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The Last Amp I Am Ever Going To Build. Maybe.

88.81.139.3
Posted on July 30, 2009 at 13:08:07
Tubelcain
Audiophile

Posts: 4
Location: Guernsey
Joined: January 6, 2008



Big thank you Kevin for your inspiration & assistance over the past 2 years & for supplying me with some of the best audio components money can buy.

Have built a few audio amps over the years, a Moscode 150, a 845 single.
Did a lot of research & was inspired by Gizmo’s rantings, A push pull 6SN7/6L6 1671 phase splitter /300b/1663 was the result. The superiority over anything else I had heard ( Even The SE845) spurred me on too build my ultimate amp, the results of which you see before you.

Much head scratching & comparisons between Kevin’s Lynn’s, Gary’s & Christen Rintelen’s designs were made & eventually decided on Kevin’s 300b/32b/1620am 3 stage direct coupled amp, with a few additional tweaks.
Wanted a more powerful driver stage than a 6BX7/1692 could muster, but couldn’t justify using DHTs so I opted for a 6EM7/1671AMpp stage instead.
Love 6EM7 a very useful little tube, cheap plentiful & great sounding, used it also for the series pass regulators on the plates of the first 2 differential pairs ( plagiarized from Mike Van Evers 1 tube regulator). Fitted Kevin’s Cascode CCS on the cathodes also, even though I needed too add a negative supply for the input stage.
Adding Kevin’s Shunt Attenuator was a great success too, between the 1690 & 6SN7. The amp has more than enough gain for my cherished Shanling SAD-300 player to drive it in balanced mode.
The PSU was made in a separate box which with hindsight was a wise move, great to get all those big noisy components in a closet & allowing me some licence to construct something a bit creative. Used a custom Sowter mains transformer 6CM3s, polypropylene motor run caps and 1638/4Hs for the choke input filters. One per channel.The 1638s are still quite mechanically noisy despite putting the quintessential 22nf caps in place.
My first mod will be to try these chokes in serial common mode connection with choke snubbers to see if this makes any audible difference. The amp itself is audibly very quite and drives my Tannoy Cheviots to perfection.

So what does’s a full Lundahl amorphous core 300B amp sound like!? I am no expert but 50 hours of burn in has definitely revealed the best amp I have ever heard by far, perhaps it that proud Papa syndrome but I eagerly await the next 200 to see how it will further improve. The Guy’s at the local Audio Society are Grrrreen With Envy!!! Have never heard a system that reveals so much tonal colour and the naturalness of the recorded environment, Puccini never sounded so real.

I don’t have many ideas for mods for this amp as I have built in as many as possible from the start. Next however I hope to buy myself a quad of Emission Labs 320B (gulp), and up the output power/quality by means of those higher VA taps on the PSU.
CCS on the output stage & snubber caps on the AC heaters would also be interesting, does anybody have any experience of these mods?

So thanks for allowing me to share my experiences with you all, don’t have many fellow amp builders here in the Channel Islands to swap notes.

Thanks again Kevin for sharing your professionalism & ideas with us Audio Amateurs. Any ideas for a two section Lundahl based tube balanced active crossover?
RE: The Last Amp I Am Ever Going To Build. Maybe., posted on August 18, 2009 at 08:41:42
tuneful
Audiophile

Posts: 87
Location: New Mexico
Joined: October 29, 2004
 Thanks for posting the update and new picture. Your build quality is inspiring.

I'd like to add my thoughts on 6SN7's. I'll admit I don't have the experience with the other tubes listed by andy evans, but I have experimented with a large number of 6SN7 varieties in my Kevin-designed 32b P-P amps. Currently configured, the 6SN7's have LL1667AM/5ma plate chokes on the top and 10 ma (5 ma/side) cathode CCS with no bypass caps. Plate-to-cathode voltage is 100 vdc.

I have tried over a dozen different 6SN7 varieties, including vintage Tung-Sol round plates, RCA red base, Sylvania (metal base, chrome tops, "Bad Boys", brown-base W's, etc,). All the good ones and some not so good. They all have different sound characteristics that can be used to "tune" the amplifier's sound. Of the old tubes, I liked the 1940's round plate Tung-Sols the best overall with their good bass, low distortion, excellent overall tonal balance, and lack of glare.

