ideas for diy tube adapters?

[rayandkerry]

Hey, all.

I just received four 9-pin bases (P/N# CON6-W-9A) and four octal bases (P/N# CON34-BKR) from here:
http://www.apexelectronic.com/connectors.htm

I was planning on making some DIY tube adapters.

I've heard that the 6SL7 octal preamp is pretty similar to a 12ax7 and sound great.

Any other ideas on what would be some good subs for 12ax7s? How about EL84 substitution ideas...

All suggestions welcome.

Thanks!

[krx]

Sorry to rain on your parade, but this is really not a very good idea for a variety of reasons:

1. An octal socket is much larger than a noval.
2. Different tubes have different pinouts, so you can't make a universal adapter.
3. Different tubes have different operating points and usually won't sound good (or even run correctly in the case of power tubes) when simply popped into the same circuit.
4. Tubes get hot and amps vibrate -- whatever you use to connect the bases needs to be able to withstand that.

[guitarmike2107]

Well you could put flying leads with tube pins soldered on the end on an octal base and use it to try different pre amp tubes, or build a board with flying leads coming off it to connect to a noval socket, but you need to know where each lead has to go and adjust if necessary.. If you wanted to changed to a specific tube long term either bolt the board assy in place or do a proper job of replacing the socket.

Most things are possible.. but might not be practical.

Mike

[rayandkerry]

Just a point of clarity...I am not trying to make some kind of universal, configurable adapters. I was thinking of these as being single-purpose, hardwired adapters. Also, I would make them strong enough to withstand shaking (although I only play at home and all my amps will all be heads...no combos).

At any rate, the bases were only $3...so I bought them on a whim and thought I'd wire them up to a different type of socket (i.e 9-pin t octal, octal to 9-pin, 9-pin to 7-pin, etc.).

Here's a link to what someone else did for a DIY adapter (although I would make mine more robust):
http://diyaudioprojects.com/Gallery/dis ... play_media

Also, I have seen several versions available for purchase. Here's one that's more robust (although I would do the reverse so I could plug an octal 6SL7 into a 12ax7 socket):
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

So...I understand that there are many tubes that have the same pinout but different operating characteristics. These are not the type of options I am looking for.

Rather, I have heard that there are some tubes that have essentially the same operating characteristics except that go into different sockets, these are the types that I would want to make...just to expand on my tube rolling possibilities.

[krx]

So...I understand that there are many tubes that have the same pinout but different operating characteristics. These are not the type of options I am looking for.

Well forget about swapping power tubes then. The only substitute for an EL84 is an EL84. For example, a 6V6 requires a different bias resistor.

Regarding preamp tubes, in a guitar amp the octals basically sound the same as the equivalent novals. They also tend to be quite a bit more expensive since hifi guys buy them. All the dual triodes also have less gain than a 12AX7. And there's no 7-pin dual triodes.

Besides the 6SL7 (which tends to be microphonic), what else were you hoping to try?

[rayandkerry]

Besides the 6SL7 (which tends to be microphonic), what else were you hoping to try?

Hey...thanks for the reply!

As to your question, I am not really sure what I am hoping to try...just looking for possibilities.

In terms of microphonics, I only play at home and have a head amp. The head is usually a good 25 feet from the speaker so I am probably not as worried about it as anormal player.

Also, in terms of power tubes, I think I read that a 6AQ5 is a 7-pin version of a 6V6 (and I happen to have have a single-ended 6V6 amp). Here's a link to an adapter for that allows you to use a 6AQ5 in a 6V6 socket:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

So...I don't really have a direction on this other than possibly making an adapter for using a 6SL7 (octal tube) in a 12ax7 socket and an adapter for using a 6AQ5 (7-pin tube, I think) in a 6V6 socket.

Also, there's a thread on this site about rewiring an EL84 socket to use an EL83...I could probably make an adapter for that. Here's that thread:
http://www.18watt.com/modules.php?name= ... light=el83

So any like ideas are welcome...I just know a little bit about the "standard" tubes used in today's guitar amps (e.g. 12A_7 types, el84, el34, 6v6, 6L6, etc.). I was hoping some of the more experienced folks around here could offer some other ideas off the beaten track that might let me play around with some different tubes...

I am building my first 18 watter now and also have the single ended amp I mentioned above that has a single 12AX7 preamp tube will take a single octal tube like a 6V6, 6K6, 6L6, EL34, KT66, KT77, KT88, etc. in the power section.

