I pulled back the throttles a bit. I am now running 450 volts at about 70 mA. I get 7 watts out at 5% distortion. There is a pale glow when the amp is driven. When you remove the input the tube gets hot. It has been running like this for about 20 minutes.
Further testing will continue, but I need to remove the expensive OPT's so that I can attempt a total meltdown.
Further testing will continue, but I need to remove the expensive OPT's so that I can attempt a total meltdown.
I've heard some folks say that the Sylvanias which were the last of the production runs from about 1987 were not very good as far as vacuum etc. Could these be from right at the bitter end of production?
I've seen a few of those being tortured in an old Fender amp with a shorted cathode cap before. Also, I had a Deluxe Reverb that I repaired once that had run with little or no bias for so long that the bottom of the chassis was dark brown from the heat. I think those were Sylvanias in that amp, so when they are good they are good, when they are not, they aren't.
I have a few Sylvania 6V6's that I bought new in 1988 they have the green writing and I've used them in several amps with great results. I better give them a good looking over before I use them again!
Great test.
I've seen a few of those being tortured in an old Fender amp with a shorted cathode cap before. Also, I had a Deluxe Reverb that I repaired once that had run with little or no bias for so long that the bottom of the chassis was dark brown from the heat. I think those were Sylvanias in that amp, so when they are good they are good, when they are not, they aren't.
I have a few Sylvania 6V6's that I bought new in 1988 they have the green writing and I've used them in several amps with great results. I better give them a good looking over before I use them again!
Great test.
The 6LQ6's are far too expensive to "test" although I do have a large (hundreds) box full of odd voltage sweep tubes, and I just bought some ceramic 9 and 12 pin compactron sockets. There are some 6GF7's in the box. Some testing will occur, but I can't say when. This was just a spur of the moment thing that allowed me to get carried away. I keep dreaming up more stupid "tests".
OK, you want to really crank some tubes. You have a nearly bullet proof power supply, those pesky cathode resistors and capacitors have been eliminated, but there is still a pair of shiny blue Edcor OPT's hooked into the amp. I would really be bummed if I blew one of those up, so they have to go. So I must ask the really stupid question, has anybody tried using an old microwave oven transformer as an OPT? I feel another experiment coming on.
The date codes read 705 vertically and ABH horizontally. I don't know how to read these. These tubes came from a lot that I got at least 10 years ago. I don't remember the details, but I probably got them in exchange for helping to load / unload a truck or two. I got some wafer based 6SN7's at the same time. I kept a few and traded the rest away. The few that I still have work fine.
If I had to guess, I would think that there was some contamination or rust on the leads that go through the glass, thus creating the path for a slow leak. I believe that all of these tubes will eventually leak, since half of them have leaked already. I haven't tested them all but some work great, and some seem to work OK but the distortion is too high. I have found a couple that are showing some blue glow inside the plate even though the getter is shiny. I am going to run these hard and see if they get worse.
Just because mine are junk doesn't mean that yours, or anybody elses are bad. These are obviously from a bad batch that got shipped when it shouldn't. I have seen worse things happen when there is a deadline to meet.
OK, I just answered the stupid question. There is no UL tap, so I used triode mode. The thing does actually kind of work as an OPT. I think that the impedance match is wrong, but there is a bigger problem. The frequency response sucks. The 3 db points are 35 Hz on the low end (good) but only 140 Hz on the top end. I can get 4 watts in triode mode (550 volts remember) at 60 Hz but only 1/4 watt at 1 KHz.
Funny you should mention...
I have a big hurky microwave tranny sitting my work room that I have been dreaming of converting to a sub amp output transformer. Thinking that if I can figure out how to non-destructively open her up I could rewind one of the windings to get proper ratio. No need for fancy interleaving as nothing above about 200Hz is really of any consequences.
Might be a good use for those 6LQ6s.
mike
Looks like you have a half decent power supply choke, but a really lousy output transformer.
I've got an old MOT that I ripped apart. I actually bothered to unwind the primary, leaving only the secondary on the core. I'm sure it's at least a couple Henries. The tough part is that one end of the secondary is tucked under the winding itself, and welded to the mounting plate. If I try to hook it up as is, I'll have 400 volts on the whole thing. I guess I could put the choke on the center tap, rather than in the more conventional location...
On a side note, has anyone ever tried use a 12AZ7 as the front tube on a Simple SE?
I've got an old MOT that I ripped apart. I actually bothered to unwind the primary, leaving only the secondary on the core. I'm sure it's at least a couple Henries. The tough part is that one end of the secondary is tucked under the winding itself, and welded to the mounting plate. If I try to hook it up as is, I'll have 400 volts on the whole thing. I guess I could put the choke on the center tap, rather than in the more conventional location...
On a side note, has anyone ever tried use a 12AZ7 as the front tube on a Simple SE?
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!
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
From:
http://www.dogstar.dantimax.dk/tubestuf/mctintro.htm
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.
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.
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.
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.
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.
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.
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!
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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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.
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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tubelab.com said:
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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