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TOF Tune on the Fly Amplifier Tuning and Protection Module

Amplifier Installation:


AL811 series

The TOF Module is available at this link: CTR Engineering Inc TOF Link


Video Link to AL80B Demonstration


The TOF system will easily fit most amplifiers that use a grounded negative grid meter system. The basic amplifier system looks like this:



If your amplifier has a grid meter at R3's position, and it has 12 volts DC available, it can be directly used. The TOF system, without overload relay, draws about 3 mA at 9 volts. Because it has a shunt regulator, the TOF draws 20-30 mA at 12 volts. This makes the TOF very easy to power.

While designed for standard grounded-negative supplies and meters, the TOF can be used in virtually any metering system when its power source is floated from ground.

The TOF, in most simple form, looks like this:


grid overload protection linear

An indicator lamp flashes when a certain peak is reached. About 20% or so beyond that warning level, and optional latching relay can be use to disconnect things. A resistor sets the value of the overload, and of the warning.

The TOF has internal provisions to represent the amplifier's grid meter internal resistance.

The internal relay system can be normally closed or normally open.

Here is the TOF board:


grid protection amplifier

The ribbon cable pins are standard .1 inch spaced header pin holes. The TOF can be ordered without ribbon.

For example, I have an amplifier bias control, transmit-receive relay sequencing, and metering board that works with amplifiers up to 10 amps plate current. That board accepts TOF modules for both plate current and grid current. The TOF is piggy backed on that board.


The TOF can easily be adapted to read peak current, voltage, or power in anything! It can be used to:

1.) read peak plate current, with a warning light and disconnect relay

2.) convert a power meter to a peak envelope power reading meter with a warning light and a latching relay

3.) read peak reflected power, with a warning light and disconnect relay


Why Use a TOF system?


History of Amplifier Metering Systems

Prior to 1982, legal limit amateur HF amplifiers typically monitored plate current on one dedicated plate current meter. Most amplifiers had a second meter that switched between grid current, relative output, and high voltage. This system was used because FCC rules required simultaneous measurement of anode supply voltage and anode current in any amplifier or transmitter exceeding 900 watts dc input power. All legal-limit amplifiers, by law, had to measure plate voltage and plate current at the same time. Since the FCC would not allow any legal power limit amplifier that did not simultaneously display plate current and high voltage, all legal limit one kilowatt amplifiers had at least two meters, one reading HV and one reading plate current.

Around 1982, FCC rules changed. The FCC changed amateur power levels from 1,000 watts dc input power to the final amplifier stage or stages, to 1500 watts peak envelope RF power. This change completely eliminated power amplifier output stage dc supply measurement requirements. Despite this FCC rule change, most amplifiers continued the old tradition and legal requirement. New amplifier were never rethought. They still continuously monitored plate current on a dedicated meter, switching a second meter between multiple functions that included HV or having additional meters.

Keeping plate current on a dedicated meter was a mistake. Grid current, screen grid current with multi-grid tubes and control grid current in grounded grid triode amplifiers, indicates proper drive and tuning far better than any other easily measured parameter. As a matter of fact, an experienced tuner can adjust an amplifier perfectly only watching the proper grid parameter.

In 1982, I switched all my amplifier designs to continuously monitor grid current, with plate current moved to the multimeter. I did this because the grid meter, more than any other parameter, indicates proper tuning and operation. This was an improvement from old-school metering, where one meter read high voltage or grid current and the other meter read plate current. Continuous grid current metering was much better than prior systems, because grid current indicates proper amplifier tuning more than any other single parameter. Even if a tuning pulser or "pecker" is used, you have no idea if you are properly tuned, or overdriving the amplifier. Despite what others might say, you still need to know the peak grid current! If grid current is not checked, there is significant risk of amplifier overdrive, amplifier damage, and/or splatter.

