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Old 5th March 2011, 11:25 PM   #1
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Default 5e7 bandmaster build q's

hey folks
I have a partially finished project i picked up for good money a while back, I never touched it before now it just sat in a box but i figure having all the parts here it will be good excersise to take on my vox build so i started at it a few nights ago.
Its pretty much a tweed bandmaster 2x 10 cab and chassis, mojotone irons and all required compnents to a degree
Having researched the part numbers i see it is actually a blackface bassman OT (2250ohms) a bassman PT ~(358v , 53v , 3.15v and 5AC) - the choke is the higher rated correct bandmaster type though.
I am going to go by the ceriatone layout as the person i bought it off said they bought the parts with that in mind so i figure why not.

I am also going to try and implement dougs grounding layout at el34world instead of the ceriatone one as it has been know to be quieter.


1 - The three main filter caps are rated at 16uf - I only have 20uf caps to go here - is that ok? Also in implementing doug's grounding layout i cannot see which of the filter caps should be grounded to pots buss wire and which to PT bolt - My apologies but i cannot differentiate between them - looking at it i guess the first 16uf sharing its ground with the 100uf bias cap ground should be grounded to PT Bolt? the second two parrelled 16uf Caps should go to the pots Buss wire ground as should the last filter stage the 8uf cap? Again i am worried about fact i am substituting 3 20uf caps instead of the 16uf's?

2 - the usuall 5e7 tweed bandmaster output trans is rated at 3000ohms with 4 and 8 ohm outoput taps. The one i have is a blackface bassman output transformer rated at 2250 ohms with 2,4 and 8 ohm outputs. I'm not worried about wiring the outs but i wonder should anything in the circuit be changed due to the 750ohm difference in what i "should" be using.

3 -should the ground for the presence pot be isolated on its own?

4 - should there be a 470r resistor from pin 4 to 6?

i literally have all the parts here in front of me - i have most of the eyelet board and fly offs done and i have the pots and input jacks soldered up too, minus a few grounds. excited about wrapping this up and doing my vox build!

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Old 6th March 2011, 02:15 AM   #2
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Well, this looks like an interesting project. I'll answer your questions and give some thoughts.

1) No worries at all regarding 20uf capacitors over 16uf. They're very close and even 30uf would be ok. Just so long as they're not lower or very much higher. Especially for the input (first) cap.

2) Using a 2250 ohm impedance transformer instead of the original 3000 ohm unit will result in the 6L6 output tubes running a little hotter with a little more distortion. It's really lower then 6L6s like to work into. There isn't too much you can to about that except for lowering the screen voltage. This will back off on the power through the tubes at the expense of lower power output. But they will last a bit longer too. An equitable tradeoff? That's up to you. Of course you could try running an 8 ohm speaker on the 4 ohm tap. That would bring the impedance up to 4500 ohms.

3) Isolate this ground? No, not necessarly. Looking at the drawing, it appears that it was grounded there for convenience since it's close enough to the central (star) ground point. Otherwise a tie point would be needed for the capacitor lead to ground.

4) The 1500 ohm resistor (or 470) shown there isn't doing anything. Pin 6 on a 6L6 is an unused pin. This is supposed to be a grid stopper resistor. The wire going to pin 5 should really connect to pin 6 to have it act as such. Either value won't make much difference because the 12AX7 driver tube probably won't be able to force an AB2 condition.

I went to EL34 World to see what they were doing with for their ground buss. A single point star ground is a very good way to assure low hum levels. It's tried and true and will work very well when done correctly. And it should be remembered that a proper ground buss is really an extention of that single point system. I've used it for neatness and convenience in making grounds. But if implemented incorrectly, it can do more harm then good.

Looking at Doug's layout I see something that I question. That's running the buss soldered to all the pots and also to the main chassis point near the transformer. If the pot covers are themselves grounded by their mounting hardware, then the single point ground is being defeated. If they are not, then it's a different matter. His way may work fine. If not, you can always wire things as shown. It isn't that much of a change.

If I were building this amp, I'd connect the grounds as pictured. All to a single point. I might move this point more to the middle of the chassis if there was a low residual hum. Sometimes just moving it will cure that. If I found that was not to my liking, then I'd redo it to the buss. But that's just me.

I also notice that there is a ground switch not wired in the diagram. This usually has a small capacitor from ground to one side of the AC line or the other via the switch. This really make a difference in hum level most of the time.
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Last edited by HollowState; 6th March 2011 at 02:23 AM.
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