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Old 26th April 2013, 02:39 PM   #691
Desmo is offline Desmo  Denmark
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Quote:
Originally Posted by sjs View Post
You only need 12uF if you retain the voltage divider network on the output, and have a SS power amp with low input impedance, as it appears to have been designed to work with loads down to about 10K.

Personally I have reasonably low sensitivity valve power amps with 100K input impedance, so I have replaced the voltage divider with a more regular 1M0 resistor, and the 12uF Obliggattos with 1uF Hovland output caps.
Yes, I know, this is obvious... If the cap just see a 100K load or equal then around 470nF is enough.

But how do you then load your triode after taking out the resistor network? They are designed to ''see'' the impedance around what the resistor network is, or what an output transformer is on the input - if one was used... Again, I think I better not try to get myself to far into the technical, I manage to get myself misunderstood every time I try ;-) But I think you know what I mean... My English is not good enough for that... Technical stuff and humor/irony is extremely difficult in a foreign language, even with 30 years of experience with electronics, and 45 years of experience with humor...
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Last edited by Desmo; 26th April 2013 at 02:42 PM.
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Old 26th April 2013, 03:10 PM   #692
sjs is offline sjs  United Kingdom
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Quote:
Originally Posted by Desmo View Post
Yes, I know, this is obvious... If the cap just see a 100K load or equal then less than 1 uf is more than enough.

But how do you then load your triode after taking out the resistor network? They are designed to ''see'' the impedance around what the resistor network is, or what an output transformer is on the input - if one was used... Again, I think I better not try to get myself to far into the technical, but I think you know what I mean... My English is not good enough for that... Technical stuff and humor/irony is extremely difficult in a foreign language...
In the Tram2 the 3 resistors which form a voltage divider network between the +6dB output and ground (2K4, 820R and 560R) and the 470R series resistor on the output were removed entirely.

The new output cap then goes from the MU out on the CCS pcb to the +6dB output phono centre pin. The 1M0 output resistor then goes from the +6dB output phono centre pin to ground. I only have a single pair of output sockets wired up, the other two pairs are not used. It is at +6dB full output, which is fine in my system.

The 1M0 resistor across the output is primarily there for safety, when the preamp is not connected to the load presented by a power amp it avoids the output of the cap rising in voltage due to electrostatic charge. In normal operation the active stage now sees the 1M0 on the output in parallel with the 100K input on the power amp, approx 91K, so a 1uF cap is more than enough.

This works ok for my application in my system, my power amps are not very sensitive about 1V to 15W full output, and my speakers are only 91dB/W. I currently have the Mk3 DIY filament boards and setup nicely I do not have a problem hum or hiss from the Tram2 compared to my other valve preamplifiers. The Rob Coleman boards will be installed in the next few weeks, followed by a change in resistors.

Last edited by sjs; 26th April 2013 at 03:12 PM.
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Old 26th April 2013, 03:23 PM   #693
Desmo is offline Desmo  Denmark
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Quote:
Originally Posted by sjs View Post
In the Tram2 the 3 resistors which form a voltage divider network between the +6dB output and ground (2K4, 820R and 560R) and the 470R series resistor on the output were removed entirely.

The new output cap then goes from the MU out on the CCS pcb to the +6dB output phono centre pin. The 1M0 output resistor then goes from the +6dB output phono centre pin to ground. I only have a single pair of output sockets wired up, the other two pairs are not used. It is at +6dB full output, which is fine in my system.

The 1M0 resistor across the output is primarily there for safety, when the preamp is not connected to the load presented by a power amp it avoids the output of the cap rising in voltage due to electrostatic charge. In normal operation the active stage now sees the 1M0 on the output in parallel with the 100K input on the power amp, approx 91K, so a 1uF cap is more than enough.

This works ok for my application in my system, my power amps are not very sensitive about 1V to 15W full output, and my speakers are only 91dB/W. I currently have the Mk3 DIY filament boards and setup nicely I do not have a problem hum or hiss from the Tram2 compared to my other valve preamplifiers.
Yes, I understand that is how you have wired it up after taking out the resistor network. And I know that 100K // 1M = 91K... And I also know that with 1uF you have a nice low corner frequency when the caps just ''see'' 91K instead of the resistor network.This is not what I'm talking about.

I'm talking about loading your triode on the output. Sorry again, I don't have the words to explain in English, but try to look up a data-sheet on a 2A3. Depending on the voltages on the tube the load on the output should be in around 3 - 6kohm or so (if memory serves me correct, I have not seen one of these data sheets for many years). If you don't load the triodes correctly you will not be in the tubes linear operating range and distortion will be high... I'm quite sure that the designers have choosen the values of the resistors in the network on the output to get the triodes in their most linear range with the voltages for the tubes in the Tram II. Now you load the triodes with 91K ohm, and I don't think this is that good...
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Last edited by Desmo; 26th April 2013 at 03:29 PM.
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Old 26th April 2013, 03:51 PM   #694
sjs is offline sjs  United Kingdom
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Quote:
Originally Posted by Desmo View Post
Yes, I understand that is how you have wired it up after taking out the resistor network. And I know that 100K // 1M = 91K... And I also know that with 1uF you have a nice low corner frequency when the caps just ''see'' 91K instead of the resistor network.This is not what I'm talking about.

