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Old 8th July 2008, 01:27 AM   #21
rdf is offline rdf  Canada
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Two resistors labeled R1, doh.
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Old 8th July 2008, 01:45 AM   #22
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Quote:
Two resistors labeled R1, doh.
To add to the confusion, the schematic printed in the magazine uses different numbers. The feedback resistor is R4.
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Old 8th July 2008, 02:37 AM   #23
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I couldn't wait until next weekend. I put in the 6GF7's and tweaked up the amp. It works best with a lower B+ voltage of 250 volts. At this voltage, with the tube current set so that the tube was running right at the maximum dissipation. Under these conditions the output power varies between 2.5 and 3 watts depending on the tube. I tried about 50 different tubes. Like the 6EM7 there is a large variability in the gain from tube to tube. I replaced the resistor - capacitor combination in the cathode of the driver tube with an LED. This seemed to lower the variability a little.

The sound (and power level) reminds me of a trioded 6V6. It sounds really good, but there is just not quite enough power for my 87db speakers. Slightly more power may be available with a lower impedance OPT, but this usually sacrifices the sound quality.

The power transformer in this photo is an Antek 1T200. I get 249 volts of B+, just right.
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Old 8th July 2008, 03:14 AM   #24
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Quote:
The first pentode in the 6T10 is intended for operation in a circuit where both G1 and G3 control the flow of current. This is a special type of tube called a dual control pentode. Pete did not need the dual control aspect, so he found the voltage on G3 that resulted in the best linearity.

Ahh! Gotcha.
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Old 8th July 2008, 04:31 AM   #25
kegger is offline kegger  United States
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Default schem

Hey Tubelab,

I'm one of the ones over at AK and doing the 6LU8 project.

I'm with yu and don't put the 6AV5 and 6LU8 pentode in the same stance.
(I've now built both, the 6AV5 is much beefier)

But still no problem running the 6LU8 in UL with a 300V supply, cathode or fixed bias.

I'm also building a paralell version using (2) 6LR8 per channel in SE,UL.

A lot of your testing and torturing of tubes saves me a lot of work, THANK YOU!

Just in case you'd like to see the schem here it is. (simple amp build project)

The amp works pretty darn well, I've had it in quite a few diferent systems as well.
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Old 8th July 2008, 05:26 AM   #26
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Originally posted by tubelab.com
This particular method of feedback has the advantage of making a pentode look like a triode with almost pentode power output. It usually requires a driver with a relatively high output impedance like a pentode.
The Pentode at the front end acts like a voltage controlled current
source. Able to impose a current onto the plate feedback network,
but not acting as a leak that affects the negative feedback ratio.

You can definately make a driver with less than infinite impedance
work, but it becomes a leaky part of the plate feedback network.
Unpredictable results if that leakage impedance is allowed to vary...

The Cathode resistor below the voltage controlled current source
should not be bypassed with a cap, as the Pentode by itself is not
able to produce a sufficiently linear ratio of current.

Bypass the heck out of the output pentode's cathode, its got all
the Schade NFB it needs already.

You can see the N and P Fetrons doing exactly the same thing,
with only slightly different bias arrangement. jFET drains are tied
high up in the dividers to insure they DO NOT operate in the triode
region. Oddly enough, the lack of a high pentode like impedance
here at this first drain would make the emulator far less Triodish...

These spuds seem to be doing almost exactly the same thing.
Making one really great Triode out of two ordinary Pentodes...

All three emulated triodes (inlcuding the Spud) are a bit wierd in
this simplified topology, they are non-inverting...
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Old 8th July 2008, 05:59 AM   #27
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Fixed weird non-inverting issue with Fetrons, and saved parts!
Not sure how you "fix" a Spud, ain't broke, don't need fixin...
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Old 8th July 2008, 12:49 PM   #28
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Kegger:

Our schematics are very similar considering independent development. Differences are:

I chose the 6LR8 over the 6LU8 because the 6LR8 and the 6GF7 (a dual dissimilar triode also made for TV vertical sweep) share the same pinout (except the screen grid is an NC on the triode). This allows two amps in one design. There may be a similar twin for the 6LU8 but I haven't found it.

I don't use the decoupling network on the driver (C2, C5, R5) it is often not needed on a two stage amp. It certainly won't hurt, and is generally a "proper" thing to do.

