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Old 26th February 2011, 06:11 AM   #61
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Well, been there, am doing that already in a parallel project ( an active cathode bias circuit with PNP Darlington + TL431). I haven't tried an LED array for cathode bias, so this project will employ a couple. I'm not really one for gaudy display, so I'm hiding my lights under a bushel and keeping the arrays out of sight. Since they're made using 1st generation GaAsP LEDs, they aren't all that bright to begin with, and ugly (construction-wise) to boot. I may use a prettier GaAsP LED (with ruby-red clear encapsulation) as a pilot, but I'm also biased towards some equally obsolete (though much more recent) SiC LEDs that are a rather fetching sky-blue, and will go well with the deep blue metallic paint I'll be using on the case. I have a feeling there won't be too much difference between an LED array (ohms of incremental impedance), and the active cathode bias (milliohms of impedance). Less experienced people will have an easier time getting the LEDs to function, as they can just string them together and not have to worry about loop stability for the bias circuit.
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Old 31st December 2014, 10:58 PM   #62
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This project is rising from a long sleep, and as such, will be affected by things I have learned in the interim.

The input stage has been modified to a differential folded cascode using the triodes of the 6LR8. Cascode XSTRs are KSA1381. Drive to the output pentodes will be direct, with no intervening coupling capacitors or followers, so don't expect any heavy class AB2 action.

Rather than putting in a pot to adjust the output balance, I'm trying the Blumlein "Garter" bias scheme, in the hope that I can use 6LR8 tubes without matching.

I'll finalize and post the schematic when I complete my SMPS to power the thing, so that I know the bias voltages I'll be using. I probably won't wind the transformer for the SMPS until I get in to work next week.
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Old 1st January 2015, 06:46 AM   #63
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I'll have to characterize the tube with a ~300V screen voltage to see what I need to do. Time for some sort of rough-cut static testing jig, I guess.
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Old 7th January 2015, 04:08 AM   #64
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It's got more of a max screen voltage rating for the power pentode than your classic horizontal deflection tube (300V), but the 6LR8 is still a deflection tube, meaning that the screen grid is wound to be touchy, regardless of max voltage rating. Live and learn...

With 300V screen voltage the 6LR8 power pentode needs a lightning bolt on g1 to tame it to plate currents within its power dissipation rating (at least 40+ volts for 350V/300V plate/screen voltage). Nope - won't go there - there's a reason that the tube is specified with 125V screen voltage. With 150V screen potential, things are much more reasonable.

I plan to wind the SMPS XFMR for this amp with a 40-50V bias winding - I'll run the screen supply via a voltage tripler off this winding. The low screen potential means low screen current, unless the tube gets really hammered so that the plate is below the screen potential for a substantial portion of the operating time. It's my job to ensure that don't happen.
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Old 18th January 2015, 10:44 PM   #65
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Here's a first cut of the schematic I'm dealing with. The overall topology will remain the same, but a few of the resistor values will need tweaking.

Resistors R3, R5, R15, and R16 are prime suspects for adjustment. I was going to try Blumlein garter bias on this amp, but I decided that that would be too much complexity/too many simultaneous new things to unravel (been burned that way before), hence pots R4 and R24 for balance adjustment. I may be able to ditch R22 and R23 and replace them with 1 ohm current measurement resistors once I get the output stage biasing sorted out. The SMPS I'm using to power this thing is all up and running, and the voltages shown on the schematic are the actual levels it delivers for plate, screen, and negative bias.

Clamp network D6-8 limits the positive bias applied to the output stage grids while the tubes are busy warming up. Another way around the problem might have been to use a capacitance multiplier or a separate small rectifier tube (like a 6X4) with a long time constant to feed the input stage, so that the output stage will initially come up in full cutoff. I may try that on another amp - perhaps the ST70 "Imposter", which will be using a similar input stage.

