Hello all, first post here on such an awesome forum. but down to business....
I have a 1950s era valve pa amp, my grandfather and I started to rebuild it, it is far from original (looks like different modifications were performed over the years.
it is push pull and uses 2x 6v6GT for the class ab output, and 1x 6au6 per input. (3) and one 12au7 for the phase inverter.
all that is on the name plate is "Audio Engineers Sydney pty ltd"
I don't have a whole lot of knowledge but am very keen to learn.
My plans for the amp are to use two of the other 6au6 valves to drive a reverb tank and use the other for the guitar input.
I have drawn a schematic of the current (sad and sorry) state of the power amplifier section, any opinions, and improvements would be greatly appreciated.
also the output transformer is crook but I've got one in the post, and am just using this one for testing...
the amp powers on, and sounds quite nice.. but just not so loud.
I know the amp is probably not correct and terribly designed, but that is why I am here.
Questions I do have are about biasing, how to setup/everything. and I also want to convert this amp to a quad of 6v6 (i have a matched quad) the power transformer is capable of running 2 kt66 at 30w so I don't think I will have any troubles with the PT. and the OT in the mail is 40w.
thanks for any help.
I have a 1950s era valve pa amp, my grandfather and I started to rebuild it, it is far from original (looks like different modifications were performed over the years.
it is push pull and uses 2x 6v6GT for the class ab output, and 1x 6au6 per input. (3) and one 12au7 for the phase inverter.
all that is on the name plate is "Audio Engineers Sydney pty ltd"
I don't have a whole lot of knowledge but am very keen to learn.
My plans for the amp are to use two of the other 6au6 valves to drive a reverb tank and use the other for the guitar input.
I have drawn a schematic of the current (sad and sorry) state of the power amplifier section, any opinions, and improvements would be greatly appreciated.
also the output transformer is crook but I've got one in the post, and am just using this one for testing...
the amp powers on, and sounds quite nice.. but just not so loud.
I know the amp is probably not correct and terribly designed, but that is why I am here.
Questions I do have are about biasing, how to setup/everything. and I also want to convert this amp to a quad of 6v6 (i have a matched quad) the power transformer is capable of running 2 kt66 at 30w so I don't think I will have any troubles with the PT. and the OT in the mail is 40w.
thanks for any help.
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Are the screens configured for pentode, i.e., connected directly to a power supply? That's often desirable in a musical instrument amp.
You'll definitely want to replace any old electrolytic caps and (shudder) selenium rectifier stacks, if there are any. Paper caps can get leaky over time and may also have to go.
You'll definitely want to replace any old electrolytic caps and (shudder) selenium rectifier stacks, if there are any. Paper caps can get leaky over time and may also have to go.
Are you really using only half of primary winding in the OT ? This thing can't work well , you need a proper OT with center tap for push-pull operation ...also screen connection is incorrect .
2.7M as grid leak ? isn't a little high ?
2.7M as grid leak ? isn't a little high ?
You are correct, I made a mistake they are infact 270k resistors, the multiplier looked green due to brownish wax on the resistors. I have another transformer on the way, I like the sound how it is, I just need it louder now. I have made a recording, I'll export it to my computer and attach it.
How should I connect the screen?
How should I connect the screen?
First off how big is The Output transformer?
6V6's are cool !!
I used to have a few different P-P types.
You ask about bias voltage?
Do you know how to measure the current that is flowing through the tubes and what the negative voltage on the grid is as well as the plate voltage?
There are some charts that you should have a look at before you go changing the bais voltage.
You can Google 6V6 for one, and the 6AU6 for another.
Those Tube are old !!
It Is Alright, if you say that it is working right now this is good.
Do you have a good multi meter?
Knowing the plate voltage and the grid voltages with a good meter you will able to find the best bias point.
It has been a while since I had messed with power tubes as they are becoming more rare
but I still have a few.
Here is a link to some cool info that I hope can help you out,
Weber Bias Calculator
http://www.diyguitarist.com/Images/BiasChart-6V6.jpg
Maybe this will help you, This says the grid voltage should be -19v I typically remember about -22v for class AB.
http://www.r-type.org/pdfs/6v6g.pdf
jer 🙂
6V6's are cool !!
I used to have a few different P-P types.
You ask about bias voltage?
Do you know how to measure the current that is flowing through the tubes and what the negative voltage on the grid is as well as the plate voltage?
There are some charts that you should have a look at before you go changing the bais voltage.
You can Google 6V6 for one, and the 6AU6 for another.
Those Tube are old !!
It Is Alright, if you say that it is working right now this is good.
Do you have a good multi meter?
Knowing the plate voltage and the grid voltages with a good meter you will able to find the best bias point.
It has been a while since I had messed with power tubes as they are becoming more rare
but I still have a few.
Here is a link to some cool info that I hope can help you out,
Weber Bias Calculator
http://www.diyguitarist.com/Images/BiasChart-6V6.jpg
Maybe this will help you, This says the grid voltage should be -19v I typically remember about -22v for class AB.
http://www.r-type.org/pdfs/6v6g.pdf
jer 🙂
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Output transformer is huge in size, but is being replaced because one of the primary windings is is open circuit. which is why I assume it was retired in the first place. I have a 40w 4k primary and 4 8 or 16 ohm secondary to replace it with, which should allow me to run 4 6v6 or 2 just by using a different secondary winding, with my 8 ohm quad box?
The plate voltage is between 410 and 420v the pt puts out 425v dc after the ss rectifier.
I've been reading a lot on the learning tubes sticky, I'd post up some photos but it won't let me from my mobile phone.
I have a few good meters, is the negative grid voltage the voltage between the earth and the grid? Or the voltage difference between the plate and the grid? I'll post that when I get home, and to measure the current flowing through the tube which pin should I disconnect and measure current? Grid? Plate? Or cathode?
Thanks for your help, I am learning a lot.
The plate voltage is between 410 and 420v the pt puts out 425v dc after the ss rectifier.
I've been reading a lot on the learning tubes sticky, I'd post up some photos but it won't let me from my mobile phone.
I have a few good meters, is the negative grid voltage the voltage between the earth and the grid? Or the voltage difference between the plate and the grid? I'll post that when I get home, and to measure the current flowing through the tube which pin should I disconnect and measure current? Grid? Plate? Or cathode?
Thanks for your help, I am learning a lot.
http://db.tt/NRTpjdht
That is a link to the sound clip I recorded. Not excellent guitar, but I enjoy it so no hate =)
That is a link to the sound clip I recorded. Not excellent guitar, but I enjoy it so no hate =)
An externally hosted image should be here but it was not working when we last tested it.
Per forum policy please post all threads about musical instrument amplifiers to Instruments and Amps. See the subheaders at the top of each forum for guidance as to what is appropriate for the forum in question.and
and
My apologies, I did read that after I had started the thread, thanks for moving it for me. .
The amp was, in its original state for a gramophone and tuner + 2 microphone inputs.. But I have a feeling that doesn't get me off the hook 😉
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