5F2a help

[xFallen]

I have a Weber-based 5F2a I built. It is largely stock with a few exceptions. I have a Sylvannia 12AX7 and a Sovtek 6V6 and swap between a NOS 5Y3 and the Weber copper cap equivalent.

I was pretty unhappy with the tone. My Mission 5E3 blows me away. The 5F2a is just "okay". I realize the speaker needs to settle in a bit, but still. The amp is very quiet. I substituted a few parts for higher quality equivalents (Alpha pots for starters and Sprague caps in the PS) to best assure that sort of signal integrity, and I think my build is decent although I am no pro by any measure. I am using metal film resistors throughout. I do need to get a better power tube in there. At least the Chinese tubes (absolute useless junk) are gone.

The tone control really has little effect over the majority of its sweep to my ears. The dynamics just are not very exciting, and the breakup is ratty and a little too harsh sounding. Some of that may be the Sovtek power tube which I have never liked anyway. The amp also doesn't have that top end shimmer and sparkle I expect from a Fender circuit. The Sylvannia preamp tube helped smoth this out a lot. Maybe a different tube would help even more, but I have a feeling it is something with the circuit or how it is implemented.

I decided to lift the negative feedback. I literally de-soldered the NFB wire at the speaker jack and fired up the amp to try it out. Wow. The amp came alive. It is very dynamic and has a nice growl when it breaks up. Of course, breakup happens very early, and the full on distortion can still be somewhat ratty. It has some sparkle and sounds very decent even compared to the 5E3.

I installed a toggle switch on the bottom of the chassis to switch between no feedback and the (stock) 22K resistor.

So, now I would like to throttle it back just a wee bit. What is the preferred way to accomplish this? I could simply put a pot in place of the resistor. I have also seen the resonance control cap/pot combination. I see how that works. I am not sure what amount of resistance at the extrema is sane (minimum and maximum). What if any fixed resistance should be there? Is there some target value for this RC network that one shoots for in the resonance control?

In addition to putting in a decent power tube what else can I do to get the bottom end a bit less ratty? I was thinking of putting a choke in the PS to (but really, there's probably no need) and maybe stiffening the PS with larger caps. How big is too big?

Are there things I can do to the tone/volume controls to open them up a bit, either changing values or types of caps? Again, it is set up with stock values right now.

Anything else I should look at? The PT and OT are whatever ships with the Weber 5F2a collection of parts.



Thanks,

Barry

[MWJB]

Your initial description of the 5F2A doesn't sound typical, especially the bit about tone pot not doing much over it's sweep. This is a great & versatile recording/practice amp. I think you have a fundamental problem that needs sortig before anything else.

The Weber OT is fine, the PT is fine if voltages are ball park (reliability has been an issue).

I went through a bunch of tubes with one & the owner - we both actually preferred the tone of the Chinese Weber supplied 6V6 compared to the other current production tubes (warmer, sweeter compression - I was surprised myself).

You could put a 250K pot, wired as a variable resistor, temporarily in place of the 22k NFB resistor, dial in by ear then replace with a fixed resistor.

[Satamax]

Myself i would keep the 22K, and put a 1 meg variable resistor in series.

[xFallen]

Your initial description of the 5F2A doesn't sound typical, especially the bit about tone pot not doing much over it's sweep. This is a great & versatile recording/practice amp. I think you have a fundamental problem that needs sortig before anything else.

The Weber OT is fine, the PT is fine if voltages are ball park (reliability has been an issue).

I went through a bunch of tubes with one & the owner - we both actually preferred the tone of the Chinese Weber supplied 6V6 compared to the other current production tubes (warmer, sweeter compression - I was surprised myself).

You could put a 250K pot, wired as a variable resistor, temporarily in place of the 22k NFB resistor, dial in by ear then replace with a fixed resistor.

Yeah, that circuit is generally pretty sweet. I have a couple of Champs of various vintages and have owned several more and they are very similar circuits.

The amp does actually sound pretty decent but the full on distortion is horribly flawed to my ears. Lifting the NFB (bypassing it completely) really makes a huge difference, opening up the top end and allowing a smoother transition from clean to dirty.

That is surprising about the Chinese 6V6. The recent 6V6 I have tried and the generic Chinese 12AX7s sure don't sound good to my ears in anything I have tried them. They sound terrible to my ears in my Champs. The old Ruby STR and a few of the others from years past have actually sounded decent generally, and the KT66 is awesome.

