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Old 11th October 2012, 09:49 AM   #561
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Have you considered 6B4G or 6C4C if you want to use DC filaments on a 2a3 type tube? I personally like the sound of the biplates.
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Old 11th October 2012, 09:55 AM   #562
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Originally Posted by andyjevans View Post
Have you considered 6B4G or 6C4C if you want to use DC filaments on a 2a3 type tube? I personally like the sound of the biplates.
I'd not considered them and will, thanks. Currently I have some Cunningham 2A3 in there, they seem very good though I once I've improved the heater supply I should be in a position judge them more critically.
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Old 11th October 2012, 10:05 AM   #563
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Hi Clive,

The TRAM circuit shows that the filament winding has a 7,5V tap as well as the one used for the standard regulator. 7,5Vrms is ideal for the normal CRC filter used to provide raw dc to the Coleman Regulators.

I believe this is the arrangement that Morten and Bjarne used to excellent effect on their TRAMs (I hope one can confirm!)

It is possible that a choke input filter & discrete trafo will provide a solution with even lower noise, and lower overall heat in the box - I will cook up a solution for that, if you prefer it.
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Old 11th October 2012, 10:13 AM   #564
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Originally Posted by Rod Coleman View Post
Hi Clive,

The TRAM circuit shows that the filament winding has a 7,5V tap as well as the one used for the standard regulator. 7,5Vrms is ideal for the normal CRC filter used to provide raw dc to the Coleman Regulators.

I believe this is the arrangement that Morten and Bjarne used to excellent effect on their TRAMs (I hope one can confirm!)

It is possible that a choke input filter & discrete trafo will provide a solution with even lower noise, and lower overall heat in the box - I will cook up a solution for that, if you prefer it.
Hi Rod, I've been conversing with Morten and yes he used the 7.5V winding on the TRAM TX. As space is tight in the chassis using a 2nd chassis could make heatsinking in particular much easier to cope with. A 2nd chassis could mean I'd need a 7.5V TX as the wiring from the original TX should I believe be as short as possible. This approach is more expensive but if the preamp would run cooler and sound even better then I'm interested! BTW, I'm guilty of writing the original review TRAM on enjoythemusic.

I'd very much appreciate your guidance. Thanks.
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Old 11th October 2012, 12:04 PM   #565
Desmo is offline Desmo  Denmark
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Originally Posted by clivetjm View Post
Hi Rod, I've been conversing with Morten and yes he used the 7.5V winding on the TRAM TX. As space is tight in the chassis using a 2nd chassis could make heatsinking in particular much easier to cope with. A 2nd chassis could mean I'd need a 7.5V TX as the wiring from the original TX should I believe be as short as possible. This approach is more expensive but if the preamp would run cooler and sound even better then I'm interested! BTW, I'm guilty of writing the original review TRAM on enjoythemusic.

I'd very much appreciate your guidance. Thanks.
Hi Rod and Clive,

@Clive: You could make a chassis under the Tram. For instance you could make a chassis that's smaller in footprint than the Tram II and then install quite tall feet / decouplers from the underside of the Tram, long enough to ''lift'' the sub-chassis of the equipment shelve.

@Rod: Yes, both Bjarne and I use the C-R-C raw supply and we use 0.15 ohm resistors.

Funny that you have introduced the idea of using coils instead of resistors. I'm now using the Full Music 2A3/SE and they use quite a lot of heater current (more than 2,5A). With other 2A3 tubes I could get 2,5V DC on the heaters with 0.22 ohm resistors in the C-R-C, but with the Full Music I can hardly get 2,5V even if I reduce the resistors to 0.15 ohm. I have 2.46V DC heater supply when turning the voltage all the way up on the trimmer, and using 0.15 ohm in the C-R-C.

Since we (Tram owners) are limited by using the 7.5V heater supply winding on the transformer I can't go higher on the AC side. And I would rather not go down to 0.10 ohm resistors in the C-R-C since this gives higher ripple current on the second set of caps. So I was actually considering to make my own small coils wound on copper wire from cross over coils. These can be small enough to fit inside the chassis, have an RDC around 0.10 - 0.15 ohm and sufficielt induction to ''protect'' the second set of caps enough from high ripple currents...

