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Old 27th November 2005, 02:41 PM   #1
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Default Which amp to DIY as a first project?

I'm a relative newbie to DIY'ing audio equipment, so I'm wondering if anyone has a suggestion of a nice-sounding yet simple tube amp project to get my feet wet with. 25-35 watts/channel is ideal, but aside from that, I'm open to suggestions...

Anyone?
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Old 27th November 2005, 03:03 PM   #2
speaker is offline speaker  United States
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Default Do you want to scratchbuild or rebuild?

If you have never built an amplifier from scratch, rebuilding and upgrading an existing model might be the most satisfying way to start. Some thoughts -

Dynaco ST70 - rebuild as stock & get to know it. Later, look to replace the front-end, upgrade the power supply, etc.

Generic Lafayette or Allied stereo integrated - they are on eBay all the time & usually have decent output tranformer size. Simple recapping makes a tremendous difference. Later, think about either eliminating the tone controls or making discrete PEC's to replace them.

Magnavox/RCA 6V6/6BQ5 console stereo amplifier - cheap & ugly, anything you do will be an improvement. I've heard a couple redone examples and they sounded quite nice. Recapping & power supply improvements would be the first things I'd do.

The other alternative is to build a kit. This way you'd be getting a known assuming that you buy one with a decent rep. Rebuilding an already done amplifier can be more time consuming than a kit as often part of the process is fixing the prior owner's mistake. Either way though, it is a good place to start.
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Old 27th November 2005, 03:17 PM   #3
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Will approx. 8 WPC be enough? If so, check out "El Cheapo". It's low budget and a preamp is not needed.

El Cheapo Project
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Old 27th November 2005, 03:53 PM   #4
edl is offline edl  Hungary
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What about Mullard 5/20?
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Old 27th November 2005, 09:54 PM   #5
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Default Re: Do you want to scratchbuild or rebuild?

Quote:
Originally posted by speaker
If you have never built an amplifier from scratch, rebuilding and upgrading an existing model might be the most satisfying way to start. Some thoughts -

Dynaco ST70 - rebuild as stock & get to know it. Later, look to replace the front-end, upgrade the power supply, etc.
Excellent idea - hadn't considered that.

Is there anything I should look for in an old ST-70? Anything to avoid?

Quote:
The other alternative is to build a kit. This way you'd be getting a known assuming that you buy one with a decent rep.
I've searched around, but only seem to find SET with very low power - any ideas? What are some well regarded kits?

Quote:
Originally posted by Eli Duttman
Will approx. 8 WPC be enough? If so, check out "El Cheapo". It's low budget and a preamp is not needed.
Nah, I need a good 25 minimum, I think, for normal dynamic speakers in a medium sized room.

Quote:
Originally posted by edl
What about Mullard 5/20?
Looks like Greek to me - can you give me more info? Off to google...

Thanks for the help so far. I really am a tweaky guy as my main monitoring system at the studio is a pair of B&W Nautilus 802s driven by a Pass Labs X250 and augmented by dual Velodyne DD12 subs. I want to make sure whatever I build actually sounds good as what I'm accustomed to sounds pretty good...
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Old 27th November 2005, 10:52 PM   #6
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Brad,

You didn't mention a preamp and it's always a consideration.

It's possible to extrapolate the "El Cheapo" concept from triode wired 12AQ5s to pentode wired 7591s. Costs go up considerably, but you will get 30 nice WPC. Also, it's an integrated; so, the preamp issue is already resolved.

Please keep the fact that the power to weight ratio of tube amps is less favorable than that of SS in mind. I have a TERRIFIC 60 WPC tubed amp, a H/K Cit. 2. It weighs over 70 pounds.
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Old 28th November 2005, 12:16 AM   #7
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Quote:
Originally posted by Eli Duttman
Brad,

You didn't mention a preamp and it's always a consideration.
Likely go passive utilizing a bridged-T attenuator...

