Hi everyone;
Sorry about the delay, but I finally got the pics in of the 6BM8 SET that I've been working on. These are actually pics of the Mk I version prior to it's first extended use trials that showed up the heating problems inherent with the 269JX in this application. The current Mk II not only uses the 270DX trafo (which still runs warmer than I would like, darn it...), but the chassis has some small but important changes, like a 3.2cm x 3.2cm vent on the sidewall nearest the dropper resistor and a vented aluminium bottom plate that is designed to also work as a heat sink for the dropper resistor. Other changes include bypass caps fitted to all electrolytics in the amp.
I'm christening this little one "The Gnat".
All the best,
Morse
Sorry about the delay, but I finally got the pics in of the 6BM8 SET that I've been working on. These are actually pics of the Mk I version prior to it's first extended use trials that showed up the heating problems inherent with the 269JX in this application. The current Mk II not only uses the 270DX trafo (which still runs warmer than I would like, darn it...), but the chassis has some small but important changes, like a 3.2cm x 3.2cm vent on the sidewall nearest the dropper resistor and a vented aluminium bottom plate that is designed to also work as a heat sink for the dropper resistor. Other changes include bypass caps fitted to all electrolytics in the amp.
I'm christening this little one "The Gnat".
All the best,
Morse
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Congrats! Looks good. A nice neat layout.
I really like the sound of mine, its great. Perfect for low volume listening. BTW NOS ECL82s are pretty easy to come by as well, you should try and pick some up. I also have read good things about the svetlanas.
What circuit did you use for that? And are those output trannies out of an Akai or something?
I really like the sound of mine, its great. Perfect for low volume listening. BTW NOS ECL82s are pretty easy to come by as well, you should try and pick some up. I also have read good things about the svetlanas.
What circuit did you use for that? And are those output trannies out of an Akai or something?
Thanks everyone! As usual I spent far too long worrying over the cosmetics of the chassis, so it's good to know that someone else likes the looks!! 
One comment about the box design for anyone else who's thinking of trying it is that (for the first time!!) I had some shrinkage in the cherry that's the top plate. It's 2 pieces of 3-1/2" wide, 3/4" thick cherry that I edge glued with a biscuit between, and there is now a 1mm wide gap between the 2 planks (*grrr!!*) that I've had to fill. If I had it to do over again I'd have laminated a 1/4" thick piece of plywood to the underside of that top deck for stability - and I'd have oven dried it first, myself, to make sure it'd done it's shrinking prior to assembly. Oh well, live and learn....
Hi ShiFtY;
The circuit and all the current data for this version can be found in a couple of posts at: http://www.diyaudio.com/forums/showthread.php?s=&threadid=14694&pagenumber=3 Thanks again Dave, for all your help with posting the schematics!!
Actually, all I can say for sure about the OPT's is that they are 5Kr->8r and that they're "...OEM surplus made by Ravenswood Electronics, 1972 date code..." Maybe from a colour TV or a small hifi? It was the tail end of US electronics manufacture, so there was more than a little nostalgia for me in buying 'em.
Hmmm, I've got one pair of RCA NOS 6BM8's (they don't match though - one bottle is a goodly bit taller than the other!) and mostly Svetlanas. This is my last pair of Ei's - one of the first pair self destructed in under 50 hours from a loose internal element, though this pair is closing in on the 100 hour mark (knock on wood!!).
All the best,
Morse
One comment about the box design for anyone else who's thinking of trying it is that (for the first time!!) I had some shrinkage in the cherry that's the top plate. It's 2 pieces of 3-1/2" wide, 3/4" thick cherry that I edge glued with a biscuit between, and there is now a 1mm wide gap between the 2 planks (*grrr!!*) that I've had to fill. If I had it to do over again I'd have laminated a 1/4" thick piece of plywood to the underside of that top deck for stability - and I'd have oven dried it first, myself, to make sure it'd done it's shrinking prior to assembly. Oh well, live and learn....
Hi ShiFtY;
The circuit and all the current data for this version can be found in a couple of posts at: http://www.diyaudio.com/forums/showthread.php?s=&threadid=14694&pagenumber=3 Thanks again Dave, for all your help with posting the schematics!!
Actually, all I can say for sure about the OPT's is that they are 5Kr->8r and that they're "...OEM surplus made by Ravenswood Electronics, 1972 date code..." Maybe from a colour TV or a small hifi? It was the tail end of US electronics manufacture, so there was more than a little nostalgia for me in buying 'em.
Hmmm, I've got one pair of RCA NOS 6BM8's (they don't match though - one bottle is a goodly bit taller than the other!) and mostly Svetlanas. This is my last pair of Ei's - one of the first pair self destructed in under 50 hours from a loose internal element, though this pair is closing in on the 100 hour mark (knock on wood!!).
All the best,
Morse
Very nice, Morse!
The proportions and symmetry are pleasant to my eyes.
