ECL84 instead 6GW8?

[tubenit]

I've gotten ahold of a Hammond AO-44 that uses  6GW8 tubes.   However, I don't have any 6WG8 tubes ....... BUT I do have some ECL84 tubes.  They both are rated for 300v on the power tubes.  Looks to me like they have about the same current draw?   However, I'm not the best at reading tube data?


TDSL Tube data [ECL84]


TDSL Tube data [6GW8]


Yrs ago, when I built the SoLow Watt using ECL84's, it turned out nice ......... so I'm thinking of using those instead and do a build of Sluckey's  amp conversion.

PeeWee Hammond (AO-44 conversion)




Other than rewiring the 9 pin socket, do you see any concerns switching to ECL84's?


With respect, Tubenit

[SEL49]

This pin assignment diagram may be easier on the eyes. Also consider the 6BM8.

[HotBluePlates]

... I don't have any 6WG8 tubes ....... BUT I do have some ECL84 tubes.
...
TDSL Tube data [ECL84]

TDSL Tube data [6GW8]
...
Other than rewiring the 9 pin socket, do you see any concerns switching to ECL84's?

I greatly dislike "TDSL tube data" because it's very incomplete & fails to highlight Significant Issues.  I use Frank's instead to search for original data sheets.

ECL86

PCL84 ---> Different heater voltage than ECL84; the ECL84 refers the user to the PCL84 sheet for complete ratings, curves, etc.

   1. Plate Dissipation:  "Limiting Values" are on Page 3 of the PCL84 sheet and Page 5 of the ECL86 sheet.  The ECL86 is rated for 9w plate dissipation, but the ECL86 is only rated for 4w.  This affects the correct output transformer primary impedance to use, and also the idle bias (which is the less-important factor here).

   2. Amplification Factor of the Triode Section:  Page 1 of the ECL84 data sheet shows the triode has a Mu of 65, and "S" (transconductance) of 4mA per volt (aka "4000 micromhos") when plate current is 3mA.  Where the ECL86 has a "12AX7 section" as its triode, the ECL84 is more like a "12AT7 section" (see tope of Page 2 of the 12AT7 data sheet, condition on the left with 100v on the plate).


I'd recommend sticking with the ECL86/6GW8.  I do see the price is way up at retail tube vendors; perhaps a high positive-feedback eBay seller might be a way to go?

[tubenit]

HBP, 


I always appreciate your weighing in on these issues!  Thanks!   I'm OK with less wattage and less gain in the triodes of
the ECL84. That would suite my goal just fine. 


I'm aiming for an inexpensive low wattage push/pull about the size of a VibroChamp.  In this amp, I'd actually prefer the lower wattage and a cleaner tone.


I guess my concern would be the output transformer?  I remember reading Kevin O'Connor's comments about OT impedence.
 

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.
A: It is awfully expensive, and it is also a ridiculous suggestion. There are two issues here, though; one is the notion of “impedance matching”, and the other is simple design preference.
As stated throughout our TUT book series, speaker load impedances and reflected loads to the output tubes are all “nominal”. An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.
A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the “power bandwidth”. If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

So my question is there any concern about damaging the OT or the power tubes with the mismatch impedence in the OT?  If not, I may experiment and see how this works out. 
I can always rewire the 9 pins for 6GW8 if I discover I need to go that route.  I understand it's less than ideal with the mismatch but for my purposes it might be just fine?   Unless there is a significant concern or caution, I don't mind experimenting with this.

A pair of 6GW8's are gonna cost almost as much as I paid for the Hammond AO-44.  :-)

With respect, Tubenit

[tubenit]

The more I'm thinking about this project, if the OT is an issue .............. it might make more sense for me to replace the OT to have something better suited for ECL84's?   


Again, I prefer the lower wattage and possibly cleaner tone that the ECL84's might offer me?  I was already considering using a 12AY7 in the V1 position that Sluckey added to his PeeWee.


If I needed to replace the OT, what might be a "better" OT transformer for the ECL84's than the OT that is on the AO-44?


With respect, Tubenit

[tubeswell]

...OT transformer for the ECL84's ...

Simply what I'd do is work out optimum load resistance for centre bias Class A, and then 'translate' that to PP.