I'm talking about loading your triode on the output. Sorry again, I don't have the words to explain in English, but try to look up a data-sheet on a 2A3. Depending on the voltages on the tube the load on the output should be in around 3 - 6kohm or so (if memory serves me correct, I have not seen one of these data sheets for many years). If you don't load the triodes correctly you will not be in the tubes linear operating range and distortion will be high... I'm quite sure that the designers have choosen the values of the resistors in the network on the output to get the triodes in their most linear range with the voltages for the tubes in the Tram II. Now you load the triodes with 91K ohm, and I don't think this is that good...
Morten, you are referring to the anode load for the 2A3. The 2A3 has a plate impedance of around 800 ohms, therefore would like to see a load of more than 2.5 times or 3 times this as a minimum, hence the classic SE OTX of 2K5 for the 2A3. This requirement is to avoid excessive distortion, and anything above this tends to lower distortion as the load line rotates and becomes flatter on the curves. It is suggested by many that the ideal is an infinitely high load, such that the load line becomes horizontal, which is what a choke load, CCS etc all try to replicate.

In the Tram2 the 2A3 anode load is the 6AS7 and CCS "Super mu" stage producing a v v high impedance load, instead of a 2K5 resistor (for example), the dynamic load seen by the 2A3 is then this anode load impedance (v v high) in parallel with the load applied across the output, i.e. the output resistors and the input of the power amp.

The basic design is setup such that the "worst case scenario", as listed in the specs, is a power amp with 5K0 input impedance, when we will have the 2A3 presented with 3K780 in parallel with 5K470 which is about 2K235, about as low as you want a 2A3 to go, and hence the 12u2 output capacitors for a 5.8Hz corner frequency, and overall a nicely setup situation to meet the needs of driving a 5K0 input power amplifier

In my view the risk is to go lower than this, not to go higher.

With the 1M0 output resistor the 2A3 will just be seeing a higher impedance load, so the dynamic load line will be closer to horizontal, I am happy with this.
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Old 26th April 2013, 06:43 PM   #695
Desmo is offline Desmo  Denmark
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It's fine sjs, if you're happy with it all is good I know what you're saying, and I understand all you say (I have worked with electronics for 30 years), I just don't have the words in English to discuss this... So I rest my case on the technical stuff...

Our experience here is just, that the load these type of triodes ''see'' on their output has big impact on the sound and that they need this load on a few K ohm to sound their best. Your experience is different, and that's fine, and with you approach the output caps are cheaper
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Last edited by Desmo; 26th April 2013 at 06:50 PM.
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Old 26th April 2013, 06:58 PM   #696
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Im glad to see the Tram is working for most of you - mine blew up after 6 hours of use (but it was 6 hours of great sound..

Anyway - it`s been a long journey in locating the fault, and I think I (with the help of an technician) that the Universal all Film Capacitor Power Supply board (FCPSU) is defect.

It is however impossible to get a new FCPSU-board from DIYhifisupply. My order from November 2012 was finally cancelled 3 months ago due to inability to supply new FCPSU-boards.

In the meantime my technician seems to have located the fault on the FCPSU-board. This is what he wrote to diyhifisupply:

"The problem is that the +B voltage from the FCPSU is missing - Thinking that Q3 is defective - this is a version 2 board. - With C1 and C2 still in place and R4 is 47 Kohm. D9 is also mounted. Could you inform me the type no. of Q3 - thanks!

There has been no answer, so i am turning to you in hope that one of you has the type number of Q3 on the FCPSU version 2...

Anyone ?

Best Wishes from Denmark

Carsten

Last edited by Nonnegativ; 26th April 2013 at 07:03 PM.
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Old 26th April 2013, 09:07 PM   #697
Desmo is offline Desmo  Denmark
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Hej Carsten, Næste gang jeg har min forforstærker åben skal jeg gerne kigge efter, hvad Q3 er for en sag..!
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Old 26th April 2013, 10:57 PM   #698
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What is wrong with Diyhifisupply..are they out of business ?
a bit sad if we not can get spare parts to the Tram...
mvh Bjarne
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Old 27th April 2013, 04:54 AM   #699
rab28 is offline rab28  Australia
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Originally Posted by beardman View Post
What is wrong with Diyhifisupply..are they out of business ?
No, no: it's just that they cannot source any more of the FCPSU boards: that's why there are no more Tram2s available at present. I am told that it can be substituted, but not by a board that fits within the Tram2 chassis!

- richard.
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Old 27th April 2013, 05:03 AM   #700
rab28 is offline rab28  Australia
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Originally Posted by Desmo View Post
Please remember to upgrade the cap + resistor on the Rod Coleman modules, it gives a nice improvement for little money and is so much easier to do before the modules are installed, than after...
Oh... it's a little late for that, the regs are already installed... What parts did you use?


Quote:
Originally Posted by Desmo View Post
The Duelund Alexander's has a slightly warmer and slightly less transparent sound compared to the more expensive models
Even with the partsconnection sale on CASTs, they are still out of my price range. Maybe in future, i will start saving my pennies...

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Originally Posted by Desmo View Post
Personally I prefer the Shinkoh tantalium resistors over the AN, but all tantaliums are really nice.
So i am told, but unfortunately i could not source Shinkos with the correct values - at least not from partsconnection! (I am doing this in a bit of a rush while my Tram2 is still with a competent technician: my soldering skills are woeful, particularly as i often have shaky hands. Not good at all for soldering small components!)

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.. Also IMHO the Obbligato tinfoils are better than the Mundorf supremes for output caps
Thanks for that Morten: I did some research and have now cancelled the Mundorfs from my order!

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Originally Posted by Desmo View Post
Besides the Philips rectifier, what kind of tubes will you be using..!?
I have EML 45s, but as yet only the cheap Chinese 6AS7... still not sure what to replace that with... are those Cunninghams available?

- richard
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