Output trafo with unusable secondaries.

[chocopower]

Hi.

i take an old spanish amp.
As you can see in the schematic is designed for a public adress sistem, hence the 125 - 250 - 500 secondary values.

Need a new OT or can i mix the values for a more usual load (16ohm, at last)

thanks in advance.

[Geezer]

I believe you are going to need to replace the OT....I don't think there is any way to get a usable output impedance from that trafo. 

G

[chocopower]

maybe the reason is the name of the amp.

PARROQUIAL.


 

[kagliostro]

May be I'm wrong but I think the only thing you can try is to find a transformer with a 500ohm primary (or 250ohm or 125ohm) and an 8ohm (or other modern standard speaker impedance) secondary to adapt the output to the speaker impedance

that way was used in the past for line outputs

alternatively 4 x 32ohm speaker will take you near 125ohm

Kagliostro

[kagliostro]

Here you can see what I mean

Kagliostro

[chocopower]

ok.
if i understand it well, a Single ended trafo with, for example, 1000 ohm primary and 16 ohm secondary, conecting to the 500ohm tap will result in a 8 ohm tap.

it´s ok or i´m totally extraviated?

[kagliostro]

Yes that is

in the example they use 3 x 1500ohm primary transformers connected in parallel to the 500ohm output tap of the OT

Kagliostro

p.s.: may be you can find transformers planned for those original Line to speaker pourpose

EDIT: Weber has this http://taweber.powweb.com/store/wzc100.jpg unfortunately this has not the right impedances, but I think you can try to use the secondary of an OT from an old PA that has both 500ohm and 8ohm tap

[chocopower]

ok.
I´ll recibe the amp next week. Ill check the options.

upss.. this is the amp.

http://www.segundamano.es/sevilla/amplificador-de-valvulas-optimus-piezasreparar/a26505311/?ca=15_s&st=a&c=22

http://www.radiomuseum.org/r/optimus_parroquial.html

thanks!

[PRR]

You need a 70V _40 watt_ line transformer (or four 70V 10W LTs to four speakers) hanging on the 125 ohm connection.

I had a longer post but the internet lost it. Get an OT replacement for a 50W Marshall, that's the best answer.

[Jack1962]

PRR is absolutely correct replace the OT , to match that output impedance would be very difficult at best.

                                                    Rock On

[RicharD]

Unless you play in a grocery store.  Guitar solo in isle 7.

[Willabe]

Unless you play in a grocery store.  Guitar solo in isle 7.

I think it's called "The Blue Light Special" !        


            Brad        

[PRR]

> Guitar solo in isle 7.

No, the point of high voltage/impedance speaker lines is to save money on LONG FAT wires, so that you can have "Guitar solo in aisle 1,2,3,4,5,6,7,8,9,10,11,12,14,15, AND 16", without making either Bogen or Belden rich.

[chocopower]

HAHA!
Amp arrive today. The schematic i posted is an older version, because mine cames with 4-8-16 plus the 125/500 secondaries taps.

Anyway, is good to learn!!

Upss..  Any safety change? (apart the obviously ground tap for the AC mains)

Can i upgrade the 10ohms resistors in the EL34 caths for a more traditional 1 ohm/1% ones?

schematic in the first post.

thanks in advance!

[kagliostro]

I think the purpose of the 10ohm resistors is to read the voltage drop as to control bias settings

so if you replace from 10 to 1 ohm the difference will be about the voltage to ma of bias current conversion as to read it in the right way

don't think a 10ohm resistor instead of 1ohm resistor will affect in some way the fixed bias settings

but I'm not 100% sure about it

hope someone can explain which is the difference (thinking to the fixed bias settings) about the use of this value for cathode resistors

Kagliostro

p.s.: Happy to read you have also some usable output impedances in the OT

[chocopower]

I have seen other 10ohm resistor for bias measure, but those ones are wrong values (15ohm one of them), burned and out of range (one of them in the 50ohm area!!).
A 1ohm resistor is disipating very few mA, but a 10-15ohm is working more like a cathode resistor in a cathode biased amp.... i dont know....
Anyway, i didnt wait for your answer and changed both them for a 2w, 1ohm, 1% ones....
Amp working but is gonna need some basic work:
cap job, pots and shockes clean an retension, new tubes. It cames with 6 ECC Brimar. Omne of them is dead, but the others sound ok in my others amps.

