I'd like to survey for some driver stage tubes that can operate well at a 5mA or less set point. I've been communicating with Cinemag, they've developed a high quality interstage transformer (for a high priced audio manufacturer) that surpasses Lundahl LL1660 and many others at a great price, that I'd like to try. The caveat is that the 50% nickel 50% steel version can handle only up to 5mA. To get more current he'd have to make them from all steel. Before I pull the trigger ordering, I'd like to determine some tube options at 5mA and get the nickel version if I can. This is to drive my interstage coupled 300B project I've been posting. Just want to get some candidate tubes in the mix from folks with experience at operating points of drivers. Finding good linearity at 5mA in a tube could open up more interstage transformer choices like this Cinemag. Physically it is a large unit they tell me, if that matters. Attached is the frequency plot model CM-22922.
https://www.cinemag.com/
What tubes run good at 5mA for this purpose?
https://www.cinemag.com/
What tubes run good at 5mA for this purpose?
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I wish I could be of more help, but I cannot think of a good tube for this application, nor a reason to build what is going to be a compromise I suspect in a lot of ways. I've designed a number of interstage based amps with 50, 300B and GM70, I've never designed driver circuits that operated at less than 15mA, and 20mA was typical.
Is parafeed operation with this transformer possible? If you are not locked into this transformer I'd recommend interstage transformers from Monolith Magnetics in Belgium, or something from Iso Tango or Tamura. (Some of this stuff is out of production) 300B amps are generally expensive, why start with a major design compromise?
More information is needed - what is the primary impedance, what rp (source impedance) does the transformer require for reasonably flat frequency response? How much gain (mu) do you need?
The 300B depending on operating point can require more than 200Vpp grid drive. (Pushed to the limits close to 300Vpp may be required)
My IT driver experience is with triode connected D3A, 6E6P-EV, and EML 20A/AM/B DHTs. All of these run at around 20mA. All but the 20 DHT have rp of <2K.
Is parafeed operation with this transformer possible? If you are not locked into this transformer I'd recommend interstage transformers from Monolith Magnetics in Belgium, or something from Iso Tango or Tamura. (Some of this stuff is out of production) 300B amps are generally expensive, why start with a major design compromise?
More information is needed - what is the primary impedance, what rp (source impedance) does the transformer require for reasonably flat frequency response? How much gain (mu) do you need?
The 300B depending on operating point can require more than 200Vpp grid drive. (Pushed to the limits close to 300Vpp may be required)
My IT driver experience is with triode connected D3A, 6E6P-EV, and EML 20A/AM/B DHTs. All of these run at around 20mA. All but the 20 DHT have rp of <2K.
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Sounds like you have a solution looking for a problem instead of the other way around. Is it a step-down IT, to get some lower Z and more current on the secondary to drive the 300B?
This is not the only question.
If you want to use this IT in two stage amplifier (VAS tube-IT-power tube-OPT), he following criteria must be met:
- enough large gain (tube+IT);
BTW 300B requiring even 180Vpp grid swing
- enough large current of VAS (driver) tube -and IT primary too- to drive power tube's Miller capacitance;
BTW 300B requiring even 1mA (70V peek, 30kHz) see there , so 5mA just enough for one 300B
- IT must be capable even 180Vpp (39dBu) signal.
I'm afraid, that this 5mA restriction limits the power tube selection.
Thanks all. I know there are other factors besides current, but that was the first one to hurdle with this particular IT. I may have him make me the all-steel one at 20mA if he can. Based on your experience driving 300B with a two stage IT topology, I'll stick to fining IT's in the 15 to 20 mA range. My schematic is here (kept updated in post #1):
And my goal is to make this project a "platform" where I can plug and play different driver boards and IT's as the focus of my hobby going forward. So scouring around for maybe 4 different IT's and 4 different driver boards to analyze and compare (eventually).
And my goal is to make this project a "platform" where I can plug and play different driver boards and IT's as the focus of my hobby going forward. So scouring around for maybe 4 different IT's and 4 different driver boards to analyze and compare (eventually).
Triode-strapped EF184 is a very capable tube, even better CCS or gyrator loaded.
That brings up another question I wanted to ask. When using an anode CCS along with an interstage, I'm assuming the CCS would take the place of the anode resistor that resides between B+ and the IT right? So basically its B+ --> CCS --> IT --> tube? I am biasing the 300B via the IT secondary as shown in my schematic. Replacing the plate resistor above the IT with a CCS would set the operating current, as long as the B+ was high enough for the EF184 coming in, mine is 380V to the 300B. That CCS would be dropping a lot of voltage right?
I also have some 6J51P Russian tubes, it appears to similar. There was another pentode that others thought was a "sleeper" 300B driver, I'd have to look for that post where people were discussing various 300B drivers. 6S45P is also similar to the 5842 I'm starting with.
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gheghe, guess what I am working on 😀
Sick minds think alike?
I made PCBs for about 15 different tubes to easily try them out, plug and play with connectors and bias chosen so they all run on 200Vdc from a universal PSU. 2P29L, 6E5P, 6E6P, 6S4P, 6S19P, 6S45P, 6J1P, 6J9P, 6J11P, 6J49P, 6J52P, EC8010, 5842, 5847, D3a and some others. But they are all configured to run between 15-30mA as I have an LL2763-40mA 2:1 interstage from a previous project. Output tube will be a 4P1L, bets are on the 2P29L but it's just good fun really.
