Johnny D's 700 Adventure

[speakerman1]

I went with Winged C 6550s. It came with 6550s. They raved about them after they biased them. Then I asked where mine were. The found them and biased them. I asked how they sound. No response. LOL They knew no tube sale here. Cary just did the tune up. I think Cary's are Electros. I have had Mullard EL-34s in a chinese amp. These do not put off heat like the Chinese did. 8 6550s were not cheap.

The other subject. I'm a good ole redneck. We had a cash crop to last the winter. Well year round really. We had woods all round us.LOL Never pay to play. I remember when Blotter, Window, Orange and Purple were household words. LOL During those days I worked on farms. Would bust my butt to get things done. Go get my fishing counterweights. ( Beer ) Fish for Bass after lunch go to the woods in the evening. Rode motorcycles every where when we weren't on horses. You could walk right up on deer on horseback. That was my 70s.

Larry

 

[soundude]

I like the SV6550C's but you have to run them hot on the bias to get some good sound, sucks the life out of them...have you tried kt88's? Robust tube alotta bottom, I like the JJ's..
I don't know your extension of electornic knowledge but I would install a adjustable feedback loop using a pot. that set at half equals the resistor in your feed back loop so you know where the factory set point would be. Alot use secondary of output transformer for the feedback, I use the plate side of the output and feed back to gain stage through a pot coupled pf cap and bypass caps. Works great and can adjust the quailty of sound to fit my genre....more or less feedback..
if you get the chance and get ahold of KT90's give them a try....they're all the rave..
As you I work my ass off too, I never really had many household words, crap as a kid I was never home and when I was it was go change the channel or move those antennas around and while your up get me a beer...where I grew up not many fishing holes around but tons of watering holes almost on every corner. Hard to fish in concrete and the only fish avalible was the local fish & chips...LMAO.. XD
No grass where I grew up, football was played in the street, manhunt games were played from roof tops and showers were greeted buy your neighbor seeing you from their shower window and sometimes a high five to your friend through the same windows.
I had a horse for a year, hard to walk on blacktop though, keep him about 5 miles away in a stable that got really expensive so I sold him and got me a ironhorse which turned out to be alot cheaper in the long run...
ride all the time, all year round just hard to try and figure out which one to ride..LOL 4 to1 coin toss....those were my 70's Now I have to try and live in this Obamanation....LMFAO!!!

 

[soundude]

Joe
Here is the outcome of the load sharing test, tell me what you think.
200hz sinewave 7volts at output with 4ohm load resistor
Left channel Right channel
t bank1 .33 bank2 .27 bank1 .33 bank2 .27
r 54mv 39mv 54mv 43.5mv
a s 51mv 46mv 51.7mv 40.6mv
n i 52mv 42mv 47mv 44mv
s d 45mv 43mv 51.6mv 41.5mv
f e 50mv 39mv 49mv 40.8mv
o
r
m
e
r

 

[soundude]

didnt show the way I wrote it, it all went to the right!!!!
well the mv's should be under the respective banks...still readable.
Just put them under columns.. thx's

 

[soundude]

Heres some pics. the pic of back plane you can see the open base circuit on the left second trasistor from top.zoom in on it...
the one pic you can see I connected the meters to the dc protect so they don't slam at power up.[attachment=3:2ltrauqj]IMG_4916.jpg[/attachment:2ltrauqj][attachment=2:2ltrauqj]IMG_4947.jpg[/attachment:2ltrauqj][attachment=1:2ltrauqj]IMG_4945.jpg[/attachment:2ltrauqj][attachment=0:2ltrauqj]Picture.jpg[/attachment:2ltrauqj]

 

[soundude]

Heres a pic of the main listening / theatre room, excuse the mess had lots of fustration with this amp so it has been my test room for a bit. also another listening room with the maggies. the other speakers you see are brewed and 5 years in the making...the small set were the proto type and such a great speaker...titanium tweets and aluminum 7' drivers. I must tell you these things can go down to 38hz easy..3 inch flare port in back, they sound unbelievable...the taller ones in the other room are the same top end just added a 8" sub on the bottom, just need to put on finish...great for music and movies.[attachment=1:romyuzp9]IMG_4946.jpg[/attachment:romyuzp9][attachment=0:romyuzp9]IMG_4954.jpg[/attachment:romyuzp9]

 

[Skratch]

Johnny, Terminating the meter wires after the relay works well. I did it on all my 400's and 700. Needles do not jump on turn on

 

[Gepetto]

Thanks for sharing all the pics John. Looks like you are ready for a light board upgrade in that 700B next

Your open base connection looked hard to find. Looked like cold solder to the bus bar and you were just making casual contact to that connection.

 

[soundude]

Yeh really hard to find, did the current sharing test and only got 9mv on that transistor and raised the suspisions, looked harder and found it. did the load test to find leaky transistor but found that instead.
Howd those loads look? they added up very close, 1 to 2.3 millivolts difference.