Standard current meters only read accurately with steady carriers. Unfortunately, metering systems to date have used a pseudo-average grid current meter. Without resorting to a carrier, standard metering systems do not even come close to indicating actual operating grid current. The common meter system requires switching to a carrier mode, and applying the maximum expected drive power to check for proper grid current and drive/tuning.  The drawbacks of using a carrier to read grid current are:

  1. A steady carrier is particularly hard on small tubes, like 811A tubes
  2. The steady carrier grid current is often not the real peak current
  3. A tuning pulser will not show grid current in normal metering systems
  4. During voice or Morse transmissions, the operator has no idea what true grid current is

I thought about a better way to tune and monitor amplifiers.

The TOF system solves the overlooked but major problems inherent with tuning pulsers or peckers. Besides a greatly improved running indication of proper drive and tuning during operation, the TOF system allows tuning (and monitoring) on the fly. The operator can adjust drive or loading while operating amplitude-varying modes, such as SSB or AM. This improved tuning method makes it virtually impossible to have two of the most damaging operating conditions, which are accidentally over-driving or accidentally under-loading the linear amplifier. This system will also detect the most damaging of antenna failure problems. This system will work with most other amplifiers, and plans are to eventually cover all amplifiers including solid state applications. 

Patent Pending

I generally do not like patents, but some have a tendency to use other's ideas or creativity to profit. They either make money, or they make up a story about developing an idea, system, or method, or a combination of those things. Historically, Ham manufacturers did not copy other manufacturers by directly "lifting" things. That seems to have changed. I decided to patent this concept or "method", which includes the basic operation of the system, because no one else has used this method.

The pending patent application is for the application, concept, and operation, rather than any specific wired unit or software implementation. The pending patent applies to solid state and multigrid tube amplifiers, as well as grounded grid amplifiers. This is my idea and my public release of the idea, and research indicates this application or concept has not appeared elsewhere before this release.


System Overview

Grid meters are the single most important indicator of proper linear amplifier operation. This includes screen grids in grid driven tetrode amplifiers, and control grids in grounded grid triode amplifiers. Almost every vacuum tube amplifier will benefit from this system, and it could even be included in certain solid state amplifiers.

This system modifies the grid current metering system in a unique way. This modification provides a constantly available indication of peak grid current. By watching peak grid current, the operator has a continuously running indication of improper tuning or excessive drive.

This system includes a bright warning LED, and provisions for an overload relay. The LED flashes when grid current peaks exceed to the optimum operating value. 

Lack of accurate reading during operation, when absent a steady carrier, causes at least three problems during operation. This also applies if you use a tuning pulser, ditter, or tuning pecker to tune:

1.) peak grid current will no longer be the same because peak exciter power changes

2.) peak grid current will no longer be the same because average amplifier voltages change with different power supply demands

3.) if the antenna system changes or drive power changes for any reason, affecting amplifier operating parameters in a harmful way, you have no indication or alert on the amplifier meters

In addition to the above, when a tuning pulser system is used and you tune exclusively for peak output power, you really have no idea what the grid current actually is. This system eliminates the largest problem that always occurs when tuning with a tuning pulser which is not knowing the true operating grid current.

This system allows the CW, SSB, or AM operator to immediately notice improper operation or tuning. With the addition of an external power meter, or with proper skill and the TOF alone, the operator can tune and monitor for optimum operation during normal voice, on any mode including AM, without external aids. It also allows tuning without loading to maximum peak power at full drive. The TOF1 also eliminates very common tuning errors caused by line voltage sag or power supply sag under carrier conditions. Once properly accustomed to tuning by grid current, nearly any operator should be able to adjust for optimum output and minimum distortion.

To make installation easy, the TOF leads are color matched to AL811 wiring. Please review the instructions below and be sure you are completely comfortable with installation.

The base TOF1 for the AL-811 amplifier series has five basic connections. The TOF1 should take about 30 minutes to install, the TOF1A and H slightly longer.

LED Warning Light

All TOF models include an LED warning indicator.

R6 sets the LED warning point. R6 is:


AL811 3.3k
AL811H 4.7k
AL80B 5.6k


The LED will occasionally blink red with normal operation. If you are overdriving the amplifier, or have the amplifier under loaded, the LED will stay on longer. If you find the LED hanging on for more than brief flashes, drive power is too high or the loading control is set too far closed.

