Ok, not much of a "design" yet but please have a good look at the picture and let me know what you think of the ingredients:
To clarify a bit: The input tube (CV1135) is a single triode more or less equal to one section of a 6SN7. 6AV5GA is a sweep tube that can been used in triode mode as a replacement for 6B4G.
you've got better than average signal-pate "iron" ;^) - - what approach might you do for a power supply?
Would the CV1135 more or less be equivalent to a 6J5? Decades ago I manufactured a small run of 300B amplifiers with 6SN7 driver stages, with the input stage parallel triodes and the second stage SRPP.
You have interstages, later designs I did would be similar to what you propose. I do recommend 2 driver stages in order to get sufficient gain. Seems like 7 - 8mA for a 6J5 would be a good operating point for a 10mA rated transformer. I am going to go against my recent practice and recommend cathode bias in the driver tubes, I would recommend fixed bias in the output stage in order to avoid a large and not so great cathode bypass cap there.
Driver plate voltages around 250V - 300V max.
Cathode resistors in driver stage maybe 1K // 100uF. I think I would recommend <horrors> polymer aluminum electrolytics in parallel perhaps 2 x 47uF or 3 x 33uF. No real need to bypass with a fancy film cap, but you can try that if desired.
In the PSU I use CLCLC (typically 60uF film and 1 - 2H chokes rated at least 2X the max current they will carry DCR <= 50 ohms) Rectifiers are SIC. Power from subsequent stages would be RC filtered.
The JJ 300B is an excellent choice I used for many years and still recommend. I am currently using EML 300B which are even better and a lot more expensive unfortunately. Shop around carefully for JJ 300B I have generally found that AES commercial branch offers the best prices, but it has been a while. I have JJ300B with > 5000 hours of service that still test at better than 80% of original transconductance, and they have been really reliable.
For heating 300B filaments I strongly recommend DC heating and Rod Coleman's filament regulators. (No affiliation, but I have used something in excess of 12 - 14 sets of them over the years in various projects after settling on them as the solution to my DHT heating woes.)
You have interstages, later designs I did would be similar to what you propose. I do recommend 2 driver stages in order to get sufficient gain. Seems like 7 - 8mA for a 6J5 would be a good operating point for a 10mA rated transformer. I am going to go against my recent practice and recommend cathode bias in the driver tubes, I would recommend fixed bias in the output stage in order to avoid a large and not so great cathode bypass cap there.
Driver plate voltages around 250V - 300V max.
Cathode resistors in driver stage maybe 1K // 100uF. I think I would recommend <horrors> polymer aluminum electrolytics in parallel perhaps 2 x 47uF or 3 x 33uF. No real need to bypass with a fancy film cap, but you can try that if desired.
In the PSU I use CLCLC (typically 60uF film and 1 - 2H chokes rated at least 2X the max current they will carry DCR <= 50 ohms) Rectifiers are SIC. Power from subsequent stages would be RC filtered.
The JJ 300B is an excellent choice I used for many years and still recommend. I am currently using EML 300B which are even better and a lot more expensive unfortunately. Shop around carefully for JJ 300B I have generally found that AES commercial branch offers the best prices, but it has been a while. I have JJ300B with > 5000 hours of service that still test at better than 80% of original transconductance, and they have been really reliable.
For heating 300B filaments I strongly recommend DC heating and Rod Coleman's filament regulators. (No affiliation, but I have used something in excess of 12 - 14 sets of them over the years in various projects after settling on them as the solution to my DHT heating woes.)
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The LL1660 has a great reputation as a line output transformer (4,5:1) but is considered mediocre as an interstage (4:4,5). I thought I might try it wired as 4:2,25 and see how it behaves, a bit of step-down can possibly tame it quite a bit. The LL1671 seems to have a more solid reputation used as an interstage transformer.
No details about the power supply yet but probably something with 6AU4 as rectifiers and fixed bias for both the output and the driver tubes. Possibly a separate PSU for the input and driver tubes. Probably around 360-380V for the outputs and maybe 260V for the rest. Coleman regulators for the filaments.
This is just one early draft based on parts that I already have, I might change the output transformers to a more reasonably sized Lundahl model and keep these 30-watters for an upcoming transmitter tube project that would make better use of their power rating.
The idea of using three stages with interstage transformers between each one is probably enough to make some people choke.
If it gets build like this, will it sound good? Probably. Would it measure well? Probably not.
To clarify a bit more: All these transformers are leftovers from old projects that never happened, this design idea is an attempt to put all of them to good use in a way that would pay homage to the very early amp designs from a century ago.
If i didn't already own both pairs of interstage transformers I would definitely lean more towards the Tubelab TSE or one of those designs with a high gm pentode as input stage.
No details about the power supply yet but probably something with 6AU4 as rectifiers and fixed bias for both the output and the driver tubes. Possibly a separate PSU for the input and driver tubes. Probably around 360-380V for the outputs and maybe 260V for the rest. Coleman regulators for the filaments.
This is just one early draft based on parts that I already have, I might change the output transformers to a more reasonably sized Lundahl model and keep these 30-watters for an upcoming transmitter tube project that would make better use of their power rating.
The idea of using three stages with interstage transformers between each one is probably enough to make some people choke.
