First post/First Design!

[Satellite Icarus]

Hello everyone!

I've been working on a hifi tube amp for the first half of my senior thesis and I'm looking for feedback on this first version of the schematic (attached). Comments, questions and suggestions welcome, I'll take any sort of help.

Thanks in advance,

SI

[billyz]

What triode is before the 12at7 PI ? Is D5 necessary? for that matter D4 ? Interesting point to bring the Negative feedback in . Why not Local feedback just around that stage ? What are your OT specs ? Obviously missing a preamp . Interesting to have a tone circuit in the Power Amp section, might be redundant. Also, most better Hifi designs shun tone circuits for better fidelity. Except of course RIAA EQ for the Phono Stage.

Interesting Ultra-linear Cathode biased design.

I am curious that Ancient Vacuum tube circuits would be the subject of a College ( or is it graduate program Thesis) in 2011 .

Keep the technology alive

[Satellite Icarus]

the PI and power amp tubes are actually enclosed in 2 PCL86 tubes, which are pentode+triode. The 12AT7 is the preamp (because I happen to have 5 of them), and the whole design is a frankenstein based on the Mullard 4-10.

D5 and D4 are fairly cheap additions to the traditional PS topology recommended by Blencowe in his "Power Supplies for tube amps" book. D4 is a fast switching diode of the UF4007 kind that should take care of fixing the switching noise of D2-3. D5 is a protection for the more sensitive preamp B+. Again, that's a recommendation from Blencowe's book.

With the 12AT7 being the preamp, the tone stack in front of it probably makes more sense. I've been told to put it between the 12AT7 half and the PI, thoughts?

OT specs are available from Edcor, basically the PT is a 275V C.T. with a 50V, two 6.3V and one 5V secondaries. The OT is a 8kohms/4,8,16 ohms PP transformer with 23% ultralinear taps to minimize distortion.

Hope that clarifies a few things.

This is an undergraduate senior thesis. I'm probably the only EE person at this school, which is why I rely on the internet a bit - my advisor is a physics teacher, but he has a fair amount of experience in tube circuits. I might do DSP stuff my second semester to make a job easier to find in the future.

Keep recommendations/questions/comments coming! It's a good test of what I actually know.

SI

[David Root]

Hi, this is most a most interesting and unusual project! You have an enlightened teacher. Some comments if I may.

First I would recommend a visit to www.turneraudio.au . This site is excellent for audio power amps and is highly educational. You would learn a lot in a very short time. It is pretty technical but you obviously have that familiarity already if you have no problems with Merlin's books, which are both excellent.

Tone circuit in the power section is redundant, if this is intended to be a power amp only.

I also agree that you don't need those two extra diodes in the power supply chain. Just use two UF or SF4007s in parallel in each side of the rectifier instead of the 1N4007s. Doubling up on the diodes in each side gives you solid voltage spike protection and current inrush protection too. Some people like a thermistor in the live side of the incoming AC line too.

What is the intended input signal? From a preamp, or directly from, say, a CD player? CD players put out around 1V, so a preamp isn't necessary, you can eliminate the tonestack and the 12AT7 and go directly into the PI.

If you must have a tonestack, put it after the preamp tube. Then, if necessary, depending on how much signal is lost in the tonestack, you can use the other half of a twin triode as a recovery stage, again I don't recommend the 12AT7. Most designs would probably opt for a 6CG7 here, or a 12AU7.

The PI would probably be better off with a constant current source arrangement instead of the 100K tail resistor. Turner discusses SS versions in detail.

If it is intended to process low level signals, then a preamp would be necessary.

Global NFB across more than two stages is a risky business, I would insert it into the input side of the PI. In that case the PI cap and the plate cap should be grounded at the same spot. You don't mention ground scheme but a double star with one star ground point for the plates, screens and PI, and the other for everything else, in a spot where its current flow thru the chassis will not intersect with that of the power/PI tubes ground point on its way to the chassis earth point ( where the green AC wire is connected).

If you are going to have a preamp tube in there, the 12AT7 is not ideal. It does make a pretty good PI though, at least in an MI amp like we build.

[rdjones]

With the 12AT7 being the preamp, the tone stack in front of it probably makes more sense. I've been told to put it between the 12AT7 half and the PI, thoughts?

