Tube Amps for Studio Monitors

[lassoharp]

Some other good stateside workhorses known to elicit drooling.

 

[alexc]

Wow !  RCA MI-9377

Wouldst that I couldst have THOSE    :'(

What beauties.
I'll bet they were also dumped once upon a time in favour of some transistor amp.
And  now cost a goodly sum  ;D

Original tubes alone would break my budget.

Thanks for the pic.

 

[CJ]

cathode bias for guitar amps will take away some of the brittleness in the sound,

i had a switch for either/or on a 6l6  amp so i could do live A/B testing and i usually left the switch in the cathode bias mode.

one of the best things about cathode bias is that it is less likely to lead to a tube meltdown, as the current goes up, so does the bias so it is a negative feedback situation, this also means you do not need to adjust bias as the tubes age,

for a house PA, i would probably go for fixed bias as you can squeeze a bit more wattage out of the tubes, however, you have to quadruple your power to double your sound level, so not thast much a difference from a loudness standpoint,

and you will get better punch at the bass frequencies in case somebody is playing a keyboard bass or someting awful like that 

you can tune an amp by playing games with the cathode bypass cap, see how a bigger value affects the bottom end, or leave it out completely, there is some canceling of signals at the cathode resistor with the push pull circuit, so you do not need as big as cap as with single ended,

 

[alexc]

Thanks for the comments regarding bias methods

My next build, a single ended parallel pair 6550 guitar amp with Edcor/Hammond iron, is going to use switched fixed/cathode biasing.

Along with triode/tetrode switching.

Then I can hear for myself 

I've already finished the nice pine speaker enclosure for it - a 15" Jensen 30W rms speaker.

It should be the least power efficient thing I ever conceived.
The iron are HUGE! The output expected 15W/25W for 200VA of input power  ;D
Just the thing for these cold climes.
Kind of a Champ on dmt 

 

[lassoharp]

Thanks for the comments regarding bias methods

My next build, a single ended parallel pair 6550 guitar amp with Edcor/Hammond iron, is going to use switched fixed/cathode biasing.

Along with triode/tetrode switching.

Then I can hear for myself 

I've already finished the nice pine enclosure for it - a 15" Jensen 30W rms speaker.

It should be the least power efficient thing I ever conceived. The iron are HUGE! The output expected 15W/25W.
Kind of a Champ on dmt 

Keep us posted on how this project works out.  I've wanted to try a single 6550 design with a good OT.  I think 12-15W would be plenty for my usage.  I have a SE amp with 6L6 that does maybe 8-9W and it will drive a 4 x 10 Marshall cab pretty well. 

15" Jensen 30W is a nice combination of big jazz tone and early breakup.  I've been wanting to build an extended, deep back combo cab out of one of the really lightweight woods and load it with a hefty 8 or low wattage 10".  Should help give everything a hollowbody flavor.  If I'm lucky the cab will resonate hard on E or A.  8)  Maybe throw in a microphonic tube like 6SL7 and let stuff happen.

 

[alexc]

For me a single 6550 not enough!  I would think around 12W in SE tetrode.

I have a stereo pair of single ended 6L6 into 8" hifi speakers of about 90dB sensitivity.
Nice but lacking in .. push.
Fabulous as an adjunct to my main monitors.
Gives the modern, mixed, compressed, effected sound from the main monitors a real, added, authentic core.
Wouldn't be without them.

But, I think they end up around 5W or so.
No tone stack but into hifi speakers with attendant crossover losses.

I like them very much but leaves me thinking 6550 not single but parallel'd is the go. (4x power for 2x volume ?)
To get some roar in the case where I have a tone stack present and a guitar speaker box (no crossover)

The driver tubes are :

6N2P as a Grounded Cathode Amp + Cathode Follower -> Tone Stack -> 6SL7 GCA (maybe single or series pair)

then to a 6SN7 GCA + 6SN7 CF as driver to the finals

How heavenly is that? 'Stuff' everywhere!

Will let you know - my last winter project to build by the fireplace.

 

[pstamler]

If you have tranformer-coupled input, how come the amp needs a phase splitter? Won't the transformer do the job?

Peace,
Paul

 

[alexc]

Well, it could have gone that way - yes. 

But I felt I wanted to stay fairly close to the Marantz 5 in this instance, being my first major hifi tube amp(s).
Except adapted to accept a balanced Finalizer output.