If it gets build like this, will it sound good? Probably. Would it measure well? Probably not.
To clarify a bit more: All these transformers are leftovers from old projects that never happened, this design idea is an attempt to put all of them to good use in a way that would pay homage to the very early amp designs from a century ago.
If i didn't already own both pairs of interstage transformers I would definitely lean more towards the Tubelab TSE or one of those designs with a high gm pentode as input stage.
Probably pretty much the same thing but with two funny looking top caps.
It would simplify things a bit, but it would also be nice to make an amp with almost no components inside 😀
The PSU will definitely get its own box, no way I'm letting any power transformers and chokes anywhere near these ITs.
Sounds like a good plan! 😀 I would up the output stage B+ to 400V, but not too much over, there is a benefit in linearity with slightly higher plate voltage.
I use fixed bias in the driver stages of some designs, via a dedicated negative supply or even batteries! (LOL) I am backing away from LED bias and CCS at this point time after a decade of fiddling around and wondering why my older designs sound better. 😀
I use fixed bias in the driver stages of some designs, via a dedicated negative supply or even batteries! (LOL) I am backing away from LED bias and CCS at this point time after a decade of fiddling around and wondering why my older designs sound better. 😀
Cool, I would definitely use it. I have a friend who uses the U.S. equivalent, they can oscillate so you may want a grid stopper right at the grid cap of say 1K and a small resistor in series with the plate lead also at the plate cap.
We align on power supply philosophy, I have been doing the separate box gig for about 30 years now. One of two amplifiers shown along with their PSU. These direct drive my horns. (the system is active and DSP based)
I'm keeping an eye open for transformers with taps on the secondaries to allow the B+ to be adjusted. I have three different brands of output tubes and I would be surprised if all of them sounded their best at the exact same plate voltage.
Regarding the driver stage, using real 6B4Gs (or russian 6S4S) is still in the cards but I still haven't fully recovered from building all the filament power supplies for my 6S4S PP amp. Only the chokes set me back a good 600 dollars and I had to call a friend over to help me lift the damned thing into the rack.
Yes, I use the same tubes in the 6S4S amp with 2,2k CC stoppers right at the top connectors. Works like a charm and sounds marvellous.
I assume the interstage would not be used with 6B4G/6C4C as the primary current rating isn't sufficient to support their use, but IT drive to the grid of the 300B is what you really want to do.
Another option if budget is not too much of a constraint would be to get Monolith Magnetics to design and build you a pair of ITs for use with the 6B4G. I have used custom IT and OPTs from them for years with great results.
Another option if budget is not too much of a constraint would be to get Monolith Magnetics to design and build you a pair of ITs for use with the 6B4G. I have used custom IT and OPTs from them for years with great results.
Look closer and you'll see one LL1660 / 10mA and one LL1671 / 50mA in the picture 🙂
The idea is to use the 1660 between input and driver and the 1671 between driver and output.
I'm coming to the conclusion despite the fact that some of my designs had 3 transformers in the signal path, that the optimum number is <= 2 LOL. Guess I am a bit jaded, and I am aware that recordings made in the early stereo era were made with many transformers in the signal path. I'd go for the LL1671 to drive the 300B and maybe RC couple to drive the 6B4G, but it is a choice and your goals might be different than mine. 😀
I'm using 1660s as IST/Phase splitter for PP 45s with a 53 driver with great results. Series connection on the primary and split windings for the 45 grids.
Definitely an option. I usually like transformers as long as I only evaluate them by ear, as soon as any test equipment is involved the fascination flatlines immediately.
As I mentioned earlier the LL1660 is probably the bottleneck here but since I already own them I'd like to try them.
I'm doing exactly the same thing in my CV1135- 6S4S PP amp. Sounds very good but I'm keeping the oscilloscope away from it 😀
Edit: I'm using the "S" version with some kind of screens for better balance when used as phase splitters.
Using X= 1/sqrt(1+ (Rp/2πf L)^2) as a first estimate
of whether driver tube with Rp and Interstage with inductance L is suitable.
X=1 is good, X=0 is bad.
CV1135 Rp is unknown to me, 6SN7 Rp is 7000. 1660S/10mA inductance is 40H.
f=20Hz, that give us 1/sqrt(1+ (70/48)^2) ~ 1/1.76
which is not so good ?
of whether driver tube with Rp and Interstage with inductance L is suitable.
X=1 is good, X=0 is bad.
CV1135 Rp is unknown to me, 6SN7 Rp is 7000. 1660S/10mA inductance is 40H.
f=20Hz, that give us 1/sqrt(1+ (70/48)^2) ~ 1/1.76
which is not so good ?
The datasheet for CV1135 does not mention the Rp but I think its safe to assume it is the neighborhood of 7k, yes.
The LL1660 datasheet on the other hand states 130H Lprim for the 10mA version, and 25Hz-40k with a 14k source impedance.
This is when wired as 4:4.5, I'm using the 2.25 windings in series as primaries in my 6S4S PP amp which should result in 150H or so at 9mA, give or take.
Hi, you mentioned the "S" mention, so I used Alt B col.
from the datasheet. Where did I get it wrong ?
from the datasheet. Where did I get it wrong ?
