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300(may)B(e) - Recs & Advice

ddroukas

Member
2016-02-15 6:38 pm
Hi everyone. I’m new to the forum and planning on venturing into the home stereo territory. I have several years experience building P2P tube guitar amps but have never built anything related to home audio. My listening scenario is mainly instrumental music (finger picked guitar/singer-songwriter), 50s/60s pop, and some more modern material such as Mogwai and Ratatat. I live in a tiny NYC studio and have little use for high-wattage designs. I’ve always favored SETs in guitar amps and would like to pursue this route in a stereo design if advisable. 300Bs are at the top of my list but due to my tastes I’m concerned if I shouldn’t go for a wider-palette design such as the ST-70. My speakers aren’t that efficient either: Sony [sorry] SS-CS5 at 6Ohms, 100W, 53Hz-50kHz, 87dB sensitivity. I know these are not ideal for a 300B but like the saying goes dress for the job you want not the one you have. In no particular order my questions are these:

General:
1. Is it ill advised in my scenario to build 300B? Should I stick with something like the ST-70?

Thorsten EL84-300B Legacy (URL: http://community.fortunecity.ws/rivendell/xentar/1179/projects/legacy/Legacy.html)
1. Anyone know the input sensitivity using an EL84 or SV83?
2. Does the 6AS7 PSU provide any significant improvement in ripple reduction compared to a more traditional LCLC filter? Thorsten reports 10mV ripple at 420VDC on full load. I’ve read an LCLC filter (5H-47uF x2) reduces ripple by a factor of 17,580 (or -84.9 dBv), but I’m not sure how these numbers relate to a mV ripple at a given voltage.
3. What is the function of the EF86 in Thorsten’s PSU design?
4. What wattage should the 1M variable resistor from B+ to ground be?
5. Any problem putting a 250K/Other value volume control on the input grid of the EL84/SV83?

Suncalc’s 300B SET (URL: DIY Audio Projects Forum • 300B SET Design Project
1. Has anyone else built from this schematic?
2. Any comments on this particular design versus the common WECO 1/2 6SN7 x 2 per channel design?

DIYAudioProjects 300B SET (URL: DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier Project)
1. Any idea where I could find a couple 200H 40mA plate anode chokes?
 
That 87 dB. speaker sensitivity is, unfortunately, a show stopper. Something like 94 dB. sensitive is what the 8 or so WPC you get from SE 300Bs should be mated to.

The bad news may not be (sorry) over. What Sony claims for the SS-CS5 speakers may require derating, depending on what the impedance curve looks like and how the 87 dB. sensitive number was established. For instance, 2.83 V. into 6 Ω is 1.33 W., not 1 W. and the "real" sensitivity is lower than 87 dB. Substantial dips in the impedance curve, particularly the power hungry bass region, lower the impedance from its nominal value. Once again, sensitivity derating is necessary.

The fact that your listening space's volume is small is a plus in the power needed equation. I strongly suggest you wade through this lengthy thread, where the OP finally settled on "cloning" the H/K Cit. 5. The "five" is a superior implementation of Mullard style topology that will drive your Sony speakers well. Give the speakers' 53 Hz. LF limit, bandwidth limiting the amp to say above 35 Hz. would make yet more power available for actually filling the listening space. GKF is Canadian and he opted to use Hammond O/P "iron". As you are in the U.S., Edcor's CXPP60-MS-6.6K on the 4 Ω tap is what I'd use. If you are considering different speakers, with better bass extension, you very well could need more capable "iron" to provide the magnetic headroom GNFB demands.
 

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wdecho

Member
2014-10-28 11:45 am
The wattage you need is going to depend on your volume expectations with your 87db speakers. I am listening with a 3/4 watt SET with 86db speakers in my kitchen with plenty enough volume for my taste. I would not build a 300B amp to start if I were you. The cost expenditure will be high for an amp you may not like with your speakers especially if you buy the better tubes. Tubelab has some great choices with detailed build guides and plenty of reviews. My favorite SET for now is the 6CB5A of Thomas Mayer, vinylsavor. I built the economy version and simply love it and the cost is not that great for a superb SET. I honestly think you may be happier with your speakers with a good push pull build with more power than what you will get from a SET.
 
