smzinno wrote:jla - I believe your speaker jack should be isolated from the chassis if you have a separate wire going back to the main ground as you mentioned.
I'm more a believer in running the main filter cap grounds to where the tube cathodes ground.
ferinstance. If C1 and C2 are the initial 40s then the negative end of C3 should terminate where the presence ground terminates, C4 should terminate where V2 cathode grounds and C5 where V1a+b cathodes ground.
I'll stress though that this most of the time doesn't make much difference at all. But sometimes it does.
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
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Thanks Dan for the information. It's good to know that I did it right because that is the exact ground scheme I used for this and every other amp I have built in the last several years.
I think my issues are minor and due to my perfectionist tendency. I can gig with the amp fine. It is generally a little noisy above 2pm, but at that volume so is everything else.
Also I gigged with the stock .0022 coupling cap and didn't like it. I prefer the .022 value, it has more tone because it allows more bass. The stock value is too chimey (thin) for my taste. I am going to put it back to my value this weekend.
Les
I wanted to post a follow-up to close this out for those having similar issues. First, my recommendation is to start by substituting tubes until you get the lowest noise floor. That was the ONLY thing that made a difference. What I was hearing was due primarily to the higher gain structure of the amp.
To save others time, here is what I tried and learned it was NOT:
1. Not the location of grounds. I used the grounding scheme recommeded in this thread and re-did it to match exactly my other builds just to make sure.
2. Component value changes. I changed the v1 to v2 coupling cap value from .0022 to .022 to increase bass response, it made a slight difference in the frequency of the noise floor, but no real change.
3. Location or orientation of OT. I moved it around with the amp turned on and it made no difference.
4. Using a DC bias to lift the heaters to 40 volts. At first I thought it helped, but realized I had changed a tube and the tube change was the difference.
5. Redress the power section of the amp. I cleaned up my lead dress, no change.
6. Redress the preamp section, I cleaned it up too, no change
7. Re-route the heater wire running to pilot light, no change.
What did make a difference was substituting different tubes. The quietest tube set-up was:
V1=GE JG 5751 or RCA 7025 (higher gain)
V2=RCA 7025
V3=RCA 7025
One thing about my build is I used some old Hammond organ iron with lower voltages. I think the lower voltages make the amp a better sounding amp for 6v6's than el34"s (IMHO). Also it really likes the .022 coupling cap instead of the .0022 (too thin) and it gets very broken up and has controlled feedback at about 11 O'clock. Past 2 O'clock the amp just gets wild and other than fainting Eddie Van Halen, it's not much use to me in a contemporary church service setting.
Thanks to all who helped me with teh build and the trouble shooting phase.
Les
Lifting the centre tap for the filament supply and running a 100 ohm hum balance pot (like those seen on the SF Fenders) works a treat on this design to help reduce noise. A good quality pot is important though, something like a higher power Clarostat model. I think Allyn may still sell them. I got a few from him a while back. Excellent quality part. Just dont' be tempted to use a standard Alpha pot though.
Apart from external lead dress problems, filament noise can be induced internally in preamp tubes as well. The hum balance pot really helps in this situation tame any 120Hz noise and even some 60Hz noise.
Hope this helps.
if you don't want to drill a hole to find this doesn't work, just hot glue the pot to the chassis and test to see if it helps. If it does, then drill a hole for it.
'67_Plexi wrote:Lifting the centre tap for the filament supply and running a 100 ohm hum balance pot (like those seen on the SF Fenders) works a treat on this design to help reduce noise. A good quality pot is important though, something like a higher power Clarostat model. I think Allyn may still sell them. I got a few from him a while back. Excellent quality part. Just dont' be tempted to use a standard Alpha pot though.
Apart from external lead dress problems, filament noise can be induced internally in preamp tubes as well. The hum balance pot really helps in this situation tame any 120Hz noise and even some 60Hz noise.
Hope this helps.
if you don't want to drill a hole to find this doesn't work, just hot glue the pot to the chassis and test to see if it helps. If it does, then drill a hole for it.
