5F2a help

[Bruce / Mission Amps]

It sounds like you have gone through it enough now that I wouldn't be surprised to find out that tiny Weber output tranny is bad.
I'd be happy to look it over if you want to send it out this way.

[xFallen]

Another oddity I noticed. With nothing plugged into the inputs, crank the volume pot to max. Very near the end of its sweep it starts to cause a crackling in the amp that rises in pitch as the pot is turned (slowly) until it becomes a consistent putt-putt-putt sort of sound. Turn the volume down below that point and it is fine again.

There's some oscillation in there I guess.


Barry

[MWJB]

"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."

"Very near the end of its sweep it starts to cause a crackling in the amp that rises in pitch as the pot is turned (slowly) until it becomes a consistent putt-putt-putt sort of sound."

Re a request for a mod, your last comment really confirms that there is a fundamental problem with the build, it's not just a case of being an amp you don't like. Therefore wait til this is definitely cured before trying any "mods".

The weber 5F2A usually comes with a 15W P-P Princeton OT doesn't it, wired SE? It's possible that this is duff, but I'm not sure it would be my first port of call.

[xFallen]

I figured as much and that does make sense. And yes, I believe the OT is reasonably stout.


Barry

[Bruce / Mission Amps]

I figured as much and that does make sense. And yes, I believe the OT is reasonably stout.


Barry

Just curious, but what is the part number on the output transformer?

[xFallen]

Hi Bruce,

The OT is marked W022913. Looks like a Champ-sized OT which I suppose makes sense given the circuit.

The PT is marked W022772.

I am about to change a few things in the lead dress such as optimize the lengths of a few wire runs and reroute a few things. After that I may be following up with you.


Barry

[Bruce / Mission Amps]

Hi Bruce,

The OT is marked W022913. Looks like a Champ-sized OT which I suppose makes sense given the circuit.
Barry

OK, not sure why they are using a push pull OT for a that amp but I'm sure anyone who has built SE amps professionally would tell you, it is the wrong OT for a single ended amp.
The 022923 is a very cheap, smallish, class AB push pull OT, not designed at all for SE class A.
See if you can find a better one.... one that is actually made for a SE Class A power tube, if you get my drift.
Hammond makes some better ones but if you go over to Satan's website, you can find a really good NOS SE OT that I've used for many years in the Soulkicker Jr. amp, a modified version of the SE tweed Princeton.
Check the very first SE OT for the Champ, Vibro Champ, Bronco:

http://www.angela.com/catalog/guitar...ANSFORMER.html

I think these are really superior to cheap Asian import iron, .... especially if you have the wrong kind installed in the first place! ha ha

[MWJB]

As Bruce says a dedicated SE OT is a great idea. However, I'd have to say that if the PP OT sounds good (and it does - it is not specifically related to Barry's problems) it's not "wrong", just different. Granted there's no air gap and again, for the builder there are plenty of other more suitable options both spec-wise & quality-wise. The PP OT might not be the 'ideal' but, (with the greatest respect Bruce), "wrong" is perhaps going a bit far.

I suspect they have gone this route as it was a Fender factory advised upgrade (along with an 8ohm speaker) for Tweed Princeton's with blown OT's. I'm not suggesting they went this route as it was the ideal, it was no doubt motivated by cost, but it perhaps explains why Weber followed suit. Would I have set out to do it that way? Probably not, but no harm in giving the PP OT a shot & upgrading IF you think it's necessary (we have to assume you bought the kit with your 'eyes open' in the first place).

For any of you who have built this amp and think that you now need to change your OT, feel free to, if you want to, but the PP OT won't stop the amp from working or sounding good.

I have A/B'd a Weber style 5F2A against amps with dedicated SE OTs... I'm still waiting for something to blow it away...

No particular affiliation to Weber and I agree with you Bruce about the quality of the OT, just saying the principle is sound enough.

[xFallen]

I have a Hammand 125 series here. I think it is a 125ESE but I need to dig it out and check. Would it be plug and play? Guess I'd need to check some specs too.

I am not sure I want to start with replacing the OT just from an expense standpoint given there's an oscillation of some sort. But, I could try an SE transformer I had on hand if it is going to make the difference, then go from there. Seems to me this oscillation thing needs to go away first.

