Let me repeat again that 56K screen grid resistor is not a pentode connection. Pentode connection is when screen grid voltage as less as possible depends on the screen grid current. Even 25K screen grid resistor in Fender Champ was added to introduce a sag for an electric guitar. Try at least to shunt it by an electrolytic capacitor for the beginning and see the difference.
No, A single ended amp must operate class A......current must flow through the single output tube all of the time since there is only one tube to do the job.
A push pull audio amp can run in class A, AB, or B, since one tube CAN be cut off while the other one does all the heavy work.
If one tube gets completely cut off for the entire half cycle that the other tube is doing all the work, that is pure class B operation and rarely used except when maximum power output is needed at the expense of distortion.
In practice there is some overlap needed during the transition (crossover) from one tube to the other, resulting in what's called class AB. The area of overlap is where the crossover distortion is the highest, since the tubes are operating at a relatively low current, where linearity is lower and mismatches in the circuit (tubes and OPT) are more noticeable. Turning the volume down forces continuous operation in the overlap region. The usual cure is to crank up the bias current which limits power output due to tube dissipation, or to add lots of feedback. All push pull amp designs involve these tradeoffs.
Many HiFi designs run enough bias current that the output tubes remain in class A for power levels that are usually seen in normal listening, then transition to class AB on loud peaks. These are technically class AB amps, but often the norm. Again this will limit the maximum power output, but is often preferable to a higher powered amp where the crossover distortion is cured with feedback.
It is possible to build a push pull amp in pure class A. The power output will be twice what a single ended amp with the same tube would make. The OPT does not need to be gaped since the DC currents cancel as do most of the even harmonics.
If you want to build a SE amp with sweep tubes you will typically run them at a much lower supply voltage and higher current than seen in a push pull design since many of them are somewhat nonlinear at low currents. Triode mode forces a low plate voltage due to the low screen voltage rating.
As pentodes, I might suggest a separate power supply for the screens.
A cheap and easy way to get there is two inexpensive filament transformers back to back, e.g
117:6.3 => 6.3:117
and rectify and filter the now isolated 117 line volts in the usual manner.
This will get you 150 ish volts, and the current you need without sag. The secondary voltage of the transformers is unimportant, since all you want is the isolated 117 volts. Good for 6DQ6, 6BQ6.
An odd ball tube with a less sensitive screen grid, despite the published ratings, is 2E26. Cheap and abundant at hamfests.
Win W5JAG
A cheap and easy way to get there is two inexpensive filament transformers back to back, e.g
117:6.3 => 6.3:117
and rectify and filter the now isolated 117 line volts in the usual manner.
This will get you 150 ish volts, and the current you need without sag. The secondary voltage of the transformers is unimportant, since all you want is the isolated 117 volts. Good for 6DQ6, 6BQ6.
An odd ball tube with a less sensitive screen grid, despite the published ratings, is 2E26. Cheap and abundant at hamfests.
Win W5JAG
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Here is an example using a similar pentode, the 6LU8. Everything in one bottle, how to build a passable amp as cheap as possible. Did this one about 20yrs ago.
The results as a SET or SEUL are reasonably good. I tried it with pentode out connexion, the results were not good. These toobz were built from the ground up as switchers, HIFI not a priority.
So measured the harmonics with one of the early Pico Tech virtual instruments shewn here. The case fof NFB was about 20 db.
H125E OPT not meant for SE, so at 100 Hz it does not do well. A lot better when I hooked up an H1628.😀
Later I did a clone of Norman Crowhurst's Twin coupled amp with PP 6LU8s. Quite good, 37 watts at clipping with 400V on the plates. Used a pair of H12Es on that one.🙂
Here is an example using a similar pentode, the 6LU8. Everything in one bottle, how to build a passable amp as cheap as possible. Did this one about 20yrs ago.
The results as a SET or SEUL are reasonably good. I tried it with pentode out connexion, the results were not good. These toobz were built from the ground up as switchers, HIFI not a priority.
So measured the harmonics with one of the early Pico Tech virtual instruments shewn here. The case fof NFB was about 20 db.
H125E OPT not meant for SE, so at 100 Hz it does not do well. A lot better when I hooked up an H1628.😀
Later I did a clone of Norman Crowhurst's Twin coupled amp with PP 6LU8s. Quite good, 37 watts at clipping with 400V on the plates. Used a pair of H12Es on that one.🙂
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Thanks for the 6LU8 data, jhstewart9. What an interesting tube. What's the screen voltage on the pentode side?
At little league tonight I was wondering if using a 0D3 to get a regulated 150V screen supply would work better than a large resistor. Or even a different regulator for lower screen voltage, leaving room for a higher plate voltage (rated up to 600V), as George suggests.
Seems it's time to learn how to draw pentode load lines.