Over-current Relay

The TOF1 can include an optional over-current relay. This relay has normally open and normally closed contacts. The contacts can serve many functions, including amplifier trip-off for mistuning.    

Later models will include a provision for ALC control. This will result in improved ALC operation in almost all amplifiers.

Cost and Availability

The base TOF has no overload relay, but does have provisions for an LED warning lamp. The base TOF1 currently sells for $55.00 US, plus $8 shipping and handling for priority mail. I will have to advise any additional cost to other countries, but most countries would fall under the $8.00 shipping price. Please specify the amplifier you have.

I accept PayPal to my normal email address. Type this in:


The TOF1 with overload relay is $68.00 plus $8.00 S&H.

Stock may be limited depending on sales. This is a new product, and price may change as my cost varies.


Typical Operation Overview

Amplifier operation with the TOF1 is normal, with the exception the grid current meter indicates peak current. Operators can observe maximum grid current on any mode, including AM or SSB. Excessive peak grid current indicates amplifier mistuning or excessive drive far better than any other system, including output envelope oscilloscopes. The results below show an ICOM 751A driving an AL811 three tube amplifier. Tuning was accomplished using only the TOF1 with normal SSB speech to the amplifier. Although the amplifier was tuned on SSB voice, the TOF works to tune with any mode. The TOF system works with any tuning pulser, and results are far more accurate than using the tuning pulser with a power output meter. As a matter of fact, a tuning pulser should probably never be used without a TOF system, because without a TOF you have no idea what the operating grid current is.



The AL811 tested was representative of any used AL811 amplifier. The was operated on 120 volts from a rather loosely regulated bench outlet. This outlet including 50 feet of number 14 AWG wire, and would be typical of most modern home wiring systems.  The radio was an IC751A ICOM, and power was measured with a calibrated Bird peak reading digital meter.


AL811 power ouput peak hold maximum SSB


I tuned the amplifier with full drive from the IC751A on twenty meters, using normal voice. The amplifier was adjusted for 140 mA absolute maximum grid current during normal SSB speech.


Tuning this way without the TOF, and without using SSB voice, will damage amplifiers or result in excessive splatter!





Tuning was adjusted so grid current just flicked up to the three tube mark. In this case the average anode current is 225 mA, and the peak grid current is around 120 mA with occasional flicks to 140 mA. Anode heat is well under safe tube ratings. If this was a carrier condition adjustment, the plate current would be nearly pinning and the tubes would be well over safe heat limits. Grid current would be the same.

The display below is a two minute storage of peak spectrum levels. This would be worse case splatter, because all peaks are stored at their peak level. Actual over the air signal would sound better, because occasional spits and spatters would not push the S meter or AGC up so high.  


AL811 three tube IMD at 850 watts PEP


  Lower side spurious was -48.28 dB peak at -3.15 kHz.


AL811 spectrum distortion SSB voice


 Upper side spurious was -37.62 dB peak at +3.68 kHz.

These results were about 10 dB better than tuning the conventional way with full drive, and 6 or 7 dB better than using a tuning pulser. The disparity between tuning with the TOF and tuning with carrier or pulser is from power supply sag. When the amplifier is loaded with continuous plate current, rather than normal voice plate current fluctuations, you will not get a good indication of peak operating grid currents.

I do not recommend full drive of 100-watts into three tubes, even though this signal was not significantly worse than the ICOM barefoot. The purpose of this is to show the improvement afforded by tuning during voice, and watching peak grid current. Typical maximum grid currents for several popular tubes is show in a table below.


All values are per tube with full airflow

 Tube Type Dissipation Peak Grid Current Typical drive power SSB PEP or CW carrier PEP SSB voice output power or CW carrier
811A 65W 50 mA 20 watts 200 watts (brief or low average)
572B 160W 70 mA 40 watts 350 watts
3-500Z 500W 140 mA 60 watts 750 watts
3CX800A7 800W 20 mA 40 watts 800 watts
8877/3CX1500A7 1500W 40 mA 90 watts 1500 watts

Note: Table is approximate values. Actual amplifier, including high voltage and other variables, determine exact values.