MOjo 5F1 - New build - Instant high frequency pitched feedback/hum

[RJM101]

Hi All, New to this and I have searched the forums and garnered a lot of good information. I have not found an exact match to the problem I am having so I am posting. First, I am mentoring a student for his senior class independent study project which is building and learning about tube amps. I have had some experience play with amps for the last 30 years or so but never getting into it as much as this project has alowed me.
Now for the problem...We applied power today and once the tubes heat up we get a high pitched hum or feedback. This happens with or without the guitar plugged in. The guitar signal does go through and come out the speaker. If we turn up the volume the speaker makes a loud low-to mid frequency hum and virbrates like it is going to come apart.
the Tubes are 5Y3GT, 6V6S and 12AX7
I get 415vac at the 1st power cap
261vac at the 2nd
224vac at the 3rd
I got 20vac at the connection between pin 6 of the 12AX7 and the .022uf cap. I think this is low. I did not check the other .022uf cap.
I am not where the amp is so I can not take any more readings but am curious if these initial voltages are in thr proper range.
Also the spec sheet that came with the output transformer told us to reverse the connections ie blue wire to B+ and Red to the 6V6 plate, could this be backwards?
Finaly, I know that each stage the signal goes through inverts the signal 180 degrees, does the transformer have any effect on phase or do I get the same phase at the out put that I get at the input of the transformer?

I should have some pics later in the day...that I can post, to check resistor values, etc...

Thanks for your help.

[JHow]

...Also the spec sheet that came with the output transformer told us to reverse the connections ie blue wire to B+ and Red to the 6V6 plate, could this be backwards?...

Reversing the OT connection seems to be a common mistake on these SE circuits. It will produce oscillation, which is what it sounds like you are describing. It is easy to switch the leads and see if that helps.

[JHow]

...I get 415vac at the 1st power cap 261vac at the 2nd 224vac at the 3rd
I got 20vac at the connection between pin 6 of the 12AX7 and the .022uf cap. I think this is low. I did not check the other .022uf cap.
I am not where the amp is so I can not take any more readings but am curious if these initial voltages are in thr proper range.

Do you mean that these are DC voltages instead of AC? After the rectifier you should be looking at DC voltages on the plates and cathodes, etc.

[RJM101]

You are correct Jhow. Those are DC voltages on the caps.

[JHow]

In general, your voltages don't sound way out of line. Not sure if you meant to say you had 20V on pin 6 (plate) of 12ax7, however. That would be way low. If true, check the value of the dropping resistor (is it really 100K Res?). For comparison, here are some DC voltages from my 5f2a kit. I listed these by pin and function, although it is probably most helpful to identify function - then you can tell right away if something is amiss:

12ax7
1 - plate 162
3 - cathode 1.41
6 - plate 166
8 - cathode 1.45
6v6
3 - plate 356
4 - screen 315
8 - cathode 19.5

Power supply
B+ 370
B++ 315
B+++ 270

My build used a weber W022905 OT, and I do have blue to the plate and red to the B+, The PT is weber W022772 300-0-300 putting out about 307VAC unloaded.

If you search on this site for 5F1 or 5F2 builds with problems, you will see many instances of people getting the OT hook-up reversed. There are also a lot of instances of people having issues with the nfb loop, with the secondaries reversed, taking the nfb off the wrong point etc. And, of course, lots of complaints about hum.

Not to say that these are necessarily your issues, but just common issues I have noticed on the board.

[RJM101]

Thanks for the info JHow. I am traveling on business till Friday evening so I wont be able to get more of the voltages or try swapping the output transformer primarys. I think I will try removing the neg feedback loop first to see what happens.

BTW, Is there a way to tell (from a measurement) which lead of the primary should go to the plate and the other to the B+?

Also, is there any inverting of the signal when it goes throught he output transformer?

Could it be that the amp is experiencing positive feedback?

I checked all of the resistors and believe they are all correct based on the color coding...

Not sure how to get the picture of the circuit board into this thread...


Thanks!

[RJM101]

Here is a pic before the final wiring was completed... I will try to post one with all the wiring as soon as I can.

Thanks.

[captntasty]

Could it be that the amp is experiencing positive feedback?

Ding, ding, ding... we have a winner! You can either remove the NFB in which case your amp will have a "hotter" output or reverse the OT leads (probably blue and brown). Either will give you the answer... I'd put money on this as the problem... I've had the same symptoms on a 5F1.

[RJM101]

Thanks Captntasty ... I will see if the student can reverse the wires on the OT while I am out of town and report back on the results...

[RJM101]

Captntasty, can you describe what happens to the signal if the OT is wired backwards? Does this invert the signal so that it would appear as positive feedback to the cathode of the 2nd preamp stage?

[captntasty]

As I understand it, is that when the signal we tap for the feedback loop is in phase with the point at which we inject it back in it is additive... so we add signal then feed that back in adding more, adding more, adding more ad infinitum. This creates a oscillation circuit... what we are doing in the negative feedback loop is sending a signal that is out of phase thus cancelling out some portion of the final output - subtractive... thus taming a portion of the final signal by means of super-imposing an attenuated version of the original signals' opposite (this can be varied by changing the feedback resistor - typically a 22k in the 5F1 I believe).

[RJM101]

So, by reverse wiring the OT primary we cause the signal at the secondary of the OT to be inverted thus cuasing the positive feedback?

[captntasty]

That's where you are now... feedback signal in phase with original signal - positive feedback = oscillation/squealing. Reverse the OT leads so signal is out of phase - it becomes negative feedback - this smooths the tone by phase cancellation. If you were to remove the feedback loop you could wire the OT either way - this would change the final tone as there is no NFB but your squealing would be gone.

[RJM101]

Thanks for the info. I will reverse the leads. Just trying to better understand the function of the OT and the windings and the effect on the signal... with out doing the math

I will post the results or progress once I get back into to town...

[CharlieP]

Yes, reverse the two outside primary wires leaving the center tap as is.