Anyway, I'm going to Green Country Hamfest when it opens this Saturday at 8am (thanks for the tip, Win!) and am drawing up quite a shopping list.
At little league tonight I was wondering if using a 0D3 to get a regulated 150V screen supply would work better than a large resistor. Or even a different regulator for lower screen voltage, leaving room for a higher plate voltage (rated up to 600V), as George suggests.
Seems it's time to learn how to draw pentode load lines.
Anyway, I'm going to Green Country Hamfest when it opens this Saturday at 8am (thanks for the tip, Win!) and am drawing up quite a shopping list.
Take an isolation transformer with 120V secondary, rectify it with a voltage doubling rectifier, and get about 150V and 300V under load, with CLC filters. If you get more it is not a problem, since about 20V would be dropped on a cathode bias resistor shunted by a cap.
Many small industrial transformers with 240V secondary would work well.
Hammond & others offer many alternatives.🙂
Cheaper, easier, and if the resistor is sized right it makes a limiter to save the screen grid in case of serious overdrive.
I use a 150 volt 5 watt zener, 1N5383BRL. Bypass it with a small electrolytic, maybe 10 to 47 uF and feed it with a resistor from B+. You want to size the resistor so the screen gets 150 volts under all conditions right up to, and slightly into clipping. You want the screen voltage to drop if the amp is run into hard clipping for an extended time. The electrolytic needs to be big enough to keep the screen at 150 volts on music peaks that run the amp well into clipping, but not keep it there for long enough to melt the screen. I have not played much with the 6BQ6 types in recent years, nor have I tried to run them in class A for SE. The bigger sweep tubes have a pretty wide range here so that the parts can be sized to suit your musical needs. The screen will draw very little current (which goes DOWN as the plate voltage is increases) until the plate voltage swings down below the screen voltage. At this point the screen current goes way up.
The 6LU8 and its brother the 6LR8 are TV vertical sweep tubes, and are not the same thing as the typical horizontal sweep tubes we talk about. They are vertical output tubes and were intended for linear operation.
The vertical sweep section of a tube TV set IS actually a single ended class A AUDIO amp, optimized for one frequency, roughly 60 Hz in the USA. I learned this early on in my youth and many of my first guitar amps were made by lifting the vertical sweep section verbatim from a TV set, breaking the feedback loop in the oscillator and inserting a guitar jack, and connecting a speaker where the deflection yoke would go. Even the OPT works for audio.
This was also exploited by the Bottlehead SEX amplifier in the 1990's. That amp like some of my early amps used the vertical sweep section from a TV, and was responsible for sucking up the worlds supply of NOS vertical output transformers and most of the 6DN7's.
As with many smallish amps it probably suffered from poor damping in pentode mode without feedback, and thus didn't do well for audio. The 6LU8 was usually run in pentode mode in the 21 and 25 inch TV's I worked on in the early 70's. If the circuit wasn't linear, circles on the TV screen weren't round, and people were tall with squished up heads. The high output impedance didn't matter when driving a deflection yoke.
Careful with VR tubes when regulating screen grid voltages of sweep tubes; when anode voltage on peaks dips about 100V below screen grid voltage, screen grid current would be enough to extinguish the VR tube, that would sound as chainsaw distortions on peaks of loudness.
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When I run a VR tube to drop B+ for screens, I usually tie it's anode to B+, use a 100k power resistor from it's cathode to ground, and take the screen voltage from that cathode. It effectively does a voltage shift (in my case VR75 so 300V B+, 225V screen).
Then there is this:
Then there is this:
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I think some of this debate on what type of screen supply you want depends on whether you want a one off just for this project, or a small power supply that you can also use in other projects. If the latter, I think the voltage doubler rectiifer stack is a good idea.
I have little doubt you will find whatever VR tube you want there. Some are really old and radioactive - I don't know if half life affects their performance.
I won't be there at 8:00 AM .... my plan is to drive up to the house at Beaver Lake on Friday afternoon / evening, and get up the next morning and drag my six year old over there. We'll be lucky to make it by 10:00 ... I'll have a name / call sign badge on.
If this is your first one ...
cargo pants are handy - you can fill the pockets with small stuff. A sturdy cloth sack is useful for heavier stuff like chokes, transformers, books, etc. If a pulled out transformer or choke smells OK, it probably is, but I have had NOS ones that look and smell pristine catch on fire. Junky looking on the outside, doesn't mean the parts inside are junk. If someone says they don't know whether or not it works, it doesn't and plan on troubleshooting or parting out. No one wants to drag stuff back home, so at closing time no offer is unreasonable. If you can buy the whole box for a few dollars more, just do it. You can trash / give away what you don't need. Check the dumpster if you can find one. I parted out an Eico quad EL-84 mono amp that someone just tossed in the dumpster at the end of one of our local hamfests.