Every resistor is a slightly different value, but I am still tweaking. The big difference is R2. It can be considerably higher. The book says 1 to 2.2 meg is OK in cathode bias. I don't go that high to avoid grid current issues in old tubes. I have 470K. This gives a little more gain. R11 is also 470K, but this is just because I am already using 470K.

I am running both stages at a higher current. I have a 680 ohm cathode resistor (R1, R7) and R12 is 150 ohm. The tube operates over spec, but this is my style. I have already reduced the current in an attempt to find a set of values that work with both tubes.

I am also running the first stage at a higher current. I have an LED in place of R4, and the plate load (R6,R8) is 22K. The feedback resistor (R3) is 150K. This works out to about the same feedback ratio. I may lower the current some because the gain is good with the 6LR8, but a little low with the 6GF7.

I am currently experimenting with the 6GF7 design. I think I like the sound better despite the lower power. I will flip flop back and forth until I find a set of components that work for both designs, or convince myself that it can't be done. The big disconnect is the supply voltage. The 6LR8 likes 300 to 350 volts, and the 6GF7 is most happy at 250 volts.

Quote:
A lot of your testing and torturing of tubes saves me a lot of work, THANK YOU!
I have a 21LR8 with a crack in the glass. The crack has grown longer (it is about 1.5 inches long now) as I have been using the tube. I guess I will "test" it before it sucks air by itself.
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Old 8th July 2008, 04:23 PM   #29
kegger is offline kegger  United States
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Tubelab

Thanks for the rundown on the schems.

I to made my choice of 6LU8 over the 6LR8 (this project) for tube swapping.
As the 6MF8 drops right in as well, I've tested them and there right at home.
(the current drops a bit on the 6MF8 and puts it right in line with dissipation)

There's the bigger guy the 6MY8 but I only have 1, then the 6JZ8, but it like
the 6GF7 wants less voltage as well so I counted it out for this simple build.

Were actually pretty much the same pentode cathode resistor as my pair is
705 ohms (or sub 1 for 680ohm) so I think we got the pentode where we like.

I went with R12 being 1.2k to make sure of no oscilation and it just happens
to drop voltage enough to where the plate and screen see the same in UL.

Also I chose both grid resistors to be the same as yah I was using the 270K
on the driver so I used it on the pentode as well, I saw those high numbers in
the (PDF) but again chose the lower value for suspect tubes or different tubes.
(got plenty of gain with using 270K, that is for the 6LU8 and 6MF8)

We are a bit different on the front tube, I went with my general rule of trying to
be 4x to 5x the plate resistance of the tube for my plate load while still getting
decent current and voltage on the tube. I shot for the most current I could get
in choosing my cathode resistor while having roughly 2v bias on that tube to.
(My thinking was I wanted to be within 1.5v - 2v bias and have it drive well)

Experimented with different amounts of feedback and settled on the current in
a realization that more didn't do to much for technical performance but did in a
sound performance, I think I found a happy medium.

I was considering experimenting with LED bias for my own version, but with the
50K plate load, biasing on the driver and the 270K feedback resistor gain works
out to be very nice and the sound is quite good for this quite simple amp project.
I will be very interested in your final schem choices, ecspecially the driver section.

What a weird world it is we choose to build such simular amps at the same time,
I'm glad to see someone such as yourself designing/creating this spudly compact.

I have found the sound of the 6LU8/6MF8 better in UL Than in Triode.
(At least on the EDCOR tranny, technical performance improves as well, so)

Hope your tubes don't Crack under the pressure before your done,
Please keep the updates coming, I'm always up for reading more of your experiments.
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Old 8th July 2008, 07:38 PM   #30
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Quote:
Originally posted by kegger
I have found the sound of the 6LU8/6MF8 better in UL Than in Triode.
(At least on the EDCOR tranny, technical performance improves as well, so)
Have you tried Pentode mode? With your plate-to-plate Schade
style Triode emulation, UL or Triode strapping the screen seems
a redundant excercise. And reduces plate performance as noted...

Or does the screen dissipation drive favorably into the UL Tap?
With beam pentodes, the screen is not supossedly in the direct
conducting path of the beams, the plate is the business end...
Reality might be a bit more fuzzy.
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