NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: Re: 1930 171-A SE ampPosted: 23 Mar 2013, 11:56
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1648
Location: US Pacific Northwest
I agree, so long as the transformer is capacitively coupled, the load is a high impedance and anything that could drive a lower impedance (like a headphone amp or a SS preamp) should be ok. An iPod (through the capacitively coupled headphone jack) should be able to drive that load no problem.

However, my iPod peaks about 2v at the headphone jack. This equates to 6v at the power tube. With a 27v bias you will still be about 13dB below full power. With the 27v bias you will need an ac drive voltage of 27/3=9v to get to full power. If you increase the bias to closer to 40v (say 4*9v=36v) you'll need 12v to get to full power but the output power will be much higher (relatively speaking). You could also use 5 9v batteries across a 100kΩ potentiometer as a bias voltage and then tune the bias to wherever voltage you want it. Independent pots would allow you balance the channels for different tubes as well. It does however mean a total of 10 9v batteries for the stereo setup.

I'm just kind of writing as I'm thinking here. The other option is that you could build the amp for current drive instead of voltage drive; but that introduces some other issues you'd have to address so maybe not. Keep us informed on the build. I'm interested to hear how it turns out.

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Matt

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 Post subject: Re: 1930 171-A SE ampPosted: 23 Mar 2013, 17:30
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
I can see I might have trouble getting a big enough swing across the grid to extract full power. The Oatley K272B only runs on 9V so there is a limit to what I can get from that. I may put the K272B on the CRO and see what sort of swing I can get into 400 ohms. At this point I'm going to leave the bias at -27V. As you pointed out I could end up with a cluster of 9V batteries just trying to get the bias up. The original build used -27V at 135V for HT. I realise I'm driving the HT to 180V.

Also I was going to leave the input and output floating. Matt (or others) can you see any good reason to ground either?

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 Post subject: Re: 1930 171-A SE ampPosted: 23 Mar 2013, 18:51
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
I ran some tests on some preamps I have. The input resistance to the 171-A amp is 390ohms DC. Firstly I loaded a K272A with 360omhs. Voltage gain was poor with 2Vpp in and a max of 3.2Vpp out. Also the sine wave is not the most symmetrical. Not sure if this was caused by the high load on the output or that is a characteristic of the preamp.

Next I tested my BoZSE which is heavily based on Pass's BoZ single jFet preamp. Again I loaded the output with 360ohms. Up until recently this has been my main preamp for years. Heavy Metal (6N7) preamp and then Menatzchim (C3m) preamp replaced it having better allover musicality and better bass. BoZSE had no problems producing a clean 1K sine wave into 360ohms at 17Vpp for 2Vpp in! BoZSE should have no problems driving Intermezzo (171-A amp) close to full power.

Where the K272A had -3db points from 10Hz to 60KHz, BoZSE's -3db points were less than 10Hz to 92Khz. Also the square wave at 100Hz and 10KHz looked better. Not as much as I would have thought but definitely better.

Also I can probably add one more 9V battery for a bias of -36V. With a HT of 180V a bias of -40V is recommended. I could drop the HT to 135 for a -27V bias but I have read where keeping the HT high will extract a little more power from the tube. Now to finished the amp which may not be for a few days now.

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 Post subject: Re: 1930 171-A SE ampPosted: 23 Mar 2013, 20:34
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1648
Location: US Pacific Northwest
It looks like the BoZSE is the winner to drive this amp.

As for setting the bias point and B+, it's good to remember that for triodes in the power tube roll, the output power is proportional to the square of the plate voltage and the reciprocal of the load resistance. The moral of the story is for maximum power out, run the B+ as high as possible without exceeding the allowed plate dissipation. Personally, I'd stick with the +180v.

I think increasing the bias is also a good idea. Especially given that the total power out for B+=180 and Vb=-40.5 is only 700mW. Assuming an 80% output transformer efficiency this equates to only ≈1/2W at the speaker. This will be a true µPower amp!

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Matt

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 Post subject: Re: 1930 171-A SE ampPosted: 24 Mar 2013, 02:37
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
Suncalc wrote:
It looks like the BoZSE is the winner to drive this amp.

As for setting the bias point and B+, it's good to remember that for triodes in the power tube roll, the output power is proportional to the square of the plate voltage and the reciprocal of the load resistance. The moral of the story is for maximum power out, run the B+ as high as possible without exceeding the allowed plate dissipation. Personally, I'd stick with the +180v.

I think increasing the bias is also a good idea. Especially given that the total power out for B+=180 and Vb=-40.5 is only 700mW. Assuming an 80% output transformer efficiency this equates to only ≈1/2W at the speaker. This will be a true µPower amp!

