'59 Bassman Clone Presence Control Problem

[airweld]

Trying to figure out a problem with the presence control on my 5F6A circuit. I built the amp from scratch using the layout (mostly) from Tino Zotolla's book, and it sounds great. I wasn't getting any change from the presence control, and on closer inspection I saw that I had neglected to make the connection from the speaker output jack to the 27K resistor of the Negative Feedback Loop (NFL). Ahah! I'm sure no one here has ever done that before . But when I make that connection I get a huge noise, no doubt due to the 15vdc supplied to the speaker from that point. This is my first experience with a presence control and I'm hoping that those here familiar with this 5F6A circuit can help me out. I have triple checked the wiring and everything appears to be right and the amp sounds great and all the other controls work fine. Is it typical to have 15vdc at the 27k resistor? Will try to attach a schematic for the original Fender Bassman. Thanks.

P.S. I have also built a "Two-Stroke" from Dave Hunter's book, and a "Micro Champ" by Adam Alpern (for a harp amp). If I can help anyone figure out something for those circuits.

BASSMAN_5F6A.pdf

[MWJB]

If you're saying that the presence pot is noisy when adjusting it, then yes, that is normal with the pot described in the original 5F6A schematic. As you have spotted there is DC on the pot. Later bassmans used the 25K pot & 0.1 uf cap in series, bypassed with 4.7K load resistor set up, as seen on the 6G12A Concert and the RI tweed bassman.

[airweld]

Hi MWJB,
I cannot make the connection from the output jack to the 27K resistor in the NFL because of the VERY LOUD noise it produces. The noise is independent of volume or presence control settings. It's not a noisy pot. Thanks for the reply.

[MWJB]

Then you have wired something wrong. 1st stop, switch the output transformer secondaries, usually green & black wires going to speaker jacks, if that doesn't work do you haveany any pics? Any voltages from around the PI & tail resistor?

[airweld]

Here are some voltages:
ECC82S PI pin 1= 244.v
pin 2= 16.v
pins 3+8= 45.5v
pin 6= 247.v
pin 7= 15.4v

at the connection of the 470R (pins 3+8) and the two 1megR = 44.2v

at the 10kR and presence pot = 15.1v

at the 27kR (that should connect to output jack) = 14.8v

the .1mfd cap connected to the wiper on the presence pot varies the voltage from 15.1v to zip when presence control is turned

I have three speaker output jacks from a Hammond 1750M, for 2,4,or 8 ohm output. Hammond 290DX PT.

JJ Tesla tubes ECC81 pre, ECC83s mid and PI., 6L6 GC power ,RCA NOS 5V4 GA rectifier

Hope that is helpful info.

[52 Bill]

I have three speaker output jacks from a Hammond 1750M, for 2,4,or 8 ohm output. Hammond 290DX PT.

I'd agree with MWJB, it sounds like you need to reverse the phase of the feedback line. What you have now is positive feedback and not negative feedback.

With that transformer, reverse the two primary leads going to the output tube plates and leave the secondary speaker winding alone.

[airweld]

Hi 52 Bill,
That was my first thought also, but I reasoned that if it worked w/o the feedback then it must be O.K. (ignorance is bliss). And I was being lazy because the lead dress of the OT primaries on the plates looks so nice the way they are now. But I guess that is what I must do in order to eliminate that from the list of possibilities.

[airweld]

Problem Solved

Well, thank you gentlemen for leading me to the solution. I swapped the OT primaries and voila, presence control now functions. I have always read that a howl when firing up a new build may be due to that, but I had already been playing the amp and did not realize the out-of-phase situation was due to the NFL part of the circuit. That's what I love about making mistakes; you get an opportunity to learn something new.

[bnwitt]

I had already been playing the amp and did not realize the out-of-phase situation was due to the NFL part of the circuit.

Never mix football with guitar amps.