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    Building, troubleshooting and testing of these amplifiers should only be
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Tube sale at AES

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#241
I didn't change anything but the tubes. The 6DQ6's draw about 65 mils per tube under the same conditions.

I'll play with them on the hi voltage tap this weekend, maybe.

Win W5JAG

They settled in to about 65 mils per tube, and sound pretty good.
 
#242
Is it possible to run 12DQ6B's UL in a slightly modified Simple SE?
Click to expand...

It is possible but there are a few obstacles:

The 12DQ6 needs 12 volts on the filament, the 12AT7 is wired for 6 volts. They are wired in parallel on the PC board. It would require some board surgery to rewire the 12AT7 for 12 volt operation. Using a 6 volt output tube would eliminate this.

The 12DQ6 needs quite a bit of bias voltage to operate in the linear region. In my experiments I needed 80 volts of negative grid bias. The Simple SE uses cathode bias, so the bias voltage is developed across the cathode resistor. This requires a cathode bias resistor with a larger wattage rating, at least 10 watts. The cathode bypass cap must be changed to at least a 100 volt cap. Since you are going to drop 70 to 90 volts in the cathode resistor, the B+ may not need to drop that much. The actual bias resistor requirement would need to be determined experimentally.

Now the good part. The 6BQ6 and the 6DQ6 have almost the same pinout as the 6L6GC. The only difference is the plate connection. This makes these tubes a drop in. All you need to do is add a plate cap and connect it to the plate terminal on the OPT connector, no soldering needed.

I have tried these tubes and I had much better luck with the 6BQ6. It takes far less bias voltage so the cathode resistor doesn't get so hot and they seem to be more linear at lower currents. Both of these tubes seem highly variable when used for audio. Every tube biases up differently, and some require too much current (they glow) to get a low distortion. Many just won't work.

I have done a lot of experiments where I use the Simple SE and the Tubelab SE as the starting point. Understand that I have several of each board that I built up when I was doing the initial testing to verify the design. I don't care if I blow them up. In fact the board seen in the previously mentioned thread has been blown up and repaired several times. It has been made into a P-P amp, and back to a SE amp. If I only had one board, and I listened to it regularly, I wouldn't be doing this.

Any time you venture from the proven designs, you must be prepared to do some experimenting, and there is always the possibility of toasted parts. In fact I blew a cathode bypass cap all over the room in the course of these experiments. That is how I know that 5 watt cathode resistors will fail taking the cap with it.
 
#244
tubelab.com said:
Now the good part. The 6BQ6 and the 6DQ6 have almost the same pinout as the 6L6GC. The only difference is the plate connection. This makes these tubes a drop in. All you need to do is add a plate cap and connect it to the plate terminal on the OPT connector, no soldering needed.
Click to expand...

Yes, I had noticed that the pinout was basically the same, darn handy.

As luck would have it, I happen to have a six-pack of 6BQ6GTB's. The 12DQ6's were only a buck each, I just couldn't resist, and I like the shape. I also got two 12BQ6GTB's in the same order.

Thanks for the info.

Jeff
 
#245
tubelab.com said:



The 12DQ6 needs quite a bit of bias voltage to operate in the linear region. In my experiments I needed 80 volts of negative grid bias.
Click to expand...

George,

For the Simple SE, do you have any suggestions for a starting value for the cathode bias resistor for 6DQ6 with B+ in the 400-450 volt range, and any suggested operating points for 6BQ6? Since these are high perveance tubes, do you think it is better to use high voltage or stick with lower plate voltages?

Neither of my tube handbooks (RCA and Tung Sol) had class A information for 6DQ6, but the Class A data for the 6GW6 (250 volts and (-) 22.5 on G1) is the same as for 6BQ6. I tried to hit close to this.

Interestingly, the Tung Sol handbook has data plates for all of the 6BQ6 family, and states that "with the exception of comparatively lower bulb temperatures and its T-11 construction the 6BQ6GA is identical to the 6BQ6GT." *

They sure look different ....