Last fall, I had the chance to try a pair of new production Russian Tung-Sol 6SN7/GTB's. Some will call this heresy, but I find the new Tung-Sols to be far superior to all the vintage types. Both bass and treble frequency responses are extended, distortion is very low, the midrange is wonderful, and they are highly dynamic without having the "punchyness" of the Sylvanias. I have also heard the Tung-Sols in a pair of Viva 845 SE monoblock amps and can report the same characteristics in that very different amplifier design. These tubes are very revealing; others on A-A have not liked them, maybe because they reveal other problems. They're cheap, and I suggest you give them a try (give them 50 hr to break in, though).

A current thread on:

RE: The Last Amp I Am Ever Going To Build. Maybe., posted on September 29, 2009 at 10:34:41
Tubelcain
Audiophile

Posts: 4
Location: Guernsey
Joined: January 6, 2008



Hello again fellow Audiophiles.

Thank you all for your comments on 6SN7 I shall keep experimenting, I am currently enjoying) a pair of Ken-Rad VT-231. This amplifier just keeps on getting better and better. I have about 200 hours running time on it now and it is sounding stunning. (with the below mods)

I had however been aware as my new 300B amplifier burnt in that there was some treble grain which wasn’t smoothing out.
I had firstly attributed this to my Tannoy Horn tweeters who have a reputation of being a bit fierce, but adding filament snubbers has made a major difference to the high/mid frequencies.
I had been reading Morgan Jones/ Things that make you go ouch from the off so I had already built my amp with an RF mains filter an interwinding screen around the HT winding plus a built in snubber network pre the 6cm3/DSE160, the quietest switching rectifiers around. (Filaments also raised 65v above ground)

My Mods were;
1/ One 1u 60v ceramic cap soldered across A tube socket filament pins. (Large positive difference.)
2/ Two 1u 60v ceramic caps soldered from each side of filament feed to centre tap of filament transformer (smoother & more detailed)
3/ One 0.10mh 8A common mode choke in filament feed. (Smoother and more detailed yet)

Overall for a few Pounds/Dollars it has made A very worthwhile improvement in the sound quality, sopranos now sound sweet as a nut, can now hear the individual members of the chorus in the mix, never heard that sort of detail before!!!!!
I am not sure but I live in a very densely populated area where there is probably much RF pollution and as the differential pair has a relatively poor rejection at RF on its cathodes it makes sense to add additional filtering to the AC filament supply. I had considered DC for the Filaments but AC now seems to be so good/quiet that I am reluctant to try with the additional switching noise problems.

Has anybody experienced the same problems/cure? And have you tried DC on the 6SN7/6BX7?

My experiments have thus far only been on the indirectly heated tubes has anybody experimented with AC directly heated tubes in this manner?

For more background see the below link:
RE: The Last Amp I Am Ever Going To Build. Maybe., posted on August 18, 2009 at 03:16:51
Tubelcain
Audiophile

Posts: 4
Location: Guernsey
Joined: January 6, 2008



Hello Again Fellow Audiophiles.

Thank you all for your kind words & advice.
Having created a reasonably flexible framework & starting with the best Iron I could afford + dial in VA I am most eager to experiment and make this an even better amplifier. I have received a few negative comments about the choice of 6SN7 as a input tube with some suggestions of some early DHT 26 or a 01A etc.
As I mentioned before living hear in the Channel Islands I don’t get a chance to listen to many tubes. There are so many out there and many personal preferences too. I am very isolated from you Guys in the Push Pull DHT REVOLUTION and am very greatfull to this forum. Do any of you have any suggestions for a superior 5-6.3v Twin triode with tightly matched halves and a gain of 20ish?
I have tried some Mullard ECC33s a higher gain tube but prefer the current Sylvania chrome top 6SN7. however I am unable to accommodate DHTs in this design, maybe in the next one!!!

Kevin thank you for your question regarding my choke mechanical noise which has now gone. ( they conduct approximately 200ma each) A combination of some mechanical decoupling & a change to Serial connection for common mode rejection + a 10k / 10nf snubber network on each half of the choke has sorted the problem nicely.
This has left me with the added bonus of a quieter blacker background to the sound stage, with more information retrieval and a more vivid tonal pallet, the bass notes now seem to come from nowhere and the ambient tails are that bit longer in the die away. This mod was well worth the effort, very difficult to give a numerical score perhaps +15%!

Thanks also for your remarks about the chassis & layout, finding a nice chassis was not easy. Bought the main chassis from Hexateq Audio Visual in Belgium, nice product if rather pricey. It is a good size however to accommodate all that iron & act as a good heat sink for the CCS’s. Still like the split level construction putting the heater wiring and regulator boards atop gave me more access to the more sensitive components below. The 1620 sit nicely isolated from the other iron with a good air flow. This also allowed me to hide the tube bases a pet hate of mine!!

