New 5F1 - Strange Problem - Low B+ with tubes plugged in

[wr7r]

I just built a 5F1 and powered it up and it kind of amplifies but mostly pops and sounds like static on an old shortwave radio. The B+ voltages seem to be low (336 vdc), but pop back up (395 vdc) when I take the tubes out. I originally suspected leaky caps and so have replaced all of the electrolytics, have swapped out the 25/25 bias cap. I have replaced the 10K and 220K voltage divider resistors with 1W (not in this picture). I pulled up an end of each of the .022 ufd caps to see if they were pulling it down. I have checked all of my grounds (ohmed them out with a VOM) I have gone back through and double checked all connections. I installed a replacement new OT ( i had a second one as I'm building 2). Now I am completely stumped!! I have swapped out both the 6V6 and the 7025. I have added the missing ground on the pot. I have replaced the pot. The only thing that hasnt been swapped is the 5Y3 and the PT.

Any help much appreciated -

I attached a short video clip where you can hear it doing it's shortwave radio impression. If you plug a guitar into you can hear audio but not very loud.


http://www.wr7r.com/pics/042.mov

My voltages are:

B+ 395
after 10K R - 321
after 220K - 230 vdc (without tubes installed)

with tubes installed:
B+ 336
after 10K 252
after 220KR 162 vdc


This gives me the following tube voltages: (without tubes installed)

7025 plate1 161 vdc
cathode 1 pin 2.55
plate 3 pin 6 145
cathode 2 pin 3 .85

6V6

pin 3 - plate 395
grid pin4 327
pin 8 cathode .4

[wr7r]

The latest - I replaced every *(*&^! part in this amp...now it does seem to amplify though not sure it is as loud as it should be...still some static crashing noise- I even replaced the pot, replaced literally very part - up'd the 470 ohm bias and 10K voltage divider resistor wattage - replaced all caps - electrolytics now 20/16/16@450 vdc. I swapped out the OT the last time, so the only thing I havent swapped out is the 5Y3 rectum-fryer. I am entirely baffled.....

my 7025 voltages are a bit off:
pin 1 - 122 pin2 - 208 pin 3 .8 pin 6-95 pin8 2.75

My 6V6 voltages are pin3: 314 pin8: 18.89 pin4: 305

Any ideas? I guess I will replace the 5Y3 as well but can't imaging that having anything to do with it....

[loudthud]

Your posts are a little hard to read. The only 220K resistor in the Fender 5F1 should have very close to zero volts on the 6V6 end and the other end should be grounded. Are you confusing it with 22K? Hard to see the colors of the bands on the resistors in your photo. Use a flash or much brighter light.

Pin 2 (and pin 7 in this amp) of a 7025 should also be very close to zero volts. Please verify these readings.

[wr7r]

The plot thickens -

Your posts are a little hard to read. The only 220K resistor in the Fender 5F1 should have very close to zero volts on the 6V6 end and the other end should be grounded. Are you confusing it with 22K? Hard to see the colors of the bands on the resistors in your photo. Use a flash or much brighter light.

Pin 2 (and pin 7 in this amp) of a 7025 should also be very close to zero volts. Please verify these readings.

I think I did mean the 22K - Well the plot may have thickened - I replaced every component save for the 5Y3 and the PT. I even swapped another OT. The 6V6 plate voltage is 390 vdc but sags to 330 vdc after it warms up. V1a cathode voltage/pin 3 is 1.1 vdc - should be 1.5. V1a plate pin 1 is 150 but sags upon warm up down to 130 (see youtube link)

The amp works somewhat but has some hiss and pop and no crunch.

V1b plate voltage pin 6 is 85 vdc instead of 150, V1b cathode pin 8 is 2.76 vdc instead of 1.5

At this point we considered something really strange. My son-in-law read abt someone else on this forum having problems with PCBs that were made from carbon fiber (really). So I did a quick check with a VOM and there are actually places on this PCB that have as little as 220K resistance from 2 locations just putting the VOM probes on the board itself. My son-in-law purchased these boards and chassis from Boothill (3 for gifts. So tonight I took a standard piece of phenolic PCB and etched the copper off and mounted turrets. I'm in the process of re-populating the board so I should know tomorrow what the verdict is. I can't believe someone would cut turret boards out of semiconductive material, but that might explain the wierd things going on.

[wr7r]

FIXED !!!

