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Old 11th March 2008, 02:35 AM   #51
GK is offline GK  Australia
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Quote:
Originally posted by tubelab.com
You are probably right. I was using 6336A's and 10 ohm resistors. I blasted a couple of mux chips before I added a series resistor and zener diodes.

With 10 ohms, my MUX would fry at >0.5A. That would be possible with the KT88's. Though The reason I went for one ohm sense resistors is to eliminate their effect on the circuit.
The INA118 isn't exactly cheap, but is an elegant solution and I've got a few in my parts drawer.



Quote:
Originally posted by tubelab.com
This is true. I was bound by the rules of the design contest, and sprinkling in a few Microchip DACs and opamps was encouraged. Still, I would rather use a few $2 chips than a bunch of real cheap ones. It makes layout easier.[/B]

Well, I woudn't make my own DAC's for stuff I build for my employer (where it's a constant struggle for fit the required functionality into the given space) but for my own stuff, I like to play around. Little black blobs with SPI inputs are BORING!


Quote:
Originally posted by tubelab.com
You have a valid point here. I didn't notice any noise related issues with my design but my output stages are cathode followers. The drive requirements are huge. A few mV of noise directly on the grid of the output tubes would not be noticed. I did have LPF's on the amp board.[/B]

I think that either way, given enough attention to detail can be made to work and I'm sure that what you're got works perfectly well, but I'm building a BIG thingie here and anything that provides a little leeway with minimal extra complication WRT to bypassing and layout is a bonus.


Cheers,
Glen
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Old 11th March 2008, 03:10 AM   #52
GK is offline GK  Australia
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Originally posted by G.Kleinschmidt
I think that either way, given enough attention to detail can be made to work and I'm sure that what you're got works perfectly well, but I'm building a BIG thingie here and anything that provides a little leeway with minimal extra complication WRT to bypassing and layout is a bonus.

ARRGGHHH.
I ******** hate it when this POS Bill Gates software covertly fixes by typos with the substitution of incorrect words.

PS
Sorry for the rant.

EDIT:
....my typos....AARRGGGGGHHHHHHHHH!!!!!
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Old 15th February 2016, 02:02 AM   #53
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Why use 6550s when TV tubes are so much better. My studio in Tracy enjoys over 400 watts of vacuum tube power; cost of the American nos glass was under $150. The massive cathodes on the sweep tube outputs have a knee current over an amp and it is a quad on each channel. Bass is as effortless as a solid state amp due to the huge current reserve and it's running McIntosh opt's so the power bw extends well past 100 kHz.
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Old 15th February 2016, 10:37 PM   #54
AJT is offline AJT  Philippines
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Quote:
Originally Posted by tubelab.com View Post
I am building one because I can, and it is different. I have breadboarded the typical push pull design using 6 X EH 6550. That is now "plan C". I got more power and punchier sound using 4 X 6LW6, now "plan B". Power output is in the 300 watt range.

The power supply is not too hard. The plate supply for the output tubes is a 480 volt to 120 volt 1 KVA industrial control transformer connected in reverse, feeding a SS bridge and CLC filtering. The choke is in the ground leg to avoid the need for a HV rated choke. Output is over 600 volts at 1 amp. Building a voltage regulator for this supply would be a serious experiment, but I am not convinced it is necessary.

I am currently experimenting with cathode follower output stages using 6LW6's or possibly 6336A's. This allows lower output impedance and distortion. I haven't got it all working right yet, so I may choose plan B or C at a later date.

I bought some Plitron toroidal OPT's that are rated for 400 watts at 20 Hz from their "surplus transformers" page last year for less money than the big Hammonds. Excellent transformers but the DC imbalance must be essentially zero, so I am using a microprocessor controller of my own design.
hey, tubelab, if 300 watts is your goal, then a pair of the QE08-200 gets you there....you need sockets for the 4-400 tube though....

https://frank.pocnet.net/sheets/030/q/QE08-200.pdf
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Old 16th February 2016, 01:08 AM   #55
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Quote:
hey, tubelab, if 300 watts is your goal
300 watts????? that was my goal eight years ago when I wrote that.....now, 500 watts, OK? Really my OPT's are rated for 400 watts at 20 Hz, so that should be the goal. The suggested tube set for this OPT is "six or eight 6550's."

Quote:
then a pair of the QE08-200 gets you there
QE08-200's are pretty rare and expensive in the USA, It takes 750 volts B+ and 3600 ohms to get 300 W. Not a good match for my OPT's. I am looking at using parts and tubes that I already have.

Quote:
Why use 6550s when TV tubes are so much better.
Gee, why didn't I say that......OH, wait a minute, I have been saying that for a while.....

The suggestion of 6 or 8 6550's and the 1250 ohm impedance implies a B+ voltage in the 600 to 650 volt range, so with those limitations, I can think of several ways to get there.....All involve TV sweep tubes. 4 or 6 X 6LW6's, 8 X 6HJ5's, 6 X 35LR6's, 10 X 13GB5's........

Many things changed or delayed my plans years ago. I have since "retired" (been laid off) and moved 1200 miles. I am in the process of building my new lab, but as you know I have already seen 250 watts flow from some transmitting tubes......more to come.

I have picked up where I left off about 5 years ago, where I saw 525 watts flow from 4 X 35LR6's using the same OPT's that I plan to use when the "big one" finally gets built.

I have designed a new driver circuit and I am currently working with 13GB5's and other expendable parts to iron out the kinks and optimize the design......which is straight G1 drive for now. Next, a 125 WPC amp gets built with Edcor iron, then the biggie!

Crazy drive and screen drive are too unconventional and still relatively unpredictable for a 500 WPC amp.
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