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New installation of my A-S OTL amps

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Posted on June 14, 2019 at 20:36:30
jeffreybehr
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Posts: 5449
Location: Phoenix, Arizona
Joined: December 10, 2004
Contributor
  Since:
December 13, 2004
I have a pair of M-60/3.3s (45 Watts into 4 Ohms) along with a pair of 96dB-sensitive Serenity Super-7 speakers.. I felt they would be an excellent pairing, and they have been; including this combo, my system has NEVER sounded better.. I also have a pair of MA-1/3.1s (135 Watts into 4 ohms) that I bought last year to drive my Quad '2905s.. Both pairs of amps have essentially identical upgrades in coupling and PS-bypass caps, and altho the 3.3s may have less measured distortion according to A-S, I consider the two versions to be equal in sonic qualty.

Altho I have the MA-1s listed for sale, I got to thinking that as often as I change speakers--on average, maybe every two years--and as convinced as I was a couple years ago that I would buy a pair of Revel Ultima2 Salon2s in the spring/summer of '18, that the MA-1s and not the M-60s might be the more-flexible OTL amps to keep.. This afternoon I switched amps and since we in Phoenix are in the armpit of summer AND the A-S OTLs will operate perfectly with fewer than full sets of tubes*, I installed only THREE pairs of output tubes (Winged-Cs this time) in each amp.. Both amps nulled to zero quickly with 6 instead of 14 tubes, and initial listening results are quite positive.

I believe this is going to work! That is, I still have a pair of excellent-sounding A-S OTLs driving my highly sensitive Super-7s, but now I have the option of succesfully driving much-less-sensitive speakers with them in addition to saving electricity money in the summer.. It's a win-win! ... :-)

What do you think?


* while, of course, delivering less-than-specified maximum power

 

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RE: What do I think?, posted on June 15, 2019 at 09:39:01
acres verde
Audiophile

Posts: 620
Location: Big Easy
Joined: November 13, 2004
I've been running M60s since 2001 nonstop on my AG Duos and tried the little experiment you're currently involved in. When the smoke cleared (figuratively) I was satisfied with the performance when using only half of the full complement of 6AS7s in all areas save the bass, which got a tad looser and flabbier in spite of my bass drivers being active. I could've lived with it but preferred not to so back in went the rest of the power tubes...electric bill be damned.

 

Fewer tubes require a fuse change..., posted on June 15, 2019 at 09:49:56
marc g.
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Posts: 2964
Location: New Orleans
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You did change the fuse (the one closest to the power inlet, I believe) to correspond with the fewer number of tubes, right? When I spoke to Ralph about running fewer tubes he advised me to change the fuse (time-delay type) as such:

For 14 tubes, use a 5A fuse as supplied
For 8 tubes, use a 3A fuse
For 6 tubes, use a 2A fuse
For 4 tubes, use a 1.5A fuse
For 2, 10, or 12 tubes...I didn't ask but sure there's a corresponding fuse value to use for each combination that Ralph could advise about.

An equal number of tubes are to be used "per row" (doesn't matter where in the row), so one has to be able to identify same on their respective amp. With my long chassis MA-1's, a row is identified as either one of the two that run parallel with the input transformer cover. Your current production chassis appears to make that identification easy.

I ask in hopes of alerting you or others that while you can run Atmas with fewer tubes, there's a little more to do than just removing tubes.

If you find the sound of MA-1 with fewer tubes to sound the same as M60's with a full compliment, then perhaps keeping the MA-1's is the way to go. For me, power is still king with my JBL's despite their higher sensitivity. The reduced heat by running fewer tubes wasn't enough to give up the available power and headroom as my air conditioner still has to run all the same. It's nice to have options though.


marc g. - audiophile by day, music lover by night

 

TY Marc; I'll find some 2A fuses., posted on June 15, 2019 at 10:56:30
jeffreybehr
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December 13, 2004
The two amps ran just finely for several hours last nite.

 

Might be worth noting that..., posted on June 15, 2019 at 14:17:30
marc g.
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Posts: 2964
Location: New Orleans
Joined: October 14, 1999
...despite having a full complement of tubes, you do have the shorting plug in V1, so you're not getting the full power output the M60 is capable of.

marc g. - audiophile by day, music lover by night

 

Hmm...I may be wrong, but V1, however many there are in the amp..., posted on June 15, 2019 at 15:49:48
jeffreybehr
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December 13, 2004
...which varies by amp, is NOT where the gain-reduction shorting plug goes, it goes in V2, the cascade 'top'.

(V3 is the CCS in amps that use one*, and the cathode folower is V4.)

Yes? No?

But you're probably correct about not getting full power while using the plug unless one has LOTS of Voltage with which to drive the amp.


* In versions 3.3, what was V3 turns into V1c, that is, one of the paralleled triodes that create initial Voltage gain.. Here's a pic of my M60/3.3 three years ago.

.

 

I think you are correct..., posted on June 15, 2019 at 18:10:28
marc g.
Audiophile

Posts: 2964
Location: New Orleans
Joined: October 14, 1999
...about where the shorting plug goes to reduce power, now that you mention it. I don't think using them requires any change in fuse value though. To clarify, I only commented about acres verde's shorting plugs since it speaks to reducing power, but doesn't require a fuse change as running fewer tubes does.

I may be worth it to verify with Ralph that the protocol for my particular MA-1s is the same as yours, since mine are considerably older.