2 Problems - Noise coming from circuit boards that the vendor had apparently fabricated from what appears to be carbon fiber! I cut a piece of 1 sided copper PCB into ferric chloride and just etched all of the copper off and put turrets in. This got rid of all of the weird popping and hissing and left an amp that was very quiet but not very loud. Voltages were still off on 7025. Then as I was re-wiring the 7025 I discovered that pins 2 and 3 were reversed...Wow what an awesome amp, now working on a Super Champ...

here's a video of measuring the resistance of these boards that was as low as 200 K in some points across the board:

[telenut62]

The 470ohm cathode resistor needs to be 5watts.....if that little resistor is still in there, your heading for trouble.

[wr7r]

The 470ohm cathode resistor needs to be 5watts.....if that little resistor is still in there, your heading for trouble.

The cathode voltage is 19 vdc and the cathode current is 40 ma thus the quiescent power P=EI is .76 watts. I now have a 3 watt resistor in there and it's cool as a cucumber.

[telenut62]

The cathode voltage is 19 vdc and the cathode current is 40 ma thus the quiescent power P=EI is .76 watts. I now have a 3 watt resistor in there and it's cool as a cucumber.

Good, you'll be sweet, I'm just a traditionalist

[hack]

I am the Boot Hill Amps guy. I've checked all boards in my inventory. I have neither continuity nor resistance to report. I have checked from turret to turret with no components connected (of course). I have probed the boards directly with the same results. They are non-conductive like any board should be. No one else has presented a curious scenario such as this. Many kits have been sold to many happy customers with successfully completed builds.

In the video, the test is unclear. To wit: There are components and leads attached to the turrets on the board. The only way to prove that any resistance or conductivity exists on the board itself would be to test it with all components removed (such as I have done with all boards in my inventory since I was first alerted to this strange phenomenon by wr7r).

Could it be that wr7r rec'd a board made of carbon fiber? Sure. Anything is possible. Sure seems unlikely.

FWIW, I offered to replace parts that wr7r had used which had become damaged or otherwise unusable in his quest to find the gremlins in his 5F1 project.

He declined and requested a discount on another kit. I agreed and I offered him that courtesy.

I hereby suggest that wr7r return the questionable board to me for a replacement. I will then check it myself and

I will post my findings here.

[Enzo]

Having discovered the conductive eyelet board myself the hard way long ago, I understand how totally surprising it is to anyone who finally figures it out.

Carbon fiber? MAybe, but in my experience usually the problem is contamination. On the Fender boards, moisture absorbs into the film coating the board, some sort of waxy dirt. It is that which is conductinve, not the actual board. I have been successful cleaning them with some alcohol, then after it evaporates (fire hazard) I go back and hit it with a heat gun. I can see the line move along the board as the heat evaporates whatever residue is left. Kinda like the frost on your windshield in winter slowly evaporating as the defrost heats up. I have saved every one I encountered. And yes, even solder flux can be involved.


And hats off to you, hack, for showing integrity. I'd be interested in your findings too.

[loudthud]

The equipment I work on in my day job measures leakage in the picoamp range so leakage is delt with on a daily basis. Repairs to circuit boards require that flux be removed to have any chance of working. Occasionaly, boards have to be baked (30 min @ 150 drgrees F) before they will pass acceptance tests. I offer the following observations.

High humidity is the enemy of low leakage. The higher the humidity, the cleaner the boards have to be to have low enough leakage. If the boards have absorbed moisture, it has to be baked.

"No Clean" solder cannot be used. A guy ask me one time if "No Clean" means you don't have to clean it, or that you can't clean it. This type solder, once used, makes it extremely difficult to get the boards to pass. Surface mount components have to be removed and the board scrubbed for a successful repair. I use an alcohol or toluene based cleaner with compressed air.

Acid core flux and the paste flux intended for wave soldering (water soluable) should not be used unless you can wash the board in a dish washer. It usually requires two passes through the dish washer for the boards I work on.

On repairs, RMA flux (Rosin Mildly Activated) is the only type that should be used. RA flux (Rosin Activated) causes to much leakage or is very difficult to clean.

Moisture absorbsion can cause problems. FR4 PCBs don't have significant leakage from the material itself, surface contamination and bad components are the cause in 99% of the cases. There can be a non-linear capacitance or dielectric effect however.

The type of material used by Fender, sometimes called Fish Paper, can absorb moisture and it is said to cause leakage, but this just may be in combination with flux which usually isn't removed from boards in guitar amps. Mositure and any type flux is a bad combination.