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Old 23rd December 2010, 06:23 PM   #1
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Default SE 6CB5A amp with 6N7 driver

Hi!

Here is the 6CB5A amp which I mentioned in various posts. This design started after some discussion in a german tube forum. There the wish for a DIY design came up which should be fairly easy to built and offer excellent sound for the cost.

Although I generally prefer DHTs, I searched for a indirectly heated tube which is rarely used in audio. This would keep the tube cost down and avoid sophisticated filament supplies since AC heating would be sufficient.

Natural candidates were TV horizontal or vertical deflection tubes in triode mode. I measured several of these in triode connection on a curve tracer. The 6CB5A showed quite remarkably linear curves, similar to a 300B:

ImageShack® - Online Photo and Video Hosting

Note that the right most curve is a bit closer to the rest. This is not caused by non-linearity of the tube, but is a glitch in my curve tracer.

The curves showed that the tube would bias up perfectly in a design suitable for a 300B. That would make it easy to compare it to 300B.

A first protoype was mocked up very quickly with some spare parts:

http://h-1.abload.de/img/cimg3439onzp.jpg

Several driver tubes were tried in this prototype: 6AM4 and 7F8 sounded quite ok, but did not offer the headroom I'm usually thriving for, so I settled for the 6N7.

Here is the schematic of one channel of the amp:

ImageShack® - Online Photo and Video Hosting

Here is the power supply (common to both channels):

ImageShack® - Online Photo and Video Hosting

It was built as a choke input. The first cap is optional in case there are mechanical buzz issues with the power transformer or choke. But they were actually never needed.

Here a pic of the first amp in a neat chassis:

ImageShack® - Online Photo and Video Hosting

ImageShack® - Online Photo and Video Hosting

This incarnation uses Lundahl 1660 interstage transformers and Lundahl XE20 output transformers. The amp has also been built with all Lundahl transformers (LL1664 works nicely) and also with all Tango transformers.

In later versions I built the amp with external PSUs and full wave bridge rectifiers using 4 TV dampers.

Also a some smaller versions got built with RC coupling instead of transformer coupling

Best regards

Thomas
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Old 23rd December 2010, 06:40 PM   #2
tomchr is offline tomchr  United States
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Curious question: Why the use of an inter-stage transformer? What are the advantages and draw-backs (other that cost obviously)?

~Tom
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Old 23rd December 2010, 06:41 PM   #3
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Some more builds of this amp...

Here is one with Tango NC20 interstage and Tango FC30-3.5S oputput transformers, external PSU using 2 6AX4 instead of the 6BY5 in the first amp:

ImageShack® - Online Photo and Video Hosting


This is the most elaborate version of this amp:

ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting

The PSU uses a full wave bridge rectifier, with two mercury vapour tubes in the upper part and 2 6AX4. Here a shot of the PSU operating in the dark:

ImageShack® - Online Photo and Video Hosting


Also mono block versions have been built:

ImageShack® - Online Photo and Video Hosting


Also 'cheaper' versions, with RC coupling:

ImageShack® - Online Photo and Video Hosting

Here is the RC-coupled schematic

ImageShack® - Online Photo and Video Hosting

Best regards

Thomas
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Old 23rd December 2010, 06:46 PM   #4
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Hi Tom

Quote:
Originally Posted by tomchr View Post
Curious question: Why the use of an inter-stage transformer? What are the advantages and draw-backs (other that cost obviously)?
This question is a good candidate to open a can of worms
Interstage transformers are widely used among DIYers and there are plenty of discussions arounf them.

Simply put: better sound.

In the context of this amp: This way the driver offers more headroom (one of my design goals). Albeit not cheap, the chosen Lundahl transformers are still reasonable, especially compared to some boutique coupling caps.

