Pics of my 6BM8 - The Gnat.... - Page 2

Mikael Abdellah · 20th May 2003, 08:34 AM

Great looking amp!

I hope my amp will look as good when it's finished. What kind of speaker do you run on it?

SHiFTY · 20th May 2003, 09:16 AM

Call me a heretic, but I really liked a little negative feedback in my amp, have you tried that?

Just a thought

Morse · 20th May 2003, 02:26 PM

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

arnoldc · 21st May 2003, 12:01 AM

hi morse, nice amp you got there! have you tried Philips Miniwatt PCL82?

fdegrove · 21st May 2003, 10:26 AM

Hi,

Quote:

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....

My two cents say: I wouldn't bother, it would probably only reduce the gain and give you a totally different distortion spectrum with predominantly odd order harmonics.

FWIW, some degree of FB is already present since the cathode R of the otpute tube isn't decoupled.

Cheers,

Bas Horneman · 21st May 2003, 03:19 PM

Hi Morse,

What does the little Gnat sound like???

I have a couple of the same transformers you have. And would like to build a Gnat as well..

Cheers,
Bas

fdegrove · 21st May 2003, 03:22 PM

Hi Bas,

Quote:

And would like to build a Gnat as well..

Shall we christen it the "GnatClone"?

Cheers,

Bas Horneman · 21st May 2003, 03:50 PM

Hi Frank,

Gnatkingclone sounds better?

Cheers,
Bas

fdegrove · 21st May 2003, 03:52 PM

Hi,

Quote:

Gnatkingclone sounds better?

LOL...yes, sounds great.

Morse · 21st May 2003, 04:58 PM

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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
6BM8 Equivalent	GlidingDutchman	Tubes / Valves	11	15th April 2009 06:40 PM
6FD7, 6BM8 or other	mashaffer	Tubes / Valves	1	9th May 2008 02:14 AM
G2-driven 6BM8	wfmali	Tubes / Valves	34	13th February 2008 05:47 PM
6BM8/ Heathkkit S.88 Has only 3 and 15 ohm O/P..?	Alastair E	Tubes / Valves	6	12th February 2008 12:05 AM
6BM8 Question	Peter Menting	Tubes / Valves	10	18th September 2004 03:08 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

20th May 2003, 08:34 AM	#11
Mikael Abdellah diyAudio Member Join Date: Jan 2003 Location: Sweden	Great looking amp! I hope my amp will look as good when it's finished. What kind of speaker do you run on it?

20th May 2003, 09:16 AM	#12
SHiFTY diyAudio Member Join Date: Jul 2002 Location: New Zealand	Call me a heretic, but I really liked a little negative feedback in my amp, have you tried that? Just a thought

20th May 2003, 02:26 PM	#13
Morse diyAudio Member Join Date: Jan 2003 Location: US for now.....	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

21st May 2003, 12:01 AM	#14
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	hi morse, nice amp you got there! have you tried Philips Miniwatt PCL82?

21st May 2003, 03:19 PM	#16
Bas Horneman diyAudio Moderator Emeritus Join Date: Apr 2002 Location: The Netherlands Blog Entries: 18	Hi Morse, What does the little Gnat sound like??? I have a couple of the same transformers you have. And would like to build a Gnat as well.. Cheers, Bas

21st May 2003, 03:50 PM	#18
Bas Horneman diyAudio Moderator Emeritus Join Date: Apr 2002 Location: The Netherlands Blog Entries: 18	Hi Frank, Gnatkingclone sounds better? Cheers, Bas

21st May 2003, 04:58 PM	#20
Morse diyAudio Member Join Date: Jan 2003 Location: US for now.....	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