The low gain staging required could surely been happy with an input transformer differentially feeding a driver stage, say a 6SN7 - kind of similar to the existing one used as a PI in this instance.

Pete Millett has a pcb called the 'universal amp' which is exactly what you mention. Traffo PI to 6SN7 driver to finals.
I do think that would be the way to go  for extra hifi-ness, being a simpler path.

So the thing was 'to input tube?  or not to input tube ? '.
That was the question.

I decided it was nobler in the mind and went with a 5687 giving the lowest grounded cathode amp stage gain I could find. As well as the ability to swallow a big input - like 10vpp on average and even more if required. Most other tubes would get blasted by the high input levels.  Only used 1 stage of it - the other is unconnected, for they can surely be a hungary tube!


It provides some 'stuff' in addition to enough gain for the attenuation pot +  feedback with  gain staging reasonably optimised for noise - ie. 'minimum boosts and cuts for the job'.

I did choose  for 'over the top' factor whilst still being somewhat engineering responsible!


Didn't get the chance to try the simpler version yet - 'you pays your money and you takes your chances'.
So far, it's working out good - sound is all there. Just preparing for more quantitative bench tests.

But yes - could easy have gone the other way 

Cheers

 

[alexc]

Testing these amps with a 2 channel preamp I previously diy'd 

http://www.groupdiy.com/index.php?topic=41543.0

Wow - finally hearing that preamp breathing fully into a compatible amp!

It has a Bassman with white cathode follower output to Edcor 10K:600 transformer putting out a good 12vpp or so in typical usage. The other channel is a Brownface Vibro with SRPP to Edcor 10K:600, somewhat hotter (wcf was a little too low output here)

Max output was around 30vpp plus if I recall. It tends to be a little 'twitchy' into the usual 10K solid state console input  ;D  And it blows high sensitivity amps to bits!

Of course, the output level varies wildly with the vol/level/gain pot settings, so it can be quite low if required too.

The preamp usally feeds my Poorman 670 limiter but I took it for test with my EL34 monoblocks feeding some Sansui 12" 70's hifi 3 ways.


Well it sounds VERY very satisfactory 8)

The preamp has a lot of breadth - clean, dirty and vibro. First time I think it's really hit is stride and not be constricted by petty earthling interfacing sensibilities  ;D  Let those Vpp run free!


Anyway - the quality of the sound is pretty humbling.

Even so, all I can really do is go UP from here  !

Cheers

 

[alexc]

And another ..

It really does sound more midrange-y than my Fenders  - thats using preamps that are very close to fender designs.

I would say EL34s definately tend more to the Marshall sound - more roar in the mids than I'm used to with my 6L6 amps.

A good thing!  Even though these are intended to be hifi, I am varying the NFB to get an idea of how the 'fidelity' ranges.
I may well just put in a rotary and keep it variable. I'll still use these to monitor instruments quite regularly.

Just working out some biasing tweeks now also - particularly the input stage - 5687. I have a round -6.5V on the grid bias at present.  I may change that higher (cooler) and adjust the plate circuit too to maintain the similar level of current.

Takes a long time to 'voice' these  ( to use a Mesa turn of phrase   )

 

[bockaudio]

I am curious as to your reasons for using fixed bias on the output tubes?

Cheers

Ian

I can't think of a serious tube monitor amp that did use cathode bias on the output......
Marantz 9 (which we used at Oceanway for mids & HF) fixed bias
McIntosh 275 fixed bias
EAR 549 or 509 fixed bias
there are probably others I've never run into

 

[ruffrecords]

I am curious as to your reasons for using fixed bias on the output tubes?

Cheers

Ian

I can't think of a serious tube monitor amp that did use cathode bias on the output......
Marantz 9 (which we used at Oceanway for mids & HF) fixed bias
McIntosh 275 fixed bias
EAR 549 or 509 fixed bias
there are probably others I've never run into

What about the Leaks that were used extensively at Abbey Road?

Cheers

Ian

 

[alexc]

Well today I integrated these amps with a Finalizer into my (lounge) studio (room with gear and speakers)

Have to say I'm impressed. It's like it was but with a live band as well!

As an adjunct to my regular hifi amp and speakers, thru a digital mixer which lets me mix'n'match sources and destinations,
and fed by a Finalizer ..

it's like my regular setup but with a live band.