Yes very uP. It's funny I tell everyone this is a 1\2W amp not really sure how much "brute force" this amp would unleash. OK I'll stay with 180V for HT and up the bias easy to do with one more 9V battery. I would guess the bias batteries will last a long time? I have test point external to the amp so real easy to check.

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 Post subject: Re: 1930 171-A SE ampPosted: 25 Mar 2013, 20:42
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
The PS and filter brd. is complete. There are two positions for limiting resistors which will be fine tuned experimentally. Each heater for the 171-A has it's own regulated 5V supply. The HT comes of the 80 rec. tube and is C-L-C-R-C filtered. There is a small 5H 50mA Hammond choke on the filter brd. A Hammond filament tranni with dual 6.3V windings will reside in the base. One winding to power the heaters for the 80 the other to share across the two 171-A tubes. Also in the base will be the HT trannies. A 2A rec. bridge is on the filter brd. for the heaters.

The filter brd. ain't pretty underneath but functional. Neato from the top though.
Attachment:
PS_I.jpg
Attachment:
PS_II.jpg
Attachment:
PS_III.jpg

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 Post subject: Re: 1930 171-A SE ampPosted: 26 Mar 2013, 21:25
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
Hope to have this amp finished today. Here is the tranni brd. which sits in the base of the amp. It supplies 230VAC for HT, 5V and 6.3V for heaters. The resistors limit the "80" filament voltage to 4.83VAC from 6.3V. The other 6.3V is rectified then regulated for the 171-A heaters to 5VDC.
Attachment:
Tranni_brd.jpg

Though the tranni brd. will never be seen I had to find some stressed pine, fine sand it and beeswax it. Why, because who would have know it was not a scrappy bit of timber? I would have, reason enough to spend some time on it. It has four soft feet and will be held in the base by a few long screws. Feet are to provide some mechanical isolation from the rest of the amp.

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 Post subject: Re: 1930 171-A SE ampPosted: 27 Mar 2013, 02:35
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
I've spent about seven hours on the amp and it is finished. It is playing and sounding good. This where the Fostex FE206 come into their own. With the Fostex drives in the double bass chamber box there is sufficient volume for relax listening at good volume. I'm keeping it simple with chamber style music or single piano, violin etc.

I have one tube with a shorted grid, and the PS failed to deliver enough HT. I upped the primary tranni and run 15V into the 12V tap of the step-up tranni. I'm only getting about 130V so bias remains at -27V. I had to resort to half wave rec. and with your ear to the Fostex there is a buzz and hum. Can't be heard from listening position or even 1M back. Not sure I'll chase it.

I have one speaker binding post wired out of phase (easy fixed) and I should upgrade the step-up tranni. When I first played it it was dead. I had wired the HT in backwards. But now enjoying some high-rez solo piano. BoZSE preamp is in control. Pleasing sound. Input and output are floating. Should I at least ground the input to the inter-stage tranni?? Can't see why I should.
Attachment:
Inermezzo_I.jpg
Attachment:
Inermezzo_II.jpg

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 Post subject: Re: 1930 171-A SE ampPosted: 27 Mar 2013, 07:35

Joined: 02 Mar 2009, 12:41
Posts: 1111
Location: Vänersborg, Sweden
That is a really nice amp with nice and odd tubes. Got a pair of 4's, but they look more like 2A3s.
However if I was to have a n opinion, Imy proposal was to make something steampunkish, omitting the metal cover on top or make it out of brass or copper. A vintage VU-meter would look nice too.
Anyway, you gave me some ideas for future projects ....

_________________
Magnus

“If you don't know where you are going, any road will get you there.”
―Lewis Carroll

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 Post subject: Re: 1930 171-A SE ampPosted: 27 Mar 2013, 07:55
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
That is a really nice amp with nice and odd tubes. Got a pair of 4's, but they look more like 2A3s.
However if I was to have a n opinion, Imy proposal was to make something steampunkish, omitting the metal cover on top or make it out of brass or copper. A vintage VU-meter would look nice too.
Anyway, you gave me some ideas for future projects ....

Magnus: Haven't heard from you for a while. I lean toward steam punk myself. This is not steam put more like peddle power. Spent a big night listening to it. My Silver Dragon 300B and VoXDeuX top this amp on many levels. But what this amp has is real history. I'm just a little proud I can be thoroughly entertained in such a great environment with other stunning DIY gear and listen to tubes which has entertained many people for many, many hours. These tubes may has served on the front line?? You never know. I can now share in their humble beauty.

I tried to keep the whole build very "old school" I think I did it. What gets me is there are no resistors or caps in this amp. Just two trannies. That's it.

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