DIY 300b

[AstromanoftheFuture]

So recently a friend of mine gave me a pair of Electro Harmonix 300b tubes. I don’t gots a 300b amp though. I do have a 2a3– a recently built Stereomour II. My friend thought maybe it was an amp where I could switch tubes. I think I read elsewhere on the forum that because of the iron in the stereomour, 300b’s wouldn’t make any more power than the 2a3’s.

So the first thing I did was look at kits- Bottlehead, Elekit, AudioNote, and then a variety of seldom seen out of Japan kits (softone, SUN, Asano). I used to live in Japan, and loved seeing the tube amps in “denden town” (Osaka’s equivalent to the better known Akihabara in Tokyo; the “den” in “denden” is from “denki” meaning“electric”).
 
Because I looked at the Japanese kits, the next thing I did was start looking at japanese schematics... I think the Japanese potted transformers look super awesome... but after pricing what ISO and Hashimoto transformers go for, I gave up on that idea quick. The cheapest tiniest ISO OT’s are $600 for a pair from the US distributor...

So the next thing I did was dig through my pile of old Sound Practices and Valve magazine (recently given to me by the same friend that gave me the tubes). I re-looked at the Joe Robert’s WE91 (and it’s Angela instruments derivative)and I looked at Paul Joppa’s 300b conversion of the original S.E.X. Amp. I found online that PJ did a version of the Robert’s WE91 as well. To quote the intro to the show Portlandia, Is “the dream of the nineties alive?” I keep coming back to these older experiments in single ended (I wish I had been hip to sound practices and valve back in the day, but I was a punk kid) and it seems all their designers have all moved on, maybe I shouldn’t be living in the past?

At this point I think I’d like to try to build something myself, meaning from a schematic, not a kit. I’m not smart enough or knowledgeable enough to create something totally on my own. I’m kinda liking Lundahl iron in terms of pricing and variety. Seems to have a good rep too. I’d like to do something integrated if I could... maybe I’m not being practical. I think I’d be willing to try to make something imperfect at this time— a quick and dirty 300b?

https://forum.bottlehead.com/index.php?topic=2598.msg21073#msg21073
https://forum.bottlehead.com/index.php?topic=11749.0

[Thermioniclife]

Go for it, you will have a lot of fun mixed with not so fun but overall I think it is a great way to express yourself. BH kits are nice
and well thought out so success is guaranteed, designing an amp and coming up with a layout and following a schematic is quite
rewarding when you are.
As far as Lundahl's are concerned I personally like them. I have used them for 21 years now on HIFI amps and never looked back.
They need to be covered as they are open frame unless you mount them on the underside of the chassis. A Hammond 1590VBK die cast
enclosure will cover a pair of Lundahl 1664's which are 3k ohms and good for a 300b. On my last 300b build I used the Lundahl
covers which look very nice but are heavy steel and are of a clamshell design that the bends were not bent correctly so you might have
to tweak the bends yourself.

[AstromanoftheFuture]

... a pair of Lundahl 1664's which are 3k ohms and good for a 300b. On my last 300b build I used the Lundahl
covers ...

I was looking specifically at the Lundahl 1664's for the Output iron. Did you build your amps from a known schematic, or cook sumpthin' up yerself? I've not seen that specific Don Garber Fi Y you've got there...

[Thermioniclife]

The Don Garber Y pre is of my own design using a modified BH Foreplay III circuit The 300b mono's are from an old Joseph Esmillsa
schematic with some changes, I also recently completed a 2a3 stereo amp in a Don Garber inspired aesthetic. All using Lundahl 1664's
as well as a parallel 1626 amp I built around 2001. The 2a3 stereo and 1626 uses Hammond 1590vbk enclosures to house the 1664's.

[Paul Birkeland]

I found online that PJ did a version of the Robert’s WE91 as well. To quote the into to the show Portlandia, Is “the dream of the nineties alive?” I keep coming back to these older experiments in single ended, when their designers have all moved on?

The 91A has its purposes, but for hifi use the top end will end up sounding pretty rolled off due to the design of the first stage.  This was intentional equalizing for theater use back in the day but is no longer particularly useful.  The amount of gain in the 91A is also not super useful.