If I was allowed to use wood I hope it'd look like that.
With regard to transformer heating, another thought came to mind:
If you use choke input rectification, the secondary current is no longer taken in short bursts, but over more of the cycle. That means that the transformer heating produced by winding resistance is reduced.
Of course you'd need a new transformer with a higher voltage secondary for this. - And a choke!
Cheers,
The proportions and symmetry are pleasant to my eyes.
If I was allowed to use wood
I hope it'd look like that.With regard to transformer heating, another thought came to mind:
If you use choke input rectification, the secondary current is no longer taken in short bursts, but over more of the cycle. That means that the transformer heating produced by winding resistance is reduced.
Of course you'd need a new transformer with a higher voltage secondary for this. - And a choke!
Cheers,
I hope my amp will look as good when it's finished. What kind of speaker do you run on it?
Hi Dave;
>>>...letting me put an article on your amp up on my site....<<<
No problem! Just attribute the Gnat to me and I'm a happy camper!
Thanks John!
Actually I've thought of a choke input and I can use the same trafo (according to PSUD II) with one - that dropper is dumping loads of voltage! Right now money's tight so I'm not spending on new stuff though.... Once I do, I'll have to find a choke that fits under the hood. Could I get away with 5H in the real world (outside of computer models), or would I need upwards of 10H?
Thanks Mikael!
I'm running 'em through a pair of Radio Shack 40-1354's (5-1/4" full range driver) in junk 10 litre cabs. I have a pair of 12 litre BR alignment cabs under construction though so hopefully it's going to be even better then.
Hi ShiFtY;
Hmmmm, I usually don't like NFB though it really did work wonders on that little Morgan Jones circuit headphone amp I built a while back. Maybe I'll give it a try one of these days....
All the best!
Morse
>>>...letting me put an article on your amp up on my site....<<<
No problem!
Just attribute the Gnat to me and I'm a happy camper! Thanks John!
Actually I've thought of a choke input and I can use the same trafo (according to PSUD II) with one - that dropper is dumping loads of voltage! Right now money's tight so I'm not spending on new stuff though.... Once I do, I'll have to find a choke that fits under the hood. Could I get away with 5H in the real world (outside of computer models), or would I need upwards of 10H?
Thanks Mikael!
I'm running 'em through a pair of Radio Shack 40-1354's (5-1/4" full range driver) in junk 10 litre cabs. I have a pair of 12 litre BR alignment cabs under construction though so hopefully it's going to be even better then.
Hi ShiFtY;
Hmmmm, I usually don't like NFB though it really did work wonders on that little Morgan Jones circuit headphone amp I built a while back. Maybe I'll give it a try one of these days....
All the best!
Morse
Hi Bas;
The sound on the Gnat has been good from the start. However there have been small issues that I've had to resolve, as well as tweaks (one of which is still in the offing) that have improved the amp markedly.
In general the sound is somewhat 'vintage'; it's great with everyone from Sinatra to Louis Armstrong, and very good at moderate volumes with everything from Beethoven's 6th to Sarah McLachlan. The biggest audible weakness is in the bass - those OPT's are just plain tiny, so don't expect Wagner's more sturm und drang filled pieces to really rattle the neighbour's windows! Running it through my FE 164's and comparing it to my 10x as expensive 2A3 was very enlightening. For one thing, this is one QUIET amp - no hum that I can hear unless I switch to 95dB efficient speakers and place my ear right up to the driver. Overall I'd say the 2A3 has a little better definition on complex orchestral passages as well as decidedly better bass slam. BUT it's not 10x better. And in some ways that little Gnat outshines my 2A3, particularly in saxophone passages (I used to play alto, tenor, and baritone sax, so I know what a sax should sound like - the Gnat's got the edge there no question). IMHO most of the sonic advantages of my 2A3 comes down to the OPT's - $9 per OPT vs $100+ per OPT - go figure.
The main issues during the development of the Gnat were related to heating - the 269JX was running at red faced maximum (and actually was over the manufacturer's spec for the HT winding) and it showed it with temperatures that were higher than I like. The 270DX does about 9 degrees C better this way and is being run within manufacturer's spec on all windings - and well under the overall VA rating; sonically the change in trafos was a bit better handling of dynamic material. Biggest disadvantage I've got going now is the amount of heat dissipated with that 1k5r dropper (used to bring the operating point down to the original one used with the 269JX). If you use this approach, I'd recommend going to a heat sinkable aluminium sheathed wirewound so that you can dump that heat efficiently into the bottom plate. For now, the MK II variant uses a aluminium 'heat shield' that partially wraps around the ceramic dropper (at a distance of about 4mm) which is heat sinked into the bottom plate. This helps a lot, along with the large 3.2cm x 3.2cm vent on the sidewall nearest the dropper resistor (it's covered with a hand made .32cm thick aluminium grille).