Voltages I observed with a tube rectifier upon initial startup and checkout, and measured after putting tubes in the amp:

12AX7 plate #1 (pin6) 190V
12AX7 plate #2 (pin1) 198V
12AX7 cathode #1 (pin 8) (forgot to measure, better do that)
12AX7 cathode #2 (pin 3) 1.3V

6V6 grid #1 (pin 5) (oops)
6V6 grid #2 (pin 4) 342V
6V6 cathode (pin 8) 20.7V

I am getting ~340VAC from the transformer. I think these voltages are right in line based on my understanding of this design. I measured each resistor before it went into the circuit and they were all well within spec. I have no fast and easy way of measuring capacitors so I never do that, but each match the values in the schematic.

Heaters and filaments are where they are supposed to be (5.2V and 6.5V respectively).

My mains voltage is about 115VAC these days and I wired the xfrmr for 120V.

I believe I have good ground and get no observed oddball behavior by temporarily enhancing the ground in various places of the circuit.

Instead of simply lilfting the NFB or possibly in addition to using a variable or fixed resistor, what about the ability to bypass it with a say 20 uF polarized cap? That should give the power tube a boost. Maybe this is not where I should start though.

What else should I look at?

Would the type of capacitor make that big of a big difference? I know it can in general. I used a good quality silver mica on the volume control, and the bypass and tone caps are yellow tubular jobbies that look like Mallory 150s but are not labelled 'Mallory' so I assume they are some Asian knock-off but I could be wrong. I have some orange drops here that I could substitute but do not have 150s in the parts stash.


Barry

[MWJB]

Personally, & if you think the tone is "nearly there", I would perhaps try some CC/CF resistors in place of the metal films, say 1W CF for the preamp plates, 0.5W for 68Ks on #2 input, 1M #1 input & 220K power tube grid load. You seem open to the idea that caps make a difference in tone, why not resistors?

Admittedly, I've never actually built an amp with MF resistors throughout, maybe they are the cause, maybe not...it's just every amp (except Cornford) I look into has CC or CFs in it?

Also try a hotter rectifier (GZ34) or the hi voltage B+ secondary taps, this will push a bit more current through the 6V6, I should think you're only getting low/mid 30's mA on the 6V6 at the moment. A bit more B+ wouldn't go a miss anyway.

[xFallen]

I do think the tone is nearly there. Clean and onset of breakup is pretty darn good really, just not anywhere near my 5E3. That is probably not a fair comparison since the speakers are very different for starters, among other things. It could also be a taste thing, but I do have a '64 VC, 8" speaker and all, that sounds a much better than the 5F2a right at the moment.

Part of this may be the speaker. I know my Weber AlNiCo Sig 8S sounded kind of bad to me until it had about 15 hours of playing into it. Now it sounds clear and nice on clean and creamy when pushed. I'd recommend it and buy it again for a Champ.

But, for the current 5F2a, the bottom end is ratty with the amp under full duress and the highs are too harsh. I can't find a good combination of volume and tone control settingsd on amp or guitar to make it sound a lot better. There's also a sort of a wooly character to the distortion with what I believe are artifacts that I do not have the correct vocabulary to describe. This leads me to believe something is not quite right. I even changed the power tube yet again to a Ruby branded black plate 6V6GT.

The input and grid stoppers are already CF now -- sorry about that.

Okay, I changed the resistors. It was fast with the turret board. They are all now CF as I had no CCs on hand in the right values. No real difference except a tiny bit more hiss, and even then nothing objectionable nor anything I would call bad or even have probably noticed had I not just had the CFs in there and listened to it just minutes ago. I can't really hear much of a tonal difference, if any. I think I would have to A/B them which is a bit difficult to pull off here I suppose.

I have a NOS 5AR4 here somewhere I think. Same as a GZ34 right? Also have various NOS 5V4 and NOS 5U4GB but I have not looked at what they might do in this circuit. I already am using the 660V secondaries (choices are 600VAC or 660VAC for the Weber 022772 PT unless I mis-understood your suggestion). I did switch back to the Weber copper cap which I think should provide a bit more voltage than the 5Y3 which yielded the above voltages -- haven't measured yet with the copper cap.


Barry

[MWJB]

What voltage are you getting on the 6V6 plate? (Sorry, misread your screen voltage as plate, as it was the highest V I could see).