Sick minds think alike?
I made PCBs for about 15 different tubes to easily try them out, plug and play with connectors and bias chosen so they all run on 200Vdc from a universal PSU. 2P29L, 6E5P, 6E6P, 6S4P, 6S19P, 6S45P, 6J1P, 6J9P, 6J11P, 6J49P, 6J52P, EC8010, 5842, 5847, D3a and some others. But they are all configured to run between 15-30mA as I have an LL2763-40mA 2:1 interstage from a previous project. Output tube will be a 4P1L, bets are on the 2P29L but it's just good fun really.
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Wow! Yes I want to concentrate on one topology (single stage driver, to IT, direct grid bias of 300B via IT secondary) for one output tube type (PSE 300B). Then try out many different driver tubes and IT's. A worthy use of time I think, on a worthy 300B. For a long time I was just building random projects then tearing them down learning. I'm thinking now I want to make this dedicated platform with very similar PCB's for the driver, same terminals, same size boards, etc. So the only variance is the driver board and what IT I use. The power section stays the same. To figure out drivers and IT's alone. My hope is to try many suitable driver tubes with resistor or ccs load with a collection of finer IT's, just see where it takes me. But first I have to build the platform shown in my schematic.
I would hope you try NOS 6S45P as well as current Sovtek 6C45Pi! Its on my list as my 300B drivers list grows.
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I tried most of the triodes about 20 years back, 6s45p is a beast to tame with its tendency to oscillate. I kept that in mind with the PCBs and added ferrites around the tube pins and/or on the pcb itself.
Working on a second set of PCBs with some more space and fixed header positions to make a modular amp basically: just plug in a different board to test a different tube. Mine are all triode strapped with auto bias parts on the pcb as well. Just B+, grid, ground and filaments.
Working on a second set of PCBs with some more space and fixed header positions to make a modular amp basically: just plug in a different board to test a different tube. Mine are all triode strapped with auto bias parts on the pcb as well. Just B+, grid, ground and filaments.
I heard that said by others too. I think for my PCB's I'm going to use chassis mount tube sockets on standoffs over a big hole in the PCB. I think that way I can solder the two grid stoppers needed on 6S45P right up against the solder lugs with less that a mm of resistor wire showing. Additionally using a chassis-type socket I may even be able to modify the socket pin to drop a short ferrite tube over the whole pin itself then solder the stopper resistor as well. With a chassis-type socket the actual leads would come off the socket then into their pads on the PCB, so I can push leads around to control oscillations too. With PCB traces and PCB type sockets there is no hope of being able to get close with the stopper or to push leads around.
2.0mm inner diameter and as small as possible outside diameter, but on a noval you can't do every pin that way or the ferrites will touch. You can add ferrite beads over resistor leads as well.
This is the bottom on a 6E6P board, but it shows what I mean. The * denotes ferrites over the noval socket pins before I solder on the socket (anode and 3 cathode pins in this case).
And on top, * means ferrite over the resistor there, as I have them all vertically to save space. the --0-- are stand-alone beads. Grids and filaments.
This is the bottom on a 6E6P board, but it shows what I mean. The * denotes ferrites over the noval socket pins before I solder on the socket (anode and 3 cathode pins in this case).
And on top, * means ferrite over the resistor there, as I have them all vertically to save space. the --0-- are stand-alone beads. Grids and filaments.
I see. But that carries current. I have some "leaded" ferrites which is just a lead with a ferrite bead molded around it. Same shape and size as a 1/2 watt resistor except the body is ferrite. Maybe those would be best for an anode stopper? Soldered close.
Those are good. I use those on the PCB when I can't put one over the socket pin (the two halo thingies just above the noval socket in the bottom picture).
Consider using 1206 SMD resistors as grid and plate stoppers, you can get them right up by the pins in your layout. I typically use 100 or 1K ohm resistors on control grid, 47 - 100 on screen grid to plate and 10 ohm as plate stoppers. 1206 resistors are small, but not extremely so. I generally use thin film, thick film are noisier but in most applications at these signal levels that's not a problem.
I'm skeptical that this will make a great IT based on the data that Cinemag sent you. @kevinkr and @euro21 already brought up that you'll need to swing significantly more voltage. Current requirements aside, this means higher distortion.
I'm also seeing about a 1.5dB insertion loss with just a 100k load. My back-of-envelope math says that this puts the transformer winding impedance at around 5k. Assuming I'm in the right ballpark, that's about 4x higher DCR than an LL1660S with windings in series, or 16x higher than the LL1671 (an interesting comparison as it uses the same core with less wire for lower level handling). If Cinemag needs to put more turns of thinner wire on their core, I suspect the core might not be so large, and that it may not have the level handling that you need. The abundance of wire might also explain the ultrasonic features.
If you do want a low current driver, the 6688 or E180F could be a good bet. Here's some info from JC Morrison; he's had good luck running it at 1-2mA, as I have. At higher current, I've found it a bit shrill.
https://www.labjc.com/?p=5734
Its gm is a bit more manageable than the 6S45Ps and 6E5Ps. You might want a follower to drive the 300b grid, but they're cheap enough that you could run a couple in parallel to a total of 4mA. If it were my amp though, I would consider some of the higher current drivers that others have suggested here.
Edit: I do think parafeed is a great option too. This would allow you to use a full nickel core. With a hybrid mu follower as a driver, you would effectively have both a driver and follower combined in a single stage.
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