Which is the right impedance of the OT for a PSE of 2 6v6 with a B+ around 350v

[kagliostro]

I was looking to the double six amp schematic



and a doubt comes to my mind

on the datasheet of the 6v6 the impedance of the OT for 250v B+ is indicated in 5k

but the double six has a PT with 275v AC that I suppose will give 385v unloaded and, may be, 340-350v loaded

so I'm not sure that 1/2 of 5k is the correct primary impedance of the OT for this amp

K

[shooter]

can't find my spec sheet for the PSE 6V6 I built, was basically a Gibson GA-8.   the Gibson schematic calls for a GA-9-0, you'll have to do the leg work on what that equals.

[VMS]

I'm not sure, but the way I read the double six schematic is that the OT is 5k because centertap is not connected and both side of it is 2,5k so 2,5+2,5 = 5k.

I would use hammond 125ESE transformer at 5k.


-v

[kagliostro]

The label is confusing, reading the spec of the OT it is 2.5k and the "CT" was never used


http://www.ampmaker.com/store/15W-single-ended-output-transformer.html


However the doubt about a 2.5k load for a pair of 6v6 in parallel with ~350v B+ remain


Franco

[VMS]

Yeah, I just visited the page and I'm also confused.

What do forum experts think of this?

[DummyLoad]

for 2 x 6v6 running @ 90% of 28Wmax; Ib=72mA Rl=4860


use 5K


--pete

[kagliostro]

The confusion is on the image, if you read the text:

The label shows the default impedances: 2.5k ohms or 3k ohms into 4, 8 or 16 ohms. So while this is aimed at single octal valves - such as an EL34 - it also allows two EL84s in parallel - using the 2.5k ohm taps.
      You have even more options available through attaching different speaker impedances to other secondary taps. For example, attach a 16-ohm speaker to the 8-ohm tap and you now have a choice of either 5k or 6k ohms primary impedance. There's also a centre-tap on the primary (although I haven't experimented yet with options for this).

They say it is a 3k with a 2.5k intake and the "CT" option was never experimented

Pete, despite of what is wrote on the spec of the OT, your math shows 5k, and thinking to the datasheet of 6v6 where a 5k is indicated for a single tube in SE the use of 5k for two tubes in PSE confuses me further ........

Franco

[VMS]

Maybe this 5e1 text will help you:

ampbooks.com/home/classic-circuits/fender-champ-5e1-power-amp/

What I took from the text is that the load determines how balanced the plate voltage swing is 

[DummyLoad]

franco, the datasheet is for 250V w/ 5K. you specified 350V.


oooohhhmms law.


Rule of thumb for maximum power output in single tube class A is you idle @ 90% of MAX anode dissipation.


90% of 14W is 12.6W.
12.6W / 350V = 36mA PER tube
360V / 36mA = Rload of 9722ohms.


you have 2 tubes so /2; 9722/2 = 4861 - 5K is close enough.


technically, you'd subtract cathode voltage if you you run in auto-bias. in this case -15V or so isn't going to make much difference.


for the OT in the picture, use the two 2.5K taps and ignore the CT. one tap goes to both 6V6 plates, the other to plate supply. 


--pete

[kagliostro]

Many Thanks Pete

now I got it

Franco

[jjasilli]

+1 to Dummyload 2X (he posted twice).  I think the center tap is a "red herring".  Franco, this is American English slang; see:  http://en.wikipedia.org/wiki/Red_herring


SE OT's (usually) don't have CT's.  See: http://www.hammondmfg.com/125SE.htm


It appears that the British are prone to do weird things with electronics.  Here, this CT might have been tapped to optionally use this SE OT in a PP configuration, maybe(???)  This description on the Danbury site seems vague enough for the Queen's English  :  http://www.danburyelectronics.co.uk/

[kagliostro]

I remember that some years ago I've seen a strange PSE connection to the OT in a Top HiFi homebuild amp

where B+ was connected to a "CT" and the plates of the tubes were connected to the extremities

I don't know if the transformer used was a PP transformer (with windings in opposite directions)

or if it was an SE OT with a 50% intake (may be it was done that way for noise rejection purpose ?)

Franco

[2deaf]

SE OT's (usually) don't have CT's.  See: http://www.hammondmfg.com/125SE.htm

Some SE OT's have UL taps (Hammond makes some).  If you hook the 2.5K tap on one end to B+ and the 3K tap on the other end to the plates, the CT could be used as a 45% screen tap.

[murrayatuptown]

Long-since solved problem, I'm sure, but...

I e-mailed extinct MCI/ClassicTone to ask what they knew about BR-9/GA-8 OPT since their EIA code was on the power transformer and I had a suspicious one (no meaningful markings and 6V written in pencil on it) in my inherited amp.

MCI/Classic Tone had specs in a folder (having bid on it for Gibson decades ago, but not being selected as a mfr.) Mr. Sacketti (Mike?) said 27.5:1 turns ratio, 2500:3-4 ohms impedance ratio (3.2 was a common voice coil impedance back then), 60-70 mA DC rating, but no such thing in their catalog to sell me.

I asked why not 40-18031 with 5k:8, 70 mA DC spec, used as 2500:4. They said 'yes, that would work, but not the first thing to come to mind, to recommend higher Z primary for lower Z use as finer gauge wire is used on higher Z primary...it might run slightly warmer'. Not at the pace I finish projects. I bought a 40-18031. He wanted a photo when I was done. Too late for that.

[Mungo Park]

Very informative discussion.
In reading the description of the amp it can use a variety of tunes.
“ designed for two 6V6 valves, but it will also run with one or two 6L6s; or one 5881/EL34/KT66/KT77/KT88“
Maybe the op transformer is such to use all these tubes but not be exactly correct for any of them, just guessing here.
I would like to ask, I can’t see it, where is the switch to provide 1,6, and 10 watts. 
                   Cheers Ron

[sluckey]

I would like to ask, I can’t see it, where is the switch to provide 1,6, and 10 watts. 

It's connected to T1.

[Mungo Park]

Thanks