Tube with Power IC Output Stage - JLTi

[Tor M]

Hi!!

My tube buffered gainclone made after Joe`s schematics has been up and running for å couple of days. And it sounds fantastic!
This was my first amp build and my first time playing with tubes.

I did some beginners mistakes ( wiring the tube socket as seen from above, not from the bottom...................) But after a couple of hours of rewireing this amp bypassed my 2000$ Electrocompaniet ECI1 (My daughter killed this one, and I`ve been out of an amp for a couple of months.) in sound quality without hardly any playing time!.

So thank you Joe!
And spesial thank to Peter Daniel for sending me isolation pads all the way from Canada to Norway!

But sadly I`m not quite in HiFi heaven yet. I`ve got hum!!!!!!!!

The hum is constatnt (and quite annoying!) and does not change if I have an input source or not.

When I turn the pot I got DC on the outputs, you can see the elements "pushing" out when turning volume down, and going in when turnig up.
Besides that it has no DC on the output when the tube buffer has stabilised.
I also have power transients coming in to the system when I turn the power section on or of, and sometimes I get transients when a light swich is turned.

I have a wooden cabinet which is not shealded, and I have no capasitor over the power swich.
Maybe it would help to add an X rated capasitor over the swich or a three legged x and y rated capasitor over the power input to kill those transients?

I have tried several ways for the grounding to see if I can reduce the hum, but it remains the same. I also tried to wrap the cabinet in aluminium foil connected to earth (no connection too star grounds) to see if this could reduce hum without any result.

At the moment I have one signal star ground and one power star ground pr channel, theese are connected by a wire. The power grounds are also connected.

The chips are not running hot at all, even when pushed hard.

If somebody have any idea of how to get my new amp hum free, I would be realy grateful if you shared them with me.

Thanks to everybody posting here sharing their ideas! This was my first amp build, I doubt it will be the last! (Oh no!! Another adiccted person!!)

Tor Martin

[fdegrove]

Hi Tor,

Happy for you, but the hum...

This was my first amp build and my first time playing with tubes.

Assuming that you are indeed new to tubes, one thing you need to be aware off:

When you feed the heaters of a tube with AC you better twist both wires together an keep them close to the groundplane and away from anything else.
This may be one reason for the hum you're experiencing.

Another reason may be capacitive coupling on the powerxfomer when the heater and main PSU share the same xformer inducing hum in the process.

In case you do use separate xformers try to put them at 90 degrees angles, in which case magnetic coupling tends to cancel.

Cheers,

[Joe Rasmussen]

Originally posted by Tor M
Hi!!

But sadly I`m not quite in HiFi heaven yet. I`ve got hum!!!!!!!!

The hum is constatnt (and quite annoying!) and does not change if I have an input source or not.

When I turn the pot I got DC on the outputs, you can see the elements "pushing" out when turning volume down, and going in when turnig up.
Besides that it has no DC on the output when the tube buffer has stabilised.
I also have power transients coming in to the system when I turn the power section on or of, and sometimes I get transients when a light swich is turned.

I have a wooden cabinet which is not shealded, and I have no capasitor over the power swich.
Maybe it would help to add an X rated capasitor over the swich or a three legged x and y rated capasitor over the power input to kill those transients?

I have tried several ways for the grounding to see if I can reduce the hum, but it remains the same. I also tried to wrap the cabinet in aluminium foil connected to earth (no connection too star grounds) to see if this could reduce hum without any result.

At the moment I have one signal star ground and one power star ground pr channel, theese are connected by a wire. The power grounds are also connected.

The chips are not running hot at all, even when pushed hard.

If somebody have any idea of how to get my new amp hum free, I would be realy grateful if you shared them with me.

Thanks to everybody posting here sharing their ideas! This was my first amp build, I doubt it will be the last! (Oh no!! Another adiccted person!!)

Tor Martin

Hi Tor

Good to hear you are so pleased, hopefully even more so when the problems have been sorted out.

We will certainly want to help you get it right, and perhaps collectively we can come up with some suggestions:

To get the ball rolling, can you confirm/answer these below:

1. Are you using separate Bridges and 1000uF x 2 per each LM3875?

2. Are you using separate Bridges and 1000uF x 2 per each for both Right and Left channels on the tube buffer.

3. Is the fault/hum the same on both channels? Is the problem when changing pot likewise on both channels?

IF the answer is yes to all three it may indicate a wiring fault(s) that has been repeated on both channels?

Give us an indication, and we will take from there. The brains trust (that us lot) can take it from there and we''ll get you home.

JR

PS: It doesn't sound (no pun intended) like a shielding problem.

[Joe Rasmussen]

Originally posted by fdegrove
Hi Tor,

Happy for you, but the hum...

Assuming that you are indeed new to tubes, one thing you need to be aware off:

When you feed the heaters of a tube with AC you better twist both wires together an keep them close to the groundplane and away from anything else.
This may be one reason for the hum you're experiencing.

