• 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.

rang out some OPTs. have a question

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The interesting thing is that even a small about of inductance gives near perfect matching in the mid to upper frequencies. The amount of inductance simply moves that matching lower in frequency so if the amplifier is intended for limited bass output (bi-ampimg for example) the inductance is not critical.

In this circuit even 4 or 5H gets you down to 100Hz before diverging by more than a couple of dB. Will have to check the effect of adding some more resistive tail.

IndTail.png

Ran a quick sim. Additional resistive tail limits the maximum deviation but not the frequency at which diversion comes.
 
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PRR

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> even 4 or 5H gets you down to 100Hz

5H at 100Hz is 3K reactance.

3K is much more than 1/Gm of a 12AU7.

OTOH, at say 10mA total tail current, a dumb/cheap 3K resistor drops 30V. So a say -24V rail (or elevated grids) and 3K resistor does the same as 5H does at 100Hz, except also all the way to zero Hz.
 
The EV is only rated at 5W v.s. the 10W rating on the other but given their reputation it might be a conservative rating. I will probably try both and see how they shake out. If they end up not being worthy of 50Hz output in the 5-10W range I can use them for the "Music Box" project that doesn't require lower than about 100-150Hz and drop $50 or $60 on a pair of Edcors.

I should also probably try to "measure" the primary inductance.
 
If you are going to use an inductor as a current source for a LTP, consider the real impedance of the 2 coupled cathodes.

At 250V P-k and 10.5mA, the 12AU7 Gm is 2200uMhos (455 Ohms), and u=17.
(Yes, with a 300V supply, the plate voltage will be lower than 250, & other values will change too, but this is just to get a ball park set of values).

Post # 22 has plate loads of 22k Ohms.
22k/17 = 1295 Ohms
The impedance of one cathode is 1/Gm + Rp/u; 455 Ohms + 1295 Ohms = 1750 Ohms.
The LTP cathode pair is 1750/2 = 875 Ohms.

I once used a Hammond 20H 100mA choke as a current source for a pair of 6C45pi tubes, running about 10mA each. The laminations that were enough for 100mA at 120Hz worked reasonably well at 40Hz and 20mA. It had quite low distributed capacitance, so it also worked well beyond 20kHz.
 
BTW, George if you are still with me have you tried these UL?

Still here, went to Florida for a while to get a break from the heat! I have not tried UL with any of the ultra cheap OPT's since my application for them is guitar amplification where UL is generally shunned.

When you hit saturation the 50C5's glow like light bulbs.

As most people know, saturation causes the OPT's inductance, and therefore the load the tube is driving to drop severely. This causes the tube current to skyrocket and the power delivered to the load to drop....most of the power supply's DC is burned up in the output tubes making them quite unhappy.

The 50C5's showed no plate color at 400 volts as long as the the amp was driven to full output. Operation at about half power causes worst case efficiency resulting in a slight plate glow in a dark room. Guitar amps are usually run relatively quietly, or at full crank, and the duty cycle is well below 100%, so the tubes may live.

What did you do with the screen voltage?

All bench testing was done with variable lab power supplies, one for the plate, one for the screen. The knobs were twiddled to maximize plate efficiency for each load impedance. Only load impedances to match common OPT's were tested (3300, 5000, 6600, 8000 ohms). The 50C5's liked 110 to 150 volts on the screen depending on load.

The trick I use in a guitar amp is to get this voltage via a resistor from the rectified iso transformer, about 165 volts. The entire preamp chain is also running from the screen side of the resistor. This affords two things. The screen voltage will be around 140 volts at idle, and so will the preamp's B+. This will remain around 130 to 140 volts as the amp is played anywhere below clipping. When the amp is thrashed the screen and preamp B+ voltage drops as low as 110 volts lowering the amps total gain, reducing the output tube's dissipation and self limiting the tube melting factor.

I am building a test amp to see how it will survive in the real world. The plate voltage will be about 330 volts since that's easy to get from doubled isolation transformer. I haven't decided on the OPT yet but the output tubes will likely be 32ET5's or 50B5's. The 32ET5 was from the last generation of tube radios. It gives near 50C5 performance with less than half the heater power. This may allow use of a 50 VA isolation transformer for power. The 50B5 is the same tube as the 50C5 except for the pinout. The 50B5 was in full production when UL adopted "creepage" rules for certification essentially banning the use of the 50B5 in table radios. There are zillions of them in stock everywhere and nobody is buying.

