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Old 5th November 2006, 03:36 AM   #1
chops is offline chops  United States
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Default Desinging my very first tube amp...

I am in the infant stages of designing a tube amp. Since this is going to be a first for me, I am going to be designing a single monoblock. If all goes well, I'll just build a second matching monoblock.

I'm not sure about much of it yet, as in if it's going to be SE or P/P and Tube or SS rectification. Heck, I'm not even too sure on what I'm going to use as output tubes yet!

I do know that I'm going to use a low MU tube for the driver stage, most likely a 12AU7 or 6SN7. I'll probably go with the 6SN7 because of the beefiness of it. If I go with a tube rectifier, it will probably be a 5U4G.

As for the output tubes, I have entertained the idea of everything from a single 6AS7 in P/P (naturally), to a bank of 6 - EL84's to a single or pair of 45's to even things like a single 211, 811 or 845 tube.

At one point, I was even thinking about running a single 6AS7 in stereo mode and run both channels off of each side of it for some nice SE power.

Please keep in mind that I am doing this project on a pretty tight budget and want to keep cost as low as possible.

BTW, I've got some questions if anyone feels up to giving some free advise...

1) So what do you all think?
2) Any suggestions on tube or SS rectification?
3) A single, pair, quad or even a bank of output tubes?
4) Who do you all use and/or recommend for PSU and output trannies? (Hammond, Parts Express, etc, etc...)
5) Any other good, reliable and inexpensive places to shop for various parts?

Thanks in advance!!!
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Old 5th November 2006, 03:50 AM   #2
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One more thing... I have also been tossing around the idea of going with an OTL design, but that still seems a little unclear to me how it works and such.

I'm definately open for info on this!
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Old 5th November 2006, 04:37 AM   #3
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The following suggestions are my own opinions in good faith. Not everyone will agree but that's OK.

1) So what do you all think?

I'd avoid designing for difficult tubes. Since you're starting from scratch, you may as well avoid tubes with poor linearity like 6AS7 and 12AU7. 6SN7 is fine, though, as is its nine-pin miniature equivalent 6FQ7. Some of the OP tubes you mention require high voltages, which can make power supply design more tricky and (of course) can be more dangerous to use.

You need to decide what speakers you're going to use and what you expect in terms of sound character and power. The SE/PP decision hinges on this.

Don't skimp on the power supply. It's as important as the rest of the amp. Use a choke, preferably in a choke-input filter if your power tranny has enough voltage. You'll get better smoothing and regulation.

It's best to avoid OTL, IMHO. It's a topology that seems unnatural, terribly inefficient and potentially dangerous to me but YMMV.

2) Any suggestions on tube or SS rectification?

SS is a less expensive way to go but I don't think it really matters, as long as you get what you need from it. For example, if you plan on a PP output stage that is in a class other than Class A, you'll need good regulation and SS rectification will help. Avoid the old noisy SS diode types like 1N4007, there are better choices. If you plan on Class A (PP or SE), then regulation isn't so critical and you may prefer tube rectification, which is reputed to sound better than SS.

3) A single, pair, quad or even a bank of output tubes?

My preference for a first design would always be PP, using one pair of OP tubes per channel. Parallel tubes need to be balanced carefully, to avoid one tube hogging the current. PP is easier to design and build, in terms of achieving success. I realise SE seems simpler, at least in terms of the number of parts involved, but it's more of a challenge to get it working well.

4) Who do you all use and/or recommend for PSU and output trannies? (Hammond, Parts Express, etc, etc...)

Lundahl trannies and chokes seem to be quite reasonably priced, compared with other top-qualilty iron. They're not cheap, though. Hammond are less expensive and are probably OK for power tranny and smoothing choke but I don't know about their OP trannies. For the power tranny, I'd plan to have some spare capacity, in terms of available current, so it doesn't run hot.

5) Any other good, reliable and inexpensive places to shop for various parts?

Can't help here.
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Old 5th November 2006, 06:24 AM   #4
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Quote:
Originally posted by ray_moth
Hammond are less expensive and are probably OK for power tranny and smoothing choke but I don't know about their OP trannies.
Great bang for the buck. Edcorusa.com has about the same Hammond quality for 1/2 the price

Of those tubes listed, 6SN7 or as Ray said, 6FQ7. The rest will put you over budget or be out of your skill since this is your first amp, or in the case of the 6AS7, not the best for linearity.

