6LW6 bias points
Any ideas what to bias a 6LW6 in pentode mode at to get ~150 watts out of a PP pair? This is for a guitar amplifier, so distortion isn't an issue.
Some basic info:
700 volts B+
350 volts screen
5K plate-plate load (if this won't work out well, it can be changed to a more suitable value)
Any help would be appreciated from anyone with any kind of experience with this tube, as I can't seem to find plate curves anywhere (apparently due to it's arrival on the market shortly before the takeover of the transistor).
Thanks in advance,
Surely someone on this forum can provide some help? :( I just want a starting point to ensure that I don't blow anything up while experimenting.
I can't help you off the top of my head, but SY did discuss an amp of similar specs with 6LF6's, which are the same tube with different base. Tubelab.com also shows a cathode follower amp that uses this tube, but it is SE and meant for audio, so distortion does matter. SY's amp is your best bet. I believe he described his OPs.
Thanks. Off to the archives...
I have been playing with 6LW6's since the first time I saw one in a TV set. I have not found any plate curves. Any published curves won't go to 350 volts on the screen since the published max is 275 volts. I think that with 700 volts on the plate and 350 on the screen you are going to need a lot of negative grid voltage. In the neighborhood of -100 volts or more. A lot of drive voltage will be needed as well.
I don't have anything set up right now so I can't just turn the knobs and read the numbers. There are a lot of versions of the 6LW6 and they vary a lot especially if operated outside the published ratings. Some can operate at 90 watts of plate dissipation without complaint, while others glow at 50 watts. I posted this last year and it contains some similar tubes with curves:
Most people know that the 6LW6 is one of my favorite tubes, but I still don't have a data sheet for it. It is the octal version of the 6LF6. Some data is here:
Similar to (but higher dissipation) the 6LX6:
Also similar to 6MH6:
I have found at least 5 different constructions of 6LW6's. Some have "40 watt" plates with no extra radiating fins. Some have similar plates with fins welded to them. I have seen two different types of the "fins" on GE made tubes. Some Sylvanias have heat radiating fins, but they are different than the GE's. Due to an unfortunate drop test, I got to examine the insides of a GE 6LW6. It also had little fins welded to the inside of the plates.
The different flavors of 6LW6's do act somewhat different in linear operation, especially as the screen voltage approaches (or exceeds) the max spec. The bias voltage can vary by 50 volts (at the same plate voltage and current) from tube to tube. The finless versions should be kept under 40 watts dissipation, but the finned Sylvanias and GE's are cool at 60 watts, with glow appearing in the 75 to 90 watt region.
Last weeks experimentation had a Sylvania running at 80 watts for 2 hours with the screen grid at 500 volts. No visible glow from the grid or plate in a dark room.
See post #20 in this thread:
I built a guitar amp that made about 150 watts with a pair of these about 6 or 8 years ago. I used a 6600 ohm OPT, but possibly had the 8 ohm speaker on the 16 ohm tap for a 3300 ohm load. I wasn't smart enough to write things down back then.
At any rate when you travel this far off the beaten path you must be prepared to do some experimentation to find out what works. If you do not have access to variable power supplies I would suggest finding some way to test at a lower voltage first. Especially the screen voltage. I don't have to tell you what the power supplies in this type of amp can do to you either. Please be very careful.
I just picked up some 6LW6's, 2 pairs of GE's and one pair of "Admirals". I'll have to see what their plates look like and report back.
I was intrigued by George's CF amp. I also think the 6C33's I have sitting on a shelf might work in this role.
The Admirals that I have are made by GE. Look for the coded dots under the type number or the EIA code 188-xx on the base of the tube.
Digging this thread out of the graveyard...
I finally got around to bulding the power supply for the amp, and a bias supply with up to -150V, but I'm still not sure how much quiescent plate current I should bias it for.
Any suggestions, especially from SY or tubelab, would be welcome.
The final quiescent plate current in an amp is one of the last things that I determine. It is usually determined by measuring distortion with an FFT analyzer. But, I realize that you need a place to start.
If you are using screen drive I would start at 5 to 10 mA and work your way down! These things work best in near class B.
If you are using conventional G1 drive:
150 watts isn't going to happen in class A, so I will assume class AB1 or AB2. I don't know if your driver circuitry can do grid current, so I will assume class AB1. Either way, I usually start out at about half of the rated dissipation and tweak upward from there. What is the "rated" dissipation for your 6LW6's. The book says 40 watts, but what tube was in the hands of the publisher. Some of the later vintage 6LW6's can handle far more. We can use 40 watts as a starting point, half of that is 20 watts. You mentioned earlier that you have a 750 volt power supply. This works out to about 26 mA per tube. My guess is that you are going to need more current, but start at about 30 mA and get the whole amp working. Then you can turn things up from there.
I assume that you know this, but:
Assemble the power supply. Test it with a suitable load. A bunch of incandescent light bulbs in series can do in a pinch.
Assemble the power amp section. Everything that is DC coupled to the output tubes. Power up without the output tubes. Check all voltages and verify that the bias can be adjusted throughout its range. Set the bias for the most negative voltage. Power off.
Put meters on B+, cathode current for each output tube, screen voltage, and anything else that you can think of.
Connect a suitable dummy load. Do not use a speaker. If no suitable load is available SHORT the speaker leads, do not power up without a load. If the amp oscillates the OPT will be fried! So will some speakers or your ears with an amp of this magnitude. A dead short will save your OPT and only heat up the output tubes. If you have a variac or other means of slowly bringing up the HV supply, use it. Do not use a variac on the filaments.
Install the output tubes. Power up the amp. If a variac is used turn it up slowly. Watch the meters. The tube current should be low or zero and stay there as the voltages are raised. once the voltages are at their final value, slowly adjust the bias for a few mA (5 or 10) of current on one tube, then the other. Let them sit there for a while. Watch the meters for bias creep (tube current that slowly increases). If there is any it should stabilize after a few minutes. If it doesn't try another tube. If the amp has been on at low current for 15 minutes or so with no unexpected events, turn the current up to 30 mA per tube. Repeat the 15 minute watch procedure. Old tubes that have not been used in a while can come up slowly, and can runaway over time. This can lead to blown up stuff real easy at these voltages.
If you have several tubes pick a pair that have the closest grid voltages at the chosen operating current.
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