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Old 7th May 2008, 08:39 PM   #21
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Join Date: Jan 2007
Location: Iowa
Thanks andyjevans, you are really starting to push me to consider DHT's, namely 10's. Any thing special, choke loads, IT coupling, or just the usual? The only thing I am worried about with DHT's is hum because of the filaments, but that can be designed for and solved. Have you by any chance heard a 26 -> 10Y -> 300B? I have wanted to try a 26 choke-loaded line stage for a while now, from what I hear people have listened to them and not gone back to anything else.

I have read too some compelling writing in favor of the 300B driven by a 417 with a interstage transformer. I suppose the 417 could be changed to something else.

The plot thickens, it seems this simple summer project has grown into something very deep and all the more mind bending. Like I said earlier I am striving to make my first 300B SET a positive one.

andyjevans, I too would like to play with some higher power tubes. I would love to do a 845, a 813 SET, and or a GM70. The 813 seems like it would a fairly easy project to tackle, seems to have more relaxed drive requirements. Mr. Millett did his with a single 12HG7, two tubes, no fancy coupling or parts, and approx. 20watts of DH Triode strapped 813 power! The problem I have is my 21yo never-shocked fingers make me nervous around 1kv!

Cheers

James
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Old 7th May 2008, 08:40 PM   #22
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I wouldn't change much of anything about my little amp except that I wish it had a little larger chassis which would allow more room for experimentation. I'd like to try some different styles of filter caps in the amp but there really isn't enough room.

I would like to experiment with a few different cathode bypass caps to see if they make any real difference in the sound.

My amp uses a Hammond mains transformer and it seems fine it's quiet and it does not hum but it does get a little warm. A slightly larger one would be good. The output transformers are the One Electron and they seem to sound good and they only get moderately warm and probably most of that is what's radiated from the tubes themselves.

I listen to the amp nearly every day and sometimes it's on for several days at a time over the weekend and stuff.

I don't have any idea how long some of the components will last but I think the life of the 300B is quite long.
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Old 7th May 2008, 08:56 PM   #23
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If you are looking for more than 8w look at Bob Danielak's EL 509 single ended amp. It is a very nice design, direct coupled and screen driven. No coupling caps, no killer HV 400v B+ and only 2 tubes per channel. Output is about 15w. One 6BM8 and a EL 509. The EL 509 is very good sounding tube. The output transformer should be around 4.8k, so output transformer isn't difficult to get.
300B amps are nice but 8w is a bit low for most speakers unless you have something over 100dB efficient or you have a small room.
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Old 7th May 2008, 08:57 PM   #24
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Have you by any chance heard a 26 -> 10Y -> 300B? I have wanted to try a 26 choke-loaded line stage for a while now, from what I hear people have listened to them and not gone back to anything else.>>

Yes - I've used several combinations, like:
201A>26>300b
26>12A>300b
201A>10Y>300b
26>10Y>300b

In every instance the 10y was slightly better than the 201A or 26, but frankly they are pretty close. You save a lot of money using a 26 or a 201A. And you'll be pretty close to ideal. You won't be disappointed with any of these.

As for joining up the dots - my preference would be transformers.
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Old 7th May 2008, 09:00 PM   #25
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by JPeitzman
Thanks andyjevans, you are really starting to push me to consider DHT's, namely 10's. Any thing special, choke loads, IT coupling, or just the usual? The only thing I am worried about with DHT's is hum because of the filaments, but that can be designed for and solved. Have you by any chance heard a 26 -> 10Y -> 300B? I have wanted to try a 26 choke-loaded line stage for a while now, from what I hear people have listened to them and not gone back to anything else.
Here's a driver based on a 6SN7 and a 10y (or 801): http://members.aol.com/sbench/testamp.html

I built the driver as an amp, and it is a very nice 400mW. I think you could sub a 26 for the N7 and have plenty of gain for a driver. Steve has a hum cancellation scheme for the 801. I tried it, and it did make a significant difference, but I didn't have a scope to tune it, so I punted and bought some filament supplies from Tentlabs (they work very well). Nice thing about his supplies is that they have very low voltage drop, so you can try AC first, then switch to DC with the same filament supply, if you need to. I use this amp with headphones and it is very quiet.

Do play with filament voltage if you can set up your amp for it. Read his section on this. I can confirm that distortion can be minimized with a little playing here.

Sheldon
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Old 8th May 2008, 04:44 AM   #26
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JimW, you make a good point about how I shouldn't write off the SRPP just because it is not in vogue. This is especially true since it is a topology that I haven't heard before. Thanks for the input.

Can the 10Y be used in a DRD setup with a 26 or 10Y input tube? I was thinking 26 choke-loaded driving the 10Y. Or can the 10Y be connected to the 300B choke-loaded, or is this a better job for a interstage transformer?

I think it is kind of down between the 91A design with a C3g or 12HG7 and the all DHT with 26 and VT-25.