"But, for the current 5F2a, the bottom end is ratty with the amp under full duress and the highs are too harsh. I can't find a good combination of volume and tone control settingsd on amp or guitar to make it sound a lot better. There's also a sort of a wooly character to the distortion with what I believe are artifacts that I do not have the correct vocabulary to describe. This leads me to believe something is not quite right." Agreed, this doesn't sound right.

I'd be surprised if it was the speaker (if it's working as intended), subbing for another would eliminate this.

Did you buy the turret board (proven layout) , or make it? Any photos that might help give us pointers?

[MWJB]

Also, given I thought your screen voltage was your plate voltage, stick to 5Y3.

[xFallen]

What voltage are you getting on the 6V6 plate? (Sorry, misread your screen voltage as plate, as it was the highest V I could see).

"But, for the current 5F2a, the bottom end is ratty with the amp under full duress and the highs are too harsh. I can't find a good combination of volume and tone control settingsd on amp or guitar to make it sound a lot better. There's also a sort of a wooly character to the distortion with what I believe are artifacts that I do not have the correct vocabulary to describe. This leads me to believe something is not quite right." Agreed, this doesn't sound right.

I'd be surprised if it was the speaker (if it's working as intended), subbing for another would eliminate this.

Did you buy the turret board (proven layout) , or make it? Any photos that might help give us pointers?

The 5F2a board came from turretboard.com. I also used their 5E3 board. I stuck to Weber's layout which is really pretty much the original Fender layout.

Here's a shot pon the bench as I was testing the circuit and swapping out the Weber supplied e-caps with Sprague Atoms, and before final wire routing was done.



And one after it was about to be buttoned up. I thought I had 8uF Spragues but as it turned out I didn't have anything better on hand than what was supplied, so they stayed put. I may replace them on principle but they should be fine, I guess. Never heard of "Tung" caps prior to this build.



My camera battery died so I can't take more, but maybe later if needed. I also put the MF resistors back in. They make it quieter but don't seem to adversely affect the tone (to my ears).

I realize looking at these pictures that those OT wires should probably be trimmed back and redone.


Barry

[xFallen]

I did actually find 8uF Sprague Atoms in sufficient voltage and installed them. I also check for any cold solder joints or questionable connections. Nothing stood out although I changed a few for good measure.

With the Copper Cap rectifier in place I get 407V at the plate and 370V at the grid of the 6V6GT. Voltages at the 12AX7 are up by a bit too.

3 - 407V
4 - 370V
5 - 0.0V
8 - ~13V

With the 5Y3 I am sure I measured ~20V at the 6V6 cathode when I first tested the thing after it was put together (at least that's what I wrote down) but now I am seeing ~12-13V. That's < 30ma which seems kind of low.

Since the circuit is cathode biased, I should NOT see voltage at grid 1, right? Or is there a hole in my understanding?

I am going to pull the board and check the wiring underneath although I think it should be fine.

I thought it was just batteries, but my camera is completely dead so no more pictures. The time of disposable electronics, I guess.


Barry

[xFallen]

I was seeing oddball voltages on the cathode of the power tube ranging from 12-20V. I turned the amp back on after checking things out and it was not working. A quick check of some voltages and I quickly shut it off. The cathode bypass cap was open and a visual check told me the smoke was let out. I will replace it with a 50V device rather than the 25V part that was in there. Not sure why this happened as prior to this the voltages were all decent except for the variance on the cathode.

I now believe this cap was doing strange things which messed with the measured voltage. Can't do anything more on this until late next week anyway.


Barry

[MWJB]

Keep the 6V6 cathode bypass cap away from the 470ohm resistor, this resistor gets hot and can cook the cap. Stand the cap clear of the board & resistor.

Looks like your wire from Vol control to V1, pin 7 is sheilded? If so, run it directly from the pot to the tube pin, not behind the board. Currently it's twice as long as it needs to be.

Wire from top of coupling cap at V1, 1 to vol pot should be short. Ditto wire from coupling cap at V1, 7 to 6V6 pin 5. Ditto wire from 68Ks to V1, 2 - your glass fibre board won't warp like a paper board (no reason to leave slack in case of warpage) so this wire can run straight from the exit point on the board to the tube pin.

I can't see a ground wire from the main & screen filters to a PT bolt, best not to ground these to the same point as the preamp.

You could probably shorten the secondaries to the rectifier?

