hi...I recently rebuilt a AWA PA amplifier....it had 2 x kt66 and 2x 12ax7....the power transformer gave around 380vdc under load ......
I thought this would make a good 6l6gc guitar amp and being so compact I named it "YE OLDE TINY TERROR"
The build went about in the usual way and then came test time.....
I cant get any appreciable volume out of it....I should a least be getting 40 watts + from it.....it should part my hair when I put my face in front of the speaker....
I have checked and rechecked my work and it all seems good but the cloud hangs over the output tranny.....this amplifier was made for 600 ohm speakers
You could run up to 20 speakers from it depending on the selected tap....and it does say on the back that a single 16 ohm speaker could be run from a selected set of taps .....so I thought this is just 3 ,7,15 ohm taps with a serious bunch of 600 ohm speakers hanging of it
But prior to that I put 7.1 vac on the primary and got .23vac from the suspected tap I need for 8 ohms..... 7.1 / .23 = 30.869.... squared = 952.93 x 8ohms =7623 ohms the tube see's
so as you can see , it all works out.....so what gives ?
The schem I am using is a fender bandmaster but cathode biased with 470 ohm on the cathode on each tube and 470k grid leak...
Is there something I am missing with this transformer ?...that's my question....
I thought this would make a good 6l6gc guitar amp and being so compact I named it "YE OLDE TINY TERROR"
The build went about in the usual way and then came test time.....
I cant get any appreciable volume out of it....I should a least be getting 40 watts + from it.....it should part my hair when I put my face in front of the speaker....
I have checked and rechecked my work and it all seems good but the cloud hangs over the output tranny.....this amplifier was made for 600 ohm speakers
You could run up to 20 speakers from it depending on the selected tap....and it does say on the back that a single 16 ohm speaker could be run from a selected set of taps .....so I thought this is just 3 ,7,15 ohm taps with a serious bunch of 600 ohm speakers hanging of it
But prior to that I put 7.1 vac on the primary and got .23vac from the suspected tap I need for 8 ohms..... 7.1 / .23 = 30.869.... squared = 952.93 x 8ohms =7623 ohms the tube see's
so as you can see , it all works out.....so what gives ?
The schem I am using is a fender bandmaster but cathode biased with 470 ohm on the cathode on each tube and 470k grid leak...
Is there something I am missing with this transformer ?...that's my question....
The schematic or at least model number for the Aiwa may help but that sounds like it was made for 100volt line applications. Ideal for a factory or large venue with speakers dotted around everywhere.
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The schematic **clearly** says it has output taps for various 600 ohm speaker combinations, from 2 to 20 of them, and a 100V line.
And that you may connect a 15 ohm speaker between the "3-5· and the "6-10" taps, which probably are wound with thicker wire to accomodate that.
All other windings are probably wound with very thin wire, suitable for 600 ohm class loads, but not lower.
That between 2 other, non recommended taps you may have a suitable voltage for lower impedance speakers, means nothing is wire is not rated for needed current.
You have 2 options:
1) get a suitable output transformer, matching what plates need to what speakers you have
2) wire 2 x 8 ohm speakers in series and connect them from "6-10" to "3-5" taps, the only connection rated for low impedance speakers.
And in any case it definitely won´t
EDIT:
How do you expect to have high output (even worse, higher than original), with relatively inefficient cathode bias and high value cathode resistors?
In any case, the original circuit used a way lower value cathode resistor: 2 preamp tube filaments in series ..... and that for a "20W" amplifier.
And that you may connect a 15 ohm speaker between the "3-5· and the "6-10" taps, which probably are wound with thicker wire to accomodate that.
All other windings are probably wound with very thin wire, suitable for 600 ohm class loads, but not lower.
That between 2 other, non recommended taps you may have a suitable voltage for lower impedance speakers, means nothing is wire is not rated for needed current.
You have 2 options:
1) get a suitable output transformer, matching what plates need to what speakers you have
2) wire 2 x 8 ohm speakers in series and connect them from "6-10" to "3-5" taps, the only connection rated for low impedance speakers.
And in any case it definitely won´t
it was a 20W amplifier when new and properly loaded, be happy to get 15/20W now ... IF properly loaded of course.
EDIT:
PLEASE build a Fender Bandmaster circuit , including grounding cathodes and fixed bias.
How do you expect to have high output (even worse, higher than original), with relatively inefficient cathode bias and high value cathode resistors?
In any case, the original circuit used a way lower value cathode resistor: 2 preamp tube filaments in series ..... and that for a "20W" amplifier.