 

[Gepetto]

Your load balancing looks good John, very close. The resistors are only 10% tolerance so you are limited by this accuracy from the start.

The only way you will get this closer (if you even care) is tighter precision on these bias resistors and matching of the transistors.

 

[laatsch55]

John, still a little fuzzy on the load sharing test. IIRC, if you don't have a variac, the manual stated to drive the amp well into clipping then take your measurements. Was there a typo or were you just outputting 7 volts?? Even at 4 ohms, 7 volts doesn't seem like much of a load. Now, the question is , does the amp have to be a half wave from destruction for the test to be valid?? If current sharing is linear in nature I would say you have a valid test . I fully expect Joe to put this another way, in fact I'm counting on it!! Good job on the rescue John.

 

[soundude]

Joe
So is this like tubes where as long as the paralleld devices are even as a whole, or should each transistor be closely matched to each other individually?

 

[soundude]

Larry
How are you? Haven't heard for you in a while. been reading the forum and see you all over, really great educational reading..
Just did low till I know the devices are close enough before i drive it to 53 volts or clipping, driving it that high and not knowing where the transistor conditions may lie and then hitting them hard could be catastrophic, so i wanted to start low to make sure none were to far out or close to failure.
Still on bench so tomorrow will drive them hard and post loads.

 

[laatsch55]

You make wayyyyy to much sense.

 

[soundude]

30years doing this, a few precautionary steps goes a long way.
Figure the time I've been doing this I would of looked for the obviously noticable cold solder connection!
Don't want to do a 100 foot bungie jump with a 110 foot bungie cord.... LOL
By the way had to use a 8amp delay fuse to keep amp from popping fuse.
Inrush to charge caps was 32 amps...

 

[laatsch55]

On the mains in??

 

[Gepetto]

John
The emitter resistors provide local negative feedback to force sharing amongst the 5 output devices that are paralleled. Bipolar transistors exhibit a negative temperature coefficient on the base-emitter forward junction voltage. Thus if an identical voltage is applied to all 5 base connections (as exists in this design) the transistor with the lowest Vbe will turn on before the other 4 transistors and hog the base and collector current. If this is allowed to happen, this device will heat up more than the remaining 4 devices. Due to the negative tempco of the base-emitter junction, the Vbe will reduce even further, leading to yet more hogging. This description is the depiction of thermal runaway in bipolar transistor devices that are paralleled without emitter resistors.

If the emitter resistors are added to each of the paralleled transistors, this same transistor that was hogging the current before receives local negative feedback from its emitter resistor. The more current that flows in the base and collector of the hogging device, the larger the voltage that is built up in its emitter resistor, effectively reducing the voltage across its base-emitter junction and in turn reducing this transistors collector current. This example is what goes on in each of the 5 paralleled transistor devices.

The emitter resistors serve to keep thermal runaway of any one device in check and to force relatively equal sharing amongst the 5 devices. Sharing accuracy, within reason, is not a critical parameter unless you intend to eke out the last bit of performance and safe operating area in each of the 5 shared transistors. If you intend to push it to the limit, it is best to bench grade each transistor and select devices that are very closely matched for Vbe at a selected base current. This is a tedious process that requires good instrumentation and time (to temperature stabilize each device under test). Once you go to this trouble, you have to put precision resistors in the emitter locations (and possibly reduce their value as well) to gain the benefit of your matching work.

You will not hear the difference resulting from all this work, but you will be able to safely eke more power out of the same paralleled devices. That old adage, a chain is only as strong as its weakest link.

Hope this help the understanding of output stage balancing and what those resistors are for.

 

[soundude]

Funny story, my Angel got stuck in the elevator at work last week and was out in a couple of minutes...everyone was joking with her about it...later the next day leaving she took the elevator with a guy coworker and she mentioned the incident. So she asked what would you do if we got stuck in the elevator...He said to her I'd go crazy ripping the elevator apart to get out!!!! What a tart, if she wasn't mine and i was in there with her i'd never want to get found.
Point is we lose sense of the obvious in a troubled situation...

 

[soundude]

Larry
yes the line in I had to do the time delay.
Joe
Excellent explination...love it....I will not ever drive it to its max in a normal operation but I do expect the sharing to become worse as the load increases. No device is perfectly linear so there will a point where the device will run outta gas and give up. Even pulse testing is for a short instance but in the long run the constant tempature changes will weaken it till it fails.
I want to build a rig to grade them on current drawn and match them that way. I have a plan for a rig with 12v supply and load them to 2 amps and rate them, would that be sufficient?...figuring transistor to pass about 24watts...?
Are my readings I have now good enough matched for a higher load test? Probably won't drie the amp over 150watts, so mabye I'll drive them to that point and see how they rate...your opinion and expertise is well appreciated...

 

[soundude]

If you have a homebrew transistor matcher....please share....can't use my Hickok for transistors..