered 390 and 430 ohm resistors to provide 75% and 85% options, so will try those when they arrive.
Overall it sounds much better, though I am finding that when I run the pre-amp gain at max volume, irrespective of master volume, there's a significant "sag" for any harder notes. I did also replace the 3rd 22uF filter cap with a 33uF cap to see if that helped, but made no difference.
Any suggestions as to what might be causing this? Could it be the switch from 50k to 22k for the first dropping resistor?

[RogerLLL]

Just to add, the voltages are as follows:
B+ 366V
6V6 anode 333v
6V6 screen (G2) 232v
Post 10k 225v
6SL7 anode 1 123v
6SL7 anode 2 129v

[RogerLLL]

I hadn't seen the most recent comments when I sent the post above - hopefully the voltages will help inform any approaches regarding G2. Makes sense to get G2 working as it should, although the G2 voltage looks to be in-line with typical values, if a touch high.

[RogerLLL]

Here's a recorded example. It's not that clear, but you can hear the immediate reduction in volume with the attack of the note, and then the volume increasing a second or so later.

[kalee20]

Super news all round.

Regarding the conditions of the 6V6... in a Class A amplifier, the average current does not change with signal level (unlike push-pull Class AB and especially Class B amplifiers) so you shouldn't get any 'sag'.

I'll listen to the clip tomorrow.

Also regarding the 6V6, increasing the current won't necessarily increase the amount of power available. You have to alter the load that the valve 'sees' at the same time. If you don't, you're just wasting current. The valve can't possibly give the load more than (HT volts) / (load resistance) in one direction, so there's no point in having a standing current more than this, because if you do, the current will never fall to zero in the other direction at the onset of distortion.

I did make a comment about the tone control, and that is that the 500pF capacitor is redundant! The series combination of this capacitor and the upper portion of the potentiometer track, are completely short-circuited by the wiper connection, so there's no point in having them.

You can demonstrate this by shorting or removing the 500pF capacitor. You shouldn't hear a shred of difference!

Happy listening!

[RogerLLL]

I didn't know that about class A vs class AB amps, so that decrease & then increase in volume at high signal level isn't sag, by definition! Could it be linked to the 6SL7 rather than the 6V6? I did wonder if the power transformer might be being pushed beyond what it can provide? Be interested to see what you think about the clip.

[RogerLLL]

Regarding the tone control, you're absolutely right! I'd drawn this incorrectly on the schematic, corrected in the attached schematic which hopefully makes more sense. It's taken from the Fender Princeton 5C2.

[RogerLLL]

Just another thing to add, I tried a 6SN7GT in place of the 6SL7GT and the volume dip wasn't there when the gain pot was on its max setting. This is a lower gain valve, so overall less overdrive and does sound good, thought it would be good to be able to use the 6SL7 just to have that high gain option.

[kalee20]

Originally Posted by RogerLLL

Just to add, the voltages are as follows:
B+ 366V
6V6 anode 333v
6V6 screen (G2) 232v
Post 10k 225v
6SL7 anode 1 123v
6SL7 anode 2 129v

The voltages look reasonable - maybe on the high-side anode voltage for the 6V6, but they are tough (it's flashover on the base or the valveholder on audio peaks that would be my main concern).

Originally Posted by RogerLLL

I also calculated plate dissipation which came out at 9.2W

OK, So, assuming 15V lost in the cathode resistor, you'll have 318V across the valve from which I deduce the anode current is 28.9mA.

This gives (assuming 2.5mA g2 current) 15.7V across the cathode resistor, so my "15V" wasn't far out...

Anyway! The 6V6 can pull its anode down to about 25V, which therefore puts about 41V minimum anode voltage on the signal troughs (valve drop + cathode resistor drop).

From the quiescent 333V anode voltage, you have a downward swing of 292V. With a current swing of 28.9mA, this puts the optimum load in this circuit as 10.1k for maximum undistorted power.

Do you know what you actually have, with your OP transformer and speaker?

Originally Posted by RogerLLL

Here's a recorded example. It's not that clear, but you can hear the immediate reduction in volume with the attack of the note, and then the volume increasing a second or so later.

I can, and it's a bit bizarre! I'd guess that the initial string pluck is putting such a massive transient in, that one of the valves (probably the second stage of the 6SL7) is getting its grid taken positive, this is charging the coupling capacitor by thje resulting grid current, which after the string-pluck transient has disappeared, is biased towards cut-off and low gain until the capacitor has re-established its normal charge.

Try adding another 22nF (or more) in parallel, which by that theory would double the 'sag' recovery time.

Originally Posted by RogerLLL

Regarding the tone control, you're absolutely right! I'd drawn this incorrectly on the schematic, corrected in the attached schematic which hopefully makes more sense.

It's now missing a ground connection from the 6V6 cathode resistor... and although it's pretty obvious what's meant, the 'amplifier' bit of the diagram needs a ground connection symbol to tie it to the 'power supply' bit's 0V.