Good luck and have fun !
AES sells the Fender 2 watt replacement.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
I lifted the heater ground and installed the 100 ohm hum balance pot and ran the heater ground from the pot to the PT bolt. It did the trick with the hum, its gone. Now all I have is high gain hiss from my ccarbon comp resisters. Thanks for the suggestion, it's good to know it could be fixed.
Last edited by jlatrace on Fri Mar 21, 2008 3:50 am, edited 1 time in total.
I lifted the heater ground and installed the 100 ohm hum balance pot and ran the heater ground from the pot to the PT bolt. It did the trick with the hum, its gone. Now all I have is high gain hiss from my ccarbon comp resisters. Thanks for the suggestion, it's good to know it could be fixed.
Pardon my ignorance. I'm extremely glad that this forum exists and that it allows me to learn from you guys, so here goes:
Francesca has two resistors (100R, I suppose) soldered to the first tube the heater wires hit and grounded with the other end. I've seen that in a number of different amps so far. How is the 100R pot an improvement over that technique? The wiper of the pot goes to ground so you can adjust the resistance ratio between the two heater wires, right? My PT has no heater center tap...will replacing the two 100R resistors with a pot and leaving the ground connection where it is (one of the mounting screws of that first power tube) work the same way than using the heater center tap?
The reason I'm asking is that I do have the two 100R resistors in place but I feel like the amp is too noisy. The noise level might be normal, I don't have any comparison, but I'd like to lower it anyway.
You are thinkin' right Tobias. Some of us have had success lowering noise by going "off campus" in this area. Welcome and good luck.
Dan
tob wrote:Pardon my ignorance. I'm extremely glad that this forum exists and that it allows me to learn from you guys, so here goes:
Francesca has two resistors (100R, I suppose) soldered to the first tube the heater wires hit and grounded with the other end. I've seen that in a number of different amps so far. How is the 100R pot an improvement over that technique? The wiper of the pot goes to ground so you can adjust the resistance ratio between the two heater wires, right? My PT has no heater center tap...will replacing the two 100R resistors with a pot and leaving the ground connection where it is (one of the mounting screws of that first power tube) work the same way than using the heater center tap?
The reason I'm asking is that I do have the two 100R resistors in place but I feel like the amp is too noisy. The noise level might be normal, I don't have any comparison, but I'd like to lower it anyway.
Tobias
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
I too put in a hum balance pot this weekend. Helped to drop noise just that much more.
With a bottom plate, good tubes, shielded V1B grid wire and a 15k resistor directly on the V1A grid my amps noise is now almost non exsistent till the master is over 3/4.
I did everything you did except the 15k resistor with good results. Can you tell me what impact the 15k had? Is it worth opening the amp back up to try?
jlatrace wrote:I did everything you did except the 15k resistor with good results. Can you tell me what impact the 15k had? Is it worth opening the amp back up to try?
Well others have put in 33k resistors to stop microphonics in V1 and RF coming in. 33k is standard on marshalls and many other circuits.
I believe skybolt put in a 33k and he said it was fine. Most/all people say it doesn't compromise the amps tone. I didn't want to take a chance so I just halfed the value and threw in a 15k.
It didn't kill ALL the microphonics but tamed them a lot, and it got rid of all the RF. I'm running a combo anyways (i just posted pics in another thread) and I had the V1 and V2 tube sockets shock mounted with some silicone rings. I also setup the V1a and V1b grid wires like on the Komet 60 so they were not touching the chassis.
The amp sounds great and will still go into controlable feedback as long as I put the guitar in front of the speakers and it's at a reasonable volume. But I'm really happy that its perfectly quite too.
I did install a Vox/Komet-style Cut control and a Komet fast/gradual switch on V1B triode. The fast/grad is very versitle but its harder to get her to get the controlable feedback in the gradual mode. The cut was good to match the amp with the brighter speakers I have.
So some/all of that may have contributed to lowering the noise floor. If I turn the cut knob up (I run it at noon like all the other controls and its quiet) it'll introduce a little more hiss.
