trouble with an old national combo

[phsyconoodler]

I have an older National combo amp on the bench. It has almost no volume. I replace a few abviously ruptured old wax capacitors right off the bat but it's got weird stuff going on. It's obviously cathode biased and has a pair of old coke bottle 6L6s. A gz34 rectifier and a 6j7 and a 6sl7.
  The voltages are weird (to me)on the power tunes.  The cathodes are 34v the plates are 348v and the screens are 250v or so.
   The funny thing is the control grids are both different. One tube has two resistors and a cap running to the 6sl7 and it reads 43v +
 The other tubes pin 5 has no reading and just has a hum when I touch it with the dvom.
  Obviously I am. Not familiar with what's going on here. Anyone with experience with this type of system know what's going on here?

[eleventeen]

Can you get an ohms reading on the cathode resistor under the 6L6's and then a voltage reading on same so you can figure on current through the pair of tubes?


Do you have *any* idea if the preamp is working?


have any spare 6L6 to throw in there?


Field coil speaker?

[phsyconoodler]

The tubes test good on my tester. It's not field coil speakers on this one.
   There is a resistor running front pin 6 to ground and from pin 6 to pin 5 on one power tube. Odd to me .it's obviously the control grid pair of 220k resistors but no coupling cap to phase inverter on one tube. ??
Trying to find a schematic to match. It has no model number o it

[PRR]

> voltages are weird (to me)on the power tunes.  The cathodes are 34v the plates are 348v and the screens are 250v or so.

Original-recipe 6L6 had ratings of plate 360V screen 250V. So 348/250 is not wrong.

34V on cathode looks high.

+43 on either grid is WRONG. Keep replacing caps, and probably grid resistors also.

Old-old-old amps have so much tarnish on the solder that it can be hard to get actual electric contact with meter-probe. An old trick is to lash a sewing-needle to your probe. Do it solid!! If the needle gets loose it will jump to the most-damage part of the amp.

[phsyconoodler]

The plate an screens are fine. Just wondering if there is a missing coupling cap from pin 5 of the one tube to the phase inverter. Not familiar with octal.phase inverter tunes.

[sluckey]

Hi-rez pics

[PRR]

Most of the Valco (National) amps used very ordinary circuits. Pre-war they favored a driver transformer, postwar we see inverter drivers.

Start with these plans. Nevermind that tubes (and pin numbers!) are different and resistor values may vary; this was THE topology for many of these amps.
http://prewaramps.org/media/nd6V6schematic.JPG
http://www.oldfrets.com/Valco/Schematics/valco_1212.pdf
Yours obviously has more drop from OT/Plates to Screens, but that's familiar from other amps.

"These circuits are not to be changed by the serviceman."

[Willabe]

"These circuits are not to be changed by the serviceman."

        They knew about our kind even back then.

[phsyconoodler]

The valco 1212 is the same circuit except this one has 6L6s instead of 6v6s.
     There is a coupling cap missing completely. That explains the odd control grid voltages. Wonder where it went?  Lol!
 

[sluckey]

It's not field coil speakers on this one.

The Valco 1212 HAS a field coil speaker. Do you see a high wattage resistor (or choke) in the amp that looks like it was added later?

There is a coupling cap missing completely. That explains the odd control grid voltages. Wonder where it went?

A missing coupling cap does not explain odd grid voltages. Both grids should have zero volts on them. One of your grids has +43v. A leaky coupling cap could explain that.

I replace a few abviously ruptured old wax capacitors right off the bat

Did you put in the same number of caps that you took out? 

[phsyconoodler]

ok it wasnt a missing capacitor after all. it was another leaking coupling cap(sluckey was right again). all is well now.However it isnt obvious how the one tube is wired up.Bizarre way of wiring if you ask me.But it works and voltages are where they are sane now and the amp sounds good.


   I replaced all the wax caps with siemens 630v replacements.

[sluckey]

That Valco amp has a 'paraphrase' phase inverter. A lot of older amps used them. They can look strange, especially if you have become used to looking at a cathodyne or LTP.

[Colas LeGrippa]

like in the old supro amps