These tubes all evolved from the 6W6 and have similar characteristics with TV vertical sweep tubes. The 12L6, 25L6 and 50L6 are all the same as the 6W6, 12W6 and 25W6. Some tubes, especially the 25W6 / 25L6 carry both numbers. The 7 pin miniatures have the same internal guts with smaller plates. Compare the characteristic curves, these guys can saturate down to tens of volts of positive grid voltage is allowed.

Here is some data from my testing of the 50C5's. The 50C5 has the same size plate as the 6AQ5 which is rated at 10 or 12 watts depending on how it's used. I am assuming that the 5 watt rating on the 50C5 is conservative since it's intended for continuous operation in class A table radios. Testing reveals that a 6AQ5 will red plate at less power than a 50C5, but this depends more on quality of construction that anything else. The 6AQ5 is a cylindrical design which requires colinearity to avoid red spots. the plate is very close to the glass so that red spots make holes in the glass!

Plate voltage was 340 volts, screen voltage ranged from 110 to 150 volts. This page shows the 6600 ohm / 150 volt data. It affords the best overall efficiency, keeping plate dissipation below 10 watts at power outputs up to 30 watts. Screen dissipation is a non issue until clipping is reached, hence the limiting system for guitar amp use where clipping is the norm.
 

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I have never seen a 50FE5 tube and can't find any data sheet for it although I didn't try too hard.

I have one used pair of 7695's. On paper they look real good, unfortunately they are rather hard to find.

Neither of these tubes are very common and I tend to use tubes that are in plentiful supply. Neither of these are in reasonable stock at my usual tube suppliers. There are less than 10 of each total in a warehouse of 2 million tubes.
 
Any progress on this project? Zenith used 50FE5 as single ended tubes in the chassis 3H01, and the 7695 in the 3K01 chassis, 12ax7 as driver tube in both. The octal 50FE5s look promising but I'd to install an isolation transformer and recap before anything else. The heaters are in series with a 125 Ohm 4W resistor. I was surprised to find the low plate voltage of 126vdc and the screen is fed 133v via UL tap off the primary winding of the output transformer. My problem is finding a transformer with 50v rated for 300mA for the 50FE5 heaters. Perhaps using 6FE5 is probable more sensible.
 

PRR

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> I was surprised to find the low plate voltage of 126vdc and the screen is fed 133v via UL tap off the primary winding of the output transformer.

With the surge rating of old solid-state rectifiers, 133V DC is pretty good for wall-valtage AC.

The lower screen voltage is NOT a problem. These are not true tetrodes, but some form of pentode. Plate voltage can fall far below screen voltage without trouble.

The 7V drop is just OT winding resistance. 5%-10% would be good commercial practice; this is on the mark.

I would bet the screen is not on a UltraLinear tap but a buzz-reduction winding. (Is there a filter cap at screen?) The plate B+ filtering is always inadequate. RCA licensed a trick of taking screen and driver power from an opposite-phase winding on the OT. The ratio is designed for the specific filter cap values used. Without mass production price pressure, you would use much bigger main cap (probably a C-R-C filter) and ignore the screen winding.
 

PRR

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There is a filter cap at screen, a huge 150u. This does not short the OT because there is 820r in the way. Assuming uniform winding gauge, the screen is near 20:1 so the screen-feed end of the 1.1K OT is like 3 Ohms, the 820r is quite negligible.

The 0.015u is the usual treble-cut needed when a naked pentode drives a simple loudspeaker. Normally connected B+ to P. I don't think the P-G2 connection works any different, maybe just convenient physically.
 
These tubes all evolved from the 6W6 and have similar characteristics with TV vertical sweep tubes. The 12L6, 25L6 and 50L6 are all the same as the 6W6, 12W6 and 25W6. Some tubes, especially the 25W6 / 25L6 carry both numbers. The 7 pin miniatures have the same internal guts with smaller plates.

I was searching something else and stumbled upon your findings. This fascinates me!
I wonder if the 6AS5 is one of these "tougher than it looks" tubes in any way. I got a couple I'd like to put to use.
 
I remember what I was looking for - it was info on the 330-040 70v-10w transformer!

I decided to make a definitive reference for it, so anyone who decides to use one in the future doesn't get lost in trying to figure it out.

Attached is a diagram with expected impedances and how to wire it up for push-pull. Cheers!
 

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