I was lucky enough to score a bunch of 6LR8s on sale at a pretty ridiculous (in a nice way) price, and want to use 4 of them for a stereo push-pull amp. The big decision is what iron to use with them. I want to make use of the high perveyance and peak current capability of the pentode section, so I'm looking at the Hammond 1620 (20W, 6600 ohm, 158 mA) and the 1645 (30W, 5000 ohm, 128 mA). Any words of wisdom on deciding between the two? I have a bunch of NOS power iron, so I can choose whatever B+ I want as long as I can feed those hungry filaments (1.5 A apiece!). I'm thinking of pushing things a bit and going for 300V B+.
I have been thinking about doing the same with the 12-pin version (6LU8), as I have several lifetime's supply. I have a pair of 7189 outputs (haven't measured them - probably 8K) that I am thinking of using - but 6.6K or 5K would give more power. I think I'll stick to class A since ultralinear isn't an option with the low screen voltage limit...
If you can't find curves, I think I decided that the 6EZ5 or 6HE5 is the equivalent of the pentode... the triode looks about like half of a 12AT7.
If you can't find curves, I think I decided that the 6EZ5 or 6HE5 is the equivalent of the pentode... the triode looks about like half of a 12AT7.
Tom Bavis said:
Tom,
2 possibilities come to mind, quickly. Regulate g2 B+ and use Bandersnatch's exolinear topology, which connects the LTP load resistors to the UL taps of the O/P trafo. The 2nd possibility is to build an "El Cheapo" variant. If g2 is very fragile, up the value of the plate tie resistors to 1.5 KOhms.
I experimented with some of these tubes in both SE and push pull modes. When experimenting with push pull, I used one of my 80 watt 6.6K ohm guitar amp OPT's. I have a lifetime supply of them, and they sound good (used in my 300Beast amp). This allows 3300, 6600, and 13200 ohm loads. I used a variable power supply so I could try any voltage up to 400 volts. Since I only have a few 6LU8's I didn't push the voltage past 300. With the 6600 ohm load I could get 12 watts at 5% distortion, on 3300 ohms distortion set in at 6 watts, and I could only get 9 watts at severe clipping. 13200 ohms was clean up until clipping, but only made 4.5 watts. My guess would be to use the 6.6K transformer, but I don't have a 5K or 8K transformer to try.
Later I found a box full of used 21LU8's, so I picked a particularly crusty looking tube, and wired the pentode section into a TubelabSE board (same setup as the 6AV5 testing earlier, 5K Edcor transformer, variable plate and bias supplies) and a variable power supply. I cranked the voltage up, and up, and up. At 300 volts I had no problem running the tube in UL mode. At 50 mA there was no plate or screen grid glow. I was getting almost 4 watts, so what do I do? Turn the knob to the right. If I turned the voltage or the current up past these levels I began to get a dull glow on the plate. I got the power supply up to 400 volts (in UL) and 40 mA, I got a dull plate glow, but no screen glow. This indicated that the screen rating may be conservative, but as with the 6AV5's there could be a large variation between tubes, I only tried one.
Later I found a box full of used 21LU8's, so I picked a particularly crusty looking tube, and wired the pentode section into a TubelabSE board (same setup as the 6AV5 testing earlier, 5K Edcor transformer, variable plate and bias supplies) and a variable power supply. I cranked the voltage up, and up, and up. At 300 volts I had no problem running the tube in UL mode. At 50 mA there was no plate or screen grid glow. I was getting almost 4 watts, so what do I do? Turn the knob to the right. If I turned the voltage or the current up past these levels I began to get a dull glow on the plate. I got the power supply up to 400 volts (in UL) and 40 mA, I got a dull plate glow, but no screen glow. This indicated that the screen rating may be conservative, but as with the 6AV5's there could be a large variation between tubes, I only tried one.
I have 4 RCA 6LR8s (neat looking, with the tip-off on top), and about 16 GEs (smooth top). The plates are about the same size for both. I also have an opportunity to locally score some Sylvanias at about $4.50 apiece, so I might snap up some of those too, as they look pretty massive. I was thinking of keeping the outputs in pentode mode, fixed bias so that I could eke out the last bit of power. The 6600 ohm, 20W Hammond 1620 is an attractive way to go as it's only about $35, which is pretty cheap for good quality iron these days.
Edit - Just for grins, I did the numbers. 6600 ohms is a ratio of 14.4:1 each side. If you drive until there's only 100V at the plate, you get about 12W RMS into an 8 ohm load at 300V B+, assuming no XFMR drop. The plate voltage would actually be less, but there are numbers in the data sheet for 40V plate voltage, though the screen current is quite substantial at that drive level.
Edit - Just for grins, I did the numbers. 6600 ohms is a ratio of 14.4:1 each side. If you drive until there's only 100V at the plate, you get about 12W RMS into an 8 ohm load at 300V B+, assuming no XFMR drop. The plate voltage would actually be less, but there are numbers in the data sheet for 40V plate voltage, though the screen current is quite substantial at that drive level.
Mach1,
I've seen the first schematic, but not the second - don't like either all that much, so I'll be doing my own thing, pentode mode, fixed bias, with a stiff screen supply. Since I'm already working on a 6AH6/1625 ultralinear single ended amp using an SMPS, I'll do a variation of the current power supply design for the 6LR8 amp, including a screen voltage output and a minus bias supply for the output tubes. I'm thinking of using 350V B+ and 275-300V for the screen supply. When I have the single ended design up and running, I'll post a separate thread for it.
I've seen the first schematic, but not the second - don't like either all that much, so I'll be doing my own thing, pentode mode, fixed bias, with a stiff screen supply. Since I'm already working on a 6AH6/1625 ultralinear single ended amp using an SMPS, I'll do a variation of the current power supply design for the 6LR8 amp, including a screen voltage output and a minus bias supply for the output tubes. I'm thinking of using 350V B+ and 275-300V for the screen supply. When I have the single ended design up and running, I'll post a separate thread for it.
wrenchone,
Sounds really interesting - please post. SMPS get such a bad rap in many audio circles, it will be good to get an alterrnative perspective.
pm
Sounds really interesting - please post. SMPS get such a bad rap in many audio circles, it will be good to get an alterrnative perspective.
pm
The power supply I'm using is a ringing choke flyback of my own design using a mosfet, two transistors, and no switcher IC. It is not for the casual experimenter, as you need access to the proper materials to make the transformer, and some knowledgeable tweaking to get the thing up and running. This approach runs in borderline discontinuous mode, which is a must for HV outputs, as the output diodes are not forced into hard recovery each switching cycle. This reduces diode power dissipation, switch dissipation, and RFI. The switch turn-on transition is relatively slow, which also reduces RFI. For output wattages of greater than about 50W, I'm thinking of using some Sanken ZVS flyback hybrid circuits that have been sitting in a bin in the local surplus output for about 3 years. These are commonly used in color TV power supplies, which need a 150-180V output of around 150W for the horizontal deflection/HV supply. I've held off buying any of the things for a while - now I have a use for them...
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