These are preliminary numbers. I haven’t finished the amp yet. But that’s in the ballpark.
The filament R is 5 Ohms/50W.
Definitely DC on the filaments. 7.5 V is far too high for AC.
I use choke input everywhere, HT and LT. For filament supplies i use Lundahl LL1694 as the first choke.
The type of resistor in the filament supply matters. I prefer vitreous enamel.
The filament R is 5 Ohms/50W.
Definitely DC on the filaments. 7.5 V is far too high for AC.
I use choke input everywhere, HT and LT. For filament supplies i use Lundahl LL1694 as the first choke.
The type of resistor in the filament supply matters. I prefer vitreous enamel.
Hi - I dug up some old designs I made and built. These are from a few years ago - I might modify them if I rebuilt the stage today, but they're in the ballpark. The resistor is definitely important. Vitreous enamel is the right kind. I actually use some Russian military resistors I found on eBay. Not vitreous enamel, but I like the sound of them. Looks like I was using starved filaments then - you can try that or use the full 7.5v as in the data. Regulators are always Rod Coleman's.
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Regarding the resistor, I am trying a Duelund Graphite-Cast 10W resistor (3R6ohm) later today on my existing amp (C3g). Never tried Duelund resistors, nor graphite cast.
FWIW: In my existing amp, all caps are Russian PIO (50uF, 0.47uF and 0.22uF). I love Russian caps. Andy, are the Russian resistors as good as the caps?
FWIW: In my existing amp, all caps are Russian PIO (50uF, 0.47uF and 0.22uF). I love Russian caps. Andy, are the Russian resistors as good as the caps?
I much prefer Russian teflon caps - FT-2 and FT-3 best of all. Not keen on the PIO ones.
I don't know about Russian resistors in general - I just like the ones I bought. 10W silver coloured, look like ceramic but sound better than the cheap box type ceramics.
The C3g can't be used in filament bias. It doesn’t have a filament, it has a heater separate from the cathode.
I haven't read through the thread very closely, but wasn't it determined that a 10Y or similar low mu tube wouldn't work? The discussion seems to have come full circle back the the 10Y mainly because other alternatives can't use filament bias.
I would think that the ability to provide enough drive would be more important that the ability to use a particular bias scheme.
If the object is to eliminate the cathode bypass cap and the use of filament bias limits your choices, why not try battery bias using a tube that can provide adequate drive?
All the "usual suspect" DHTs seem have a mu of less than 10 but there are some that have a higher mu. I haven't tried them yet, but I recently picked up a few 1G6GTs which are dual DHTs with a mu of 30.
I have no idea if they can be used as a driver, though, since they are designed to operate as Class B outputs with 0v grids. Any thoughts on their feasibility?
https://tube-data.com/sheets/127/1/1G6GT.pdf
I believe there are some other DH tubes with higher mu too.
I had a bunch of 1J6G until I sold them all. They were very poorly matched between sections - only a small number in the bunch matched at all. This is a similar dual DHT but with a mu of 14. It sounded quite good, quite a full sound. But not in 26 class. I also had some 3B7 with a mu of 21, which didn't sound that good at all. And then some 3a5, mu=15, which sounded quite nice - at least as good as the 1J6G. For some reason I don't remember ever having 1G6GT. If it's like 1J6G the sections may be poorly matched - just guessing here.
I think this is a "no no", but I will ask anyways. My power transformer has 2.5Vac that are currently used for the output tube (45B) filaments. IF I replace the C3g with a DHT that also uses 2.5Vac filaments, can both the driver and output tube share the same filament power supply?
As an alternative, what is the best way to drop the filament voltage to 2.5Vac if I use the existing 6.3Vac?
As an alternative, what is the best way to drop the filament voltage to 2.5Vac if I use the existing 6.3Vac?
A DHT needs its own supply - you should not share supplies.
Surely you need a gain of 30-40 for the input tube? That's not a DHT unless you use a step-up input transformer but that will affect the purity of the sound so you're probably better off with an indirectly heated tube. Anyway, with a DHT I'd use Rod Coleman regs and they need headroom to work, so you might need more than 6.3v in fact when converting to DC. If you have 6.3v as a starter voltage you can top it up by connecting another transformer in series. e.g. 6.3 + 6v takes you to just over 12v. If you do this you need to check that the windings are the right way around, or you'll get a much lower voltage.