I don't know what power supply parameters you're thinking of so I'm just gonna assume a nominal B+ voltage of (say) 320V and plug in into a formula for load resistance i.e.: Zout = Va/(Pa/Va) where Va is idle voltage on the plate, and Pa is maximum plate dissipation rating


We can see from the data on Frank's site that Pmax for an ECL84 pentode section is 4W, therefore:


Zout = 320/(4/320) = 25,600R


So that translates to a push-pull OT (with speaker plugged in) giving a plate-to-plate load resistance of 25.6k. i.e., if you find an OT with a 2,000:1 Pr:Sec impedance ratio and plug 12 ohms worth of speaker impedance (or 3 x 4R speakers in series) on that, you'd be in the ballpark.


Or if your B+ was more like 250V, then you'd be looking for about 16k plate-to-plate load resistance - which is a 1000:1* OT ratio with a 16R speaker plugged in, or a 2000:1 OT ratio with a 8R speaker plugged in.

*1,000:1 is a bog standard PP OT ratio for OTs in lots of amps needing 8k from an 8R speaker.


and an OT max power handling of (say) 5-10W.

[HotBluePlates]

I'm aiming for an inexpensive low wattage push/pull about the size of a VibroChamp.  In this amp, I'd actually prefer the lower wattage and a cleaner tone.

I guess my concern would be the output transformer?  ... So my question is there any concern about damaging the OT or the power tubes with the mismatch impedence in the OT?

I went looking further at the PCL84 data sheet and noticed a Cathode Current Limit of 40mA average, and the need to keep screen volts to 250v or less.  I also reached some of the same conclusions that Tubeswell pointed out.

All that made me wonder if it would take too many changes to the power supply to force ECL84s to work.  I don't think the OT would be in danger, as much as maybe the ECL84s might redplate.

SEL49 has a good idea with seeing whether 6BM8 looks like it might work (you certainly have more experience with those than I do).

[tubenit]

Gentlemen,

I'm grateful for the information and advice!  Thank you!  It's becoming clear that I didn't understand the tube data enough to head the direction I was hoping.

Keeping with my goal of low watt, inexpensive and a small cab ................. I can consider  a SE Champ like design.  I can punch it out for a 6V6 or even use a 6K6. I can still use the PT and the chassis and head that direction.  I can pick up a Champ OT for around $30

OR go with the 6GW8?

Appreciate the knowledge you guys are willing to share.   With respect, Tubenit

[tubeswell]

If you're doing SE ECL84 pentode for the output stage, then I'd keep the B+ voltage to 250 and use a standard champ 5-10W single-ended OT (i.e. with a 1000:1 impedance ratio) and a 16R speaker.


Otherwise what HPB said

[tubenit]

I think (?) I have enough parts lying around to build this with only having to buy an output transformer that's SE for a champ.

Transformer - Output, 5W, 5k & 8k Primary Options, Upgrade for Fender® Champ | Antique Electronic Supply

So, I'll head this direction.  The 6GW8 tubes are more expensive than I'd want to spend and I think they are no longer manufactured.  So, this approach uses tubes that are still being manufactured and that I'm likely to have around for my other amps.

Again, thanks guys for the help on this!  Grateful for your generous sharing of knowledge.


With respect, Tubenit

[tubeswell]

Note that to get a B+ of 250, a PT secondary winding of 250-0-250 will be too high.

Assuming a cathode bias voltage around about 4V (from the datasheet) and plate to cathode voltage of 246, then plate current will be about 16mA. Add about 3mA for screen current and that’s 19mA cathode current. 4V/19mA is 210R cathode resistor (so try 220R to start with)

[bmccowan]

For an ECL84 amp I built - with Tubenit's help, I used a 195-0-195 PT and a 5Y3 to get a B+ of 225.
BTW, I like SEL49's suggestion of rewiring for 6BM8s. Somebody named Sluckey suggested the same a few years back when I picked up an AO-44 You love that tube, Tubenit - just looking for something different? My AO-44 project is still in my crowded waiting room.

[DummyLoad]

Since it seems like you're not opposed to using odd-ball tubes, may I suggest that you try a pair 6T9? - Made for audio, yes, they are compactrons, slightly more power handling than the 6BM8 and 6GW8. I sent you a PM where to source from. Also, they make a great one tube reverb. 

6T9 is basically a 6V6GT and 1/2 12AX7 in one envelope. Use your existing AO-44 OT & PT just change the sockets. I've been buying them for about $15 each. Compactron sockets use same hole dims as most octal socket - Get those off eBay. 

6T9 Dalasheet

--Pete

[tubenit]

That's an interesting tube and a great idea! 


Thank you!   With respect, Jeff