pmcuk

Building a 300b SE amp.

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I thought I'd start a thread for sharing information about 300b builds. I'm in the middle of building 5 amplifiers with different OPTS. These are O-Netics Commercial 3.3K (Used in Wright Sound 2a3), O-Netics Level 1 3.5K, NP Acoustics amorphous core 3.5K, Lundahl LL1664/70 3K and Lundahl LL1682 5K into 5R. First job will be to do a shootout of the OPTs with a separate PSU,  just the 300b stage and as much as possible the same parts. The O-Netics are probably NLA but the others are current, the NP from Vietnam. So far I'm loving both O-Netics. The LL1682/50 is good as well at a lower current of 50mA.

I'm using a 46 driver stage with a large NP Acoustics amorphous plate choke, 180H, 40mA. Love this plate choke, and prefer the 46 to a 10Y stage I also tried. 46 more subtle, 10Y more dynamic. I will be experimenting with several driver stages, including using Ale Moglia's Gyrator. In the past I've also used 4P1L and 2P29L in triode. I want to try EL33 and one or two more indirectly heated tubes with a gain of 20 or over. At present the 46 stage needs more gain into it than the usual 2v, so I'm boosting my DAC with an extra NE5534 stage giving x5.5 gain.  

I decided against using the usual 2 stage high mu driver stages, having not had great results from the tubes I tried. Also I've ruled out 3 tube stages as unnecessary and adding colourations, e.g. 6SN7 designs. The great drivers like 46, 47 and 10Y are low gain so some added gain is required - my choice is an op-amp stage but Ale Moglia has a design on Bartola Valves with a step-up transformer into a 47 driver stage which should sound wonderful.  

But 300b builds are common and cover all kinds of design solutions, so please add your thoughts and experiences so we can pool our collective wisdom for present and future 300b amp builders!  

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I'll be following with interest, how are you setting up your 300b, ie anode volts , cathode volts, auto or fixed bias 

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Posted (edited)
1 hour ago, steve 57 said:

I'll be following with interest, how are you setting up your 300b, ie anode volts , cathode volts, auto or fixed bias 

I have a separate PSU (or in fact several) which was designed for PSE 4P1L which was my previous go-to output stage. Like a 2a3 this was designed for 250v a-k. I haven't modified this yet, so operating point is around 250v a-k, 50-60mA. Low for a 300b but it's enough for my room. I don't want to stress the 300b needlessly and it makes it easier to drive at a lower operating point. I'll increase the HT at some point just to see what that changes, but I want all 5 output stages working first. Cathode bias with polypropylene bypasses. 

I have various different chassis types. Since these kind of amps are heavy I've been using separate chassis for driver and output stage, though with the Lundahl OPTs I've put them on the same chassis. I'll probably sell the 2 Lundahl amps and keep the other 3. 

Edited by pmcuk

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So your power supply is around 300v that gives you around -50 vt on the cathode 250 vt a-k 

Roughly 3 k transformers 

About 4 watts

That's quite easy to drive 

I've used both small output valves and voltage amplifier triodes, cosser 41mhl, Mazda ac/hl, osram mh4 are my favourites choke loaded  or the cosser 41mp transformer coupled (all the 30s mesh)

I also tend to build quite conservatively when it come to valve output stages.

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Fascinating choice of tubes - very esoteric. You must be quite a collector. I must admit, there's a mystery and romance about these rare old tubes. I bet they sound good too. 

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The vast majority (as I'm sure you already know) simply sound better. I have to confess to being a bit 'non-precious' about using old valves, because I don't see the point in having them hidden away in boxes. A few have commented about the 'waste' of using a pair of pre-war Osram KT66s as drivers in the current 845, but why not? Pentodes actually work quite well, because they can manage the decent voltage swing that 845s need. Not to mention the relatively cheap phono stage board that's populated with Telefunken ECC803S.

I too have an rather eccentric selection of older bits and pieces, plus of course a few of the additional DHTs that came from you. My biggest issue is the constant lack of time to get on with things. There are at least three fairly large projects in various stages of incomplete, but whenever I go to pick up the soldering iron, there always seems to be something else that needs doing round the house and garden...

