ampex 6793 stereo

[pwdiya12]

hi,

I am about to recap and re-resistor this amp and wondered if anybody had any comments/advice.

One article I read suggested converting the power supply to full rectification from half wave to gain better tube life and have a wider selection of tubes

any input greatly appreciated

peter

 

[Eli Duttman]

A schematic or a link to a schematic would help.

 

[planet10]

Beefy looking iron

from the tubes poking up, i'd guess PP EL84.

dave

 

[kevinkr]

PP or SE 6973? The 6973 is rare and can be expensive these days so take care of them.

Schematic and/or pix would be helpful.

Internal chassis shots too?

Sure that this thing does not use a full wave voltage doubler which would have been de rigeur in the 6973 era?

 

[rmyauck]

Would these be good? They've got about the best price. NOS probably better, but like you say hard to find so pricey.

6973 Tube Types

Randy

 

[pwdiya12]

schematic

hi

I will post some pictures shortly.

I have attached a schematic that appears to match the actual circuit, and will need to replace the at least the 100Mfd 250V Electro as they are leaking

Regards

peter

 

[planet10]

I cleaned that schema up, and made it easier to read if anyone wnts me to post it. I have seen one of those, my buddy Scott scored one. Very nice looking piece of vintage kit.

My advice at the time was to convert it to an El Cheapo variant (using EL84) since 7199 is getting very scarce.

dave

 

[pwdiya12]

schematic and el84

Hi dave

thanks, I would appreciate if you would post the cleaned up schematic and aany information about converting to EL84 operation

Regards

Peter

 

[Triodethom]

Using the russian 6p14p-ev the high voltage one not just 6p14p is a drop in replacement . I do not stand for what they sound like in this amp just that they will not burn up at 385 like the el-84 and its family will. Other than that the 7189 will take the voltage . Second chose is to drop the plate voltage down to about 290 and run any of them . look like very good iron .
think about putting it on a larger chassis that would give room for choke or other voltage control devices.

 

[planet10]

 

[planet10]

Second chose is to drop the plate voltage down to about 290

The PS in this is in need of some serious work, adding a CLC filter and some R between the trafo and the bridge at a minimum.

dave

 

[Triodethom]

Found some chassis on ebay by
lkschmit

in the pro audio guitar section 17 by 8 by 2 for 17 dollars 10 shipping for 1 to 4 12gauge that I approve very much of . That or the 19 by 8 by 2 would give you room for some asc caps and a choke to drop the power supply down to a good voltage level and clean also . Even think about damper diodes for the power supply .

 

[pwdiya12]

7199 substitutions

hi

on tubedepot I found a circuit for substitution of 6U8 for 7199, with the claim of "as good as or better"....any thoughts?

also thnaks for the PS upgrade thoughts, and circuits/pointers would be appreciated.

I am also planning to sell the unit once refitted, so any thoughts as to the degree of mods to undertake in light of potential value would also be appreciated

I must thank you all for your inouts and comments, I will post pics of the original in the next day or so

Regards

Peter

 

[pwdiya12]

before pictures

Hi

here are the before pictures, I have decided to stay with the original tube configuration 7199/6973 initially, though I am investigating cleaning up the power supply.

I will provide updates and further pictures as works progress, and and all input and thoughts welcome...

Regards

Peter

 

[Triodethom]

6u8 vs 7199

hi

on tubedepot I found a circuit for substitution of 6U8 for 7199, with the claim of "as good as or better"....any thoughts?

also thnaks for the PS upgrade thoughts, and circuits/pointers would be appreciated.

I am also planning to sell the unit once refitted, so any thoughts as to the degree of mods to undertake in light of potential value would also be appreciated

I must thank you all for your inouts and comments, I will post pics of the original in the next day or so

Regards

Peter

H H Scott used the 6bl8/ecf80 . 6gh8 also 6u8 in my listening is better than 7199 and easier to find different make so you can tube role some . go to dunkanamps web site and down load the tdls it will give you a number pin compatible tube to play with.

 

[Frank Berry]

I have converted my HH Scott model 299C to use a 6U8A rather than the original 7199.
The 6U8A performs very well.

 

[pwdiya12]

update

Hi

So I have replaced all the components in the 6793 and found some original NOS 7199 (GE)....so far so good.

However, the B+ is now over 400v (starts at 430 and then drops to about 404/408) on a 110v supply and I am keen to drop this to the 385v specified before debugging the unit.

I did purchase a 2.5h choke as suggested, but this is physically too big and I am thinking more along the line of adding a regulator chip to the PS to clean it up and make it more stable. Also considering the MOSFET/Diode approach, but your collective wisdom would be much appreciated...

The alternative is to move the whole thing to a new chassis, however after hours of soldering, I am keen to not do that as yet!

thanks in advance

Peter

 

[TubeHead Johnny]

I wouldn't worry about the voltage too much. Some Bogen amplifiers ran their 6973's at over 450V!

http://members.shaw.ca/pacifictv4/bogendb125.jpg

However, I would insert 1K resistors between the screens and the UL taps of the OPT's.

How about some new pics?

 

[pwdiya12]

Pics

Hi

thanks for the heads up, attached are a couple of "work in progress" pictures, though I am debating changing all the tube sockets (as opposed to the 2 6793 I already have) and would like to "clean up" the power supply.

also where did you suggest for the 1k, between UL Tap (yes) and OPT (?)

and I am currently chasing a wild feedback/oscillation on one channel...

thanks

Peter