What power do you want? If you have efficient speakers, a simple SE could be the way to go.
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Old 5th November 2006, 06:38 AM   #5
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Hi ray_moth!

Thanks for the input.

I failed to mention this before, but my father will be doing the majority of the building (a.k.a. - ALL of the building) as he's been building tube amps and crystal radios out of cigar boxes and old cookie tins since he was 8 years old!!!

In fact, he was just telling me the other day that back in the late 60's or very early 70's (before I was born) that he built from scratch his own design integrated amp using something like a pair of 6L6G output tubes per channel in PP which was good for around 35wpc. Tube rectified of course, and it had 3 inputs for tuner, reel-to-reel and phono with RIAA built in. And all of the other details of that amp that he told me about, it was like reading the description of a modern tube amp in Stereophile Mag or something!

So you see, I won't be totally in the dark doing this project. On the other hand, I still need to ask these questions because I want to get the proper parts and because my father isn't up to date on the most current "hi-fi" amplifier designs.

This is why I was thinking of a more advanced amp design. We've been talking about this for the last month or so and I'm really impressed with some of the stuff he's come up with so far for this amp. It's pretty much like anything I can think up, he can build it! I'm kind of just there for the "gathering of information" and for designing the layout and visual design of the amp. The final circuit design and build is totally up to him.

Again, thanks for the input so far and I look forward to getting some more soon from other members as well! I'm really looking forward to getting things rolling.

BTW, his website is in my Sig below.
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Old 5th November 2006, 06:45 AM   #6
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Darn it, I keep forgetting things! LOL

As for output and the speakers being used...

This amp(s) will be used on speakers ranging from my brother's 92dB 1w/1m Infinity's to my 106dB 1w/1m Altec 511B/902-8B combo on my Klipsch Cornwalls (99dB). They are actively crossed over and bi-amped, but I could build a simple 2-way passive network for them, that much I CAN do.

But I'll be trying this amp on a wide selection of speakers over time, so I'm guessing at least 10 watts or so. One of the reasons I was looking at those 845 tubes is because they are good for a solid 20 watts each, if not more!
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Old 5th November 2006, 01:45 PM   #7
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BTW, here are a few pics of my father's handy work. He built this shortwave radio about a year ago for one of the guys on the Klipsch forum, all from scratch and with no schematic to go by... Well, except for the one thought up in his head at the time.

As you can see, he is quite capable!

Click the image to open in full size.

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Old 5th November 2006, 01:51 PM   #8
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Chops,

PP "12" W. multigrid O/P tubes switch selectable between triode and ultralinear modes should be fine for the speaker range you mentioned. If the nominal impedance of your brother's Infinitys is 4 Ohms, they might be problematic. If the impedance curve has significant dips below 4 Ohms, a tube amp may not be able to control the voice coils (too small a damping factor).

If you restrict the small signal circuitry to 'SN7 family tubes, you will wind up using Williamson topology. While "tried and true", Williamson amps have more than their share of phase shifts. REALLY good (COSTLY) O/P trafos are a must. OTOH, if you are open minded about the small signal tubes, I suggest you peruse the "El Cheapo" Project thread. "El Cheapo" is exactly the sort of low cost/solid performance project you seem interested in. If you build an "El Cheapo" with DynaClone Z565 O/P trafos, you will be pleased.


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Daddy's work is SUPERB. You can rest easy about the end result.



edit: added remark about father's work
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Old 5th November 2006, 03:31 PM   #9
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Quote:
Originally posted by Eli Duttman
Chops,

PP "12" W. multigrid O/P tubes switch selectable between triode and ultralinear modes should be fine for the speaker range you mentioned. If the nominal impedance of your brother's Infinitys is 4 Ohms, they might be problematic. If the impedance curve has significant dips below 4 Ohms, a tube amp may not be able to control the voice coils (too small a damping factor).

