670 signal amp/control amp woes

[DaveP]

I must say you have not chosen the easiest of projects to learn on.

I started my own Fairchild and then decided I needed to learn a bit more before I finished it.  I did an Altec 436B and converted it to a RS124 when the schematic became available, it was a good place to start.

Still, full marks for perseverance and you obviously have got the right attitude to see it through to the end.

Good Luck
DaveP

 

[skal1]

Dave p , i would not call learning more like blundering

Skal

 

[skal1]

ok i have been messing about with the signal all day,  and i can get about 267 Vac of gain from an input voltage of 17.67 across the the step up transformer secondary to grid to primary of output transformer ,the output secondary voltage is 72.2 Vac unload,  how would i work out how much GR i can achieve with this value.

cheers

skal

 

[DaveP]

As far as I remember, there is a 3.9k load resistor followed by a bridge rectifier. so if your 72V is rms, then its 72 x 1.4 = -100VDC.

This is probably more than you need, its usually about -70V max for full GR.  Check the negative DC on your setup

Someone will correct me if I'm wrong

DaveP

 

[skal1]

Hi Davep  cheers again just a couple of ? ,what is the 1.4 multiplier for ,is it to convert the output voltage to peak vdc,  i get wot  your saying about the output voltage ,i think i will need to reduce the  amplification  of the signal amp .

skal

 

[DaveP]

!.4 is actually root 2 which is 1.414.  This is the conversion factor when you use a full wave bridge rectifier.

Don't reduce the output until you have measured the -ve DC with the rest of the timing components, it might be lower under load.

DaveP

 

[skal1]

cheers,  ;D

skal

 

[skal1]

Don't reduce the output until you have measured the -ve DC with the rest of the timing components, it might be lower under load.

DaveP

What do mean by  -ve DC.

skal

 

[DaveP]

You will be connecting a bridge rectifier to the output of the control amp, but instead of connecting the -ve to earth (like a power supply) you will be connecting the +ve to earth, so you will be measuring the -ve with respect to earth.

DaveP

 

[skal1]

ok, so just trying understand where i am going with the signal amp voltage gain.

Well at the moment  my signal amp is putting out 50VAc  rms  from the 10k:600 transformer secondary , from their it is met with a pad  300r 1200k 300r, this pad is across the control amp primary  of a 1:7 step up tx ,the voltage at the primary is  46.5vac rms ,and  at the secondary  it is 187.2vac  rms ,this is with curve switch at postion 2.

The voltage from the control amp is 50Vdc  ,this turns down the signal amp output voltage to about 1.23Vac rms which is -30db  ish i believe.

The control amp should be putting out 70Vdc but delivering 50Vdc instead, is the  ratio between control amp and signal amp incorrect or is it the control amp...

cheers

skal

 

[DaveP]

This is probably just the difference between off load and on load.

-30dB gain sounds good, you should never need that much in practice.

DaveP

 

[skal1]

I  thought the 670 did 30db gr ?  should i back it off a bit ...

it sounds good, but the signal amp is burning a lot of current about 0.070Ma  hum might have to rethink this..



skal

 

[abbey road d enfer]

  I did an Altec 436B and converted it to a RS124 when the schematic became available, it was a good place to start.

[hijack ] I've tried to find what the RS124 mods are, but ended up with a multitude of contradictory advices. Where can I find the definitive answer.
Regards. [/hijack]

 

[skal1]

;D no problem abbey.

Anyway heres my twist on dave p control amp just slight changes voltages..

Have a look any comments welcome

cheers

skal

 

[skal1]

ok ,i have been doing a bit more experimenting with the control amp and signal amp  together,
what i have found is  it with the the same input voltage of 28.894 to the dual 100k pot to grid's of the 12ax7 i  can not produce the voltage of 50Vac Rms  for  curve  2 of dave p 's chart{see attachment}

is the control amp is not set up correcly? .What procedure would use to test if the amp is set up correctly.

This is with the signal amp loading the cv.

cheers

skal

 

[abbey road d enfer]

ok ,i have been doing a bit more experimenting with the control amp and signal amp  together,
what i have found is  it with the the same input voltage of 28.894 to the dual 100k pot to grid's of the 12ax7 i  can not produce the voltage of 50Vac Rms  for  curve  2 of dave p 's chart{see attachment}

is the control amp is not set up correcly? .What procedure would use to test if the amp is set up correctly.

This is with the signal amp loading the cv.

cheers

skal

If you want us to help you, you need to measure the voltages at the secondary of input xfmr, at the plates of all six tubes and the volatges across each winding of the output xfmr.

 

[skal1]

ok ,i have been doing a bit more experimenting with the control amp and signal amp  together,
what i have found is  it with the the same input voltage of 28.894 to the dual 100k pot to grid's of the 12ax7 i  can not produce the voltage of 50Vac Rms  for  curve  2 of dave p 's chart{see attachment}

is the control amp is not set up correcly? .What procedure would use to test if the amp is set up correctly.

This is with the signal amp loading the cv.

cheers

skal

If you want us to help you, you need to measure the voltages at the secondary of input xfmr, at the plates of all six tubes and the volatges across each winding of the output xfmr.

Are talking about the signal or control amp  ,with  threshold pot cw or ccw

regards
skal

 

[abbey road d enfer]

Are talking about the signal or control amp

All your last posts have been about the control amp, so I'm following on.

,with  threshold pot cw or ccw

In order to check gain, it's the AC voltages that we need, so it has to be with the threshold pot open enough to get correct measurements, but not too much as not to clip the output.

 

[skal1]

Ok

so i inject a  3.65vrms signal @ 1khz in to the side chain input tx,  with pot ccw i read 28.9vrms across secondary .

Now with the threshold pot pointing towards 10 oclock  the voltage across the  12ax7  grids is 0.674v rms  and ross the dc blocking cap it is 2.89v rms from here it hits the 12hb7 grids , output voltage across  12hb7  blocking  caps is 1.80v rms.

the power tubes receive this  1.80v rms across their grids  and  the  Ac voltage at the primary of out tx is
330 vrms , from the secondary of the side chain out tx  is just over 59vac  ,which is connect to the bridge and loaded with 3900K

To me something looks abit fishy around the  12hb7 and power tubes.
Cheers

skal

 

[DaveP]

It looks like your 12BH7's are outputting less than their input

If you really have 1.8v input into your output tubes and are getting 330V across the primary then that has to be a record with a gain of 183!!!  You could get that from a small signal pentode, but an output pentode??

Are you measuring the input voltage grid to grid?  Have you checked the scale? Sure it's not 18V?

DaveP