Been listening and playing along with a bunch of Detroit blues persons, San Fran vocal groups and trippy Brit stuff

No joking - adding this stereo tube channel at the 'front of the house'  (other stuff at back and sides) is like my usual

..  but with band as well !

Magic Alx was right - it's all about 'lots of speakers'  'all over the place'


Cheers

 

[bockaudio]

I am curious as to your reasons for using fixed bias on the output tubes?

Cheers

Ian

I can't think of a serious tube monitor amp that did use cathode bias on the output......
Marantz 9 (which we used at Oceanway for mids & HF) fixed bias
McIntosh 275 fixed bias
EAR 549 or 509 fixed bias
there are probably others I've never run into

What about the Leaks that were used extensively at Abbey Road?

Cheers

Ian

My ignorance. What are the Leaks, what was their power, what were they driving and what era?

 

[ruffrecords]

My ignorance. What are the Leaks, what was their power, what were they driving and what era?

Check out this link:

http://44bx.com/leak/PointOne2.html

and this:

http://en.wikipedia.org/wiki/LEAK

Cheers

Ian

 

[bockaudio]

My ignorance. What are the Leaks, what was their power, what were they driving and what era?

Check out this link:

http://44bx.com/leak/PointOne2.html

and this:

http://en.wikipedia.org/wiki/LEAK

Cheers

Ian

Interesting links. I didn't see the schematics or references to self bias output stages. So, state of the art for 1946-7? (Not that there's anything wrong with that)
I was thinking more of the modern control room, where  100W (at least) amps are needed, and 25 watters just won't cut it, except for small speakers, and even then you might find your tube amp had been placed out in the hall while you were out.
For example, the 3 way passive ATC's I have NEED 200W amps. But the little 8" two ways are fine with the McIntosh 225, though, in fairness, I never push them where most other people push their small speakers.

 

[bockaudio]

BTW the amps in the first link look beautiful, especially the power tubes themselves. For anyone who has not put some really well made NOS power tubes in their amps, get thee to ebay and experience the joy!

 

[emrr]

Back in the 10W era, they were almost all self bias.  In America at least, broadcast and recording tended to use 10Wish amps and the higher power stuff was used in theaters and industrial installs.  You'd get 50W PPP 807 in cutting amps like the Presto line, alongside the 10W monitoring types. 

 

[alexc]

I started to do the detail testing recently with these, after using them for a few months ..


On the bad side :

- I have a 100Hz hum at more dB than I care for - it's just audible in use, which is quite a bit too high
  It appears to be ripple on the supply B+ at the input tube. Not dependant on volume setting.
  Messing about now to reduce it.

- the input tube, Phillips Jan Nos 5687 is very microphonic - it pings like a pure and pristine little bell when tapped, quite loud.
  Subbing a 12B4A tube instead,  running at around 150V plate and 20mA.
  Hopefully I can find one with less microphonics
 

On the good side :

- the dc voltages all track very well with what I calculated
- the gain staging thru the amps all track very well with what I calculated

On the very good side :

- no oscillations at all, even at full output of 32Wrms with no visually detectable distortion 1KHz sine wave
- the frequency response is amazing : at around 10Wrms output into an 8ohm load box
    -1.5dB at 20Hz and -1.5db at 20KHz with 3dB points at something like 10hz-30Khz ruler flat

  I haven't done any feedback compensation yet - just a single 15K resistance from 8ohm tap to cathode divider at input
  That Edcor output traffo paired with the Jensen at the input is a great combo, especially considering the price.

Still very happy with the sound. I actually swapped speakers  to some cheap but good sounding dual 15"+mid+high units. I paid f* all for these but they are actually pretty good. Being so large and powerful, they have high senstivity and do the bottom extension of the amp justice. Running at modest levels, they sound relaxed and full.

I have finished reorganising my test kit - a high powered load box with a feed to the audio analyzer and an upgraded 96KHz audio interface as well  capacitance and resistance substitution boxes.

Once I sort out the hum and microphonics, I will start auditioning the feedback network and do the next level of detail testing.

Cheers

 

[bockaudio]

Thanks for the comments regarding bias methods
My next build, a single ended parallel pair 6550 guitar amp with Edcor/Hammond iron, is going to use switched fixed/cathode biasing.

I did this with a single EL34 & 12AT driver. Ended up with cathode bias. Did NOT like xfmrs made for SE on this, an old Chicago made for PP 6V6 did the trick & lived to tell.