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sharing eBay find: OPT from Kustom Double Cross
sharing eBay find: OPT from Kustom Double Cross
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Old 30th March 2018, 05:40 AM   #11
PRR is offline PRR  United States
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The 1,500r is an excellent fit to the 6C33 push-pull (two bottles). About 80 Watts out. Damping Factor about 4. Potentially a fine hi-fi amp. (So we may get thrown out of this section.)

For "normal audio" tubes, a first-crack at bias and drive is K*Vp/Mu. K may be 0.6 for class A and 0.8+ for class B.

(Why? Vg=0V is normally the max current we can use. To get all the power we paid for, we want to run from there to cut-off. Cut-off grid voltage is nominally Vp/Mu. In A we bias "halfway"; which with Child's Law tends to be 0.6 of the way on the grid. Pure B would be cutoff at idle. We don't go that far.)

300V on plate and Mu of 2.7 is 111V on G1!

And 6C33 is not "normal audio", it is a Pass Tube aimed at low drop in operation. The ratings get funky over 250V. For better design, go to the curves.
http://www.jogis-roehrenbude.de/Russ...atasheetMB.pdf
Rated Pdiss at 300V is 30W and to hit 100mA at 300V you need to put G1 between the 110V and 120V lines. In push-pull the lines will curve from this point to the active load-line. Which indeed skims the 60W Pdiss allowed at lower swing voltage and is safe.

On another thread with a guy with 350V supply I penciled drivers to make 60V-70V peak. (His original plan barely did 32V.) A 115V peak from 300V supply is possible with high-V transistors in push-pull; with resistor loading you can only get there with huge driver current, drain loads nearing 10K (to drive the 6C33's rated 200K grid leak). Like 9 Watts heat in a push-pull driver.

I would not be picking your driver tubes before you fully outline what they have to do to justify stoking coal into those 6C33 bottles.

> PP-self-split cascode driver stage card

DOUBLE the drive voltage (at least!).

> reduced drive.

If 115V makes 80 Watts, and you can reasonably make 57V drive, now it is a 20W amp. There are easier ways to get there. (FWIW, six 6L6 (a 666 amp?) is still an option even at 300V-400V. 6V6 Mu(g2) is 10, so only 30V-45V drive is needed to slam over 80W.)
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Old 30th March 2018, 05:55 AM   #12
multi-volti is offline multi-volti  United States
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Yeah, that pesky mu and drive situation...same with 6AS7 or 6080’s (I forget which) I bought that same summer, but not as stubborn.

Thank you for more support.
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Old 30th March 2018, 01:18 PM   #13
Tubelab_com is offline Tubelab_com  United States
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Quote:
The only thing I found peculiar in the listing for the item I bought was the 'translation' of 120 W to 120 VA. I get where that logic came from, but it's so out of context I don't think it could be misleading or misinterpreted. Nobody measures power factor speaker impedances, do they?
I bought a batch of OPT's from an Ebay seller nearly 20 years ago. They were custom made by Schumacher for ADA. They look and are packaged just like these, except mine are 6600 ohms to 0-4-8-16. They also look like the transformer in my 80's vintage Schumacher automotive battery charger, which is 6 or 12 Volts, 6 Amp.

NOS Woodward Schumacher Output Transformer 4, 8, 16 Ohm Taps Bassman Super 2 6L6 | eBay

The transformers you linked were made by Bassler electric. Both of these companies primarily make power transformers, but will do custom work. I had the Schumacher spec sheet for my transformers but it has been lost over 20 years and two moves. It was a single page with mostly mechanical specs, dimensions, weight, wire color....The only electrical specs were 6600 ohms to 0-4-8-16, 80VA, 80Hz to 5KHz. These are the type of specs you would provide to a company who makes power transformers.

I took one of these transformers apart to see how it is made. One entire half - primary is wound on the paper bobbin, followed by the entire tapped secondary with the entire remaining half - primary, wound over it with slightly thicker wire due to the extra winding length. Laminations are thick, as you would expect in a power transformer. There is zero interleaving.

How do they work? The 80Hz to 5KHz spec would tend to rule these out for HiFi work, yet I had a pair in my 300B P-P amp. Measured frequency response was 30 Hz to 20 KHz at 25 watts. Feed the same OPT's with EL84's and you will get more like 40 Hz to 12 KHz. The low primary inductance, and high capacitance mean that these OPT's work best when driven by a low impedance source. Mine also perform better when used as a 3300 ohm to 0-2-4-8 OPT. In this configuration I can drive them to 60 watts from 30 Hz to 18 KHz with a pair of big TV sweep tubes using local feedback around the output stage to lower the tube's output impedance. I have also been to 175 watts at 1KHz without issue. I do not use any GNFB around these transformers due to phase shift in the audio range. They sound good playing music, or as a rock group's PA just touching clipping in a 125 WPC amp. Just don't let the bass player run through the PA......He had an SVT anyway.

