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 Post subject: 300B Amplifier Schematic
PostPosted: October 1st, 2019, 8:07 pm 
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Guys,

I want to build a 300B Amplifier. There are a million circuits available online but they are all designed for full range. Moreover, I have no experience with SETs.

So, Need suggestions from the SET gurus here.

Goal is to use this amp approx between 100Hz to 500Hz driving horn loaded Altec 416-B.

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PostPosted: October 1st, 2019, 8:54 pm 
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Joined: February 28th, 2013, 1:19 pm
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Shashi,

I personally tend towards a high transconductance single stage driver, such as a D3A, 6688, 7788, 6C45P, 5842, etc., pentodes triode strapped, and direct coupled like Jack Elliano's DRD design. However, this design avoids the use of coupling caps. In your case, a coupling cap between the driver and the 300B grid can be put to good use as a crossover filter cap for the 100 Hz crossover point, so a cap coupled amp may be a better choice. You can really save alot on the output transformer and use something really cheap since the frequency range is so limited. Look at the open frame Hammond universal (125ESE or 125FSE) or Edcor XSE25-3K here. They don't have alot of inductance, but you don't need alot. You will still need to have a filter to roll off the top end. Now if you are planning to use an active crossover for both points, then you will want to have more inductance on the bottom (i.e. wider range transformer) and use a larger coupling cap so the low end frequency and phase response is flat at the crossover point.

I would avoid two stage drivers (i.e. cascaded 6SN7) since they have too much gain, especially driving horns. You will really have to attenuate the signal into the amp. Still, that could be useful if you are trying to match a much lower efficiency bass speaker.

SE amp are very simple beasts.

David


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PostPosted: October 1st, 2019, 9:05 pm 
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One other thing. Filament (Heater) supply for the 300B. This is a big issue to address, since the heater noise can be an issue using high efficiency speakers like horns. I would recommend DC rather than AC heaters with humbucker pot. Use a CLC filter circuit, with a power resistor in line with the choke to adjust voltage to 5V at the filament. Remember you will be pulling 1.2A through the choke, so plan on the voltage drop through both the choke, resistor and rectifier to determine the transformer voltage. You will see a number of examples out there.

David


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PostPosted: October 1st, 2019, 9:10 pm 
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Out of curiosity, what amp are you planning to use for the higher frequencies?

Ray


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PostPosted: October 2nd, 2019, 7:56 am 
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Thanks David.

I found two versions online. Don’t understand the operation. Won’t the DC reach grid of 300B?

Image

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PostPosted: October 2nd, 2019, 8:11 am 
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ratbagp wrote:
Out of curiosity, what amp are you planning to use for the higher frequencies?

Ray


Haven’t decided on other frequencies. Either 300B or 6c33c for JBL LE-2 between 500 and 1500/200Hz approx. Above that, small tubes. Got a radio receiver with built in EL84 SE output section from David B. Some thing for me play with.

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PostPosted: October 2nd, 2019, 8:26 am 
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Years ago I created a 'tube taster' linestage where I could easily switch between different tubes and hear the differences. And just like single malt whiskeys, there are different flavours. It would be worth thinking about using the same type of input tube for each frequency range amp. You might even extend that idea to output tubes as well. Here is an example from Romy the Cat.

http://www.goodsoundclub.com/Site_Images/6-Chennal_Melquiades_DSET_Amplifier_Rev3.jpg

More about the tube taster here. Don't be too embarrassed if you feel that you can't quite match the level of craftsmanship on display here.

http://audioratbag.blogspot.com/2012/02 ... stage.html

ray


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PostPosted: October 2nd, 2019, 2:52 pm 
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Joined: February 28th, 2013, 1:19 pm
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Shashi,

If this is your first foray into SET, I recommend a cap coupled driver as I discussed in the second part of my email. It makes more sense for what you are trying to do since you can build the high-pass crossover into the amp using the coupling cap. If you do not understand DC drivers like the DRD circuit, then it is like throwing you into the deep end of the pool to learn to swim.

That being said, the DRD arrangement is a interesting one. Basically, you set the cathode of the 300B to a high voltage (i.e. a large cathode resistor) and feed the plate of the driver tube through a choke from the cathode of the 300B. You want enough voltage drop from the 300B cathode through the choke (you need additional resistance, hence the choke connection from a voltage divider below the cathode) to bias the 300B. with the plate of the driver direct coupled to the grid of the 300B. The choke acts to linearize the driver tube (like a current source), as well as acting as a low impedance grid choke on the 300B. You end up with an amp that can output more power than a classic 300B SET (i.e. drive into A2). It is a neat circuit, but you have to work out the biasing of the 300B (hint, all about the operating point of the driver tube).

You need a midrange and tweeter horn? The midrange horns have the LE2. Using a Fostex on the top. I think right now it has a passive crossover between the mid and tweeter, but need to check. I am full in my Elsinore project, so really do not intend to go back to horns. If someone else can have fun playing with them, I am happy.

I do have a 45 amp that would be good for the tweeters if you are interested. Saves you the effort. It is based on the Rankin Bugle. Again, an amp I will not be needing with my current setup.

David

David


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PostPosted: October 4th, 2019, 7:32 am 
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Ray,
My goal is building the DSET around Altec 416-B.
Found this power transformer for $70 on eBay capable of driving the DSET.

https://www.ebay.com/itm/Huge-Tube-Amplifier-Power-Transformer-6550-20-pounds-3/193063959113?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3Ddf7cccb2c7cc4064841ce2fce77812cf%26pid%3D100675%26rk%3D1%26rkt%3D15%26sd%3D193063959113%26itm%3D193063959113%26pmt%3D0%26noa%3D1%26pg%3D2481888&_trksid=p2481888.c100675.m4236&_trkparms=pageci%3A744e1dd0-e696-11e9-9e8f-74dbd1803b62%7Cparentrq%3A966f6e4b16d0a4e884438bf5fffec0e1%7Ciid%3A1

Tube comparer idea is neat.

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PostPosted: October 4th, 2019, 8:06 am 
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David,

DC on the grid provides the bias offset for AC signal. I will build the 300B DRD amp first. Ordered a pair amorphous core OPTs from eBay. Hopefully, they are good.

https://www.ebay.com/itm/3-5K-SE-8C-CORE-amorphous-core-AUDIO-OUTPUT-TRANSFORMERS-pair-FOR-300B/254171118771?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I was looking for alnico drivers for the mids. Could not find JBL LE8T or LE5-2 in good condition. So, bought a pair of JBL 2105 and 2118 (ferrite) to try out. I am going to try these drivers in open frame first. Once I know 2105 (rebranded LE5-2) mates well with 416, I might take your Edgar horn. I will be driving these inky upto 2kHz max. Above that I have a nice pair of DuKane 5A540 compression drivers with exp. horns rated for 500Hz. Since the low end is raised, 3D printing of Tratric or LeCleach horns becomes feasible.

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