Hey there folks,
I'm thinking about building a modified version of Gordon Rankin's Bugle amp to use as a headphone amplifier (mostly for a pair of Hifiman Ananda - 25ohm and 95db / mW).
First and foremost, I'm keen to use a pair of 46 tubes instead of the classic 45, with G2 tied to the plate. I'm not yet sure if I should install UX4 or UX5 sockets by default, but I'm leaning towards UX5 (and use an adapter to mount 45s if needed). This shouldn't modify the design much - I just have to make sure the power supply can handle the current requirement.
Now, for the drivers, I'm interested in going with a 6SN7 setup either in SRPP configuration (which again wouldn't modify the original design much) or in an Aikido configuration. I really like my Aikido preamp and I would try to replicate this here, but I'm aware it would complicate things a bit due to the need for a balanced power supply.
I'd start putting together some schematics soon, but I'm looking forward to the opinions here - even if the replies will be along the lines of "this will never work as you hoped". Better to fail early, right?
I'm thinking about building a modified version of Gordon Rankin's Bugle amp to use as a headphone amplifier (mostly for a pair of Hifiman Ananda - 25ohm and 95db / mW).
First and foremost, I'm keen to use a pair of 46 tubes instead of the classic 45, with G2 tied to the plate. I'm not yet sure if I should install UX4 or UX5 sockets by default, but I'm leaning towards UX5 (and use an adapter to mount 45s if needed). This shouldn't modify the design much - I just have to make sure the power supply can handle the current requirement.
Now, for the drivers, I'm interested in going with a 6SN7 setup either in SRPP configuration (which again wouldn't modify the original design much) or in an Aikido configuration. I really like my Aikido preamp and I would try to replicate this here, but I'm aware it would complicate things a bit due to the need for a balanced power supply.
I'd start putting together some schematics soon, but I'm looking forward to the opinions here - even if the replies will be along the lines of "this will never work as you hoped". Better to fail early, right?
Hi there,
If you have access to a tube curve tracer I'd try the following, I don't know if it was attempted before...
I'd use the 46 with Schade local feedback to get a good distortion vs power balance (you will get bigger power than triode strapped while benefiting from the triode sound finesse).
Schade feedback rises the input impedance of the output tube so I'd bias the SRPP at 8mA to be sure it can drive the schaded 46.
Schade feedback gets you lower output impedance and wider frequency response.
For 46 filaments I'd use Rod Coleman regulators. For the SRPP I'd use SiC diodes in the cathode bias scheme and for the 46 I'd bias it trough the grid with a regulated negative supply. Also I would use a regulated supply for the 46 second grid (In case the Schaded feedback thing works).
For inspiration purposes: 45 SE Amplifier – revisited – Bartola(R) Valves
Reards,
Silviu
If you have access to a tube curve tracer I'd try the following, I don't know if it was attempted before...
I'd use the 46 with Schade local feedback to get a good distortion vs power balance (you will get bigger power than triode strapped while benefiting from the triode sound finesse).
Schade feedback rises the input impedance of the output tube so I'd bias the SRPP at 8mA to be sure it can drive the schaded 46.
Schade feedback gets you lower output impedance and wider frequency response.
For 46 filaments I'd use Rod Coleman regulators. For the SRPP I'd use SiC diodes in the cathode bias scheme and for the 46 I'd bias it trough the grid with a regulated negative supply. Also I would use a regulated supply for the 46 second grid (In case the Schaded feedback thing works).
For inspiration purposes: 45 SE Amplifier – revisited – Bartola(R) Valves
Reards,
Silviu
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That's a very carefully designed circuit. Changing to a 6SN7 and a 46 would drastically alter the whole topology. You're basically starting from scratch.
Can you please elaborate on this?
From what I read so far the 46 is a drop in replacement for the 45 if you connect the second grid to the plate, so no topology change there.
As for the 6SN7 - if I run them in SRPP mode I think I would have to change a few part values, but can't exactly see how the topology would change?
Thanks,
Catalin
It's different device that can get same performance but it works a rather different way: different drive, different supply, different output transformer.