As you can see in my later post, this amp was also suggested in a RC coupled verions for lower budget verions

Best regards

Thomas
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Old 23rd December 2010, 07:50 PM   #5
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Quote:
Originally Posted by tomchr View Post
Curious question: Why the use of an inter-stage transformer? What are the advantages and draw-backs (other that cost obviously)?
Advantages:

*) Greater output swing for the DC rail voltage than you'd get with other forms of passive or active plate loading.

*) Voltage gain: you can use a voltage step up IST

*) Less (much less) DC resistance in the grid return path -- eliminates problems of blocking on overdrive.

*) Less tendency to produce higher order harmonics, better distribution (i.e. "waterfall") of harmonic distortion.

Disadvantages:

*) Much worse phase performance that pretty much precludes including the IST in the feedback path.

*) Limited choice of driver types, as IST performance depends on using VTs with very low r(p) -- that means no pentodes at all, no high-u triodes, excludes a lot of even medium-u triodes. (Unless you include esssss-loads of NFB to get the Zo down.)

*) Nasty resonance distortion problems are likely to result from cathode bias of driving VT, as capacitive currents in the cathode mean capacitive currents in the plate, and that can cause resonances in the IST. These sound real nasty. You'll probably need fixed bias on the driving stage.

*) Increased driver difficulties since voltage step up means impedance step down.

*) Really good ISTs ain't cheap, and cheap ones sound like Click the image to open in full size.
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Old 23rd December 2010, 08:16 PM   #6
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[QUOTE=Miles Prower;2408828][b]
Disadvantages:

*) Much worse phase performance that pretty much precludes including the IST in the feedback path.

Not necessarily so. A good quality 1:1 interstage transformer driven by a triode with Rp of some 2k will have straight frequency bandwidth over 200 kHz without nasty resonances.

*) Limited choice of driver types, as IST performance depends on using VTs with very low r(p) -- that means no pentodes at all, no high-u triodes, excludes a lot of even medium-u triodes. (Unless you include esssss-loads of NFB to get the Zo down.)

High-mu triodes like triode-connected pentodes D3a, E810F, and pure triodes like EC8010, 5842, EC88 work flawlessly with interstage transformers.


*) Increased driver difficulties since voltage step up means impedance step down.

That's why I prefer no step-up ratio for interstage transformers.
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Old 23rd December 2010, 08:28 PM   #7
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Hi Miles, Peter,

I pretty much agree with both of you. If you are looking for best possible phase response, bandwidth, square wave response, etc. transformer coupling might not be the right choice. For my preferences the advantages of transformers exceed their draw backs by far.

Usually I prefer lower rp tubes for transformer coupling, like those Peter listed. I'd add EC8020, WE437, 10Y, 801A...

For the particular design presented in this approach I looked for a compromise. The 6N7 paralleled has quite high rp. Just on the edge to be still usable with interstage transformers, but it needs high quality ones. I found the Lundahls to work nicely with them. The NC20 still has excellent bandwidth with the 6N7. I'm sure there are other suitable transformers out there, but I can't try all of them...

I wanted to stick to a two stage design to keep it simple. Still the amp should have enough input sensitivity. Input transformes were out of the question since the amp should present an easy load to the preamp to be usable with just about any preamp out there.

Best regards

Thomas
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Old 23rd December 2010, 08:37 PM   #8
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Old 21st March 2011, 03:26 AM   #9
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How does it sound compared to your favorite dht?
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Old 21st March 2011, 08:17 AM   #10
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Hi!

Quote:
Originally Posted by jimmyd53 View Post
How does it sound compared to your favorite dht?
Of course I prefer a well implemented DHT amp. But that would cost much more. If the budget is below about 2000 Euros, I'd rather use a 6CB5A amp with top quality parts than a DHT where a large amount of the budget needs to go into the tubes themselves and the filament supply.

At the end it's a matter of tastes. There are poeple whoe replaced their 300B amps with a 6CB5A.

BTW: more details about various versions and schematics can be found on my blog in the single ended amplifer concept articles

Best regards

Thomas
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