I tried the 1J6G in my currently breadboarded project, which is an "inverted" SET in that it uses a DH input tube with an indirectly heated output tube, the 6N6G, which is very easy to drive and puts out ~4 wpc. I also tried numerous other DH input tubes.
I really liked the 1J6G too but I found I preferred the 26, which is the tube I was originally using. I could easily live with the 1J6G but the 26 sounds a bit more holographic to me.
If the 1G6GT sounds as good as the 1J6G it would be an excellent choice for a situation where a higher mu is needed. I wasn't planning on trying the 1G6GTs in this amp since the 6N6G is so easy to drive.
I have four 1J6Gs and only one has sections that aren't quite closely matched for transconductance. I did, however, use them with the two sections in parallel.
I also have four of the 1G6GTs. One has perfectly matched sections, the second matches within 4.7%, the other two are at 10.3% and 12.3%. But, again, I'd probably run them in parallel.
The reason I wondered about using the 1G6GT as drivers is because the data sheet only shows an operating point with 0v on the grid, whereas the data sheet for the 1J6G shows operating points with negative grid voltage in addition to one with 0v.
I've been using battery grid bias on all the various input tubes I've tried and heating them with DC from a little Meanwell SMPS with a little additional filtering (L-C) added.
For some perspective, I also have a 45 SET I built years ago using the highly regarded Bugle design by Gordon Rankin. This breadboard amp sounds much better.
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True, but very expensive. OK for deep pockets but for the economically minded there are better ways of spending the money, like good 300b outputs and OPTs.
I am thinking carefully about the EML 30A. Andy is correct: expensive. That said, I am pursuing my personal all out assault on a DHT --> DHT monoblocks, so the EML 30A is within realm of possibilities. I was checking the price of 10Y tubes, holy cow. Very expensive.
Ale Maglia has a very nice article (EML20A driving EML45B – Bartola(R) Valves) on 20B --> 45B. He states " EML20A. With a gain of 20, it’s a good candidate but it hasn’t got just enough gain to drive the output stage to full power... The EML30 is an alternative...With the voltages involved and swing required, probably better to run it with choke load."
So, I need help defining the specs for an choke for the 30A. Please help me understand the math for this. Is this correct:
anode current = 25mA
frequency cutoff = 20hz
Vrms = 176 (per Ale Maglia's article for the 45B)
Vrms=4.44 x i x L x f = 4.44 * 0.025 * L * 20
Then L = 176/ (4.44 * 0.025 * 20) = 79.3H
Giving ourselves 10%+ headroom, should we specify the choke at 90H?
Also, could the 30A direct couple to the 45B, the eliminating the need for an IT or RC circuit?
Ale Maglia has a very nice article (EML20A driving EML45B – Bartola(R) Valves) on 20B --> 45B. He states " EML20A. With a gain of 20, it’s a good candidate but it hasn’t got just enough gain to drive the output stage to full power... The EML30 is an alternative...With the voltages involved and swing required, probably better to run it with choke load."
So, I need help defining the specs for an choke for the 30A. Please help me understand the math for this. Is this correct:
anode current = 25mA
frequency cutoff = 20hz
Vrms = 176 (per Ale Maglia's article for the 45B)
Vrms=4.44 x i x L x f = 4.44 * 0.025 * L * 20
Then L = 176/ (4.44 * 0.025 * 20) = 79.3H
Giving ourselves 10%+ headroom, should we specify the choke at 90H?
Also, could the 30A direct couple to the 45B, the eliminating the need for an IT or RC circuit?
I couldn't find a data sheet for the EML30. Is it extremely rare or are you referring to the proprietary tube made by Emission Labs called the 30A on their website? I didn't see any prices, how expensive are they?
The 1G6GTs are $3 at the dealer near me.
Hi FlaCharlie - thanks for chiming in on the conversation. Yes, the EML30A is the tube made by Emission Labs.
Regarding the 1G6GT, is this a twin DHT?
Regarding the 1G6GT, is this a twin DHT?
The 10Y doesn't have enough gain. You are looking for a triode with mu of 30-40. Or a pentode like C3m. Or a pentode in triode like C3g. I haven't tried the EC86 yet or the Russian 6S3P equivalent. They would work but be careful to avoid oscillation - search on EC86 for how to attach 3 grid stoppers together like a tripod.
i've actually heard the EML20 in Ale's system when we did a preamp shootout and it was quite disappointing. The other DHTs were much better sounding. I certainly wouldn't pay money for it, no way. Read about it here...
DHT Audio Shootout @ London – Bartola(R) Valves