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45 minutes ago, pmcuk said:

Fascinating choice of tubes - very esoteric. You must be quite a collector. I must admit, there's a mystery and romance about these rare old tubes. I bet they sound good too. 

I had at one time about 10,000 valves probably still got 2/3rds of them, been into them since the mid 90s when I first discovered px4s and other british valves.

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Well, today finished off the last of my 5 SE amps with 300b output stages. 

- O-Netics Wright Sound OPT 3.3K. Clean, detailed, engaging, neutral towards sensuous. Very likeable with a good soundstage. NLA

- O-Netics Level 1 OPT 3.5K. Big fruity sound with good detail and a large soundstage. Very engaging, more towards sensuous. Again, very likeable. NLA

- NP Acoustics amorphous core 3.5K. This is a different sound altogether, being amorphous. Lots of low level detail, a big soundstage and a very sensuous sound. It's somewhat lacking in precise focus but makes up for this with the kind of revealing detail that brings out parts of the music you don't usually get to hear. Available from Vietnam and quite well priced. The winder is a very nice guy. 

- Lundahl LL1682 5K into 5 ohms. Neutral and clean sound with quite good detail and a good soundstage. This is a likeable and quite engaging OPT and suits the 300b well. This is the 50 mA version so won't take as much current as the others, but I'd rate this as a good buy. 

- Lundahl LL1664, 3K. Not quite as successful as the LL1682, but still a clean and neutral sound with good focus. A good baseline and a quality OPT. 

I intend to sell the two Lundahl amps when they are completed with input stages. For the purpose of testing I used my 46 driver stage with a large NP amorphous core plate choke. All the amps had the same EH tubes, the same outboard PSU and operating point (except the LL1682) and polypropylene cathode bypass caps with 820 ohm Russian precision military resistors. The O-Netics OPTs are unavailable since the winder retired, and the NP Acoustics OPT has to be ordered from Vietnam. So I'm hanging on to these three rather esoteric OPTs for my own use - I like all three in different ways. The Lundahls can be replaced, so these amps can go to new homes to make space in my rather over-full London apartment. 

More on driver choices when I start building those and comparing them.

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Time to start sorting out driver tubes. My current driver is a 46 DHT with large plate choke. My goal is to find a driver tube which is more available and has more gain, like 20 or more. I rigged up a line stage with LL1660/18mA wired up as a plate choke with two Russian teflon coupling caps FT-2 giving me 0.2uF. 

First out of the blocks is a pair of Sovtek EL84 in triode. I'm totally surprised at how good this is sounding. Good detail and timbre - almost DHT like in clarity without quite having the silkiness of the best DHTs, but it's close. Well beyond my expectations. Now I'll have to find the best operating point. 

Given that the EL84 is sounding this good, it opens the door for its cousins the SV83 and 6P15P. I'm very encouraged by this - I wasn't expecting this good a sound out of an indirectly heated tube. I like what I'm hearing. 

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I think theres very little between indirectly heated and directly heated, I have a few idh triodes which sound very similar to the equivalent dht's.

I just think in general the better output valves are dhts.

Dont forget the el84 has a slight natural upper end peak, making it sound detailed, never the less it's a good valve 

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Hi Steve - any idea where exactly the peak is and how far off flat it is? I'm getting a very faint edginess, nothing too obvious but it's not totally smooth. Playing around with the operating point and plate choke might help. I have a 126B and a NP Acoustics amorphous plate choke. 

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If you could live with just a little less gain, 5687s are rather lovely.

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Dont forget the el84 has a slight natural upper end peak


Sorry to ask but what do you base this on?

Sent from my BLA-L09 using Tapatalk

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10 hours ago, Jazid said:


 


Sorry to ask but what do you base this on?

Sent from my BLA-L09 using Tapatalk
 

I've lots of old valve books at home one of them describe this, ( trouble is I'm in my motorhome in south Germany at the moment) 

I've always thought it was right as that's what I hear when I listen to el84 amps, including the ones I've built in the 90s.. 

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