If you restrict the small signal circuitry to 'SN7 family tubes, you will wind up using Williamson topology. While "tried and true", Williamson amps have more than their share of phase shifts. REALLY good (COSTLY) O/P trafos are a must. OTOH, if you are open minded about the small signal tubes, I suggest you peruse the "El Cheapo" Project thread. "El Cheapo" is exactly the sort of low cost/solid performance project you seem interested in. If you build an "El Cheapo" with DynaClone Z565 O/P trafos, you will be pleased.


-----------------------------------------------------------------------------------


Daddy's work is SUPERB. You can rest easy about the end result.



edit: added remark about father's work

Hi Eli,

Thanks for the complement on my dad's work. Everyone has always been impressed with his work. In fact, his last job right before he retired a couple months ago was desinging pipe organs for Foley-Baker, Inc up there in CT, and he was also the one updating and rewiring all of the organ consoles to include anything from a full restore to incorporating a MIDI interface.

Way back when, when he was just a kid in the army (late '60s), he was the one teaching the electronics classes (the instructor realised that my father knew more than him and left the teaching to him!!) and was in charge of the Signal Corps maintaining and opertating everything to keep communications up and running while in France and Vietnam.

******

Anywho... My brother's Infinitys are 8 ohms, so that shouldn't be a problem there. My Altec topend is 8 ohms as well, however the Klipsch K33-E woofer is 4 ohms, but is still an easy load to drive, even with flea-powered 2A3 amps.

I am planning on building a custom bass bin dedicated to the Altec 511B/902-8B's which will incorporate 2x 15" drivers of about 98 - 101dB each per driver. So I can have up to 101 - 107dB 1w/1m bass bins (if not more) somewhere between 4, 8 or 16 ohms depending on what drivers I go with. Also, I'm not sure if I'll be going sealed, ported or even Open Baffle with the bass bin. Since those 15's have to reach up to 600Hz or so, the OB design would help with the midrange.

So what would you recommend going with other than 6SN7's? I want to use a low MU tube just keep noise and such at a minimum, plus I feel they give a warmer, smoother, even cleaner sound. Would a 12AU7 be a better choice?

Yes, I know iron is NOT cheap and I do not want to skimp on that, especially not for the PSU. If the O/P trannies roll off around 30Hz or so, I won't worry too much about that as I doubt my bass bins will go much lower than that anyway, not to mention that I have my mains crossed over at 50Hz anyway to a push/pull sub. Although, I did see a trans on Hammond's site that did go down to 20Hz, I'm not sure on the price though.

Also, my father has a bunch of good sources on various parts and such, including trannies, so he might be able to get a good deal on something as well. Eventhough those sources are msotly tube radio based, a lot of the bigger old radios used some major quality iron. And talk about a large supply of 45 tubes!

I know the majority of cost for this project is going to be for iron. My father informed me of that early-on. Possibly the next biggest cost would be the O/P tube(s).

I would really like to go with something like a 211/845 tube, but my father hasn't had much if any experience with them. I like the idea of getting a solid 15-20 watts of SE power, but that might run into more money than I can spend right now. So with that in mind, I should keep my "wants/needs" in balance and go with smaller signal tubes as you mentioned. Have any in mind?

I'm assuming you're referring to the 12AQ5s as the O/P tubes. What really sucks is that I found seven 6BQ5s in my father's stash and all of them tested bad. The filament in one of them didn't even fire up! What a shame.
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Old 5th November 2006, 05:30 PM   #10
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Chops,

The 12AU7 is INFERIOR to the 'SN7. Mu is the same (20); everywhere else, the 'U7 can't lift the 'SN7's jockstrap.

Stay away from transmitting triodes until you accumulate experience. 1 KV. B+ rails can get you killed!

There is MUCH to be said in favor of 2 stage amps, like "El Cheapo". Noise, hum, and phase shifts are reasonably easy to control. Also, "motorboating" is unlikely.

Yes, we were cost conscious in designing the "El Cheapo" PSU. However, the design is UNCOMPROMISED. A crappy PSU inevitably results in a crappy amp. We wanted something cost effective that works well. Add a little more money for good O/P "iron" and it works VERY well.

See if you can find a set of 4 6V6s in dad's stash. The 6V6 is perfect in "El Cheapo". The 12AQ5 is an inexpensive 6V6 "equivalent". Triode wired 6V6 family tubes are incredibly linear and they aint bad UL.



edit: removed schematic remark, as it's too big to upload
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