I will try to make some small coils to experiment with.
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Old 11th October 2012, 01:39 PM   #566
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Quote:
Originally Posted by clivetjm View Post
Hi Rod, I've been conversing with Morten and yes he used the 7.5V winding on the TRAM TX. As space is tight in the chassis using a 2nd chassis could make heatsinking in particular much easier to cope with. A 2nd chassis could mean I'd need a 7.5V TX as the wiring from the original TX should I believe be as short as possible. This approach is more expensive but if the preamp would run cooler and sound even better then I'm interested! BTW, I'm guilty of writing the original review TRAM on enjoythemusic.

I'd very much appreciate your guidance. Thanks.
Hi Clive,

I recommend short wiring from trafo-rectifiers-caps, because long wires (which carry the heavy pulse currents) will risk coupling to the signal wiring.

If you buy a 7,5V rms trafo, you can use the standard CRC raw dc circuit - this is given in detail in the application notes (please click my name -> send email for the PDF documents).

Alternatively, a choke input circuit can be formed from a 10V 4A trafo (from JMS, or a Hammond 266L20 -- and a Hammond 159ZJ choke. this would be a kind of Rolls-Royce of a raw dc supply.
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Old 11th October 2012, 01:54 PM   #567
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Originally Posted by Rod Coleman View Post
Hi Clive,

I recommend short wiring from trafo-rectifiers-caps, because long wires (which carry the heavy pulse currents) will risk coupling to the signal wiring.

If you buy a 7,5V rms trafo, you can use the standard CRC raw dc circuit - this is given in detail in the application notes (please click my name -> send email for the PDF documents).

Alternatively, a choke input circuit can be formed from a 10V 4A trafo (from JMS, or a Hammond 266L20 -- and a Hammond 159ZJ choke. this would be a kind of Rolls-Royce of a raw dc supply.
Thanks a ton Rod,

I'll measure up how close I can get the heater outputs to the 2A3 sockets, my initial estimate is that I should be able to arrange things so the heater wire to the valves are about 50mm to 60mm, it this ok? The trafo wiring can be really short, far shorter than it would be if it used the TRAM trafo.

If I go this more expensive route, I would I hope derive benefits from the choke input PS and by reducing the load on the TRAM trafo. Given that a DHT preamp is a very sensitive beast I'd hope any improvement would be audible. Of course I'd never know as I can't do a side-by-side comparison. If the improvement is likely to be debatable then I might as well stick with a single chassis which would look more attractive - overall sound quality is the more important factor.
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Old 11th October 2012, 02:11 PM   #568
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Hi Clive, provided the filament wiring avoids running too close to power transformers, the length can be 200mm. The only risk is picking up common-mode EM radiation, so in quiet locations, it can be longer still. The Regulator stability (with the rev 4 design) is completely independent of cable length.

Use fairly heavy cable, and twist lightly.

It's hard to predict whether the choke input will improve the preamp - the difference will not be on account of the filament supply, so much as the improvement in the B+ supply, where the filament rectifier recharge pulses will no longer be coupled into the trafo, and indirectly into the signal wiring. And you are right, a DHT amplifier in a high resolution system can highlight almost any improvement!
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Old 11th October 2012, 02:21 PM   #569
Desmo is offline Desmo  Denmark
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Originally Posted by clivetjm View Post
Thanks a ton Rod,

I'll measure up how close I can get the heater outputs to the 2A3 sockets, my initial estimate is that I should be able to arrange things so the heater wire to the valves are about 50mm to 60mm, it this ok? The trafo wiring can be really short, far shorter than it would be if it used the TRAM trafo.

If I go this more expensive route, I would I hope derive benefits from the choke input PS and by reducing the load on the TRAM trafo. Given that a DHT preamp is a very sensitive beast I'd hope any improvement would be audible. Of course I'd never know as I can't do a side-by-side comparison. If the improvement is likely to be debatable then I might as well stick with a single chassis which would look more attractive - overall sound quality is the more important factor.
Hi Clive,

The wires from the DHT regulators to the tube sockets must be short because they are -basically- in the signal path. But it's the wires from the transformers, to the rectifiers and from the rectifiers to the first set of cap's that are really critical regarding ripple currents = noise... ((EDIT... Ups, just noticed that Rod has also replied on this))

The load on the Tram II trafo is no problem. I think that the transformer cover is (something like) 40 degress or so after several hours of operation with 2A3 tubes.
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Old 11th October 2012, 02:28 PM   #570
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I'll stack the Tram on top of the identical phono chassis to judge the looks. This will probably be the deciding factor as to which route I go. The separate trafo seems like it ought to be a benefit but whether it's worth the effort and expense is hard to assess.
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