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Here's a rare day when I get to add to a JM post
3) Get a 20W PA line transformer and attach that to the 100V line and a speaker to the 8ohm tap on the PA trannie
I know what ur sayin...but
first off...the maths does not lie.....secondly I didn't want fixed bias ,I prefer the sound of overdriven cathode bias... thirdly ,I have 16 ohm speakers to test with.....I looked for possible 8 ohm tap area to take into consideration ... most setups these days run 8 ohms..... the cathode bias setup is from another fender model... and yes it was set up for 600 ohm speakers , so if you run 20 , 600 ohm speakers that would equal 30 ohms load.....I get all that.
Thing is its an otherwise useless transformer if it cannot be used in any modern application , so I done the maths to see what I could get from it , and the results are on the first post..
the only thing that is helpful so far is that its possible to assume that the wire is so thin that its unable to carry the current...if this is the case then it will be scrapped......I just wondered if anyone else has had this issue ?
and lastly.....its rated 20 watts clean but quite often one can squeeze a few more watts from these old tranny's and in the case of a guitar amp it wouldn't be the first amp I've seen with an under rated tranny , perhaps 40 watts is optimistic 30 is probably the real number...got to remember it was running KT66 in the first place !
at the end of the day ,with these old units are we not just trying to make something useful with what we have to start with.....
thanks folks
first off...the maths does not lie.....secondly I didn't want fixed bias ,I prefer the sound of overdriven cathode bias... thirdly ,I have 16 ohm speakers to test with.....I looked for possible 8 ohm tap area to take into consideration ... most setups these days run 8 ohms..... the cathode bias setup is from another fender model... and yes it was set up for 600 ohm speakers , so if you run 20 , 600 ohm speakers that would equal 30 ohms load.....I get all that.
Thing is its an otherwise useless transformer if it cannot be used in any modern application , so I done the maths to see what I could get from it , and the results are on the first post..
the only thing that is helpful so far is that its possible to assume that the wire is so thin that its unable to carry the current...if this is the case then it will be scrapped......I just wondered if anyone else has had this issue ?
and lastly.....its rated 20 watts clean but quite often one can squeeze a few more watts from these old tranny's and in the case of a guitar amp it wouldn't be the first amp I've seen with an under rated tranny , perhaps 40 watts is optimistic 30 is probably the real number...got to remember it was running KT66 in the first place !
at the end of the day ,with these old units are we not just trying to make something useful with what we have to start with.....
thanks folks
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impedance works out !
Well it's what I figured.....the old amp used a rectifier tube where I have gone solid state....just a personal taste thing...so now I have increased the voltage to the plate .....depending on the time of day it can be as high as 400vdc....
as per the 1st post you can see what the impedance is...also I am using 6l6gc and the calculated A-A was worked out at 7623 ohms at 8ohms load on y / g and the ohms for that section is .9
I got another transformer I can A/B with it.....see what happens😉
Well it's what I figured.....the old amp used a rectifier tube where I have gone solid state....just a personal taste thing...so now I have increased the voltage to the plate .....depending on the time of day it can be as high as 400vdc....
as per the 1st post you can see what the impedance is...also I am using 6l6gc and the calculated A-A was worked out at 7623 ohms at 8ohms load on y / g and the ohms for that section is .9
I got another transformer I can A/B with it.....see what happens😉
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found it !
Indeed the transformer does work and does it well......so what was all the fuss about ?.....it was just a stupid mistake 470k resistor on the cathode on the LTPI....it should have been 470r
Sorry guys for the hindering , but at least it supports the fact that these transformers are not always useless
As to cathode bias , in this age most cafe's , bars etc don't want "loud" so its pointless having a loud amp....I had a fender 100watt and sold it because it was useless.....I use a vox clone ac30 with 6gk6 fitted and its sweat...but they even ask me to turn down with that !!!!
There is an old data sheet with cathode resistor selection amongst it , could be helpful.....I could rebias with 390r on each tube and squeeze a little more out of them....
cheers ,rifraf50
Indeed the transformer does work and does it well......so what was all the fuss about ?.....it was just a stupid mistake 470k resistor on the cathode on the LTPI....it should have been 470r
Sorry guys for the hindering , but at least it supports the fact that these transformers are not always useless
As to cathode bias , in this age most cafe's , bars etc don't want "loud" so its pointless having a loud amp....I had a fender 100watt and sold it because it was useless.....I use a vox clone ac30 with 6gk6 fitted and its sweat...but they even ask me to turn down with that !!!!
There is an old data sheet with cathode resistor selection amongst it , could be helpful.....I could rebias with 390r on each tube and squeeze a little more out of them....
cheers ,rifraf50
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