Author Topic: Re-Tubing a Mesa 400+ (Read 581 times)

adriaan · « **Reply #15 on:** May 05, 2004, 01:32:07 AM »

My English maths expressions are not up to scratch, and I know there are others who know much better than I do, but this is what I remember from my schooldays:

Loads wired in series add up, like (R1 + R2).

Loads wired in parallel go like (1/R1 + 1/R2) and then you turn the resulting fracture on its head and you have the end result. Loads wired in series, as a branch of a parallel load, are regarded as a single load.

If you put the 4 ohm cabs in series, and wire that in parallel with the 8 ohm cab, you get:
1/(4+4) + 1/8 = 1/8 + 1/8 = 2/8, make that 8/2 - that's 4 ohms.

If you put the 3 cabs in parallel, you get
1/4 + 1/4 + 1/8 = 2/8 + 2/8 + 1/8 = 5/8, make that 8/5 - that would be 1.6 ohm, not 8 ohm.

Hollis is right that Mike's formula (R1xR2xR3)/(R1+R2+R3) translates to (8x4x4)/(8+4+4) = 128/16 = 8 ohm - but I just don't know if that's the appropriate formula.

son_of_magni · « **Reply #16 on:** May 05, 2004, 10:05:16 AM »

adriaan is exactly correct...

Loads wired in series add up, like (R1 + R2).

Loads wired in parallel go like (1/R1 + 1/R2) and then you turn the resulting fracture on its head and you have the end result.

To be more clear 1/(1/R1 + 1/R2 + 1/R3...) = n
where n is the resulting parallel value.

For example if you have two 4 ohm and one 8 ohm speaker:

1/(1/4 + 1/4 + 1/8) =
1/(.25 + .25 + .125) =
1/.625 =
1.6 ohms

(Message edited by son_of_magni on May 05, 2004)

hollis · « **Reply #17 on:** May 05, 2004, 02:40:46 PM »

Ummmm.... hmmmm...... I'm sooo confused!

adriaan · « **Reply #18 on:** May 06, 2004, 02:20:36 AM »

It's better to be confused than to have your speakers fried or your amp up in flames of glory, is it not?

hollis · « **Reply #19 on:** May 06, 2004, 10:53:08 AM »

That is something that I am fully upright and at attention about. Yessirree, you are correct sir!

Thanks for the detour sign along my thought's highway....

wcv · « **Reply #20 on:** May 06, 2004, 12:42:34 PM »

I used to think ohm as a mantra...not something to cause a headache...although I do feel, some how enlightend...ohm...

I get it, I think...at least I should be able to prevent an unexpected pyrotechnic display and cancellation of a gig.

Bill

adriaan · « **Reply #21 on:** May 06, 2004, 01:43:00 PM »

Hollis,

When in doubt, skip my contributions. It works for me too.

adriaan · « **Reply #22 on:** May 07, 2004, 07:24:47 AM »

Beware, this is the voice of inexperience speaking.

The above formulas are for wiring up resistors in parallel and in series, and any combination thereof, but only in a laboratory situation with everything hooked up to a single power outlet.

The same principles apply to connecting speakers to an amp, except that if the amp has multiple outlets then they are probably wired in parallel, and when you connect cabs to separate outlets then they are included in separate branches of the parallel circuit.

In any case, any halfway decent amp will have markings telling you the minimum and maximum load per outlet and/or channel, and probably also the minimum total when using more than one outlet.

If you start daisy-chaining your cabs, then you'll have to check the documentation of the cabs if that creates a parallel or a series circuit, then do the maths to see what load it creates for the amp.

basso · « **Reply #23 on:** May 07, 2004, 01:27:23 PM »

I used svetlana russian tubes when i had my boogie, both for preamp and power amp,the first time i replaced them i bought the boogie brand,and then found out that they are indeed svetlana,they just put their name on them,the tubes did have made in russia printed on them!i didn't have any problems at all with them,hope this helps.

wcv · « **Reply #24 on:** May 07, 2004, 04:13:11 PM »

Adrian,

Thanks for the heads up on which formula to use and the bit about the wiring of the cabinets. I'm not sure it would have occured to me, to check if the in/out of the cabinets wired series or parallel. This gives me a good start, and with a little patience, I think I can work the math out.

wcv · « **Reply #25 on:** May 07, 2004, 04:30:41 PM »

Adriaan, sorry about the typo...I'm quite new to this, is there a way to edit a post once it is up?
Bill

Never mind, I found it.

(Message edited by wcv on May 07, 2004)

wcv · « **Reply #26 on:** May 07, 2004, 04:38:45 PM »

basso,
Thank you. From the responses to my original post about tubes, it looks like a shoot out between Svetlanas and JAN-Phillips. With an edge to the Svetlanas, but the review of the Phillips tubes sounds interesting.

hmmm.....

hollis · « **Reply #27 on:** May 07, 2004, 04:41:10 PM »

Bill,
Click on the pad and pencil in the upper right of your post to edit.

adriaan · « **Reply #28 on:** May 08, 2004, 04:23:56 PM »

Hey Bill (wcv),

You must be the first American to even notice there's three A's in Adriaan - you score big time! Others have been known to think I'm a female of the species - but no, I'm just (not) funny that way.

I've owned two tube amps. One had no less than 2 EL84 tubes in the end stage, not sure about the brand though - may well have been Svetlana - and they put out about 30 watts of pure tone into an 8 ohm 80 watts full-range speaker (Fane Disco 80) in an undersized cab. If it was anything, it was not loud. But it had the sweetest highs - no sizzles! I've been told the high end purity was due mostly to having a relatively high-end end transformer (Unitran) coupling the amp's end stage to the speaker. The rest of the amp was - spartan, for want of a better word.

The other tube amp was a Marshall 100 watts, presumably a late 1970s model. It was loud enough for what I was playing at the time, but don't ask about the tone. Though it could have been the 15 EV ruining the tone, I don't (care to) know.