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Old 13th June 2007, 09:32 PM   #1
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Default Help with amp layout

Hi everybody, I'm writing now because I finished the design and audio test of my amp. (EF86 - 12AU7 with CCS - 2xKT88 UL)

Click the image to open in full size.

Now I want to make it beautifull...

This is what I was thinking.

Click the image to open in full size.

what do you think? Any suggestions / advices / corrections are welcome.
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Old 13th June 2007, 09:53 PM   #2
EC8010 is offline EC8010  United Kingdom
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Hmmm. There's no way you're going to do a phono stage with a single EF86. Even if you did, there's no way you'd stop it from humming on a power amplifier chassis.

I like the layout you built better than your proposed layout. But what's the green light?
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Old 13th June 2007, 10:23 PM   #3
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Quote:
Originally posted by EC8010
I like the layout you built better than your proposed layout. But what's the green light?
LED voltage reference for the CCSs?
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Old 14th June 2007, 03:03 AM   #4
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Quote:
Originally posted by EC8010
Hmmm. There's no way you're going to do a phono stage with a single EF86. Even if you did, there's no way you'd stop it from humming on a power amplifier chassis.

I like the layout you built better than your proposed layout. But what's the green light?
The green light is the led of the cascode current source.


Besides the phono preamp, what else you don't like of my new design?

the changes are

1- the "irons" are horizontal instead of vertical (visual change)
2- the output irons are 90degree rotated
3- the choke isnt inside the chassis anymore
4- more space between the KT's (datasheet!!)

I'm planning to put a microcontroller & display to control the amp & measure that everything is going well (only for fun, I'm studying microcontrollers)
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Old 14th June 2007, 04:02 AM   #5
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personally, i don't like how bulky your amp will be. anything el34 or above, i think, benefits greatly from monoblock design. if you want to stay with a stereo chassis, your layout looks fine. what kind of choke are you using? got any PSU schematics?
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Old 14th June 2007, 04:24 AM   #6
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here are the schematics. there are little modifications that aren't in the schematics, tomorrow I will update.

Amp:
http://img116.imageshack.us/img116/8533/ampqu3.gif

PSU:
http://img402.imageshack.us/img402/2889/psuwh7.jpg

look at this post!!
http://www.diyaudio.com/forums/showt...48#post1170648
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Old 14th June 2007, 06:41 AM   #7
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Your'e looking at roughly 80-100W pout? Do you really want this ?
At 520V the output section will get hot. Large bulbs at least 100mm apart centre/centre. One can relax this with reduced B+.
The psu choke is an orientation headache. Somehow further away from pentode input stage. with at least 26dB gain this will pickup 100Hz mod hum.
What make is the o/p tranny ?
R15 could lead to instability as one is lifting the cathode off ground.....try it as it is but I use way down 0.1 ohm or 1 ohm.
Screen grid values on data sheets specify typ values. On some o/p trannies I find 1K gives better results. Depends on quality of op tranny.
You might have to fit Zobel network on o/p stage screen to anodes.
The power supply will hit 530V or more on switch-on. No B+delay fitted or interlock to protect o/p tubes if neg bias fails ?


richj
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Old 14th June 2007, 02:07 PM   #8
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Your'e looking at roughly 80-100W pout? Do you really want this ?
At 520V the output section will get hot. Large bulbs at least 100mm apart centre/centre. One can relax this with reduced B+.


I'm looking for 60 - 70W output. The +B isn't 520 anymore, now it's 450V. That is one of the changes.

The psu choke is an orientation headache. Somehow further away from pentode input stage. with at least 26dB gain this will pickup 100Hz mod hum.

I'm really lucky, with the volume pot at 100%, there is no hiss or humm audible. Only if I stand just in front on the speakers and put my ear on it.

What make is the o/p tranny ?

I dont know what you mean exactly but I asume that you mean about who build the tranny. They are home made, all of them. The o/p is made with silice-Fe ("oriented grain"??), "sectorized winding" ... really good tranny!! look here http://www.diyaudio.com/forums/showt...48#post1170648

R15 could lead to instability as one is lifting the cathode off ground.....try it as it is but I use way down 0.1 ohm or 1 ohm.

When you say lead to instability, you mean that can start doing some oscillations?

I used 10 ohm because I took that value from another circuit. I use it to adjust the bias. I can lower it to 1 ohm.


Screen grid values on data sheets specify typ values. On some o/p trannies I find 1K gives better results. Depends on quality of op tranny.

What should change if I change the value of the screen grids?

You might have to fit Zobel network on o/p stage screen to anodes.

I dont know what is a zobel network... I will google it

The power supply will hit 530V or more on switch-on. No B+delay fitted or interlock to protect o/p tubes if neg bias fails ?

I will build a protection circuit with a microcontroller. It's kind of weird to use microcontrollers & tubes but I dont mind, I will put a display in front of the amp with "usefull" information (just to keep learning about control and microcontrollers)

What I dont know how to do is to switch on and off the +B. I cant find a high voltage relay... what do you recomend?



edit: OMG!!! I forgot to say.. THANKS!

