My 12B4A CF amp continues to impress me, so I´ve decided to build a bigger SE version before I start with the PL519 PP project.
Setup will be like this:
PL36 (25E5) triode wired output tube.
(http://www.tubes.mynetcologne.de/roehren/daten/el36pentode_as_triode_v2.pdf)
James 6112HS OPTs (http://euphoniaaudio.netfirms.com/ea/nfoscomm/catalog/pdf/6112HS.pdf)
I bought a pair a while ago to build a 6S45PE spud but I changed my mind as I don´t need another 1W amp.
A pair of choke (40H 50mA) loaded 12B4A will take care of the driver stage duties, with >300V on the plates and fixed bias these tubes can deliver enormous voltage swings.
Haven´t decided what input tubes to use yet, but anything that can swing +-50Vp will do.
A Hammond 370HX power transformer together with a 5AR4 rectifier and a 10H 200mA choke will deliver B+ in the range of 320-350V.
As I´ve mentioned before the output tubes will operate with cathode bias (to drop some voltage) while the driver stage will have fixed bias to allow maximum output swing.
Setup will be like this:
PL36 (25E5) triode wired output tube.
(http://www.tubes.mynetcologne.de/roehren/daten/el36pentode_as_triode_v2.pdf)
James 6112HS OPTs (http://euphoniaaudio.netfirms.com/ea/nfoscomm/catalog/pdf/6112HS.pdf)
I bought a pair a while ago to build a 6S45PE spud but I changed my mind as I don´t need another 1W amp.
A pair of choke (40H 50mA) loaded 12B4A will take care of the driver stage duties, with >300V on the plates and fixed bias these tubes can deliver enormous voltage swings.
Haven´t decided what input tubes to use yet, but anything that can swing +-50Vp will do.
A Hammond 370HX power transformer together with a 5AR4 rectifier and a 10H 200mA choke will deliver B+ in the range of 320-350V.
As I´ve mentioned before the output tubes will operate with cathode bias (to drop some voltage) while the driver stage will have fixed bias to allow maximum output swing.
Addendum:
As I´ve mentioned before I have already run some bench tests on the PL36-12B4A combo. That time I used a 125DSE OPT wired to 2,5k/8 and got 7,5W out with 260V 60mA across the output tube.
This time I´m going to use a 3,5k OPT and run the tubes at a slightly higher voltage and lower current, the calculated output power is 6,9W and the Zout and distortion should be slightly lower.
The James transformers are definitely higher quality unit that the cheap Hammonds which should be reflected in the overall sound quality.
There are two things "wrong" with the PL36 tube in this application though: With four cats in the house I would prefer a tube without top cap, and a little higher gm wouldn´t hurt either.
As I´ve mentioned before I have already run some bench tests on the PL36-12B4A combo. That time I used a 125DSE OPT wired to 2,5k/8 and got 7,5W out with 260V 60mA across the output tube.
This time I´m going to use a 3,5k OPT and run the tubes at a slightly higher voltage and lower current, the calculated output power is 6,9W and the Zout and distortion should be slightly lower.
The James transformers are definitely higher quality unit that the cheap Hammonds which should be reflected in the overall sound quality.
There are two things "wrong" with the PL36 tube in this application though: With four cats in the house I would prefer a tube without top cap, and a little higher gm wouldn´t hurt either.
My latest contribution to the world of CF power amps:
Even though I got my PL36 amp up and running I never liked the sound of it, the 12B4A amp sounded much better even though it has much cheaper iron. What to do? Well, I obviously tore the thing apart (shame on a fairly nice chassi) and rebuilt the thing with a whole new tube lineup.
Instead of 1/2 6SN7 - 12B4A - PL36 per channel I went for only the 6SN7 and the 12B4´s, this time each channel employs both sections of a 6SN7 in a cascade to drive two paralell 12B4A´s.
The first 6SN7 section works as a resistor loaded gain stage with a non-decoupled cathode resistor (to decrease the overall gain) and the second acts as a driver stage with Hammond 156C (150H 8mA) chokes as plate loads (two in series).
The output stage consists of two 12B4A+s in //, each with a 1k2/100uF combo on the cathode for self bias. This stage runs on a 280V rail delivered by a 5AR4 rectifier and a CLC filter while the input and driver section operates at 360Vdc to get maximum swing and linearity. Both voltage rails are taken from the same PT winding but the higher voltage is rectified separately with silicon diodes and filtered by an IRF 840-based capacitance multiplier.