So, if there are any other preamp tube equivalents for a 12ax7 or any other power tube equivalents for an el84 for any of the octal power tubes I mention above (whether they have the same socket with a different pinout or an entirely different socket), I would love to hear about it.

Thanks!

[krx]

There's a ton of different cheap tubes out there that can work well in a guitar amp. 7-pin tubes are especially cheap as are some of the tubes with weird heater voltages (e.g. NOS 5V6 tubes go for about $3-4 and are identical to 6V6s except for the heaters).

Consider your single-ended amp. Sure you can plug in a 6V6, KT66, EL34, whatever. But the circuit was actually designed around only one of those (probably the 6V6). To sound its best, each tube should have one or more of the following changed: cathode resistor, load impedance, plate voltage, grid reference, screen grid resistor, driver stage, etc.

Same thing with preamp tubes. Sure, you can plug a 12AT7 into a 12AX7 slot and it will work but it won't be operating ideally and thus won't sound as good.

So you could make an adapter but really you need to do more than just hook the pins to the right spots to make using a different tube worthwhile.

[Dana-L]

Here's an adapter I made which allows a 6AQ5 to be used in place of a 6BQ5:

-Dana

[krx]

Here's an adapter I made which allows a 6AQ5 to be used in place of a 6BQ5:

-Dana

Looks pretty solid. Did you change the circuit too? The 6AQ5 is rated for 250V max and needs a different bias resistor. I know they're cheap, but you'd burn through them pretty quick in a 6BQ5 circuit.

[rayandkerry]

Here's an adapter I made which allows a 6AQ5 to be used in place of a 6BQ5:

-Dana

Cool. How did you secure 9-pin the base to the 7-pin socket?

[Chappy]

I salvaged some old relays from an old military flight simulator. They use octal pins, and have a clear plastic covering. I removed the guts of the relays which provides a nice empty enclosure with an octal base. Enclosure keeps all of the electrical connections safely covered Here is what I have made so far

Bias adjust circuit:
drilled hole in top and mounted another octal socket. wired it up so i can put my digital fluke ammeter in series in order to measure cathode current flow. Meter leads attach to a few screws i put in the side which act as terminals.

Solid state rectifier:
wired up one to replace rectifier tubes with a diode rectifier bridge. sure beats that copper can rectifier thing from you know who.

6SL7 to 12AX7 coverter:
drilled holes in the top of a few and mounted 9 pin sockets. now i can use 9 pin 12a*7 series tubes on my 6sl7 homebrew. Still experimenting to see if I can hear a difference proving the "octal fatness" claims which have read about on the net.

As for using a 6aq5 in place of a 6v6 i did just that. I built a SE amp with sockets for either one, as well as a 6BQ5. Separate cathode resistors for each tube. I can switch in different dropping resistors in the B+ to give me different voltages for the output section as well. It works real nice and I can't tell the difference in sound between the 6aq5 and 6v6 on either of the two different B+ settings. I got the idea from the "chamelion" schematic which IIRC I pulled of the AX84 website a while back.

Chappy

[rayandkerry]

Hey, Chappy.

Thanks for the ideas. I had thought of making an adapter at some point that would let me measure voltages without opening up the chassis, but hadn't thought of the SS recitifier (although I have seen the product you refer to). Thanks for the idea.

As to the 6SL7 to 12AX7 converter, do you just have wires maping the octal pins to corresponding parts of the 9-pin socket?

Thanks!

[zaphod_phil]

I've heard that the 6SL7 octal preamp is pretty similar to a 12ax7 and sound great.

A 6SL7 isn't particularly similar to a 12AX7. Gain-wise it's closer to a 12AT7. So it's not particularly suited for use in a Classic/Trem 18W or Lite II. However 12AT7s can generate some unpleasantly muddy sounding high-odd harmonics when overdriven (as I discovered), which hopefully 6SL7s wouldn't. And it is true that 6SL7s are said to have good tone. However, they can suffer from microphony issues and may need DC heaters.

You can also use an 6SJ7 as a substitute for an EF86 in something like an EF86 Lite 18W, say. However, you would either need to use a 470k grid reference resistor on the input grid, instead of the usual 1M, or else a 0.1uF DC blocking cap at the input jack. Otherwise you will get scratchy guitar volume pot issues.