I'm sure others will have other suggestions ...
You never know what will turn up at these things; sometimes they are a complete bust, sometimes they are pure gold. I was at a podunk one and some guy showed up with PALLETS of depot overhauled, mil surplus, test equipment. I got an overhauled, calibrated, TV-7/D in a depot sealed carton for $50 bucks.
Win W5JAG
I have little doubt you will find whatever VR tube you want there. Some are really old and radioactive - I don't know if half life affects their performance.
I won't be there at 8:00 AM .... my plan is to drive up to the house at Beaver Lake on Friday afternoon / evening, and get up the next morning and drag my six year old over there. We'll be lucky to make it by 10:00 ... I'll have a name / call sign badge on.
If this is your first one ...
cargo pants are handy - you can fill the pockets with small stuff. A sturdy cloth sack is useful for heavier stuff like chokes, transformers, books, etc. If a pulled out transformer or choke smells OK, it probably is, but I have had NOS ones that look and smell pristine catch on fire. Junky looking on the outside, doesn't mean the parts inside are junk. If someone says they don't know whether or not it works, it doesn't and plan on troubleshooting or parting out. No one wants to drag stuff back home, so at closing time no offer is unreasonable. If you can buy the whole box for a few dollars more, just do it. You can trash / give away what you don't need. Check the dumpster if you can find one. I parted out an Eico quad EL-84 mono amp that someone just tossed in the dumpster at the end of one of our local hamfests.
I'm sure others will have other suggestions ...
You never know what will turn up at these things; sometimes they are a complete bust, sometimes they are pure gold. I was at a podunk one and some guy showed up with PALLETS of depot overhauled, mil surplus, test equipment. I got an overhauled, calibrated, TV-7/D in a depot sealed carton for $50 bucks.
Win W5JAG
Yes, it does a voltage shift, but makes G2 voltage dependence on B+ higher than would be if powered from a rectifier.
I usually use a parametric stabilizer, like a voltage divider with a resistor and a VR tube, as a reference voltage source for a source follower using a high voltage MOSFET. When I need more voltage, I just add a Zener in series with the VR tube.
OK, clearly I need to do more research on VR operation as well as understanding pentodes, so I'll work on that while I finish a pair of speakers (trapezoidal single driver design by Planet 10) and will come back to this in a few days.
Win: thanks for the hamfest tips. Do they allow bags? I was thinking I'd just bring my big old messenger bag to fill with goodies.
There's a chance I'll still be there at 10:00, but I do have to get back to Tulsa for my 6-year-old's little league game.
I'll be looking for VR tubes as well as 1626 (for a headphone amp idea I've been mulling), 8532, lots of tube sockets, plus whatever output tubes might look interesting (EL34, 6550, 2A3) and a 12.6V heater transformer.
Beyond that, I'll go with an open mind and more dollars in my wallet than my bride would probably like. ;-)
Win: thanks for the hamfest tips. Do they allow bags? I was thinking I'd just bring my big old messenger bag to fill with goodies.
There's a chance I'll still be there at 10:00, but I do have to get back to Tulsa for my 6-year-old's little league game.
I'll be looking for VR tubes as well as 1626 (for a headphone amp idea I've been mulling), 8532, lots of tube sockets, plus whatever output tubes might look interesting (EL34, 6550, 2A3) and a 12.6V heater transformer.
Beyond that, I'll go with an open mind and more dollars in my wallet than my bride would probably like. ;-)
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Yes, bags are fine.
My recent experience in these parts is that you are most likely to run across sweep tubes - totally relevant to this thread - and 807 variants including 5933 and 1625, and small power pentodes likes 5881, 6K6, 6V6, 6Y6, etc. 47 is pretty common, 46 less so, but I would give even odds on finding some of those. Octal and miniature dual triodes are still pretty common. Rectifier tubes are common. Small signal tubes are common, but I'm not familiar with the two you mention.
All of the sweep tubes in the below picture, plus a sack of small tubes, came from a $5 box of tubes last week. The boxes are in terrible condition, but the tubes appear unused.
Also relevant to this thread - the old tube style variable regulated high voltage power supplies are in pretty high supply, and pretty low demand, and you might see some of those. Easy to fix if something is wrong. Both of mine came from Hamfests. One was NOS.
Junky old transmitters and power supplies often have giant power transformers with taps for screen voltage ( sometimes multiple taps for screen voltage ), bias, and 10+ amp heater taps, in addition to B+.
Audio signal generators, distortion meters, and scopes are pretty common. They can be any thing from elderly lab grade to Grief Kit.
I am only looking for 1E7 - I'm sure I'll be the only person there looking for those, and a 15MF8 ( got one of those in that box of tubes last week, and it looks pretty interesting ), and some more teflon hook up wire so I can be leisurely getting over there ...
Win W5JAG
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