Yesterday I noticed that as it warmed up there was a tone that started subaudibly and increased in pitch until it went away over the course of maybe 5-6 seconds. I do not hear this all the time, and the only time it has ever happened is when the amp has not had anything plugged into an input.

Timewise, it sounded like it might be correlated to the charging of the electrolytic caps.


Barry

[Bruce / Mission Amps]

"With nothing plugged into the inputs, crank the volume pot to max. Very near the end of its sweep it starts to cause a crackling in the amp that rises in pitch as the pot is turned (slowly) until it becomes a consistent putt-putt-putt sort of sound. Turn the volume down below that point and it is fine again."

******************************************************
It does sound like you have some kind of ultra sonic oscillation.
Not sure where that is coming from.
I've built MANY SE 6V6 amps over the years but it is hard to tell from written text what yours is sounding like or what is wrong.
However, if you have the 125ese... I'd still stick it in there when you get chance.
Set the output taps so it gives you around 5K-6K ohms with what ever speaker you have.

I have pretty strong convictions but I don't think "wrong" OT is too strong of an opinion... for best results SE amps require a SE OT.
Although I know it will work, I don't think any professional builder would go out of their way to use an Asian rim push pull OT in their SE amps.

Regardless Barry, I submit that you try to tweak or build your amp to sounds the best it can with that small Princeton Reverb PP knock off OT and then try a real SE OT and I think you'll be convinced.

[xFallen]

Got it. I think I'd like to try a different OT as per your suggestion at some point. The 125ESE should be a good fit.

The oscillation does seem like it should be indpendent of the OT and I should probably quash it first. Time has been a bit of an issue of late but hopefully I can dedicate some time to finding the oscillation. I already shortened all signal and power leads to their minimum and made sure the routing was okay at least as far as my (beginner) level of experience can tell.


Thanks,

Barry

[xFallen]

Update. Oscillation no longer present (mostly).

I finally took some time to put the 5F2a output on the silly scope. After some fiddling I found a way to reliably reproduce the oscillation.

From my crappy 'scope display I estimated the frequency to be around 137.5 kHz. I have a broadband DC to daylight receiver and sure enough I found the signal right around 138 kHz.

Long and short of it, it was a combination of stray capacitance and lead dress. Just moving my hand around and even theproximity iof my body to the amp wold cause the oscillation to start or stop, so comprehensive testing took quite a while.

In the end this is what did it:

- slightly shortened and re-routed the control grid wire form the board to pin 5 of the 6V6GT output tube
- re-routed the input jack input wires so they ran orthoganol to each other and to the grounds
- pulled the normal channel input wire up away from the chassis, while rerouting the high input close to the chassis (both were close to chassis prior)
- changed the preamp input leads orientation although that in the end did not make much difference (they were already reasonably well routed)

There are no oscillations audible or visible on the scope at any volume or tone/volume pot positions. PLaying the amp is much more enjoyable now. I stil.l prefer the tone of my 5E3 or my existing Champs, but maybe that's just the way this ciruit is, or, maybe it is the OT, speaker, or some other problem I have yet to detect.

There is still an oscillation present only at a certain time as the amp warms up. Even then, this only happens with nothing plugged into the inputs. Very strange. I can see it on the 'scope and I can hear it manifest itself in the speaker and then it goes off the 'scope. Since it only happens as the amps fires up it seems that it could be potentially tricky to diagnose. If it is a very high frequency oscillation it is beyond the capability of my 20 MHz scope to detect using standard probes.

Q: any pointers as to best way to try and resolve this one?

Touching my hand to the chassis, especially around the tone and volume controls, makes the amp dead quiet at idle. The ground plate is bonded to grounds on the board in three places. The control and input grounds are strung together then bonded to the ground plate. I cannot simulate what happens placing my hand on the chassis by better grounding. I have tried running temporary grounds to the transformer mounting lugs and directly to the chassis at various points and there was no change in sound.

Q: I have seen this in other amps commerical and otherwise. I have also seen amps that did not do this. Is this something I should try to resolve? It is not objectionable.

I can now hear microphonic interaction from the brand new EH 6V6GT at certain frequencies. Holding the tube makes them subside. The microphonics go away with a different tube. At least that issue is easily addressed.


Barry