Fender 65 Twin Reverb

[bsco]

Got a question for somebody......I have this annoying backround hum and it is not the tubes......looks like it is not power supply related either.....however, when I scoped the bias pot wiper arm, I get a little less than 1 volt AC.....isn't this bias line supposed to be clean??....before I start tearing everything apart I just wanted to check here first......Schematic attached......Fender 65 Twin Reverb Schematic.pdfFender 65 Twin Reverb Schematic.pdf

[Mick Bailey]

The bias line has to be clean otherwise any noise will be injected onto the output tube grids. The supply diode could be leaking under operating conditions. A multimeter check may not show up reverse leakage, though.


What does the waveform look like?

[bsco]

It looks like a sawtooth.....I also noticed that when I pull the phase inverter, the hum goes away but it does this with another 12AT7 installed as well...so it is not the tube...I also checked the resistors to see if any have drifted but they check fine....there is a filter cap...(C36)... connected from the wiper of the bias pot to ground.....I am thinking that might be the problem....the hum does not increase in volume...it stays at the same level and can't be heard if you play....when you stop playing then you can hear it....
Cheers

[Leo_Gnardo]

As Mick said the bias supply has to be clean. I hope somebody isn't trying to make up for a failing bias filter by sticking a second filter on the adjustment pot. Nothing wrong with a second filter, I've done it for customers who want a background noise minimized amp. Make sure the main bias filter is OK, heck a cheap cap and a minute's work, there you are, good for the next 20 years at least.

Also consider the possibility of a shorted choke in the HV supply. Had one in an old Marshall 100W amp a couple weeks back. Measured about an ohm and a half - I knew that couldn't be right. When I subbed in another choke, hum and gargling notes cleared up right away. Never saw a choke do that before - there's always a first time. So stick an ohmmeter across your choke & if it's below @ 100 ohms by a lot, swap it out.

[The Dude]

You might also try removing other preamp tubes. By pulling the PI, you are effectively removing all of the other preamp tubes from the chain by interrupting the signal path from the input to the output. Your problem could actually be back further. I would think if your problem was with the bias circuit, the noise would still be there when you pulled the PI.

[Jazz P Bass]

Stick your meter on the speaker leads, set it to read Vac & tell us how much hum is there.

4 mv's is about the best you will get.

8, 9 10 you have a problem.

[bsco]

Thanks for this added info guys. Leo....this cap that I am referring to is actually stock...there are two caps in the associated circuit...a 100uf at 100V and a 22uf at 63V...the latter one is connected to the wiper arm of the bias pot and it is here that I get this sawtooth type waveform...as far as the choke goes, I scoped both sides....the input side of the choke has some ripple but the output side is smooth.....hey Dude.....I had done as you suggested earlier.....I pulled all the pre amp tubes one at a time and when I got to the the PI the backround hum disappeared.....Ok Jass P....I'll check that out...pretty sure I checked that but can't remember what the reading was...I will check it again...and let you know......and Leo....I will do a ohm measurement on the choke as well but I did not see any hum there with the scope.......Thanks guys....
Cheers,
Bernie

[g1]

So the sawtooth/ripple is at C36 but not C35?
Then I would expect the source to be something coming in at the grids, not the bias supply itself.

[The Dude]

^^^^^^Yes, what g-one said. My logic here is that pulling the PI made the noise go away. If it were the bias supply, one would think the noise would be there even with the PI removed. Are you possibly biased so hot that a small noise is amplified that much? Where are your tubes biased? Maybe you just have the thing setup too "hot"?

[bsco]

Thanks Dude....The bias is set at -45V at the test point....The cathodes of all four tubes are grounded....there are no one ohm resistors there.I was thinking of putting two 1 ohm,1w 1% resistors in.....one for each set of tubes....Jass P....the mV reading at the speaker is 17.....I knew that I had measured something there earlier but couldn't remember the amount.......but did realize it was too much...I will take another look tomorrow and see what could be causing the noise to appear at the grids of the output tubes...and install a couple of 1 ohm resistors in the cathodes of the output tubes.....Thanks for the replies......
Cheers,
Bernie

[The Dude]

10-4 Bernie. It would be helpful to know the bias current of the tubes rather than a somewhat arbitrary bias voltage. You may want to invest in or build a bias probe. They don't really cost that much and are very helpful for these sorts of things. It sure beats soldering in resistors all of the time!

[bsco]

Hey Dude...I was just downstairs sizing this up again....as long as the PI is in circuit, the hum is there...all other pre-amp tubes are taken out......I did notice that when I measured the voltage at pin 7 of the PI it read 62VDC and at pin 3 it was 56VDC....funny, when the meter probe was placed on pin 7 all you could hear in the speaker was a very low crackling or static type sound when the meter lead was connected to that pin....however, when the meter lead was attached to pin 2 of the PI, there was a fairly loud thump in the speaker every time the lead was connected and I also picked up one of our local radio stations while measuring the voltage........I am going to add a couple of cathode resistors to the output tubes tomorrow morning and I'll let you know what the bias reading is.....and I'll do a search for some info on bias probes....
Cheers,
Bernie
P.S. with the PI pulled, the ACV across the speaker is 4 mVAC...with the PI installed it is up to 17mVAC

[g1]

Have a look at the ripple at point Y of the supply, or the PI plates.

[bsco]

Ok g-one...I'll do that tomorrow morning and let you know what I find....

[bsco]

OK. Back again.....here is what I found......the supply line for the PI tube has .2V of ripple if that.....Installed two 1 ohm resistors and the bias is 117mV on one side and 119mV on the other side....can't get it to go any lower than that......plate voltage is 447V....that's 53Watts of power dissipation for 2 tubes....a little over 25 watts per tube...and that hum is still there.....let me know your thoughts....
Cheers,
Bernie
P.S. I noticed that as you increase the bias, the hum gets more noticeable...