If you opt for a wooden chassis, I'd strongly advise that you both check that the wood has done all it's shrinking ahead of time (i.e. it's COMPLETELY dry) and that you laminate a .635cm thick sheet of plywood to the underside of the top deck for additional stability. Also, if possible, you should use a plate joiner to make sure that all the joints are strong.
One option that I'm considering is the use of a 10H choke and reconfiguring the PS to a choke load. So long as I can keep my B+ within a couple volts of it's present value it would have the advantage of keeping the trafo even cooler and not having the excess heat to dissipate under the chassis. That would be a MK III variant though, and I'm not sure it's worth the extra effort and expense.
The biggest sonic improvements came from some cheapo tweaks.
1. .1uF "X2" rated, UL Listed 250VAC safety cap across the mains as an RF filter (ONLY use "X" rated 250VAC caps from a reputable manufacturer that have been UL Listed here! Anything else is a fire hazard, pure and simple....).
2. .01uF 600V Sprague Orange Drops fitted in parallel to all filter caps.
3. .068uF 630V polypro caps fitted in parallel to the decoupling caps for the screen grids and as a bypass on the 3300uF cap that filters the DC heaters.
Hope all this hasn't put you off. Frankly it's really a sweet little amp that's making me very happy! But like all projects there have been things learned along the way, so I really like to make sure everyone knows not to make my mistakes.....
Good luck and all the best,
Morse
The sound on the Gnat has been good from the start. However there have been small issues that I've had to resolve, as well as tweaks (one of which is still in the offing) that have improved the amp markedly.
In general the sound is somewhat 'vintage'; it's great with everyone from Sinatra to Louis Armstrong, and very good at moderate volumes with everything from Beethoven's 6th to Sarah McLachlan. The biggest audible weakness is in the bass - those OPT's are just plain tiny, so don't expect Wagner's more sturm und drang filled pieces to really rattle the neighbour's windows! Running it through my FE 164's and comparing it to my 10x as expensive 2A3 was very enlightening. For one thing, this is one QUIET amp - no hum that I can hear unless I switch to 95dB efficient speakers and place my ear right up to the driver. Overall I'd say the 2A3 has a little better definition on complex orchestral passages as well as decidedly better bass slam. BUT it's not 10x better. And in some ways that little Gnat outshines my 2A3, particularly in saxophone passages (I used to play alto, tenor, and baritone sax, so I know what a sax should sound like - the Gnat's got the edge there no question). IMHO most of the sonic advantages of my 2A3 comes down to the OPT's - $9 per OPT vs $100+ per OPT - go figure.
The main issues during the development of the Gnat were related to heating - the 269JX was running at red faced maximum (and actually was over the manufacturer's spec for the HT winding) and it showed it with temperatures that were higher than I like. The 270DX does about 9 degrees C better this way and is being run within manufacturer's spec on all windings - and well under the overall VA rating; sonically the change in trafos was a bit better handling of dynamic material. Biggest disadvantage I've got going now is the amount of heat dissipated with that 1k5r dropper (used to bring the operating point down to the original one used with the 269JX). If you use this approach, I'd recommend going to a heat sinkable aluminium sheathed wirewound so that you can dump that heat efficiently into the bottom plate. For now, the MK II variant uses a aluminium 'heat shield' that partially wraps around the ceramic dropper (at a distance of about 4mm) which is heat sinked into the bottom plate. This helps a lot, along with the large 3.2cm x 3.2cm vent on the sidewall nearest the dropper resistor (it's covered with a hand made .32cm thick aluminium grille).
If you opt for a wooden chassis, I'd strongly advise that you both check that the wood has done all it's shrinking ahead of time (i.e. it's COMPLETELY dry) and that you laminate a .635cm thick sheet of plywood to the underside of the top deck for additional stability. Also, if possible, you should use a plate joiner to make sure that all the joints are strong.
One option that I'm considering is the use of a 10H choke and reconfiguring the PS to a choke load. So long as I can keep my B+ within a couple volts of it's present value it would have the advantage of keeping the trafo even cooler and not having the excess heat to dissipate under the chassis. That would be a MK III variant though, and I'm not sure it's worth the extra effort and expense.
The biggest sonic improvements came from some cheapo tweaks.
1. .1uF "X2" rated, UL Listed 250VAC safety cap across the mains as an RF filter (ONLY use "X" rated 250VAC caps from a reputable manufacturer that have been UL Listed here! Anything else is a fire hazard, pure and simple....).
2. .01uF 600V Sprague Orange Drops fitted in parallel to all filter caps.
3. .068uF 630V polypro caps fitted in parallel to the decoupling caps for the screen grids and as a bypass on the 3300uF cap that filters the DC heaters.
Hope all this hasn't put you off. Frankly it's really a sweet little amp that's making me very happy!
But like all projects there have been things learned along the way, so I really like to make sure everyone knows not to make my mistakes.....Good luck and all the best,
Morse
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