Announcement

**RWood** · 08-01-2011, 08:36 PM

Pretty common heater technique for early-ish Fenders but it has nothing to do with your amp's symptoms. It is more prone to heater hum than a twisted parallel pair to each socket, which is a better practice.

I don't follow your diagnosis on the PT, which if bad would give you no sound on either channel and no heater voltage supply either.

**64deluxe** · 08-01-2011, 10:32 PM

My attempt at a diagnosis was based on a recent incident with a Super Reverb that was getting nothing on the first channel and turned out to be missing AC on one side of the first preamp because of a bad solder joint. That amp still functioned on the other channel. I know this is a different case in that I was seeing no voltage on one side of all the tube filaments.

It turns out the problem was an incorrectly placed underboard wire. Works fine now, although it does hum a lot. The oscillations are directly related to the Presence pot and I will troubleshoot that now.
I think I will delete this thread if I can because I was so far off the mark initially.

**RWood** · 08-02-2011, 03:00 AM

Nah, let's get your amp working the way it should. That's what this place is for. And Blondes do not like to be ignored.

So far you've got hum and you've got oscillations, possibly unrelated but who knows at this point. You've narrowed down the oscillations to the Presence control, which works off the Negative Feedback line from the OT secondary. So, this begs the question, has the OT been replaced? If so, the plate wires might have been reversed and what you really have is Positive Feedback (not good). So lets rule that in or out. If it is a replacement OT, try swapping the plate wires on pin 3 of each power tube.

After that, if you still have osc, a good chopsticking of that circuit, tapping firmly on each solder connection on the pot, the ground connection from the cap, everything in that part neighborhood, might reveal a problem. If in doubt reflow the joints. Using the chopstick to gently reposition the wires that effect the oscillations might cure it, too.

Let us know what you find out. If it still hums after you fix the osc, we'll tackle that next.

RWood

**Enzo** · 08-02-2011, 03:01 AM

No, don't delete anything. other people servicing similar amps may benefit from the discussion. So many people leap to the conclusion that their transformers are bad, when there is no evidence to that effect. Finding out that what seems like funny voltage readings can come from wiring problems instead of transformer defects, would be valuable to many people. That saves them from ordering a new, expensive transformer, and installing it to find the same problem remains.

**64deluxe** · 08-02-2011, 03:36 PM

Thank you, brother for your encouraging advice. This blonde is now having more fun after I followed your advice and switched the plate leads.

It was a Hammond replacement OT I installed. The oscillation always occurred when I adjusted the Presence knob and it is gone now but any effect that knob is having is very slight to my ears. It is the original pot and blue Mallory .1uf cap and I replaced the resistor with the correct value. Something definitely happens when I turn the knob but not a whole lot.

Quick history of the amp: Very late '64 6g6-B [NG on tube chart]. Total hacked disaster, only salvageable parts the PT, preamp sockets, some pots, circuit board and chassis. I carefully cleaned and saved what I could and then rebuilt the whole thing with new caps and resistors.

Regarding the hum, I have chopsticked it to death but no sign. I was meticulous with my lead dress but I replaced the grid wires with shielded cable grounded at one end and heatshrinked at the other. I had seen this on silverface amps and I have done it before to silverface amps that didn't have it with great success.

When I grounded the wire to the BASS channel treble pot it had a cap/resistor combo to ground so I grounded to the ground on the volume pot, along with the ground to the volume pot's shield. My logic tells me this should not be a problem but it is the one exception from my previous experience with using shielded wire.

The hum increases from about 2 or 3 on the dial and virtually vanishes around 8. Same on both channels. If I dime both it is dead quiet.

When I flailed around about the heater voltages, I removed the 2x 100 ohm resistor virtual center tap and regrounded the heater centretap from the transformer. I am going to put the virtual center tap back and see if that makes a difference to the hum.

Other than the hum and a wimpy Presence pot it sounds pretty damn fine! I made it exactly stock and was concerned the BASS channel would be too "bassy" for guitar but with a Strat through it it sounds superb.

So I appreciate your ongoing help with those two issues so we can fully resurrect this extraordinary amp.

**64deluxe** · 08-02-2011, 04:37 PM

Thanks Enzo, as per my reply to RWood I appreciate the encouragement and will keep the thread going. I know how valuable other threads have been for me.

I am grateful that after my first complete rebuild i only have a few minor problems.

**RWood** · 08-02-2011, 05:55 PM

Glad to hear you cured the oscillations - check that off the list!

Regarding the Presence, it might not be your amp, it might be the circuit itself. Others chime in, but I have worked on two Brownface amps in the last year with this same Presence scheme and neither one did squat for me. In defense of the circuit, I will say that the values in a presence circuit are chosen based on which OT tap is used (2Ω, 4Ω, 8Ω etc) and both of my examples had non-original OTs; yours does too. I believe the originals only had a 4Ω secondary.

Originally posted by 64deluxe View Post

When I grounded the wire to the BASS channel treble pot it had a cap/resistor combo to ground so I grounded to the ground on the volume pot, along with the ground to the volume pot's shield. My logic tells me this should not be a problem but it is the one exception from my previous experience with using shielded wire.
The hum increases from about 2 or 3 on the dial and virtually vanishes around 8. Same on both channels. If I dime both it is dead quiet.

So when the treble control is out of the circuit, no problem. Good info, and here's what I would try. Instead of grounding the volume and treble on the pot casings, run a wire from the pot's ground terminal to the ground point of that tube's cathode resistor / bypass capacitor network. Ideally you want all of the grounds from a particular triode to be in the same place (filter cap, too!). On this amp, see on the layout how the Bass channel's input tube grounds are together in a star, on the right side of the board? Go there with your control grounds.
Then the Normal channel's grounds are in a second grouping towards the middle of the board. Go there with that channel's tone and volume controls ground connections.

It's okay for the pot casings to be shielded, and they will be here, but I would not build an amp using that as a circuit ground. Too many different ground points invites ground loops, and that is what we hear as hum.

Easy enough to try - no parts count!

RWood

**64deluxe** · 08-03-2011, 04:30 PM

Superb advice!

I did as you suggested and the BASS channel was great but the NORMAL still hummed. Then I realised that the volume pot on that channel was still grounded to it's own casing so I unsoldered and bent the lug back up and grounded it with the other grounds on that channel and that did the trick.

So it is now virtually hum-free and I can hear the Presence control now and it is all in all a truly amazing amp. Now I know why people are so hot on these blondes!

Thanks for all your help.