Crazy 5f1

[longo3]

I built a 5F1 with a switchable 25 uf gain cap on primary of the 12ax7.
The problem is the weirdest thing, when I plug in a guitar with a 250k volume pot there is hum.
With a 500k guitar volume pot at half volume there is the same hum.
The amp sounds great through a celestion 10 but the 250k hum is crazy.
Has anybody ran into this?
I took the volume and tone out of circuit no hum 2 different guitars though has to be amp input resistors or coupling cap I think


Les paul 500k .22 cap

fender strat 250k .47 cap
Ken Long

[loudthud]

Let me guess. You used some kind of magical star ground with a bunch of spade lugs on a transformer bolt. Post a layout showing all the ground connections including this "switchable 25 uf gain cap on primary of the 12ax7". What kind of input and speaker jacks did you use?

[longo3]

I actually point to point wired it like a 5 c1 ( no circuit board no star ground ) . speaker jack is in the wood off the cassis.
I used a schematic that had a switchable 25 uf gain cap on pin 3 of 12ax7.
guitar with no volume control ( pickup strait through rocks no hum ) and the humbuckers with 500k volume at full volume.
its a very dull hum
Ive tried 1 meg volume pot 500k and 250k all do it.


strat does it worst but take its volume and tone ciruit out and its gone .
jackson dont do it
dot dont do it
warlock dont
nothing with a 500k volume pot

[longo3]

Got it

I recieved an email from another post to remove the death cap if I had a 3 prong cord and wallah no hum anywher on volume silent on 10 . It was part of schem so i put it in but the 3rd prong wasnt on schem. I gues we only need the "death cap" if we are useing 2 prongs.

[Bruce / Mission Amps]

Although your actual build is pretty helter skelter... I really can not imagine why that buzz cap, one way or another, would have made any difference what so ever in your hum problem.

[longo3]

dont know either

I dont know either but now when I set palm on bridge or touch the strings all ground hum is gone. before touching strings did not make a difference

[longo3]

I kow its helter skelter but I found an old tube radio and gutted it to tinker. allready had a 5c1 in there but went up to higher gain 12ax7

[longo3]

[longo3]

it might have been in my head other guitars still do it half volume on the guitar.a faint hum

[loudthud]

A couple of things you can do to clean up the wiring. There are two wires that connect the heater supply (6.3VAC) from the 6V6 over to the 12AX7. Replace those wires with a pair that are tightly twisted together and route them up in the air away from other wires and components. You can use a smaller wire.

There is a red wire that goes from the volume control to the 12AX7. Keep that wire away from the heater wires. Route it down against the chassis. The same goes for the wire that connects the junction of the two 68K resistors to the 12AX7.

I don't see the 22K resistor that goes from one side of the speaker back to the 12AX7. Also, I don't see a ground connection to the other side of the speaker. These shouldn't affect the hum issue but can cause other problems.

[longo3]

got those

I twisted the pair of heater wires and made sure positive stayed with positive pins.
I got the 22k resistor in the 0 feedback loop hooked up.
I noticed burned mark on 10 k replaced it with a 5 watt and the 22k in filter nodes went up to 3 watt ( it was crumbly).
the ground out of output transformer is ran to speaker jack if I hook it to chassis to i get a squell.
had a stroke 2 years ago and dont move much but im pretty proud of my circuit in the air.
there is no 60 cycle hum, amp is silent with your hand over strings on 10, and 10.
Its when I turn any of my guitars with a 500k volume pot and 500k tone circuit to half volume on the guitar there is a faint buzz.
straight pickups and guitars with only a volume knob do not create this buzz at half volume.
I wonder if I need bigger than 68k input resistors.
with the volume knob at half , half of the signal is shunted to ground? if so do I need to isolate the guitar ground through the circuit?
I invested in some wire everything else was done with on hand stuff im gonna try to clean up my helter skelter
I have it recorded not hitting a note just turning the knob up and down and right in the middle you can hear the noise.

longo3@yahoo.com

drop me a line ill send it and see if you heard this kind noise

[longo3]

one of those thigs worked but I did them all at once I believe it was the input wire
does the input wire have to be big I have sheilded guitar wire.
plus I have a classictone 40-18019
should I use zeners to get 340 or is 397 allright

[longo3]

replaced the mylar with mallory 150's nice oh yeah

[Bruce / Mission Amps]

Yes, the input wire from the jack to the first preamp stage is very sensitive.
The squeal was probably because you applied negative feedback to the second triode, correctly but the OT is wired or just wound out of phase with this build.
Try reversing the two wires on the OT primary (B+ and power tube plate) and that should stop... also I have had hums when the audio low (sometimes called ground) speaker lead is not connected to a chassis.
I would try to get the plate voltage a little lower, but where it is, well it is not the end of the world.

added:
By the way, one of my favorite odd ball builds was an old S-38 Hallicrafters, table top, radio receiver which I gutted and turned into a 12 watt head with a pair of reissue Princeton Reverb trannys, SS rectifiers, two cathode biased 6V6s and a pair of octal 6SL7s!! Left the greenish dial lights in and and the whole thing is flat black...loved that goofy thing.

[longo3]

I love gutting and the rebirth process.

22k resistor from 4ohm tap to pin 3 on 12ax7.

I did read on that reversed ot before but colors matched so i left it alone.

the ground from speaker output jack to chassis is what makes the squell