Quote:
It's possible to extrapolate the "El Cheapo" concept from triode wired 12AQ5s to pentode wired 7591s. Costs go up considerably, but you will get 30 nice WPC. Also, it's an integrated; so, the preamp issue is already resolved.
Well, the qulaity of the power amp is most important, not the cost. Being this is a hobby piece that is likely to end up in the lounge for casual listening, it's not a requirement that it be 'The Greatest Tube Amp Ever', but I'd like for it to sound really good. Cost is secondary...

Quote:
Please keep the fact that the power to weight ratio of tube amps is less favorable than that of SS in mind. I have a TERRIFIC 60 WPC tubed amp, a H/K Cit. 2. It weighs over 70 pounds.
Oh, I'm aware! But I won't be lugging it around, so that's not an issue for me.

Thanks for all your help so far, guys, this place is amazing!

Any other suggestions on what I should look at?
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Old 28th November 2005, 01:14 AM   #8
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Quote:
Likely go passive utilizing a bridged-T attenuator...
The grid leak resistor at the I/P of the "integrated" is 100 KOhms. That mates well with 10 KOhm passive level controls. In the bargain basement configuration, a low cost Alpha Carbon pot. is used. The bridged "T" is obviously an improvement. IMO, the only place for passive level controls is inside the power amp, which makes it an "integrated". Otherwise cable capacitance can cause PROBLEMS.

Quote:
Well, the qulaity of the power amp is most important, not the cost. Being this is a hobby piece that is likely to end up in the lounge for casual listening, it's not a requirement that it be 'The Greatest Tube Amp Ever', but I'd like for it to sound really good. Cost is secondary...
The 7591 "integrated" is anything but bargain basement. It will sound GOOD, BETTER than a "mere" lounge unit. You seem willing to part with some cash. That allows you to obtain high quality O/P trafos, which are essential for superior performance. In the bargain basement "El Cheapo", steps, including bass bandwidth limiting and O/P power limiting, were taken to work around the weaknesses of the low cost O/P "iron". Better "iron" allows those sort of measures to be side stepped. The 7591 amp's bass 3 dB. down point will be approx. 17 Hz.
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Old 28th November 2005, 03:33 AM   #9
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Quote:
Originally posted by Eli Duttman
IMO, the only place for passive level controls is inside the power amp, which makes it an "integrated". Otherwise cable capacitance can cause PROBLEMS.
Sure, but I have a passive monitor controller in my mastering room and have zero issues with HF response. Just gotta be careful!

Quote:
The 7591 "integrated" is anything but bargain basement. It will sound GOOD, BETTER than a "mere" lounge unit. You seem willing to part with some cash. That allows you to obtain high quality O/P trafos, which are essential for superior performance. In the bargain basement "El Cheapo", steps, including bass bandwidth limiting and O/P power limiting, were taken to work around the weaknesses of the low cost O/P "iron". Better "iron" allows those sort of measures to be side stepped. The 7591 amp's bass 3 dB. down point will be approx. 17 Hz.
Well, that sounds like a winner - I have no objection to an 'integrated'. Is there some place I can find a kit for this 7591 integrated or is this going to be way more involved? Is it simple enough for a relative newbie to take on?
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Old 28th November 2005, 04:53 AM   #10
speaker is offline speaker  United States
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Quote:
Is it simple enough for a relative newbie to take on?
I like automotive analogies -

Can you change a tire?

Can you change a crankshaft?

The level of difficulty for you will be somewhere in-between those extremes. That is the main reason I would recommend starting on ground that has been already trod until you get acclimated. Scratchbuilding is very rewarding or very frustrating depending on your level of expertise. Although I've never recommendend a ground-up amplifier build to someone who has never done it before, if you do decide to go that route you'll get plenty of learned advice & commentary here. In other words, you won't be out there without a net!



As far as things to avoid on an ST70, the only thing I would be sure of is that both output transformers, the power transformer, & the power supply choke are good. Everything thing else is cut & paste. The amp will deliver a solid 30+ watts too. The other thing going for the Dynaco is that not only has nearly every tube afficionado in the USA had one or more, there is a myriad of parts available for it. It is the small black Chevy of the vacuum tube amplifier world. Any audio forum out there will have knowledgeable users of it. One hell of a resource base. It is nearly the equivalent of JBL in that regard.

Good Luck in your quest!
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