Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on August 29, 2016, 10:28:49 am

Title: AMI model C - What is doing the 6SA7 tube ???
Post by: kagliostro on August 29, 2016, 10:28:49 am

Some time ago I've find this old schematic

http://i.imgur.com/Yr9FiY2.jpg (http://i.imgur.com/Yr9FiY2.jpg)


here a better resolution of only the schematic
(http://i.imgur.com/zDRswF8.jpg)

occasionally I give it a look as to try to understand the signal path and always I stop around the 6SA7 + the 6SN7 that is on the left bottom of the schematic

What are doing there those tubes ??

Thanks

Franco

Title: Re: AMI model C - What is doing the 6SA7 tube ???
Post by: shooter on August 29, 2016, 10:51:14 am

I'm in your boat, but a stab in the dark, the A7 is *standard* TS recovery/driver for the PI, + signal tap for the N7 to provide a conditioned signal for the remote output?

Title: Re: AMI model C - What is doing the 6SA7 tube ???
Post by: drgonzonm on August 29, 2016, 11:15:37 am

If you look at the s7, both of these triodes, are cathode followers, which provide a low impedance signal to the downstream components. 

I believe the a7, is converting the low impedance signal to high impedance, as well as amplifying the signal, as shooter, stated recovery/driver is likely correct.

The remote output, appears to be unconditioned (different interpretation than shooter), low impedance signal off the cathode of half of the s7. 

Title: Re: AMI model C - What is doing the 6SA7 tube ???
Post by: PRR on August 29, 2016, 11:35:24 am

What is it??

Record player?

Juke box?

Much detail around 6SA7 was scanned-away.

OK. Cathode follower. Then two paths, highs and lows. High side has a scratch filter. Low side has a triode hanging on the signal. Triode had a 1N34 diode in the grid, taking output signal. I suspect the bass is normally boosted (for full sound at low volume) but compressed down when output is large (saves amplifier power and speaker slap).

Where is the volume control? Actually outside the amplifier, at the bar where the bartender can control it. This varies the bias on variable-gain 6SA7 to control volume (over a limited range).

NFB from output is applied back to 6SA7 plate! Which is really the same as driver grid. Odd but not strange.

The multiple output windings are for the in-box speaker and also long wires to speakers at the tables.

For more fun: the record player motor won't turn-on until the 6L6es get hot and draw current.

Title: Re: AMI model C - What is doing the 6SA7 tube ???
Post by: kagliostro on August 29, 2016, 01:56:10 pm

Many Thanks PRR

Interesting explanation about bass control and the delay on motor, the NFB as you say is really a bit crazy, I've never seen one as that

Quote

What is it??

I think it is this

(https://www.jukeboxmanuals.com/uploads/1/8/2/8/18281459/6438784_orig.png?1055)

Franco