At the start: Triodedick´s Monobill Classic KT88 Monos - diyAudio
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Old 29th January 2015, 04:54 PM   #1
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Default At the start: Triodedick´s Monobill Classic KT88 Monos

Hi there,
what a great forum.
For the several last months I used to read here a lot and got a lot of input due to your comments.
That made me think of starting a new project.
After building some tube based preamps, one integrated tube amp and one F5, I´m actually walking down Triodedick´s Monobill Classic street and want to document it step by step at this forum.
Nearly all parts are sourced. That means tubes, resistors, caps, connectors, …
So it´s time to get one step further: the chassis design.
First I thought of making it easy and build the amp like Triodedick did, but is it an ideal arrangement?
The input tubes are located in front of the power transformer and are far away from one output tube.
What do you think? Would a dual mono design with a separated psu be better or is it a dumb idea?
All the best,
Rents



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Last edited by Renton76; 29th January 2015 at 05:24 PM.
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Old 30th January 2015, 06:10 AM   #2
AllenB is offline AllenB  Australia
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Providing isolation between channels for stereo separation is good up to a point. Other issues are largely practical, such as sizing the components to suit one or two channels.

Consider a power supply from two perspectives ie: filtering the noise from the supply to the amp, and going the other way decoupling the amp from the power supply. In its simple form this means filtering ripple leading into a reservoir with clean DC... then some nice caps bypassing the amp but reasonably isolated from the reservoir.

It's not too hard to provide extra channel separation at this point by bypassing them individually from a single reservoir, even though a low source impedance from a single bypass capacitor would already be providing some isolation.

Considering that some of us like to use smaller values of capacitance for the last stage for one reason or another, separate bypassing is simple and effective. It could even be two stage.
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Old 30th January 2015, 07:59 AM   #3
Keit is offline Keit  Australia
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In solid state amplifiers, separate power supplies are essential if hum and distortion is to be of the highest order. However, with tube amps it isn't, because the output stage currents drawn from the power supply are so much lower. And in Class A push pull they balance out, where as with Class B used in solid state they do not. For these reasons the problem is 100 to 200 times worse in solid state, yet many ss designs do get away with a common power supply.

Similarly, with solid state amps, channel separation is important. It is NOT important for stereo separation as the ear is not sensitive to channel crosstalk. It IS important in solid state amps because what gets from one channel to the other gets there because of capacititive and inductive coupling and both the favour harmonics that are generated in Class B. So, for a signal in one channel, what gets reproduced in the other channel is high in distortion. You hear the distortion.

As tube amps Class A harmonics are much lower, so the inter-channel crosstalk is unimportant.

Designing a good layout with tubes is however a matter of a great deal of complexities. I notice the transformers in your photo are offset wrt each other. Normally, the output transformer magnetic axis should be orthogonal to the power transfomer axis. This ensures that the stray filed from teh power transfgormer does not induce hum in the output transformer and is more important in an amp without neg feednack. However, it is equally valid to do this by having an offset. It depended on how the transformers are oriented within their cans. And the cans help. So do leakage flux shorting straps.

Why not try and find out what the hum levels are in the designer's original. Really, it doesn't matter what the layout is, if he got inaudible hum, he got it right, end of story. All you have to do is copy it exactly, using the same transformers and tubes.

One thing I notice - the low level tubes are not in screening cans. This is a common thing in modern audiophile amps, and probably doesn't matter. In commerical equipment made during the tube era, unwanted hum and feedback was guarded against by tube sockets incorporating screening cans for the input stage. But who wants to look at an aluminium can? One wants to see the glowing tube.

Last edited by Keit; 30th January 2015 at 08:05 AM.
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Old 31st January 2015, 06:55 PM   #4
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Hello Keit and Allen,
many thanks for your infos.
You´re right Allen. Never touch a running system. :-)
So Dick´s layout will be used.
I knew about transformer orientation. But how would it be this case? Think it´s right. What would you say (left potted power tranny, right choke and opt)?
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Old 1st February 2015, 12:05 AM   #5
Keit is offline Keit  Australia
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It's good but not the best.

The most important are the releative orientations of the power tx and the output tx. If possible you want zero coupling to the outpout tx.
Coupling from the powerr tx to the choke is less important, as the 2nd HT coupling cap will keep HT down, and the input stage HT rail will have additional filtering as found necessary.
Cpupling from the choke to teh output tx also less important, as the AC current component is the choke is very much smaller than that of the power tx.

The pic shows the power tx is oriened with the core axis east-west, laminations parallel to chassis.
The output tx is oriented with lamistions orthogonal to chassis, which is thus good, but with core east-west, not good. But it is offset, which is good.

Taking the power tx as a starting point, it has core axis east-west, lams parallel to chassis.
Therefore, ideally, the output tx should be lams orthogonal to chassis, core axis north-south.
This leaves the choke to be lams orthogonal to chassis, axis up-down.

But as I say, if it has inaudible hum, then it doesn't matter.
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Old 1st February 2015, 01:29 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by Renton76 View Post
So Dick´s layout will be used. I knew about transformer orientation. But how would it be this case?
Think it´s right. What would you say (left potted power tranny, right choke and opt)?
Looks like the two original amplifiers are mirror imaged. Instead, I'd build two identical mono amps,
copies of whichever one of the two has less hum and noise.
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Old 1st February 2015, 11:38 AM   #7
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What Keit said, and you can use this layout to separate power, input and output. If you use modern "snap" power capacitors, they are usually short enough to fit inside the chassis.

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Old 1st February 2015, 12:45 PM   #8
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Thanks guys.
The amps will not be mirrored.
Costis, I thought about this option, with opt, choke and power transformer in a line. Pro: the signal lines don´t run through the chassis.
Keit, don´t you expect problems, cause the choke and opt won´t be in a twisted 90 degree position? And how about the KT88s? No probs with induction?
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Old 1st February 2015, 02:09 PM   #9
Keit is offline Keit  Australia
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I would lay it out in the manner I suggested. I prefer mirror imaging 2 monoblocks. It looks nice. But I don't use PCB's in tube designs. If you use PCB's there are less problam if you don't mirror image. It is NOT an issue I regard as important though.

With push-pull, and the spacing depicted in your photos, I certainly do not anticipate problems with the KT88's.
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Old 2nd February 2015, 08:13 AM   #10
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Keit, again many thanks.
I'll place the transformers as you recommended.
All the best,
Rents
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