[PRR]

> a more traditional 1 ohm/1% ones?

10 ohms is traditional. When tubes ruled the world there were no fancy voltmeters on service benches. Your $2.99 DMM has a hundred transistors, will read 0.199V full-scale and easily read 30mV or 32mV. A traditional (pre-DMM) bench meter had 1.5V full-scale on the lowest range, and readings below 100mV were squinty and fairly dubious. 10 ohms gives say 300mV or 0.3V which is easily read on a 1.5V scale.

> burned and out of range (one of them in the 50ohm area!!).

Then you were wise to replace them. And since DMMs are now cheeper than beer, 1 ohm is as good as 10 ohms.

> 10-15ohm is working more like a cathode resistor in a cathode biased amp

Cathode bias resistors are rarely less than 100 ohms. 10 ohms is much less than 100 ohms.

Taking a simple analysis, for maximum gain you want Rk much less than 1/Gm. EL34 Gm at 100mA is 11mA/V, 0.011A/V. Invert, 91V/A, or 91 ohms. 10 ohms is much less.

Going futher: tube gain increases with current. This causes distortion. Some Rk will reduce the nonlinearity. 1/Gm for EL34 is about 80 at 200mA but 200 at 30mA. With another 10 ohms, this becomes 90 and 210.... gain does not change so much. The drawback is lower gain overall, increased drive needed.

[kagliostro]

Thanks PRR

Cathode bias resistors are rarely less than 100 ohms. 10 ohms is much less than 100 ohms.

I was thinking to the old Vox AC30 cathode bias resistor which was in the order of 47 / 50 ohm don't remembering that this is a very unusual solution and usually cathode resistors are larger

----

about a "mixed" bias circuit with both cathode resistors and fixed bias voltage it seems to me there was (but don't remember well) a tread long time ago about an old PA amp that used this hybrid technology, but don't remember about the reasons for that mixed solution, can someone give infos about ?

Kagliostro

[PRR]

> AC30 cathode bias resistor which was in the order of 47 / 50 ohm

For _four_ tubes, right? So 200 per tube. 10 ohms is much-smaller.

[kagliostro]

For _four_ tubes, right? So 200 per tube. 10 ohms is much-smaller.

I agree - I was focusing my attention on a  wrong way - sorry 

---

Nobody has news about mixed bias circuits use (both at the same time - fixed bias more a bit of cathode bias) ?

Am I wrong remembering an amp that used that architecture ?

Kagliostro

[sluckey]

My Maggie M10A uses fixed and cathode bias on the PA.
http://home.comcast.net/~seluckey/amps/magnatone/Magnatone_M10A.pdf

There was an amp that used both discussed on the forum but I don't recall which.

[chocopower]

I think some late silverface fenders use this mixed technique.....

[kagliostro]

I'm not sure but seems to me that PRR and HBP at the time of the old tread about this type of mixed bias told something about the reason for that choice

Kagliostro

[octal]

Well, the most obvious reason is that it will add bias stability (i.e. a mixed bias amp will self-regulate bias like a cathode biased amp does and compensate to an extent for variations in the output tubes, mains voltage shifts, etc.) You can probably get away with plug & playing replacement output tubes without rebiasing in most cases.

I think the sliverface Fenders also allowed user adjustable bias balance to compensate for mismatch between output tubes. So it's a scheme to make the amp easier to field maintain with whatever tubes are on hand.

The advantage over just straight cathode biasing is more power output, less voltage drop & power wasted in the cathode resistor. It probably comes closer to a fixed bias design in 'punch' as well.