If you want same power as the 45 without positive grid drive you need higher plate voltage around 320/30mA. 8K output transformer. Required swing 80V peak-to-peak. If the tubes are NEW you might get some 1.7-1.8W power otherwise around 1.5W. So as you can see it's not a drop-in replacement at all.
Power output is not required to be the same, but I just pulled up the datasheet for the 46 and I see what you mean regarding the output transformers. Question begs to be asked now... if I put in an 8k output trafo and then use 45s, would that be an issue? I know there's a minimum primary resistance, but what about maximum?
Nope. You can actually use the 45 with 8K and 320V supply if dissipation does not exceed 10W. Plate voltage regulation (and also bias if you use fixed bias) is highly recommended in such case.
P.S.
Beware that is you use the 45 with 320V plata voltage and 8K you will need 140V peak-to-peak to fully drive it in class A1. So you need a really capable driver. However output power will be about 2.5W (THD =5% or less)! With new tubes of course....
P.S.
Beware that is you use the 45 with 320V plata voltage and 8K you will need 140V peak-to-peak to fully drive it in class A1. So you need a really capable driver. However output power will be about 2.5W (THD =5% or less)! With new tubes of course....
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I didn't notice it was for headphone use. Then it can run much lower. Still the 45 and 46 are not identical. Some adjustments are needed.
If you only need few mW then why not use something like a 49? It's cheap, just as linear as a 45 in triode mode and only needs 2V/120mA filament supply. Max Pout can easily exceed 150 mW.
If you only need few mW then why not use something like a 49? It's cheap, just as linear as a 45 in triode mode and only needs 2V/120mA filament supply. Max Pout can easily exceed 150 mW.
I would tune it for the 46 to be honest. If it works "good enough" with a 45 without adjustments that's fine, as a backup. I'll go over the datasheets this weekend and try to figure it out.
I had no idea about the 49 to be honest and I already have a pair of 46s. I see they're much cheaper, but just as rare as a pair, so I'm not too inclined to switch now.
Yup, that's where I found out about the 46 to begin with.
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Right now I'm just a bit concerned about noise, considering I'm targeting headphones. Do you guys see any issues with the standard design here?
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Not difficult to find really. It's just that they aren't popular among DIYers and so you don't see them on display but lots of tube sellers have them. In matched pairs too, of course.
I personally have 2 RCA quads, 1 Cunningham quad (RCA I think), 2 Sylvania quads. All matched and tested in their original boxes. Plus 2 used like new pairs that I use for testing...
I don't know the design you are talking about. As general rule, use DC heater supply. Better if regulated for both high voltage and heaters.
I’ll admit, I was only looking on eBay now for the 49. I’ll check some of the reputable tube sites as well, but I’m not sure something just a bit over 150mW is enough (especially if I’ll use it with higher impedance headphones)
Also, by standard design I was referring to the Bugle’s power supply circuit.
Also, by standard design I was referring to the Bugle’s power supply circuit.
Significant difference in Vg1 for same Ip.
(even after I found the right curves...
)Attachments
Significant difference in Vg1 for same Ip.
(even after I found the right curves...
I'm going through GR's article on the Bugle and redoing the calculations for the 46. It seems that for Vg at -33V and Ip at 22mA, we get Rbias at 1k5, just like in the original design.
For a 2380 Rp, let's consider using an 8K output transformer. Using the new values, I got a total gain of 4.135 and then a gain for -3db @ 5Hz to be 2.92. Using these in the formulas Gordon provided, I get the value for an unbiased R2 at 2164 ohms. This would yield a bypass cap of at least 14uF - I take it using the original 24uF suggestion would work just fine, right?
Looking at the datasheet for the 45 you posted, I would conclude that switching the 46 with a 45 in this setup should work "good enough" - the op points would be close enough (even at 250V Vplate)
Second opinions are welcomed!
Either way, I'm looking to build the amp with the 46 as an output tube and keep the 45 as a backup solution in case I can't find any more tubes.
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