Altec 1568A modifications for comments and help…. again - diyAudio
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Old 28th January 2009, 02:06 AM   #1
martyh is offline martyh  United States
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Default Altec 1568A modifications for comments and help…. again

After a few days playing around with tube curves and spreadsheets and taking into account the advice I got here, this is what I came up with for mod’s to the front end and PS of the 1568A

One thing I took into consideration thanks to your advice, open loop gain as near as possible to the original. The two stages shown are overall ~12 new ~10 original (I plan to make up the difference in overall gain by switching to triode operation of the outputs and adjusting the feedback OR adding a small amount of local feedback on the output tubes in pentode mode.)

Some things I wanted to accomplish were differential operation of the input and driver for better AC balance, operating the input and driver tubes well below max dissipation to retain some of the reliability, not adding too much current draw (the new is +6% over original as far as I can see) minimizing DC imbalance on the output transformer for better headroom, better filtering on the PS, setting the input LF knee frequency above those of the following stages and operating the input and driver stages at a more linear point.

I have been playing around with tube circuits for years so I don’t qualify as a noob but I am a bit thick when it comes engineering so I know enough not to kill myself (hopefully) but not enough to have things work the first time. So, please let me know if I have made a silly mistake and thanks for taking the time to look at this stuff again.

Marty

The schematics....

power supply


input and driver
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Old 28th January 2009, 02:22 AM   #2
martyh is offline martyh  United States
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Default Oops……. the PS schematic link

ps
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Old 29th January 2009, 01:13 AM   #3
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hey-Hey!!!,
I built my first amp with a cascaded LTP/diff amp front end. It started out 6SL7 to 6SN7, and I dropped the gain with split-section 6DN7; input stage of the higher gain section and driver with the lower mu sections. OTOH, I do like the 12AY7/6072. It is a very nice tube. Consider 12B4 drivers? 12A4 maybe?
cheers,
Douglas
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Old 30th January 2009, 02:43 AM   #4
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Hi Marty ,

At first sight , every thing seems OK .
The result , to apply NFB at the second grid of a LTP amp
Is very good .
I usually operate the 6CG7 at 4 ma , plate load of 36 K ohms
and + B = 250 Volts ( EACH TRIODE ) , and I always could
get a very nice sound . The resultant grid bias would be – 3 Volts .
In your case I think that + B = 379 Volts is too high , BUT this
is only a comment , may be you ( and others ) think different .


Only two things :
1 – The diode in the bias supply is inverted !!!
2 – In my oppinion , 330 uf each filter cap is very large
for a 2 Hy 15 ohm filter choke , if I was you , I would
start with 100 uf , for the first cap after choke , 100uf
for the second , and 47 uf for the third and fourth , this is
more than enough .

Regards ,

Carlos
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Old 30th January 2009, 11:30 PM   #5
martyh is offline martyh  United States
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Quote:
Originally posted by Bandersnatch
[BConsider 12B4 drivers? 12A4 maybe?
[/B]
Douglass,

Thanks for taking the time to look at this. I have never seen the curves for the 12B4 before you mentioned the tube. The 150 Volt operating point looks nice. I am stretching the existing power transformer as it is so this tube may not work in this case but I have often wished I had known of a small tube with lowish RP and Mu for driver service. I am going to get a few of these for my stash for the next time I try a totally new amp.

Quote:
Originally posted by refference


Only two things :
1 – The diode in the bias supply is inverted !!!
2 – In my oppinion , 330 uf each filter cap is very large
for a 2 Hy 15 ohm filter choke , if I was you , I would
start with 100 uf , for the first cap after choke , 100uf
for the second , and 47 uf for the third and fourth , this is
more than enough .
Carlos,

Thanks for the tips. I plugged those big capacitors into the schematic just because I have a bunch around and wanted to use them. I never really gave a thought how hard they would be on the transformer. That and the reversed diode should prove the point I made in my original post, I suck at engineering.

Thanks for your help with all of this, you guys have an awesome forum going here.

Marty
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Old 31st January 2009, 02:32 AM   #6
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Quote:
Originally posted by martyh



That and the reversed diode should prove the point I made in my original post, I suck at engineering.


Marty
Nah...not at all. Only fools don't learn. Engineers recognize limitations and continue forward. You're doing just fine...
cheers,
Douglas
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