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Old 2nd June 2011, 08:52 AM   #141
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Hi,

Quote:
Originally Posted by jacklee View Post
Initially I used TJ Globe 45s and WE 274B rectifier tubes as I happen to have them on hand. This setup works fine except for a low-level hum that I cannot get rid of. After some trouble-shooting I concluded the hum is generated by the amp itself and not due to other components or ground loops.
It is picked up via the radiated field from the rectifier tube. Some combinations do not work too well. Especially recent TJ 45's are very prone to picking up noise, which is why we no longer recommend for the Tram II.

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Originally Posted by jacklee View Post
I also tried shielding the rectifier and the output triodes with a graphite tube shield connected to the signal input ground but no luck. Then I found a RCA 5U4 in my tube stash and tried it on replacing the WE 274B and voila the hum is gone completely.
The field is radiated and received via the top of the tubes, we tried "shielding hats" made from copper and grounded, these kill the hum completely.

Quote:
Originally Posted by jacklee View Post
So in this case the hum is either due to EM leak or different operating points resulting from the WE274B. To investigate this further I installed a UX4 test socket adapter to the TJ45 and measured:

- Filament voltage is 2.56V
- Bias current is 31mA (w/ 274B)
- Grid to filament ground is ~-8V (w/ 274B) and ~-10V (w/ 5u4)
- Plate to filament ground is ~80V (w/ 274B) and ~100V (w/ 5u4)

So it seems changing the rectifier could change the operating point of the output tubes. Is this normal?
Yes, there is no voltage regulation, different rectifiers have different voltage drops. The WE 274 IIRC is closer to a 5R4 in terms of electrical behaviour than a 5U4, so a 5U4 would have more voltage. We don't have any WE 274 at hand, so I cannot confirm if the difference is expected to be like what you have.

Otherwise the voltages and currents look okay.

Quote:
Originally Posted by jacklee View Post
The RCA 5u4 does measured much stronger than the WE274B in my tube tester. Also, are the measured voltages/currents within spec? In particular is the plate-to-ground too low (~100V vs 120V)?
The plate voltage and the voltage at the output cap are around 20V apart, so with a 5U4 things are spot on.

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Originally Posted by jacklee View Post
I also noticed that upon turn-on the plate-to-ground and grid-to-ground voltages are much higher, ~200V and ~-60V respectively, and then slowly converge to the values listed above.
Yes, this is due to operation of the Active Bias System.

At startup the current through the tube is cut off totally (hence the rise of the Anode voltage to essentially equal +B) and is then slowly increased to nominal. This makes sure the cathode is up to temperature by the time we draw full current. You may have noticed that the heater voltage is also soft-started, as is the HT... You may have noticed that the final Anode current is the same, regardless of Anode voltage, this is due to the adjustment of the ABS.

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Originally Posted by jacklee View Post
One final question: do I need to adjust anything if I replace the 45 output tubes with 2A3? The manual seems to suggest it is plug and play but I want to make sure first.
It normally is plug/play, but it will do no harm checking the heater and anode voltages and re-adjusting.

In principle the CCS is set to 31mA and the ABS has it's internal thresholds (the tolerances in both are why adjustment is needed to get things "just so"), however if your 2A3's for example are far of normal spec, adjustment may be needed.

Ciao T
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Old 2nd June 2011, 10:40 AM   #142
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Location: UK
I found the heater voltage needs resetting when switching between 45 and 2A3. The CCS thankfully didn't need adjusting.
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Old 3rd June 2011, 02:55 AM   #143
jacklee is offline jacklee  Hong Kong
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Hi Thorsten,

Thanks for your quick reply. It was a pleasure meeting you the other day I was at the workshop auditioning the Tram2.

Quote:
Originally Posted by ThorstenL View Post
Hi,
It is picked up via the radiated field from the rectifier tube. Some combinations do not work too well. Especially recent TJ 45's are very prone to picking up noise, which is why we no longer recommend for the Tram II.
My TJ45 was bought from Brian many years ago. I think the WE274B rectifier just radiated too much noise.

Quote:
The field is radiated and received via the top of the tubes, we tried "shielding hats" made from copper and grounded, these kill the hum completely.
Interesting. There's probably a market for a mesh shield with hat. I bought graphite ones made by a local vendor but would have preferred a wired-mesh one if that's available.