Another reason may be capacitive coupling on the powerxfomer when the heater and main PSU share the same xformer inducing hum in the process.

In case you do use separate xformers try to put them at 90 degrees angles, in which case magnetic coupling tends to cancel.

Cheers,

I wouldn't suggest AC when you have 30dB of gain after that stage. If that is is the problem then the answer is simple... but is it AC? We'll hear from Tor. About txs, same again.

JR

[Tor M]

Hi!
Thanks for the answers.

I have four transformers in my gainclone.
For the LM3875 I use 2X 300VA 25V transformers, 2 bridges pr chip, one for + and one for -.

For the filament I use 15VA 9V transformer. The cicuit is the same as shows on Joe`s www, besides that I use LM317, a multiturn pot and a 330 ohm resistor, to adjust the voltage to exactly 6.3 V.

For the tube supply I use 30 VA 25 V transformer. One bridge and 2 X1200uF Panasonic FC used for both "sides" of the tube buffer.

All the diodes are Onsemi Mur 860.

All the transformers are of this type:

http://www.en.rs-components.no/cgi-b...oid=-990928665

The hum is the same in both channels, and the problem when changing pot is also the same in both channels.

Unfortunaly I can`t post any pictures yet. Have to borrow a digital camera first.

Hope I can get the amp hum free. I`ve allready fallen in love with it!

Do you think x-rated capasitor over the power swich, and y rated capasitors from N and L to ground on the IEC socet will help me get rid of the power supply transients?

Thanks for the help!

Tor Martin

[cjunk]

This is my Tubed GC prototype following Joe's schematics.
The amp is assembled using industrial standard components.
PSU is 2x220 VA xformers with 25 V rails for the LM3875, one 15VA
for the heater and one 50VA for both sides of the triode.

I mounted the PSU in a MDF sheet, whereas the amp is built joining
an aluminium plate with two heatsinks for the chips. I used parts
that I had at hand. Also the tube shares the same plate, so it is
a very compact arrangement.

I was exactly in the same situation described by Tor Martin in his post,
i.e. first experience with tubes and hum in both channels either with
input connected or not and independent of the volume pot level.

Yesterday night I split the supply of the left and right channels
of the tube buffers (separate bridges and caps, still sharing the
same transformer). This has helped in reducing the hum,
which is still present but much more acceptable. I have not
tried to change the ground routing yet.

The amp has worked few hours, and I think it needs to break in
properly. I just compared it with my other diy amp, which is a
10W Son of Zen. I found the deep and mid bass performance of the
tubed gainclone very impressive. The bass is deep and very controlled.
The Tubed GC made me discover how powerful can be the bass performance
of my speakers. It was much better than the SOZ bass. In the mid and
high range I found that the SoZ is much more forward and lively.
The Tubed GC provide a much softer sound, may be some people could prefer it
(and my wife likes it very much!!). But sometimes seemed to me a bit dark.
I also find that overall the dynamics of the SOZ is much more superior.
Another point is that as a "summer" amp the Tubed GC is much more fresh
(my SOZ produce an awful quantity of heat).
I would appreciate some suggestion to tweak the project in order to
clarify a bit mids and highs. However, I think that I should listen again
after the break in.

I just want to thank all the contributors to the Forum and Joe for
sharing this project which I enjoyed so much.

Carlo

[cjunk]

Lower side, p2p wiring

[Tor M]

Hi Carlo.

Good to hear that I`m not the only one having problems, but sorry for you.

I think I will rewire the tubebuffer this weekend and try split supply also and se if it can help my amp as well.
I actually have a spear transformer I can use. I ordered one 25 V 30 VA transformer, I payed for one, but RS sent me two!!
Maybe they knew I was gonna need it!!

If I get the time I`ll try to bypass the tube section to night to see if the problems get less without it. Then, maybe I know what to focus on.

Nice amp Carlos. Looking forwart to hear about your impressions when the amp has broken in.

Have a nice weekend!

Tor Martin

[Tor M]

Hi folks!

I bypassed the tubebuffer in my amp by disconnecting the two trafos and inserting two 330 ohm resistors in the tube socket between K and G. Now the amp is absolutely 100% hum free!

Now I know for sure the tube buffer is the problem. That helped alot!

Tor Martin

[fdegrove]

Hi Tor,

I bypassed the tubebuffer in my amp by disconnecting the two trafos and inserting two 330 ohm resistors in the tube socket between K and G. Now the amp is absolutely 100% hum free!

Not quite sure exactly what is was you did here...

If, as you said, you bypassed the tube buffer stage and ran the splitter xformer straight from the source (as I understand it) then I am at a loss on how putting a 330 R between cathode and grid of the CF is going to have any influence?
Or lets's just say that I don't have the foggiest on how a 300R between cathode and grid is going to solve a hum prob.

Probably my ignorance but I'm curious nonetheless.

Do you mind elaborating on this?

Ta,