[Phil_S]

I don't know that much, but I'd differ with Zaphod Phil a bit on the 6SL7. Its gain factor and plate resistance are pretty darn close to a noval 5751. If you bias it right, you might get it to work OK in an 18W build. I'd be concerned that raising Rk will result in a sound that isn't "authentic". Then again, maybe no one will notice. You really need to give it a try to find out for sure.

Consider this: many people turn to the 5751 in place of a 12AX7 because they want to clean up the tone. This is moving away from 18W territory, where, as someone here says, "the distortion IS the tone."

[zaphod_phil]

I actually think we're saying roughly the same thing, as 5751's gain is in about the same ballpark as a 12AT7's. I therefore wouldn't really say either of those was a particularly close substitute for a regular 12AX7.

[Chappy]

Hey, Chappy.

Thanks for the ideas. I had thought of making an adapter at some point that would let me measure voltages without opening up the chassis, but hadn't thought of the SS recitifier (although I have seen the product you refer to). Thanks for the idea.

As to the 6SL7 to 12AX7 converter, do you just have wires maping the octal pins to corresponding parts of the 9-pin socket?

Thanks!

Hi Rayandkerry

Yes that is exactly what I did for the converter. A short run of leads mapping from one set of pins to the other. Hardwired. All safely inside the clear plastic enclosure of the plastic relay cover. The only possible drawback to these covnverters is that they are about 1 3/4 tall. Not an issue for my homebuilt head but I guess they would look pretty ugly hanging upside down in a combo.

Chappy

[Chappy]

I've heard that the 6SL7 octal preamp is pretty similar to a 12ax7 and sound great.

However 12AT7s can generate some unpleasantly muddy sounding high-odd harmonics when overdriven (as I discovered), which hopefully 6SL7s wouldn't. And it is true that 6SL7s are said to have good tone. However, they can suffer from microphony issues and may need DC heaters.

I've heard these issues before about the 6SL7's but I have not experienced these problems personally. I made a head that uses 3 6SL7,s and it seems quiet enough to me. I have tried numerous used mil spec tubes in it and I haven't found one that is harmonic yet. I've also tried some NOS commercial tubes and they were fine too. I don't know, but maybe I just got lucky with what I have.

I actually built the 6SL7 to 12AX7 converter(s) to help me troubleshoot this amp. I wasn't happy with the overdriven sound and thought that maybe the octal tubes were the culprit. On intial experiments with the smaller bottles, I still get the ugly distortion so I don't think the 6SL7's are at fault.

When I get a chance (back home next month) I'll play with it some more and do some hum noise and distortion comparison tests and report back.

Cheers
Chappy

[rayandkerry]

Thanks, Chappy.

I think my first try will be the 12ax7 to 6SL7 adapter!

[Dana-L]

Here's an adapter I made which allows a 6AQ5 to be used in place of a 6BQ5:

-Dana

Looks pretty solid. Did you change the circuit too? The 6AQ5 is rated for 250V max and needs a different bias resistor. I know they're cheap, but you'd burn through them pretty quick in a 6BQ5 circuit.

I built this to run a 6AQ5 in a modded Valve Junior. The plate voltage in mine is about 360 and I've not seen any failures. I made no changes to the circuit; my adapter simply connects the like pins.

To answer another question, I ran a machine screw up through the bottom of the male 9-pin section and into the center metal piece of the 7-pin female section to hold everything together.

Cheers,

-Dana

[rayandkerry]

To answer another question, I ran a machine screw up through the bottom of the male 9-pin section and into the center metal piece of the 7-pin female section to hold everything together.

Cheers,

-Dana

Thanks, Dana. That was kind of the last open question I had on construction.

If I can get some time this weekend, I think I'll try and build an adapter so I can see what a 6SL7 sounds like in my SE amp (which takes a 12ax7).

Also, while I understand the many comments about the tubes not performing at their best unless they are in a circuit designed specifically for them, I am by no means a purist and really enjoy playing around.

I recently tried a 12ay7, 12at7 and 5751 in my SE amp and it sounded good in many cases. It all depends on the combination of pickups (single coil versus humbucker) and output tubes I am using too (6V6, 6L6, KT-66, EL34, etc.). The given amp just has a volume and a tone control (no master volume or VVR options). So swapping out the one preamp tube kind of allows me to fine-tune the relationship between pre-amp grind and power tube saturation/distortion.

Once I get my 18Watt up and running, I am sure I'll have fun listening to different substitutions in the first pre-amp position.

At any rate, it's all fun and I'm learning what I like and don't like every time I try something new.

So thanks for the tips everyone...I'll be using them soon!

Ray