Tube Amps for Studio Monitors

Hi

Have a look at my lastest diy - a pair of tube amps for my studio monitors.

Here's the insides

They are el34 push pull mullard topology with Edcor iron.

It is based on the Marantz 5 with some things from an Eico 86 too.

I used a jensen 10K:10K input transformer for balanced inputs feeding a 5687 grounded cathode amp which has a low sensitivity - around 10vpp for full output, as it designed to hang off a TC Finalizer which is on the outputs of my desk.

It has first tube stage direct coupled to 6sn7 phase splitter with an ac balance adjust pot and uses fixed bias with  seperate adjust pots for each el34. An ammeter shows bias current for each tube on a DPDT switch.

The 100mA ammeter monitors the cathode current and is in series with a 10R 1W resistor - a little less actually. Meter resistance is some tenths of an ohm so it makes up the difference to 10R resistance.

For more accuracy I would need better low resistance measurement of the cathode resistances and a lower value bias pot on the grids - 5K instead of 10K or even lower for a finer adjustment. 

When switched to each side, another 10R is substituted in opposite and vice versa.
I chose 10R resistors  which were a little more than 10R. 
So the monitored cathode and unmonitored cathode are still balanced as far as I can easily measure. (10ths of an ohm).
The fixed bias method and measurement also allows me to confirm that the tubes remain balanced.

Avoids the need for the original Marantz monitoring which measures on HV. Safer in a diy build not to have HV metering and switching !

I am biasing to 40mA, 460V plate (18.4W) and 45mA each tube. The whole thing uses around 12mA for the input tube, 16mA for the PI and 90mA for the finals -> 120mA of HV at B+ 466Vdc

Uses elevated heaters to 65V dc referenced from HV.

Feedback is currently around 12dB or so and still looking at that with my cap/resist sub boxes. The circuit is quite low gain overall, as you could imagine with a low senstivity to full output.

Just building some dummy load boxes for the performance bench tests.

I'm still tweaking but early indications are good. 

These are my first tube amp builds where I calculated and sim'd  the operating points of the circuits.

And finally, a frontal pic

very nice looking amps!!

How is the dampening in the low end?

Cheers,
Zach

I haven't done any detailed audio measurements yet, am still working on the feedback settings,
so I still don't know the output impedance, damping factor, noise and distortion etc.

Bass sounds all there and noise is pretty reasonable so far. No oscillation yet, but I've been pretty timid with it.

I have a lot of gain I can yet give up to feedback, so i think I can improve things alot, once I quantify some stuff.

I will be working on all those things shortly. I'll post some performance figures when I figure out the next stage.

Oh, and I'm using it with some 70's Sansui SP-2500 speakers. They are a 12" woofer, dual 5"mids and a 2" tweeter with a horn shaped 'acoustic lens'    Sensitivity is 92dB and they are rated at 80W rms.
You can see them in the background of the first pic. The most amazing woodwork on the solid trellis grills. They are built like a tank, weigh a tonne and sound really nice to these middle aged ears.

I think the amp is around 30W rms or so.

Cost was around $1400usd for the pair all up with jan nos 5687 and modern tungsol 6sn7 and sed/winged c el34s and sovtek 5AR4s.

Chassis are from an ebay seller I really like
http://www.ebay.com/itm/Tube-Amp-Chassis-DIY-17x10x2-Aluminum-Project-Box-Enclosure-Stereo-Amp-Preamp-/251008771286?pt=Vintage_Electronics_R2&hash=item3a7149e4d6

Took me for each unit, about 8hrs  for the mechanical assembly and about 8hrs to wire.
Unfortunately, I am not an artisan  :


Cheers and thanks for the kind words. 

Cool! I have some old Allen Organ mono blocks the use 4 6L6's each and huge air design?
output transformers. I have used them before in the studio and the low end is soo nice...

Beautiful  build. How do they sound??

Cheers

Ian

Wow, great work!

They sound pretty darn good so far  8)

Quite a big bottom end and a very solid midrange. The top end is all there too!

Hard to describe, but so far not much eq needed so far. The Finalizer Plus (which can be had pretty cheap these days) is very good as a preamp for these - a bunch of subtle attenuations/boost and eq/compression curves can be programmed for instant comparisons.

Compared to my standard monitoring, some Tannoy concentric 8 monitors and an 80s Yamaha transistor amp, it does seem to have more ... heaviosity      (they weigh around 17kg each)

Compared to my bench monitoring, some 8" JBLs hifi speakers and an 60s pp EL84 tube amp, the bottom end is way more focussed and less woolly.

All up, very worthwhile and I think it will certainly see use in the studio (ie. music room - I'm no pro!) particularly in the early stages of tracking a song.

