Confusion about Power Transformer Supply for Filaments

[gheorge77]

So I'm working on a Matamp GT120 clone and I purchased a set of transformers from one of the original manufacturers in the UK.  On their data sheet it lists 2 sets for filament wiring.  The first is 3.15-0-3.15 @4.2A and the second is 3.15-0-3.15 @1.06A.  When I ordered the PT, the manufacturer stated that "the transformer can supply 4.2A across the 6.3v CT ie. 4.2A at each 3.15v leg."  The amp uses 4 EL34s (1.5A per tube) and 2 12AX7s (~150mA). Am I correct in believing that I can just use the filament wiring with the larger current supply?  I've attached the sheet from the manufacturer (albeit without the final description of having the 1.06A filament wiring with an added center tap). 

Also seems strange that the PT has a earth/screen wire...not something I've seen before.

Last q, should I be concerned at all about the bias tap of 0-100V @ 80mA?  Thank you for any advice/guidance!!

-Ian

[PRR]

> 4 EL34s (1.5A per tube) and 2 12AX7s (~150mA)

No, a 12AX7 wired at 6.3V is 300ma, 0.3A.

4 * 1.5A = 6A
2 * 0.3A = 0.6A
==============  6.6A

The EL34s alone sure seems to exceed the 4.2A rating of the PT. Even if you put the 12AX7 on the other winding.

Are you sure you got the right PT? For a four-EL34 amp, not a two-EL34 amp?

The bias winding is used in the plans I found, 80mA is more than ample.

[d95err]

The bigger winding can supply a total of 2x4.2A=8.4A at 3.15V. However as you need the double voltage (6.3V), you only get half the current, so 4.2A.

As the previous post said, this transformer cannot do 4xEL34.

[gheorge77]

This seems so weird. The manufacturer of this PT specifically attached it to the design for the GT120 (and has done so for years) as a 4 EL34 amp. I sent another email to them asking for additional clarification. Is there a way I could measure the total current supply prior to wiring the tubes?

[sluckey]

The only way to verify that PT is capable of supplying the needed filament current is to connect the filament winding to the amp's filament circuit. You can use gator clips to do this. Apply power and then measure the filament voltage under load. If the voltage is still 6.3VAC, let it run for a long time, periodically checking the voltage and temperature of the PT. This is about as scientific as you can get.

It will be interesting to hear what the manufacturer has to say about this. The info on that data sheet is conflicted. If the PT can only provide 4.2 amps then the PT is clearly not capable of powering that GT120 amp. OTOH, if the PT can be used in the GT120 amp, then the 4.2A spec is clearly wrong. What will they say???   

[gheorge77]

I'll be sure to update.  I am pretty sure it will be the 8.4A across 6.3V.  I am hoping that their original email, which stated that "the transformer can supply 4.2A across the 6.3v CT ie. 4.2A at each 3.15v leg," is a mistake.  Even if that was just for the 4 EL34s it would be in adequate.  It's basically the equivalent of a JCM800 PT based upon other PTs I looked at.  Both the Classic Tone and Hammond equivalent PTs offer about 7A on the filament wirings, which would be in line with the 8.4A. 

[kagliostro]

It will be interesting to hear what the manufacturer has to say about this.

OH, sorry, 4 x EL34 ...... we were wrong, we mean 4 x 6L6GT ............ (??????)

---

I expect something like that

Franco

[sluckey]

OH, sorry, 4 x EL34 ...... we were wrong, we mean 4 x 6L6GT ............ (??????)

No one has mentioned 6L6s in this thread.   

[PRR]

This what Franco *imagines* the vendor *might* say. "Humor".

(And sorry for touching your post-- I don't think I changed it.)

[sluckey]

Duh! Now I get it.   

[kagliostro]

    only a jest

--

4 x EL34 = 4 x 1.5A = 6A

4 x 6L6GT = 4 x 0.9A = 3.6A

so with a 4.2A filament current disposable the amp can be build using 6L6GT tubes

But I don't think this is in your plans (EL34 = 11mA/V - 6L6 = 6mA/V)

Franco

[gheorge77]

No definitely not.