Due to the high gain in the input-driver sections I had some problems with oscillations and noise pickup that took me several days to fix. PSU decoupling, copper screens between the 6SN7 sections and rearranged ground buses took care of most of the problems but the amp is still a bit touchy when it comes to picking up noise.
Even though I got my PL36 amp up and running I never liked the sound of it, the 12B4A amp sounded much better even though it has much cheaper iron. What to do? Well, I obviously tore the thing apart (shame on a fairly nice chassi) and rebuilt the thing with a whole new tube lineup.
Instead of 1/2 6SN7 - 12B4A - PL36 per channel I went for only the 6SN7 and the 12B4´s, this time each channel employs both sections of a 6SN7 in a cascade to drive two paralell 12B4A´s.
The first 6SN7 section works as a resistor loaded gain stage with a non-decoupled cathode resistor (to decrease the overall gain) and the second acts as a driver stage with Hammond 156C (150H 8mA) chokes as plate loads (two in series).
The output stage consists of two 12B4A+s in //, each with a 1k2/100uF combo on the cathode for self bias. This stage runs on a 280V rail delivered by a 5AR4 rectifier and a CLC filter while the input and driver section operates at 360Vdc to get maximum swing and linearity. Both voltage rails are taken from the same PT winding but the higher voltage is rectified separately with silicon diodes and filtered by an IRF 840-based capacitance multiplier.
Due to the high gain in the input-driver sections I had some problems with oscillations and noise pickup that took me several days to fix. PSU decoupling, copper screens between the 6SN7 sections and rearranged ground buses took care of most of the problems but the amp is still a bit touchy when it comes to picking up noise.
Attachments
It looks cool!
I have done some experimenting with 6S4's and 12B4's. I went looking for a 9 pin miniature tube that had a little more grunt. The 7233 is easily equal to two 12B4's but they are not too easy to find. I have done some experiments, and the results are promising, but I only had one tube. I got 10 more of them at a hamfest yesterday, so there will be an amp built in the next few months. The Bendix 6877 looks like a contender too, but they are even harder to find and not cheap. I got some at the hamfest. I also found one Bendix 6080 with graphite plates for $4. It is lonely and needs a friend!
I have done some experimenting with 6S4's and 12B4's. I went looking for a 9 pin miniature tube that had a little more grunt. The 7233 is easily equal to two 12B4's but they are not too easy to find. I have done some experiments, and the results are promising, but I only had one tube. I got 10 more of them at a hamfest yesterday, so there will be an amp built in the next few months. The Bendix 6877 looks like a contender too, but they are even harder to find and not cheap. I got some at the hamfest. I also found one Bendix 6080 with graphite plates for $4. It is lonely and needs a friend!
Thank you!
I sort of fell in love with the sweet little 12B4A years ago and I´ve used them in a bunch of different applications ever since and they´ve never let me down.
Aside from chasing RCA 808´s for my future big A2 SET project I´m trying to stay away from expensive and/or hard-to-get tubes.
What I´m looking fo next is a cheap and easily available lowRp, high Gm tube capable fo delivering >8W into a 750R load at <120mA plate current.
The thing is that I have a pair of Lundahl LL1664 OPT´s that I´m planning to use in a quite unorthodox way. These are 3k/8R transformers with two identical primary sections that are supposed to be wired in series, my plan is to wire them in // to get a 750R/8R impedance ratio with twice the current handling capability.
Not sure it will work as intended, but the datasheets for some of Lunahl´s interstage transformers shows application examples where the windings can be wired in series or in parallel to achieve different winding ratios.
E130L would probably be the perfect tube for this application, but I´ll try to find something less rare and expensive.
I sort of fell in love with the sweet little 12B4A years ago and I´ve used them in a bunch of different applications ever since and they´ve never let me down.
Aside from chasing RCA 808´s for my future big A2 SET project I´m trying to stay away from expensive and/or hard-to-get tubes.
What I´m looking fo next is a cheap and easily available lowRp, high Gm tube capable fo delivering >8W into a 750R load at <120mA plate current.
The thing is that I have a pair of Lundahl LL1664 OPT´s that I´m planning to use in a quite unorthodox way. These are 3k/8R transformers with two identical primary sections that are supposed to be wired in series, my plan is to wire them in // to get a 750R/8R impedance ratio with twice the current handling capability.
Not sure it will work as intended, but the datasheets for some of Lunahl´s interstage transformers shows application examples where the windings can be wired in series or in parallel to achieve different winding ratios.
