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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:

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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:

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

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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:

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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
 
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:

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This is the most elaborate version of this amp:

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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:

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Also mono block versions have been built:

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Also 'cheaper' versions, with RC coupling:

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Here is the RC-coupled schematic

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Best regards

Thomas
 
Hi Tom

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;)
 
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 [IMGDEAD]http://img225.imageshack.us/img225/6457/pukeface3kz.gif[/IMGDEAD]
 

pieter t

Disabled Account
2009-04-12 11:13 am
www.tribute-audio.nl

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.
 
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
 
Hi!

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
 

pso

Member
2010-05-29 8:53 pm
Valencia
Hi Thomas.

Currently I have a SSE (Tubelab) with 6P3S-E and EDCOR GXSE15-8-5K, in spite of the simple components, sounds great.

But I think I have the ''SE fever'' and I want more.

I would like a 300B, but only the tubes are taken much of the budget, for the price of a one tube 300B buy four 6AX4GTB, five 6N7 (Metal) and six 6CB5A, all RCA and NOS.

Apparently the sound is close to the 300B's, and I can spend more on iron and I do not mind running the tube pretty hard or that any fail.

For now I will go with the version without inter-stage transformer (I can always upgrade in the future) I prefer to spend on better OPT and PSU.

For the PSU I will go with the rectifier bridge (Graetz) and oil caps, for the iron, LUNDAHL.

Upgrade to EDCOR (25W) becomes a pain due to shipping and VAT.

There is an authorized dealer in Spain and the shipping is free or almost nothing, and although LUNDAHL is far more expensive than EDCOR , with this, there is not much difference.

Besides the tubes, I have the two chokes (LL1673H10), for the OPT I'm thinking about the LL1620, sounds like a good option?

My doubt is in the PT, I'm looking for something like this,

0-500V at 200mA.

6,3V at 7a.

6,3v at 5A

But for now I find nothing, if the price is not prohibitive, I would like to have one that you use, where you get them?

Thanks.
 
Hi Thomas,

Ultimately did not order the chokes, I will buy all the iron to you ( Besides some small parts).

I will wait for you design momoblock to decide.

Some questions ...

PSU + amplifier and monoblocks, one of the two designs have any advantages over the other?

Your smaller transformer for this monoblock, also has several voltage options?

This amplifier can be driven at full power with CD player, or need a preamplifier?

Thanks.
 
Hi!

PSU + amplifier and monoblocks, one of the two designs have any advantages over the other?

There is no real advantage to mono blocks. If you are on a limited budget, going for mnonoblocks is the silliest thing you could do. Monos almost double the cost for almost no advantage in sound. I do it with my upcoming low cost 6CB5A amps for two reasons. One is I happen to have wooden chassis available which fit monos, not a stereo amp. Second, this project might get published in a german DIY mag. Monos are popular, so let's serve the taste of the crowd ;-)
Smart people go for stereo and use the money saved from the reduced chassis cost to get better parts.

Going for the transformer coupled version will bring the sound a bigger step forward than anything else.



Your smaller transformer for this monoblock, also has several voltage options?

Yes

This amplifier can be driven at full power with CD player, or need a preamplifier?

Yes the typical CD level of 2V RMS is enough to drive this amp to full power

Best regards

Thomas
 
Hi!

I have some oil caps 50uf, I can use these in the PSU instead of 30uf of the scheme?

The RC coupled version as I porposed it uses electrolytic pas for cost reasons.
Of course you can use oil caps and they will certainly yield an improvement. I used 200-250uF. The schematic can be found in this article:

VinylSavor: Single Ended Amplifier Concept, Part 4

Best regards

Thomas