DIY 300B SET Update

Billfort

Addicted Member
A lot has changed with my DIY 300B SET amps so I thought I would document things before I fundamentally change the layout & circuit.

Driver Stage

A friend who built this same amp has tried endless variations of single tube driver stages and the one I liked best was his 6SN7 version so I went in this direction. All the Solen plastic caps are gone now (accept on the 300B humpot), I used a Blackgate on the 2nd stage cathode and all Tantalum resistors. I eliminated the first stage cathode bypass cap to reduce gain, making for a better (lower apparent hum) match with my tube pre-amp. A balanced heater ground was also added in the quest to further reduced hum.

So far I’ve tried various 6SN7 (GE side getters, Sylvania green & yellow label chrome domes), a CBS-Hytron 5692 and Brimar CV1988. The 5692 was very nice but my favorite by far is the 1988.

300B

Changed out the ceramic wafer socket with fork pin clips for a NOS Amphenol piece with tube style clips. I probably just got some bad parts but the fork clips where never real tight and the tensioning clips started braking and falling off leading to loose connections and intermittent hum; the Amphenols work much better.

Have only tried a few 300B’s and I keep going back to the Western Electrics. Maybe not “flavor of the month” but I like em and I’m done looking. The driver changes & even coupling caps seem to make big differences in this amp’s balance and tone so I’ve concentrated on those areas to flavor to taste – a hell of a lot cheaper than messing around with 300B’s.

DC heat on the 300B. This is one thing I wanted to avoid but I’m resigned to the fact that I will never be happy with 5v AC heat in this circuit and layout with my 100db speakers. I am probably over-reacting, I hear way more hum in many other tube systems, but it bothers me every once in a while and that is once too often. I built a DC board consisting of Schottky diodes, C-R-C filtering (10,000uF caps) and adjusted R to get close to 5v on the heaters. My next change will be to add a choke and try C-L-C filtering. The hum bastard is finally dead but I think this change made the amps sound a touch drier which seems to have been nicely resolved by re-flavoring to-suit with driver tubes and coupling caps.

Coupling Caps

Before these changes, I liked MultiCap RTX or Auricaps best overall, even though the midrange with the Vitamin-Q PIO’s was stunning. I now prefer the Angela/AudioNote copper foil in oil caps by far, and don’t care for any of the plastic caps I have tried.

The Future

These amps are my first DIY electronics undertaking and seem to never be “done” as I keep changing them to learn and hopefully, improve. My next experiment with them will be a complete redesign of the circuit and chassis top plate. I want to try a 2 stage driver arrangement using a 76 indirectly heated triode first and ½ a 6SN7 (or a 6J5) second stage. The power supply will be beefier and use Cerafine caps and an additional choke. I’m hoping to get even better dynamics and bass control with these changes. I’m going to keep all the 120v AC and the rectifier in the back corner of the chassis away from the amp section and try different grounding layouts. I’ll again try to get AC heating to work on the 300B but I’ll prep the chassis for simple bolt-in of the DC board if (when) the hum gets to me again. The chassis top will again be stainless but the tubes will mount in rubber isolated sub assembly plates.

This is a pretty drastic change but it’s easy to redesign the layout with CAD and I have access to a sheet metal shop with an NC turret press. I’m going to wait till my 45 SET is done before embarking on this – my wife won’t let me “wreck” the amps anymore unless I have something else to listen to :) .

Billfort
 
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Billfort, that is one beautiful amp! What a great job you've done.

Have you taken a look at the Larry Moore Design 300b amp? it uses dc on the heaters...for virtually no hum (at least in his amp:rolleyes: ) and uses a triode for a driver...and a 6sn7 for the input. You can change the circuit a little to fit a 76. Only one problem would be having a shelf strong enough to hold an amp with all the iron :eek:

Then again...if I had the amp you've just built...I can't say I'd be in too much of a hurry to do anything :D


nice job!

(if I ever get up enough money to buy some OPT's...I'll be asking ya many questions ;)
 
Yes Fishin, I sat in on Moore's amp design session at the last MAF and heard his amp with the Supravox speakers; very nice but that 10Y scares me off, they seem to be un-obtainium now. I'm going to try all AC heat again but will have my trusty DC board ready to parachute in on the 300B if I hear any hum.

I going to build the JELabs circuit this time around as I have rounded up a bunch of 76's in anticipation of the Berman 76 pre-amp I'm building. My mentor buddy is going to build a similar circuit with the 2.5v 56 since he has a box full of them, I might go that route too after hearing his (assuming he'll share his tube stash :) ).
 
Bill,
The VT-231s are supposed to be great 6SN7s. If they are of the quality of my VT-229 6SL7s (RCA) they really are the best for quiet background which makes for a better sound stage. If you can get a balanced pair you might want to give them a whirl. I think the VT-229s make more of a difference than anything else I've done but upgrading to TJ mesh from OLD Valve Arts. WEs are just out of my price range, but the rolling of the drivers improved the sound on parr with the 300Bs.
 
Yes Thatch, I suppose I should try the VT-231's. I've stayed away from these so far as they seem to be "the" internet favorite right now and are priced accordingly. A friend who has tried several versions of these told me he prefers the CV1988 so I went for them as my choice in the "stupid" money arena - we tend to like the same amp flavors so it seemed the safe bet. Next time I see him I'll ask for a few 231's to try.
 
Originally posted by Billfort
Yes Fishin, I sat in on Moore's amp design session at the last MAF and heard his amp with the Supravox speakers; very nice but that 10Y scares me off, they seem to be un-obtainium now. I'm going to try all AC heat again but will have my trusty DC board ready to parachute in on the 300B if I hear any hum.


I didn't mean the amp in particular...I was talking more about his use of DC, if you have to go that way.
Do you use the 6sn7 metal based? I'm just curious how they sound compared to other sn7's?

take care...and if we ever do hook up (one of these days) I'll buy the first round!
:beerchug:
 
Ok Dan, I misread your post. I see he uses DC on the 300B and 10Y, AC on the 6SN7. I suppose this is encouraging as I quite liked his amps and mine are dead quiet now with DC on the 300B. I picked up a couple small chokes I'm going to try tonight in a C-L-C heater filter arrangement - I hope this gets rid of my last reservations about this DC thing.

The CV1988 is a ceramic base tube - you can see them in the picture above. I might try the VT-231 as I mentioned but have to admit I really like these CV's - they just seem to have a great symmetry with this circuit (at least today's version of it :) ).
 
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