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Old 12th April 2012, 07:31 PM   #11
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Join Date: Nov 2010
Location: Muscat
Default Finally I built something!

Heres where I have got with my design...

1. Having read a lot of posts on these forums I decided to use 6SN7s / 6SL7s in a Mullard topology.

2. Decided now much gain I would need from the driver (basically 1.5V RMS from a modern D/A converter pre-amp to give about 120V P/P out of the driver to take PP KT88s at 480-500 V B+ to clipping). I then discovered (via LT Spice sims) that anything with a 6SL7 would have too much gain. So I decided on a 6SN7 triode gain stage to a 6SN7 LTP and 6SN7 cathode followers.

3. Having decided to use cascade CCS for the triode input stage and LTP of the Mullard topology I played with DN2540 cascodes. I had problems with getting stable DC performance from the classic circuit posted many times on this forum. Having read around this subject and excluded oscillation as a cause of the problem I discovered that the bottom MOSFET in the cascode should have lower Vgth for the degenerative feedback in the cascode to work properly. All of my DN2540s were from the same batch and all had very similar Vgth of about 2.6V. When I substituted the BSP135s for the lower MOSFET (these have Vgth around 1.4V) my DC stability problems went away.

4. I then breadboarded something very close to the schematic posted, but without the feedback loop or the zobel network. Open loop response was -3dB at 10Hz and 72kHz, but with gain rising again to another broad peak at 120-150kHz. I mention this because although sine wave performance was good (125W RMS 2kHz at clipping, with less than 1% THD at 50W with no feedback) the amp rang like a bell when driven with a fast 5khz square wave. The ringing was 50% of the square wave amplitude at 130khz and not damped out within a half-cycle. Output transformer is a Sowter UO71 and the KT88s are Electro Harmonix.

5. The ringing was almost totally eliminated with the zobel values shown (thanks SY for a post which pointed to putting these from plate to screen). 630V orange drop caps in this network exploded (literally) and 1W resistors smoked at high outputs (yes I admit 50W of 10kHz square wave may not have been a good idea but..). Big 2kV Arcotronics caps and 2W resistors work well. I may still optimize these values vs the HF roll-off caps in the first stage.

6. I am now pretty settled on the schematic shown, but have posted it looking for suggestions for improvement from the gurus. Having applied 18 or so dB of GNFB it gives 125W at clipping from 480V b+ medium power distortion (I just have not gotten round to measuring it at high powers) is good I measure -60dB THD+noise at 2kHz. Frequency response is -0.8dB at 20khz and -3dB at 60khz. Input sensitivity is 1.5VRMS to fully drive the output, and square waves now show minimal overshoot. Ive run the amp for long periods at high power with music inputs into a 8R / 0.47u load with no signs of instability.

I am currently running the bread boarded design from a lashed up variac to power transformer to CLC filter power supply.
My next steps are going to be to design a regulated PSU for B+ and bias.
The posted design shows DC heaters for the input tubes my current plan is to run an output protection circuit from this, I will use comparators to sample the voltage across the KT88 cathode resistors and switch off B+ it exceeds a safe value.
Attached Images
File Type: png kt88 ppp v5.png (38.4 KB, 974 views)
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Old 23rd May 2012, 01:54 PM   #12
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Great job!
Looks a bit like Crystal Palace. Is it going to built with external Powersupply? As monoblocks?
Grid stoppers?

Last edited by costis_n; 23rd May 2012 at 01:56 PM. Reason: typo
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Old 23rd May 2012, 02:27 PM   #13
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Location: Alps:Tube amp designs over 150W, SMPS guru.
Those 10 R o/p stage cathode resistors....use wirewounds.....modern metal oxide/films will give up on transients.

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Old 27th May 2012, 04:45 PM   #14
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Default Regulating B+ in high power push pull amp?

OK I will use small wirewound resistors for the 10R cathode resistors.
I can't see a need for grid stoppers on the 6SN7s during extensive testing I have not seen the slightest signs of instability without them.
I am planning to build these as monoblocs separate power supplies might be a step too far domestically. I do plan to keep the PSU very separate in construction terms.
My idea of what the PSU needs is:
6.3V @ 1.5A for the triode gain stage and the LTP ground referenced.
6.3V @ 0.7A floating for the cathode follower elevated heaters.
6.3V @ 8A (to be on the safe side) for the KT88 heaters
80V @ a few mA for the bias circuits
480-500V @ 0.6A for B+ - I need 230mA quiescent current, and twice that for peak output. Add 30mA for the preamp stages and a margin for headroom.
At this power level I am very concerned about burn ups and whilst I dont have the skills / time to build a microcontroller watchdog I intend to implement fail-safes.
I am planning DC heaters on the preamp stages.
I have read these forums on B+ delays and the various 'cathode poisoning' controversies and do not intend to implement this.
I will softstart the PSU overall using a NTC thermistor on the mains.
My implementation plans are:
The power transformer will be custom wound something like 6.3V 2A, 6.3V 2A, 6.3V 8A, 60V 0.1A, 380-0-380V 600mA
Watchdog circuits I plan 4 off comparators looking at the cathode resistors. These will shut down B+ via either a MOSFET driven by a high-side driver or by shutting down the regulator if I do go for completely regulated B+
I am currently playing with the Gary Pimm / Swenson high voltage regulator. This gives me the option of shutting down the B+ with an optocoupler (look at the Gary Pimm circuit and replace the current-limit transistor with the optocoupler). This requires drop out voltage of 25V minimum. If I double that to take account of ripple / transformer internal resistance at 600mA out this means 30W of dissipation in the regulator entirely doable.
I am planning to use LT1185s to regulate the 6SN7 heaters, and current limit them for soft start of the heaters.
I think the TL783 looks like a good option to regulate the bias, which I will have to do if I regulate B+.
The $64000 question to me is if it is worth regulating B+?
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Old 27th May 2012, 07:11 PM   #15
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Got any square wave pics ?