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Old 14th June 2007, 03:48 PM   #9
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Much hass been said about high B+ on switch-on causes degraded tube life by gradual cathode stripping. That's why indirectly heated rects i.e GZ34 are often used. Any change increases complexities.
450V+ makes life alot easier. One then has a possibility of using 6550C's which are cheaper and give same Pout. and I use more 6550's in equipment than 88's.
When one builds an amp, using high gm tubes, one has to watchout for oscillations which can occur i.e leakage reactance of tranny can excite oscillation parasitics within the gain of the o/p stage usually 75Khz upwards. One might "getaway" with nothing happening until a capacitive load is connected i.e multisectioned LS cross-over network. Or more risky if the secondary becomes o/c and this must be avoided. This is where the anode to screen grid dampers (zobel) are used. My experience is values give in old books are fine. 1000pF+1K.
An o'scope is most useful tool.

If you stand infront of the speakers and hardly hear hum etc....not everyones hearing is the same. I suffer from tinnitus but hear hum better than top noise.

After the amp is running up and seems happy on power, some amp builders are quite happy to leave things as they are even with instant B+ with switch on and just listen to it..finished.Do no more.
Later on one can improve things with increased learning curve.
With Sowter o/p trannies screen grid vaues are sim to what you have 270-470R, but I've another amp which has a Majestic tranny (also very good) but output stage prefers 1K screens for min thd. Transformers vary so much.
If you use a HT relay directly in the sec winding for delay, the discharged low impedance capacitive input filter will prob blow the input fuse and an anti-surge fuse won't last long. There are some gimmick circuits around.

More anon
richj
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Old 14th June 2007, 06:17 PM   #10
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Quote:
Originally posted by richwalters
Your'e looking at roughly 80-100W pout? Do you really want this ?
At 520V the output section will get hot. Large bulbs at least 100mm apart centre/centre. One can relax this with reduced B+.
The psu choke is an orientation headache. Somehow further away from pentode input stage. with at least 26dB gain this will pickup 100Hz mod hum.
What make is the o/p tranny ?
R15 could lead to instability as one is lifting the cathode off ground.....try it as it is but I use way down 0.1 ohm or 1 ohm.
Screen grid values on data sheets specify typ values. On some o/p trannies I find 1K gives better results. Depends on quality of op tranny.
You might have to fit Zobel network on o/p stage screen to anodes.
The power supply will hit 530V or more on switch-on. No B+delay fitted or interlock to protect o/p tubes if neg bias fails ?


richj

Quote:
Originally posted by richwalters
Much hass been said about high B+ on switch-on causes degraded tube life by gradual cathode stripping. That's why indirectly heated rects i.e GZ34 are often used. Any change increases complexities.
450V+ makes life alot easier. One then has a possibility of using 6550C's which are cheaper and give same Pout. and I use more 6550's in equipment than 88's.
When one builds an amp, using high gm tubes, one has to watchout for oscillations which can occur i.e leakage reactance of tranny can excite oscillation parasitics within the gain of the o/p stage usually 75Khz upwards. One might "getaway" with nothing happening until a capacitive load is connected i.e multisectioned LS cross-over network. Or more risky if the secondary becomes o/c and this must be avoided. This is where the anode to screen grid dampers (zobel) are used. My experience is values give in old books are fine. 1000pF+1K.
An o'scope is most useful tool.

If you stand infront of the speakers and hardly hear hum etc....not everyones hearing is the same. I suffer from tinnitus but hear hum better than top noise.

After the amp is running up and seems happy on power, some amp builders are quite happy to leave things as they are even with instant B+ with switch on and just listen to it..finished.Do no more.
Later on one can improve things with increased learning curve.
With Sowter o/p trannies screen grid vaues are sim to what you have 270-470R, but I've another amp which has a Majestic tranny (also very good) but output stage prefers 1K screens for min thd. Transformers vary so much.
If you use a HT relay directly in the sec winding for delay, the discharged low impedance capacitive input filter will prob blow the input fuse and an anti-surge fuse won't last long. There are some gimmick circuits around.

More anon
richj
Thanks a lot both of you... I've made some changes in the schematic.
1- R15 (kt's cathode to ground) from 2x22R to 2x2R2
2- zobel network between Anode and screen of KT's
3- corrected the +B values
4- updated to real CCS circuit
5- updated to real Rf and Cf values

Here is the new version...

Click the image to open in full size.

I was thinking of using a relay on B+ not in the sec winding. Other to select 4 or 6 ohms output and other to connect and disconnet the speaker output (switch between the real speakers and a R 4/6ohms).

so, with the microcontroller, do this:
at power on, wait for the capacitors to charge, then connect the +B to the o/p trans. Then, a delay of 1sec and switch the speakers relay to connect the speakers and disconnect the 4/6 ohm R's.

Also I was thinking to use relays to select between 2 or 3 audio inputs.

I'm not sure but I heard that isn't good to use relays in series with speakers.. what do you think?
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