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Old 12th September 2011, 08:05 PM   #1201
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....O, and with Filament Bias, the negative Filament terminal must always be used. Using the positive side will mean the bias is reversed, and anode current too high!
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Old 13th September 2011, 06:57 PM   #1202
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OK, thanks Rod.

B r u n o
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Old 18th September 2011, 05:13 AM   #1203
iko is offline iko  Canada
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Finished a prototype 26 preamp and have a strange problem with hum, which I solved, but can't explain. The setup is as follows:
HT - 220VAC transformer, solid state bridge, CLC filter (470u->6H->470uF), solid state regulator (my own design).
Filament supply - 30VAC, solid state bridge, RCL filter (400R->100uH 2A->33mF), solid state double gyrator (my own design).

At the other side of the gyrator I use a 50W 10R resistor, for biasing.

Star ground for all power stuff, and another star ground for the signal, then both joined to the case.

The 26 tubes are each loaded by a Hammond 156C choke.

At idle, the scope showed a wiggly trace of about 8mV p-p, between choke and the plate, which ends up as audible hum.

Here's the part that puzzles me. If I replace the chokes with 4K resistors, there is no hum. The plate chokes were mounted at the very front of the amp, about 30cm away from the nearest transformer. The transformers and chokes are all in the same case. Any ideas?
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Old 18th September 2011, 05:24 AM   #1204
regal is offline regal  United States
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Quote:
Originally Posted by ikoflexer View Post
Finished a prototype 26 preamp and have a strange problem with hum, which I solved, but can't explain. The setup is as follows:
HT - 220VAC transformer, solid state bridge, CLC filter (470u->6H->470uF), solid state regulator (my own design).
Filament supply - 30VAC, solid state bridge, RCL filter (400R->100uH 2A->33mF), solid state double gyrator (my own design).

At the other side of the gyrator I use a 50W 10R resistor, for biasing.

Star ground for all power stuff, and another star ground for the signal, then both joined to the case.

The 26 tubes are each loaded by a Hammond 156C choke.

At idle, the scope showed a wiggly trace of about 8mV p-p, between choke and the plate, which ends up as audible hum.

Here's the part that puzzles me. If I replace the chokes with 4K resistors, there is no hum. The plate chokes were mounted at the very front of the amp, about 30cm away from the nearest transformer. The transformers and chokes are all in the same case. Any ideas?
For headphone amps and preamps with output transformers, a separate chassis for the power transformers is a requirement. Your chokes are no different. You can try turning your chokes 90 degrees, but a separate chassis is best.

Some may get lucky and not hear the hum with thier system, but I can guarantee that a decent soundcard will always show hum pickup in an opt or choke from the power transformers, even when they are separated 12" or more. When dealing with a preamp you are on the level of a 24bit soundcard's noisefloor because of the post amplification, especially if you have sensitive speakers.
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Old 18th September 2011, 10:37 AM   #1205
mogliaa is offline mogliaa  United Kingdom
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Hi all,
Just another one who fall into this great preamp. Having had the opportunity to listen to Andy Evans' 26 preamp and finished my 45SET, is now time to embark myself on the DHT preamp quest.
After my conversation with Andy yesterday, I've made the decision to go for filament bias. Also have some DHTs at hand that I want to try: CX112a, 01a, 26, 30, 30sp, 10Y and 4P1L. I know that people may suggest to keep with the 26, but I want this to be a learning experience for me.

Hopefully tomorrow will arrive a pair of Lundahl 1660 I bought from Thomas. So will be breadboarding the preamp soon.

I did a quick simulation in Ltspice. See attached. I'm not entirely sure if I modeled the Lundalhn in 4.5:1 connection in the right way. I biased the 26 at 163V and 5mA with a filament bias voltage of -10.5V. The stage gain is around 1.7 and the frequency response is 15Hz-15kHz. Is the anode inductance responsible for this response? Any way of improving this a bit?

Thanks
Ale
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File Type: png 26 Preamp LL1660.png (32.0 KB, 605 views)
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Old 18th September 2011, 11:50 AM   #1206
regal is offline regal  United States
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Quote:
Originally Posted by mogliaa View Post
Hi all,
Just another one who fall into this great preamp. Having had the opportunity to listen to Andy Evans' 26 preamp and finished my 45SET, is now time to embark myself on the DHT preamp quest.
After my conversation with Andy yesterday, I've made the decision to go for filament bias. Also have some DHTs at hand that I want to try: CX112a, 01a, 26, 30, 30sp, 10Y and 4P1L. I know that people may suggest to keep with the 26, but I want this to be a learning experience for me.

Hopefully tomorrow will arrive a pair of Lundahl 1660 I bought from Thomas. So will be breadboarding the preamp soon.

I did a quick simulation in Ltspice. See attached. I'm not entirely sure if I modeled the Lundalhn in 4.5:1 connection in the right way. I biased the 26 at 163V and 5mA with a filament bias voltage of -10.5V. The stage gain is around 1.7 and the frequency response is 15Hz-15kHz. Is the anode inductance responsible for this response? Any way of improving this a bit?

Thanks
Ale
It would either be interlayer capacitance or leakage induction, can't imagine spice would be able to predict HF reponse out of an amorphous transformer.

So you are floating the Colemane regulator at -10.5V, using a battery?.
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Old 18th September 2011, 01:44 PM   #1207
mogliaa is offline mogliaa  United Kingdom
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No floating, will be the regulator running from a stabilised 18 Vdc source so I can have sufficient voltage to drop in the 10 ohm filament resistor.
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Old 19th September 2011, 06:26 AM   #1208
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Question: Are there any interesting Russian DHT’s for preamp use? I only see DHT pre’s with the well known 01A, 26, 10Y ect., but never Russian types.
Could it be that during the early ages of tube technology the quality of Russian tubes was (far) behind western tubes? Or is the reason that, because almost all of the Russian stuff is from military surplus, these stocks are from later years when IDHT were already common?

Peter
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Old 19th September 2011, 06:45 AM   #1209
mogliaa is offline mogliaa  United Kingdom
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Hi Peter,
I'm not the right person to answer that specific question, perhaps Anatoly?
I can say that I will try the triode strapped 4P1L and 6P21... There are various opinions around these as are very linear albeit highly microphonics for others....
Will try to test myself and post results, but firstly I will build my 26 preamp.
Cheers
Ale
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Old 19th September 2011, 10:04 AM   #1210
hihopes is offline hihopes  South Africa
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Has anyone compared a 4p1l preamp to a 26 preamp yet? I am waiting for some 26 and listening to 4p1l in the meantime. Their clarity is astounding.

Last edited by hihopes; 19th September 2011 at 10:33 AM.
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