Driving the tonestack from an unknown source may give unpredictable results.
Better to have it after a purpose built buffer, so it has a consistent source impedance.

rd

[Kagliostro]

With the 12AT7 being the preamp, the tone stack in front of it probably makes more sense. I've been told to put it between the 12AT7 half and the PI, thoughts?

The use of the not shown 1/2 of the 12at7 BEFORE the ToneStack is an obviously choice

Look to some old schematics ................ you can learn a lot only reading it

the NFB, usually, is connected directly to one of the output tap, if you connect it as is, you must give it regulation each time you change the attached speaker impedance

Kagliostro

[rdjones]

The use of the not shown 1/2 of the 12at7 BEFORE the ToneStack is an obviously choice

I thought the same thing, but remember this is a stereo amp.
The "not shown" 1/2 is used with the not shown channel.

rd

[Satellite Icarus]

rdjones is right about the other half of the 12AT7

Kagliostro: I've been looking at a number of old schematics. This design is based on the Mullard 4-10. Would you have any other designs in mind that I could use as comparison points? Also, do you have some details as to the regulation depending on output impedance issue you mention?Thanks in advance!

Mr. Root: This is intended to be a multipurpose ipod/Soundcard out/Ipod/record player amplifier (Record player these days usually have a preamp that takes care of RIAA eq, so not attempting to do any of that here).

tone stack could be useful since the speakers this amp is going were have a fairly pronounced bass response. the Mullard 4-10 had a tone stack, so I thought I might as well keep it as well.

lastly, newcomer question: What's an MI amp?

I made a mistake in one of my previous post: the PT has a tap on the 275-0-275 secondary at 55V, not 50.



For grounding, I'm thinking single ground star if possible. does that seem unlikely/not recommended?

[rdjones]

tone stack could be useful since the speakers this amp is going were have a fairly pronounced bass response. the Mullard 4-10 had a tone stack, so I thought I might as well keep it as well.

lastly, newcomer question: What's an MI amp?

Definitely keep the tone stack, or some kind of tone controls., since it's an "integrated amp" of sorts.
Maybe add a source switch on the front end in keeping with this concept.
For HiFi use the Baxandall tone circuits might be worth considering.

M.I. = musical instrument

Other circuits contemporary to the Mullard would be the Dynacos, and anything based on the Williamson design, and possibly Stancor.

rd

[Kagliostro]

I thought the same thing, but remember this is a stereo amp.
The "not shown" 1/2 is used with the not shown channel.

I forgot that

----

do you have some details as to the regulation depending on output impedance issue you mention?Thanks in advance!

See how is connected the NFB to the transformer in this project

http://singleended.web.fc2.com/PCL86PPDif.htm

(EDIT) I forgot

here you can find some explanation about Baxandall TS (perhaps the site is engaged in guitar amps and not in HiFi amps)

http://amps.zugster.net/articles/tone-stacks#Three


Kagliostro

[Satellite Icarus]

Here's something I've been thinking of: since the sources are going to be either CD, ipod, computer direct out or Record player with built in preamp, do I even need a preamp? Would it be conceivable for me to have: In>unity gain Buffer with NFB fed to Cathode>Power amp?

[Ian444]

Many modern popular DIY small stereo amps are using either a LTP input stage with CCS, or a DC-coupled cathodyne (usually a 12AT7). No preamp or tone controls are used. You might find the output of a computer or ipod to be a bit lower than a CD or turntable with RIAA preamp. For some people this can be a problem and for others it is not. Some amp schematics worth looking at are Sy's Red Light District, Tubelab Simple PP, Poindexter's Musical Machine, Eli Duttman's El Cheapo, and Gingertube's Baby Huey (which was derived from Yves Monmagnon's ECL86 amp). A search on these will come up with more info than you can read in 3 months, especially if you end up at diyaudio.com

If you keep the tone controls, it might be an idea to have a defeat switch so you can switch them out if not required.

Edit - a few links make it easy.
http://www.diyaudio.com/forums/tubes-va ... post552739

http://www.diyaudio.com/forums/tubes-va ... post645244

http://www.diyaudio.com/forums/tubes-va ... 4-amp.html

http://www.diyaudio.com/forums/tubes-va ... ost1883565