The wattage it takes through speakers of a given sensitivity to reach a satisfying volume level is highly subjective. I play a 1.5 watt per channel SET through my own speakers at maybe 89 or 90 dB sensitivity, and it will play loudly enough to stress my ears. I don't need more volume than that. This amp was my first venture into building a tube amp and I was leery of how much volume I would have with only 1.5 WPC, but I've been amazed. An eight WPC amp now sounds huge to me.

I suggest to build the amp you want and then if there's not enough volume for you, try some more sensitive speakers than what you have. Yours are on the low side.
 
Build the amp. You can always upgrade the speakers. Dont let a medicre set of speakers that you own now determine what you will build.

As one of the other posters said, it is COMPLETELY subjective when people say "oh it sounds like this ..that sounds better....blah". This is how you figure out exactly what you do and do not like.

Bite the bullet, build it, and if you like it great. If not then sell it and build something else. You are in a great location to do something like that.

Most importantly......and i cannot stress this enough......have fun doing it. That is what diy is all about anyway. :)
 
I'm going to give you a different take on this. I'm another musician - play upright bass, mostly jazz but also acoustic music. I've always built amps for listening to music with two priorities - transparency and accurate timbre on acoustic instruments. I've built countless amps in my search for this - started with PP EL84 designs, then a ST70 and some DHT PP amps, then into SE with 300B and 2a3. I thought that was the end of the road, but it wasn't. I started to get better results using 4P1L tubes. Yep - those cheap Russian military tubes are little miracles.

First used them as drivers, and they were great. Then on a whim used a pair in PSE as outputs. Whoa! Great sound and a gain of 10! So I could build 2-stage amps which were 4P1L into 4P1L and that gave me enough volume for a small/medium room with 2v input from a DAC. In fact, with my full-range Alpair 10 speakers I'm now listening with just a single 4P1L in the output. I couldn't be happier - best sound I've ever had. All the transparency and timbre are there.

A 2-stage 4P1L amp is a very simple circuit, but it's quite a complex build since the amp uses filament bias, and the DC filament supplies must be uber-clean. Rod Coleman filement regs are mandatory (see Lyrima website) and you need 4 separate filament supplies, preferably choke input. If you are up to that you'll get something special. All the build instructions are on two DIY Audio threads, one for "4P1L DHT line stage" and the other for "One more 4P1L SE". Here's my latest circuit. Note that my speakers are around 88-89db sensitive, so in your ballpark.

You can use a Hammond 126B just as the plate choke for the first stage - works very well. The LL1663 would be a good OPT. You can get them in the USA from K&K. I use a LL1682 but just because I had a pair. If a single 4P1L isn't enough for you, you can double up and use 2 in the output. That is the equivalent of a 2a3. 4P1L tubes are cheap, and will save you a lot of money and grief over the likes of 300b. I bought 150! Don't need any other DHTs. Have a look at Ale Moglia's site to find out more about using the 4P1L. In an amp you'd use either a plate choke or an interstage e.g. LL1660 in 1:1. I've used both, and I'm pretty happy with a 126B.

http://www.bartola.co.uk/valves/2014/04/21/4p1l-dht-preamp-siberian-gen3-finished/

Transformers:
K&K Audio | Transformers
 

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wdecho

Member
2014-10-28 11:45 am
The wattage it takes through speakers of a given sensitivity to reach a satisfying volume level is highly subjective. I play a 1.5 watt per channel SET through my own speakers at maybe 89 or 90 dB sensitivity, and it will play loudly enough to stress my ears. I don't need more volume than that. This amp was my first venture into building a tube amp and I was leery of how much volume I would have with only 1.5 WPC, but I've been amazed. An eight WPC amp now sounds huge to me.

I suggest to build the amp you want and then if there's not enough volume for you, try some more sensitive speakers than what you have. Yours are on the low side.

I have done much the same as you. I went from 200 watts per channel SS to being perfectly happy with 1.3 watts SET with my horns. I have another SET with a tremendous 5 watts that does have more authority but it could also just be a better circuit. With SS I like the Firstwatt clones with much more wattage 25 watts. In my world 1 watt is a lot.