The no-brainer project, some questions

[taj]

Hey,

I've was searching around for a Mullard topology circuit using EL34s, that uses REALLY commonly available parts - particularly the transformers. The DynaMull (attached) is a good example. But it has some curious features like 2 feedback points, low gain LTP tubes, pentode input.

Your opinion of the technical circuit, please? I'm not a tube electronics whiz, so keep it simple, I'm still reading/digesting.

Another option that caught my eye was the Leak Stereo 60. Same topology but uses a triode input, hi gain diff amp, and only one FB point (those make more sense to me). But because of the amp's different voltage structure, it uses a much less common power transformer (800VCT is common, 720VCT is much less so) and I'm not too sure what OPT primaries it would need (?).

My interest here, is to find a 25-50W amplifier project: commonly available and preferably inexpensive parts; easy to build; stable; sounds great. Something anyone could build as a first tube project, but isn't scoffed at as being "flawed" in some obvious way -- a "no-brainer" if you will (suites me perfectly).

..Todd

[rotaspec]

Your post is quite timely, as I am a relative beginner looking to progress beyond the easy low-power SE and PP designs, so I am also looking at circuits similar to these. I only need a phase splitter for the input, as I am already building an Aikido line stage, but I am quite happy to look at a fully integrated build as well.
As far as feedback loops go, while I am far from knowledgeable about them (I have deliberately avoided them in my amps so far), I think what you may be seeing as 2 loops is not actually so in the Dynamull. The feedback from o/p stage screen via 12pF would be some form of anti-parasitic filter, there won't be much audio along that path. I'll defer to the experts though.
I'm sure the output stage can be adapted to run happily with 800CT tranny, as shown in the Dynamull schematic. EL34s seem to be able to cope
There are plenty of choices of iron from Edcor or Hammond to suit EL34 in PP at the lower end of the cost spectrum.
There is an interesting and informative thread going on here:
Need Help understanding a triode fed LTP.
on the exact style of input stage topology of the Leak 60.

Gary

[taj]

Originally Posted by rotaspec

I'm sure the output stage can be adapted to run happily with 800CT tranny, as shown in the Dynamull schematic. EL34s seem to be able to cope

Hi Gary. I assume you mean the Leak design can be modified to use a 800VCT transformer. Yes of course, but I would need to recalculate the amp's operating points, and I'm not tube-fluent enough (yet) for such an undertaking.

Nor am I convinced I have found the perfect circuit for this project, so no sense planning to modify anything yet. Hence the request for opinions on these circuits (and any other similar ones that might match my parameters.)

The feedback from o/p stage screen via 12pF would be some form of anti-parasitic filter

I think you are probably correct. Thanks.

There are plenty of choices of iron from Edcor or Hammond to suit EL34 in PP at the lower end of the cost spectrum.

Yes, and a closer look at all their product lists will show plenty of 800VCT options at all price points, but fewer options at other values. That's the goal: plenty of choice at all price points. That applies to all the components involved.

..Todd

[wakibaki]

Originally Posted by taj

My interest here, is to find a 25-50W amplifier project: commonly available and preferably inexpensive parts; easy to build; stable; sounds great. Something anyone could build as a first tube project

Never happen.

w

[Nanook]

a 6BM8 based amp in push pull configuration...

or if looking for something like the Dynaco ST70, Classic Valve Design - Vacuum Tube Printed Circuit and Turret Boards will put you in touch with our very own Geek , genuinely a nice guy. He offers some circuit boards etc. Quite reasonably priced.

Or check out decware

They offer very good sounding amps, and a kit (2 watts/channel but oh so nice...)

In particular to the decware, I was looking for an amp for my brother to build, but he found a Decware SE84C local to him. 2 Watts doesn't sound like much, but it plays very loud (with 94dB speakers), and he cranks the little puppy. The kit is essentially a SE84C without an enclosure. At USD$299 this could be a real no-brainer.

I'm no expert on tubes, and thus defer to those that are

[rotaspec]

Originally Posted by wakibaki

Never happen.

w

That's a bit of a harsh judgement I think. If one has access to compactron tubes (and I don't), then Pete Millett's Engineer's PP makes a simple build at this power level as it's all on one PCB, and sounds good by all reports. Edcor even offer the iron at about $165 all up (code PT-100, look under project specials).
But keeping to simple pentode tube circuits, I'm sure there are plenty of choices.
There's a thread going on at the moment about taking the El-Cheapo design a step up in power (El-Cheapo Grande). That may fulfill the original poster's requirements.
But, given all that, cheap and good-sounding are very much relative. One man's muck is another man's meat, as they say.

[Eli Duttman]

My interest here, is to find a 25-50W amplifier project: commonly available and preferably inexpensive parts; easy to build; stable; sounds great. Something anyone could build as a first tube project, but isn't scoffed at as being "flawed" in some obvious way -- a "no-brainer" if you will (suites me perfectly).

Map "El Cheapo" on to UL 7591s, in the O/P "holes". You'll get 30 honest WPC. Fisher and Scott leveraged their investment and knowledge, by using the same small signal circuitry with 7591s, as was used with "12" W. tubes. What those 2 respected firms did, you can do too.

As for buying parts, without selling off some of your vital organs, remember what Claude Rains said. "Round the usual suspects up."

[boywonder]

Taj: The power supplies in both of these designs are quite vintage; they both are using tiny caps and tube rectification, since that's what was available back in the day.

360V CT transformers are reasonably common, check out the ST-70 replacement PA-060 (and other Dynaclone transformers) here:

Dynaco Tube Amp Replacement Transformers

Of course that one requires a large hole in the deck to mount.....

or the Edcor XPWR002......

For some reason, most of the transformers I have in the parts pile are 360-0-360......so my impression is that they are the most common, but that may just be my beginner-ness impression.

Also, in a CLC topology, you can vary the value of the first cap a bit to adjust the B+ output, so there are plenty of alternatives for the 720V CT transformer.

These are both cathode bias so that makes them slightly more "no-brainer" than fixed bias designs.

[Eli Duttman]

Also, in a CLC topology, you can vary the value of the first cap a bit to adjust the B+ output, so there are plenty of alternatives for the 720V CT transformer.

Another thing to consider is a small 1st cap. allows more B+ current to be drawn.

Look at the 7591 Data Sheet. Pay close attention to the "fixed" bias UL condition set. That setup requires a 400 V. B+ rail. The Edcor XPWR005 power trafo will provide B+ and heater power. Something low cost from AnTek will take care of the negative voltages needed.

The EH 7591 is intolerant of grid to ground resistor value cheating. Hold the 7591 grid to ground resistance under 200 KOhms. That means the LTP needs buffering with FETs. The ZVN0545A will do here.

[taj]

Originally Posted by wakibaki

Never happen.
w

There are few things that pis$ me off as much as meaningless, unhelpful snarky two-word replies. Childish and gutless. But thanks for giving us everything you've got.

..Todd