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Old 12th June 2008, 02:22 AM   #31
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Quote:
Looks like you have a half decent power supply choke, but a really lousy output transformer.
There have been a few threads on this forum where several users have reported success using MOT's for parafeed chokes. I found two identical microwaves in the trash so I relieved them of their transformers.

In almost ever microwave that I have dissected (I developed this habit of taking things apart at a very young age) one end of the HV winding is grounded. The transformers that I have used a self tapping screw into the transformer core for the ground. Undoing the ground was easy. How much can the insulation stand is now the question.

I have not tried the 12AZ7, but if I have any i'll plug one in. I haven't blown anything up this week!
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Old 12th June 2008, 04:02 AM   #32
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I can't vouch for the information's accuracy, but I did find this tidbid that gives an interesting comparison, emphasis added:

From:
http://www.dogstar.dantimax.dk/tubestuf/mctintro.htm

Quote:
If you intend to experiment with different tubes to get "just that sound" also keep an eye out for type 12AU7 and 12AT7 (and their European cousins ECC82 and ECC81), as well as 12AY7 and 12AZ7. The 12AT7 and 12AZ7 will give comparable gain, but their clipping characteristics are a little different. The 12AU7 will give considerably less gain, and will therefore only be generally useful if the overdrive unit is placed later in your effects chain.
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Old 12th June 2008, 04:05 AM   #33
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Quote:
Originally posted by Zap
I can't vouch for the information's accuracy, but I did find this tidbid that gives an interesting comparison, emphasis added:

From:
http://www.dogstar.dantimax.dk/tubestuf/mctintro.htm

I can and it's bang on
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Old 12th June 2008, 04:21 AM   #34
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Seeing how much joy you get splodin stuff with wacky voltages...
http://w5jgv.com/hv-ps/index.htm

1300V 1KW Switcher.. And he winds his own 25KHz transformer.
From the guts of six PC power supplies.
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Old 12th June 2008, 04:52 AM   #35
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On microwave transformers -
One - They ain't gapped, though there is a magnetic shunt, which may explain why the things work at all in SE mode. The shunt adds considerable leakage inductance, which screws up the HF response. Knocking out the shunts (a cold chisel does the trick) will improve HF response somewhat, but no doubt ruin any real pretense at SE operation (parafeed, anyone?).

Just about every microwave transformer I've seen has had the lams welded together, which puts paid to any notion of deconstructing them for other nefarious purposes.
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Old 12th June 2008, 10:57 AM   #36
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A bandsaw will make deconstruct welded cores if an airgap is ok...
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Old 12th June 2008, 01:12 PM   #37
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One - They ain't gapped, though there is a magnetic shunt, which may explain why the things work at all in SE mode.
I know about the shunts, and haven't knocked them out yet. The lams are welded. I mentioned the lousy frequency response, but I didn't mention the distortion. There were some saturation effects visible on the low frequencies (below 60 Hz) and the distortion was about 5% minimum and went up from there. I decided rather quickly that even for "destructive testing" these things are worthless as OPT's.

I didn't get home from work until 9PM last night so there was very little time for experimenting. I did throw a 6K P-P guitar amp OPT into the amp using the entire primary winding with the center tap used for the UL (screen) connection. Of course there is some saturation on frequencies below 100Hz but the distortion at 1KHz is under 3% at 5 watts. Maximum (fully distorted) power output is now 18 watts.

Quote:
Seeing how much joy you get splodin stuff with wacky voltages...1300V 1KW Switcher.. And he winds his own 25KHz transformer. From the guts of six PC power supplies.
Like the guy in the movie says " I gotta git (make) me one of those". Of course it would be a lot easier to just put a variac on the input of the MOT.
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Old 15th June 2008, 03:11 AM   #38
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I had some fun blowing up a couple of already useless 6V6 tubes, and I made a few questionable tubes a little bit more questionable. The fun lasted for a few hours but I got tired of it. I put the 6V6's back in their box (so that a few more could die) and put the box back in the far corner of the closet. In the process I discovered a box full of tubes that I had forgotten about. It contained some tubes that I had saved for "future experiments". I opened the box and decided that it was time to visit the future and my distant past at the same time.

When I was quite young (12 to 14) I was making guitar amps out of old TV sets. I had cooked up a single ended design that used a 6BQ6 or a 6DQ6, the power transformer from the donor TV and the vertical output transformer from the TV as an OPT. I made these amplifiers and even sold a few without really understanding what I was doing. I don't think I fully understood this "bias" thing. I didn't know what a load line was or even understood "impedance matching". I didn't understand why the small tube (6BQ6) was louder that the big tube (6DQ6) in an SE amp but the big ones worked better in a P-P amp. Guess what was in the box. I decided to re-explore these tubes. What would it take to wire one into the Simple SE? A quick trip to the tube manual revealed that it was a drop in! All I had to do was make a pair of wires with plate caps on one end, and screw the other end into the plate terminal on the OPT connector.

NOTE: DO NOT TRY THIS YET. IT WILL BLOW UP THE CATHODE BYPASS CAP. THESE TUBES DO NOT BIAS UP LIKE THE TUBES NORMALLY USED IN A SIMPLE SE. I will post more info as I experiment further.

I already had my amp wired up for an external bias supply. This allows me more freedom to adjust the operating point.

At first I didn't know what to expect, so I picked the crustiest dirtiest, ugliest tubes in the box. There was a Philco and an Admiral 6BQ6GA (both GE's). Both were well used with much of the getter used up, so I decided to "test" these tubes.

There is another user on this forum that is fond of these tubes. He says that they are like a 6V6 only better. I'll say!
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Old 15th June 2008, 03:19 AM   #39
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It was really easy to get 5 watts in UL mode without really warming up the tubes. 10 watts came without glow. I don't think that you can do this with a 6V6.

Now what if you just ignored the complaints from the tube and turn the knobs for best power at a low distortion. It really is possible to get 20 watts per channel at 3% distortion from a 6BQ6 in SE UL mode with no feedback whatsoever. There is however one little drawback. You can read by the bright orange light coming from the plates, and you can feel the radiant heat from 3 feet away. I think tube life can be measured in minutes! Note the reflections in the OPT's.

More experiments tomorrow. It is late here and I don't like to play with high voltage late at night.
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Old 15th June 2008, 05:23 AM   #40
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Originally posted by tubelab.com
There is another user on this forum that is fond of these tubes. He says that they are like a 6V6 only better. I'll say!
That would be me.

Le Renard

6BQ6GTBs really are one of the premiere audio finals, even if the spec sheet doesn't mention audio useage. These really do sound good, and don't need any extra help in the form of local NFB. The main problem is that they like to pull some big currents, hence, the project is in monoblock form otherwise the power xfmr I used (NOS Stancore) would be stressed if required to power two channels instead of one. You really have to use 'em in push-pull since you can't hit the most linear part of the plate characteristic without red plate destruction in SE.

Though rated like a 6V6 (Pd= 12W) I run 'em a good deal hotter. With a cathode current of 55mA, that's more like 18W of Pd. The plates don't show any colour in the dark at that current, and the sonics benefit noticeably from getting them farther towards Class A operation. The 6BQ6GTAs, however, don't seem to be quite so robust, and are built differently (cathode is a good deal thinner) and red plate obviously at the same Q-Point that the 'GTBs tolerate without problems.

I'm getting almost 40W, and if you really want to bust some specs, these can do 70W.
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