There are a series of modifications for the DTR-2000L which will incrementally improve operational performance and enhance safety. None of these are radical and do not entail sawing up the original chassis.

I have grouped these mods as follows:

  • 10M enable and input circuit
  • 40M efficiency enhancement
  • Filament voltage and monitoring
  • Meter protection diodes
  • Improved plate current monitoring
  • Time delay relays
  • Status lamps
  • Glitch and fuse resistors

10M Enable and Input Circuit - The amplifier was originally shipped with 10M disabled. This was the epoch of CBer's using amateur radio amplifiers for 11M use. Consequently, all amplifiers had 10M disabled or circuitry for operating on 10M missing.

The DTR-2000L was not an exception to this rule; however, the tank circuit is already set up for 10M and a simple mechanical modification will enable it. The band switch actually has a position for 10M. It is one position to the right of 15M on the bandswitch. There is no front panel labeling but it is there.

A mechanical stop prevents the bandswitch from moving to the 10M position. If this stop is moved one hole to the right on the switch frame then the switch can be turned one position to the right. The photo's below show the before and after photos.

         The Bandswitch showing the threaded pin in the original position                             The Bandswitch pin moved one hole to the right

This pin is threaded on one end and secured to the switch frame with a nut. With the tank circuit in the amplifier removing the pin can be a bit of chore. I did mine with the tank circuit out of the amplifier. If you have a hemostat or other skinny long nosed tweezers/pliers you should be able to do it with the tank circuit in the amplifier.

If you trace out the RG-174/U cable from the antenna relay you will find it goes to a PCB mounted on the interior bulkhead on the power supply side. Some amateurs believe that this is an input matching network for the amplifier. It is not. This is a low-pass filter with about a 23 MHz cutoff. It is designed to also prevent 10M operation. Remove this filter and discard it. Simply run a length of RG-174/U from the input side of the antenna relay to the tube box. The tube box end is terminated in a RCA plug. The nominal input impedance of the 8877 is 54 ohms and my amplifier exhibits under 2:1 input VSWR (typically 1.5:1) on all bands without input tuning. I have used it with several types of exciters, including  an Elecraft K3 and Yaesu FT-2000D with excellent results.

The 40M Tank Circuit Modification - Dentron amplifiers have in general suffered from a common problem when operating on 80M and 40M. The tank circuit loading variable capacitor is usually too small. The DTR-2000L is no exception and if the stock circuit is used efficiency will suffer. C21, the loading capacitor, is 800 PF. This is much too small for proper tuning. One can instantly tell the issue when tuning up. If the load capacitor is all the way to the left on 40M and 80M and efficiency is in the low to mid 30 percent range then a remedy is in order.

Strangely enough, this usually does not crop up on 160M as additional capacitance is switched in with the original circuit at both ends of the tank circuit when operating that band.

The solution on 80M/40M is actually quite simple. A 1000 PF leaded doorknob capacitor is switched in across the loading capacitor. Switching is done with a Jennings RJ1A vacuum RF relay. A small switch is mounted on the back panel. One side of the switch goes from the junction of D16 and C47 on the power supply board (this is the control voltage supply). The other end goes to the coil of the RJ1A. The relay coil has a .01 UFD capacitor across it. The nominal control voltage is 17.5 VDC so a RJ1A with a 26 VDC coil will work fine. The remaining coil terminal is grounded.

When operating 40M or 80M push the switch toggle down. Pushing the toggle up returns the amp to normal operation. The logic of the switch sense is simple; up - higher frequencies, down - lower frequencies.

You will find that when enabled the tuning is a bit broader with the loading control. 40M Efficiency will jump at least 15% and at 1 KW RF output it's in the high 40's.

On the left, the vacuum relay and the doorknob capacitor below it are shown mounted on a homemade aluminum bracket. They are mounted on the bulkhead between the tube and tank circuit compartments. The photo at the right shows the miniature toggle switch mounted on the back panel of the amplifier.

Heater Voltage - The 8877 has a rated heater voltage of 5.0 V +/- 0.25 V. Modern line voltages have crept up over the years and my DTR-2000L had 5.6 VAC on it's heater, well above what is recommended. Over time this kind of voltage will shorten tube life, something you don't want to do with an expensive tube.

To monitor the heater voltage I installed a green tip jack as shown on the back panel in the above photo for the 40M mod. I also installed another feedthrough cap on the tube box that is connected to the tip jack. To get the heater voltage more in line with the specs I installed a 0.05-ohm 10W resistor in the tube box. It is in series with the applied heater voltage. At the 10A nominal heater current the voltage was reduced to 5.1 VAC. You can see the interior of the tube box with the installed resistor and feedthrough capacitor in the photo below.

Finally, please do not attempt to measure heater voltage with the interlock switch defeated. It could be a dangerous mistake.

The bottom of the tube box showing the heater voltage dropping resistor and other components.