Have fun with your experiments!


marc g. - audiophile by day, music lover by night

 

RE: Hmm...I may be wrong, but V1, however many there are in the amp..., posted on June 15, 2019 at 19:16:26
acres verde
Audiophile

Posts: 620
Location: Big Easy
Joined: November 13, 2004
V2 in my M60s

 

RE: For sure I have less gain..., posted on June 15, 2019 at 19:23:29
acres verde
Audiophile

Posts: 620
Location: Big Easy
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...by shorting V2 but by still using 8 output tubes per channel vs 4, the amps output impedance is halved as compared to 4 6AS7s per channel. Obviously, this is advantageous for the performance of fully tubed up version.

 

Finding fuses in Phoenix is increasingly frustrating...., posted on June 20, 2019 at 12:10:23
jeffreybehr
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Location: Phoenix, Arizona
Joined: December 10, 2004
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  Since:
December 13, 2004
...with Radio Shacks long gone and Fry's Electronics allowing stock to disappear while reevaluating their marketing strategy.. I did find some fuses labled 3A. slowblow, but one of those popped, so I'm back to 4s.. I'm going to order from Mouser some genuine Littelfuse 313(3AG)s in 4A. and 5A. so I don't have to chase around town--or pay $3 each at the hardware store.

My MA-1 uses 5s in both the 1st (output-stage B+) and 2nd (filament) positions, so they'll be useful.

I'm quite happy with the sounds from using just three pairs per channel; perhaps when I'm heating the house during Phoenix's short nonsummer I'll increase the pair-count to four.. :-)
.

 

RE: Finding fuses in Phoenix is increasingly frustrating...., posted on June 20, 2019 at 14:08:54
marc g.
Audiophile

Posts: 2964
Location: New Orleans
Joined: October 14, 1999
Those fuses might be the last thing I ever bought at my local Radio Shack before they closed up. If not there, I probably went to an auto parts store.

Being in south Louisiana, pulling a few tubes to reduce heat just doesn't make a big enough difference to be bothered. I just know for couple months I'm going to peg the thermostat down a little lower than usual. Of course, in the winter, all the heat is welcome!

Thank you for sharing the details of your experiments.







marc g. - audiophile by day, music lover by night

 

I received the 3AG-313 Littlefuses in 4A. and 5A., posted on June 27, 2019 at 12:20:11
jeffreybehr
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Contributor
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December 13, 2004
I liked the looks of them immediately because they're built with a filament wound around some sort of whitish core.. Have now set aside all the 3s, 4s, and 5s I got from Fry's El.; some kept blowing prematurely as I was experimenting with fewer-than-7-pairs of OTs in the MA-1s.

 

RE: Hmm...I may be wrong, but V1, however many there are in the amp..., posted on July 1, 2019 at 15:22:13
Posts: 1
Location: Cincinnati, OH
Joined: July 1, 2019
If you decide the MA-1 meet your needs, and decide to sell your M60 Mk3.3, please let me know. I'm in the market.

 

"135 Watts into 4 ohms", posted on July 1, 2019 at 16:44:25
Lew
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Location: Bethesda, Maryland
Joined: December 11, 2000
Are you sure of this much power into 4 ohms? Sounds optimistic to me, but it makes no difference if you are happy with the resulting SPLs. I would have guessed more like 135W into 8 ohms, maybe.

 

Yup. (NT), posted on July 1, 2019 at 23:05:33
jeffreybehr
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December 13, 2004
.

 

Am now using only 2 pairs of OTs per amp, and the amps still sound..., posted on July 2, 2019 at 14:57:47
jeffreybehr
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...excellent driving my 96dB-sensitive speakers.

.

 

RE: Am now using only 2 pairs of OTs per amp, and the amps still sound..., posted on July 4, 2019 at 09:44:41
Caucasian Blackplate
Industry Professional

Posts: 8312
Location: Seattle
Joined: June 18, 2004
This will drive the output impedance of the amp up substantially.

 

Indeed, but it's driving only MR and treble into the 96dB-sensitive..., posted on July 4, 2019 at 11:38:10
jeffreybehr
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...Super-7s, so everything works very well, at least as far as I can hear.
.

 

RE: Indeed, but it's driving only MR and treble into the 96dB-sensitive..., posted on July 4, 2019 at 12:12:48
Caucasian Blackplate
Industry Professional

Posts: 8312
Location: Seattle
Joined: June 18, 2004
If the midrange and treble have large changes in impedance, the amps output will tend to follow those impedance changes more with fewer output tubes installed. Though this is most audible with low frequency drivers, it's not absent for most midranges and tweeters.

 

RE: For sure I have less gain... but the same power nt, posted on July 8, 2019 at 10:48:37
Ralph
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Posts: 3966
Location: Minnesota
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-

 

Here's a newer pic with only two pairs in the amp., posted on July 8, 2019 at 16:47:08
jeffreybehr
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They continue to sound EXCELLENT driving the Super-7s.


.

 

One of the strengths of that amp is its ability to play bass, posted on July 9, 2019 at 10:12:18
Ralph
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If the Super 7s have a woofer array that is as easy to drive as the mids and highs, you might consider running them full range (with more power tubes and a passive crossover). That amp has full power bandwidth to 1Hz.

 

Ralph, the Super-7s have an active, dedicated bass amp., posted on July 9, 2019 at 11:07:06
jeffreybehr
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December 13, 2004
The MR and tweeter are B-G mag-planars without impedance peaks.
.

 

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