You may find the same tricks apply to the Basslers. My experience's with 6C33's weren't so great, so my preference would go to TV sweep tubes for an under 100 watt amp into a 1500 ohm load. You could drive them with a 12AX7!
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Last edited by Tubelab_com; 30th March 2018 at 01:44 PM.
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Old 30th March 2018, 04:11 PM   #14
PRR is offline PRR  United States
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You can "easily" make >115V peak drive if you go 700V on the driver. This could be as "simple" as using a 550V CT transformer to make 350V each side of CT, grounding the - end to make +350V and +700V. The 350 to the big jugs and the 700 to the driver.

Or one big PT for the big jugs and another 350V supply stacked on top.

Given ample supply voltage, the driver can reduce to a half-12AX7 and a modest MOSFET cathodyne. I've modeled this but with a way too big old MOSFET and it was fading at 1KHz. When I tried to hack a smaller newer MOSFET my SPICE blew-up.

The same could be done with a 3W triode but it needs a floating heater supply or a very high heater-cathode rating.

An alternative is a stretch 5F6a longtail, but lunch is ready.

And there is the Bereskin 3,000-Watt driver.

Last edited by PRR; 30th March 2018 at 04:18 PM.
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Old 30th March 2018, 08:07 PM   #15
multi-volti is offline multi-volti  United States
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Thanks George & PRR.

G: The xfmr construction is both disappointing and inspiring...(-): looks like a power transformer tweaked to pass low bar (transformer equation & a little more) for audio performance perhaps with desired shortcomings. (+): reality check: If one winding over the other with no interleaving ‘functions’, then double-digit # of interleaves, pie-wound, etc. I don’t need. Somewhere in between is a step up.

PRR: Your Pspice must have been the low-voltage edition��.

Multiple supplies & elevated filament supply are OK...I’m obsessed with cascodes anyway so I anticipate the need.

I tend to get parts first so when I have time I can get moving, but it does result in some cart-before-horse planning...who needs the horse, anyway?
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Old 30th March 2018, 08:08 PM   #16
multi-volti is offline multi-volti  United States
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Ok, no emojis from dumfone. They don’t translate.
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Old 30th March 2018, 08:36 PM   #17
Printer2 is offline Printer2  Canada
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I was thinking maybe the 1500 ohm for KMG's Fet amp design. A little high though.

Fet power amplifier with transformer output
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Old 30th March 2018, 09:04 PM   #18
PRR is offline PRR  United States
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> a half-12AX7 and a modest MOSFET cathodyne

This works but wants 750V supply, and bias-trim on the triode because at best it just-misses bottoming or topping-out.

That IRF part is WAY too fat; I had to scale down to 100Hz to avoid top-loss. Much more suitable parts are available, and the caps do not need to be so big.

The MOSFET needs the usual ~~1K gate stopper and (if not internal) a 15V Zener. This also limits the 12AX7's cold-voltage to 370V which means it is within all ratings.
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Old 1st April 2018, 04:00 AM   #19
danFrank is offline danFrank  United States
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sharing eBay find: OPT from Kustom Double Cross
Quote:
Originally Posted by multi-volti View Post
So I agree with the buyer beware part, but not the value part. I assume yours was priced similarly.
Yeah, he was selling the "40 watter" for the same price as the "120 watter". The 120 watter seems like a good deal, not so with the "40 watter".
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Old 1st April 2018, 01:30 PM   #20
Tubelab_com is offline Tubelab_com  United States
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I tend to get parts first so when I have time I can get moving, but it does result in some cart-before-horse planning....
It has taken me several years of collecting, but I have all the big stuff needed for "the last amp ever," or "the big one," or whatever I call it on any given day. The big one would be a stereo HiFi amp in the 1 KW range. It must also be capable of eating my guitar playing and be suitable for outdoor PA use at full crank for hours, even though it may never see such use.

Obviously the OPT's would be the big ticket item, and one pair were the first parts I collected. These were found in post # 6 here, almost 11 years ago:

Eight 807's in Push Pull?

I got a second larger set of these OPT's from another forum member a couple of years ago. They have the same guts potted into a shiny metal can.....much heavier though.

I have acquired 3 X 600 VA isolation transformers with a pair of 120 volt windings on each side. The primaries can be fed with 120 or 240 volts. Each 120 volt secondary will have a bridge rectifier and a cap for 6 X 160 volt supplies which can be series connected Tektronix style for any B+ voltahe needed at big power levels. There is a 200 amp breaker panel 10 feet from my bench with ample 120 or 240 volts available.

I now have driver boards capable of driving nearly any tube well beyond their limits, and a few boxes full of big TV sweep tubes......so the cart is nearly full.

Quote:
.....who needs the horse, anyway?
The "horse" is the knowledge, skill, patience, and workspace necessary to pull this off. We have seen a few BIG amp builds attempted on these forums, but very few successes. I believe that I would have made something that might have worked, and might have lasted for a year or two, or might have just blown up on the initial power up, if I had jumped into it 10 years ago. I believe that I can pull it off today, but I still have two untested technologies that I will work with at a lower power level with budget friendly parts first.

One is a new concept for an output stage that I have been tinkering with for several years, and the other is an efficiency improvement scheme that I developed at Motorola for use in RF power amps. Both have technological hurdles that must be overcome.
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