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 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 amp
PostPosted: 07 Mar 2013, 18:31 
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I now have all my tubes (2 pairs) and all the parts to start the amp. I cleaned-up and oiled the wooden base. It came up better than I expected. No need for a strip back, just as is will more than do.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 08 Mar 2013, 08:36 
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Great choice to just clean and refresh the oil as you would have to spend extra time getting it to take it back to period. I say it already has a great look. Sometimes you do come across items that have to be stripped back. I have found that chemical strippers do less damage than grit. I will use some fine grit paper and steel wool to knock off the raised fibers and leftover finish flakes. A light dusting (for the experienced) in the blasting booth might help too. Cool project to follow.


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 Post subject: Re: 1930 171-A SE amp
PostPosted: 09 Mar 2013, 01:48 
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AlabamaCajun wrote:
Great choice to just clean and refresh the oil as you would have to spend extra time getting it to take it back to period. I say it already has a great look. Sometimes you do come across items that have to be stripped back. I have found that chemical strippers do less damage than grit. I will use some fine grit paper and steel wool to knock off the raised fibers and leftover finish flakes. A light dusting (for the experienced) in the blasting booth might help too. Cool project to follow.

A speaker building buddy said if I blasted it back I may lose the " old" wood. He suggested a rub with fine steel wool and beeswax. I just went with the clean and cedar oil.

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PostPosted: 09 Mar 2013, 02:56 
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Here is a schematic of the amp (Intermezzo). This is based on an amp I saw in an AudioXpress mag. The builder used different trannies but similar voltages. The -27V will be supplied by 3 x 9V batteries. The 180V HT from step down tranni used backward to recreate 230V. Diode rec. bridges will be used with a 5H Hammond choke. A double filter of C-Choke-C-R-C should provide enough filtering: 47uf-5H-100uf-1500oms-180uf.

The heater voltage supplied from the first power tranni (15V tap) rectified, filtered then two 5V regs.

All comments welcome particularly the ones that start "this will not work..."
Attachment:
Intermezzo.jpg


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 Post subject: Re: 1930 171-A SE amp
PostPosted: 13 Mar 2013, 00:47 
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I have had builders block for about a week but though I didn't hold a soldering iron I did a pen and did some design and calcs. on the PS. Also (while recovering) I designed the top plate for the amp (Intermezzo). I have spent the whole afternoon cutting out the top pate and getting to fit. Cutting it out was easy but fitting it was a nightmare.

As you can see I stayed with the timber theme. I had thought about a metal plate and even ABS. I was going to paint the wooden top plate Hammertone blue (also known as Ocean Blue). The three ply the top plate is made from has not the best grain in the world (I can see Matt nodding his head) and that is why I was going to paint it. But I like wood and staying with the 20's feel. I should have called this amp "Woody".

Usually I would drill then paint. But I have given the top plate a polyurethane spray coat and in 24 hours I'll lightly sand and re-coat. Only the two tubes and grey housing for the OPTs will be on the top plate. Speaker connections and RCAs on the back of the OPT housing. Long way to go.
Attachment:
TopPlate.jpg
Attachment:
BaseToplate_II.jpg


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 Post subject: Re: 1930 171-A SE amp
PostPosted: 18 Mar 2013, 23:31 
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I have started assembly of the amp. I'm going to use 3x9V batteries to generate the bias. I thought I must include a switch to turn the batteries off when the amp is not in use. Thinking about the schematic when the amp is off there is no cct. For the bias current to flow. Therefore no switch needed. Also you can see the issues if the HT is up with no negative bias. Fortunately there will always be negative bias as long as the batteries have charge. To monitor the batteries I am mounting a RCA blue socket on the top plate as a battery test point similar to other valve amps with bias adjust.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 20 Mar 2013, 00:02 
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I guess when you are just making it up as you go along with no more than an image in your head of the finished item it is always hard and slow going. Now I think about it 99% of all my builds are like this. Slow progress with many logistical layout considerations.The top plate (on the top of the top plate only) is complete. The 4pin sockets are gold pinned and ceramic and quiet substantial.

The switch switches HT (bias from batteries is always on) on and off. A bias battery test point is in front of the switch. The trannies for the PS will reside in the base of the stand the smoothing caps will sitting on a cct. brd. under the top plate. The inter-stage trannies are underneath (image to come).

The dark metal housing on the top plate holds the OPTs and acts as a secure point for the speaker binding post and the input RCAs. I used this idea in the Be Bamp (KT120 bread board amp). The metal case has rubber feet so there is a little vibration isolation built in. I think open frame anything is ugly and should be hidden. This is a simple solution and offers additional functionality by securing the IO of the amp. Power cord drops out of the bottom of the base (no visible holes) and will have an inline switch in it.
Attachment:
IMG441.jpg
Attachment:
IMG443.jpg
Attachment:
IMG445.jpg


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 Post subject: Re: 1930 171-A SE amp
PostPosted: 20 Mar 2013, 02:13 
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Partly assembled and partly wired. About another two days work but this is how it will look when finished. I'm proud daddy.
Attachment:
Intermezzo_I.jpg
Attachment:
Intermezzo_II.jpg
Attachment:
Intermezzo_III.jpg
Attachment:
Intermezzo_IV.jpg
Attachment:
Intermezzo_V.jpg


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 Post subject: Re: 1930 171-A SE amp
PostPosted: 22 Mar 2013, 19:38 
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Mark;

Looks good so far. I like the physical build.

I am however a little concerned about your schematic. Output transformer and power tube topology look ok. Although I would think that with 180v on the plate you would want to bias closer to -40v; -27v seems far too low (based on the curves provided in the linked data sheet). My real concern however is with the open ended input to the interstage transformer.

Because the transformer is grounded, I am assuming that you are interested in driving the amplifier from a typical grounded AC source. To get to full power, the AC voltage across the transformer primary is going to need to get to 1/3 of the bias voltage at peak drive. The hammond 128B has an input impedance of 10kΩ but a DCR of only 403Ω. This immediately rules out preamps like the 4S line because the output impedance is far too high. In fact, if using a tube driver you're going to need to use a cathode follower stage with relatively low O/P impedance capacitively coupled to the transformer. This is almost like trying to drive a transmission line. These restrictions mean that the driver will likely need to be some type of op-amp or transistor based stage with a suitably low output impedance which has the capacity for driving the 403Ω input at idle. This is kind of a specialized drive requirement. Your typical preamp will probably not suffice.

The other option is to simply go with a typical tube transformer driver with the transformer in the plate circuit. In this case midband gain is simply µn (in this case n=3 with the 124B). But this of course requires a dual triode (or two triodes) which haven't been included yet. They could go inside the case however.

I'm sorry to rain on your parade (so to speak) :( but I think the drive issue needs to be addressed.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 23 Mar 2013, 02:51 
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Matt: the original builder of this amp used a different input tranny and drove the amp with an iPod. The iPod would expect a low load so 400 or so ohms was probably OK. I was thinking on driving the amp direct with my Oppo. Most CD players can drive into low impedance but 400 may just be too low.

There maybe a "get out of jail free" for me: the latest Oakley K272B preamp\headphone driver (JAN 6418 tube) has no problems driving 33ohm phones, has some gain, high impedance input and chip (OAP2134) driven output buffer. I have one built up. I could use this as my preamp.

What do you think?

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