Author Topic: KT88 push-pull amplifier with integrated phono stage  (Read 2252 times)

Offline Mars

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KT88 push-pull amplifier with integrated phono stage
« on: August 24, 2016, 06:53:49 AM »
Hi All

I have amplifier coming out of the shed, that is a KT88 push-pull amplifier with fixed bias and phono stage. Me and the client decided to use octal tubes throughout, including the phono stage.

I demonstrated the Mars Stereo 70, but the conclusion was: we need more power! So I suggested we go for a more complex build with fixed bias on this one, and there will be enough mojo to rock the town.

The power limiting criteria on this amp turned out to be the physical size of the transformers. The customer requested transformer covers, and to fit the whole kaboodle on my chassis design, the power transformer will have to be a EI114 (114mm) type.

Moving on to the output transformers: Ampdog pointed out that 5K at about 420V should give us the power we need. According to this graph:



one can get more than 55 watts out of two KT88's in push-pull. We should be able to squeeze that kind of power out of my regular design output transformer and still have decent bandwidth.
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Offline Mars

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #1 on: August 24, 2016, 10:11:14 AM »
I have 6 dual triodes available for the combination of phono stage (uses 3 dual triodes) and preamp/driver circuit for the KT88's. I am considering using a 6SL7 dual triode shared between channels L&R driving a 6N7 phase splitter. That is 3 tubes and should have enough gain to drive the KT88 grids into clipping.

Something vaguely like this:



Input stage one half of 6SL7 and Phase Splitter 6N7. The rest would probably be similar. I am considering dual chokes and dual power supplies to separate the current demands of 4 x KT88's running at max. Has anybody used center tapped chokes to split HT. This scenario does promise smaller core size for effective induction.

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Offline Ampdog

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #2 on: August 25, 2016, 03:38:09 AM »
Karel,

Few suggestions, if I may:

No, I have not used center-tapped chokes. Yes, the choke is a rather hefty (thus expensive!) component. I am wondering though regarding two chokes plus two sets of filter caps, vs. 'combining' the lot and using a single choke plus double the value of caps. Economically - maybe you must do the arithmetic. Not sure here about which topology to choose, will be interested in comments from others. (I am in the same boat with the 100 + 100W 'Hallé' monster - I use a single choke plus caps, but at present not saying that is definitely better than two separate sets of filtering.)

Then another matter just for the record (since you are using a triode input rather that the EF86 - but my comment points to a general mistake made in several valve amplifiers in the past):  By passing the screen to common (earth) complicates matters - simple pentode operation requires the screen to be bypassed to cathode. It might not matter greatly in a straight circuit, but in a feedback amplifier, the equivalent diagram will show that you have the fed-back signal voltage as a voltage source in the screen circuit, in anti-phase to the input. (It resembles a kind of UL topology, though different because there is no inductive coupling here.) But simulating will show a supposed pentode stage not very far from in fact triode gain, with the fed-back signal plus its distortion now included in the screen circuit, giving a rather queer state of affairs. I have asked around on other forums and nobody (including writer Morgan Jones) could come up with a reason for earthing the screen in this way, except lack of understanding by the designer.

This OT because you are not using a pentode input, but thanks for allowing this comment since you show such a circuit!

Another (technical again!) point: Again in the input circuit as shown, rather put the bias components (1,5K and 100µF in this example) inside the feedback loop i.e. between the cathode and the feedback point, not at the earth side. (Reason: Outside the NFB circuit as here, the impedance of the 100µF cap will become significant at some low frequency, increasing feedback with possible ill effect, although this will depend on coupling caps. Within the loop as I suggest, it will improve stability by decreasing first stage gain and thus loop gain at some low f-point, apart from phase considerations.)

Lastly I always suggest a resistor shunting the loudspeaker output, say something like 820 to 1000 ohm, to keep some sort of load on the tubes and improve stability should the loudspeaker load become open for whatever reason.

Thanks for tolerating me - happy building! (And watch out for that 525V!  I had 600V between hands some months ago, fortunate to still be with you! Muscles ached for a several days from the jolt ....)
Creativity requires the courage to let go of certainties.  (Erich Fromm)

Offline Mars

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #3 on: September 13, 2016, 09:23:17 PM »
The project is starting to come together. Customer is happy with the look, and all the parts fit the holes :Whoohoo:







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Offline ScottulusMaximus

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #4 on: September 13, 2016, 09:27:39 PM »
Wow!
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Offline fredeb

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #5 on: September 14, 2016, 06:55:23 AM »
For some reason I can't see the pictures , except the schematic . Router rules blocking them ?
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Offline King_Julian_S

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #6 on: September 25, 2016, 08:44:17 PM »
For some reason I can't see the pictures , except the schematic .

Same here
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Offline entropyagent

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #7 on: September 26, 2016, 05:37:18 PM »
I see only a valve-characteristics graph and a schematic. Is there anything more?

Offline ScottulusMaximus

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #8 on: September 26, 2016, 05:39:35 PM »
I see only a valve-characteristics graph and a schematic. Is there anything more?

There was... Was gorgeous
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Offline Mars

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #9 on: September 28, 2016, 07:12:34 PM »
Can you see this?:

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Offline El Sid

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #10 on: September 28, 2016, 07:56:11 PM »
 :drool: :drool:

Offline JonnyP

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #11 on: September 28, 2016, 08:14:52 PM »
Like the Millenium Falcon going into hyperdrive, that is PR0N of note.   :drool: :drool:
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Offline fredeb

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #12 on: September 29, 2016, 12:12:33 PM »
Holy Moly ! That is beautiful - well done man Karel ! I take it the black knobs are for bias adjustment ?

I like the ventilation holes next to KT88's too . Are there rubber grommets beneath ( the KT88 ventilation plates ) to curb vibrations too ?
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Offline iondb

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #13 on: September 29, 2016, 12:33:25 PM »
That is beautiful!!!!    :drool: :drool: :drool:

Offline peterc

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #14 on: September 29, 2016, 02:33:42 PM »
Love the brushed look!

V Nice