[2wo]

I have been dinking around with the Angela 300b for over 20 years now I have the Lundel transformers and I have tried any number of driver stages.  Never got quite what I was looking for...John

[Jamier]

Lee, your 2A3 homage to Don Garber is beautiful.

Jamie

[Paul Joppa]

So recently a friend of mine gave me a pair of Electro Harmonix 300b tubes. I don’t gots a 300b amp though. ...

Was there a question here?

[Paul Birkeland]

So recently a friend of mine gave me a pair of Electro Harmonix 300b tubes. I don’t gots a 300b amp though.

Oddly enough we do sell one...
https://bottlehead.com/product/kaiju-300b-amplifier-kit/
We even have a preamp you could plug them into!

[AstromanoftheFuture]

Was there a question here?

Sorry PJ, I’ll try to focus my rambling, and give you a direct question. Among the things I’m interested in is your 300b circuit as outlined in that old Valve magazine (300b S.E.X. change). I don’t see much on the ‘net about it. I guess one clear question to ask is do you feel that this is still worth trying out? I’ve gleaned from other posts that you’ve “moved on” which I do not mean in any sort of negative way. Seems like a lot of “hip” amp ideas of the 90’s are long dead. I liked the idea of the simple WE91, but reading about it and talking to knowledgeable people such as yourself have lead to the conclusion that it was very imperfect and left a lot to be desired.
What I’d like to find is a relatively simple, relatively decent 300b topology that also ideally would be integrated. It doesn’t need to be my reference system. I want to learn a thing or two, and have fun making my own amp. I’m fighting the urge to ramble more. I’ll stop here for now. 

[Paul Joppa]

Thanks; I understand much better now. I dug up the article and reviewed it. Incidentally, I still have this amp (now 24 years old!).

The design is still good, though today I would make the following changes:

1) Run a little more current in the 300B to reduce the distortion. Basically that means change the 300B cathode bias resistor from 1200 ohms to 1000 ohms.

2) Take the output of the 6SN7 driver stage from the lower tube plate rather than the upper tube cathode. We tried both and it sounds slightly better this way - but it's an easy change to make so you can do the experiment yourself to see.

3) Increase the capacitance in the power supply filter to reduce the hum.

This design needed 2.5vRMS input to achieve full power, so a driver tube of greater gain, or a preamp with 10-20dB gain, would be needed.

I hope that helps!

[AstromanoftheFuture]

@Paul Joppa:

Thanks for that reply Paul. It gives me a bit of direction.

In your original 300b conversion was the mains transformer a magnequest PGP-8? I’ve gathered in my research that there were several different mains transformers used for the original S.E.X. With Magnequest being gone, (and I don’t currently see any on the used market) I think I’ll have to find something else (and make circuit alterations accordingly).

In regards to “This design needed 2.5vRMS input to achieve full power, so a driver tube of greater gain, or a preamp with 10-20dB gain, would be needed.” —> I think you said in the original article that the 6SL7 wound up with about 10db more gain, right?

Thanks again!

[Paul Joppa]

I used the original S.E.X. amp transformer at first, but it was unreliable so I replaced it with the deYoung when Bottlehead made that switch. We later switched again to the PGP8; and after that to a custom Hammond for the Paramour. None of these are in current production, but suitable transformers are not too hard to find. Look for 350v/150mA or thereabouts for a stereo amp. Selecting and sourcing components is one major skill you'll need to learn if you plan to go beyond kits (the other major skill is layout and wiring).

Yes, the 6SL7 has more gain. Most people thought the 6SN7 sounded slightly better, but the difference was pretty small. With that you'd just need a volume control (and maybe a source selector switch) to make an integrated amp.

[AstromanoftheFuture]

”... suitable transformers are not too hard to find. Look for 350v/150mA or thereabouts for a stereo amp. Selecting and sourcing components is one major skill you'll need to learn if you plan to go beyond kits (the other major skill is layout and wiring).

I loooooove hunting through catalogs. I just did a major bender to find crossover parts for my Altecs!