yamaha t50c hum

[Marioverkill]

Hello all!

I've been working/modding a yamaha t50c.When I got the amp,it would develop a hum/ringing after the amp is on for about 20 minutes.I did some mods and have gotten the sound pretty close to where I like it minus that ringing/hum.Here are some observations:

1)It sounds like a 60 cycle hum
2) You only hear it when you play a note
3) It sounds like a harmonized lower note,more prominent on the 12-20 frets on the 2nd string.(you can hear it better here)
4) I have isolated it to the preamp part of the circiut from input to V3.I did this by plugging the effects send of the t50c into the effects return of my PT100 and it is still there.
5) It is there on both clean and overdrive channels.
6) New tubes,new power supply E-Caps and some mods


Like I said before,I am new at tube amps.I am going to order new smoothing caps.

Because you only hear it when you play a note,I am thinking something is being coupled with the signal?

Fire away!!

Everything appreciated!!

Mario

[roberto]

All four electrolytic caps into the power supply board are new?
What other mods have you done? Can you show us a picture of the inside?

[Marioverkill]

Roberto,

I could post pics.Keep in mind that this problem was there before mods,from the beginning.I know I should have addressed this first but oh well...

The 4 caps in the power supply are new and bigger as per Mike and Bill at Soldano.

Any suggestions from the info I mentioned?

Does it make sense to suspect the smoothing caps?

Thanks
Mario

[Marioverkill]

Nobody has had this problem before???

I have ordered smoothing caps.Any suggestions would be welcome.

Mario

[VacuumVoodoo]

Generally speaking, if you have 60 or 50Hz hum the issue is with heater wiring and/or relative positions of power and output transformers.
120 or 100Hz buzzing hum is rectifier and smoothing capacitor related or caused by a ground loop.
On this particular amp heater supply is grounded via winding center tap. There is no easy way to install a hum balancing trimpot because of the way heater AC supply is rectified to provide DC for relays.
These are the first obvious causes, there can be other hidden faults.

[billyz]

I just did a t100c for a client. Same amp except 4 6l6's. I replaced the 220k plate resistors with 100k and changed the 120k pi plate r with 100k. Also found several bad resistors at the reverb switching circuit.
If the sound is present on both clean and overdrive channels it must be V1 as v2 is the od fed from v1. Could be a bad solder, cap in that part of the circuit. Double check that power supply node too.

[Marioverkill]

Generally speaking, if you have 60 or 50Hz hum the issue is with heater wiring and/or relative positions of power and output transformers.
120 or 100Hz buzzing hum is rectifier and smoothing capacitor related or caused by a ground loop.
On this particular amp heater supply is grounded via winding center tap. There is no easy way to install a hum balancing trimpot because of the way heater AC supply is rectified to provide DC for relays.
These are the first obvious causes, there can be other hidden faults

Thank you.
I only hear that other note/sound when I play a note;nothing when no input.

I just did a t100c for a client. Same amp except 4 6l6's. I replaced the 220k plate resistors with 100k and changed the 120k pi plate r with 100k. Also found several bad resistors at the reverb switching circuit.
If the sound is present on both clean and overdrive channels it must be V1 as v2 is the od fed from v1. Could be a bad solder, cap in that part of the circuit. Double check that power supply node too

Thank you.
I also thought what you said about V1.I thought of replacing the .02 coupling caps.Do you think I would get noise only when inputting sounds if the coupling caps where bad or would I get hum even with no signal(leaking dc) ?

Why did you replace the 220k?Redesigning the stage for different sound?I did this for the clean channel.I changed the Pr for a 47K and the Cr for a 1.5K.The Pi is one area I did not alter.I did a lot more on the drive channel for more gain.

Did you revert the amp back to original from those weird production updates in the reverb circuit?

Sorry for so many questions.I am trying to analyze as much as I can before I start plugging parts. LOL

Mario

[roberto]

If the noise appears only after 20 minutes, it's difficult that the noise is related to heaters, maybe with thermal effects on some components (the more obvious were supply caps, but he has changed them..).
At the same time, if the noise is the same (intensity and period after it appears) it's related to something that you have not changed.

