• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Those Magnificent Television Tubes

Do you know if the Spice model is showing (non linear) current draw from g1?

Hello Smoking-amp, I looked at the current feeding G2 via 68 ohm series resistor from source follower as compared to the current in the resistor from G2 to G1 and there appears that there is no compression. The current is simply lower and still nice an sinusoidal in shape. Mickeystan
 
I can ship you one 6P41S, just give me your address......13GB5 and EL509 I don't currently have I'm afraid. But I will put them on my acquire list.

PM me the address.....I have plenty of 13GB5's...NIB Mullard / Sylvania 13GB5/XL500.

is that at any level or just low level where cross-over distortion is present?

I have found that there is a limited range of plate voltages that can be applied to a given set of tubes when trying to extract maximum power output if we are to stay within the published maximum ratings, and provide low distortion.

At the lowish plate voltages, the plate load impedance must be reduced to make big power. There is a limit in how far this can go to stay within the peak cathode current ratings. My experiences with violating this rating has shown that you shouldn't do this.

At higher plate voltages, you will eventually run into plate dissipation limits at higher output power levels. Maximum plate dissipation does not occur at maximum power output. It is usually around half power. This is the limiting factor if the amplifier is designed to remain in dissipation spec when tested with continuous sine waves.

If we accept that most music has a peak to average ratio (crest factor) of at least 10 db, and therefore the amp will be operated below an average power output of 10% of max when pushed to clipping on peaks, we can willfully violate the dissipation spec for continuous sine wave testing. Keep such testing to brief periods. Bob Carver made his career on this fact.

As the plate voltage is increased, the idle current through each tube must be reduced to avoid excessive dissipation at idle. As the plate voltage is increased the idle current must be reduced to keep the tube's dissipation at a level where tube life is acceptable. There will be a point where the crossover distortion at low volumes can't be tolerated. This is the limiting plate voltage factor on most conventional G1 driven sweep tube amps.....hence dual drive, crazy drive, or warp drive! Screen drive can inherently operate at lower idle currents without excessive crossover. This is very tube dependent.

Using Pete's Engineer's Amp as a test board, I found that for 6HJ5's/ 6HD5's 600 volts and 30 to 35 mA of idle current with a 3300 ohm load will result in about 130 watts of power output with a faint red plate at about 75 watts continuous for 5 minutes, and no visible or audible crossover. The amp will play for hours at the edge of clipping with loud music into a speaker. 650 volts gets into trouble at both ends of the power spectrum.

The same amp with bigger tubes (I used 35LR6's) hits 250 watts with 650 volts and 2500 ohms. These are the limits of my testing ability. My biggest power supply goes to 650 volts and 1.7 amps. My biggest OPT is 1250, 2500 or 5000 ohms. No single tube works on 1250 ohms. 650 volts is about as far as it was intended to go too. Pete's board was not laid out to eat 650+ volts either.
 
Address messages sent. Thanks for the tube offers guys!

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"hence dual drive, crazy drive, or warp drive!"

Warp Drive!

Hmmm, started me thinking. (dangerous) Seeing as how the current fed grid 1 is largely able to compensate the grid 2 distortion for "Crazy Drive", I wonder if one could do the current fed grid 1 operation in a previous tube, and then use that output to drive the final tubes via normal -g1 voltage.

Two problems though, the pre-distorter has the desired compressive distortion inverted at its plate. (pre-distorting cathode follower instead?) The other might be that the Crazy Drive is more than just a simple sum of g1 and g2 effects, since the currents induced by each grid affects the gm of the other as well (something like Space Charge tubes, which might be a product rather than a sum). Also, pre-distorting would be giving a product rather than a sum, so maybe OK. Well something to think about anyway.

Some dark Winter night, the evil experimenting begins.... Tubes turned to the dark side.
 
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Good thing most of these only cost $1 !!

Much of my extreme testing is done on tubes that ranged from free to $1. I got the 13GB5's from ESRC for $1 and that was a credit earned by bringing him a truckload of tubes.

I got a bunch of 6BQ6's for an average price of 66 cents. They were supposed to be NIB, but some ere ugly and well used. They entered the zone! How many watts can you get out of a 6BQ6GT in SE? I was over 20 watts when the cathode bypass cap exploded!

