Magnatone 15 voltage question

[Rphancock1]

Hi, I've used this board for years but haven't registered til now. 

I'm working on a Magnatone 15, the one with separate reverb output and speaker (schematic linked) that I found in non working condition.  I've done some very minor repairs (replaced power supply filter caps, all new tubes, replaced two tube sockets, rebuilt reverb circuit) and it's sounding lovely, except... When it's turned up all the way and you hit something like a low e hard it sounds voltage starved.  Classic "squash swell" like you can get with a 5Y3 sometimes.

On the schematic you can see that the voltages going to the preamp section should be 240, 260, 300V.  In this case, although the power supply voltage is accurate, the preamp has less than half that, at about 90V to the D nodes (first preamp tube plates).  With the 5 preamp/trem tubes pulled this voltage doubles to a little over 200V.  When I put the 5 preamp/trem tubes back in one at a time the voltage drops nearly the same amount per tube, between 20-30 volts each, so I didn't see that any one section had a massive current.

My first check was the power supply dropping resistors, but they were in spec when I swapped the filter caps so I didn't replace them.  I'm regretting that now as it would eliminate a suspect, but they do still measure in spec.  I didn't change the rectifier diodes, so I suppose that would be the first step here?

Any suggestions welcome as to the most likely culprit.. I haven't found any leaking coupling caps either fyi..
Thanks in advance
Ryan

[shooter]

and it's sounding lovely

your B+ is probably ok, if not it probably wouldn't like a C# either.  I'm guessing something like a blocking distortion in the PI? 

[jjasilli]

With the 5 preamp/trem tubes pulled this voltage doubles to a little over 200V.  When I put the 5 preamp/trem tubes back in one at a time the voltage drops nearly the same amount per tube, between 20-30 volts each, so I didn't see that any one section had a massive current.

The D node at the last filter cap is 220V.  Preamp tubes draw little current.  There should be at least 180 - 200V at the preamp tube plate pins off the D node. 


There may be a partial short; possibly a bad tube???

Always suspect tubes first. 

[Rphancock1]

I thought he same, but felt like if one was bad it would’ve shown up as a much larger drop than the others, no? 2 out of the 5 are 12au7’s driving the term oscillator which in this amp is super complex, being both stereo and pitch shifting. I don’t understand that part of the circuit which doesn’t help my troubleshooting..

You’re right though, I should sub all known good tubes as a precaution.

[sluckey]

Check the 4.7K between S1-12 and node D. It's located somewhere on the main chassis.

[Rphancock1]

Man, how did you know that was there?! I’ve been so puzzled because I can’t find that on the schematic... there’s also an 8uF 450V filter cap on the tube side of that resistor there that I can’t find on the schematic either. The resistor is right at 5k, and the cap seems fine, but I wonder why they are there and not on the schematic?

[Rphancock1]

EDIT Sorry Sluckey, I guess the schematic I have that I took from the inside of the amp cabinet is slightly different than the clearer one I found to post here... Glancing at that schematic it's obvious of course that the 4.7k and cap are there... I'll print that one out and see fi I missed anything else since it seems more accurate to this amp than the one included WITH the amp haha.

[Rphancock1]

Also, would the first filter cap being 100uF rather than the spec'd 40uF cause any of this?  I put that larger value there because it's what I had several months ago when I was working on this and forgot until I was just going over it again that it's over spec...  I would think if anything it would do the opposite but I ask anyway...

[jjasilli]

In theory, SS rectification should be able to handle the larger cap.  However, if an amp- isn't working right, the best rule of thumb is to restore it to spec and get it working. 

[Rphancock1]

So I've subbed known good tubes in the 5 positions fed by B, C and D nodes, changed the first filter 100uF cap back to spec'd 40uF, measured all plate and cathode resistors which are within tolerance, measured power resistors in the filter chain.. still getting between 20-30v drop across each plate resistor.

One thing I should have mentioned earlier is that plugging in to channel two and diming it doesn't show any voltage sag, and is louder than channel one considerably.  Channel two doesn't pass through the vibrato section, so that would indicate to me that the problem is likely in that area, yes? 

This amp is a "challenging" layout, all point to point construction (see attached), and the vibrato section with it's two "cone" circuit boards is the worst part.

Is it worth replacing the diodes if I'm getting the right voltage at the power amp?  The only reason I wonder is that I've never gotten the right voltage readings from a schematic from the 60's, because the 10V rise in AC since then always make the voltages higher than they were originally.  Seems odd that the voltage for the power section should be right at the schematic... 

The diodes shown on the schematic are IN1697, around 600v 600ma.. I'm sure I can find a replacement easily enough.