Thanks again for listening, will report back about the output stage CCS + Heater snubber mods when time allows.
Now that IS impressive, congratulations! (nt), posted on August 5, 2009 at 17:04:00
twystd
Audiophile

Posts: 1663
Location: Austin,Texas
Joined: December 9, 2000
 .

RE: The Last Amp I Am Ever Going To Build. Maybe., posted on August 1, 2009 at 12:17:01
andy evans
Audiophile

Posts: 997
Joined: October 20, 2000
 "couldn’t justify using DHTs so I opted for a 6EM7/1671AMpp stage instead" ????

I'm curious how you "couldn't justify" something like a 46, 10Y or 71a as a driver!!!

And something like a 26 or a 01A instead of the 6SN7?

With such an ambitious build of such quality I'd whip out some 26s and 10Ys before you could say "Jack Robinson"!

andy
RE: The Last Amp I Am Ever Going To Build. Maybe., posted on August 17, 2009 at 13:24:16
yang43057@yahoo.com
Audiophile

Posts: 1
Joined: August 17, 2009
 6sn7 definitely is not the tube for true high fidelity amps. 26, 56 or even px4 would be better choices.

On the other hand, as a driver tube, I think 6EM7 is a nice sounding tube, better than 71a and 46 (triode mode), second to 10(not 10y). It has very liquid sound and can stand abuse.

RE: The Last Amp I Am Ever Going To Build. Maybe., posted on August 1, 2009 at 06:19:45
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 Thanks for the nice post.

I am puzzled about your mechanical noise with the LL1638/4H chokes, as I have used dozens and never once had one that was mechanically noisy. What DC current does your amplifier actually draw?

Kevin Carter
K&K Audio
www.kandkaudio.com
RE: The Last Amp I Am Ever Going To Build. Maybe., posted on July 31, 2009 at 13:20:11
tuneful
Audiophile

Posts: 87
Location: New Mexico
Joined: October 29, 2004
 Congratulations on your beautiful build, and I'm sure it sounds wonderful with the design and components you have selected. On the output CCS, I agree with nerdorama and suggest you try it. The enhanced detail with full differential output is very nice. I recently added CCS's to my 32b PP amps.

RE: The Last Amp I Am Ever Going To Build. Maybe., posted on July 31, 2009 at 12:43:26
nerdorama
Audiophile

Posts: 328
Location: Seattle
Joined: February 2, 2002
 Hi,

Great project. Any schematics to share?

Re: your ideas for mods i.e. "CCS on the output stage ... would also be interesting"

I've done a couple of amps where I put CCS's in the output stage cathodes. I think it's a great mod but you have to stay away from clipping since they force a hard limit. The micro detail from a differential output stage is really something. My amp uses some of Lynn Olson's ideas: PP differential driver stage with LL1660S. That was a great mod when changing from a LTP cap coupled driver. My outputs are triode connected 6AR6's but my long term plan is to try 300b's. What do you think of the EH's shown in the picture?

Thanks,
John
De gustibus non est disputandum
RE: The Last Amp I Am Ever Going To Build. Maybe., posted on August 1, 2009 at 06:16:31
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 My experience with clipping performance with this kind of output stage is different from yours. If you look at the clipping performance of the ST-70 mod on the 'scope, you see a nicely curved transition to a flat top on the sine wave, with no over or undershoot. All of the other amps like this I've built look similar. Sonically, all of the many amps I've built with common cathode CCSs in the output stage have overloaded in such a way that they never "bark", but just exhibit the sonic effects of mild compression, rather than hard limiting.

Kevin Carter
K&K Audio
www.kandkaudio.com
RE: The Last Amp I Am Ever Going To Build. Maybe., posted on August 1, 2009 at 11:11:44
nerdorama
Audiophile

Posts: 328
Location: Seattle
Joined: February 2, 2002
 Hi Kevin,

Thanks for the comments. I've never looked on my scope at the full power limits of my amp. I was only thinking of the hard current limit imposed by the CCS. Once one of the tubes in the pair is at the current limit of the CCS, I assume that's it. My amp seems to sound harder or harsher (hard to describe but it looses it fluid natural feeling) when I push it toward it's limits. It never sounds like a SS amp at clipping.

Regards,
John
De gustibus non est disputandum

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