Meter Protection Diodes - A great deal of explanation is not needed for this mod. Back-to-back 3A rectifier diodes are placed across the terminals of each meter. This helps protect the meters should a tube-arc occur (those who have experienced this event know how exciting this can be).

​Back-to-back meter protection diodes are shown across the terminals of each meter.

Improved Plate Current Monitoring - The original circuit was a bit odd. It consisted of an extremely low value resistor from the bottom of the power supply capacitor stack to ground. The value of this resistor was approximately 0.133 ohms. It acted as a shunt for the plate current meter. While it would be possible to make such a resistor I decided to go with a more conventional circuit.

First, this resistor was removed and discarded. It was replaced with a 1 ohm 5W resistor (mounts on the PS board). In this way we will be measuring the voltage drop across the 1 ohm resistor. I made a simple PCB from a scrap piece of board and cut severals lands with an Exacto knife. Each major land corresponding to a meter terminal. Next I cut a land on the + side. This allows me to connect a series resistor between the meter and the top end of the 1 ohm resistor.

Defining the value of the meter multiplier resistor (we have essentially built a voltmeter) is really quite simple. Not knowing the meter internal resistance and not wanting to build a test circuit I did it by trial and error. It only takes a few minutes. Take a fresh D-cell battery and measure its voltage with a DVM. This is a convenient calibration source since the full scale reading is 1.5A (1.5V across 1 ohm). I started with a 100k series resistor and adjusted its value downwards until the plate current meter (being used as a 1.5V full scale voltmeter). reads exactly what the DVM indicated. In this case the resistor value was about 34K ohms. This was made by taking a 39K resistor and paralleling it with a very high value resistor. It works great. The image below shows the homemade board and multiplier resistor combination.

The new plate current meter circuit

Time Delay Relays - This is not really a modification but more of a discussion on the time delay relay used to allow for 8877 heater/cathode warm-up. Since the 8877 has a cathode it must warm-up for a minimum of three (3) minutes (per the  data sheet). This is accomplished in the DTR-2000L by using an Amperite time delay relay. These look like an octal based vacuum tube and in this case is a normally open SPST relay that takes the amp out of standby when enabled.

The relay used in the DTR-2000L is an Amperite 5NO180. The "5" signifies 5 volts, the "NO" for normally open and the "180" for 180 seconds (3 minutes). These are still made but can be very expensive if a current new unit is purchased. They can cost between $150 and $175 each depending on where they are ordered. Fortunately they are available surplus and on eBay for reasonable prices ($10 to $30).

Unfortunately, the 5NO180 is almost impossible to find. The 6 volt version, the 6NO180, is much more available. The downside is that some long-delay 6 volt relays will not pull-in at the voltage used in the amp even though the 6 volt relays are rated at 4 to 8 volts.

I ended up buying several types of these relays on eBay and surplus and I could never get a 6NO180 to work. Strangely enough the 6NO300 (5 minutes) seem to work fine but the pull-in time is about 5.5 minutes at the lower voltage. The 6NO120 does work; however, and it is just about three minutes at the lower voltage.

I went through all this as during the course of my restoration I accidentlly tapped the glass envelope of the 5NO180 in my amp with a screwdriver and broke the glass. So, as a word to the wise, remove the relay before doing anything mechanical in the amplifier.

Status Lamps - This is not much of a mod but it will make the amplifier more of a pleasure to look at when it is on. There are three status indicators on the front panel. Each of these use military style T-1 3/4 lamps that slide into the jewel of the indicator (the colored part). Originally, these were no. 382 lamps which are rated at 14V each.

The problem is that because of modern line voltages the actually voltage on each lamp is about 16V. I didn't have one burn out on me the but the indicators were so bright that they quickly became a nuisance. The solution was simple. Remove the 382 lamps and replace them with type 370. The no. 370 lamp is rated at 18V and the brightness level is now back in the comfortable range.

Glitch and Fuse Resistors - This is a primary safety mod that most modern amplifiers have. The glitch reisitor is normally a 10 - 20 ohm resistor in series with the power supply. It's purpose is to limit current should the tube develop a grid to plate short. Having one of these installed can save your power supply. A grid to plate short is an exciting event on a big tube that you want to avoid. This one resistor can save your power supply. 

The fuse resistor is a 1 ohm resistor in series with the glitch resistor. If all else fails it will open and hopefully prevent further destruction.

In this amplifier the glitch resistor is a 10 ohm 20w resistor. The fuse resistor is a 1 ohm 3w part. You will notice that the fuse resistor is covered with a piece of shrink tubing that has NOT been shrunk. This is done for a reason. should the fuse resistor come into play the initial heat will shrink it down and the shrink tubing prevents resistor debris from being sprayed all over the amp interior. 

The fuse and glitch resistor are mounted via ceramic insulators on the bulkhead will between the power and RF sections.

The glitch and fuse resistors mounted on the RF-Power Supply bulkhead wall

NR7N's Modifications