Next up is installing the guts to the chassis for a first power-up test - right now the amp is all assorted bits and pieces and sub-modules.
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Last edited by wrenchone; 18th January 2015 at 10:46 PM. Reason: another concept
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Old 21st January 2015, 06:54 AM   #66
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Just got the sheet metal bent and holes spotted for the SMPS shield. This will go over the PS and 1) keep wires and parts from sticking where they shouldn't go for safety's sake 2) Help (along with other commonplace and proprietary measures) keep the SMPS racket from coupling into the nearby amp guts.

Once this is in place, I can spot and remount the output transformers and start locating the other circuit modules.
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Old 22nd January 2015, 02:12 AM   #67
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Quote:
Originally Posted by 20to20 View Post
What is the load impedance for the pentode section? All I can find is the plate resistance. 14K. Perhaps the data sheet is using the two terms interchangably.
Given that this is a vertical deflection type with no audio data, you have to figure audio loads from loadlines. (Attached)

It looks like this type should perform quite well as an audio final. That it includes small signal triodes in the same bottle is an added bonus as you can always use these as grid drivers. They're also good enough to serve as an LTP splitter.

The 6K2 OPTs he has are ideal for this type, under this loading.

I have a bunch of 21LR8s, and I can definitely use 'em.
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Old 22nd January 2015, 05:19 PM   #68
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Miles -

The bias on the setup you show is more aggressive than I'll end up using, as the power pentode in the 6LR8 is only good for 14W or so. But yeah, George (Tubelab) and I had this discussion a few years ago - he was the one who publicized this tube, possibly to his regret. I have another amp using the 6LR8 that is floating around waiting for completion. It has a hybrid cascade front end using the triodes and some differential JFETs. The transformers I'll be using in that case are the 20W Hammonds (I forget the P/N).
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Old 24th January 2015, 12:20 AM   #69
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Miles -

The bias on the setup you show is more aggressive than I'll end up using, as the power pentode in the 6LR8 is only good for 14W or so.
That'd be fine, as you have a lot of leeway there with the h3 estimate. Colder bias might mean a bit more h3, and less P1, but NFB can fix that without compromising the sonic performance. Hotter bias with the 6BQ6s and busting the PD= 11W rating definitely led to better sound without blowing the finals or red plates.

Since audio duty isn't "brick on the key" like deflection duty, the ratings tend to the conservative side anyway.

Quote:
But yeah, George (Tubelab) and I had this discussion a few years ago - he was the one who publicized this tube, possibly to his regret. I have another amp using the 6LR8 that is floating around waiting for completion. It has a hybrid cascade front end using the triodes and some differential JFETs. The transformers I'll be using in that case are the 20W Hammonds (I forget the P/N).
Yeah, that happens. After doing the write-up for Le Renard, 6BQ6GAs went up about 100%. I used to be able to get 'em for less than a buck a pop. Not anymore.

The good thing here is that these TV types come in with odd heater voltages, and you can get those types for significantly less than the 6.3V heater versions. Don't have any 6LR8s myself, but rather 21LR8s with the 21V heaters. Not a prob since I can wind my own PTXs, and give 'em whatever heater voltages I like. Same deal with the 36LW6s I have.
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Old 24th January 2015, 01:07 PM   #70
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Quote:
he was the one who publicized this tube, possibly to his regret
That whole thing was an eye opening experience. The 21LR8 and 31LR8 were on the dollar menu, now they are extinct. I had a board designed for a little 2 tube amp, and was in the process of purchasing the parts for kits, when a job change and other issues put things on hold. Another forum member copied the design, redid the PCB, and is now selling his version, so the 6LR8 spud is now a dead end. I have maybe 100+ 6LR8's stashed away. I will probably sell them.

That was several years ago and I have been very closed lipped since then. I am now unemployed, and have started burning through some of the unfinished projects that I started back then.

I will not have a lab, or even a workbench for several more months, and all my test equipment is packed away in storage, but last Thursday the FedEx guy brought me a fresh stack of what will be Tubelab's next PC board.

I can't even get to my huge parts stash right now, so I am placing an order with Mouser to build up a couple of boards. I will rob some transformers from an existing amp, build up the entire amp, stand back and plug it in.......
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