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Fred Nachbaur SET amp: input capacitor
Fred Nachbaur SET amp: input capacitor
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Old 19th May 2013, 07:16 PM   #11
lexx21 is offline lexx21  United States
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Fred Nachbaur SET amp: input capacitor
Keep in mind what he said in the article about the output transformer... he took it apart and modified the secondary winding. You may want to go with a different output transformer if you are not comfortable doing this.
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Old 19th May 2013, 09:10 PM   #12
DF96 is offline DF96  England
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Originally Posted by Soonerorlater
ps I have a lot of low value (<5k) NOS carbon comp. resistors. It would be a shame to throw them away just because it is thought they are so much worse than other resistor types.
But they are so much worse than other resistor types. Carbon comp should only be used in three places:
1. when repairing an old item and wanting to maintain 'authenticity'.
2. when requiring some resistor distortion, as in some guitar amps.
3. when serious transient overload is likely (which is why they are still made) - probably never in most audio circuits.
There is no need to use carbon comp for grid stoppers - this is just an audio myth.
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Old 19th May 2013, 10:30 PM   #13
Soonerorlater is offline Soonerorlater  United Kingdom
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Fred Nachbaur SET amp: input capacitor
Thankyou DF96.
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Old 20th May 2013, 01:46 AM   #14
TB is offline TB  United States
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Hello,
Thanks all for replies. I did not get direct answer for my question, but I understand CC resistors are out. Really, I want to use: all carbon film resistors, polycarbonate input cap, and carbon PEC potentiometer. For power transformers....... oh no, I would not have any courage to take X-former apart. It was mentioned in text to use slightly more powerful transformers for higher output. I have already collected four PL30-12-130B power transformers by Tamura: PL30-12-130B Tamura | MT3111-ND | DigiKey
Output transformers are Hammond's 125CSE. I still lack of capacitors and time to finally finish the amp. Since you are willing to help..... does it make any sense to use all carbon film resistors in this amp? I am not "systemphile", but "musicphile". I do like the warmer sound of vacuum tube radios, the ones which I grew up with. I do not want to open another ridiculous topic for people to fight above my head. I did try to look into many threads on the topic and I......gave up. So, will the use of the same type of resistors account for overall sound or there are some places when certain type of resistor will be beneficial for the sound which I look for. I do understand this amp is not HiFi, but this project will be my first attempt at vacuum tube design. I also expect I will live with this amp for a long time before I will try again to build another tube amp. I do not mind to use any suggested type of resistor, but besides above additional questions my main one was not answered. For R1 metal film or metal oxide? I would like to place an order for the missing components and resistors are on the order list.
Thanks again for the help !!!!!

Last edited by TB; 20th May 2013 at 02:03 AM. Reason: add explanation
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Old 20th May 2013, 01:51 AM   #15
TB is offline TB  United States
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Hello JPS,
Very nice amp. I like it very much. Now, in your answer you refer to the "output cap". Which cap do you mean? I see output transformer, but no output cap.
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Old 20th May 2013, 02:34 AM   #16
rfengineer2013 is offline rfengineer2013
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Hi TB, to directly answer your question re: 150ohm resistor in the power supply, the best option IMO would be a wirewound (the kind that come in the cement blocks). They're cheap, and wirewound resistors only contribute thermal noise (kTBR) with none of the other noise mechanisms associated with other types of resistors.

For the rest of the resistors, metal film will give you the best results in terms of noise - especially in the first stage where most of your gain resides. Resistors in the output power amp (PA) stage have minimal noise contribution because the audio signal + noise applied to the PA will generally mask any noise contribution from resistors in the PA stage.

As far as carbon composition resistors as grid stoppers, that is a holdover from the days when tubes were used as RF amplifiers. The grid-stopper resistor is in series with the signal, and at high frequency (>1MHz or so) any impedance other than the resistance affects circuit performance.

Carbon comp (CC) resistors have extremely low self-inductance, which is a big deal at RF but not so much a consideration at audio frequencies.

In fact, using film resistors for the grid stopper might improve performance as the added inductance will a) improve the lowpass filtering performance (at RF) of the grid stopper working into the tube Miller capacitance and b) better isolate the grid from the rest of the circuit (again at RF) to prevent oscillation of the tube.

Especially in the case of (a), film resistors will help keep RF out of the tube and prevent introduction of the stray electromagnetic interference (EMI) into your amplifier, thus improving performance!

As for the rest of the resistors, I suggest that if you have the values on-hand and want to use them then don't be afraid of doing so.

Stereo amplifiers were built for decades using CC resistors, and granted the performance of those amplifiers isn't up to the finest amplifiers built today using modern parts you will probably be pleasantly surprised at how good an amp can sound even with CC resistors.

If you have the latest 103dB speakers and ultra-gold oxygen-free speaker wires than yeah, you might hear a difference - but if you're just driving an old pair of 83dB Zeniths using zip-cord or something probably not. (BTW - I'm of the old Zenith/zip-cord school FWIW )

If you are purchasing resistors, then the metal film types are the way to go. They're dirt cheap (1/2 watt, 1% leaded resistors run 10-cents each at Mouser, I just ordered a bunch for another project ) and available in pretty much any value you need. They also come in 1W and 2W flavors, with perhaps less choice in values but still a good value.

When you get into building that tube phono-preamp is the time to really worry about resistor noise - for this amp, especially if you're running an iPod into it - go with what you have and have a blast!

~ Sam

EDIT - Besides, once you get the amp built and working you can play with different resistor types to see what difference, if any, it makes to your ears. For a good guide on parts selection, check out TubeLab's parts lists. He has provided suggested parts for his designs, along with manufacturer part numbers, that are commonly available from Digi or Mouser and won't break the bank!

Stick with parts in those families (even if the value TubeLab uses is different from yours) and you can't go wrong IMHO.
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Last edited by rfengineer2013; 20th May 2013 at 02:40 AM.
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Old 20th May 2013, 03:19 AM   #17
TB is offline TB  United States
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Hello Sam,
Thank you very much for the explanation. I want this amp to work with single-drive speakers. CD player will be the source.
Regards TB
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Old 20th May 2013, 03:45 AM   #18
rfengineer2013 is offline rfengineer2013
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Should sound great!

I've been running a new TubeLab SE for the past month or so, and it does make the old music sound _so much better than I had heard in years.

Even songs that I knew by heart sound fresh and new - I think you will be pleasantly surprised with your result.

Best luck to you!
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Old 20th May 2013, 07:04 AM   #19
JPS is offline JPS  India
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Quote:
Originally Posted by TB View Post
Hello JPS,
Very nice amp. I like it very much. Now, in your answer you refer to the "output cap". Which cap do you mean? I see output transformer, but no output cap.
The MiniBlok SET Amp: How It Works

Sorry for incorrect wording. I used Russian PIO for C7 and Black Gate for C8. I used custom wound power transformer instead of two transformers as used by FN . I also used a choke instead of R1.

Here is another picture of smaller MiniBlok SET amp driving large size speakers which are normally used with GM70 SE amp which is also shown in picture.
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Old 20th May 2013, 07:16 AM   #20
bullpeters is offline bullpeters  Australia
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I made one of these with an Ipod dock, taking the sound out thru the 30 pin plug for my 12 year old daughter, with some Zigmahornets. I have tried to pry it from her warm compressed pop listening ears ever since. Its a great sounding amp. Loud enough for a small bedroom. I built it as per schematic but put a bigger PT because it was convenient
there are pics on this forum somewhere
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