Win W5JAG

*Tung Sol Electric, Inc., Electron Tube Division Jan 1, 1957, plate #4882
 
#246
isaacc7 said:
I seem to remember seeing that the *DQ6 was similar to the 6146. Any truth to that?

Isaac
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I wouldn't think so.

The 6146 is a transmitting RF tube pretty well suited to operation well into the VHF range with only minimal derating because of its internal construction (short plate connection, interelectrode capacitances, and multiple cathode pins). Transmitting tubes generally have a higher temperature bulb specification, as well.

6DQ6's were used in a few 50-60 watt AM/CW transmitters. I am thinking there was an early Eico SSB transceiver that used a pair of 6DQ6's, but it only went up to 14 MHz, IIRC.

Sweep tubes are difficult to manage as you go up in frequency, but brave firms have tried it. The Drake TR-6 (50 Mhz with three (3) 6JB6A in parallel) was a notable example.

Win W5JAG
 
#247
w5jag said:


I wouldn't think so.

The 6146 is a transmitting RF tube pretty well suited to operation well into the VHF range with only minimal derating because of its internal construction (short plate connection, interelectrode capacitances, and multiple cathode pins). Transmitting tubes generally have a higher temperature bulb specification, as well.

6DQ6's were used in a few 50-60 watt AM/CW transmitters. I am thinking there was an early Eico SSB transceiver that used a pair of 6DQ6's, but it only went up to 14 MHz, IIRC.

Sweep tubes are difficult to manage as you go up in frequency, but brave firms have tried it. The Drake TR-6 (50 Mhz with three (3) 6JB6A in parallel) was a notable example.

Win W5JAG
Click to expand...


Ah, sorry, I meant in audio applications. I admit to being someone on the sidelines, I have never been able to get my amp building off the ground. All of my stuff is in storage... Anyway, maybe it was on Grimwood's site. He has some questionable stuff on there, but he claims that the 6146 (and the 2e26 of all things) are capable of really nice audio performance if they are run correctly. He then says, about two thirds of the way down this page:

http://www.webace.com.au/~electron/tubes/ulo.html

that the 6dq6 series is the "receiving tube" version of the 6146... So has anyone done similar experiments with the 6146 family?

Isaac
 
#249
OH NO IT HAS STARTED AGAIN!!!

This statement came in an email today. This is where it started last summer. Then there was a 50% off list..... Then I had 3 boxes of tubes.

"For a limited time, almost all New-Old-Stock Tubes are 25% off!"

I went to their web site and it seems overloaded. Searches are very slow and "no channel available" error messages come often. I guess we wiped out their entire supply of 6BQ6's because they still don't have any. Fortunately the error messages exceeded my lack of patience, which saved me some money......for now.
 
#252
I really need to buy is time to mess with the ones I already have
Click to expand...

Yeah, I googled that one and came up empty handed. I still have a tray full of tubes from last years sale and the recent dollar sale that have not been opened yet. I did look up a few numbers that are on my "curiosity list" and not one of them showed up. The error messages helped me control my tube habit for now.
 
#254
At this late date, I've finally gotten around to using some of the tubes I so greedily glommed from AES during the multiple waves of feeding frenzy. I dropped a pair of the $2 6CB5A sweep tubes that were highlighted by George during the first sale into my "Miz Piggy" SE screen driven amp to replace a pair of 6CD6G that looked very cool and retro but might have been getting a bit too hot for their own good. I was careful in that case to select RCA and Raytheon tubes that had large plates with fins, not like older incarnations of the tube that used two parallel cathodes and a short, stubby plate with vestigal fins. At any rate, the 6CD6Gs played very nicely, but got pretty hot (no glow, though).

The 6CB5As are a drop-in replacement. They biased up at 55 mA each with no fuss whatsover, and also sounded quite nice to boot. I may try the 6CD6Gs again with lower bias current, as the old bottles give the amp a cool retro-chic look.
 
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