well done.
Don't force the issue with 10Khz square wave at power..remember squarewave harmonics are rich and one is asking the large o/p transformer to punch through the 3rd harmonic at little loss; this depends on the leakage parasitics of the transformer versus the output stage primary impedance. These are much higher than for smaller transformers. The first victims to be punished will be the series screen resistors of the o/p tubes as more energy is pushed through them as o/p stage inefficiency rises vs. frequency.
Expect a squarewave similiar to the pic; although I used 6550's new edit which weren't up to the job; but used a sim output transformer.
Be careful where one places the osc scope probe earth on high power square wave measurements; as multiple equipment earths will display untrue waveforms.

The Majestic output transformer/amp at 150W per ch using EH KT90's is sim except I used a Williamson diff driver... expect sim curves if circuit is optimised.

Attached Images
File Type: jpg 10Khz 50W 8659 6550-12BY7-ECF80.jpg (31.9 KB, 771 views)
File Type: jpg Majestic D5301-02 Modified op trans THD vs Pout.JPG (52.3 KB, 704 views)
File Type: jpg spectrum 150W 4xKT90.JPG (68.1 KB, 684 views)
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Old 27th May 2012, 08:40 PM   #16
DAK808 is offline DAK808  United States
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Default KT88 ppp upadate?

Have you built this amp? I am looking for a similar type of amp to build using Dynaco MK6 output trannies and would appreciate your input. tyvm.
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Old 28th May 2012, 04:00 PM   #17
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Default Here's a square wave

Have you built this amp?..

I certainly have built it to the prototype stage - it took me six months to get this far (alongside my full time job) and a lot of reading of this forum! I've attached a picture of the amp breadboarded to give you some idea of what you are getting yourself in to - what you can't see on the picture is the temporary power supply containing amongst other things a 8kg power transformer, a 4kg choke and some pretty sizeable capacitors on the metal rack to the right of the driver and output stages.

I have just finished designing a PCB for the driver stages, and am currently having that fabricated. This is so I can concentrate on the PSU, and test it together with the amp in a reasonable amount of space - I don't have enough space to breadboard both at the same time! I expect the PSU to be as complex as the amplifier...

Got any square wave pics ?

Attached is a 10kHz square wave output into a 8R + 100nF load - higher powers do not look any different. This is from the exact schematic posted. I'm not going crazy about the square waves other than as a way of deciding when the amp is more or less damped right - I recognise that any big output transformer has it's limitations. a completely clean 10khz square wave would imply the kind of HF response that my reading clearly tells me does not come from such devices.

I was quite proud of the fact that my messy breadboard only had a few mV hum.

I've torn down the amp now as I am breadboarding the PSU, but the distortion products from a 2khz sinewave at 50W output into 8R were:

2nd - -38dB
3rd - 48db
4th << -55db

I'm getting there with your help!!
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File Type: jpg breadboard.jpg (81.2 KB, 703 views)
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Old 28th May 2012, 05:57 PM   #18
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Default Oops

Oops - here is the trace of the 10khz square wave.

Also the distortion figures are dbW, so the second harmonic is -38dB on + 17dB output = about 0.2% etc.
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File Type: jpg V4 10khz sq wave.jpg (96.5 KB, 399 views)
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Old 28th May 2012, 06:50 PM   #19
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Your'e getting there fine: the technical learning curve is considerable. I could go further, by doing the open loop curves, plot what's actually happening vs. frequency with the response etc.
The next job is to put all what's loose on the bench in a box, make it look good....I chose aluminium with reinforcements....others choose different. However, it's when the ferrous components get close then proximity & technical problems can begin. The photo section of this tubes forum has many build examples.

Here is my 250+250W Williamson...completely stable and virtually hum free; some top and under pics. This amp is an extremity; it can be used in bridge mode i.e 500W o/p; however there are caveats....the power supply (PFC) design gets complex and the weight of the whole amp doesn't come light.
65kg /143Lbs if one wants to go down to 15hz as this one does. Notice the "tennis court" matrix earth bus...(traditionally many amps in the past were done like this) with tinned 4mm copper wire. The output stages is where the currents flow and the "court matrix" effectively neutralises them from the input sections. Earthing is only at the input sections and where possible a symmetrical layout used for both channels. As the power goes up, so does the protection too.

Keep it up !

Attached Images
File Type: jpg DSC06721red.jpg (621.0 KB, 427 views)
File Type: jpg DSC06730 red complete.jpg (329.6 KB, 478 views)

Last edited by richwalters; 28th May 2012 at 06:56 PM. Reason: spelling
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Old 28th May 2012, 07:06 PM   #20
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Originally Posted by richwalters View Post
Notice the "tennis court" matrix earth bus...(traditionally many amps in the past were done like this) with tinned 4mm copper wire. The output stages is where the currents flow and the "court matrix" effectively neutralises them from the input sections.

Care to elaborate? I have never met this term...
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