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Old 31st October 2011, 05:29 PM   #21
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Yep, I don't live by the seaside. If I did have a notion to use some protection, I'd probably use some clear polyurethane spray from the hardware store.
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Old 31st October 2011, 10:55 PM   #22
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Quote:
Yep, I don't live by the seaside.
I live about 12 miles from the ocean in an area that is known for its heat, humidity and salty air (Fort Lauderdale Florida). I have home made PC boards that I made back in the 70's. None have been protected and all are still alive.

The picture shows a few Plastic Tigers and Universal Tigers I made in the late 70's. The two complete ones spent a few years in the sound system in my 70's van.

Here are two pictures of the first two Tubelab amp prototypes. They still have their home made PC boards and are both over 7 years old. Both still work. The 845SE operates on 1100 volts and the power supply is all PC board based. Note the swimming pool in the background.
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File Type: jpg FirstTSE_A.jpg (353.7 KB, 298 views)
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Old 10th November 2011, 08:05 AM   #23
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Here's the preliminary schematic of what I want to try. The outputs tubes are direct coupled, so I don't expect blocking distortion, though there will be gentle clipping if the grids on the outputs go positive. All the stuff inside the dotted lines is going ( has gone, as the layout is done) on to one fairly small PCB, so the insides of the amp will not be as cluttered as before. The pots will be used to tweak the overall bias on the output tubes. I took advantage of the direct coupling to put in place a "garter" bias circuit to help balance the output tubes, hoping to get rid of an extra couple of pots that way. Since the input stage runs on low voltage. I can bias it up with some bench supplies and check it out before I mate it with the rest of the amp.

If you substitute tubes for the jfets in the input stage (6EW6s or some hi-mu VHF triodes, I'm not sure which) , it's the topology I'm considering for the driver card on my ST70 not-clone (just the output XFMRS and the chassis will be like the original).
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Old 10th November 2011, 07:58 PM   #24
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Ok, how does it work? The circuit utilizes a differential jfet input with folded cascode loading. The input diff pair is Q6 and Q7. The 2SK117 is a part with about 1/2 the transconductance of the 2SK170 and much lower input capacitance, still with respectable noise performance. The differential pair tail is fed by current source Q8, a Supertex depletion mosfet. Q8 is also the "boss current" source, setting the levels for the rest of the circuit. Tail current for the diff pair is set at 6ma, meaning that Q6 and Q7 will be each pulling 3mA. Current source Q8 also feeds a current mirror (Q9 and 10), which in turn feeds the top current source loads (Q1-Q3). Q1-Q3 form a current mirror that provides current source loading for Q6 and Q7. The ratios between resistors R2, R5/R27, and R3 are set such that Q1 and Q3 each drive 4ma into the drains of Q6 and Q7. The excess current (1mA) feeds cascode transistors Q4 and Q5, and is used to establish the bias levels of the output tubes. Pot R27 is used for fine setting of the output bias.

The attached circuit is the fancy-pants implementation, and uses a fair number of parts. An extreme stripped-down version would keep the Q8 diff pair tail current source, lose Q9, Q10, and Q1-Q3. The drain load for Q6 and Q7 would be replaced with a pair of 1.54k resistors. This would work ok, as long as a stiff voltage (D11 in this case) is used to clamp the bias on the folded cascode transistors. It would be interesting to see if there is any perceived difference in performance between the two approaches.
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Old 10th November 2011, 09:42 PM   #25
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Looks like there are two D11's. Is the bottom D11 (39V) drawn backwards?

If you used some P type input Fets, you could avoid some of the folding transistors for the neg. voltage shift.
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Old 10th November 2011, 10:41 PM   #26
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Good spot, the 39V zener in the bottom should be the other way ( It's shown correctly in the layout, which was done free-hand and was not coupled to this schematic). It's true that I could use a p-channel input stage for a simpler approach, but the folded cascode separates the bias current in the input stage vs the current in the gain resistors and allows one to optimize for more loop gain. I may try the p-channel approach on another amp. Having said that, appropriate n-channel input devices are more available.

Regarding P-channels, if you want to ignore the 2SJ74 as unobtainium ( I have some, but I don't know where I'd get more), this leaves devices like the 2N5460-62 (low gain) or short channel devices like the J175-J177, or small signal mosfets from Supertex and Zetex/Diodes, Inc. I'll fix up the current schematic and gin up one of the p-channel approach when I get some time.
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Old 10th November 2011, 11:01 PM   #27
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Wouldn't a short channel device behave a bit more like a triode? I have some 2SJ74 around too. I've heard of some outfit in Ca. making the dual N channel parts, like 2SK389 I think, don't know about the P channel parts.
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Old 10th November 2011, 11:32 PM   #28
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The short channel is more like a crappier pentode. The long channel is like a good pentode. One thing I omitted to mention is that wringing out the folded cascode stage for the Lil' Devil is a preliminary to adapting it to tube inputs for my "ST-70" amp, where no vacuum-state p-channel devices are available. For input tubes, I'm limiting myself to 7 pin devices (less space, which is critical, as I'm using 4 envelopes intead of two). I'm wavering between a pentode like the 6EW6 or a triode like the 6HA5.
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Old 10th November 2011, 11:45 PM   #29
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P tube emulation:
and a real P tube(ion gauge tube):

I recently found that there are actually sealed ion gauge tubes available for calibrating an ion gauge controller. Around $150 each though, and only good for microamps I think. Just a gassy radio tube should work though if the typical voltages are inverted.
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Last edited by smoking-amp; 10th November 2011 at 11:59 PM.
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Old 10th November 2011, 11:50 PM   #30
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Nice, but it doesn't fit what I want to do.
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