E130L would probably be the perfect tube for this application, but I´ll try to find something less rare and expensive.
I recently acquired a few of these but haven't experimented with them yet. I am running a 600 ohm load in my augmented CF board. I have tried all of the usual dual regulator triodes. The 6336A of course works the best making about 10 watts, but these fall into the expensive category. I also like the 5998A or the 7236, but they fall into the rare category. The common and cheap 6AS7 (or 6080) works well at the 5 watt level, but you could parallel both halves (seperate cathode resistors) to double the Gm and 8 watts would be easy.
My simulator says that you will need to run about 150 mA to get 8 watts into a 750 ohm load. I am running 150 to 250 mA depending on the tube in my augmented cathode follower. I can get about 6 watts out of a 6AS7 (the two tube sections are in series) into a 600 ohm load at 150 mA (over the limit, I know). This is exactly what the simulator shows.
There are a few on Ebay now and then but most of the time the prices are silly.
PL504 would be an alternative, especially as I´m planning to use power Mosfets to keep the voltage across the output tubes at a constant 100V or so.
120mA seems to be enough for "only" 5,4Wrms. Perhaps the current rating can be pushed a little bit, but 5,4W is good enough for me anyway.
revintage: PL519 is definitely an option, but due to the current limits of the OPT´s there´s no need for such a big tube.
The constant voltage circuitry will keep the plate dissipation down so what I need is a tube with a big cathode, not big cathode AND big anode.
I´m saving those 519´s for my PP CF project.
The constant voltage circuitry will keep the plate dissipation down so what I need is a tube with a big cathode, not big cathode AND big anode.
I´m saving those 519´s for my PP CF project.
I am running constant voltage circuitry in my augmented CF amp. I run 100 to 125 volts across the CF. I had 6AS7's in it for several days. They sounded very good and had distortion figures in the 0.5% range at 5 watts. These should be cheap worldwide. I received some suggestions via email that claim the Russian 6AS7's could easilly out do the American versions in OTL amps, so they may be worthy of a look. I have a lifetime supply of crusty old 6AS7G's, 6AS7GA's and 6080's, so I experiment with them.
The same email suggested Russian 6S19P instead of the 7233. This looks promising and they are cheap on Ebay, so I may try some of them in a hybrid augmented CF with mosfets for the constant voltage circuit.
The same email suggested Russian 6S19P instead of the 7233. This looks promising and they are cheap on Ebay, so I may try some of them in a hybrid augmented CF with mosfets for the constant voltage circuit.
It´s about time to start building my PL519 PP monster but I still haven´t found a suitable power transformer, so the question is:
Is it bad engineering practice to stack two or more power supplies to achieve the right voltages?
A shallow dive in the junkboxes revealed the following items that might be useful:
1x 2x15V 300VA toroid
2x 2x36V 200VA toroids
50x 1000uF 63V caps
14x 680uF 160V caps
Various chokes, rated up to 3ADC
My idea is to rectify the secondary of each transformer separately giving 40Vdc and 2 x 100Vdc then stack them to get +40V for the heaters and +240V for the output tubes.
Using 12B4A and PC86 for driver and input stages I can wire the heaters in series and use the same 40V supply along with a resistor.
A small(ish) 230:230V toroid rectified to 320V should be enough for the input and driver stages B+.
Good or bad idea?
Is it bad engineering practice to stack two or more power supplies to achieve the right voltages?
A shallow dive in the junkboxes revealed the following items that might be useful:
1x 2x15V 300VA toroid
2x 2x36V 200VA toroids
50x 1000uF 63V caps
14x 680uF 160V caps
Various chokes, rated up to 3ADC
My idea is to rectify the secondary of each transformer separately giving 40Vdc and 2 x 100Vdc then stack them to get +40V for the heaters and +240V for the output tubes.
Using 12B4A and PC86 for driver and input stages I can wire the heaters in series and use the same 40V supply along with a resistor.
A small(ish) 230:230V toroid rectified to 320V should be enough for the input and driver stages B+.
Good or bad idea?
I guess the results should be similar, although it would be easier to use my 160V caps if I rectify each transformer separately.
Two per channel, not carved in stone yet though.
Not sure if miswiring the Hammonds to use the 16R taps into 8 ohms works well.
It would be if I had lower impedance transformers.
With 1,9k p-p a little more voltage gives a significant increase in output power.
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