DIY Tube Monitor Amps

I didn't want to derail alexc's interesting thread over here about his DIY tube monitor amps, so I thought I'd start a new one.

I've been thinking about building a tube-based amp for my hifi, and I was looking at going down the usual clone route of the Western Electric 91a.  However, over on the other thread there's a list of tube amps that were used in music studios in the days of old, and building one of those is quite tempting (just to be awkward and different!).

Is recreating one of these tube monitor amp designs worth it for a hifi set up?  I'm drawn to them by the same indescribable urge that I have towards other 1950's kit, but are they designs that stand up to modern tube amp design, or even the WE 91a?

I'm slightly worried that no one really seems to build them, which makes me think they know something about them that I don't (highly likely!).

Cool 

Western Electric 91A - that would be a single ended amp with a single 300B power tube per channel.

I've not done one of these, so I'm speaking opinion based on readin', but I think you can get a great result providing you have sensitive speakers.

The output power you can expect from a single 300B is not huge, so good quality, high sensitivity speakers are a must.

That said, many regard these as a holy grail kind of thing. Quality will depend on parts for sure and the costs will mount quickly.

Also, the directly heated tubes have more complexity than a lowly Mullard topology, but there is so much great info and support it is entirely feasible.

To me, it comes down to 'what speakers do you have', 'how loud do you need it' and 'how much spare cash you got?'

I will say that diy tube amps can easily rival many commercial units and often at a fraction of the cost. Providing you have the commitment to see it through.

I proved to myself that even a fairly haphazaard approach to layout and construction can yield some truly rewarding audio performance. Even on a fairly modest budget.

Now using my tube amps with some Tannoy 8" concentrics and I'm I'm definately hearing things I didn't using my previous studio amp.  And I don't *think* it's just my imaginatin'

I can just imagine what some of those 15" Tannoy Golds must be like   Or better yet for a Western Electric SE 300B, some of them Altec horns. The BIG ones!  Probably any of the horn loaded waveguide thingys would be the go.

Cheers and good luck
AlexC

I think I would be tempted to go for one of the well tried push pull designs to get the power to a level where really sensitive speakers are not essential. The other factor/barrier to success is the output transformer iron. It is key to the performance of tube amplifiers and as a rule you pays your money and takes your choice. Fortunately Sowter and other respected transformer makers still make output transformers for some of the classic designs.

I guess I am saying you should start with something for which the major parts of available.

I disagree that no-one makes these. A quick trip to diyaudio.com will find lots of people doing it.

I have been toying with the idea of doing this for my studio monitors because I have a pair of Tannoy 12 inch monitor golds. They sound really good anyway because of their dual concentric design but I quite fancy driving them with some tubes.

Count me in.

Cheers

Ian

Thanks for both of your replies!

When I say no one makes these, I mean no one seems to be building amps based on vintage studio amplifiers. For example the LEAK or the old RCA studio amp. Why do you think that is?

Plenty of makers - it's just they are 'boutique' cause they cost big, big dollars.
Not unusual to see high end for 5K+ usd.

And a lot of people mad enough to want to spend  a couple of K on a 7W per channel amp tend to DIY

The cheapest 300B SE amp I've seen for sale is around 1K usd.
A reasonable one starts at around 2K and rapidly escalates. Skywards!

I think a DIY of  'Edcor' monoblocs that I favour could be done for around 1.5Kusd or so.
Maybe some less. Likely some more

Candidates for iron could be Edcor, Transcendar, Hammond then James and after that, the higher price stuff.

There's no shortage of top end product if that's the arena you like to exist in.

Cheers

Probably said it in the other thread; I currently have a pair of Gates SA-10's (PP 6L6) driving Klipsch La Scala's.  10W into big (2' x 2' x 3') horn loaded high efficiency speakers.  The amps didn't do very well with modern speakers, but they really shine with the LS.  I have a variety of other low power tube amps in the restore pile, so I really need the high efficiency speakers as permanent fixtures.  Things like early RCA broadcast PPP 45 amps, Collins PP 6B4, various PP 2A3, RCA SE 6L6, parts for a 1930's style PPP 2A3.  No plans here for anything as pricey as the world of 300B's.  45's and 2A3's are enough. 

You want to run modern speakers?  You are going to need power and good damping factor.  The antiques generally won't do it, unless you get into higher power ones like MacIntosh, PA amps, or vinyl cutting amps like the Presto, etc. 

If you want something single ended & are not afraid of very high HT votages how about a pair of monoblocks using the 211 triode.  Maybe something like this
http://www.diyparadiso.com/211cools1.htm

I have to say, after serving my early years in the world of electrical engineering on REALLY hv traction motor and generators, I have a healthy *fear* of anything more than 800VAC CT 

Those very high hv amps really do make me nervous. Strange tings begin to 'appen after 1KV  :-X

A

On the subject of out-of-the-ordinary studio amps, if I had some more bucks, enought to justify YATA  (yet another tube amp), I would do a massively parallel triode SE  style 

Like 8x 6S4A in parallel for some 12W or so SE for each channel. 
I believe there is a company out there that did this.
Interesting to see if the whole wonderful triode-with-power thing would overcome the negative parallel-tubes-means-trouble thing.

Only one way to know if it would work  good 8)

A

How about $85,000 for a stereo power amp which does no more than 30 W/channel?

http://www.alumrocktech.com/prices.shtml

http://www.alumrocktech.com/amplifier.shtml

Bri

;D

Triode connect the output for sound or go ultra linear for power.

Thanks Analag for the interesting circuit.

Balanced signal path throughout, first stage with mosfet current source (or is it locked cathode voltage ?), no global nfb,  using some local nfb at the finals. Non ultralinear connection to output transformer.

Gain, I guess would be quite high with the AT7 in the first position, with a high sensitivity meaning a relatively low input voltage for full output ?

What size would the choke be on cathodes of the 5687? More of a  ferrite type of thing or a chunky high inductance ?

I'm not sure I'm a big fan of 5687 anymore : my ones have been troublesome in terms of noise and vibration sensitivity. I think it is not so easy to find good ones, at least for someone without a good tube source and an ability to select for good quality and match too, used in a differential pair.

I'd like to try a balanced amp like this one day. But I fear my diy spend is drawing to a close!

Chrs

5687 could be 6SN7 or equivalant. That is a tertiary winding across the cathodes...forgot to make that clear.

Oh, right.

So that would be feedback into the cathodes of the second stage from a tertiary winding of the output transformer.

I guess you wind your own transformers to suit.

I think I have seen some modern Sowter units which provide a tertiary winding.

Very popular method of providing feedback in vintage tube amps for sure. I've seen several of the Marantz units, Altec and others go that way.

I have to do some research to better understand the advantages of tertiary winding derived feedback over the more basic methods.

And then try again to understand McIntosh designs, which as we all know, was derived from captured alien technology.

Thanks
A

Heres a revision fer ya. Got rid of the tertiary winding just simplifying the feedback paths in general. Went ultra linear with the output.  Making use of the bias supply to increase CMMR and injecting voltage feedback into the extra input, besides I'm partial to VF over CF in tube devices.

did you abandon 3W 300b SE?

I have some 604E Altecs with some Dynaco MkIII's.  The Dynaco's have their driver boards pulled and a replacement boards by ADI added .  Its been so long since I looked at the circuit since this was all done back in the 80's.

When I switch from my 1031's to the Altecs its a big difference but you really hear vocal peaks that need tweaking on the Horns that seem to be ok on the 1031s.  I have a Jam session on Sundays and can use the Altecs for the PA for vocals.  It's Nice hearing the ALTECS with the Tube amps.