5F2A gain questions

[Groover]

I have built an amp basically using the 5F2A circuit. Differences include more filtering in the B+ and no negative feedback. I find it breaks up at very low volume settings - starting at around 1/4 way up on the linear taper volume control.

Is this normal behavior for this circuit?

[Bruce / Mission Amps]

I have built an amp basically using the 5F2A circuit. Differences include more filtering in the B+ and no negative feedback. I find it breaks up at very low volume settings - starting at around 1/4 way up on the linear taper volume control.

Is this normal behavior for this circuit?

Yes...
Put an audio taper volume pot in and remove one of the cathode bypass cap from one of the 12AX7 triodes and see if that helps.
You can use a little NFB with good results.

[Groover]

Thanks for the info, Bruce.

Changing the taper of the VOL pot will not change the sounds I can get, only where on the dial I get them, correct?

I have tried it with switching the bypass caps in & out on both the 1st & 2nd stages, and reduced the bypass caps on both to 11uF. (2 x 22uF in series makes 11uF, correct?) But from my research it looks like 11uF might still be large enough to be fully or nearly fully bypassed.

I also have wired in a dpdt to bypass the first stage and VT completely and connect the input signal directly to the grid of the 2nd stage (by way of 33K to grid and 1meg to gnd.) I wanted to see what the driver and power section sounded like without the first stage and controls. (I posted about this in another sub-forum.)

I guess what I am experiencing has to do with the lower amount of negative gain through this VT circuit as compared to the BF style TMB tone stack and Volume. It is a much more dramatic difference than I would have expected. (I posted a question about this in another sub-forum.)

I think I will try adding adjustable negative FB and see what that gets me.

This is really a great learning experience for me.

[tubeswell]

You won't notice any difference using 11uF instead of 22 uF for the bypass caps in that amp. You have to go to around 2.2uF for more noticeable bass roll-off, and 1.1uF starts to sound more middy. Or as Bruce said, take out one (or both) of the pre-amp bypass caps completely. Adding NFB will smooth out the crunch. You could make the NFB switchable (and same thing with the bypass cap(s)).

[Bruce / Mission Amps]

The audio taper pot only changes where the knob position is on the chassis.
It also changes a little where the simple tone control starts to bypass the highs around the pot so there will be slight difference in where you will set the tone control.
You can make the vol pot better by using a 2 Meg to 1 Meg resistor on the middle lug to ground.

OK... look at a stock 5F2A schematic and you'll note the NFB circuit.
What you should do is lift the second cathode biasing resistor, (1K5), from ground with a 47 to 100 ohm resistor and insert NFB at their junction.
Use a 2700-3900 ohm resistor at the speaker jack and over to a 10K-25K linear pot wired as a variable resistor.
Then from the NFB pot, connect a wire to the junction of the 47-100 ohm resistor and the 1500 biasing resistor.
You can still bypass the 1500 ohm resistor with a 1uF to 4.7uF cap but only bypass the 1k5 resistor not the 47-100 lifting one... make sure the cap is not grounded except by the lifting cap.
Now you can tone tweak with different bypass caps and have a little or a lot of NFB because it is variable.
The mod I do is a 3900 ohm resistor on the speaker jack hot lug over to a small 10Kb pot mounted on the rear chassis, a 47 ohm resistor under the standard 1k5 and a 1uF to 2.2uF bypass cap and that's all.

[JHow]

Bruce:
I have been lurking here, and I will definitely try this mod on my 5f2a, but what should we expect tonally from this variable nfb?

[Bruce / Mission Amps]

Bruce:
I have been lurking here, and I will definitely try this mod on my 5f2a, but what should we expect tonally from this variable nfb?

The NFB mod will allow you to dial in a cleaner tighter loud over drive vs a loud and juicy over drive tone with a little more bass and treble.

[Groover]

Bruce, I noticed on a 5E3 thread you referring to the 5D2 Vol & Tone circuit. I think I might try that (along with adding some NFB).

[Groover]

OK, I wired in NFB via: + of the speaker jack > 3k3 resistor > SPST switch > the junction of the 1k5 cathode resistor (no bypass cap) and a 47 ohm resistor to ground.

I can't hear much if any difference at all switching it in and out. (This is with the 1st stage cathode bypassed @ 11mF, and at various settings of tone & vol controls.

Am I doing something wrong?

[Groover]

I was poking around with my (non-powered up) 5F2a build to try and see why I couldn't hear much difference switching the NFB in and out, and in testing continuity of the switch etc. I discovered that the OT secondary DC resistance reads 0.7 ohms hot to ground. So then I measured the primary, it read 226 ohms across the primary. Both these measurements were made without lifting either end from the circuit. Tubes were installed but as I said there was no power applied.

Are these readings normal? What am I doing wrong?

[Bruce / Mission Amps]

I was poking around with my (non-powered up) 5F2a build to try and see why I couldn't hear much difference switching the NFB in and out, and in testing continuity of the switch etc. I discovered that the OT secondary DC resistance reads 0.7 ohms hot to ground. So then I measured the primary, it read 226 ohms across the primary. Both these measurements were made without lifting either end from the circuit. Tubes were installed but as I said there was no power applied.

Are these readings normal? What am I doing wrong?

I don't know what you ar doing wrong.. I don't even know what you are doing right.
The measurements you made seem reasonable to me.
The secondary winding is grounded and there will always be around 1 ohm or less then 1 ohm of resistance in most OTs... we're not dealing with DC resistances in reactive components like an OT.
Look at my scratch drawing and tell me if this is what you did.
Try a smaller value resistor from the speaker jack too, like 2200 ohms.

[Groover]

Yes, your drawing does match what I wired in. I will try a smaller R.

Should I hear a pronounced or a subtle difference? I guess the answer depends on the R value, huh?

[Bruce / Mission Amps]

Yes, your drawing does match what I wired in. I will try a smaller R.

Should I hear a pronounced or a subtle difference? I guess the answer depends on the R value, huh?

With the right resistor combinations it is quite obvious, even in simple SE 6V6 Champ type amps.

[Groover]

Well I wired in a 220 ohm resistor on the + of the output jack in series to a 10k ohm (linear) variable resistor then to the V1b cathode circuit 47 ohms above ground.

Measuring the total of the 220 ohm + the variable resistor:
I hear an audible change from minimum up to about 2.7K in the feedback circuit. Increasing the R in the feedback loop above ~2.7k doesn't seem to make much audible difference. I might try bypassing the 220 ohm resistor and just using the pot. (I was going to ask if I needed the resistor there sort of like a grid stopper, but I guess I can just try it and see.)

I used the amp (with Weber 8F150 fitted) today for a lap steel track and the artist said it sounded "very 50s". Which makes sense, since it is a 5F2 circuit.

[Groover]

I measured the voltage across the cathode cap of the 6V6 - 21.5V / 460 ohms = 46mA at idle. Measuring the voltage across the 470 ohm screen resistor I got 1-2 volts which works out to 2-4mA, also at idle. Screen voltage is 375+/-1V. Plate voltage is still 394V.

More tweaking:

I lifted the cathode bypass cap on V1a. I now have switchable 11mF bypass cap on V1b.

In an effort to tighten up the flabby low end I changed the coupling cap coming off the V1a plate from .02 to a .0056.

In an effort to tame the fizzy high end I added a 100p snubbing cap across the plate resistor coming from V1b.