Has anyone built this amp?

[krzanik]

Hi,

My first post. I've been looking at the "Five Tube Stereo Amplifier", designed by Wes Kinsler (you can see it at: www.wkinsler.com/radios/5tubeamp.html ). I was thinking of trying to build it without the tone controls but had a few questions. Mr. Kinsler lists the parts required to build the amp but labels the capacitors mF. Does he meen uF (microfarad)? And capacitor No. 5 has a value of mmF? Is this maybe a typo or does he possibly meen pF (picofarad)? If someone can help with this it would be appreciated. Also, any general opinions regarding the design would be great. I can't find a transformer (Hammond or any other) that seems like it would work for the power transformer (600v CT @ 80ma?). Suggestions would be great. I've built the S-5 11MS8 kit and have messed around some with electronics. It would be nice to try to build something simple and cheap for a first amp. Oh yah I built some pretty decent speakers (foxtex FE207E 95/db efficiency).

Thanks in advance. Dylan.

[Yvesm]

Hi Dylan.

Looked at the schemo and part list, and, yes, mF is for microfarads and mmf for picofarads.

I think there is a typo in that the grid of the first tube is floating.
Since they have the cathode grounded, I suggest to add a 5 to 10 megohms resistor from each grid to ground in order to implement a so called "grid leak" biasing.

Do not simply short the link cap (C1) !

Yves.

[Giaime]

For the Power transformer, it looks like a Hammond 272BX or DX is good. 600VAC center tap means 300-0-300.

[krzanik]

Thanks for your replies.

The transformer question has effectively revealed my ignorance so I have nothing to lose. Thanks to Giaime for the transformer information. I looked over the specs of both transformers and the specs on the various tubes and it looks like either one would work fine (and they'll fairly inexpensive too) as Mr. Burns would say "excellent".

Yves,

Grid leak biasing eh? I looked over some circuits that use this and I think I understand you. But lets make sure. I don't have any children (that I know of) yet and I don't want the Snufnerowski name to die out just yet.

The grid of the first tube is floating

. I take it we are talking about the 6SF5GT tube? So, to grid leak bias the grid on each of the 6SFGT tubes I would connect a 5 or 10 megohm resister from pin 3 to ground? How would I determine which (5 or 10) megohm resistor to use? Or would it not really matter much? Oh, and what watt resistor would I use? Thanks for your help.
This is all very interesting.

Dylan

[Yvesm]

Hi Dylan,

So, to grid leak bias the grid on each of the 6SFGT tubes I would connect a 5 or 10 megohm resister from pin 3 to ground? How would I determine which (5 or 10) megohm resistor to use? Or would it not really matter much? Oh, and what watt resistor would I use?

Yes, you get it.
Use what you have at hand between 5 and 10 M, there are no watts dissipated inside, don't care !

Yves.

[Giaime]

2 suggestions: first use a 1M metal film 1/4W or 1/8W resistor between grids to ground. 1M and not more because more resistance means more noise with high values. And also doing this you will reduce input impedance (not too much), that will not matter in audio but will in RF in case you'll picking up interference or oscillations. Solder the resistor nearest to the grid pin on the socket.

Second: use the DX tranny. The other one seems a bit underrated for filaments...

[krzanik]

Well, here it is. I've altered the circuit to remove the tone controls and included the considerations for grid leak bias and power transformer. I put in the Hammond 125 ESE as the output transformer. If this is not appropriate let me know. The origional schematic called for a prim Z of 7600 Ohms. I figured with the many choices for primary impedance on the 125 ESE, that it would be a good choice. I think I would have to connect wires 3 & 6 to get a Prim Z of 6800 Ohms on the 125 ESE. Hopefully this will be close enough? Let me know what you think about the circuit. thanks for everyones help. Dylan.

[IMG]5tubeamp.cwk[/IMG]

[krzanik]

Well, I tried to attach the circuit to the last reply and it doesn't appear as if that worked very well. I'll try to convert the file to another format and attach again. Here goes:

[IMG]5tubeamp[/IMG]

I converted the image to a jpeg image. Hopefully that will work. If you can open it please let me know what you think. Cheers.

Dylan

[Giaime]

Hello! No the images won't load... I hope you fix it.

About the OT, 6800ohm seems fine. Make sure it is a SE tranny....

[jaencer]

Just finished building this amp , and I have been listening to it all day, cant stop! This tube amp should be more popular than it is, I was half expecting it to suck due to the schematic's lack of feedback from others, but man, i'm glad to have built it anyway.

I traded the two input tubes for a single 6SN7 (it has two triodes in it, and has a much better reputation). Rather than using the power supply resistor R9, i used a 15H choke for better regulation and efficiency, and rather than two 10uF supply caps C6 and C7, i used a 15uF before the choke, and 40uF after it. Both my supply caps are big old oil capacitors at 450V. Everything else is the same from the schematic. I highly recommend the choke and the oil caps.

My power transformer is 95VA and still gets quite hot, i had to thermal epoxy some heatsinks to it. The output transformers should be as big as possible for better bass.

I also built an LM3886 chip-amp by carlosfm's schematic (also amazing) and I don't know wich is better. This tube amp definitely easier to lisen to at the end of the day sometimes. Its so warm and airy, even sounds good with mp3 files (unlike the chip amp).

There really aren't any problems, this is the first of 4 amps i've built that didn't begin with noise that needed to be diagnosed right away. There's no noise and no clear frequency aberrations.

I have noticed that it doesn't like metal too much, like, metallica sounds better on my chip-amp. But test it out with Jack Johnson, and you'll have a hard time turning it off.