The AM Forum

Having problems with R26 (33kohm 2 watt) over heating on the Modulator 6DE7 (V4).  I have verified the wiring connections and check the other components and they are ok.  Also tested the tube.

The voltage on Pin 2/6 is supposed to be 25vdc but mine is a little 100vdc.  Puzzling to say the least.

Any ideas?

73,
Bill KO4NR

I would suggest checking to determine that pin 8 of V4 is securely grounded.  I suggest this because the grid voltage on pin 2 is determined by the current through R25, namely the plate current of the previous triode stage.  It might seem unrelated at first glance, but please examine the wiring layout as follows:

Looking at the schematic and the pictorial, it appears that R33 and C35 use pin 8 as a ground tie point.  Pin 8 appears to be connected to the shielding post in the center of the socket.  I assume this is also grounded to the ground lug bolted to the chassis at the other side of the tube socket, via pin 5, completing the filament supply return circuit.  Pictorial 3 in the DX-60B assembly manual illustrates the proper path of the ground wire to the grounding terminal labeled "EC".

If grid pin 2 has a higher voltage than expected, this is likely caused by insufficient current through the other V4 triode (pins 6, 7, and 8 ).  A leaky capacitor at C33 (grid pin 7) would cause the opposite problem.  If the cathode on pin 8 is raised more positive than the grid on pin 7, this would reduce the current through the tube, raising the plate voltage on pin 6, and the grid voltage on pin 2.  It would not take much voltage on pin 8 for the tube to reach cutoff, so check the solder joints, re-solder if in doubt, and ensure the ground lug is secure.  If pin 8 is not grounded through the path assumed above, determine the ground path and make sure the resistance is nil.  A small amount of resistance on the ground return of R26 could raise the cathode voltage on pin 8 significantly.

EDIT:  If you find the ground on pin 8 is satisfactory, it is also possible that the tube socket is not making secure contact with tube cathode pin 8.  Either fault could cause the symptom you report.  If all else fails, do not trust the tube tester; instead substitute a different 6DE7.

Thank you!!
73,
Bill KO4NR

I resoldered the connections and ensured Pin 8 was grounded by adding a ground wire to another ground point.  No change.  The resistor still overheats.   Did some more checking of the wiring but didn't find any problems.

Frustrating to say the least.

73,
Bill KO4NR

Down to thinking that the 1 megohm resistor (R25) on pin 2/6 of V4 may not have a high enough voltage rating and is therefore not dropping the voltage as it should.
Bill

Well, I discovered the Audio Gain was not set correctly.  Cleaned the pot and readjusted it several times before the modulated Plate current settled down to peaks of about 75ma.

Resistor overheating is now greatly reduced.

Thanks to everyone!!

73,
Bill KO4NR

75mA is scary.

I have actually measured screen grid voltages and currents for many screen modulated transmitters with many different finals.

For a 6146 with a 65-70V Sg voltage, the Average screen current with no modulation is about 5 mA and at modulation peaks it is 12 mA MAX.

If you decide to keep the Mic gain (audio gain) pot at 100k, then C31 needs to be 0.05 to 0.1 uF. C32 needs to be changed to 220pF and connected across R19, not at its wiper.

Phil - AC0OB

The grid current is 2.5ma on peaks.  That was what the manual said to set it to.  The 75ma is the Plate current.

Thanks for the response.

73,
Bill KO4NR

Thanks for the clarification.

For further clarification, the DX-60 measures cathode current across the 8.2 resistor R13, so if the meter is indicating 75mA, the Plate current is actually
Ip = Cathode Current - Control Grid current - Screen Grid Current;
Ip = 75mA - 2.5mA - 10mA = 75mA - 12.5mA = 62.5mA.  

Addendum: The DX-60's audio can really sound good with a few modifications to the speech amplifier and the modulator. I have had QSO's with no less than 3 people with a DX-60 last week and they sounded really good.

Phil - AC0OB

Any specific modifications?
Thanks
Carl

Hi Carl,

I would personally recommend the WA1QIX  DX-60 version for modifications.  I have heard it on the air and it sounds flawless.  As with any screen mod system you choose, be sure to load the plate heavily (less C2 loading cap, unmeshed) for the best and cleanest audio peaks.  It will have very inefficient plate power performance, but that is the price we pay for clean screen modulation.

http://amfone.net/Amforum/index.php?topic%3d26603.0

T

  I used this one: http://amfone.net/Amforum/index.php?topic%3d26603.0
  It's the Steve, WA1QIX, mod, and I can honestly say that it's the best mod I've ever put into any radio I've ever owned. While I'll be the first to admit that I don't have a particularly outstanding "radio voice", I've been told many times that my DX-60B, with this mod, sounds like an e-rig. Sine sweeps from 20hz to 15khz nice and clean (see the pictures in one of my posts in that thread).
  I highly recommend that if you're going to do this mod - and Steve advises this too - go all the way and set it up for line-level input. Use a decent external mic-amp with line-level out; it doesn't need to be an Orban or anything pricey - I use a DBX-286S. I get around 12 watts out - enough to drive my amp - and easily 130% positive peaks, all sparkling clean.
  An inexpensive entry-level studio mike rounds things out; I used an Audio-Technica AT2020 ($99 on Amazon), but later upgraded to a Bluebird SL, a little more expensive but an awesome mic.
  I can't recommend this mod highly enough - it turns a competent yet uninspiring controlled-carrier transmitter into a rig that will earn you a constant stream of compliments on the air.

Hi Carl and All.

Here are my enhancement schematics:

Phil - AC0OB

Thank you all

Good bunch of guys on here!!
73,
Bill KO4NR