[xFallen]

Keep the 6V6 cathode bypass cap away from the 470ohm resistor, this resistor gets hot and can cook the cap. Stand the cap clear of the board & resistor.

Makes sense.

Looks like your wire from Vol control to V1, pin 7 is sheilded? If so, run it directly from the pot to the tube pin, not behind the board. Currently it's twice as long as it needs to be.

Yes, shielded.

Wire from top of coupling cap at V1, 1 to vol pot should be short. Ditto wire from coupling cap at V1, 7 to 6V6 pin 5. Ditto wire from 68Ks to V1, 2 - your glass fibre board won't warp like a paper board (no reason to leave slack in case of warpage) so this wire can run straight from the exit point on the board to the tube pin.

These can all certainly be shortened although the wires really are not much if any longer than my 5E3, Champ or other similar amps.

I can't see a ground wire from the main & screen filters to a PT bolt, best not to ground these to the same point as the preamp.

It is grounded to the ground plate as per the original design, and the same as my 5E3 and Champs. There is a green wire above the second filter cap near the yellow tubular 0.0047 uF tone cap.

You could probably shorten the secondaries to the rectifier?

Yes, definitely. I could do the same with the OT also, and will. No need for the extra wire.

I suppose combining all of the above may resolve some of the issue, but I now suspect the cathode bypass cap was bad. I base this on the range of voltages read at the cathode even without similar fluctuations at other supply points in the circuit.

I also have some new 6V6s coming. The ones in my stash of spares are suspect.


Barry

[xFallen]

Just as a point of followup, I wanted to clsoe the loop on this build for now at least.

I replaced the 6V6 cathode bypass cap which had let the smoke out. The original device was rated for 25V which I thought was too low. I replaced it with a 50V device.

I also found a couple of wires that had questionable conductivity of all things. It turns out the wire was cracked beneath the insulation. The wire is cloth covered with a plastic insulation and is not real push back cloth insualtion. I do not know the origin of this wire but it sucks. It is unusually brittle. One little bend and it can crack or break.

I replaced the junk Chinese 6V6 with a recent production EH and selected a low microphonic Sylvania 12AX7 to replace the junk Chinese preamp tube. This all made a lot of difference. The amp still breaks up very early but does have a more pleasant overdrive than before. The Chinese preamp tube actually is not bad with the EH 6V6 at lower volumes, but the amp breaks up far sooner and more harshly with the Chinese glass.

The amp is very quiet, just like it was prior, but now tonally seems to be a bit more like what I was expecting. It certainly doesn't blow my socks off like my 5E3, but it is decent and more closely approximates a loud Champ, which is basicly what it is I suppose.

I am still not impressed with the NFB circuit (it is back to stock) and may address that at some point if I don't end up just selling this amp. It would make a great amp for the livingroom player as it gets nasty without rattling the windows too badly (it is till pretty darn loud though), or plays clean at listenable levels.


Barry

[xFallen]

After installing the cathode bypass cap, the amp seems to beahve better. It also sounds better with a 5Y3 rather than the solid state copper cap. I suspect this is due to slightly lower voltages with the 5Y3.

A couple of tings. The distortion is better but is still a little bright. The amp sounds decent, but just doesn't have the character and depth that I find in my 5E3 and my BF VC.

The bottom end is nowhere that of the 5E3, even when I play the 5F2a circuit through the 5E3 cabinet and 12" speaker. I now know that the 10" Weber is decent but not as nice to my ears as the C12Q that came in my Mission 5E3 cabinet.

On generally lower notes there is also a bit of fizzy/fuzzy/wooly distortion that is definitely in the signal path and is an artificat, not a product. This can sometimes happen even at modest volume settings. I may be using the wrong terminology, but by that I mean it shouldn't be there. It is also somewhat intermittent. I do not believe it is chassis or cabinet buzz (had and resolved that issue already a few times). I have been over the wiring any number of times and am quite convinced all connnections are physically and electrically secure. I have checked sockets and tubes. Could be there but am not yet convinced as I have changed the tubes out to other amps and they are fine in them.

Anyway, I am wondering what the best bang for the buck modifcation might be to smooth out the distortion a bit and get a bit more bottom end into the output. The 5F2a circuit is at stock specifcations as it stands with a Weber OT and PT, stock value Sprague electrolytics, Mallory 150s, and a 500 pf mica cap in the tone stack.



Barry