Name that Tube! Mystery mini-amp with 6AV6 and 6AQ5.

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skipwave

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Joined
Jul 8, 2005
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Location
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I picked up a bargain bin tube amp with a nice Hammond power transformer, dual secondary 120V 75mA and 6.3V 1.5A, and a little output transformer that says 2.5K:3.2 ohms (no manufacturer just "JAPAN").

The two tubes on it are unmarked. Both are 7 pin. Based on the pinout the output tube is a 6AQ5. I can't figure out the first tube, but it is probably a medium to high mu triode.

Pin 1 is grid, Pin 7 is plate, Pins 2/5/6 are cathode (and whatever else you connect to the cathode resistor in this funky tube). 3 and 4 heaters, 6.3VAC. In the mystery circuit I'm looking at the cathode resistor is 2k2 and the plate resistor is 220k.

Plate out goes through a .01uF capacitor to a 6AQ5 power stage. The B+ is 120VAC through a 4R7 resistor, then half-wave rectified for around 160VDC.

I'll keep scratching my head and searching the web.
 
prob a 6AV6, hi mu triode/dual diode. the triode is equivalent to one side of a 12AX7.
http://home.netcom.com/~wb2jia/tubes/6av6.pdf
its usual for the diode plates to be simply strapped to the cathode if theyre not being used.
 
I would agree with it being an 6AV6 or 6AT6, a slightly lower gain 6AV6. They are very easy tubes to pick out if they are there and the labels are worn off. You will have a cylindrical plate in the center with two support looking things on either side, these are the diodes. They are great sounding tubes and cheap.
 
Thanks! It's gotta be that. I couldn't find anything else with appropriate pinout. That's why I love this place. When you're faced with something that has you stumped and think, "I bet somebody on the forum knows." And they do! :thumb: :grin: :guinness: :guinness: :guinness:
 
Here's a quick schematic. There is an input pot that I omitted.

The interesting thing is the feedback arrangement around the plate/cathode/output transformer. The pot between the plate and B+ is not even wired as a rheostat, the ground lug is just unconnected.

They must have been connecting this to an 8ohm speaker to present a workable load to the 6AQ5.

6aq5amp.GIF


First order of business is to replace the two-prong power plug with a grounded cord, then that single diode with a fullwave diode bridge. Then that resistor on the AC side of the diode. There has to be a better way to accomplish the voltage drop across it. Does that also act as a filter to clean up the AC?
 
>>>Then that resistor on the AC side of the diode.

Be careful there. That resistor is there to limit the surge current to charge the C as well as filtering.

Does the amp hum or something now? If you like the way it sounds better leave well enough alone. More rectification will raise the B+ which might ruin the way it sounds now...

Uhh...where is the heater voltage? I think you have a problem with your power supply schem there...! Maybe just the way the transformer is labeled?

HTH!
Charlie
 
Oh, right. Yeah, I forgot to even draw the heaters and the separate winding. I just listed the full transformer spec on one winding. It's just 6.3VAC fed to heaters in series (EDIT: errr, parallel). :green:
 
Yes, parallel. I realized that slip after posting and didn't have time to correct it last night. I am just on a role with posting nonsense. :roll:
 
AC. I updated the schematic. Drawing a silly looking schematic in WinPaint is a fun way to kill time at the day job. :green:

Any thoughts on the feedback around the 6AQ5?
 
This amp had a dead 6AQ5, replaced it with a good one and played a bit. It sounds fantastic! Amazingly (to me anyway) it is dead quiet, though overall gain is not very high.

Now I'm going to pull more gain out of the 6AV6 stage and possibly add another stage before it.
 
you could first try to make stage one into grid leak, 5 or 10 meg from grid to ground, ground the cathode by simply jumpering it with a 10 ohm resistor, or piece of wire, this is sim[per than punching a new socket.
 
The grid leak options (in the middle range B+ where my amp is) show less gain in the Resistance Coupled Amp tables in the 6AV6 datasheet. There is definitely a little more gain to be had in common cathode, and Rk was unbypassed.

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AV6.pdf

It's unusual that it calls for a 100k grid resistor. I don't think I've seen another tube that wants one that low in value. The amp has 470k now.
 
It's unusual that it calls for a 100k grid resistor. I don't think I've seen another tube that wants one that low in value. The amp has 470k now.

Grid resistor just sets input impedance for the most part. These charts were designed with radios in mind, not guitar amps. Unless you go to Grid leak the grid resistor can be anything from 1k to 1M for most tubes.

You pretty much have all the gain you can get from that tube. You can squeeze out a couple more volts, but it is not going to change much. If you really want more gain switch the 6AV6 out for a 7pin pentode like a 6AU6 or 6BA6 or stick a booster in front of it.

Edit: You can also make sure it is a 6AV6, it could be an 6AT6, they look identical. Also a fresh 6AV6 might give you abit more gain, they are cheap tubes any, $2 or so. Have you measured the gain of the stage?
 
Thanks Adam. I'll have to try a new 6AV6 (no markings on the existing tube, so could be a 6AT6). I just installed a new Rp of 470k and Rk 4k7 (bypassed with a 47uF), and gain increased noticeably. The Rk should really be 3k8 to 4k2, though, B+ is 163V and the plate sits at 65V. Nice distortion sound, plenty for blues or punk rock, with a booster it goes into full on crunch.

I'm really pleased with the sound of this amp, and it's so quiet. Through a little 8" Jensen speaker it's perfect for recording. I'll try to post a sample when I get home later tonight.
 
I just installed a new Rp of 470k and Rk 4k7 (bypassed with a 47uF), and gain increased noticeably

Its odd no one noticed the lack of the bypass cap. That gives you another 6db plus a couple volts from the new Rp and Rk. We all over complicated it. This is making me want to dig my little 6AQ5 amp out of the box of random projects it ended up in.
 

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