[RogerLLL]

Thanks - schematic now corrected - missed that!
I have a very old Rola 8-Z speaker which is in great condition, and appears to have an alnico magnet. It has an OT mounted to the chassis, and sounds lovely, seems to handle the power and tonal range pretty well, though a little lacking in bass response. I assume this is an 8 ohm speaker.
I'll try a 47nF capacitor in place of the current second stage 22nF.

[kalee20]

Originally Posted by RogerLLL

I'll try a 47nF capacitor in place of the current second stage 22nF.

No, just tack it across (easy!) to prove the point! The fix may be something else...

[RogerLLL]

This just got weirder! I added another 22nF cap tacked onto the existing, and was immediately met with the oscillation sounds as in the attached zipped audio file. The oscillation disappeared when I plugged a guitar into the input, but the same "sag" was present. I removed the additional cap and was presented with exactly the same phenomenon. Weird as this was then exactly as per the circuit previously, so its like this has just come from nowhere.
The recording was taken with no guitar plugged in, MV set at max and going from zero to max on the gain pot. There's a strange pop or click at one point. I repeated this and the same pattern was there, as in the recording.
I tried replacing the 6SL7 with another 6SL7 and it was exactly the same.
I tried with a 6SN7 in place of the 6SL7 and the whole thing was completely normal, but with lower gain, no spurious oscillation and no "sag".
So I get the feeling that there's something the 6SL7 doesn't like in the circuit, just not sure what.

[RogerLLL]

The oscillation issues discussed in this thread from the EL34 world forum sound pretty much identical to mine:
https://el34world.com/Forum/index.php?topic=28853.0#google_vignette
This seems to point to positive feedback as a potential cause.
If I hold the valve, the oscillation changes significantly, and even more so if I then ground my other hand to the chassis.
Thinking a simple thing to try is a shielded input cable, so I'll give that a go.

[RogerLLL]

Tried a shielded input cable, and disconnecting the tone control. Both changed the nature of the oscillation but its still there. When chop-sticking, the only wiring that seems to affect the oscillation seems to be the anode connectors, so I might try re-routing these. I'm tempted to change the valve socket, just in case there's some internal resistance affecting this issue. I've cleaned it but no change. I have two spare, and its an easy enough thing to try.
I have to say that the 6SL7s sound absolutely excellent, so its worth pursuing.

[kalee20]

Originally Posted by RogerLLL

This just got weirder! I added another 22nF cap tacked onto the existing, and was immediately met with the oscillation sounds as in the attached zipped audio file.

It's a good, descriptive sound snippet, with commentary.

It's high-frequency oscillation, caused by stray capacitive coupling. It's a completely different effect from the original, low-frequency soft motorboating.

There's quite a lot of gain there...

I thought maybe the extra capacitor, simply due to its bulk, might have added enough stray capacitance to provoke this, but you say it remained when you removed the extra capacitor.

There's no HF roll-off deliberately added anywhere in the amplifier. I'd put a 2.2nF capacitor across the output transformer primary. It'll have to be a high voltage one, to cope with audio peaks, though almost anything will do to prove the point. (Just don't expect a 250V one to last very long ).

You don't say if the sag is the same time frame or longer. (Incidentally, just to check - the extra capacitance added is between the sections of the 6SL7 isn't it, not between 6SL7 and 6V6? I may not have been clear with my suggestion.)

Also, can you try a different input source? Something like an organ which doesn't have the pluck transient of a guitar, but is just a uniform tone?

Using 6SN7's where the sag isn't present, does make sense - they take more current so the cathode bias voltage will be greater - grid current and resultant partial blocking is much less likely. But once the cause has been confirmed, there may be a way to use the 6SL7 all the same!

[RogerLLL]

I agree there's a lot of gain, possibly more than is needed/ healthy! Was also wondering if trying higher value dropping resistors to anodes 1 & 2 of the 6SL7 might be worth a go? I think that generally 220k is used as a standard, but I chose 150k as that appeared in a couple of 6SL7 schematics I saw online.
I'll try the 2.2nF cap over the OT primary. Not sure if I have a 2.2nF but definitely have a high voltage 4.7nF bought for exactly the same purpose for another build. Should provide a good test of this.
I added the extra capacitance between anode 2 of the 6SL7 and the 6v6, so will try the same between the gain stages. And I'll make a recording for comparison.
I'll also try a different input with less attack than a guitar string. Though I have to say that if I do pluck a string with less attack, the "sag" is much less. So it does seem to be that high level transient that causes the issue.
What's weird is that this issue wasn't there before I tried the extra coupling cap. It's like fitting that kick started something which resulted in this oscillation, even though the circuit was returned to an identical state to previously! It's clearly linked to the 6SL7 for some reason as its just not there at all with the 6SN7.

[kalee20]

OK. The 2.2nF should tame that high-frequency oscillation in the last sound clip.

Then it's return to the 'sag' issue.

Adding capacitance between the 6SL7 stages should enable a time-comparison - when solving an issue, finding what makes it worse often provides a clue! The actual fix may be rather different, and in this case I think it will be.

The load resistors 150k are about right... we may need actually to go lower in value rather than higher, but at the moment that's jumping the gun.

Look forward to your next report!