As for my mods, referring to this schematic:
http://www.prowessamplifiers.com/schema ... 0_t100.pdf

pag.1
- substitute two 82µF 450V with two 220µF 300V (fi 22mm, pin distance 10mm)

pag.2
- substitute all Rp with same value but 1W
- 1st and 2nd OD channel's coupling caps from 22n to 4n7
- change all 1µF with poly 100V
- remove 47nF from 2nd clean stage's cathode
- sostituire la resistenza da 5M6 con la coppia 1M//270pF
- 470pF snubber caps on 2nd and 3rd lead gain stages
- 470k 470k voltage divider before the 3rd lead gain stage
- 3rd stage Rk from 39k to 10k
- optional boost switch 1µF in parallel with 1k8 on V3

pag.3
- clean eq to 56k slope and 270pF treble cap
- substitute all Rp with same value but 1W (100k 82k on PI)
- substitute two 82µF 450V with two 220µF 300V (fi 22mm, pin distance 10mm)
- substitute 500R with a 3H 120mA choke (you have to drill the chassis)
- substitute the 39k NFB with 47k plus 100k//4n7

[billyz]

He wanted less distortion and better clean sound. It was not an extensive reworking , better pi balance. The od channel is pretty obnoxious and needs further refinement, which we will in do the future. Try the coupling cap replacement. It was easy to work the board without removal. Use a better film cap.

[billyz]

Now that I look at it. Try the .02 coming from v1a . Also, I would remove the treble peaking circuit to v2a. , then plan on dropping some gain. Recheck the input circuit resistors and the function of the jack . Sorry, I am typing on an iPhone and it sucks .

[Marioverkill]

Thanks Billy !

I will replace the the coupling cap(s) from V1A & V1B while I'm at it.I will also replace the three smoothing caps(20u,20u,40, and the bias circuit 47u).

I am waiting for the parts which should be in this week.

Mario

[Marioverkill]

Roberto !

Glad to share with someone who has also reworked this amp !

substitute two 82µF 450V with two 220µF 300V (fi 22mm, pin distance 10mm)

I did this.

substitute all Rp with same value but 1W

I did not do this,but will.I just ordered parts and will try to add them.

[/quote]1st and 2nd OD channel's coupling caps from 22n to 4n7

I changed the 22n caps on V2 to 1n

change all 1µF with poly 100V

I used ecaps 100v new

I also did:
V2 39k to 16k
V3 1Uf//1.8k
Bootstrapped V3 with 47nf to plate R in between 137k and 110 k(resistors I had in //)
1nf snubbing cap at V3 // with Pr(dampened high gain shrillness)
Choke mod to replace 500
added resonance(4.7nf//1M pot)between the 4ohm tap and 39k feedback.
Got a good deal on a eminence Governor and replaced the stock speaker
The first 2 80uf power supply caps(near bridge) with 220uF 450V ecaps and the 2 80uf near output with 100uf(as per Bill a soldano)

I am 90% happy with the drive channel.
The clean I would like to improve but feel limited because it shares V3 and I don't want to mess up the sound I've achieved on the drive channel.

What were the sonic effects of your following mods:
1)5M6 with 1M//270pF ?
2)clean eq to 56k slope and 270pF treble cap ?
3)substitute the 39k NFB with 47k plus 100k//4n7 ?

Thanks again.I am learning so much by working on this amp.

Mario

[roberto]

I changed the 22n caps on V2 to 1n

Isn't it a bit too small? Do you have still enought low-mids? With a 1n cap here you can "engl-ize" mids by raising Ck of 2nd od stage to 2µ2 to 10µF.

The clean I would like to improve but feel limited because it shares V3 and I don't want to mess up the sound I've achieved on the drive channel.

All the amps I've built share major part of the stages, this helps the amp to have similar feel in clean (usually plexi like) and lead channels. These are my latest six amps: http://img441.imageshack.us/img441/6854/001dhv.jpg

What were the sonic effects of your following mods:
1)5M6 with 1M//270pF ?

5M6 dampens mids and hi-mids, the amp sounds dull (this is the reason of the partial bypass of the 2nd clean stage). 1M//270p will add more hi-mids, helping the sound to cut through the band's mix. You can try 470k//470p if you need more british midrange.

2)clean eq to 56k slope and 270pF treble cap ?

a more scooped clean, if you like.

3)substitute the 39k NFB with 47k plus 100k//4n7 ?

47k helps to add some harmonics to the power amp, 100k//4n7 is a fixed resonance, like a soldano slo100 depth control on 5.

Thanks again.I am learning so much by working on this amp.

It's a nice amp to work on. I learned alot on one T100H some years ago.

[Marioverkill]

Thanks again Roberto !
I can't wait to try your mods on the clean channel.I mis-typed the caps on V2.I meant to say I changed them from 22nF to 10nF

Mario

[Marioverkill]

Oh your amps !!Italian fit and finish !!They immediatly reminded me of Ducati motorcycles !!

Mario