Just out of interest how would the JE Labs Deluxe with the 6SN7 driver setup as a cathode follower work? I think I am still a little drawn to this design just because it is my first 300B build it is easy. Plus I have all the tubes but the 300B's. Of course I know all the designs use low parts counts. I have heard good things from other sources too about the all DHT amps, my biggest concern is getting all the hum out, especially with the 26 input. I could try my own methods though and if that didn't work I could do like Sheldon and buy some Tentlabs units.

Hopefully very soon I can start gathering the parts that will not change, output and power section and what not. Also hopefully I can start on the case work. I am thinking white cherry with a aluminum or copper top plate. I do however have to get a dovetail jig, I have the bit but not the jig. My father is going to help me as well with the case and both he and I have been busy with work and updating rental properties. I think we are going to do a couple trial runs of both the case and the dovetails to make sure we have it down. I build the audio circuits and he spent a solid 20 years of his life working with tubes, the rest has been spent running his current business, in other words most of our combined wood working skill is building walls and framing doors, not small finish work. We do have all the tools and knowledge to do it, just not the experience, hope we are fast learners. Once the case is done, hopefully in a months time tops, I fully take over and start building. I am hoping that once we build the first case it will be easy to reproduce. I am planing on building up a few of the raw wood frames for other projects. I think I may use this same style case for a 26 or VT-25 line stage as well. For that I can spin down some aluminum volume and selector knobs...or maybe some copper ones. Anyways if all goes as planed they will be done by July, the only problems I foresee are financial ones involving the iron. This is my target though because I want July and August to listen to them before school starts again. I would also like to fire them up on the 4th of July for friends and family along side my JLH and soon-to-be Zen amps; I can have myself a small July 4th amplifier shoot out, the battle of the 8-10watt amps.

Thanks for all the input and keep coming with any feedback of opinions. I think a lot of it now is up to me deciding which path to take and ultimately what my overall budget is, both time wise and money wise. The 26 line stage may come first to the test bench so that I can play a little with it and the hum. Hopefully in the next few weeks I can switch this from a discussion to pictures and questions. No guarantees though as far as the time frame, this project is a long time coming but it still will take a little to get off the ground. A lot of love goes out to this community for all the time and help. I know this thread has only been here for a few days but I have had question on other ones as well. Everyone is so helpful and friendly, more than likely because we are all united by a love for audio. I started this hobby on a whim and out of boredom, stumbling across a schematic of the JLH Class-A I said "I can build that". I have meet some amazing people and artists along the way even though my journey is just beginning. I think I have found my new passion and my new home, thanks to all.

Cheers

James

BTW1: Has anyone custom ordered caps through ASC?

BTW2: Sorry to take my own thread off topic but how does a C3g -> 300B -> GM70 sound? More than likely the 300B would go through a interstage transformer. The other option I have considered is replacing the 300B with a KT88 or EL34...this project is a ways away though, just thinking.
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Old 8th May 2008, 08:26 PM   #27
JosM is offline JosM  Netherlands
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Dear James,

Hereby I send you two schemes of 300B ampifiers. One with 76, 46 and 300B I built for myself. A foto of this amp can you find in this forum. And one with 6SN7, 6C45 and 300B. This amp is my next project. When you built a SE 300B amp the most important parts are the transformers and the tubes. The other parts comes later. Use a Interstage between the driver tube and the output tube. The Tango NC20 is a very good IT. Use no condensator between the input tube and the driver. Use for the input and the driver tubes tubes with high mu, low ra and high Ia, such as 6C45, WE 317 etc.

best regards

Jos
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File Type: pdf 300b met 6sn7amp.pdf (16.5 KB, 271 views)
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Old 8th May 2008, 08:32 PM   #28
JosM is offline JosM  Netherlands
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Dear James,

Hereby the other scheme.

Jos
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File Type: pdf 300b met 76 amp.pdf (16.6 KB, 241 views)
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Old 8th May 2008, 08:49 PM   #29
JosM is offline JosM  Netherlands
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Hello James.

In my first reply I send you the wrong scheme. I have to make a PDF of the right scheme. I only can do that at my work. so tomorrow I send you the right scheme.

Jos
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Old 8th May 2008, 09:43 PM   #30
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Not a problem, thank you very much JosM! I always appreciate input and schematics, especially of amps that people have actually built. I have always been a little worried about using interstage iron, but I suppose if you use quality iron it wouldn't be any different than RC or LC coupling. If I did a design like this I would more than likely use custom iron from Electra-Print. These and Hammonds seem to be the easiest for me to get and I feel for the money that the Electra-Prints are high quality units.

andyjevans - I am surprised you haven't mentioned the 71A, I thought you liked this tube, end up liking the 10Y better. How do you think the 71A would sound in a choke loaded line amp compared to the 26 or 10Y? You really have me reconsidering DHT's now, I have been reading a lot about them. Do you think though that between the driver and the 300B a interstage transformers is the best or would DRD or choke-loading work?

Cheers

James
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