I heard back from Pennine Radio, which has apparently manufactured these tranny sets for Matamp since the 70s:

"The 3.15 - 0 -3.15v is rated at 4A total ie. 3.15 @ 4A + 3.15 @ 4A totalling 6.3v @ 4A not 6.3v @ 8A

the second winding 3.15 - 0 -3.15v is rated at 1.06A total ie. 3.15 @ 1.06A + 3.15 @ 1.06A totalling 6.3v @ 1.06A not 6.3v @ 2.12A

You cannot run 4 x 1.5A tubes + 2 x 150mA tubes as this is more than the 4A available"

So now I need another PT. Going to go with the Classic Tone 40-18053, which has 7A available on the 6.3V tap and matches the HT more or less at 350VCT, compared with 340VCT on the Pennine.

This has me thinking now about the OT....its rated at 2.5k on the primary side with 0-4-8-16 on the secondary. Should I be concerned? I've tried researching on the significance of the primary side and am confused.  The Classic Tone equivalent OTs all run at either 1.7k or 2.2k....Any advice would be greatly appreciated.

-Ian

[jjasilli]

As an aside & FWIW:  this is the wrong analysis: "The 3.15 - 0 -3.15v is rated at 4A total ie. 3.15 @ 4A + 3.15 @ 4A totalling 6.3v @ 4A not 6.3v @ 8A


Each "side" of a center-tapped tranny is not rated separately for current handling capacity in the specs; the whole winding (both "sides") is so rated -- in this case 4A.  This is determined mainly by the gauge of the wire which makes up the winding.  Hence, each "side " of the wiring can handle that same amount of current -- they do not add together.  So, each side of this winding 3.15 - 0; and 0 - 3.15 can handle 4A; and the whole winding with no CT connection 0 - 6.3 can handle 4A.

[shooter]

rated at 2.5k on the primary side with 0-4-8-16 on the secondary. Should I be concerned

Wouldn't worry, once you add AC to the mix everything becomes dynamic, your speaker impedance changes with frequency enough
that 1.7K or 2.5K become ~~~ =

[PRR]

> the OT....its rated at 2.5k ...The Classic Tone equivalent OTs all run at either 1.7k or 2.2k....

For a *pair*, traditional loadings are 4k for 6L6, 3.4k for EL34. (However I know an amp with EL34 at 6.6k and high B+ which kicks butt.)

Transposed for *four* power tubes these would be 2k for 6L6, 1.7k for EL34 (to 3.3k at high B+).

Since there seems to be little information for GT120 (and maybe little factory consistency), I don't know. If the GT120 runs farly high B+ then 5k/pair 2.5k/quad may be right. OTOH ClassicTone claims "authentic" so their opinion should be considered.

And in a Real World it may not matter a whole lot. OTOH this is a *big* build and you like it to come out right.

[gheorge77]

The HT on this is supposed to be around 470, which really isn't that far off from the Plexi I built, which had a plate voltage around 465.

The GT120 is a real interesting amp. Cathodyne phase inverter, James tone stack, and a cool cap filter after the first gain stage. It's my most desired amp at this point. I can't get the Plexi clean enough for playing live and it doesn't really take fuzz pedals well for me. I'm also building a two channel Hoffman ab763 as a vibroverb with a Weber tone cvlt 15, which I'm excited to finish and hear. I can lost a schematic if anyone is interested. I have the Hilbish one but it's scaled for a giant sheet. I'm working on revising a good Orange GRO 100 schematic instead, which is the closest amp I've seen to the Matamp. The Orange OR120 is definitely different, with different caps in the filter, different location in connection with gain stages, a different filter cap arrangement, and different coupling caps.

[shooter]

found this on my drive
think they copied a bunch of amps n glued together 
was the rotary cap filter thing Ampeg   

[gheorge77]

Slightly different.

I've included the Matamp schematic and the better drawn Orange GRO100, which was the first amp Orange produced after splitting with Matamp.  From what I've discerned the biggest difference between the GRO100 and the GT120 is the location of the FAC controls (which goes after the first gain stage and the first 0.047uF coupling cap) and flipping of the James tonestack, which seems weird.  Should there be any difference in tone with a James tonestack if it goes through the treble portion before the bass portion?  There is also the addition of an inductor in the H.F. Drive/Presence controls and removal of a 100p cap in the power amp stage.

[PRR]

> flipping of the James tonestack, which seems weird.  Should there be any difference in tone with a James tonestack if it goes through the treble portion before the bass portion?

The B and T are in parallel. The wires have "zero" impedance. The only difference is the drawing. Wire each plan, you will see they come out the same.