I also like to use multiple amps/speakers so I will use it a lot for monitoring certain instruments while using the regular chain for the main song monitoring.  I find that way I get best results when playing/recording in the same control room  (ie. music room!)

It really will be interesting when I hook it up with an input source selector to accept some of my tube diy gear - I'm thinking the Orange86s, gPultecs and dLa2as.  That should be as 'old school' as I can get 

Thanks

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

Cheers

Ian

I used fixed bias on the finals just to have complete control over the operating point of the el34s.

Sometimes to run a bit hotter, sometimes a bit cooler.
Sometimes I like to sub other tubes in there  ;D

I could have used cathode bias because I do have some B+ to spare and am actually running them quite conservatively so as to give less distortion and extend life.

That said, I've not noticed much difference in my guitar amps between fixed and cathode bias, except my cathode biased Fender has failed several times with a burnt out 6L6. It is a positively evil 90s PCB based  amp. Sounds OK and is relatively cheap, though!

My fixed bias, point-point amps have lasted more than 40years without failures so far.

Probably just an indulgence really  ;D

Yes, failure mode was uppermost in my mind. So performance was not crucial to biassing. Interesting. I have been thinking about building myself a couple of tube monoblocs for monitoring.

Cheers

Ian

Apparently cathode bias' requirement of an electrolytic cap is one of the negatives people speak of.

According to Claus Byrith, it is 'not optimal'.

I would highly recommend tube monitor amps as an alternative to your regular ones.
For the guys who just can't resist diy, that is.

For those who like the pcb approach, I think Pete Millet has some very interesting designs, particularly the one which uses the Sowter traffo front end as a phase splitter going to 6SN7 driver and on to the finals which can be a range of types. Very tidy and the finals are not mounted on the pcb. It's called the 'universal amp' or something.
For the high tech types, he has the 'Engineers Amp' which I think would prpvide superior performance on a budget.
Edcor sells 'iron packages' at a very good price for it.

The main issue is adapting designs to take a very hot input. Most designs have a high sensitvity which is not useful when your tube channels are putting out +20dBu plus 

Anyway, back to optimising them ...  ;D

How big of an electrolytic cap are we talking about for cathode bias?  If it's in the 25 uF or so range, you can get Solens that will work just fine.  I've done this in guitar amps before, and it works great.

Cheers,
Zach

On the subject of push pull output stage cathode bias method cathode bypass cap sizes,  The Valve Wizard site http://www.freewebs.com/valvewizard/ on page 'Push Pull' says :

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Cathode bias often lends a natural compression or 'squishiness' to the sound, due to the increase in bias voltage when one valve enters Class B conditions, though the larger the bypass capacitor, the less will be this effect.

A small capacitor (less than 100uF say) also increases the non-linear distortion, which may be significant in hifi.
Using a large capacitor (greater than 470uF say), or using no capacitor at all reduces this effect.

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So I guess that it's way out of the range of anything but electros.

He also states :

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If using cathode bias, each valve can be given it's own bias resistor and capacitor, or they can both share the same one ... , which tends to encourage extra second harmonic distortion.

Fixed bias, on the other hand, .... the reduced compression gives faithful transient response, and a stiff or 'barking' overdriven sound.

Furthermore, there is no reason why we cannot use a little fixed bias *and* cathode bias simulataneously to achieve the desired mix of compression and 'bark'.

----

I guess there would be much opinion on the choice of cathode/fixed bias. I've read lots of them at 'The DIY Audio' forum 'Tubes/Valves' http://www.diyaudio.com/forums/tubes-valves/

Having only built a half a dozen or so tube amps, I can't really comment as to a sonic difference.

But I'll go with Claus in this instance, who speaks of electrolytic phase shift and distortion.
... (electrolytic) 'condensors are far from ideal and should be avoided wherever possible'   

Cheers

Most of the antique cathode biased PP amps skip the electrolytic. 

Fixed bias, on the other hand, .... the reduced compression gives faithful transient response, and a stiff or 'barking' overdriven sound.

Click to expand...

I have somewhat held this feeling regarding fixed bias, but in the context of guitar amps which are purposely driven well into overdrive.  I don't ever recall getting this sonic impression from my Altec 1568s which I use as studio monitor amps, but I'm well below overdrive in that context.  There's no denying the brittleness of many older Marshalls and some Fenders, all of which use fixed bias.  Likely other factors there too but worth noting Valve Wizards remarks.

Got it.  My head must have been in guitar amp world yesterday.  Oops.

Cheers,
Zach

> Altec 1568s

mmmm ......  (drool ....)

Sometimes I really do wish I was an American  ;D