Quote:
Otherwise the voltages and currents look okay.

The plate voltage and the voltage at the output cap are around 20V apart, so with a 5U4 things are spot on.
Good to hear that.

Quote:
It normally is plug/play, but it will do no harm checking the heater and anode voltages and re-adjusting.
I'll try that this weekend. I have a pair of Sovtek 2A3 and a pair of NU 2A3 somewhere in my stash.

I have two minor suggestions for the Tram2:
(a) Add a mute function;
(b) Add a power-up/power-down mute circuit to prevent thumping noises. I have two active subs connected to the Tram2 and I have to manually turn them on-off instead of using the auto-on feature to prevent thumping at Tram2 power-up/down.

Thanks.
Jack
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Old 7th June 2011, 02:08 AM   #144
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Join Date: Jan 2008
I've just gotten caught up on this thread. I've had a Tram since December (I posted in the thread about how much I loved it after getting it), built by DIYHiFi.

I have had EXACTLY the same problem described by two others in the thread - the "hum pulses". I was sent parts to "fix" it but after reading of others' experiences it's pretty clear this is a design issue with the filament supplies - right? Heat sinks not big/effective enough?

I've also been running 2A3s. I was never told to use 45s instead.

Why not enlarge the heat sinks and/or provide for better cooling? Why not make it a 45-only pre if they draw less current and eliminate the problem?
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Old 7th June 2011, 03:27 AM   #145
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Hi Paul,

Quote:
Originally Posted by Paul Folbrecht View Post
I have had EXACTLY the same problem described by two others in the thread - the "hum pulses". I was sent parts to "fix" it but after reading of others' experiences it's pretty clear this is a design issue with the filament supplies - right? Heat sinks not big/effective enough?
The Heatsinking of the "Filament Supply" is sufficient 2A3 use if:

1) The correct Heatsink is fitted

and

2) For extremely high mains voltages (> 120/240V) and 2A3 use the dropper resistors are fitted.

The problems that surfaced appear mainly because some Filament supplies that made their way into the Tram (diyhfs are still looking why and why this was not noticed), that had been fitted by the supplier with heat sinks that appeared superficially identical to the ones specified but in reality could handle only around 2/3rds of the needed dissipation and hence were actually insufficient.

This affected a small number of Trams Kits (and build up kits) during Dec to early March and AFAIK all these units have been fixed. This was for example the case for your unit, hence you received new supplies manufacturerd to the correct spec., which as you note fixed the problem.

Ciao T
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Old 7th June 2011, 03:34 AM   #146
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Thanks for the reply, T. The thing is that after reading what I just did I had no way to know if the problem was really fixed for good as I haven't been using the pre much and it seems to only occur when it's left on for quite a long while.

I'll take your word for it that it's licked.
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Old 7th June 2011, 03:38 AM   #147
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Paul,

Quote:
Originally Posted by Paul Folbrecht View Post
Thanks for the reply, T. The thing is that after reading what I just did I had no way to know if the problem was really fixed for good as I haven't been using the pre much and it seems to only occur when it's left on for quite a long while.

I'll take your word for it that it's licked.
Please do test fully and revert to diyhifisupply with any issues.

Also, what is you local wall voltage?

Ciao T
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Old 7th June 2011, 04:52 AM   #148
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Quote:
Originally Posted by ThorstenL View Post
Paul,



Please do test fully and revert to diyhifisupply with any issues.

Also, what is you local wall voltage?

Ciao T
Not positive but actually I just bought a PS Audio Premier AC regenerator and will be using that once it comes in a week.
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Old 7th June 2011, 07:19 AM   #149
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Paul,

Quote:
Originally Posted by Paul Folbrecht View Post
Not positive but actually I just bought a PS Audio Premier AC regenerator and will be using that once it comes in a week.
This should do nicely, you can just check the wall voltage using a normal multimeter set to AC...

Ciao T
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Old 8th June 2011, 11:24 AM   #150
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It should be stated that although it is possible to make Tram to use 2A3 valves, there is significantly more background buzz compared to 45 valves. Also, in my opinion, large heatsinks are not optional - they have to be used regardless of the mains voltage.
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Last edited by DECKY999; 8th June 2011 at 11:28 AM.
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