Note the absence of getter on these 6V6GT's. They were in a sealed 100 unit bulk pack that I got from NASA at an auction in Cape Canaveral Fla. Over half of them were unusable, and most of the rest have since died due to air leaks. Sylvania made some real trash in the last years of their vacuum tube manufacturing. I have many other examples to support this statement.

They made some excessive power, and distortion for a while. I performed an autopsy on one of each.
 

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I'm not familiar with Darius's version. Loftin-White I thought was about stacked DC coupling to avoid coupling caps (or transformers). Anything with DC coupling will need some kind of DC feedback loop to keep operating points stable. Wavebourn is an expert on that. Otherwise, the Crazy drive thing needs like 0 to +60V (or more, depending on output tube) swing from the driver stage, through some kind of follower, to drive the screen current(s).

DC Bias drift:
 

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Hi Smoking Amp,

many thanks for all the efforts! I must admit I am a slow builder, but interested in transconductance amplifiers for a long time as some initial (subjective) experiments showed they just sound better for me - therefore looking into linear high output impedance devices, and these curves do indeed look very good!

Best regards, Erik
 
Re: Erik

The equal spacing of the tube plate curves using "Crazy Drive" indicate a constant gm versus input signal. They are however still relatively high impedance plate curves as far as Rp goes. Some local feedback like Schade, or back to the driver, would allow that output impedance to be lowered in value. Since the local feedback network would be setting a new Mu, or gain, for the stage, and gm is constant as well, then Rp = Mu/gm should also be constant.

Some of the aural benefits of transconductance stages may simply be due to an effectively constant output impedance toward the speaker. (most amps have varying output Z with signal level)

I hope to get some Schade/Crazy Drive plate curves put together on the tracer soon, but so far have been held back by the limited grid sweep voltage limits here (been trying to fake a current output with a series resistor). I may have to resort to a degenerated Mosfet to develop the current drive for the tube with Schade feedback. Plus the variable input Z of the Crazy Drive has to be handled as well, so some buffering needed.

---------------------------

note: that busted tube pic above was actually hit by a hammer, but only after the tube got "smoked".
 
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Using Curve Captor I captured (manually traced) the data from Curve Set #4 in post #25 (triode mode) which produced the following model from the attached fitted curves (I added the 26W Pa line based on the datasheet):

* 6cb5a macro model
.subckt 6cb5a P G K
koren8(0.01338107979,-0.06422320865,0.1403707944,69.59565876,-27.12750462,38.45402696,-9.115093614,1.596498127)
.ends 6cb5a



JM
 
I got some more 6CB5A tubes out to curve trace since you were making a model for them.

As I often find, a bunch of tubes of the same type are not always hugely consistent, but these were closer than some I've seen. The earlier 6CB5A was marked Sylvania in a plain white box, with plain white lettering on the tube. The other two "Sylvania" types here are the same white box types. I also found an exactly identically constructed tube marked "National", but printed in red on the base instead. Never seen Sylvania tubes marked with plain white letters before, so I think these are all "National". They all have black rectangular plates.

Then an RCA with a rectangular grey plate. And a real Sylvania with a rounded grey plate.

Triode curves attached, 50 mA/div Vert., 50V/div Horiz., -7 V steps on grid 1 from 0V

1) original "Sylvania"
2) "Sylvania"
3) "Sylvania"
4) National
5) RCA rectangular grey plate
6) real Sylvania round grey plate

To show how much variation can occur even within a same brand tube type, here are some RCA 36MC6 in triode, a typical, and a best of 30 tubes. Then some RCA 26HU5 that don't look so similar either.

7) 36MC6 RCA typical
8) 36MC6 RCA best found

9) RCA 26HU5
10) RCA 26HU5

Which all goes to show that for selecting similar triodes, you need a curve tracer. Fortunately Schade "triode" versions look quite similar, and Crazy Drive pentodes look similar (so should make extremely similar, and linear, Schade "triode" versions).
 

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I found two more 6CB5A types. A second RCA rectangular grey plate and an RCA round grey plate. (the plate looks identical to the Sylvania round plate, but the getter is very different) Same triode scales: 50 mA/div Vert., 50V/div. Horiz., 7 V grid 1 steps

1) 6CB5A RCA rectangular grey plate
2) 6CB5A RCA round grey plate
 

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