Bandmaster questions

[bluesbear]

First, Hi, all! I'm still dealing with the same issues with my wife's illness as I was 5 years ago. I guess that's a good thing, considering the alternative! I've been fiddling around with stuff but now it's time to actually finish some projects.


I have an AB763 Bandmaster chassis w/transformers. I made a combo cab for it a few years ago (no cabinet). I have 2 questions before I can proceed. I'm adding a 5 volt recto tranny and I'm trying to place the tube. I don't know if they all did, but this Bandmaster has 4 holes for power tubes. If I had to guess, I'd say it was also used for the Showman. 2 are covered with little plates. The sockets are about 2" center to center. If I put the rectifier tube in the first and 6V6's in the 3rd and 4th, will the 6V6's be too close together? I guess another way to put it would be: how far apart do 6V6's need to be in a circuit?


The OT is wimpy for 6L6's but it ought to be monster for 6V6's. I know it would work for 6V6's into an 8 ohm load but I'm trying to understand the math. Since the tube to speaker ratio is 4200 to 4 for 6L6's, does that mean it would be 6600 to just under 6.3 for 6V6's? If I have that right, I've got it. If not, more work is required!


Thanks to all!!!
Dave

[Ed_Chambley]

Haven't seen you post in a while.  Good to see ya!


If you have a winding of 4.2k to 4 for your Plate load on 6L6 if you change the secondary to an 8 ohm the ratio would become 8.4K.  Should be fine to use for 6V6 since I have used 6.6K to 10K primaries on 6V6 tubes.  It has to do with how much voltage you will be putting on the 6v6.


Not sure what you are asking about the spacing of tubes tho.  If you are planning a tube rectifier and 2 6V6 tubes you will be fine, actually you should have more room than necessary.  I have used a 4 hole Fender chassis and I just put a cover on the last octal hole and used the 3 left.

[bluesbear]

That's perfect! Thanks for your help… and I'm glad to be back.
As far as the tube spacing, I've never put tubes that close together. It makes me nervous!!
Thanks again,

Dave

[John]

Hey,  nice to see you back! Best luck and wishes for you and your wife!

[drgonzonm]

Check the data sheets, and the RCA tube books, as there are some recommendations on minimum tube distances. (The data is hard to find).   If necessary. place a vertical sheet of aluminum between the rectifier and the 6V6s.  This will cut down on the radiant heat transfer. 
Also, check the data sheets for maximum surface temperature,  Some tubes have a max temp of 250 C while others have a max temp of 200 C. 
If max temp is 250 C, you need more distance

For 6v6 tubes, I have seen recommendations like 1 tube diameter and 1-1/2 diameter, but these are builders recommendations, not tube manufacturers. 

And make sure your enclosure is well ventilated. 

[pompeiisneaks]

You also mentioned there were 4 holes, Just put the rectifier in one hole, skip the next and then put the other tubes in the last two (if they're not there already) that should likely give you enough distance.  (or hopefully, if I'm understanding what you're asking).

~Phil

[drgonzonm]

I looked at a KT88 spec sheet.  It recommended a minimum of 4" center to center distance between tubes, considering that the tube is 54mm in diameter, the actual spacing is from tube edge to tube edge is about 2" or about 1 tube diameter.  The tube also has a surface temp of 250 C. 

I hope this is helpful

[sluckey]

Really has nothing to do with this thread.

[drgonzonm]

Really has nothing to do with this thread.

   
The question was "is 2 inch on center too close to mount a rectifier, and two 6v6 tubes".  Data on other tubes, is relevant, as it shows how one manufacturer states minimum distance while providing other relevant data as tube surface temperature.    The data also puts quantitative information to a previous post. 

After looking up the RCA data on a 6v6, I would say the holes are too close.  (maximum tube diameter is 1-9/16.)  It means the tubes will be really tight.   If there is 2" of metal between holes, then distance is probably not relevant. 

[sluckey]

The question was "is 2 inch on center too close to mount a rectifier, and two 6v6 tubes

So what does KT88 data have to do with any of this?

The sockets are about 2" center to center. If I put the rectifier tube in the first and 6V6's in the 3rd and 4th, will the 6V6's be too close together?

No, they will be fine. I built a 6V6 plexi about 1.5 years ago. My sockets are on 1-3/4" centers and are fine. Heat is not an issue. Besides, Fender built that amp to also hold 4 6L6s side by side.

[drgonzonm]

The question was "is 2 inch on center too close to mount a rectifier, and two 6v6 tubes

So what does KT88 data have to do with any of this?

See post #5, Manufacturer recommendations are hard to come by.  KT data addressed both tube surface temperature and minimum distance.

I will take manufacturer's recommendations over builder's recommendations including Leo,  accountants are just too influential.

When you are the builder, you live with the consequences of the compromises. 

[sluckey]

Well, since the OP wants to put 6V6s in this amp none of your KT88 data has any relevance whatsoever. Just more of your useless library bull...

[drgonzonm]

Well, since the OP wants to put 6V6s in this amp none of your KT88 data has any relevance whatsoever. Just more of your useless library bull...

[bluesbear]

These are closer than anyone has mentioned, less than an inch apart. The question (rhetorical) is, why did they drill them at the factory, then cover 2 of them? Doesn't make a lot of sense!
I guess if I'm gonna do this, I'll have to open up the phase inverter hole. We'll see.
Thanks, all!
Dave

[sluckey]

The question (rhetorical) is, why did they drill them at the factory, then cover 2 of them? Doesn't make a lot of sense!

That chassis was likely used for more than just a Bandmaster amp. Probably a 4x6L6 twin also.

I guess if I'm gonna do this, I'll have to open up the phase inverter hole. We'll see.

Why? You had all the answers in your original post?

The pic I posted only has 3/4" between sockets. There's no problem using 5AR4 and two 6V6s. Well, I obviously can't use old coke bottle style tubes. 

[mresistor]

bluesbear - i recently built a single channel dr and it only has 1/2" between big bottle sockets - no problems at all 

[PRR]

On convection issues, I would not go below 3/8" between surfaces.

1/2" between tubes seems fine to me.

Several hot bottles 1/2" apart will make that area of the chassis a hot-spot.

Also there's more parts than tubes, and stuffing too many parts in too small space is a bad idea. For building but especially for checking(!) and debugging.

[bluesbear]

Well, it seems I'll have to give it a try, as soon as I'm done with the one I'm working on and restoring an old Crosley for my wife. Should be fun... Bakelite!
Thanks,
Dave

[PRR]

> restoring an old Crosley for my wife.

??

[mresistor]

That car will raise some eyebrows...  :-)

[shooter]

Reply #18

Your grocery getter?

[PRR]

Crosley also made radios, refrigerators, and other stuff.

But my mind jumped to the car because they were so quirky.

(First OHC in a production US car.)