Twin Reverb UL hums even when all controls are set to 0

[mogazi!]

Ok, so i finally got myself a soldering iron and multimeter and started working on the amp.

Here is what i did so far:

• Replaced the two blue caps in the doghouse (that had been previously changed for wrong value ones)
• Replaced the burned tube socket and a couple of burned wires connected to it
• Replaced the 470Ω resistor on all the sockets (except on V9 where the value was still right)
• Replaced the 1.5kΩ resistor that was on the burned socket (but left the other ones since their value seemed ok)

I think the buzz was reduced after these operations but still present when I turned the master volume up even with normal and vibrato channels to 0.

I had a look at the little board behind the jewel lamp that Lowell mentioned but the resistor values seem to be more or less correct. Not entirely sure why but my multimeter goes up to their actual value rather slowly (gradually). I'm assuming it's because it's a high value or maybe something related to their position. Right?

I tried the exclusion method and found that the buzz disappears when I remove V4. Any ideas about that? (I've already tried different tubes in that position to make sure it wasn't a bad tube)

The other rather concerning noise which remains even after removing V4 is a sort of very faint internal buzz (like a fridge) mostly noticeable when putting your ears close to the amp. It seems to come from the transformer area or at least that side of the chassis. It starts as soon as the amp is turned on and stays the same whether in standby not. It is not affected by volume or other pots. When the chassis is removed you can also feel a slight induced overall vibration along with the buzz (again, like a fridge).

The previous owner told me that he had the transformer replaced about a year/year and a half ago so technically the transformer should be new but the amp has been sitting unused for quite a while since then.
Any ideas on this?

[pdf64]

'The other rather concerning noise which remains even after removing V4 is a sort of very faint internal buzz (like a fridge) mostly noticeable when putting your ears close to the amp. It seems to come from the transformer area or at least that side of the chassis. It starts as soon as the amp is turned on and stays the same whether in standby not. It is not affected by volume or other pots. When the chassis is removed you can also feel a slight induced overall vibration along with the buzz (again, like a fridge).

The previous owner told me that he had the transformer replaced about a year/year and a half ago so technically the transformer should be new but the amp has been sitting unused for quite a while since then.
Any ideas on this?'
Unless the transformer is getting extremely hot, don't worry about this.

'I tried the exclusion method and found that the buzz disappears when I remove V4. Any ideas about that? (I've already tried different tubes in that position to make sure it wasn't a bad tube)'
Like Sweetfinger says, it's most likely a bad plate resistor. Looking at your 1st pic, see the V formations of brown/black/yellow/silver chunky resistors. Try replacing the 6th resistor from the left. 100k 1 watt 500V carbon film.
Of course it's your call whether to replace amplifier critical components, but flameproofs for the 470 ohms and replacing the 1k5 and bias supply components is insurance for your amp.
Peter.

[txstrat]

I'd say the buzz (like a fridge) actually IS the transformer. Transformers do this more or less (mostly the cheaper ones but others as well). Sometimes you can hear it, sometimes not. Depends on the quality of the wire, the laminate, the winding job and if they got end covers or not. Did you ever own a cheap stereo? They do hum sometimes even when they are switched off. Same thing.

[lowell]

Mogazi,
It's good that the components in the bias supply measure right. However, I'd still replace them. There are just some components in an amp that should be replaced every so often as preventative maintenance. Especially if the sum of the components is only around $2, and it'll only take 5 minutes to replace them. If the diode in your bias failed, or the bias capacitor at that, you would not only be measuring and finding those bad but would probably at least need all new power tubes, if not worse. It's only 3 components and can save you lots 'o money in tube failure.

[mogazi!]

Hey lowell,
Thanks for the sound advice. I'll make a part list of the things you said and order them.

This is the board with three components located behind the Jewel lamp

If I'm not wrong these are three resistors. Which diode and Cap are you referring to?
Which ones are the bias feed resistors and bias supply?

[pdf64]

Ahh, well spotted, this TR model doesn't have the seperate bias board Lowell was thinking of, behind the jewel light.
Instead, the bias supply components are on the small board to the left of the power transformer, and capacitor board.
Your schematic is
http://www.schematicheaven.com/fende..._135_schem.pdf
To the rh side of small board there's a 'square' arrangement of diodes and caps. That's the bridge rectifier for the B+. The 1st bias resistor and diode is just to the left of that square. Best to replace them.
Cap board, the dark blue caps were the bias supply caps which you've replaced anyway? The resistor there should have been 2k2, according to schematic, but is 1k2 in your pic. That resistor or the 1st one can be replaced by different values to getting suitable bias settings, so we can look at the suitability of that later and replace it with same of different value, as appropriate.

Finally, to rh end of main board are the 47k and 68k bias feed resistors to the power tube grids. These should be replaced also.
But do the 1k5 ones on the power tube sockets before those, as they get all the heat and wiggle stress from the power tubes.
Peter.

[lowell]

oops my bad, thanks peter.

[phydauex]

I'm working on the same amp right now. That big power resistor is to drop some voltage before the reverb circuit (primitive regulator) gets super hot. The two 100k carbons are the 2 resistors that bypass the 100uf/350v caps in the first stage. the red wires connect them to the doghouse supply. wires running everywhere. Check the integrity of that big resistor. I'm going to replace it with a 25 watt metal body power resistor that I can connect to the chassis for a heat sink, plus the leads were broken inside of the solder joint.
Matt